Changes in salted yoghurt during storage

Application of tea polyphenols as additives in brown fermented milk: Potential analysis of mitigating Maillard reaction products

Brown fermented milk (BFM) is favored by consumers in the dairy market for its unique burnt flavor and brown color. However, Maillard reaction products (MRP) from high-temperature baking are also noteworthy. In this study, tea polyphenols (TP) were initially developed as potential inhibitors of MRP formation in BFM. The results showed that the flavor profile of BFM did not change after adding 0.08% (wt/wt) of TP, and its inhibition rates on 5-hydroxymethyl-2-furaldehyde (5-HMF), glyoxal (GO), methylglyoxal (MGO), Nε-carboxymethyl lysine (CML), and Nε-carboxyethyl lysine (CEL) were 60.8%, 27.12%, 23.44%, 57.7%, and 31.28%, respectively. After 21 d of storage, the levels of 5-HMF, GO, MGO, CML, and CEL in BFM with TP were 46.3%, 9.7%, 20.6%, 5.2%, and 24.7% lower than the control group, respectively. Moreover, a smaller change occurred in their color and the browning index was lower than that of the control group. The significance of this study was to develop TP as additives to inhibit the production of MRP in brown fermented yogurt without changing color and flavors, thereby making dairy products safer for consumers.

Comparison of volatile metabolic profiles in fermented milk of Streptococcus thermophilus during the postripening period at different incubation temperatures

Streptococcus thermophilus has been extensively applied in fermented milk. This study used gas chromatography-ion mobility spectroscopy to determine and evaluate the volatile metabolites in raw milk, milk fermented at 37°C, and milk fermented at 42°C. Ten discriminatory volatile metabolites were identified at different incubation temperatures: acetone, 2-heptanone, 2-pentanone, 2-hexanone, butanal, hexanal, ethyl acetate, 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropanoic acid, indicating that fermentation temperature affected the spectrum of volatiles in milk fermented by different strains of S. thermophilus. Specifically, fermentation at 37°C led to accumulation of short-chain fatty acids, whereas fermentation at 42°C enriched ketones and other flavor substances in the fermented milk, enhancing the flavor of the product. This work examined the differences between the volatile metabolites produced by different S. thermophilus strains fermented at different temperatures to evaluate the effect of temperature on the metabolic pathways.

Design and Volatile Compound Profiling of Starter Cultures for Yogurt Preparation

Stable symbiotic starter cultures were created using selected strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus with antimicrobial activity against pathogens and necessary antibiotic sensitivity, growth kinetic parameters, and metabolic profiles. The volatile compound profiles of the obtained starter cultures were determined and their specificity was proven depending on the ratio of monocultures in each combination. The influence of the freeze-drying process on the starter cultures in relation to the production of aromatic components was investigated and it was demonstrated that this process had a significant effect on the content of the aroma-forming substances in the fermented milk. However, the influence of the pre-cooling process and crude fat content from 1.5 to 3.0% did not notably affect the levels of volatile compounds synthesized by the selected starter cultures. Comprehensive data for all volatile aromatic metabolites in the fermented milk were also obtained. These designed symbiotic starter cultures can be used to produce traditional Bulgarian yogurt with increased functional and probiotic properties.

Spoilage Potential of Contaminating Yeast Species Kluyveromyces marxianus, Pichia kudriavzevii and Torulaspora delbrueckii during Cold Storage of Skyr

This study investigated the spoilage potential of yeast strains Kluyveromyces marxianus (Km1, Km2 and Km3), Pichia kudriavzevii Pk1 and Torulaspora delbrueckii Td1 grown in skyr in cold storage. Yeast strains were isolated from skyr and identified by sequencing of the 26S rRNA gene. K. marxianus yeasts were grown in skyr to high numbers, generating large amounts of volatile organic compounds (VOC) associated with off-flavours, among them were alcohols (3-methyl-1-butanol, 2-methyl-1-propanol and 1-hexanol), esters (ethyl acetate and 3-methylbutyl acetate) and aldehydes (hexanal, methylbutanal and methylpropanal). Growth of P. kudriavzevii Pk1 led to moderate increases in several alcohols and esters (mostly, 3-methyl-1-butanol and ethyl acetate), whereas only minor shifts in VOCs were associated with T. delbrueckii Td2. The levels of the key aroma compounds, diacetyl and acetoin, were significantly decreased by all K. marxianus strains and P. kudriavzevii Pk1. In contrast to the other yeast species, K. marxianus was able to utilize lactose, producing ethanol and carbon dioxide. Based on the overall results, K. marxianus was characterised by the highest spoilage potential. The study revealed the differences between the yeast species in fermentative and spoilage activities, and clarified the role of yeast metabolites for off-flavour formation and quality defects in skyr during cold storage.

