Influence of Lactobacillus plantarum on yogurt fermentation properties and subsequent changes during postfermentation storage

This study aimed to evaluate the influence of 9 Lactobacillusplantarum with broad-spectrum antibacterial activity on fermented milk, including changes to the fermentation characteristics (pH, titration acidity, and viable counts), texture profile, relative content of volatile compounds, and sensory evaluation during 28-d storage at 4°C. First, L. plantarum IMAU80106, IMAU10216, and IMAU70095 were selected as candidates for further study because of their excellent coagulation and proteolytic activities. Subsequently, these L. plantarum strainswere supplemented to fermented milk produced by commercial yogurt starters (Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) and a panel of parameters reflecting product quality was subsequently monitored during 28 d of postfermentation storage. The pH value and titration acidity of the fermented milk mildly fluctuated, whereas the L. plantarum viable counts remained stable along the storage period. Fourteen key volatile compounds were detected in the fermented milk by gas chromatography-mass spectrometry, and some flavor compounds were uniquely present in the L. plantarum-supplemented fermented milk (including 2,3-pentanedione, acetaldehyde, and acetate). No significant difference was shown in the sensory evaluation scores between samples with or without L. plantarum supplementation, but a gradual decrease was observed over storage in all samples. However, when L. plantarum was added, apparent shifts were observed in the overall quality of the fermented milk based on principal component analysis and multivariate ANOVA, particularly in the texture (adhesiveness) and volatile flavor compound profiles (acetaldehyde). Compared with L. plantarum IMAU80106 and IMAU10216, both the texture and volatile flavor profiles of IMAU70095 were closest to those of the control without adding the adjunct bacteria, suggesting that IMAU70095 might be the most suitable strain for further application in functional dairy product development. The current work has explored the potential of applying L. plantarum in fermented milk by performing thorough physical and chemical characterization. Our work is of intense interest to the dairy industry.     

Antibacterial activity of selenium-enriched lactic acid bacteria against common food-borne pathogens in vitro

Selenium (Se) is an essential trace element for human health and animal nutrition. The aim of this study was to evaluate the inhibitory activities of Se-enriched lactic acid bacteria (LAB), Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus, against pathogenic Salmonella typhimurium, Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes in vitro. The results indicated that the accumulation amount of Se by Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus reached 12.05 ± 0.43 µg/mL and 11.56 ± 0.25 µg/mL, respectively, accompanied by the relative maximum living cells when sodium selenite was 80 µg/mL. Oxford cup double plate assay showed that bacterial culture solution and cell-free culture supernatant (CFCS) from Se-enriched LAB exerted stronger antibacterial activity than those from the non-Se strains. The growth of pathogenic bacterial culture with CFCS at any growth stages was worse than that without CFCS; moreover, the inhibiting effect of CFCS of Se-enriched LAB was more significant than that of non-Se strains. Results from a scanning electron microscope equipped with energy dispersion X-ray spectrometry showed that elemental Se nanoparticles, which characteristically energy peak around 1.42 keV, were deposited on the cell surfaces of Lactobacillus delbrueckii ssp. bulgaricus. In addition, CFCS of Se-enriched LAB induced more serious cell structure damage of pathogenic bacteria than did non-Se LAB.     

Genetic diversity of Lactobacillus delbrueckii isolated from raw milk in Hokkaido,Japan

Lactic acid bacteria (LAB) play important roles in acid production and flavor formation in fermented dairy products. Lactic acid bacteria strains with distinct characteristics confer unique features to products. Diverse LAB have been identified in raw milk and traditional fermented milk prepared from raw milk. However, little is known about LAB in raw milk in Japan. To preserve diverse LAB as potential starters or probiotics for future use, we have isolated and identified various kinds of LAB from raw milk produced in Japan. In this study, we focused on Lactobacillus delbrueckii, one of the most important species in the dairy industry. We identified L. delbrueckii subspecies isolated from raw milk in Hokkaido, Japan, by analyzing intraspecific diversity using 4 distinct methods, hsp60 cluster analysis, multilocus sequence analysis, core-genome analysis, and whole-genome analysis based on average nucleotide identity. The subspecies distribution and a new dominant subset of L. delbrueckii from raw milk in Japan were revealed. The discovery of new strains with different genotypes is important for understanding the geographic distribution and characteristics of the bacteria and further their use as a microbial resource with the potential to express unconventional flavors and functionalities. The strains identified in this study may have practical applications in the development of fermented dairy products.     

Immunomodulatory effects of polysaccharides produced by Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1

The extracellular polysaccharides (EPS) produced by lactic acid bacteria (LAB) are associated with the rheology, texture, and mouthfeel of fermented milk products, including yogurt. This study investigated the immunomodulatory effects of EPS purified from the culture supernatant of Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) OLL1073R-1. The crude EPS were prepared from the culture supernatant of L. bulgaricus OLL1073R-1 by standard chromatographic methods, and were fractionated into neutral EPS and acidic EPS (APS). Acidic EPS were further fractionated into high molecular weight APS (H-APS) and low molecular weight APS (L-APS). High molecular weight APS were shown to be phosphopolysaccharides containing D-glucose, D-galactose, and phosphorus. Stimulation of mouse splenocytes by H-APS significantly increased interferon-γ production, and, moreover, orally administered H-APS augmented natural killer cell activity. Oral administration of yogurt fermented with L. bulgaricus OLL1073R-1 and Streptococcus thermophilus OLS3059 to mice showed a similar level of immunomodulation as H-APS. However, these effects were not detected following administration of yogurt fermented with the starter combination of L. bulgaricus OLL1256 and S. thermophilus OLS3295. We conclude from these findings that yogurt fermented with L. bulgaricus OLL1073R-1, containing immunostimulative EPS, would have an immunomodulatory effect on the human body.     

