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Article

The Effects of Leuconostoc mesenteroides RSG7 Exopolysaccharide on the Physicochemical Properties and Flavor Compounds of Set Yoghurt

by
Baomei Wu
,
Yanru Guo
,
Linlin Hao
,
Kaiyue Zuo
,
Yufei Du
,
Ruixin An
and
Binbin Wang
*
School of Life Sciences, Shanxi Normal University, Taiyuan 030000, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(5), 1442; https://doi.org/10.3390/pr13051442
Submission received: 7 April 2025 / Revised: 2 May 2025 / Accepted: 6 May 2025 / Published: 8 May 2025
(This article belongs to the Section Food Process Engineering)
Browse Figures
Versions Notes

Abstract

:
Leuconostoc mesenteroides RSG7 was previously isolated from pepino, and its exopolysaccharide has potential bioactivities. To better understand the function of RSG7 exopolysaccharide (RE), its effects on the stability and flavour characteristics of set yoghurt were comprehensively investigated. RE was incorporated into milk at 0% (control), 0.05%, 0.10%, and 0.15% (w/v), respectively. Subsequently, samples were fermented and stored at 4 °C for 24 h. The pH, water-holding capacity (WHC), texture profiles, rheological properties, microstructure, and flavour characteristics were analyzed. The results showed that the addition of RE significantly enhanced the WHC; improved hardness, gumminess, chewiness, springiness, adhesiveness, apparent viscosity, and storage modulus (G′) and loss modulus (G″); and reduced the cohesiveness and loss tangent (tan δ) of set yoghurt in a dose-dependent manner, which might be attributed to the interaction between RE and proteins based on the compact microstructure. These results suggested that RE endowed yoghurt with better gel properties and more stability. No differences were observed in the pH of set yoghurt, while RE significantly improved flavour characteristics such as sourness, according to an electronic nose and tongue and gas chromatography–mass spectrometry analyses. Consequently, our results suggest that the bioactive properties, such as its interaction with milk proteins and flavour modulation capabilities, make it a promising functional ingredient for designing yoghurt formulations with enhanced texture, stability, and sensory profiles. This study deepens the understanding of RE functions and shows potential applications in the dairy industry.

1. Introduction

Yoghurt is strictly defined by the Food and Agriculture Organization (FAO)/World Health Organization (WHO) as a milk product obtained by fermentation through the action of symbiotic starter of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, which shall be viable and active in the final product based on the Codex standard for fermented milk products (adopted in 2003). This dairy product is a globally prevalent functional food with potential health benefits [1]. Although the abovementioned two bacteria are considered as a conventional yoghurt-fermented starter [2], Lactobacillus, Bifidobacterium, or other probiotic microorganisms are also supplemented to improve the benefits during dairy fermentation in commercial yoghurt products [3]. Emerging studies illustrate that yoghurt is a nutritious food that contains bioactive minerals, proteins, lipids, and carbohydrates and improves immune function through gut-mediated mechanisms directly and indirectly [3,4]. However, some reports have suggested that the health benefits and textural properties of yoghurt are significantly impacted by the fermentation starter [1,5,6], indicating that screening and exploring special lactic acid bacteria (LAB), such as high-yield exopolysaccharide (EPS) strains, possess great potential for application in the yoghurt industry. Meanwhile, in order to make the yoghurt more nutritious and improve the physicochemical properties, minerals, organics (such as polyphenols), fruits, vegetables, and animal products are supplemented or added to enhance the quality of the dairy product [7].
EPS is an important and bioactive organic, and it is not only produced by LAB but also by yeasts, algae, fungi, moulds, plants, and animals [8]. LAB EPS is considered safe and possesses diverse physicochemical properties and biological activities. It is usually regarded as a stabilizer, thickener, emulsifier, and gelling agent to improve the appearance, rheological properties, taste, and texture of fermented food [9,10]. Meanwhile, LAB EPS is also reported as antioxidant, immunomodulator, anticancer, anticoagulant, anti-inflammatory, antiviral, anticholesterol, and even antidiabetic agents [8,9,10]. For yoghurt, gel formation significantly affects its texture. The supplementation of a stabilizer could improve gel formation through the strengthening of the casein network, the delay of milk protein aggregation or precipitation, and a reduction in yoghurt syneresis [11]. Previous report suggested that EPSs produced by Levilactobacillus brevis UCLM-Lb47 and Leuconostoc (Leu.) mesenteroides subsp. mesenteroides 6F6-12 and 2F6-9 were able to enhance the water-holding capacity (WHC) and improved the viscosity in the mouth of the non-fat set yoghurt [12]. Similar results were also found in the yoghurt with the addition of Leu. mesenteroides XR1 EPS, which showed an improvement of the WHC, an alteration of the yoghurt microstructure, and an enhancement of viscoelastic properties [13,14]. Another study showed that EPS produced by starter cultures enhanced the sensory quality of goats’ milk set yoghurt with reduced syneresis and high apparent viscosity [15]. Meanwhile, EPSs were also reported to play important roles in the stability and flavour characteristics of yoghurt [1,16]. Consequently, LAB EPS possesses varied functions to improve yoghurt quality during fermentation.
RSG7 EPS (RE) with favourable physicochemical properties was previously isolated and purified from Leu. mesenteroides RSG7, which was isolated from pepino [8]. The RE was identified as dextran with a molecular weight (Mw) of 5.47 × 106 Da and is composed of α-(1→6) glycosidic linkages as the backbone and α-(1→2), α-(1→3), α-(1→4), and α-(1→6) glycosidic linkages as the side chains [8]. Further analysis suggested that RE could promote the growth of Bifidobacterium animalis ATCC27673 and Bif. longum LTBL16 at the stationary phase and the logarithmic phase in in vitro experiments, respectively [8]. Additionally, during in vitro fecal fermentation, RE could increase the relative abundances of beneficial bacterial genera, such as Phascolarctobacterium and Faecalicoccus, to improve gut bacterial eubiosis [17]. Since previous studies indicated that the functional properties of EPS are closely associated with its molecular structure, composition, mass, configuration, and charge polarity [10,18], the effect of RE on the physicochemical properties of set yoghurt remains unclear. To address the functional gap, different RE concentrations were designed and added to the milk to evaluate the texture and flavour changes in the obtained set yoghurt after fermentation and low-temperature (4 °C) storage. The texture analyzer, rheometer, scanning electron microscope (SEM), electronic nose (e-nose) and tongue (e-tongue), and gas chromatography–mass spectrometry (GC-MS) were utilized to analyze the texture and flavour of the set yoghurt after 24 h of low-temperature storage. This study provides an understanding of RE application as a potential ingredient to improve the texture and flavour characteristics of dairy products.