Metabolic footprint analysis of volatile metabolites by gas chromatography-ion mobility spectrometry to discriminate between different fermentation temperatures during Streptococcus thermophilus milk fermentation

Streptococcus thermophilus is widely used in the dairy industry to produce fermented milk. Gas chromatography-ion mobility spectrometry-based metabolomics was used to discriminate different fermentation temperatures (37°C and 42°C) at 3 time points (F0: pH = 6.50 ± 0.02; F1: pH = 5.20 ± 0.02; F2: pH = 4.60 ± 0.02) during S. thermophilus milk fermentation, and differences of fermentation physical properties and growth curves were also evaluated. Fermentation was completed (pH 4.60) after 6 h at 42°C and after 8 h at 37°C; there were no significant differences in viable cell counts and titratable acidity; water-holding capacity and viscosity were higher at 37°C than at 42°C. Different fermentation temperatures affected volatile metabolic profiles. After the fermentation was completed, the volatile metabolites that could be used to distinguish the fermentation temperature were hexanal, butyraldehyde, ethyl acetate, ethanol, 3-methylbutanal, 3-methylbutanoic acid, and 2-methylpropionic acid. Specifically, at 37°C of milk fermentation, branched-chain AA had higher levels, and leucine, isoleucine, and valine were involved in growth and metabolism, which promoted accumulation of some short-chain fatty acids such as 3-methylbutanoic acid and 2-methylpanprooic acid. At 42°C, at 3 different time points during fermentation, ethanol from glycolysis all presented higher levels, including acetone and 3-methylbutanal, producing a more pleasant flavor in the fermented milk. This work provides detailed insight into S. thermophilus fermented milk metabolites that differed between incubation temperatures; these data can be used for understanding and eventually predicting metabolic changes during milk fermentation.

Metabolic footprint analysis of volatile metabolites to discriminate between different key time points in the fermentation and storage of starter cultures and probiotic Lactobacillus casei Zhang milk

Interest has been growing in the co-fermentation of starter cultures with probiotic bacteria in milk. However, the representative metabolites and metabolic changes at different key time points during milk fermentation and storage in starter cultures and probiotic bacteria are still unclear. In this study, we used gas chromatography/mass spectrometry-based metabolomics to identify volatile metabolites and discriminate between 6 different time points [fermentation initiation (FI), fermentation curd (FC), fermentation termination (FT), storage 1 d (S1d), storage 7 d (S7d), and storage 14 d (S14d)] during the fermentation and storage of starter cultures and Lactobacillus casei Zhang milk. Of the 52 volatile metabolites identified, 15 contributed to discrimination of the 6 time points. Then, using the profile from the different time points, we analyzed pairwise comparisons (FI vs. FC; FC vs. FT; FT vs. S1d; S1d vs. S7d; S7d vs. S14d); these time-lapse comparisons showed metabolic progressions from one fermentation stage to the next. We found representative and exclusive metabolites at specific fermentation and storage time points. The greatest difference in metabolites occurred between FC and FT, and the metabolic profiles between S7d and S14d were most similar. Interestingly, decanoic acid, octanoic acid, and hexanoic acid reached their highest level at storage 14 d, indicating that the post-fermentation storage of fermented milk with L. casei Zhang may add more probiotic functions. This work provides detailed insight into the time-specific profiles of volatile metabolites and their dynamic changes; these data may be used for understanding and eventually predicting metabolic changes in milk fermentation and storage, where probiotic strains may be used.

