Assessment of Masticatory Performance: A Systematic Review of Current Assessment Methods and Measurement Properties

Abstract

To systematically evaluate the measurement properties of current masticatory performance (MP) assessment methods in adult populations. A systematic search was performed in the PubMed, Cochrane Library, and EBSCOhost databases for studies published from January 2000 to September 2025. This review included adult populations aged ≥18 years and was not restricted to a specific age subgroup. Eligible studies were those that assessed MP by mixing ability, comminution tests, or other objective approaches. The measurement properties and corresponding levels of evidence for each included study were systematically evaluated. Data extraction and methodological quality appraisal were conducted in accordance with the COSMIN guidelines. A total of 40 studies met the inclusion criteria. MP assessment methods can be categorized into mixing ability, comminution, and other tests. The mixing ability tests, particularly those using colour-changeable or two-colour chewing gums, demonstrated the most consistent evidence, with sufficient reliability and strong validity. Comminution tests, including sieving, optical scanning, and gummy jelly-based tests, revealed moderate reliability and validity. Emerging digital and image-based analyses have shown promising potential but remain insufficiently validated. Current MP assessment methods provide partial but insufficient evidence across measurement properties, and no standardized, clinically validated test exists. Future research should prioritize methodological standardization and incorporate digital and image-based technologies with rigorous validation to establish reliable and universally applicable MP assessment tools.

Background

Mastication is a complex neuromuscular process that involves the integration of the coordinated movements of the tongue, cheeks, teeth and masticatory muscles.^1,2 Efficient mastication is essential for adequate nutrient intake, safe swallowing, and maintenance of overall systemic health.³ Impaired mastication has been associated with malnutrition, frailty, cognitive decline, and reduced quality of life, particularly among older adults and individuals with compromised oral function.4, 5, 6 To evaluate masticatory function objectively, masticatory performance (MP), which is defined as the ability to comminute or mix test food during chewing, has become widely recognized as an important clinical indicator.⁷

Over the past few decades, numerous MP assessment methods have been developed, with a focus on different components of the masticatory process.⁷ Generally, MP can be assessed by mixing ability or comminution tests. Mixing ability tests evaluate the degree of colour mixing in colour-changeable or two-coloured chewing gums and waxes and are well known for their simplicity, with high clinical applicability.8, 9, 10, 11 Comminution tests measure how effectively food is broken down and quantify the reduction in particle size in natural or artificial test foods by sieving, optical scanning, or digital image analysis.12, 13, 14, 15 Additionally, photographic image analysis and smartphone-based applications have introduced digital and automated approaches that measure particle size, bolus uniformity, colour changes in test foods, and surface area, offering accessible options for MP evaluation.16, 17, 18 Despite the availability of multiple tests, MP assessment methods differ substantially in terms of test materials, analytical procedures, and reported outcome measures. Consequently, studies vary in how they examine key measurement properties such as reliability, validity, responsiveness, and measurement error, making direct comparison difficult. These inconsistencies also create challenges for clinicians and researchers when determining which methods provide the most accurate and clinically meaningful assessment of masticatory function.

Therefore, this systematic review aimed to systematically evaluate the measurement properties of MP assessment methods and test materials used in adult populations aged ≥18 years. Using a structured methodological framework, including the COSMIN guidelines, this review synthesizes evidence from comminution and mixing ability tests to identify which methods demonstrate a reliable and valid assessment of MP.

Methods

Protocol registration

This systematic review was reported in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) guidelines.¹⁹ The review protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) database (registration number – CRD420251165166).

Eligibility criteria

This review focused on full-length articles published in English in scientific journals that evaluated the measurement properties of objective MP assessment methods in adult populations aged ≥18 years, without restriction to a specific age subgroup. Studies conducted on animals or in vitro models, as well as those that did not objectively measure MP, were excluded. Studies that used only interview-based or self-report questionnaires and those focused exclusively on patients with craniofacial anomalies, neuromuscular disorders, or other conditions unrelated to masticatory function were also excluded. In addition, studies that used MP only as a clinical outcome without evaluating the measurement properties of the assessment method were excluded. Reviews, meta-analyses, study protocols, conference abstracts lacking full data, editorials, letters, commentaries, case reports or case series with ≤10 participants were also excluded.

Information sources and search strategy

Electronic searches of the entire database were performed in MEDLINE via PubMed, the Cochrane Library, and EBSCOhost (Dentistry and Oral Sciences Source). Potentially relevant articles were identified through Google Scholar. The databases were searched for studies published between January 1, 2000, and September 30, 2025. The detailed search strategies and keyword combinations are provided in Supplementary File 1. After the electronic search was complete, a targeted manual search of the reference lists of the included studies was conducted. Titles and abstracts retrieved from all sources were reported, and duplicates were removed prior to screening.

Study selection

Two reviewers independently screened the titles and abstracts against the prespecified inclusion and exclusion criteria. The full-text articles were then assessed independently by the same reviewers. Disagreements were resolved through discussion; when a consensus could not be reached, a third or fourth reviewer adjudicated. The reasons for exclusion at the full-text stage were recorded, and the study selection process is presented in a PRISMA 2020 flow diagram.

Methodological quality evaluation of the included studies

The methodological quality of the eligible studies was evaluated using the COSMIN risk of bias (RoB) checklist.²⁰ As this review focused on performance-based instruments for MP, only the relevant domains, reliability, validity, measurement error, and responsiveness, were assessed. Each domain contains 5 to 18 methodological items rated using the COSMIN’s four-point scale (very good, adequate, doubtful, inadequate), and the ‘worst score counts’ principle was adopted so that the lowest item rating determines the overall methodological quality for each measurement property. Two reviewers independently appraised the studies, and any disagreements were resolved through discussion; when consensus could not be achieved, a third or fourth reviewer adjudicated.

Evaluation of the quality of measurement properties

The quality of each measurement property was rated using the criteria proposed by Terwee et al.²¹ According to these criteria, each measurement property (reliability, validity, responsiveness, and measurement error) was rated as positive (+), indeterminate (?), or negative (−) based on the reported statistical outcomes (Table 1). Two reviewers independently evaluated all measurement properties, and the results were confirmed by the review team.

Table 1.

Quality criteria for rating the results of measurements properties.

Property	Rating	Quality criteria
Reliability
Internal consistency	+	Cronbach’s alpha(s) ≥0.70
	?	Cronbach’s alpha not determined
	−	Cronbach’s alpha(s) <0.70
Reliability	+	ICC/weighted kappa ≥0.70 OR Pearson’s r ≥ 0.80
	?	Neither ICC/weighted kappa, nor Pearson’s r determined
	−	ICC/weighted kappa <0.70 OR Pearson’s r < 0.80
Measurement error	+	MIC > SDC OR MIC outside the LOA
	?	MIC not defined
	−	MIC ≤ SDC OR MIC equals or inside LOA
Validity
Content validity	+	The target population considers all items in the questionnaire to be relevant AND considers the questionnaire to be complete
	?	No target population involvement
	−	The target population considers items in the questionnaire to be irrelevant OR considers the questionnaire to be incomplete
Structural validity	+	Factors should explain at least 50% of the variance
	?	Explained variance not mentioned
	−	Factors explain <50% of the variance
Construct validity (hypothesis testing)	+	Correlation with an instrument measuring the same construct ≥0.50 OR at least 75% of the results are in accordance with the hypotheses AND correlation with related constructs is higher than with unrelated constructs
	?	Solely correlations determined with unrelated constructs
	−	Correlation with an instrument measuring the same construct <0.50 OR <75% of the results are in accordance with the hypotheses OR correlation with related constructs is lower than with unrelated constructs
Cross-cultural validity	+	Original factor structure confirmed OR no important DIF between language versions
	?	Confirmatory factor analysis not applied and DIF not assessed
	−	Original factor structure not confirmed OR important DIF found between language versions
Criterion validity	+	Convincing arguments that gold standard is ‘gold’ AND correlation with gold standard ≥0.70
	?	No convincing arguments that gold standard is ‘gold’ OR doubtful design or method
	−	Correlation with gold standard <0.70, despite adequate design and method
Responsiveness
Responsiveness	+	Correlation with an instrument measuring the same construct ≥0.50 OR at least 75% of the results are in accordance with the hypotheses OR AUC ≥0.70 AND correlation with related constructs is higher than with unrelated constructs
	?	Solely correlations determined with unrelated constructs
	−	Correlation with an instrument measuring the same construct <0.50 OR <75% of the results are in accordance with the hypotheses OR AUC <0.70 OR correlation with related constructs is lower than with unrelated constructs

ICC, intraclass correlation; LOA, limits of agreement; MIC, minimal important change; SDC, smallest detectable change.

+, positive rating; ?, indeterminate rating; −, negative rating.

