Instant stevia powder as a novel potential additive for enhancing nutritional value and quality characteristics of yogurt

Abstract

In the current study, yogurts containing instant stevia powder (ISP) at varying proportions (0.1, 0.2, 0.3, and 0.4 g/100 mL) were perused in terms of physicochemical attributes, textural behavior, antioxidant activity, and sensory acceptability during 14 day storage at 4 °C. For this, bioactive components extracted by using microwave-assisted system were spray dried in optimum conditions (11 mL/min flow rate and 167 °C inlet air temperature) and then incorporated into yogurts. The minimal syneresis value (17.09 g/100 g) at the day of 14 was detected in ISP (0.4 g/100 mL)-supplemented yogurts while this value was reached to 19.45 g/100 g in control counterpart without stevia powder. Enriching yogurts with powders was a plausible way for boosting their mechanical properties. The antioxidative parameters namely total phenolic content (TPC), DPPH, ABTS, FRAP, and CUPRAC values were tendency to increase with ISP increment in yogurts. Low scores in sensory evaluation were detected in yogurts loaded with ISP above a certain amount (more than 0.2 g/100 mL). Sum up, the findings proved that the hypotheses (fabricating innovative dairy product rich in bioactive substances and maintaining quality parameters of yogurts during storage) predicted for this study were successfully achieved.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05892-z.

Introduction

The production and consumption of yogurt, one of the most considerable dairy products, has a long history all over the world. In terms of nutrition, this dairy product is rich in proteins, lipids, vitamins (riboflavin, B6, B12 etc.), and minerals that are excellent health benefits. Yogurt assists to strengthen the immune system and inhibit diseases thanks to its probiotic and prebiotics components together with these structures (Fazilah et al. 2018). However, typical problems for yogurt as in other fermented milk products relate to rheological and textural attributes namely low viscosity, syneresis, or liquid consistency (Domagała et al. 2013). Moreover, this product is poor in phenolic compounds with beneficial effects on health. For this reason, yogurt products are enriched with bioactive components by adding multifarious plant extracts e.g., argel extract (Mohamed Ahmed et al. 2021), pineapple peel (Sah et al. 2016). Also, these components have positive impact on the mechanical properties and consumer preference (Mohamed Ahmed et al. 2021).

Stevia (Stevia rebaudiana) is a perennial herb belonging to the Compositae family. This plant is grown in many parts of the world while its homeland is South America. Out of 200 species, one type (S. rebaudiana Bertoni) is sweet. This characteristic feature is because of steviol glycosides with hundreds of times sweeter than sucrose. Therefore, the industrial utilization of stevia as a sweetener substitute has been widespread, recently. Moreover, the presence of vitamins, minerals, essential amino acids, fatty acids, flavonoids, phenolic compounds, and phytosterols in this plant was noted (Wölwer-Rieck 2012; Ahmad et al. 2022). Many studies have been reported regarding the extraction of these special structures in stevia and their applications. In a previous study, the microwave-assisted system conditions were optimized for functional compounds extraction from it (Yılmaz et al. 2021). However, the utilization of the obtained structures is limited due to some reasons. One of them is their low stability against environmental stresses namely temperature, oxygen etc. (Ahmad et al. 2022). Also, integrating them into the food matrices in liquid form is not an effective approach because of homogeneity problem, solubility, unknown concentration, and undesirable flavor (Chranioti et al. 2016). Therefore, converting extracts into powder form is a sensible approach for overcoming these shortcomings. Spray drying system is widely used for the fabrication of powders thanks to short production period, final product with the desired quality, a simple and economical method (Poozesh and Bilgili 2019). For example, lemon balm (Tülek et al. 2021) and mango (Siacor et al. 2020) powders were fabricated using this system. In these previous studies, both extracts were fed to spray dryer after incorporation of maltodextrin (a drying aid material) into them. Maltodextrin is frequently used in this system because it is abundant, cost-effective, and dissolves well in water, preventing disadvantages such as agglomeration in final products (Singh et al. 2019). In addition to all this, it is necessary to optimize the spray drying conditions since the nature of extracts is dissimilar from each other. For example, the convenient air inlet temperature for drying process was 125 °C for lulo (Solanum quitoense L.) pulp (Igual et al. 2014) and 185 °C for Jamun (Syzygium cumini L.) pulp (Singh et al. 2019).

Studies on the evaluation of instant plant-based powders as ingredients in food matrices are limited. Increasing such studies will be a guide for scientific literature and food industry. All of these mentioned reasons, in the current study, set-type yogurts containing different concentrations of instant stevia powders (ISPs) produced using spray dryer were prepared. The efficacy of powders on physicochemical attributes, textural behavior, microbial loads, antioxidant activity, and sensory quality of yogurt was investigated for 14 days.