Volatile Profile in Yogurt Obtained from Saanen Goats Fed with Olive Leaves

The aim of this study was to evaluate the development of volatile compounds in yogurt samples obtained from goats fed a dietary supplementation with olive leaves (OL). For this purpose, thirty Saanen goats were divided into two homogeneous groups of 15 goats each: a control group that received a standard diet (CG) and an experimental group whose diet was supplemented with olive leaves (OLG). The trial lasted 28 days, at the end of which the milk of each group was collected and used for yogurt production. Immediately after production, and after 7 days of storage at 4 °C in the absence of light, the yogurt samples were characterized in terms of fatty acid profile, oxidative stability and volatile compounds by the solid-phase microextraction (SPME)-GC/MS technique. Dietary OL supplementation positively affected the fatty acid composition, inducing a significant increase in the relative proportion of unsaturated fatty acids, mainly oleic acid (C18:1 cis9) and linolenic acid (C18:3). With regard to the volatile profile, both in fresh and yogurt samples stored for 7 days, the OL supplementation induced an increase in free fatty acids, probably due to an increase in lipolysis carried out by microbial and endogenous milk enzymes. Specifically, the largest variations were found for C6, C7, C8 and C10 free fatty acids. In the same samples, a significant decrease in aldehydes, mainly heptanal and nonanal, was also detected, supporting-at least in part-an improvement in the oxidative stability. Moreover, alcohols, esters and ketones appeared lower in OLG samples, while no significant variations were observed for lactones. These findings suggest the positive role of dietary OL supplementation in the production of goats' milk yogurt, with characteristics potentially indicative of an improvement in nutritional properties and flavor.

Evaluation of Volatile Compounds in Milks Fermented Using Traditional Starter Cultures and Probiotics Based on Odor Activity Value and Chemometric Techniques

The volatile components of milks fermented using traditional starter cultures (Streptococcus thermophilus and Lactobacillus bulgaricus) and probiotics (Lactobacillus lactis, Lactobacillus bifidus, Lactobacillus casei, and Lactobacillus plantarum) were investigated by means of gas chromatography-mass spectrometry (GC-MS) combined with simultaneous distillation extraction (SDE). A total of 53 volatile compounds were detected, being 10 aldehydes, 11 ketones, 10 acids, 11 hydrocarbons, 7 benzene derivatives, and 4 other compounds. The starter culture was found to significantly affect the composition of volatile components in the fermented milks. Ketones and hydrocarbons were the dominant compounds in milk before fermentation, while acids were dominant compounds in the fermented samples. Compared with probiotics, there was greater abundance of volatile components in fermented milks with traditional strains. The importance of each volatile compound was assessed on the basis of odor, thresholds, and odor activity values (OAVs). Of the volatile compounds, 31 of them were found to be odor-active compounds (OAV > 1). The component with the highest OAVs in most samples was (E,E)-2,4-decadienal. Heatmap analysis and principal component analysis were employed to characterize the volatile profiles of milks fermented by different starter cultures. The results could help to better understand the influence of starter cultures on the odor quality of milks.

Physicochemical and Microbiological Properties of Synbiotic Yogurt Made with Probiotic Yeast Saccharomyces boulardii in Combination with Inulin

Saccharomyces boulardii is a unique species of yeast previously characterized as a probiotic strain (CNCM I-745) among a few probiotic yeasts reported to date. Inulin is one of the most common prebiotics that exhibit twisted hydrocolloidal properties in dairy products. The present study was designed to develop a synbiotic yogurt by incorporation of S. boulardii and inulin at 1%, 1.5%, and 2% (w/v), comparing with the probiotic and control plain yogurts. Microrheological, microstructural, microbiological, sensory properties, and volatile compounds of the yogurt samples were evaluated. Microrheological analysis showed that addition of inulin to yogurt slightly reduced the values of G' and G″, while solid-liquid balance (SLB) values confirmed more solid properties of the synbiotic yogurt (0.582~0.595) than the plain yogurt (0.503~0.518). A total of 18 volatile compounds were identified in the synbiotic yogurt, while only five and six compounds were identified in plain and probiotic yogurts, respectively. Physiochemical parameters such as pH, acidity, and protein content were in the normal range (as with the control), while fat content in the synbiotic yogurt decreased significantly. Addition of 1% inulin not only reduced syneresis but also maintained viability of S. boulardii after 28 days of storage. Microstructural and microrheological studies confirmed the dense, compressed, homogeneous structure of the synbiotic yogurt. Thus, addition of inulin improved the textural and sensory properties of the synbiotic yogurt, as well as survival of S. boulardii with viable count above 6.0 log CFU/g in yogurt, as generally required for probiotics. Therefore, novel synbiotic yogurt with desirable quality was developed as an effective carrier for delivery of the probiotic yeast exerting its beneficial health effects.