Adapted from Terwee et al., J Clin Epidemiol 2007.²¹

Levels of evidence

Following the quality appraisal, the overall level of evidence for each measurement property was determined using the evidence synthesis approach proposed by Terwee et al.²¹ Studies that used the same MP assessment method were synthesized by considering their methodological quality, the number of available studies, and the consistency of the results. The level of evidence for each measurement property was classified as strong, moderate, limited, conflicting, or unknown (Table 2). Two reviewers independently determined the levels of evidence, with any disagreements resolved through discussion and consensus, and the results were confirmed by the review team.

Table 2.

Levels of evidence for the quality of the measurement property.

Level	Criteria
Strong	Consistent findings, multiple studies of good methodological quality OR ≥ 1 study of excellent methodological quality
Moderate	Consistent findings, multiple studies of fair methodological quality OR ≥ 1 study of good methodological quality
Limited	One study of fair methodological quality
Conflicting	Conflicting findings
Unknown	Only studies of poor methodological quality

Adapted from Terwee et al., J Clin Epidemiol 2007.²¹

Results

Study selection and characteristics

The study selection process is illustrated in Figure 1 according to the PRISMA 2020 flowchart.¹⁹ The final electronic database search yielded 192 records. After 74 duplicates were removed, 118 records remained for title, abstract, and study-design screening. In the second phase, 41 full-text articles were assessed for eligibility. During this stage, three articles were excluded because they did not meet the inclusion criteria. Additionally, two relevant articles were identified through reference checking and manual searching. The last search was conducted on October 20, 2025. In total, 40 studies met the eligibility criteria and were included in the review. The characteristics of the included studies are summarized in Table 3, which presents the study objectives, methodological approaches, participant characteristics, test foods or materials used, and measurement properties evaluated.

Fig. 1.

2020 PRISMA flow diagram of the literature search, selection, and inclusion of the studies.

Table 3.

Characteristics of included studies.

