Table 3.

(a) Sensory methods commonly used to investigate whey protein-derived mouthdrying. (b) Physiochemical analysis commonly used to investigate whey protein-derived mouthdrying. (c) In vivo analysis commonly used to investigate whey protein-derived mouthdrying.

(a) Sensory methods commonly used to investigate whey protein-derived mouthdrying.
Method	Food Matrix	Description	Limitations
Sensory methods using trained panel or consumers. Key limitation: unable to explain the cause of mouthdrying
Descriptive analysis using a trained sensory panel 1,2	Cakes and biscuits [90], WPB [83,86,91,92,123,124,125,126,140,141,142], rye bread and cream cheese [94]	oProvides an objective sensory measure of mouthdrying	oStudies have used different methods (such as SpectrumTM and QDATM), scored differing numbers of attributes (2 to 36) and there are potential issues with providing a standard mouthdrying reference to ensure consistency across studies
Threshold using a trained sensory panel 1	WPB [123,139,140]	oEvaluates mouthdrying intensity strength compared with protein concentration	oStudies have rated mouthdrying intensity using different methods (for example: 0–5 and 0–7 scales, SpectrumTM and scalar scoring), different types of whey protein beverages and studies have used varying number of panelists (7–12 panelists)
Sequential profiling and time intensity methods using trained sensory panels 1	WPB [86,122,123,124]	oSequential profiling measures changes in sensory attributes with repeated consumption and time intensity provides data on time, duration and intensity of mouthdrying	oTypically, sequential profiling methods have not solely focused on mouthdrying and there are also potential issues with providing a standard mouthdrying reference to ensure consistency across studies
Sensory methods using consumers 1,2	WPB [83,89,91,92,143], cakes and biscuits [90], muffins [93], rye bread and cream cheese [94]	oProvides feedback on products using the target consumer population. Common methods to evaluate mouthdrying include focus group sessions, 9-point hedonic liking, Just-About-Right (JAR), generalised linear magnitude scale (gLMS), visual analogue scale (VAS) and two-alternative forced choice test (2-AFC).	oLimited studies have tested mouthdrying using consumers and there are potential issues with test sensitivity of methods used. Carter et al. noted consumers are untrained and potentially less able to quantify mouthdrying objectively [46].
1 Refers to studies using a whey protein liquid model (whey protein beverage: WPB); 2 refers to studies using a whey protein solid model.
(b) Physiochemical analysis commonly used to investigate whey protein-derived mouthdrying.
Method	WPB Model	Description	Limitations
Physiochemical analysis. Key limitation: requires sensory data to provide correlations
Taste sensor 1*	WPI, PWP, aPWP [139]	oMeasures the change in membrane potential as a result of adsorption	oAnalysis has been carried out in low pH WPBs; therefore, this method may not be suitable for neutral pH WPBs
Turbidity 1*#	β-LG [123], WPI [124], β-LG and LF 126]	oMeasures aggregation of protein and saliva	oSaliva has been mixed artificially with whey protein and this may differ to saliva samples collected post beverage consumption oSaliva samples in the referenced studies were only collected from 2–5 volunteers; however, saliva is considered to vary between individuals oTurbidity in isolation is unlikely to explain the cause of mouthdrying
Electrophoresis analysis 1*#	β-LG and LF [125,126]	oDetermines protein composition using SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis)	oAs for turbidity: saliva has been mixed artificially with whey protein; saliva samples only collected from 2–5 volunteers
Dynamic Light Scattering 1*#	WPC [86], β-LG and LF [126]	oMeasures the size and distribution of protein and/or with saliva	oAs for turbidity: saliva has been mixed artificially with whey protein oParticle size in WPB increases with heating time, however, mouthdrying is also present in unheated WPBs. Therefore, particle size in isolation is unlikely to explain the cause of mouthdrying
Zeta potential 1*#	WPC [86], β-LG and LF [126,140]	oMeasures electrostatic interactions of protein, with or without saliva	oBull et al. identified within a neutral pH that WPBs (samples varying in levels of heat treatment) had similar zeta potential scores, therefore proposed mouthdrying in this study was not related to electrostatic interactions and proposed other mechanisms could be involved. However, saliva was not collected in this study [86].
Portable infrared spectrometer 1*	WPI, WPC, WPH [141]	oPredicts mouthdrying in low pH WPB	oThis method was only tested in low pH WPBs; therefore, this method may not relate to mouthdrying from neutral pH WPBs
Tribology 1*	β-LG [128]	oMeasures friction and lubrication	oIn some conditions (i.e. increasing protein concentration from 0.5 to 4%) sensory results were unable to correlate with tribology data
1 Refers to studies using a whey protein beverage (WPB) model (whey protein isolate (WPI), process whey protein (PWP), acidic process whey protein (aPWP), whey protein concentration (WPC), whey protein hydrolysate (WPH), β-lactoglobulin (β-LG) and lactoferrin (LF); 2 refers to studies using a whey protein solid model; *denotes studies using a low pH WPB model; #denotes studies using a neutral pH WPB model.
(c) In vivo analysis commonly used to investigate whey protein-derived mouthdrying.
Method	WPB Model	Description	Limitations
In vivo analysis. Key limitation: requires sensory data to provide correlations
Saliva flow 1*	β-LG [123]	oEvaluates saliva flow following different stimulants and relating this to whey protein-derived mouthdrying	oStudies have been limited by the number of saliva samples which can be collected within one session and this referenced study was limited by a relatively small sample size (10 volunteers) with a gender imbalance (2 males and 8 females)
Animal models 1#	β-LG [131]	oMeasures the adhesion of proteins to porcine oral mucosa tissue	oMethods need to be adapted to enable human investigation
Oral retention 1#	WPC [89,134]	oMeasures the protein remaining in saliva samples post beverage consumption	oPrevious limitations were small subject size and no non-protein control; more recent limitations include the link between mucoadhesion and mouthdrying within the same method have not been investigated
Dynamic in vivo models 1*	WPI [127]	oAims to replicate in-mouth beverage consumption by measuring whey protein and saliva interactions	oModels were estimated based on limited data from the literature, therefore may not fully reflect individual variability
1 Refers to studies using a whey protein beverage (WPB) model (whey protein concentration (WPC), whey protein isolate (WPI), β-lactoglobulin (β-LG); 2 refers to studies using a whey protein solid model; * denotes studies using a low pH WPB model; # denotes studies using a neutral pH WPB model.