Materials and methods

Materials

Dried stevia was procured from a local market in Şanlıurfa province of Turkey. It was pulverized with the help of a laboratory type grinder (DMS253, Demsan, Turkey) and stored at + 4 °C for further uses. All chemicals were of analytical purity and obtained either from Sigma or Merck unless otherwise stated.

Preparation of extracts, powders, and yogurts

Microwave-assisted extraction

The microwave-assisted extraction step was carried out according to a previous study regarding the extraction of stevia with some modifications (Gençdağ et al. 2021). For this, 100 mL distilled water was added into a glass balloon containing 10 g stevia (1:10, w/v). This balloon was placed in the relevant part of a microwave extractor device (Sineo, Mass II Plus, Shanghai, China). Microwave watt (600 W), extraction temperature (50 °C), and extraction time (20 min) were fixed during the process period. After centrifugation at 4000 rpm for 10 min (Nüve, NF 1200R, Ankara), the supernatant was filtered through filter paper. The filtrate was stored at + 4 °C until analysis.

Instant stevia powder production

The water-soluble dry matter content of stevia extracts was set to 7 g/100 g by using distilled water. The ratio of water-soluble dry matter content in extracts to drying aid material was adjusted as 1:1 (w/w). For this, 7 g maltodextrin was added to a glass beaker containing stevia extract. Next, this mixture was homogenized at 12,000 rpm by using Ultra-Turrax (IKA-T18 Basic, Japan) for 5 min at room temperature and fed to a spray dryer (Unopex B15, Bak-On, Mak. Muh. Ltd. Sti., Izmir, Turkey).

The spray drying step was optimized by using response surface methodology with thirteen trials (two factors and five levels). Flow rate (5, 10, 23, 35, and 40 mL/min) and inlet air temperature (120, 129, 150, 171, and 180 °C) of spray drying were shifted to obtain maximum powder (Table S1).

Yogurt preparation

Method of preparation with some modification was used to prepare yogurt (Korkmaz et al. 2021). Raw milk (fat content: 3.10%) was obtained from the local market in Şanlıurfa province of Turkey. Heat treatment was applied to this raw material at 85 ± 2 °C for 20 min. After it was cooled to 42 °C, 2% yogurt culture (L. delbrueckii subsp. bulgaricus and S. thermophilus, 1:1 v/v) (YC-350, Chr Hansen, Istanbul, Turkey) and different ratios (0.1, 0.2, 0.3, and 0.4 g/100 mL) of ISP were added, respectively. The control sample was produced without ISP. The mixtures were filled in polystyrene cups (100 mL) and incubated until pH 4.6 at 42 °C. Yogurts were stored at 4 °C (Fig. S1) and analyzed on the 1st, 7th, and 14th days of storage.

Analyses

FTIR spectroscopy

The samples placed in the relevant part of the FTIR spectroscopy (Shimadzu Corporation, Japan) were analyzed at 1 cm⁻¹ resolution between 400 and 4000 cm⁻¹ wavelengths at ambient temperature (Arriola et al. 2019).

Physicochemical properties

Moisture content, water activity, Hausner ratio, Carr index, color, and solubility (Başyiğit et al. 2022) were investigated accordingly.

Powder yield

Powder yield (%) was detected by using the total solids content in the feeding solution and final powder weight. The calculation was conducted according to the following equation (Eq. (1)) (Başyiğit et al. 2022).

$$Powderyield(\%)=(1-\frac{Finalpowderweight}{Totalsolidcontentinthefeedingsolution})x100$$ 1

Process efficiency

Process efficiency (%) was calculated by a method of Tülek et al. The total phenolic content (TPC) and surface phenolic content of ISP were determined, and the efficiency was calculated using the following equation (Eq. (2)) (Tülek et al. 2021).

$$Processefficiency(\%)=(1-\frac{Steviapowdersurfacephenoliccontent}{Steviapowdertotalphenoliccontent})x100$$ 2

Phenolic content and antioxidant activity

Sample preparation

For powders, 0.5 g sample was dissolved with 10 mL distilled water and vortexed for 1 min. After this mixture was centrifuged at 4000 rpm for 5 min (Nüve, NF 1200R, Ankara), the supernatant was used for analyses.

For yogurts, the mixture of 1 g sample and 10 mL 80% ethanol was centrifuged at 4000 rpm for 5 min. Next, the supernatant was exposed to the evaporation process up to dryness. The dried extracts were blended with 5 mL distilled water and used for analyses.

Total and surface phenolic content

TPC was measured using the Folin-Ciocalteu reagent (Alsataf et al. 2021). Briefly, 0.4 mL diluted sample, 2 mL diluted Folin–Ciocalteau reagent (1:9 v/v), and 1.6 mL sodium carbonate (7.5%, w/v) were mixed in a glass tube and incubated for 60 min in a lightless environment at room temperature. Absorbance was measured at 765 nm. Results were expressed as mg gallic acid equivalent per g sample (mg GAE/g).