2. Materials and Methods

2.1. Preparation of RE

The crude RE was extracted using a previously described protocol [8]. A solution containing 10% (w/v) trichloroacetic acid was used to remove proteins. Then, the obtained crude RE was purified using gel-filtration chromatography with a Sephadex G-100 column (1.6 cm × 50 cm, GE Healthcare, Fairfield, CT, USA). The purified RE solution was finally lyophilized for further studies.

2.2. Preparation of Set Yoghurt

Set yoghurt containing RE was obtained according to a previous report with slight modifications [19]. RE was added into commercial milk (fresh mill with 5% proteins, 6% fats, 2% carbohydrates, 3% sodium, and 14% calcium; size: 2L; MENGNIU, Inner Mongolia, China) at concentrations of 0%, 0.05%, 0.10%, and 0.15% (w/v) and then stirred with a stirrer until it was completely dissolved. The mixed milk was pasteurized at 95 °C for 2–3 min and subsequently cooled to 43 °C. The yoghurt starter (YO-PROX BA986, BIOPROX, Paris, France) was supplemented into the cooled milk with the content of 0.01% (w/v); then, it was transferred to sterile containers with lids for incubation at 43 °C for 5 h. Finally, all samples were stored at 4 °C for 24 h.

2.3. Determination of pH and WHC

The pH values of set yoghurt were measured with a digital pH metre (S220-K, Mettler Toledo, Zurich, Switzerland). Five measurements were performed and recorded.
The WHC of set yoghurt was analyzed based on a previous method with slight modifications [20]. The 10 g set yoghurt (as W0) from each sample was centrifuged at 845× g for 30 min, and the supernatant was obtained and weighted as W1. The WHC was calculated with the equation as follows: WHC   % = 1 − W 1 W 0 × 100 % .

2.4. Texture Profile Analysis

The texture analyses of set yoghurt samples were carried out according to a previous study with slight modifications [21,22]. A texture analyzer (TA-XT plus, Stable Micro Systems Ltd., London, UK) equipped with a 36 mm cylindrical P/36R probe was used to evaluate the texture profiles, and device parameters were set as follows: 1 mm/s probe speed, 5.0 g trigger force, 30% compression ratio, 50 mm return distance, and 20 mm/s return speed [19].