Study	Study objective	Participants	Test food Test materials	Methods	Measurement property
Mixing ability methods
1. Hama et al17	To develop a smartphone application for evaluation of the colour-changing degree of chewing gum.	Development phase: 60 chewed gum samples (Xylitol, Japan). Field validation phase: n = 100 (aged 20-39 y)	Colour-changeable chewing gum	Chewed gum samples were flattened and imaged with smartphones under fluorescent light. Colour was measured using a colorimeter and image analysis with a correction algorithm. Additional samples and 100 adults were used for field validation.	Reliability, Validity (criterion validity), Measurement error
2. Nrecaj et al9	To evaluate the reliability and interchangeability between the two methods (two two-coloured chewing gum mixing-ability tests) and the gummy jelly glucose elution test.	n = 12, 6 M, and 6 F; mean age = 28.3 ± 4.1 y)	Two two-coloured chewing gums, gummy jelly	Each participant completed MP assessments using both two-coloured chewing gums (five samples each at 10, 15, 20, 25, 30 chewing cycles per gum) and the gummy jelly test (five samples at 10, 15, 20, 25, 30 s). The gum samples were scanned and colour-mixed analysis performed; gummy jelly glucose elution was measured.	Reliability, Measurement error
3. Yashiro et al10	To evaluate the visual measurement of MP by assessing chewing gum colour, using a colour scale compared to a colorimeter, to determine masticable foods for older adults.	n = 50, 24 males and 26 females, mean age: 82.6 ± 7.8 y	Colour-changeable chewing gum	Participants chewed the gum for 120 s. The chewed gum was evaluated by (1) a colorimeter (a* value in CIELAB colour space, mean of 5 points) and (2) a 10-grade visual colour scale. Participants were categorized into three groups of food masticability (meat, soft side dish, insufficient) based on a* or scale values.	Reliability, Validity (criterion validity)
4. Imamura et al11	To compare three alternative masticatory tests (colour-changing gum, two-coloured gum visual and digital) with the Jelly-scan glucose extraction test as a reference for assessing MP in older adults.	n = 61,23 M and 38 F, mean age: 82.4 ± 6.8 y, 34% denture wearers.	Colour-changing gum, two-colour Hue-Check gum, gummy jelly	Each participant underwent four masticatory tests: Jelly-scan test: gummy jelly was chewed 20 s and glucose concentration measured by colorimetric device. Colour-change gum: Chewed 60 times and degree of colour change was visually scored. Two-colour gum – visual: Chewed 20 times; mixed-colour level visually graded (SA1- SA5 scale). Two-colour gum – digital: Same gum flattened (1 mm), scanned, and analysed in ViewGum software for variance of hue (VoH).	Validity (hypothesis testing and criterion validity)
5. Carneiro et al24	To develop a new colour-changing test food based on an acid-base reaction for clinical assessment of MP.	n = 10, 5 M and 5 F, mean age: 21.1 ± 1.7 y	Test food with two overlapping halves (acidic + basic), Optosil silicone cubes	The test food was chewed for 10-100 cycles. Colour changes were quantified using the CIELAB system (L*, a*, b*) from standardized photos, averaging nine surface points. A 10-point colour scale was developed from mean colour values for each cycle. Thirty samples were rated by three examiners. Colour change was compared to Optosil cube fragmentation (X50 value).	Reliability, Validity (criterion validity)
6. Fankhauser et al29	To compare smartphone camera vs flatbed scanner images for analysing the colour mixing of two-coloured chewing gum.	Bench study using 5 chewed two-colour gum samples from one fully dentate adult volunteer	Two-coloured chewing gum	Specimens were flattened to 1 mm wafers, scanned with a flatbed scanner, and photographed 20 times using eight smartphones. Images were analysed for variance of hue (VoH) and visual scores (SA1- SA5) to compare digital and visual assessments.	Measurement error
7. Yousof et al33	To develop a custom-made two-coloured chewing gum and an image-processing method for objective evaluation of mixing ability.	n = 20, 10 M and 10 F, mean age: 20.9 ± 3.3 y	Custom-made red-green (RG) two-coloured chewing gum Ingredients: gum base, softener, powder filler, and water-insoluble dyes	Each participant chewed RG gum specimens for 3, 6, 9, 15, 25 cycles (repeated 3 times; n = 15 per subject) Chewed boluses flattened to 1mm wafers, scanned on flatbed scanner. Digital images analysed with ImageJ using a custom macro to measure geometric dispersion of red segment (CIELAB colour space). Hardness and mass loss of RG, commercial gum also measured pre-& post-mastication.	Validity (hypothesis testing and criterion validity)
8. Buser et al16	To validate a purpose-built two-coloured chewing gum and test the accuracy of a custom smartphone app for a colour-mixing ability test.	n = 20 dentate, 13 M and 7 F, mean age: 24.8 ± 8.2 y and n = 20 edentulous with implant overdentures, 5 M and 15 F, mean age:72.6 ± 8.2 y	Two-coloured chewing gum.	Participants chewed the gum for 5-50 cycles. Colour mixing was analysed using VOH_scan software and a smartphone app (VOH_app). Maximum bite force was also measured.	Validity (hypothesis testing and criterion validity)
9. Silva et al30	To evaluate the two-coloured chewing gum mixing-ability test for assessing MP in complete denture wearers, using both visual scoring and electronic colourimetric (VOH) analysis.	n = 75 complete denture wearers, 51 F, mean age: 67.1 ± 8.5 y	Two-coloured chewing gum	Participants chewed gum for 5-50 cycles (random order). The chewed gum was assessed visually (5-point scale, two raters) and electronically using ViewGum software (variance of hue, VOH).	Reliability, Measurement error
10. Wada et al25	To determine whether a colour-changeable chewing gum test can predict the types of regular foods older adults can masticate safely.	n = 30, 12 M and 18 F, mean age: 81.6 ± 8.6 y	Colour-changeable chewing gum and five regular foods (boiled rice, sliced white bread, ginger-fried pork loin, boiled fish-paste, rice cracker).	After 120 s of chewing, gum colour (a* value) measured at five points (CIELAB system) using a colorimeter. Food bolus texture (hardness, adhesiveness, cohesiveness) analysed with texture analyser at swallowing threshold.	Validity (hypothesis testing)
11. Vaccaro et al34	To select the optimal image processing method for assessing MP using two-coloured chewing gum.	n = 250 dentate, 120 M and 130 F, mean age: 25 ± 6.3 y	Two-coloured chewing gum	Each subject chewed 8 samples (3-25 cycles). 2000 samples scanned and 122 Image Processing Methods (IPMs) tested across 4 colour spaces.	Validity (hypothesis testing)
12. Schimmel et al31	To evaluate three two-coloured chewing gums for a colour-mixing ability test and validate new purpose-built software (ViewGum)	n = 20 dentate, 10 M and 10 F, mean age: 30.8 ± 6.7 y and n = 20 edentulous with dentures, 10 M and 10 F, mean age: 74.6 ± 8.3 y	Three two-coloured gums: Gum1: Hubba-Bubba Tape (control), Gum2: Lotte gum (test), Gum3: Vivident Fruitswing (test)	Participants chewed each gum for 5-50 cycles. Colour mixing was assessed visually (5-point scale, SA1- SA5) and opto-electronically using ViewGum software (variance of hue, VOH).	Reliability, Validity (hypothesis testing)
13. Hama et al26	To clarify the basic properties of a colour-changeable chewing gum and determine its applicability for evaluating MP in different dental conditions.	n = 10, 70% M, mean age: 27.7 ± 1.5 y (colour progression), n = 42 dentate, 52% M, mean age: 26.8 ± 2.6 y and n = 47 edentulous, 43% M, mean age: 74.9 ± 10.5 y (group comparisons)	Colour-changeable chewing gum	Gum chewed for 20-600 strokes; colour changes measured using colorimeter (L*, a*, b* in CIELAB system).	Reliability, Validity (hypothesis testing)
14. Hama et al27	To develop a new visual colour scale for evaluating MP using colour-changeable chewing gum	n = 10, age: 26-30 y for colour calibration; n = 18 for evaluation (6 dentists, age 25-27 y), (6 students, age: 21-23 y), (6 elderly; mean age: 68-84 y)	Colour-changeable chewing gum	Gum chewed for several 20-200 strokes; colour change quantified by colorimeter (CIELAB L*, a*, b*), linear regression used to generate theoretical colour-scale levels. Thirty-two chewed gums assessed three times by 18 raters using visual colour scale.	Reliability, Validity (criterion validity)
15. Endo et al36	To compare subjective and objective assessment methods using a two-coloured chewing gum test, and to determine whether these approaches can effectively distinguish MP differences between sexes.	n = 31, 16 M and 15 F, mean age: 24.8 ± 2.3 y with full natural dentition and Class I occlusion	Two-coloured chewing gum	Participants chewed 5 gum samples for 5, 10, 20, 30 and 50 strokes. Subjective indices were visually scored (1-5 scale). Objective analysis (OCMR-W) performed by scanning 1 mm wafer images and quantifying unmixed blue pixel ratio using Photoshop.	Reliability
16. Weijenberg et al35	To develop and fully digitalise a two-coloured chewing-gum mixing-ability test using spatial heterogeneity analysis.	Exp 1: n = 14, 7 M and 7 F, age: 19-63 y, Exp 2: n = 10, 4 M and 6 F, age: 20-49 y, Exp 3: 13 F age: 21-31 y	Two-coloured chewing gum, wax cubes	Mixing ability was assessed using digital image software, Mathematica. Exp 1: Participants chewed the gum in stepwise-increased cycles (5, 10, 15, 20, 25, 30); rest as needed; assessed for response to more cycles. Exp 2: Participants chewed the gum habitually for 20 s, repeated after 1 h. Exp 3: Participants chewed wax cubes (10 strokes each), then two gum samples (20 s each); outcome: agreement between gum and wax mixing measures.	Reliability, Validity (hypothesis testing)
17. Halazonetis et al32	To evaluate a new image-analysis software (ViewGum) for two-colour chewing-gum mixing ability test.	n = 20, 11 M and 9 F, mean age: 27.5 ± 3.4 y.	Two-coloured chewing gum	Each subject chewed 5 gum samples for 5, 10, 20, 30, and 50 cycles. Each chewed gum was flattened to 1 mm thickness, scanned on both sides (500 dpi), and analysed with ViewGum software. The software calculated the standard deviation of hue from HSI colour-space values (smaller SDHue = better mixing).	Validity (hypothesis testing), Measurement error
18. Van der Bilt et al22	To compare digital image processing and visual assessment of chewed two-colour wax in evaluating mixing ability.	n = 60 (3 groups × 20, age- and sex-matched): (Natural dentition/ Maxillary denture + implant-supported mandibular overdenture/ Full denture wearers)	Two-coloured wax tablets	Each subject chewed two-colour wax for 5-20 strokes. Wax was flattened (2 mm) and scanned (Epson V750). Mixing index calculated from red-blue intensity histograms (Photoshop). Five examiners visually scored 1-5.	Reliability, Validity (hypothesis testing)
19. Kamiyama et al28	To access self-implementable MP test using colour-changeable chewing gum and a dedicated colour scale as an alternative to colorimeter-based evaluation.	18 examiners in 3 groups (6 dentists, 6 adults, 6 elderly adults; all with normal colour vision).	Colour-changeable chewing gum	Gum was chewed for 40-200 strokes, flattened to 1.5 mm, and colour measured at five points by a colorimeter (a* value, CIELAB). Examiners rated 30 randomized samples using the colour scale.	Reliability, Validity (criterion validity)
20. Sugiura et al38	To compare a mixing ability test (two-coloured wax) with the tests using a brittle food (peanuts) and an elastic food (gummy jelly).	n = 72, 32 dentate, 18 M and 14 F, mean age: 25.1 ± 2.8 y and 40 removable partial denture wearers, 18 M and 22 F, mean age: 65.5 ± 9.1 y	Two-colour paraffin wax cube, Peanuts, Gummy jelly	Mixing Ability Index calculated from colour and shape parameters after 10 strokes using CCD camera and digital image analysis. Peanut test analysed by multiple-sieving method and gummy test by glucose concentration using blood glucose meter.	Validity (hypothesis testing), Measurement error