The same procedure explained for TPC analysis for powders was applied in the surface phenolic content but ethanol instead of distilled water was used as a solvent.

Antioxidant activity

In the DPPH method, 3.9 mL 25 mg/L methanolic DPPH solution and 0.1 mL diluted sample were mixed. This mixture was incubated in the dark for 30 min and absorbance was read at 515 nm (Çam et al. 2009).

In the ABTS method, ABTS radical solution containing 2.46 mM potassium persulfate was diluted with PBS (phosphate buffered saline, pH 7.6) until absorbance value at 734 nm reached to 0.700 ± 0.02. This solution (2 mL) was then mixed with different concentrations of diluted samples (60 µL) and absorbance values were read after 6 min (Çam et al. 2009).

In the CUPRAC test, 0.1 mL diluted sample were added to a test tube containing 1 mL ammonium acetate buffer solution (1.0 M), 1 mL ethanolic neocuproin solution (7.5 × 10⁻³ M), and 1 mL copper (II) chloride solution (0.01 M). Final volumes were then adjusted to 4.1 mL with distilled water. Absorbances were measured at 450 nm after 30 min (Alsataf et al. 2021).

The FRAP analysis was carried out by the method of Alsataf et al. Sample (150 µL) was mixed with 2850 µL FRAP reagent (25 mL 30 mM acetate solution, 2.5 mL 10 mM 2,4,6-Tris(2-pyridyl)-s-triazine, and 2.5 mL 20 mM iron (II) chloride). After 30 min, absorbances were read at 593 nm (Alsataf et al. 2021).

Absorbances were measured by a UV–VIS spectrophotometer (Model UV-1280, Shimadzu Corp., Kyoto, Japan) and DPPH, ABTS, CUPRAC, and FRAP values were expressed as mmol Trolox equivalent (TE) per g.

pH and titratable acidity

The pH and titratable acidity were determined according to a previous study (Hamid et al. 2022).

Syneresis

Syneresis in yogurts was detected by using the method of Korkmaz et al. with slight modifications. Yogurts (25 g) were stored at + 4 °C in a refrigerator. After 3 h, sample was passed through coarse filter paper and the serum layer was measured as volumetric (Korkmaz et al. 2021).

Textural profile

Textural behaviors of yogurts were studied by using a TA-XT plus texture analyzer (Stable Micro Systems Ltd., Godalming, Surrey, UK) fitted with a 5 kg load cell. After the calibration and program was set up, hardness, gumminess, and chewiness tests were performed using the HDP/90 platform and P/36R probe at a temperature of 5 °C. Pre-test speed, test speed, and post-test speed was 2.00, 5.00, and 5 mm/sec, respectively. Five measurements were done for each sample.

Microbiological analysis

For lactic acid bacteria (LAB) analysis, 10–15 mL M17 (Streptococcus thermophilus) and MRS (Lactobacillus bulgaricus) agar mediums were added to a petri dish containing 1 mL yogurt diluted with peptone water. These petri dishes were incubated in aerobic and anaerobic environment at 37 °C for 72 h. For yeast-mould analysis, petri dishes were incubated at 25 °C for 5 days after 10–15 ml PDA with 10% (w/v) L-tartaric acid was added to petri dish containing 1 mL yogurt diluted with peptone water (Korkmaz et al. 2021).

Sensory analysis

Sensory analysis was carried out by the method of Hamid et al. with some modifications (Hamid et al. 2022). Fifteen trained panel members from graduate and doctoral students of Food Engineering Department at Harran University evaluated yogurts in terms of sensory attributes. Panelists consisted of 10 female and 5 male members. Their age was between 25 and 50. Yogurts were appraised by panelists for color, texture, taste–aroma, and overall acceptability. Sensory quality scores ranged from 1 to 5 for each category; 1: unacceptable, 2: hardly acceptable, 3: acceptable, 4: good, and 5: excellent.

Statistical analysis

All analyses were performed in triplicate and results were expressed as mean ± standard deviation of the mean. Design expert 7.0 (Stat-Ease Inc., Minneapolis, MN) was used for optimization process. Differences between means were determined at 95% (p ≤ 0.05) confidence level using one-way analysis of variance (ANOVA) and Tukey multiple comparison test. Datasets were performed using Statistical Package for the Social Sciences (SPSS) software (version 22.0 for Windows, SPSS Inc., Chicago, IL, USA).

Results and discussion

Optimization of instant stevia powder production

In this section, stevia extracts obtained using microwave-assisted system (600 W, 20 min, and 50 °C) were converted into instant powder form by a spray dryer. The effect of spray drying parameters including flow rate (5–40 mL/min) and air inlet temperature (120–180 °C) on powder yield using response surface methodology by central composite design (CCD) was evaluated. Trial design and response (powder yield) results are presented in Table 1. Powder yield varied between 0.2 and 53.31% appertaining to experimental states.