2.5. Rheological Properties Analysis

Rheological testing was performed based on a previously described method with minor modifications [21]. Briefly, the rheological properties of set yoghurt samples were analyzed using a rheometer (Discovery HR20, TA Instruments, New Castle, DE, USA) equipped with a 60.0 mm parallel plate. The viscosity of the set yoghurt samples was determined at a shear rate range from 0.1 to 100 s−1. Strain sweep tests were initially carried out at a frequency of 1 Hz from 0.01 to 100% to determine the linear viscoelastic region before the oscillatory measurements. Then, frequency sweep tests were performed with a frequency range of 0.01 to 16 Hz to determine the storage modulus (G′), loss modulus (G″), and loss tangent (tan δ) after 24 h of low-temperature storage.

2.6. Microstructure Analysis

The microstructure of set yoghurt was observed using a SEM (TM3030Plus, HITACHI, Tokyo, Japan). The set yoghurt samples were diluted using deionized water at a ratio of 1:6 and then frozen. The frozen samples were lyophilized and subsequently sputtered with gold power for 2 min. Afterward, the SEM was utilized at magnifications of 500× and 1.0k×.

2.7. Flavour Analysis with E-Nose and E-Tongue

The scents of set yoghurt samples were detected with a PEN 3 e-nose (AIRSENSE Analytics, Schwerin, Germany) according to a previous report [19]. A 5.0 g set yoghurt sample was placed in a sealed headspace vial of the e-nose for 30 min. Then, the analyses of samples were performed. The testing and cleaning time were both set as 2 min.
The taste of set yoghurt was analyzed with an SA402B Taste Sensing System (Insent, Kyushu, Japan). Each set yoghurt sample (40 g) was diluted with distilled water at a ratio of 1:5, and it was filtered and subsequently analyzed with the e-tongue [19].

2.8. Volatile Compound Analysis

Volatile components of the set yoghurt were extracted using a solid-phase microextraction (SPME) fibre (DVB/CAR/PDMS, 50/30 μm; fibre length, 1 cm), separated, and analyzed using a GC-MS system (QP 460, Bruker, Karlsruhe, Germany) equipped with a DB-WAX column (30 m × 0.25 mm × 0.25 μm, Agilent, USA) according to a previously delineated technique with slight modifications [4,19]. Each sample (3 g) was added into headspace bottles with 0.5 g of sodium chloride. Then, 1-octanol (40 μL, 1.13 μg/L) was taken as an internal standard. After preconditioning the fibres at 250 °C for 30 min, it was inserted into the headspace flask for 30 min at 50 °C for incubation. The fibres were then held in the injection port and desorbed at 260 °C for 8 min. The column temperature programme was set according to a previous description [23]. The volatile compounds were identified and quantified by searching the National Institute of Standards and Technology (NIST) database, comparing the retention indexes and mass spectra of the standard substances, and calculating the relative contents using the area normalization method.

2.9. Statistical Analysis

All samples were prepared in triplicate, and the results were expressed as the mean ± the standard deviation (SD). Graphs were plotted using the GraphPad PRISM 8 (GraphPad Software, Inc., La Jolla, CA, USA) and the OriginPro version 2024 software (OriginLab Corporation, Northampton, MA, USA). Statistical significance analysis was conducted using the SPSS Statistics 23 software (IBM Corporation, Armonk, NY, USA), with a significance level of p < 0.05. Additionally, principal component analysis (PCA) of the e-nose and e-tongue’s data was performed in OriginPro 2024. Pearson correlation was used to analyze the correlation among the selected physicochemical characteristics of the set yoghurt.

3. Results and Discussion

3.1. pH and WHC of Set Yoghurt

Figure 1a presents the pH of fermented set yoghurt with different RE additions. All set yoghurt samples showed similar pH values without significant differences. The pH could affect the microbial activity of the final product significantly [24]. Our results demonstrated that the addition of RE had no effect on the change in the activity of the yoghurt starter bacteria, which was consistent with previous reports [20,25].
WHC is a critical parameter in the texture and flavour of set yoghurt [19]; i.e., the higher the value, the better the curd stability [25]. As shown in Figure 1b, the WHC values of set yoghurt with 0.05%, 0.10%, and 0.15% RE were 44.09 ± 2.90%, 46.74 ± 1.90%, and 47.48 ± 2.53% after 24 h of low-temperature storage, respectively. The results indicated that an increasing trend was observed with an increase in RE concentrations, although no significant difference was found among the three samples. However, compared with the control (40.54 ± 1.19%), the WHC values of the set yoghurt with 0.10% (46.74 ± 1.90%) and 0.15% (47.48 ± 2.53%) RE showed a significant difference. Several studies reported that the addition of polysaccharides significantly increased the WHC values in the set yoghurt after 24 h of low-temperature storage [19,20,22]. Based on previous investigations, the improvement of WHC of set yoghurt after the addition of polysaccharides might be attributed to the strong hydrophilicity of polysaccharide molecules, which resulted in the rigidity of the protein gel network by absorbing water [26], and the interaction between proteins and polysaccharides, which led to the formation of stable complexes to improve the structure of protein gels [25]. The honeycomb-like porous structure with spike-like expansions around the pores of RE [8] might be beneficial for the increase in WHC during the low-temperature storage of set yoghurt in this study.