21. Schimmel et al23	To compare different assessment methods of a two-coloured chewing gum test	n = 20, 9 M and 11 F, mean age: 27.5 ± 3.4 y	Two-coloured chewing gum	Each participant chewed five gum samples for 5, 10, 20, 30, and 50 cycles on the preferred side. Gums were assessed visually (bolus and wafer) and by digital image analysis using Adobe Photoshop Elements. Maximum bite force was measured.	Reliability, Validity (hypothesis testing)
22. Salleh et al37	To compare the hardness of a wax cube used in the mixing ability test with common natural foods and to examine the effect of paraffin ratio on hardness.	n = 30, 15 M and 15 F, mean age: 26.9 ± 2.2 y	Two-coloured paraffin wax cube and 11 foods: cream cheese, boiled fish paste and beef, apple, carrot, peanut, rice cracker, jelly, chocolate, chewing gum.	Objective hardness: Tensipresser compression test (40% compression rate, 10 mm/s) – peak force (N) at 37 °C. Subjective hardness: Visual analogue scale (VAS, 0-100 mm) rated after one bite on preferred side.	Reliability, Validity (hypothesis testing)
23. Sato et al39	To assess mixing ability test for evaluating masticatory function using a two-coloured paraffin wax cube compared with the sieving method used as a ‘gold standard’ test.	n = 44, (Group A: 7 M and 4 F, age: 25-32 y), (Group B: 2 M and 18 F, age:42-75 y) (Group C: 5 M and 8 F, age: 55-77 y)	Two-coloured paraffin wax cube	Participants chewed cubes for 5-30 strokes. Images from six surfaces were captured with a CCD camera and analysed digitally. A Mixing Ability Index (MAI) was calculated using discriminant function combining colour-mixing and shape parameters. The Comminuting Ability Test (CAI) served as gold standard.	Reliability, Validity (hypothesis testing)
Comminution methods
24. Aiyar et al48	To evaluate the glucose concentration method and its applicability in orthodontic patients with braces.	n = 16, 7 M and 9 F, mean age: 26 ± 5 y, (orthodontic patients), n = 16 control, 5 M and 11 F, mean age: 26 ± 4 y.	Gummy jelly, colour-changing chewing gum	Participants chewed gummy jelly and colour-change gum for 5, 10, and 15 s. Glucose concentration measured using Glucosensor GS-II; colour mixing analysed using ViewGum software (variance of hue, VoH). Questionnaires (OHIP, MPQ, JFLS) evaluated perception and pain.	Validity (criterion validity)
25. Carneiro et al13	To assess capsules containing fuchsin-based beads for the evaluation of MP.	n = 60, mean age: 41.0 ± 13.2 y and divided into four groups by number of occlusal pairs (n = 15 each):16-14 (control), 13-10, 9-6, and 5-1 groups.	Fuchsin-bead capsules, Optosil silicone cubes	Subjects chewed each test food. Fuchsin released was measured by UV-Vis spectrophotometry. Median particle size for cubes determined by multiple-sieve analysis.	Validity (criterion validity) Measurement error
26. Murakami et al15	To verify the compatibility and between two gummy jelly based MP tests: the glucose-concentration method and the 10-step visual scoring method.	n = 134, 51 M and 83 F, mean age: 74.1 ± 8.8 y.	(1) Glucolumn 2 g gummy jelly for glucose-based test, GC, (2) UHA Mikakuto 5.5 g gummy jelly for 10-step visual scoring, VS (3) ISA method.	GC: chewed 2 g gummy jelly for 20 s; glucose (mg/dL) < 100 = low MP. VS: chewed 5.5 g gummy 30 times; visual score ≤ 2 = low MP. ISA: The comminuted pieces of gummy jelly from VS methods were photographed and analysed for surface area (mm²); cut-off < 1687 mm².	Validity (hypothesis testing and criterion validity)
27. Sano et al14	To assess inter-measurement variation of the MP test.	n = 40, 20 M and 20 F, mean age: 31.1 ± 4.7 y.	Gummy jelly	Two groups: Group A (no prechewing) and Group B (pre-chewing). Each chewed one gummy jelly for 20 s on the habitual side, repeated three times (1-min intervals). Glucose extraction was measured with Glucosensor GS-II.	Reliability
28. Salazar et al18	To develop a photographic image-analysis method for measuring MP using gummy jelly.	75 chewed gummy-jelly samples (50 for model construction, 25 for verification).	Gummy jelly	Digital images captured with a smartphone under three lighting conditions. Image parameters (area, perimeter, colour average, colour deviation, side area, surface roughness) were extracted and entered into a multiple-regression model to estimate increased surface area and weight. Accuracy was validated against a fully automated β-carotene extraction system.	Validity (criterion validity)
29. Igarashi et al49	To access the visual scoring method using gummy jelly by testing its correlation with MP measured by a fully automated device.	n = 1234, 504 M and 730 F, mean age: 77.1 ± 4.8 y.	Gummy jelly	Participants chewed the gummy 30 times, and chewed pieces were visually scored on a 10-point scale (0-9) based on standard photos (actual VS). The same specimens were photographed and re-scored (photo VS). A fully automatic device measured β-carotene elution to calculate MP (MP was calculated as increased surface area)	Reliability, (hypothesis testing and criterion validity)
30. Sumonsiri et al44	To evaluate frankfurter sausage as a suitable test food for measuring MP by examining its correlation with almonds.	n = 27, mean age: 23.9 ± 3.8 y.	Frankfurter sausage and almond	Subjects chewed each food for 5 and 15 cycles on both sides. Almonds were air-dried; sausage samples were freeze-dried for 48 h. Chewed particles were sieved through 8 mesh sizes; median particle size (X₅₀) was calculated from cumulative weight curves. Six subjects repeated the test after 2-4 wk.	Reliability, Validity (hypothesis testing)
31. Sanchez-Ayala et al40	To evaluate fuchsin beads as test food for assessing MP.	n = 20, 5 M and 15 F, mean age: 23.3 ± 0.7 y.	(1) Fuchsin beads encapsulated in polyvinyl acetate capsule (2) Optosil silicone cubes as gold standard.	Each participant performed 5 MP trials (3 by examiner 1, 1 each by examiners 2 & 3). Each trial = 20 chewing cycles. Fuchsin release quantified by UV-Vis spectrophotometry (absorbance AU). Silicone-cube comminution analysed by multiple sieves – median particle size (X₅₀).	Reliability, Validity (criterion validity)
32. Sanchez-Ayala et al41	To evaluate silicone Optosil Comfort as an artificial test food for MP evaluation using single, double, and multiple sieve methods.	n = 20, 5 men and 15 F, mean age: 23.3 ± 0.7 y.	Optosil Comfort	Each participant performed three trials (1-wk intervals) by the first examiner and one each by two others. After 20 chewing cycles of 17 cubes, particles were sieved (0.5-5.6 mm), weighed, and analysed using single, double, and multiple sieve methods.	Reliability, Measurement error
33. Sugimoto et al43	To develop an objective image-analysis method for evaluating mastication using a mixed natural test food simulating daily meals and to determine whether large particle analysis can discriminate deficient mastication.	n = 10 F, mean age: 27.6 ± 2.6 y with ≥ 28 natural teeth, normal occlusion, and no masticatory disorders.	Mixed foodstuffs (total 8.6 ± 0.4 g): cooked rice, sausage, hard omelette, raw cabbage, and raw cucumber.	Participants chewed mixed foods under four conditions: 100 %, 75 %, 50 %, and 25 % of individual natural masticatory strokes. Bolus samples were collected and washed in 0.032% fatty alkanolamide + 0.06% benzalkonium chloride solution, and analysed by digital image analysis under double dark-field illumination. Particles > 2 mm were analysed for area and virtual diameter.	Validity (hypothesis testing)
34. Nokubi et al50	To evaluate a 10-stage scale visual scoring method for MP using gummy jelly.	n = 1, 50 raters (26 M and 24 F. mean age: 33.4 ± 10.6 y, range 23-68 y.	Gummy jelly	A 10-stage visual score scale (based on glucose concentration 20-600 mg/dL, 50 mg/dL increments) was constructed from photographic images of chewed samples. Each rater assigned scores (1-10) for 50 images on a laptop three times under controlled conditions.	Reliability, Validity (hypothesis testing and criterion validity)
35. Sugimoto et al42	To develop a new image-analysis method focusing on large particles (>2 mm) in food bolus and to determine whether it can discriminate hampered mastication.	n = 20 F, mean age: 23.4 ± 4.3 y, with ≥ 28 natural teeth, normal occlusion, and no masticatory disorders.	Natural foods: raw carrot, peanuts, and cooked beef	Participants chewed each food under four conditions: (1) normal, (2) half strokes, (3) quarter strokes, and (4) reduced force (20% EMG of masseter). Chewed samples were washed and dried; particles >2 mm were analysed using a double dark-field image-analysis system to measure particle area and virtual diameter. For validation, the same samples were also subjected to multiple sieving (2.0-0.25 mm mesh sizes).	Validity (hypothesis testing)
36. Eberhard et al45	To evaluate optical scanning method for assessing MP comparable to conventional sieving method.	n = 20, 10 M and 10 F, mean age: 24 ± 2 y	Optosil comfort	After 15 chewing strokes (17 cubes / test), samples were collected and analysed by (1) sieving through 10 sieves and (2) flatbed scanning followed by ImageJ analysis to estimate particle area and volume. Median particle size (x₅₀) was calculated via Rosin–Rammler distribution for both methods.	Validity (criterion validity)
37. Van der Bilt et al46	To compare single and multiple sieve methods for determining MP and evaluate their sensitivity to dental state, age, and gender.	n = 176, 53 M and 123 F, age: 19-70 y.	Optosil Plus	Each subject chewed 17 cubes for 15 cycles, then expectorated, washed, and dried the comminuted particles. Multiple-sieve method: stack of 12 sieves (0.5-8 mm). Single-sieve method: calculated % weight passing 1 mm, 2 mm, or 4 mm sieve.	Validity (hypothesis testing)
38. Ohara et al47	To develop a simplified sieve method using standardized hydrocolloid impression material as a test food, and comparing it with peanuts.	Exp A: 15 dentate adults 10 M and 5 F, age: 23-36 y. Exp B: 15 new dentate adults, 10 M and 5 F, age: 24-35 y.	Cylindrical hydrocolloid impression material and peanuts.	Exp A: Participants chewed 5-35 strokes of both test foods and the specimen was then washed and put on seives. 10 sieves (0.71-4.75 mm). Exp B: Chewing (hydrocolloid impression material only) repeated 3 different days/ 1-wk interval.	Reliability, Validity (hypothesis testing)
Other methods
39. Goto et al51	To assess the odour-based masticatory efficiency test using odour intensity released during chewing as a non-invasive indicator of MP.	n = 20, 12 M and 8 F, mean age: 26.9 ± 3.3 y.	Odor compound-containing chewing gum, gummy jelly	Participants chewed the gum for 10, 20, 30, and 40 s. Odor intensity measured immediately, and 1, 5, and 10 min after chewing with a portable odour sensor (OMX-SR). Odor intensity value = difference between pre- and post-chewing readings. Gummy jelly masticatory efficiency was measured using glucose extraction (mg/dL).	Validity (hypothesis testing) Measurement error
40. Ikebe et al52	To investigate the association between posterior occlusal contacts (Eichner Index) and MP.	n = 1288, (640 M and 648 F, mean age: 66.2 ± 4.2 y.	Gummy jelly	Participants classified into 10 Eichner subgroups by posterior occlusal contacts (group A 1, 2, 3; Group B 1, 2, 3; group C 1, 2, 3). Maximum occlusal force was measured using pressure-sensitive sheets. Masticatory performance was assessed by glucose concentration (mg/dL) eluted from chewed test gummy jellies (30 strokes).	Validity (hypothesis testing)