Table 1.

Central composite design and responses for spray drying process

Trials	Independent variables		Dependent variable
	Flow rate (mL/min)	Inlet temperature (°C)	Powder yield (%)
1	22.50	180.00	44.89
2	10.13	128.79	48.12
3	22.50	150.00	27.84
4	5.00	150.00	53.31
5	22.50	120.00	8.18
6	22.50	150.00	36.58
7	10.13	171.21	50.86
8	22.50	150.00	28.74
9	34.87	171.21	1.00
10	40.00	150.00	0.50
11	22.50	150.00	28.34
12	22.50	150.00	29.46
13	34.87	128.79	0.20

The model statistics data for the optimization step are given in Table S2. Statistical data (maximum R²_Adj: adjusted coefficient of determination, maximum P-R²: estimated coefficient of determination, the highest degree of importance in p-value, and the value greater than 0.05 for lack of fit were considered in the selection of model. When cubic model was applied to experimental design, the probability of data losses was high according to the evaluations made on the program, resulting in inconsistent graphics. As for 2FI and quadratic models, lack of fit values (less than 0.05) were incompatible with the specified criteria. In addition, significance of p value was the lowest in the quadratic model. According to the examinations and model statistics results, linear model was the most suitable model to explain the dependent variable (powder yield, %) in this study.

Analysis of variance (ANOVA) results were used to show possible effects of independent variables (feed rate and air inlet temperature of spray dryer) on powder yield (%) on a statistical scale. The analysis of variance results is presented in Table S3. Both flow rate (p < 0.001) and air inlet temperature (p < 0.05) possessed a statistically significant impact on powder yield (%).

Based on statistical results, optimum production conditions were 11 mL/min flow rate and 167 °C air inlet temperature. Statistical differences between theoretical and experimental data were negligible. Ultimately, the fabrication of ISP was conducted under optimum conditions and used for further applications.

FTIR spectroscopy

FTIR spectroscopy is one of the methods used to define the specific peaks and functional groups of compounds (Arriola et al. 2019). In the current study, FTIR analysis was performed to identify successful spray drying process and spectrums of the samples are presented in Fig. 1. One of the strongest main peaks in stevia extract was observed at a wavelength of 3311.42 cm⁻¹. This band (3600–2500 cm⁻¹) attributed to intra- and intermolecular O–H stretching vibration in the hydroxyl groups (Chranioti et al. 2016). A remarkable decrease in the width and broadness of the band in the wavelength of 3459.71 cm⁻¹ was detected in the spectrum of ISP compared to extract. This phenomenon was associated with separation of aqueous phase from product during drying (Arriola et al. 2019). Bands detected at wavelengths of 1646.21 cm⁻¹ in stevia extract and 1662.09 cm⁻¹ in stevia powder could be named as stevia fingerprints band. These bands originate from steviol glycosides (Chranioti et al. 2016; Arriola et al. 2019). For ISP, peak at 1528.42 cm⁻¹ between 1600 and 1400 cm⁻¹ wavelengths was attributed to aromatic, stretching mode C=C or C–O stretch vibration of flavonoids. Various bands at a wavelength of 1300–1000 cm⁻¹ in FTIR spectra are related with stretching vibration of esters, ethers, carboxylic acids, and alcohols (Chranioti et al. 2016). In addition, other defined bands were identified at 1064.54 cm⁻¹ and 1023.98 cm⁻¹ wavelengths in stevia extract and stevia powder corresponding to the vibration of C–O–C and C–C glycosidic bonds (Arriola et al. 2019). Sum up, all these comments confirmed successful drying process.

Fig. 1.

FTIR spectrums of stevia extract and instant stevia powder

Physicochemical and bioactive characteristics of instant stevia powders

Physicochemical and bioactive properties of ISP are shown in Table S4. Moisture content and water activity of ISP were determined as 0.98% (wet basis) and 0.21, respectively. Weak flow behavior (Carr index: 28.57 and Hausner ratio: 1.40) was detected in stevia powder (Başyiğit et al. 2022). As for color, L* (75.95), a* (0.68), and b* (23.52) values of ISP was established. High solubility (94.98%) provides notable advantages to powder when they are evaluated in food matrix as an ingredient. Powder yield was 55.91%. Moreover, process efficiency relating to surface (uncoated) phenolics was 93.53%. This value confirmed the effectual loading of phenolic component into the polymeric system.

Antioxidant activities of ISP at optimum point are presented in Table S4. TPC and other analyses results of powder were given as mg GAE/g and µmol TE/g, respectively. TPC, DPPH, ABTS, FRAP, and CUPRAC in ISP were 51.04 mg GAE/g, 400.64 µmol TE/g, 551.75 µmol TE/g, 291.78 µmol TE/g, and 2288.04 µmol TE/g, respectively.