Volatile Flavor Compounds Profile and Fermentation Characteristics of Milk Fermented by Lactobacillus delbrueckii subsp. bulgaricus

Lactobacillus delbrueckii subsp. bulgaricus is one of the predominant lactic acid bacterial species used as starter cultures in industrial fermented dairy manufacturing, as it strongly affects the quality of the products. Volatile flavor compound profiles and fermentation characteristics are considered to be the most important indicators for starter culture screening. In the present study, volatile compounds in milk fermented by 17 test strains of L. delbrueckii subsp. bulgaricus and a commercial strain used as a control were identified using solid-phase microextraction (SPME) methods coupled with gas chromatography mass spectrometry (GC-MS). In total, 86 volatile flavor compounds were identified in the fermented milk upon completion of fermentation, including 17 carboxylic acids, 14 aldehydes, 13 ketones, 29 alcohols, 8 esters, and 5 aromatic hydrocarbon compounds. Various volatile flavor compounds (acetaldehyde, 3-methyl-butanal, (E)-2-pentenal, hexanal, (E)-2-octenal, nonanal, 2,3-butanedione, acetoin, 2-heptanone, 2-non-anone, formic acid ethenyl ester) were identified due to their higher odor activity values (>1). In addition, of the 17 test strains of L. delbrueckii subsp. bulgaricus, IMAU20312 (B14) and IMAU62081 (B16) strains exhibited good fermentation characteristics in milk compared with the control strain. The combination of the volatile flavor compound profile and fermentation characteristics in this work could be useful when selecting lactic acid bacteria that may serve as important resources in the development of novel fermented milk products.

Assessment of Freeze-Dried Immobilized Lactobacillus casei as Probiotic Adjunct Culture in Yogurts

Freeze-dried immobilized Lactobacillus casei ATCC 393 on casein and apple pieces were assessed as a probiotic adjunct culture for novel probiotic yogurt production. The effect of probiotic culture on physicochemical characteristics, probiotic cell survival, volatile aroma compounds, and sensory quality were evaluated during 28 days of storage at 4 °C. The use of L. casei resulted in lower pH values (3.92–4.12), higher acidity (0.88–1.10 g lactic acid/100 g of yogurt), and lower syneresis (40.8%–42.6%) compared to traditionally produced yogurt (pH 4.29; acidity 0.83 g lactic acid/100 g of yogurt; syneresis 44.1%). Microbiological and strain-specific multiplex PCR (Polymerase Chain Reaction) analysis confirmed that immobilized L. casei ATCC 393 cells were detected in yogurts at levels >7 log cfu g⁻¹ after 28 days. In addition, probiotic supplementation significantly affected the concentrations of key volatile compounds, like acetic and other organic acids, 2-ethyl-1-hexanol, acetoin, and 2-butanone, as revealed by GC-MS (Gas Chromatography–Mass Spectrometry) analysis. Finally, the sensory evaluation demonstrated that the new products exhibited improved characteristics compared to traditionally produced yogurts.

Screening of aroma-producing lactic acid bacteria and their application in improving the aromatic profile of yogurt

The development of yogurt flavor is a complicated and dynamic biochemical process. In addition to traditional starter cultures, adjunct cultures could also make significant contributions to the flavor profiles of yogurt. In the current study, two Lactobacillus plantarum strains (1-33 and 1-34) were isolated based on their abilities to produce acetaldehyde and diacetyl. In co-fermentation with traditional starters, these isolated strains were able to maintain viability without affecting the yogurt's acidification profiles. Furthermore, they positively influenced the aroma quality of the yogurt samples. They promoted the formation of volatile metabolites, especially acetaldehyde, diacetyl, and acetoin, which are recognized as characteristic compounds. The results of this work provide novel knowledge about the contributions of isolated strains on the flavor profiles of yogurt, which will help to improve the organoleptic properties of the final products. PRACTICAL APPLICATIONS: Using lactic acid bacteria (LAB) as adjunct cultures co-fermented with traditional yogurt starter cultures can increase the quantities of flavor compounds in yogurt. This study enriches our understanding of the effects of adjunct cultures on yogurt flavor. Researchers and manufacturers that specialize in yogurt making can use the results of this study to improve the aromatic profile and organoleptic quality of yogurt.

The combined effect of probiotic cultures and incubation final pH on the quality of buffalo milk yogurt during cold storage

The combined effects of starter culture type (SCT) and incubation final pH (IFpH) on the physicochemical and organoleptic properties of buffalo milk yogurt containing 3 g·100 g⁻¹ milk fat were investigated throughout 20 days of storage at 4°C. The postacidification kinetics fitted to zero‐order reaction for all buffalo milk yogurt samples. The reaction rate constants of the buffalo milk yogurt samples containing YC‐X11, ABY‐2, and ABT‐4 cultures were 0.010, 0.007, and 0.004 g·100 g⁻¹·day⁻¹, respectively. Regardless of the IFpH, the absence of Lactobacillus delbrueckii subsp. bulgaricus in the starter culture increased the syneresis. L*, a*, and b* values were not affected by the IFpH and the SCT. ABY‐2 culture increased the amount of organic acids during cold storage in comparison with the YC‐X11, while its effect on the proportions of saturated and unsaturated fatty acids was not significant. The results of sensory evaluation revealed that a more acceptable buffalo milk yogurt can be manufactured by using probiotic ABY‐2 culture.