Masticatory performance assessment methods

The assessment methods of MP can generally be categorized into three main groups: mixing ability tests, comminution tests, and other assessment methods (Table 4).

Table 4.

Measurement properties and level of evidence of the included studies.

Study (Author)	Test method	Measurement outcomes					Key findings
		Reliability	Validity		Responsiveness	Measurement error
			Hypothesis testing	Criterion validity
Colour-changeable chewing gum (mixing ability)
Hama et al17	Assessment of colour-changing degree of chewing gum. (smartphone based)	Adequate (+)		Adequate (+)		Inadequate (−)	ICC ≥ 0.97, r ≥ 0.92 vs colorimeter, Bland-Altman LoA ±6 ΔE.
Level of evidence		Moderate		Limited		Limited
Carneiro et al24	Assessment of MP using acid-base reaction gum from image colour analysis	Adequate (+)		Adequate (+)			k > 0.8 intra-/inter-examiner reliability. a* inversely and b* directly correlated with X₅.
Level of evidence		Limited		Limited
Wada et al25	Assessment of MP from chewing gum using colorimeter		Doubtful (+)				Higher a* values were significantly associated with mastication ability and prepare food with suitable texture for swallowing.
Hama et al26	Assessment of MP from chewing gum using colorimeter	Inadequate (+)	Inadequate (+)				ΔE increased with chewing strokes until plateau. ICCs > 0.70 at 60-160 strokes. Significant difference between dentate and edentulous groups (P < .001).
Level of evidence		Limited	Moderate
Yashiro et al10	Assessment of MP with chewing gum using visual colour scale compared to colorimeter.	Adequate (+)		Adequate (+)			Inter-rater reliability (k = 0.94) and strong agreement with colorimeter (k = 0.91). Visual colour-scale accurately classified masticatory levels.
Imamura et al11	Assessment of MP with chewing gum using visual colour scale compared to colorimeter.		Adequate (+)	Adequate (+)			Strong correlation with glucose extraction; AUC 0.872; specificity 100%; improved kappa after cutoff adjustment.
Hama et al27	Assessment of MP with chewing gum using visual colour scale compared to colorimeter.	Adequate (+)		Doubtful (+)			ICCs > 0.90 and rs > 0.95 between visual scale and colorimeter.
Kamiyama et al28	Assessment of MP with chewing gum using visual colour scale compared to colorimeter.	Doubtful (+)		Doubtful (+)			ICCs = 0.86-0.94, strong correlation (r = 0.979) between colour scale and colorimeter. a* increased with chewing strokes.
Level of evidence		Strong	Limited	Strong
Two-coloured chewing gum (mixing ability)
Nrecaj et al9	Assessment of MP using chewing gum with digital colour-mixing analysis (ViewGum software) and gummy-jelly (glucose extraction test).	Inadequate (−)				Inadequate (−)	Correlation between two gums and gummy jelly tests (r = 0.74-0.86). Low measurement error, Maximum occlusal force - not related with tests.
Imamura et al11	Assessment of MP using chewing gum with digital colour-mixing analysis (ViewGum)		Adequate (+)	Adequate (+)			Strong correlations with visual and glucose tests; AUC 0.835; specificity 100%; improved kappa after cutoff adjustment.
Fankhauser et al29	Assessment of MP from chewing gum, smartphone images analysed for colour variance with visual 5 point scales and ViewGum software.					Inadequate (−)	VoH values from smartphone and scanner highly correlated (r > −0.96); small mean difference (−0.044 to −0.006); consistent trends across SA1-SA5.
Buser et al16	Assessment of mixing ability, Validated with smartphone app against scanner-based VoH.		Adequate (+)	Adequate (+)			VOH decreased with chewing cycles (P < .0001) and showed strong agreement between app and scanner (R² = 0.80-0.83)
Silva et al30	Assessment of mixing ability of gum by using visual 5-point scales and ViewGum software.	Adequate (+)				Adequate (?)	ViewGum (VoH) and visual 5-point scale analyses showed strong reliability (k > 0.8) and agreement (r = −0.89). VoH decreased with more chewing cycles (P < .001).
Schimmel et al31	Assessment of colour mixing ability with visual 5-point scale and ViewGum software.	Adequate (+)	Doubtful (+)			Doubtful (?)	ViewGum (VoH) – ICC 0.95-0.97, agreement with visual scale (r = −0.97). VoH decreased with increased chewing cycles (P < .001).
Halazonetis et al32	Assessment of mixing ability using image analysis software ViewGum.		Doubtful (+)			Doubtful (?)	Intra-/inter-rater reliability (LoA < 0.02). SDHue decreased significantly with chewing cycles (R² = 0.94, P < .01).
Level of evidence		Strong	Strong	Strong		Moderate
Yousof et al33	Assessment of MP from custom-made gum using image-processing method: Image J, geometric dispersion (GD).		Adequate (+)	Adequate (+)			GD increased with chewing cycles (r = 0.90, P < .001). Significantly discriminated cycle groups (P < .001).
Level of evidence			Limited	Limited
Vaccaro et al34	Assessment of colour mixing ability using digital software, MATLAB 2015b.		Very good (+)				Variance of histogram of Hue (VhH) best for MP: correlation with cycles rho = 0.79 (P < .0001); automated analysis (n > 2000) reliable, no error.
Level of evidence			Strong
Weijenberg et al35	Assessment of mixing ability using gum with fully automated digital analysis (Diffpix, Mathematica).	Inadequate (+)	Inadequate (−)				DiffPix decreased with increasing chewing cycles (F = 57.27, P < .001). ICC = 0.714. No significant correlation with wax cube test (rₛ = 0.429).
Level of evidence		Limited	Unknown
Endo et al36	Assessment of MP with chewing gum using subjective colour-mixing index (SCMI) and objective colour-mixing ratio (OCMR).	Doubtful (+)					Significant differences among chewing strokes (P < .001); SCMI-W and OCMR-W (k = 0.42-0.90); OCMR-W better detected sex differences (P = .007); reliable at ≥ 20 strokes.
Level of evidence		Limited
Imamura et al11	Assessment of MP using mixing ability visual assessment (SA1-SA5 scale)		Adequate (+)	Adequate (+)			Strong correlation (P = .625); AUC 0.864; sensitivity 73%, specificity 90%; improved diagnostic agreement.
Schimmel et al23	Subjective assessment (SA1-SA5 scale) for gum bolus and colour mixing ability of wafer	Inadequate (−)	Inadequate (?)				Unmixed Fraction significantly decreased with chewing strokes (P < .001); intra-/inter-examiner reliability (k = 0.60-0.88); reproducible on retest; no correlation with bite force; 20 strokes recommended for reliable assessment.
Schimmel et al31	Subjective assessment (SA1-SA5 scale) for gum bolus and colour mixing ability of wafer	Doubtful (+)				Doubtful (?)	Visual scale showed agreement with VoH. mixing values increased with chewing cycles (P < .0001)
Silva et al30	Assessment of mixing ability of gum by using visual 5-point scales and ViewGum software.	Adequate (+)				Adequate (?)	ViewGum (VoH) and visual 5-point scale analyses showed reliability (k > 0.8) and agreement (r = −0.89).
Level of evidence		Moderate	Limited	Limited		Limited
Two-coloured wax (mixing ability)
Van der Bilt et al22	Assessment of visual 5-grade scale and digital image analysis (mixing index from histogram intensity distributions)	Adequate (?)	Adequate (+)				Strong correlation between visual and digital methods (r = 0.95, P < .001); digital image analysis discriminated denture types better; intra/inter-rater k = 0.55-0.78; 20 chews recommended for reliable assessment.
Level of evidence		Unknown	Moderate
Salleh et al37	Assessment of mixing ability by evaluating physical and sensory hardness of wax cube compared with 11 foods.	Adequate (+)	Adequate (+)				Wax cube hardness closely matched soft food texture; strong correlation between objective and subjective hardness (r = 0.90, P < .001) and repeatability (ICC > 0.68)
Level of evidence		Limited	Limited
Sugiura et al38	Assessment of Mixing-Ability Index (MAI) (wax) with comminution (peanut) and glucose-releasing (gummy jelly) tests.		Adequate (+)			Adequate (?)	MAI significantly correlated with peanut comminution (r = −0.56 to −0.70, P < .001) but not with gummy-jelly glucose (ns); confirmed wax-based MAI reflects brittle-food comminution ability.
Sato et al39	Assessment of mixing ability using two-coloured paraffin-wax using MAI and compared with sieving method.	Inadequate (+)	Doubtful (+)				MAI showed significant correlation with sieving (rₛ = 0.56-0.72, P < .001) and reliability (ICC = 0.89-0.99).
Level of evidence		Limited	Moderate			Unknown
Sieving and optical scanning (comminution tests)
Carneiro et al13	Assessment of MP from Optosil silicone cubes using multiple-sieve analysis.			Adequate (+)		Inadequate (−)	Optosil cube tests differentiated MP by occlusal pair count (P < .05), cube AUC = 0.887 (sens/spec 80%);
Sanchez-Ayala et al40	Assessment of Optosil Comfort as artificial test food for evaluating MP using sieve method.	Adequate (+)		Adequate (−)			Optosil Comfort comminution test showed excellent intra- and inter-examiner reproducibility (ICC > 0.90) and consistent particle-size measurement (X₅₀) across trials.
Sanchez-Ayala et al41	Assessment of Optosil Comfort as artificial test food for evaluating MP using sieve method.	Adequate (+)				Doubtful (?)	Intra- (ICC = 0.89-0.98) and inter-examiner reproducibility (ICC > 0.93, P < .001) across single, double, and multiple sieve methods; random error <1 mm.
Level of evidence		Strong		Moderate		Unknown
Sugimoto et al42	Developed a new digital image analysis system for large food particles (carrot, peanut, beef); validated against sieve-measured weight.		Adequate (+)				Image-measured area strongly correlated with sieve weight (r = 0.83); threshold values (0.10, 1.62) distinguished normal vs suppressed mastication with sensitivity 0.86, specificity 0.90.
Sugimoto et al43	Extended the same digital image analysis to mixed real foods (rice, sausage, vegetables) to simulate daily diet.		Adequate (+)				Homogeneity and size indices (HI ≤ 0.10, SI ≥ 1.62) effectively classified full vs deficient mastication (P < .001; sensitivity 0.90, specificity 0.77).
Level of evidence			Moderate
Sumonsiri et al44	Assessment of MP using natural test foods using sieving method.	Adequate (+)	Adequate (+)				Frankfurter sausage (15 chews) showed high reliability (ICC = 0.91) and strong correlation with almond (r = 0.76, P < .01). Median particle size decreased with chewing cycles.
Level of evidence		Limited	Limited
Eberhard et al45	Assessment of MP using optical scanning method by comparing to conventional sieving method.			Adequate (+)			Validated optical scanning (ImageJ) method for Optosil test food produced x₅₀ values highly correlated (r = 0.919-1.0) with sieving.
Level of evidence				Unknown
Van der Bilt et al46	Assessment of MP with Optosil Plus using sieve methods.		Doubtful (+)				Both single (1-4 mm) and multiple sieve methods detected occlusal-unit effects (P < .001; r = 0.91-0.98). Multiple sieving (X₅₀ = 3.32 ± 0.76 mm) provided more reliable and interpretable results (R² = 0.16) than single sieving (R² = 0.14).
Level of evidence			Limited
Ohara et al47	Assessment of impression material (alginate) as a test food for MP using sieve method.	Adequate (−)	Inadequate (+)				Impression material showed strong linear correlation (r = 0.92-0.99, P < .01) between particle count and chewing strokes, with high intra-/inter-test reproducibility (r = 0.65-0.90). Two-task (10 & 20 strokes) protocol equivalent to seven-task.
Level of evidence		Unknown	Limited
Fuchsine beads (comminution tests)
Carneiro et al13	Assessment of MP using encapsulated fuchsine beads as a test food.			Adequate (+)		Inadequate (−)	Fuchsin-bead test differentiated MP by occlusal-pair count (P < .05); AUC = 0.744 (sensitivity 73%, specificity 70%), closely matched Optosil Comfort sieving results.
Sanchez-Ayala et al40	Assessment of MP using encapsulated fuchsine beads as a test food.	Adequate (?)		Adequate (−)			Fuchsin-based bead test showed good intra- and excellent inter-examiner reproducibility (ICC = 0.65-0.76), but lower reliability and validity than the silicone-cube reference (mean difference = −31.8 %, P < .001).
Level of evidence		Unknown		Moderate		Unknown
Gummy jelly (comminution tests)
Aiyar et al48	Assessment of MP using gummy jelly glucose concentration test.			Adequate (+)			The glucose extraction method showed a strong correlation (R² = 0.965) with the colour-changing gum test. Glucose concentration values increased linearly with chewing duration (R² = 0.711).
Sano et al14	Assessment of MP using the glucose-extraction gummy-jelly test.	Adequate (+)					Repeated glucose-extraction tests (20 s) showed high inter-measurement reliability (ICC = 0.924-0.950) when participants pre-chewed the gummy jelly, eliminating first-trial bias.
Murakami et al15	Assessment of MP by comparing glucose-extraction test (GC) with the visual scoring and photographic ISA methods.		Adequate (+)	Adequate (+)			GC showed strong agreement with visual scoring (k = 0.86) and strong correlation with ISA (r = 0.70); ISA cut-off 1687 mm² matched 100 mg/dL GC threshold.
Imamura et al11	Assessment of MP using the glucose-extraction gummy-jelly test (Jelly-scan).		Adequate (+)	Adequate (+)			Correlated strongly with all gum-based mixing tests (P = .53-.63), showed expected decreases with age and fewer teeth, and diagnostic agreement improved markedly with adjusted cutoff.
Level of evidence		Limited	Strong	Strong
Salazar et al18	Assessment of MP using photographic image analysis method. (smartphone base)			Adequate (+)			Photographic image analysis strongly correlated with the fully automated method (ICC = 0.96) and visual scoring (r = 0.96).
Murakami et al15	Assessment of MP using photographic image analysis method. (smartphone base)		Adequate (+)	Adequate (+)			Increase surface area (ISA) correlated strongly with glucose-extraction (r = 0.70) and visual scoring (k = 0.86); ISA cut-off 1687 mm² corresponded to 100 mg/dL GC threshold for decreased masticatory function.
Level of evidence			Limited	Strong
Igarashi et al49	Assessment of MP using visual scoring method (actual and photo VS) by comparing with a fully automated β-carotene system.	Adequate (+)	Adequate (+)	Adequate (+)			Actual-VS and photo-VS showed strong correlations with fully automated MP (r = 0.86-0.87) and inter-rater reliability (ICC = 0.93).
Murakami et al15	Assessment of MP using visual scoring method by comparing with glucose extraction test (GC).		Adequate (+)	Adequate (+)			VS showed excellent agreement with GC (k = 0.86; sens 88 %; spec 97 %) and correlation with ISA (r = 0.70).
Nokubi et al50	Assessment of MP using visual scoring method compared with GC.	Adequate (+)	Adequate (+)	Adequate (+)			10-stage visual scoring scale showed excellent correlation with glucose method (rₛ = 0.91-0.98) and inter-/intra-rater reliability (ICC 0.946-0.947).
Level of evidence		Strong	Strong	Strong
Other methods
Goto et al51	Assessment of odour-based MP test using odour sensor (OMX-SR).		Inadequate (+)			Inadequate (?)	Odour-based test correlated well with glucose test (r = 0.68) and showed lower variation and better reproducibility. Odour intensity measured immediately after 10 s chewing best reflected masticatory efficiency.
Level of evidence			Unknown			Unknown
Ikebe et al52	Assessment of MP using Eichner Index.		Doubtful (+)				Occlusal force and MP decreased with fewer occlusal contacts; bilateral (B2) and unilateral (B3) premolar contacts were critical for maintaining function.
Level of evidence			Limited