3.2. Texture Profile Analysis of Set Yoghurt

As one of the most pronounced factors, texture influences the quality and acceptance of yoghurt significantly [27]. Several parameters including hardness, cohesiveness, gumminess, chewiness, springiness, and adhesiveness play vital roles in the textural assessment of yoghurt [28]. Hardness is the most commonly assessed characteristic in determining the texture of set yoghurt and could reflect its gel state directly. Previous study reported that greater hardness implied better coagulation [29]. According to Figure 2a, the hardness of set yoghurt with 0.05%, 0.10%, and 0.15% RE was significantly higher than that of the control after 24 h of low-temperature storage, indicating that the addition of RE enhanced the gel-forming properties of set yoghurt. Additionally, the hardness exhibited increasing trend with RE concentration, although no significant difference was observed between the 0.05% and 0.10% RE samples (Figure 2a). Similar findings were reported during the functional exploration of Pleurotus ostreatus (oyster) mushroom polysaccharides in yoghurt fermentation and storage [30]. There is a relationship between the microstructure of set yoghurt and hardness; a compact structure could cause more hardness and lower water drainage [31]. Consequently, RE could stabilize set yoghurt and improve the WHC (Figure 1b).
Cohesiveness is commonly defined as the degree of force required to pull the probe from the sample and depends on the strength of the internal bonds in the set yoghurt [32]. The smaller the cohesiveness of the set yoghurt, the better the smoothness [29]. As shown in Figure 2b, the cohesiveness of set yoghurt between the 0.05% RE and control samples was not significantly changed after 24 h of low-temperature storage, as well as that between the 0.10% and 0.15% RE samples. However, the cohesiveness of set yoghurt with the 0.10% and 0.15% RE samples was significantly lower than that of the control sample (Figure 2b). These results suggested that the addition of RE could improve the smooth texture of set yoghurt significantly. Similar results were also reported in the studies on the effects of polysaccharides isolated from Pleurotus ostreatus (oyster) mushroom and Auricularia cornea var. Li on the yoghurt after 24 h of low-temperature storage [30,33].
Gumminess is related to the hardness of set yoghurt [34]. Therefore, this parameter is generally defined as the energy needed to fragment a semisolid food until it is ready to swallow [28]. As can be seen in Figure 2c, the gumminess of set yoghurt showed an increasing trend with RE concentration, although no significant difference was found between the 0.05% and 0.10% RE samples. This trend was consistent with hardness (Figure 2a). The outcome is similar to a previous study where inulin could increase the gumminess of yoghurt with the increasing concentration of the polysaccharide after 24 h of low-temperature storage [28], while it is contrary to that of β-glucan investigations [35]. In addition, chewiness is the time or work required to masticate a sample to reduce it to a state ready for consuming [28]. The parameter showed a similar increasing trend to gumminess as the RE concentration increased (Figure 2c,d). Proper gumminess and good chewiness are beneficial for improving the human eating experience and increasing consumer satisfaction and re-consumption willingness [36]. These results indicated that RE possessed the potential for commercial applications in yoghurt.
Springiness is the rate or extent to which a deformed sample returns to its native dimensions after the removal of a force [28]. This parameter of set yoghurt was found to be influenced by the addition of RE and increased with the increase in RE concentrations (Figure 2e), which was consistent with the abovementioned parameters. In addition, adhesiveness was also explored to further analyze the texture of set yoghurt with the addition of RE. As can be seen in Figure 2f, the adhesiveness value was also showed an increasing trend with the increase in RE concentrations after 24 h of low-temperature storage. This parameter is considered as the force needed to remove the adhered sample in the mouth while eating [37]. The greater the adhesiveness indicated, the thicker the set yoghurt [29]. These results suggested that the addition of RE was able to improve the palatability of set yoghurt. Overall, RE had a positive effect on the texture improvement of set yoghurt based on the aforementioned outcomes.