Diversity of lactic acid bacteria associated with traditional fermented dairy products in Mongolia

Spontaneous milk fermentation has a long history in Mongolia, and beneficial microorganisms have been handed down from one generation to the next for use in fermented dairy products. The objective of this study was to investigate the diversity of lactic acid bacteria (LAB) communities in fermented yak, mare, goat, and cow milk products by analyzing 189 samples collected from 13 different regions in Mongolia. The LAB counts in these samples varied from 3.41 to 9.03 log cfu/mL. Fermented yak and mare milks had almost identical mean numbers of LAB, which were significantly higher than those in fermented goat milk but slightly lower than those in fermented cow milk. In total, 668 isolates were obtained from these samples using de Man, Rogosa, and Sharpe agar and M17 agar. Each isolate was considered to be presumptive LAB based on gram-positive and catalase-negative properties, and was identified at the species level by 16S rRNA gene sequencing, multiplex PCR assay, and restriction fragment length polymorphism analysis. All isolates from Mongolian dairy products were accurately identified as Enterococcus faecalis (1 strain), Enterococcus durans (3 strains), Lactobacillus brevis (3 strains), Lactobacillus buchneri (2 strains), Lactobacillus casei (16 strains), Lactobacillus delbrueckii ssp. bulgaricus (142 strains), Lactobacillus diolivorans (17 strains), Lactobacillus fermentum (42 strains), Lactobacillus helveticus (183 strains), Lactobacillus kefiri (6 strains), Lactobacillus plantarum ssp. plantarum (7 strains), Lactococcus lactis ssp. lactis (7 strains), Leuconostoc lactis (22 strains), Leuconostoc mesenteroides (21 strains), Streptococcus thermophilus (195 strains), and Weissella cibaria (1 strain). The predominant LAB were Strep. thermophilus and Lb. helveticus, which were isolated from all sampling sites. The results demonstrate that traditional fermented dairy products from different regions of Mongolia have complex compositions of LAB species. Such diversity of LAB provides useful information for further studies of probiotic strain selection and starter culture design, with regard to the industrial production of traditional fermented milk.     

Exploring the industrial potential of Lactobacillus delbrueckii ssp. bulgaricus by population genomics and genome-wide association study analysis

Lactobacillus delbrueckii ssp. bulgaricus is one of the most commonly used starter cultures for yogurt production. However, how its genetic background affects acid production capacity is unclear. This study investigated the industrial potential of L. delbrueckii ssp. bulgaricus using population genomics and GWAS analysis. To meet our goal, population genetics and functional genomics analyses were performed on 188 newly sequenced L. delbrueckii ssp. bulgaricus strains isolated from naturally fermented dairy products together with 19 genome sequences retrieved from the NCBI database. Four distinct clusters were identified, and they were correlated with the geographical sites where the samples were collected. The GWAS analysis about acidification fermentation results with sucrose-fortified reconstituted skim milk revealed a significant association between l-lactate dehydrogenase (lldD; Ldb2036) and the bacterial acid production rate. Our study has broadened the understanding of the population structure and genetic diversity of L. delbrueckii ssp. bulgaricus by identifying potential targets for further research, development, and use of lactic acid bacteria in the dairy industry.     

Comparison of Associative Growth and Proteolytic Activity of Yogurt Starters in Whole Milk from Camels and Cows

Growth and proteolytic activities were studied using yogurt starter cultures incubated in pasteurized whole milk from camels and cows at 42°C as single and mixed cultures. In general, the growth of four strains of Streptococcus thermophilus and three strains of Lactobacillus delbrueckii ssp. bulgaricus was higher in cow milk than in camel milk. However, proteolysis was higher in camel milk than in cow milk. Lactobacillus delbrueckii ssp. bulgaricus LB12 in combination with streptococci had lowered pH more than did the other lactobacilli. Mixed cultures released the same amount of free amino groups as the corresponding single cultures, except for L. delbrueckii ssp. bulgaricus LB12.     