Mixing ability tests

The mixing ability refers to a person’s capacity to knead and blend a test food or material during chewing.^22,23 The mixing ability tests commonly use test materials such as colour-changeable chewing gum, two-coloured chewing gum, or two-coloured wax, and the degree of colour mixing is evaluated to quantify the MP.

Colour-changeable chewing gum

Eight studies assessed MP using colour-changeable chewing gum.^10,11,17,24, 25, 26, 27, 28 These studies analysed gum colour progression after chewing and evaluated measurement properties such as reliability, validity, and measurement error. Previous studies^10,17,24,26, 27, 28 have examined the reproducibility of colour-change measures. Test-retest evaluations, repeated ratings, and examiner-based assessments consistently demonstrated good reproducibility, indicating that colour-changeable chewing gum provides stable measurements across repeated trials and raters. Criterion validity was evaluated by comparing gum colour values with reference measures such as colorimeters or tests of food fragmentation.^{10,11,17,24,27,28} These comparisons revealed that the gum colour measurements closely corresponded with established reference indicators. Construct validity has been reported in studies that examined the expected relationships between gum colour change and factors such as masticable food categories, dental conditions, or variations in chewing strokes.^11,25,26 These studies supported the expected directional changes in colour according to the MP. One study reported measurement error using colorimetric methods and suggested that the method provides precise and consistent quantitative colour measurements.¹⁷

Two-coloured chewing gum

A total of 12 studies evaluated MP using two-coloured chewing gum.^9,11,16,23,29, 30, 31, 32, 33, 34, 35, 36 These studies evaluated the degree of color mixing after chewing through either digital image analysis or visual scoring. Reliability has been consistently reported, with digital tools such as ViewGum, variance of hue (VoH) analyses, and image-processing methods^{9,30,31,35,36} consistently reporting reliability, demonstrating stable colour-mixing outcomes across repeated measurements and different examiners. Studies that compared subjective visual scoring and objective digital scoring^23,30,31 have also shown reliable consistency between raters and across sessions. Criterion validity was evaluated by comparing gum-mixing measures with reference standards such as colorimeters, scanner-based image analyses, glucose-based tests, or comminution outcomes from alternative test foods.^11,16,33 These studies demonstrated strong correspondence between two-coloured chewing gum outcomes and reference measurements. Construct validity has been examined in studies assessing the expected relationships between colour mixing and chewing cycles, dental status, age, or masticatory ability.^11,16,23,31, 32, 33, 34, 35 These studies consistently revealed that an increased number of chewing strokes or better masticatory function resulted in improved colour mixing. Studies using digital colour mixing or colorimetry VoH analyses^9,29, 30, 31, 32 reported low measurement errors, indicating precise and repeatable digital colour measurements.