3.3. Rheological Properties Analysis of Set Yoghurt

The apparent viscosity changes in set yoghurt containing different RE concentrations with different shear rates are shown in Figure 3a. A negative correlation between viscosity and the shear rate was observed, indicating that the set yoghurt exhibited a non-Newtonian shear thinning behaviour due to the breaking of protein bonds [38]. In particular, the apparent viscosity of set yoghurt showed an upward trend with increasing RE concentrations, demonstrating that RE had a significant impact on the apparent viscosity of set yoghurt. The most likely explanation for this phenomenon was attributed to the formation of a strong network due to the interaction of polysaccharides with casein micelles [39], as well as the increase in WHC in the set yoghurt. Viscosity affected the texture, mouthfeel, and acceptability of the set yoghurt significantly; higher viscosity indicated better texture, flavour, and acceptability by the consumers [40]. These results were consistent with the aforementioned studies of set yoghurt texture.
The variation in the G′ and G″ of the set yoghurt containing different RE concentrations with frequency was explored. As shown in Figure 3b, the values of G′ and G″ increased with frequency in the entire range of the test frequency in all samples; meanwhile, G′ values were always higher than G″, which indicated that the yoghurt system showed a solid-like behaviour. Interestingly, the values of G′ and G″ in the set yoghurt supplemented with RE were higher than those of the control and exhibited an upward trend with increasing RE concentrations, which suggested that RE could improve the hardness of the set yoghurt with a reasonable explanation that the polymer entanglement in the solid-like gel increased significantly [41]. The good viscoelastic gel structure of the set yoghurt with the addition of RE was consistent with the results obtained by the WHC, apparent viscosity, and texture profile analysis. A similar result was reported by a previous study where the increase in guar gum concentrations increased the complex viscosity of yoghurt [42].
The value of tan δ as a function of frequency was subsequently analyzed. As can be seen in Figure 3c, all the tan δ values of the set yoghurt samples were less than one after 24 h of low-temperature storage, which indicated that the set yoghurts showed solid-like behaviour due to the higher G′ values and lower G″ values [43]. With the increase in RE concentrations, the tan δ value exhibited a downward trend, and a similar result was also reported by investigations of Tremella fuciformis polysaccharides [41]. Additionally, the tan δ values of the set yoghurt samples showed an upward trend with the frequency, which was consistent with previous exopolysaccharide (cEPS and fEPS) studies and contrary to the reports of Potentilla anserine and Lactobacillus helveticus MB2-1 polysaccharides [44,45,46], although both the tan δ values of yoghurts in the three experiments were less than one. It has been reported that the set yoghurt with a lower tan δ value possessed the properties of difficult dehydration [46]. Consequently, the addition of RE improved the stability of the set yoghurt.

3.4. Microstructure Analysis of Set Yoghurt

Scanning electron microscopy is a powerful tool to illustrate and characterize the surface morphology and texture of biopolymers [8]. The microstructures of set yoghurt are shown in Figure 4. The set yoghurt samples with the addition of RE were more compact than the control yoghurt, and the compact state was gradually strengthened with increasing RE concentrations (Figure 4 marked with yellow ellipse). Meanwhile, three-dimensional (3D) network connections formed by the linkages between protein micelles were also observed with the increase in RE concentration due to the yoghurt’s special honeycomb-like porous structure. Polysaccharides were considered to facilitate the aggregation of casein at anchor points and are beneficial for 3D network development [47], which resulted in the improvement of set yoghurt hardness, WHC, and viscosity [48]. Taken together, RE could improve the stability as well as some of the physicochemical properties of set yoghurt through the formation of a more stable microstructure.