Physicochemical properties of yogurts

Physicochemical properties of control and ISP-enriched yogurts were followed from first day to fourteen day and results are presented in Table 2. Yogurts produced in the presence of ISP possessed a higher dry matter content than that of control. No remarkable changes in terms of this value for all samples were detected during storage (p > 0.05). Unlike dry matter content, pH and titration acidity values tented to decrease during storage period. This phenomenon could be ascribed to microbial activities of LAB. Syneresis values of control and ISP-fortified yogurts are presented in Table 2. Close values in terms of syneresis were obtained for fresh samples. No notable changes for these values in all samples were detected up to 7 days (p > 0.05). However, longer storage period was influenced syneresis value and the addition of ISP led to reduce syneresis in yogurts. Moreover, higher ISP provided lower syneresis rate to these products. Minimum syneresis was observed in yogurts containing 0.4 g/100 mL ISP (17.09 g/100 g) while the maximum value was detected in control group (19.45 g/100 g) on the last day of storage. Positive impact of maca powder and propolis extract in yogurts on the syneresis rate was reported in elsewhere (Korkmaz et al. 2021). The reason could be related to the interaction of polyphenols in ISP and yogurt proteins. This interaction may prevent syneresis by strengthening yogurt gel structure (Kwon et al. 2019).

Table 2.

Physicochemical properties of yogurts during storage period

Analyses	Treatment
Storage time (day)	A	B	C	D	E
Dry matter (%)
1 7 14	17.59 ± 0.04dA 17.51 ± 0.02eA 17.14 ± 0.05 dB	17.96 ± 0.01cB 18.67 ± 0.03bA 17.64 ± 0.10cC	18.08 ± 0.02abA 17.79 ± 0.04 dB 17.79 ± 0.03bcB	18.01 ± 0.01bcA 18.00 ± 0.02cA 17.92 ± 0.04bA	18.13 ± 0.04aB 18.80 ± 0.01aA 18.12 ± 0.07aB
pH 1 7 14	4.65 ± 0.01aA 4.33 ± 0.01aB 4.05 ± 0.01cC	4.61 ± 0.01bA 4.33 ± 0.01aB 4.12 ± 0.01bC	4.64 ± 0.02abA 4.34 ± 0.01aB 4.15 ± 0.01aC	4.55 ± 0.01cA 4.32 ± 0.01aB 4.13 ± 0.01abC	4.60 ± 0.01bA 4.21 ± 0.01bB 4.07 ± 0.01cC
Titratable acidity (%) 1 7 14	0.66 ± 0.01cC 0.79 ± 0.00abB 1.01 ± 0.02aA	0.65 ± 0.01cC 0.79 ± 0.01abB 0.91 ± 0.01bcA	0.69 ± 0.00bC 0.78 ± 0.01bB 0.86 ± 0.00dA	0.70 ± 0.01abC 0.78 ± 0.01bB 0.87 ± 0.01dcA	0.72 ± 0.01aC 0.81 ± 0.01aB 0.92 ± 0.01bA
Syneresis (%) 1 7 14	22.13 ± 0.14aA 21.10 ± 0.35bA 19.45 ± 0.21aB	21.82 ± 0.52abA 23.08 ± 0.30aA 17.87 ± 0.30bB	20.51 ± 0.17cA 21.63 ± 0.38abA 17.51 ± 0.17abB	20.09 ± 0.25cA 22.16 ± 0.82abA 17.63 ± 0.16cB	20.90 ± 0.35bcA 22.39 ± 0.82bA 17.09 ± 0.47 dB
Color (L*) 1 7 14	88.71 ± 0.23aA 86.55 ± 0.77aAB 86.12 ± 0.23abB	87.57 ± 0.03aA 85.22 ± 0.47abB 86.70 ± 0.50aAB	85.31 ± 0.17bA 86.37 ± 0.49aA 85.06 ± 0.07bcA	85.05 ± 0.99bA 85.81 ± 0.02aA 85.87 ± 0.33abA	84.48 ± 0.38bA 83.60 ± 0.19bA 84.44 ± 0.09cA
a* 1 7 14	-0.40 ± 0.01aA -0.44 ± 0.04aA -0.35 ± 0.03aA	-0.56 ± 0.02bA -0.84 ± 0.03bB -0.51 ± 0.01bA	-0.71 ± 0.03cAB -0.75 ± 0.03bB -0.62 ± 0.03bcA	-0.75 ± 0.01cA -0.73 ± 0.03bA -0.67 ± 0.06cA	-0.91 ± 0.04 dB -0.72 ± 0.06bA -0.69 ± 0.01cA
b* 1 7 14	8.73 ± 0.08cB 8.88 ± 0.01dAB 8.95 ± 0.02eA	10.18 ± 0.04bA 8.61 ± 0.07eC 9.78 ± 0.06 dB	10.18 ± 0.06bB 10.74 ± 0.12cA 10.23 ± 0.11cB	11.68 ± 0.18aA 11.57 ± 0.03bA 11.33 ± 0.20bA	11.99 ± 0.17aA 12.29 ± 0.07aA 11.92 ± 0.15aA