Two-coloured wax

Four studies evaluated MP using two-coloured wax.^22,37, 38, 39 These studies used colour-mixing or wax fragmentation analyses and reported reliability, validity, and measurement error as the main measurement properties. Three studies^22,37,39 assessed the reliability of wax-based mixing ability tests and reported reasonable repeatability. Repeated chewing trials and repeated measurements of colour-mixing indices showed stable values across sessions and examiners, indicating that two-coloured wax provides consistent and reproducible results. Construct validity was assessed in all four studies, which demonstrated that two-coloured waxes responded predictably to differences in dental status, the number of chewing strokes, and participant characteristics. One study reported measurement error.³⁸ This study evaluated repeated measurements of colour and shape parameters after wax cubes were chewed and found to have acceptable levels of error, indicating indeterminate precision for both clinical and research use.

Comminution tests

Comminution tests are used to evaluate MP by assessing the degree to which a test food is broken down or fragmented during chewing.⁷ These tests generally involve a range of test foods, including gummy jelly, optosil silicone materials, fuchsine-bead capsules, natural foods (eg, almonds, carrots, beef), and mixed-food boluses. Comminution tests can be categorized into sieving or optical scanning tests, fuchsine bead tests, and gummy jelly-based tests.

Sieving and optical scanning tests

A total of 9 studies evaluated MP using sieving or optical scanning tests,^13,40, 41, 42, 43, 44, 45, 46, 47 which assess the particle size distribution after being chewed through single or multiple sieves or through digital image analysis. Most studies have examined the reproducibility of particle size measurements. Studies using repeated trials or multi-examiner assessments^40,41,44,47 have consistently reported reliability, demonstrating stable particle size distributions and median particle size values across repeated measurements, days, and examiners. Criterion validity was evaluated by comparing optical scanning or UV–Vis based particle quantification with sieving procedures.^13,40,45 Eberhard et al⁴⁵ demonstrated a strong correspondence between optical scanning outcomes and the sieving method. Two studies confirmed criterion validity through comparisons with comminution of alternative test foods or with reference materials such as Optosil silicone cubes.^13,40 The construct validity was examined in studies evaluating the expected relationships between particle size reduction and masticatory ability, dental condition, chewing strokes, or food hardness.42, 43, 44^,46,47 These studies consistently showed that participants with better MP or an increased number of chewing strokes produced smaller and more uniform particles, supporting the discriminative ability of sieving and optical scanning tests. Two studies reported low levels of variability for sieve-based and optical scanning outcomes, indicating precise and repeatable particle size measurements.^13,41

Fuchsine bead tests

Fuchsine beads are small, dye-containing beads enclosed in a capsule that release fuchsine dye when crushed during chewing. Two studies evaluated MP using fuchsine bead capsules.^13,40 These studies measured the degree of fragmentation by quantifying the amount of fuchsine dye released after chewing, typically using UV–Vis spectrophotometry. Sanchez-Ayala et al⁴⁰ examined the reproducibility of dye release measurements. The repeated trials conducted in this study consistently demonstrated good reliability, with stable absorbance values and consistent dye release profiles across multiple assessments and examiners. Both studies compared dye release values with the median particle size obtained from Optosil silicone cube comminution tests. These comparisons demonstrated a reliable correspondence between fuchsin release and particle size reduction, supporting the criterion validity of the test. The measurement error was quantified in one study.¹³ Although the reported error values were small and the UV‒Vis measurements showed narrow limits of agreement, the study did not establish interpretability thresholds.

Gummy jelly tests

A total of seven studies used gummy jelly to assess MP.^11,14,15,18,48, 49, 50 These studies assessed comminution through glucose concentration, visual scoring, or photographic image analysis methods. Four studies^11,14,15,48 examined MP through glucose concentration measurements. One study reported good reliability when repeated trials or repeated chewing sessions were used.¹⁴ The visual scoring test also showed consistent intra- and interrater agreement,^49,50 supporting the reproducibility of score-based gummy jelly evaluations. Criterion validity is typically evaluated by comparing gummy jelly outcomes with reference standards such as automatic β-carotene extraction, image-analysis surface area, two-coloured gum tests, or glucose sensors.^11,15,18,48, 49, 50 These studies demonstrated strong correspondence between glucose concentrations or visual scores and established MP measures, confirming the criterion validity of gummy jelly tests. Construct validity was assessed in studies evaluating expected differences in masticatory ability according to dental status, occlusal contact, chewing time, or functional classification.^11,15,49,50 These studies consistently showed that individuals with better MP produced higher glucose concentration values, more advanced particle breakdown, or higher visual scores.

Other assessment methods

Goto et al⁵¹ evaluated MP using an odour-based method that measures odour intensity released from odour compound-containing chewing gum with a portable odour sensor. The construct validity was supported by a significant correlation between odour intensity and glucose concentration values from chewed gummy jelly, indicating that individuals with better masticatory efficiency released stronger odour signals. The measurement error was small, indicating that the odour-based method provided consistent and precise readings. Ikebe et al⁵² assessed MP by examining occlusal support using the Eichner index and evaluated the associated validity properties. The construct validity was confirmed, as the study revealed that both the occlusal force and the MP decreased systematically with fewer occlusal contacts. Participants in Eichner groups B2 and B3, representing bilateral and unilateral premolar contact losses, showed significant reductions in function compared with those with more complete posterior support. These findings were consistent with the expected functional deterioration linked to occlusal support loss.

Measurement properties and level of evidence

The methodological quality, measurement property ratings, and level of evidence for each included study are summarized in Table 4 and Supplementary File 2. The levels of evidence were determined by synthesizing the methodological quality (COSMIN RoB) with the sufficiency ratings of their measurement properties. The summary results of the measurement properties and levels of evidence for MP assessment methods in the included studies are shown in Figure 2.

Fig. 2.

Summary of the measurement properties and levels of evidence for masticatory performance assessment methods in the included studies.

Colour-changeable chewing gum (mixing ability)

Studies using colour-changeable chewing gum have assessed MP through visual scoring or colorimetric analysis.^10,11,17,24, 25, 26, 27, 28 Reliability was consistently rated sufficiently (+), with a limited to strong level of evidence, indicating stable colour-change measurements across the raters and repeated trials. The criterion and construct validity were also sufficient (+) and limited to a strong level of evidence, with colour-change values showing strong correlations with reference measures such as glucose extraction, digital colour analysis, and known functional variations. The measurement error was rated limited because most studies lacked the smallest detectable change (SDC) or minimal important change (MIC) reports. Overall, the level of evidence ranged from limited to strong, with the colorimeter and visual scoring tests supported by strong evidence due to consistent positive findings across multiple studies. These results indicate that colour-changeable chewing gum provides a valid and reliable method for assessing mixing ability in MP research.

Two-coloured chewing gum (mixing ability)

Many studies have evaluated MP using two-coloured chewing gum, employing digital colour-mixing analysis or visual mixing scores.^9,11,16,23,29, 30, 31, 32, 33, 34, 35, 36 Although reliability findings have not been uniformly rated (+), with a limited to strong level of evidence, both visual scoring and digital measures have demonstrated reliable repeatability and stable outputs across repeated assessments where examined. A limited to strong level of evidence was observed for validity. Some studies rated criterion validity as sufficient (+), supported by strong and consistent correlations between mixing outcomes and established reference tests, including glucose extraction, colorimetric measurements, and particle-size indices. The construct validity was sufficiently rated to be sufficient (+) in most studies, and both visual and digital methods showed expected differences across dental status, occlusal support, age groups, and known MP levels. Measurement error was examined in only a subset of the studies and frequently lacked complete reporting of key indices such as SDC or MIC. As a result, the overall synthesis of evidence for measurement error remains inconsistent, with a limited to moderate level of evidence. Overall, the two-coloured chewing gum test, both visual and digital, demonstrates reliable and valid performance, which is supported by multiple studies with positive findings, although the reporting of measurement error remains insufficient in several studies.

Two-coloured wax (mixing ability)

Some studies have assessed MP using two-coloured wax to evaluate mixing ability.^22,37, 38, 39 Reliability ranged from unknown to limited levels of evidence, with some evidence of reproducible mixing scores but variability in examiner agreement and reporting quality. Construct validity was also rated from limited to moderate, supported by modest correlations with alternative MP measures and by expected differences between groups with varying functional statuses. However, the findings were less consistent than those for two-coloured chewing gum, partly due to heterogeneity in wax materials and scoring approaches. Measurement error is generally unknown, as none of the studies reported the MIC, SDC, or comparable indices required by COSMIN to evaluate error. Overall, the level of evidence for two-coloured wax tests was limited to moderate, indicating that while wax-based mixing tests show potential, they are supported by fewer and methodologically weaker studies than gum-based assessments.