Profiles of Volatile Flavor Compounds in Milk Fermented with Different Proportional Combinations of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are key factors in the fermentation process and the final quality of dairy products worldwide. This study was performed to investigate the effects of the proportions of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus isolated from traditionally fermented dairy products in China and Mongolia on the profile of volatile compounds produced in samples. Six proportional combinations (1:1, 1:10, 1:50, 1:100, 1:1000, and 1:10,000) of L. delbrueckii subsp. bulgaricus IMAU20401 to S. thermophilus ND03 were considered, and the volatiles were identified and quantified by solid-phase microextraction and gas chromatography-mass spectrometry (SPME-GC-MS) against an internal standard. In total, 89 volatile flavor compounds, consisting of aldehydes, ketones, acids, alcohols, esters, and aromatic hydrocarbons, were identified. Among these, some key flavor volatile compounds were identified, including acetaldehyde, 3-methylbutanal, acetoin, 2-heptanone, acetic acid, butanoic acid, and 3-methyl-1-butanol. The of L. delbrueckii subsp. bulgaricus IMAU20401 to S. thermophilus ND03 influenced the type and concentration of volatiles produced. In particular, aldehydes and ketones were present at higher concentrations in the 1:1000 treatment combination than in the other combinations. Our findings emphasize the importance of selecting the appropriate proportions of L. delbrueckii subsp. bulgaricus and S. thermophilus for the starter culture in determining the final profile of volatiles and the overall flavor of dairy products.

Enhanced Microbial, Functional and Sensory Properties of Herbal Yogurt Fermented with Korean Traditional Plant Extracts

This study evaluated the effects of two Korean traditional plant extracts (Diospyros kaki THUNB. leaf; DK, and Nelumbo nucifera leaf; NN) on the fermentation, functional and sensory properties of herbal yogurts. Compared to control fermentation, all plant extracts increased acidification rate and reduced the time to complete fermentation (pH 4.5). Supplementation of plant extracts and storage time were found to influence the characteristics of the yogurts, contributing to increased viability of starter culture and phenolic compounds. In particular, the increase in the counts of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus was highest (2.95 and 1.14 Log CFU/mL respectively) in DK yogurt. Furthermore, supplementation of the plant extracts significantly influenced to increase the antioxidant activity and water holding capacity and to produce volatile compounds. The higher antioxidant activity and water holding capacity were observed in NN yogurt than DK yogurt. Moreover, all of the sensory characteristics were altered by the addition of plant extracts. Addition of plant extracts increased the scores related to flavor, taste, and texture from plain yogurt without a plant extract, as a result of volatile compounds analysis. Thus, the overall preference was increased by plant extracts. Consequently, supplementation of DK and NN extracts in yogurt enhanced the antioxidant activity and physical property, moreover increased the acceptability of yogurt. These findings demonstrate the possibility of using plant extracts as a functional ingredient in the manufacture of herbal yogurt.

Assessment of technological characteristics of non-fat yoghurt manufactured with prebiotics and probiotic strains

Microbiological, physicochemical, aroma and organic acid characteristics of non-fat yoghurt incorporated with β-glucan and probiotic Lactobacillus plantarum strains (AB6-25, AC18-82 and AK4-11) combination as adjunct culture were investigated during a 21 day storage period at 4 °C. Four treatment yoghurts contained 0.25, 0.5, 1 and 1.5 % β-glucan. Treatments also included probiotic combination and commercial culture. Treatments were compared with three controls produced containing commercial culture, commercial culture and probiotic combination, and commercial culture, Lactobacillus acidophilus and inulin. The results indicated that β-glucan promote the viability of lactobacilli. However, the addition of β-glucan (except 0.25 %) resulted in enhanced syneresis (P < 0.05). In general, the use of 0.25 % β-glucan had no significant effect on pH, fat, protein and organic acid content of non-fat yoghurt. The results obtained from this research demonstrated that the use of 0.25 % β-glucan has no adverse effect on the characteristics of non-fat yogurt produced with probiotic combination.