Data are expressed as a mean ± standard deviation of three replicate (n = 3). A: control yogurt, B: yogurt containing 0.1 g/100 mL instant stevia powder, C: yogurt containing 0.2 g/100 mL instant stevia powder, D: yogurt containing 0.3 g/100 mL instant stevia powder, E: yogurt containing 0.4 g/100 mL instant stevia powder

^a−e Means with different characters in the same row were significantly different between samples in the related storage day (p < 0.05)

A–C Means with different characters in the same column were significantly different during storage time in same sample (p < 0.05)

Color properties of yogurts

Color properties (L^*, a^*, and b^*) of yogurts were investigated over fourteen days and the results are shown in Table 2. Color shifts during storage time were at negligible levels. However, L^* (lightness) value of the control group was higher than those of yogurts containing ISP at the beginning of the storage. On the other hand, the maximum values in terms of a^* (greenness) and b^* (yellowness) were detected in ISP-fortified yogurts. Moreover, in parallel with the increase in ISP concentration in samples, L^* value diminished and the enhancement was observed in the other color parameters namely a^* and b^*. Similar findings were reported by another study (Pelaes Vital et al. 2015). Color pigments from plant materials could be responsible for these phenomena.

Texture profile of yogurts

One of the primary objectives of scientific literature and industry is to produce yogurts with desirable texture (Brückner-Gühmann et al. 2019). Therefore, effect of ISP addition on the textural attributes (hardness, gumminess, and chewiness) of yogurts was investigated in the current study and results are given in Table 3. The incorporation of ISP at different rates into yogurts significantly affected their hardness, gumminess, and chewiness. ISP-fortified yogurts except for yogurt containing 0.4 g/100 mL powder possessed superior hardness (a gel network less sensitive to breaking off) on the first day of storage compared to control. Gel strength of yogurts evolve when phenolic-rich substances are supplemented their formulations (Kumar and Mishra 2003). More compact structure could be ascribed to hydrophobic interaction of aromatic rings in phenolics and amino acid side chains in milk protein (Hernández-Rodríguez et al. 2016). Moreover, hydrogen bonds promote these interactions, resulting in desirable shifts in hardness (Charlton et al. 2002). At day of 1, observing a notable decline in gel strength of yogurts containing the maximum concentration of powders could be ascribed to impairment of the three-dimensional structure because of the increased interaction between phenolics and proteins. The adverse impact of polyphenols on textural parameters of yogurts were also explained by the similar comments in elsewhere (Ning et al. 2021). Moreover, fluctuations in terms of this parameter for all samples were observed during storage period. However, in general, their hardness (except for samples containing 0.4 g/100 mL powder) behavior diminished at the end of storage. Similar findings were reported for the hardness of yogurts prepared in the presence of argel leaves extract (Mohamed Ahmed et al. 2021). On the contrary, the situation in samples with a denser/more rigid structures or texture integrity may be related to low storage temperature. Low temperature triggers the strengthening of the gel structure (Paseephol et al. 2008). Other interpretation could be the rearranged interactions between phenolics and proteins during storage. Yogurts containing ISP displayed superior gumminess and chewiness behavior compared to control counterparts at the beginning of the storage. Positive effect of plant extracts on these textural parameters was reported in the scientific literature (Mohamed Ahmed et al. 2021). The gooeyness nature of plant-based extracts could be cited as a reason for explaining this situation (Mousavi et al. 2019). Consequently, supplementation of yogurts with powders contributed to the improvement of their textural properties.

Table 3.

Texture profile of yogurts during storage period

Analyses	Treatment
Storage time (day)	A	B	C	D	E
Hardness (g)
1 7 14	156.44 ± 2.00bcA 165.70 ± 2.57bA 138.45 ± 3.90cB	168.40 ± 2.05bB 187.07 ± 3.39aA 167.89 ± 3.67aB	168.35 ± 4.88bA 137.04 ± 3.41cB 147.89 ± 2.54bcB	184.57 ± 4.09aA 139.61 ± 2.95cB 168.64 ± 2.72aC	145.39 ± 3.41cB 188.88 ± 5.76aA 159.50 ± 4.15abB
Gumminess 1 7 14	81.51 ± 1.35dA 62.31 ± 1.30cdB 54.29 ± 2.36cC	104.99 ± 0.69aA 68.36 ± 0.58bB 56.91 ± 0.74cC	83.82 ± 1.76cdA 63.30 ± 1.03bcB 63.36 ± 0.42bB	99.92 ± 1.20bA 57.25 ± 1.49dC 72.76 ± 0.41aB	87.07 ± 1.17cA 83.85 ± 2.18aA 54.07 ± 1.57cB
Chewiness 1 7 14	77.60 ± 1.45eA 58.45 ± 1.41cB 51.02 ± 2.41cB	103.04 ± 0.31aA 64.43 ± 0.45bB 52.47 ± 0.57cC	81.42 ± 1.42dA 59.78 ± 0.75cB 58.83 ± 0.17bB	92.82 ± 0.51bA 52.43 ± 1.13dC 66.10 ± 0.70aB	85.14 ± 0.54cA 79.02 ± 1.59aB 49.58 ± 1.11cC