Sieving and optical scanning (comminution tests)

Several studies have investigated MP via sieving or optical scanning tests, which evaluate the particle size distribution after chewing.^13,40, 41, 42, 43, 44, 45, 46, 47 Reliability ranged from unknown to strong levels of evidence, with digital image analysis systems and standardized multi-sieve protocols showing greater consistency than manual procedures do. Criterion validity was rated unknown to moderate, as supported by correlations with glucose concentration, colour-mixing tests, and occlusal function measures, although strength varied with test-food type and measurement protocol. Construct validity was also rated as limited to moderate, as both sieving and optical scanning tests demonstrated expected differences according to dental status, occlusal support, age, and MP levels. Two studies assessed measurement error, and the level of evidence was rated as unknown. While some optical scanning studies reported low variability, most comminution studies did not provide the MIC or SDC values required for COSMIN evaluation. Overall, the level of evidence for sieving and optical scanning tests ranged from unknown to strong for reliability and validity, indicating that comminution-based assessments are supported by multiple studies, although reporting of measurement error remains insufficient.

Fuchsine beads (comminution tests)

A small number of studies have evaluated MP using fuchsine bead capsules, which quantify dye release after bead rupture during chewing.^13,40 In these studies, reliability was rated unknown, reflecting limited reporting and variability in repeated measures. The criterion was rated moderate, supported by correlations between dye concentration and other MP measures, including glucose extraction, particle comminution, and functional status. One study evaluated measurement error, but the level of evidence was rated as unknown, and the results indicated insufficient measurement error.¹³

Gummy jelly (comminution tests)

Many studies have evaluated MP using gummy jelly tests, including glucose concentration, particle size distribution, surface area analysis, and photographic image analysis.^11,14,15,18,48, 49, 50 Reliability ranged from limited to strong, with glucose concentration and visual scoring tests showing the highest repeatability. The criterion validity and construct validity ratings ranged from limited to strong, supported by consistent correlations with established reference measures such as colour-mixing tests, sieving outcomes, and dental status and by expected differences across groups with varying masticatory ability, age, and occlusal support. Overall, the level of evidence for gummy jelly ranged from limited to strong, with strong support observed for glucose concentration, photographic image analysis, and visual scoring tests. These findings indicate that gummy jelly-based tests are robust comminution approaches, although future studies should address measurement error.

Other assessment methods

Unknown to limited levels of evidence were identified for construct validity in studies evaluating alternative approaches to MP.^51,52 The odour-sensor method provided insufficient information to establish construct validity, as predefined hypotheses were not tested and associations with reference measures were not adequately reported. For the odour-based approach, measurement error also remains unknown because essential metrics such as the MIC or SDC are not presented. Similarly, the Eichner Index demonstrated only limited evidence for its ability to discriminate MP across occlusal support categories, and further methodological shortcomings contributed to an overall uncertain evidence rating.

Discussion

This systematic review synthesized evidence from 40 studies to evaluate the measurement properties of current masticatory performance (MP) assessment methods in adult populations aged ≥18 years. The present review indicates that while numerous methods are available, the strength and consistency of evidence vary substantially across test types, particularly with respect to reliability, validity, and measurement error. Overall, mixing ability tests, especially those using colour-changeable or two-coloured chewing gum, demonstrated the most consistent and clinically applicable evidence, whereas comminution-based tests showed greater methodological heterogeneity. These findings are consistent with earlier reviews, which reported colour-changeable chewing gum as a valid and reliable method for evaluating masticatory function⁵³ and identified moderate-to-strong levels of evidence for mixing ability tests using two-coloured chewing gum, particularly when assessed with visual or digital analysis tools.⁵⁴

Clinical interpretation of current MP assessment methods

From a clinical perspective, gum-based mixing ability tests currently demonstrate the strongest evidence for reliability and validity and represent the most supported tools for assessing MP. Moreover, their simplicity and ease of standardization make them well suited for clinical and epidemiological use. Recent studies using smartphone-based colour analysis, visual colour scales, and colorimetric assessments further support the feasibility of these methods in routine clinical settings, particularly when examiner experience or access to specialized equipment is limited.9, 10, 11^,16,17 In contrast, two-coloured wax has shown less consistent evidence, likely due to variability in material properties, scoring protocols, and limited methodological reporting.^22,37, 38, 39

Comminution tests, including sieving, optical scanning, and glucose concentration, have strong theoretical validity but show less consistent methodological quality. Although several recent studies have reported promising diagnostic performance for gummy jelly-based glucose concentration and photographic image analysis methods,^{14,15,18,48,49} variability in test foods, chewing protocols, and analytical thresholds hinders their immediate clinical standardization. Consequently, these methods may be better suited for research settings or structured screening programs than for routine clinical assessment at present.

Other approaches, such as odour-based assessment⁵¹ and occlusal contact indices,⁵² lack sufficient evidence across key measurement properties. While preliminary associations with established MP indicators have been reported, incomplete validation and limited reproducibility prevent their recommendation as standalone MP assessment tools.

Influence of age, dentition, and test food characteristics on MP assessment methods

The evidence from this review suggests that age and dentition status influence MP measurement outcomes, as well as their reliability and validity. Lower MP values have been reported in older adults and in individuals with reduced posterior occlusal support.^5,15,52 Moreover, age-related variability in chewing patterns and muscle coordination may increase measurement variability, especially in tests requiring brittle food fragmentation. Test food characteristics also play a critical role in MP assessment. Differences in test food hardness strongly influence chewing behaviour, force generation, and measurement variability.⁷ Elastic materials such as chewing gum generally demonstrate greater reproducibility, as their mixing-based outcomes are less sensitive to individual variations in occlusal force and chewing stroke patterns. In contrast, viscoelastic foods such as gummy jelly and brittle test foods are more sensitive to occlusal force, which may increase measurement variability across individuals. Together, these factors directly affect reliability by increasing measurement variability and validity by influencing the test’s ability to accurately reflect true masticatory function. These findings are consistent with earlier MP assessment studies^25,37,38 highlighting the need for standardized test materials and protocols, particularly when MP is used to monitor longitudinal changes or intervention effects.

Relationship with previous literature

To date, only a limited number of reviews have addressed methods for assessing MP. Tarkowska et al⁵³ provided an overview of available assessment methods using colour-changeable chewing gum but did not systematically evaluate measurement properties using a validated appraisal framework. A systematic review by Elgestad Stjernfeldt et al⁵⁴ applied a validated methodological framework to assess MP measurement properties. However, it was conducted before the widespread adoption of digital and automated assessment technologies. As a result, objective methods incorporating photographic image analysis, smartphone-based applications, and automated particle analysis were either not included or not critically appraised. The present review extends the literature by providing an up-to-date, structured, and systematic evaluation of contemporary MP assessment methods, incorporating recent technological developments and current standards for measurement property evaluation.

Implications for research and clinical practice

On the basis of the synthesized evidence, gum-based mixing ability tests are currently recommended as the most clinically reliable MP assessment methods. Comminution tests retain conceptual value; however, methodological variation limits reproducibility. Standardization of test food, particle-analysis techniques, and reporting thresholds is essential before they can serve as reference measurements. Across all methods, measurement error remains insufficiently reported, limiting interpretation of longitudinal changes and intervention effects. Future research should prioritize standardized protocols, explicit reporting of measurement error parameters (eg, MIC and SDC), and further validation of digital and automated technologies to establish reliable, scalable, and clinically meaningful MP assessment tools.

Limitations

This review has several limitations that should be considered when interpreting the findings. First, substantial heterogeneity was observed across the included studies with respect to test foods, assessment protocols, outcome measures, and analytical approaches. Variations in the mechanical properties of test foods (eg, elastic, viscoelastic, or brittle materials), chewing instructions, and analytical thresholds limit direct comparisons between studies and make quantitative synthesis difficult. Second, participant characteristics vary widely across studies, including age distribution, dentition status, and occlusal support, which may influence MP outcomes and contribute to variability across studies. In addition, differences in study design and reporting quality affect the overall strength of evidence assessment. Finally, this review was limited to full-length English-language articles, which may have resulted in the exclusion of relevant studies published in other languages. Despite these limitations, the structured COSMIN-based appraisal and comprehensive synthesis provide a clear overview of the current evidence and highlight priorities for future methodological research.

Conclusion

Although multiple methods are available for assessing masticatory performance, evidence across measurement properties remains limited, and no standardized clinical test with established diagnostic accuracy is currently available. Based on current evidence, chewing gum-based mixing ability tests demonstrate the most consistent evidence for reliability and validity, whereas gummy jelly-based comminution tests show promise but require further methodological standardization. Future research should focus on the integration and comprehensive validation of digital and automated assessment approaches, together with established methods, to develop reliable, standardized, and clinically applicable tools.

Data availability

The datasets generated and/or analysed during the current study are available from the corresponding author upon reasonable request.

Funding

The authors declare that there are no sources of funding for this review.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.