3.5. Flavour Characteristic Analysis of Set Yoghurt with E-Nose and E-Tongue

To further explore the effect of RE on the mouthfeel and acceptability of set yoghurt, the changes in flavour characteristic were evaluated using an e-nose and an e-tongue without the separation of individual compounds. As shown in Figure 5a, the response values of W1W (sulphur compounds), W2S (broad alcohol), W2W (aromatics compounds, sulphur organic, and chlorine compounds), and W5S (broad-range compounds, polar compounds, and nitrogen oxides) sensors were significantly enhanced with the increase in RE concentrations in the set yoghurt after 24 h of low-temperature storage, whereas those of the other six sensors (W1C, aromatic compounds; W3C, aromatic compounds; W6S, hydrogen; W5C, aromatic-aliphatic; W1S, broad methane; and W3S, methane-aliphatic) did not change significantly. Consequently, W1W, W2S, W2W, and W5S were the main contributors to the flavour of set yoghurt with the addition of RE. Other polysaccharides increasing the W1W, W2W, and W5S of the yoghurt samples were reported by previous studies [22,49]. For the taste analysis of set yoghurt samples with an e-tongue (Figure 5b), sourness was significantly enhanced in the samples added with 0.15% RE, compared with other samples after 24 h of low-temperature storage. No significant differences in sweetness, saltiness, richness, umami, aftertaste-A and -B, astringency, and bitterness among the varied set yoghurt samples were observed. These results indicated that the addition of RE could improve the sour characteristic flavour of the set yoghurt.
Principal component analysis (PCA) is an important tool to analyze the similarity and distinctiveness of set yoghurt by extracting and displaying the systematic variation in the acquired data [22,50]. Figure 6a showed that the PC1 and PC2 of the e-nose contributed 76.0% and 13.3% of the variation, respectively, which could better distinguish different samples. For the control set yoghurt, the 0.05%, 0.10%, and 0.15% RE samples were, respectively, distributed in the four quadrants, and the control samples were clearly separated from the RE samples. These results were similar to the effect of rose flower extract on the set yoghurt [51]. The PCA of the e-tongue is shown in Figure 6b, which displayed that 81.5% of the total taste signal variance was described by PC1 (57.2%) and PC2 (24.3%). Set yoghurt containing the 0.15% RE samples showed clear differences from the control and the 0.05% and 0.10% RE samples, while set yoghurt with the 0.10% RE samples had overlaps with the control and the 0.05% RE samples, respectively. Overall, the addition of RE, to a certain extent, had an effect on the flavour characteristics of set yoghurt.

3.6. Correlation Analysis of Physicochemical Properties with Significant Differences

Pearson correlation analysis was performed on physicochemical characteristics with significant differences. As presented in Figure 7a, significantly positive correlations were observed between W2S and cohesiveness, as well as W2W and W1W, in the control samples after 24 h of low-temperature storage. Gumminess negatively correlated with springiness and W5S, and sourness showed similar correlations with W1W and W2W (Figure 7a). Interestingly, there were no significant correlations among the physicochemical characteristics in the set yoghurt with the 0.05% RE samples (Figure 7b). With increasing RE concentrations, W1W displayed a strong positive correlation with WHC in the 0.10% RE samples, and significantly negative correlations were shown between adhesiveness and gumminess, as well as W5S and springiness (Figure 7c). When the concentration of RE increased to 0.15% in the set yoghurt, W2S showed a significant correlation with hardness, while significantly negative correlations were presented between adhesiveness and springiness, W5S and W2W, and sourness and W2S (Figure 7d). Based on the correlation analysis, there were significant differences in the correlations among varied physicochemical characteristics under different RE concentrations, indicating that RE concentrations should be selected according to the physicochemical properties of the ultimately obtained set yoghurt.

3.7. Volatile Compounds Analysis of Set Yoghurt

The concentrations of volatile compounds in the set yoghurt samples are presented in Table 1. Generally, a total of 10 compounds including one ketone, four acids, and five alkanes were successfully identified using solid phase microextraction and GC-MS. Acetoin plays an important role in yoghurt aroma and could induce the production of a butter-like flavour [2]. However, there was no significant difference in the content of acetoin among the set yoghurt samples with and without the addition of RE, which indicated that RE had no impact on the butter-like flavour of the set yoghurt. On the contrary, four acid compounds showed an upward trend with increasing RE concentrations. Butanoic, hexanoic, n-decanoic, and octanoic acids were reported to endow set yoghurt with acidic and cheesy characteristics [52]. However, another report suggested that the longer-chain molecules of n-decanoic acid could result in a rancid characteristic [53]. Fortunately, n-decanoic acid could be considered negligible due to its low content in the set yoghurt with RE. These results indicated that the addition of RE might be beneficial in improving the acidic and cheesy properties of set yoghurt. The majority of alkanes have high flavour thresholds; a higher concentration of alkanes could favour fermented products such as set yoghurt with a fruity flavour [54,55]. As shown in Table 1, three alkanes, decamethyl cyclopentasiloxane, dodecamethyl cyclohexasiloxane, and tetradecamethyl cycloheptasiloxane, showed an upward trend with increasing RE concentrations. Another report also suggested that alkanes correlated with the improvement of the set yoghurt’s taste [54]. Consequently, the addition of RE possessed the potential to enhance the flavour of yoghurt through the improvement in bacteria metabolism or its interactions with milk components.