Data are expressed as a mean ± standard deviation of three replicate (n = 3). A: control yogurt, B: yogurt containing 0.1 g/100 mL instant stevia powder, C: yogurt containing 0.2 g/100 mL instant stevia powder, D: yogurt containing 0.3 g/100 mL instant stevia powder, E: yogurt containing 0.4 g/100 mL instant stevia powder

^a−e Means with different characters in the same row were significantly different between samples in the related storage day (p < 0.05)

^A–C Means with different characters in the same column were significantly different during storage time in same sample (p < 0.05)

Microbiological analysis of yogurts

Results regarding microbiological findings are shown in Table S5. No yeast-mould was found in all samples throughout storage time (data not shown); indicating that products were hygienically acceptable. As for LAB, there was a positive correlation between enhancement of LAB counts and ISP concentration in yogurts. This enhancing effect could be ascribed to prebiotic effect of phenolics (Sanches Lopes et al. 2016). Beneficial effect of argel leaf extract to LAB growth in yogurts was noted in a previous study (Mohamed Ahmed et al. 2021). Moreover, LAB count in yogurts produced in the present study was in line with standards and was not less than 7 log cfu/g (Mohamed Ahmed et al. 2021). Viable counts ranged from 8.20 to 9.10 log cfu/g at the beginning of storage. These counts decreased gradually during storage and reached to minimum levels (7.50–8.60 log cfu/g) at the end of storage. This phenomenon could be associated with the increase in acidity and decrease in pH during storage period. Similar comment was also reported by a previous study (Arriola et al. 2019).

Antioxidant activity of yogurts

The addition of natural plant extracts having functional structures to yogurts is one of the ways to increase its health benefits impact. Antioxidative behavior of yogurts were evaluated by using five different methods including TPC, DPPH, ABTS, FRAP, and CUPRAC during storage period (Fig. 2). TPC in control group was 7.86 mg GAE/g on the first day of the storage. A remarkable increase was observed in phenolic content of yogurts with the addition of ISP (p < 0.05). Moreover, correlation between ISP concentration and TPC was expressive. At the beginning of the storage, phenolic content of yogurts fortified with 0.1, 0.2, 0.3, and 0.4 g/100 mL was found as 93.34, 100.11, 116.69, and 142.85 mg GAE/g, respectively. All samples exhibited similar behavior in terms of TPC during storage period and the decreases in this value happened. Free radical scavenging activity (DPPH and ABTS methods) and reducing power (FRAP and CUPRAC methods) results were expressed as µmol TE/g and are presented in Fig. 2. On day first of storage, DPPH, ABTS, FRAP, and CUPRAC values were detected as 17.81, 1.58, 0.76, and 1.78 µmol TE/g for control, respectively. These values shifted higher in the presence of ISP. As expected, the superior antioxidant activity was determined in yogurt containing 0.4 g/100 mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 136.86, 25.11, 2.95, and 7.20 µmol TE/g, respectively) followed by 0.3 g/100 mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 127.53, 17.67, 2.80, and 6.05 µmol TE/g, respectively), 0.2 g/100 mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 117.03, 12.08, 2.71, and 4.70 µmol TE/g, respectively), and 0.1 g/100 mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 108.48, 7.85, 2.06, and 4.36 µmol TE/g, respectively). Antioxidative behavior of yogurts was enhanced with the addition of argel leaf extract (Mohamed Ahmed et al. 2021), pomegranate peel phenolic extract (Hamid et al. 2022), and chia seed extract (Kwon et al. 2019). Compared to at the beginning of storage, changes in DPPH (6.80 µmol TE/g), ABTS (0.29 µmol TE/g), FRAP (0.24 µmol TE/g), and CUPRAC (0.60 µmol TE/g) values in control group were remarkable levels on the last day of storage. Similarly, notable decreases were observed in other samples, namely yogurts fortified with 0.4 g/mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 110.09, 19.07, 2.11, and 5.07 µmol TE/g, respectively), 0.3 g/mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 102.37, 14.81, 2.06, and 4.16 µmol TE/g, respectively), 0.2 g/mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 100.26, 8.99, 1.88, and 3.12 µmol TE/g, respectively), and 0.1 g/mL ISP (DPPH, ABTS, FRAP, and CUPRAC: 98.51, 5.96, 1.18, and 2.95 µmol TE/g, respectively) after fourteen day. The reason behind this decrease could be attributed to interaction between phenolic compounds and yogurt proteins (Pelaes Vital et al. 2015).

Fig. 2.

Antioxidant activities of yogurts during storage period. Data are expressed as a mean ± standard deviation of three replicate (n = 3). Error bars indicates standard deviation of the mean values. A: control yogurt, B: yogurt containing 0.1 g/100 mL instant stevia powder, C: yogurt containing 0.2 g/100 mL instant stevia powder, D: yogurt containing 0.3 g/100 mL instant stevia powder, E: yogurt containing 0.4 g/100 mL instant stevia powder. a−e Means with different characters were significantly different between samples in the related storage day (p < 0.05). A–C Means with different characters were significantly different during storage time in same sample (p < 0.05).

Sensory properties of yogurts

Sensory properties were evaluated according to four different criteria, namely taste–aroma, color, texture and general acceptability. Changes in these parameters were evaluated according to both the ratio of ISP in yogurts and storage time. The actual images of yogurts and results are shown in Fig. S2 and Fig. 3, respectively. Significant differences were observed in terms of sensory attributes in samples (p < 0.05). The superior scores for namely taste–aroma, texture and general acceptability were detected in presence of 0.2 g/100 mL ISP followed by yogurt fortified with 0.1 g/100 mL ISP. Compared to these samples, a remarkable decrease in taste–aroma, texture and general acceptability of yogurts enriched 0.3 and 0.4 g/100 mL ISP was detected. The color scores were maximum (8.50) in yogurts (control and 0.1 g/100 mL ISP), and minimum in yogurt (4.00) enriched with 0.4 g/100 mL ISP. Also, taste–aroma, color, texture and general acceptability of yogurts shifted during storage time (p < 0.05). On the first day of storage, the maximum scores were obtained in terms of sensory properties for all samples. However, the scores tended to decrease on the 7th and 14th days of storage. The lowest taste–aroma was found in the yogurts containing 0.3 and 0.4 g/100 mL ISP except for control during storage period. This phenomenon could be ascribed to intense sweetness of stevia. A trend to decrease in color properties was observed in yogurts throughout storage. The reason behind this situation could be related to the partial syneresis, fat oxidation, protein degradation (Hamid et al. 2022). On the other hand, color values of yogurts were within the acceptable range (4–9). Storage time also exhibited a negative impact on the texture and general acceptability in the current study. The similar notes were reported in the previous study regarding yogurt fortified with argel leaf extract (Mohamed Ahmed et al. 2021). Consequently, addition of ISP up to a certain concentration (0.2 g/100 mL) improved sensory properties of yogurts. In other words, further additions were not considered reasonable.

Fig. 3.

Sensory properties of yogurts during storage period. A: control yogurt, B: yogurt containing 0.1 g/100 mL instant stevia powder, C: yogurt containing 0.2 g/100 mL instant stevia powder, D: yogurt containing 0.3 g/100 mL instant stevia powder, E: yogurt containing 0.4 g/100 mL instant stevia powder

Conclusion

There is an effort to increase the use of stevia in different sectors, especially in the food industry because of its special structures that possess positive effects on health. However, it could not be said that this effort has not yet reached its final success. In this context, the up-to-date dataset presented novel information regarding the usage of stevia. The presence of instant powders rich in stevia extracts in yogurts positively affected quality parameters from their textural behavior to functional and sensory properties. Also, there is a need for scale-up studies on this subject in the future. ISP could be used as an ingredient not only yogurt but also other dairy products, such as ice cream in these further studies.

Supplementary Information

Below is the link to the electronic supplementary material.

Abbreviations

ISP

Instant stevia powder

FTIR

Fourier transform infrared spectrophotometer

LAB

Lactic acid bacteria

TPC

Total phenolic content

DPPH

2,2-Diphenyl-1-picrylhydrazyl

ABTS

2,2-Azino-Bis (3-ethylbenzothiazoline-6-sulfonic acid)

FRAP

Ferric ion reducing antioxidant power

CUPRAC

Cupric reducing antioxidant capacity

PDA

Potato dextrose agar

CCD

Central composite design

TE

Trolox equivalent

GAE

Gallic acid equivalent

Author contributions

Investigation, MA, KBA, BB, MŞK, MY, AK, and MK; Formal analysis, MA, KBA, MŞK, and MY; Software, MA, KBA, BB, MŞK, and MY; Visualization, MA, KBA, BB, MŞK, MY, and AK; Writing – original draft, MA, KBA, BB, MŞK, MY, AK and MK; Writing – review & editing, BB and MK; Conceptualization, BB and MK; Funding acquisition, MK; Resources, MK; Supervision, MK.

Funding

The authors have not disclosed any funding.

Declarations

Conflict of interest

The authors have not disclosed any conflict of interests.