4. Conclusions

The effects of RE on the physicochemical properties of set yoghurt were comprehensively explored. During 24 h of low-temperature storage, the addition of RE significantly and dose-dependently reduced cohesiveness and improved the hardness, gumminess, chewiness, springiness, adhesiveness, and rheological performance of set yoghurt. Meanwhile, the addition of RE effectively improved the flavour properties of set yoghurt based on the analyses of the e-nose and e-tongue data, and the microstructure was more compact compared with the control samples, which indicated that the WHC and the interaction between RE and proteins were significantly enhanced. In conclusion, the addition of RE improved the gel and flavour properties of set yoghurt and further provided greater stability. Consequently, RE possessed the potential to be an ingredient with favourable thickening and gelling properties for stabilization in dairy products. However, further research should be conducted to explore the acceptance of consumers for the RE-added set yoghurt.

Author Contributions

Conceptualization, B.W. (Binbin Wang); methodology, B.W. (Binbin Wang) and B.W. (Baomei Wu); validation, B.W. (Binbin Wang) and Y.D.; formal analysis, Y.G., L.H., K.Z. and B.W. (Binbin Wang); investigation, L.H., K.Z., Y.D. and R.A.; data curation, Y.G., L.H. and R.A.; writing—original draft preparation, B.W. (Baomei Wu); writing—review and editing, B.W. (Binbin Wang); supervision, B.W. (Binbin Wang); project administration, B.W. (Binbin Wang); funding acquisition, B.W. (Binbin Wang) and B.W. (Baomei Wu). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 32101923; the Basic Research Programme of Shanxi Province, grant number 20210302124066; Research award fund for the outstanding doctor of the Department of Finance of Shanxi Province, China, grant numbers 0110/02010008 and 0503/02010189; and the Doctoral initial fund of Shanxi Normal University, grant numbers 0505/02070532 and 0505/02070485.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Acknowledgments

We thank Weizhong Liu and Qiang Zhang for their enthusiastic help during the experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 01442 g001
Figure 1. pH and WHC of set yoghurt after low-temperature storage of 24 h. (a) pH; (b) WHC. Different lowercase letters indicated significant differences (p < 0.05) among different yoghurts using one-way ANOVA analysis with a Tukey test.
Figure 1. pH and WHC of set yoghurt after low-temperature storage of 24 h. (a) pH; (b) WHC. Different lowercase letters indicated significant differences (p < 0.05) among different yoghurts using one-way ANOVA analysis with a Tukey test.
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Figure 2. Texture profile analysis of set yoghurt after low-temperature storage of 24 h. (a) Hardness; (b) cohesiveness; (c) gumminess; (d) chewiness; (e) springiness; (f) adhesiveness. Different lowercase letters indicated significant differences (p < 0.05) among different yoghurts using one-way ANOVA analysis with a Tukey test.
Figure 2. Texture profile analysis of set yoghurt after low-temperature storage of 24 h. (a) Hardness; (b) cohesiveness; (c) gumminess; (d) chewiness; (e) springiness; (f) adhesiveness. Different lowercase letters indicated significant differences (p < 0.05) among different yoghurts using one-way ANOVA analysis with a Tukey test.
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Figure 3. Rheological properties of different set yoghurt samples stored for 24 h in a low-temperature condition. (a) Apparent viscosity of set yoghurt; (b) storage modulus (G′) and loss modulus (G″) of set yoghurt; (c) loss tangent (tan δ) of set yoghurt.
Figure 3. Rheological properties of different set yoghurt samples stored for 24 h in a low-temperature condition. (a) Apparent viscosity of set yoghurt; (b) storage modulus (G′) and loss modulus (G″) of set yoghurt; (c) loss tangent (tan δ) of set yoghurt.
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Figure 4. The SEM observation of set yoghurt samples with different concentrations of RE after 24 h of low-temperature storage. Changes in surface morphology were marked with yellow ellipse.
Figure 4. The SEM observation of set yoghurt samples with different concentrations of RE after 24 h of low-temperature storage. Changes in surface morphology were marked with yellow ellipse.
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Figure 5. The aroma and taste characteristics of set yoghurt after 24 h of low-temperature storage. (a) The radar graph of response signals in the e-nose analysis of set yoghurt aroma; (b) The radar graph of the taste signals of the e-tongue analysis.
Figure 5. The aroma and taste characteristics of set yoghurt after 24 h of low-temperature storage. (a) The radar graph of response signals in the e-nose analysis of set yoghurt aroma; (b) The radar graph of the taste signals of the e-tongue analysis.
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Figure 6. Principal component analysis of the electronic nose (a) and electronic tongue (b) after 24 h of low-temperature storage.
Figure 6. Principal component analysis of the electronic nose (a) and electronic tongue (b) after 24 h of low-temperature storage.
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Figure 7. Pearson correlation analysis of 12 tested physicochemical characteristics with significant differences in the set yoghurt after 24 h of low-temperature storage. (a) Control samples; (b) set yoghurt with 0.05% RE; (c) set yoghurt with 0.10% RE; (d) set yoghurt with 0.15% RE. * and ** indicate significance at the p < 0.05 and 0.01 levels, respectively.
Figure 7. Pearson correlation analysis of 12 tested physicochemical characteristics with significant differences in the set yoghurt after 24 h of low-temperature storage. (a) Control samples; (b) set yoghurt with 0.05% RE; (c) set yoghurt with 0.10% RE; (d) set yoghurt with 0.15% RE. * and ** indicate significance at the p < 0.05 and 0.01 levels, respectively.
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Table 1. Concentration of volatile flavour compounds in yoghurt.
Table 1. Concentration of volatile flavour compounds in yoghurt.
KindsCompoundsControlContent (μg/kg)
0.05% RE0.10% RE0.15% RE
KetoneAcetoin0.026 ± 0.0040.023 ± 0.0010.020 ± 0.0040.024 ± 0.010
AcidsButanoic acid0.008 ± 0.001 ab0.005 ± 0.001 a0.010 ± 0.001 b0.021 ± 0.004 c
Hexanoic acid0.025 ± 0.001 a0.025 ± 0.004 a0.035 ± 0.003 b0.053 ± 0.003 c
n-Decanoic acidNN0.009 ± 0.0010.008 ± 0.005
Octanoic acid0.011 ± 0.000 a0.014 ± 0.001 a0.023 ± 0.002 b0.022 ± 0.003 b
AlkanesDecamethyl cyclopentasiloxane0.012 ± 0.001 ab0.006 ± 0.002 a0.018 ± 0.004 b0.027 ± 0.006 c
Dodecamethyl cyclohexasiloxane0.026 ± 0.002 a0.026 ± 0.005 a0.046 ± 0.003 b0.052 ± 0.008 b
Hexadecamethyl cyclooctasiloxane0.003 ± 0.0020.004 ± 0.0010.006 ± 0.0020.004 ± 0.001
Octadecamethyl cyclononasiloxane0.007 ± 0.001NNN
Tetradecamethyl Cycloheptasiloxane0.010 ± 0.002 a0.010 ± 0.001 a0.016 ± 0.004 b0.015 ± 0.001 b
Values are given as the mean ± SD (n = 3). “N” means not detected. Values in a row with different superscripted letters differ significantly (p < 0.05) according to one-way ANOVA analysis with a Tukey test.
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Wu, B.; Guo, Y.; Hao, L.; Zuo, K.; Du, Y.; An, R.; Wang, B. The Effects of Leuconostoc mesenteroides RSG7 Exopolysaccharide on the Physicochemical Properties and Flavor Compounds of Set Yoghurt. Processes 2025, 13, 1442. https://doi.org/10.3390/pr13051442

AMA Style

Wu B, Guo Y, Hao L, Zuo K, Du Y, An R, Wang B. The Effects of Leuconostoc mesenteroides RSG7 Exopolysaccharide on the Physicochemical Properties and Flavor Compounds of Set Yoghurt. Processes. 2025; 13(5):1442. https://doi.org/10.3390/pr13051442

Chicago/Turabian Style

Wu, Baomei, Yanru Guo, Linlin Hao, Kaiyue Zuo, Yufei Du, Ruixin An, and Binbin Wang. 2025. "The Effects of Leuconostoc mesenteroides RSG7 Exopolysaccharide on the Physicochemical Properties and Flavor Compounds of Set Yoghurt" Processes 13, no. 5: 1442. https://doi.org/10.3390/pr13051442

APA Style

Wu, B., Guo, Y., Hao, L., Zuo, K., Du, Y., An, R., & Wang, B. (2025). The Effects of Leuconostoc mesenteroides RSG7 Exopolysaccharide on the Physicochemical Properties and Flavor Compounds of Set Yoghurt. Processes, 13(5), 1442. https://doi.org/10.3390/pr13051442

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