Probiotic viability and storage stability of yogurts and fermented milks prepared with several mixtures of lactic acid bacteria

Currently, the food industry wants to expand the range of probiotic yogurts but each probiotic bacteria offers different and specific health benefits. Little information exists on the influence of probiotic strains on physicochemical properties and sensory characteristics of yogurts and fermented milks. Six probiotic yogurts or fermented milks and 1 control yogurt were prepared, and we evaluated several physicochemical properties (pH, titratable acidity, texture, color, and syneresis), microbial viability of starter cultures (Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus) and probiotics (Lactobacillus acidophilus, Lactobacillus casei, and Lactobacillus reuteri) during fermentation and storage (35 d at 5°C), as well as sensory preference among them. Decreases in pH (0.17 to 0.50 units) and increases in titratable acidity (0.09 to 0.29%) were observed during storage. Only the yogurt with S. thermophilus, L. delbrueckii ssp. bulgaricus, and L. reuteri differed in firmness. No differences in adhesiveness were determined among the tested yogurts, fermented milks, and the control. Syneresis was in the range of 45 to 58%. No changes in color during storage were observed and no color differences were detected among the evaluated fermented milk products. Counts of S. thermophilus decreased from 1.8 to 3.5 log during storage. Counts of L. delbrueckii ssp. bulgaricus also decreased in probiotic yogurts and varied from 30 to 50% of initial population. Probiotic bacteria also lost viability throughout storage, although the 3 probiotic fermented milks maintained counts ≥10⁷ cfu/mL for 3 wk. Probiotic bacteria had variable viability in yogurts, maintaining counts of L. acidophilus ≥10⁷ cfu/mL for 35 d, of L. casei for 7 d, and of L. reuteri for 14 d. We found no significant sensory preference among the 6 probiotic yogurts and fermented milks or the control. However, the yogurt and fermented milk made with L. casei were better accepted. This study presents relevant information on physicochemical, sensory, and microbial properties of probiotic yogurts and fermented milks, which could guide the dairy industry in developing new probiotic products.     

Characterization and association of bacterial communities and nonvolatile components in spontaneously fermented cow milk at different geographical distances

In the ecosystem of spontaneously fermented cow milk, the characteristics and relationship of bacterial communities and nonvolatile components at different scales of geographical distances (provincial, county, and village levels) are unclear. Here, 25 sampling sites from Xin Jiang and Tibet, 2 provinces of China, were selected based on the distribution of spontaneously fermented cow milk and used for metagenomic and metabolomic analysis. At the provincial geographical distance, the same predominant species, Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus, were detected in Xin Jiang and Tibet. Further, the richness of the bacterial composition of samples from Tibet was higher than those from Xin Jiang; specifically, at the species level, 28 species were identified in Tibet samples but only 7 species in Xin Jiang samples. At the provincial geographical level, we detected significant differences in bacterial structure, shown in principal coordinate analysis plots, and significant differences (Simpson index) in bacterial diversity were also detected. However, at the county and village levels, no significant differences were detected in bacterial communities and diversity, but a difference in bacterial compositions was detectable. This indicates that bacterial communities and diversity of spontaneously fermented milk dissimilarity significantly increased with geographic distance. For the nonvolatile component profiles, the partial least squares discriminant analysis plot (R²Y > 0.5 and Q² > 0.5 for the goodness-of-fit and predictive ability parameter, respectively) showed that samples from different geographical distances (provincial, county, and village) were all separated, which indicated that all the discriminations in nonvolatile components profiles were from different geographical distances. Investigating relationships between lactic acid bacteria and discriminatory nonvolatile components at the county level showed that 9 species were positively correlated with 16 discriminatory nonvolatile components, all species with low abundance rather than the predominant species L. delbrueckii ssp. bulgaricus and Strep. thermophilus, which indicates the importance of the selection of autochthonous nonpredominant bacteria.     

Use of gases to improve survival of Bifidobacterium bifidum by modifying redox potential in fermented milk

The aim of this work was to study the effect of the oxidoreduction potential, modified using gas, on the growth and survival of a probiotic strain, Bifidobacterium bifidum, and 2 yogurt strains, Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. Three fermented milks were manufactured with an initial oxidoreduction potential value adjusted to +440 mV (control milk), +350 mV (milk gassed with N₂), and −300 mV [milk gassed with N₂ plus 4% (vol/vol) H₂ (N₂-H₂)]. Acidification profiles, growth during milk fermentation and survival during storage at 4°C for 28 d were determined. This study showed that fermented probiotic dairy products made from milk gassed with N₂ and, more particularly, those made from milk gassed with N₂H₂ were characterized by a significant increase in B. bifidum survival during storage without affecting the fermentation kinetics and the survival of Strep. thermophilus and L. delbrueckii ssp. bulgaricus.     

Effect of lactose hydrolysis on the milk-fermenting properties of Lactobacillus delbrueckii ssp. bulgaricus 2038 and Streptococcus thermophilus 1131

Yogurt is a well-known nutritious and probiotic food and is traditionally fermented from milk using the symbiotic starter culture of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. However, yogurt consumption may cause health problems in lactose-intolerant individuals, and the demand for lactose-free yogurt has been increasing. The standard method to prepare lactose-free yogurt is to hydrolyze milk by lactase; however, this process has been reported to influence the fermentation properties of starter strains. This study aimed to investigate the fermentation properties of an industrial starter culture of L. bulgaricus 2038 and S. thermophilus 1131 in lactose-hydrolyzed milk and to examine the metabolic changes induced by glucose utilization. We found that the cell number of L. bulgaricus 2038, exopolysaccharide concentration, and viscosity in the coculture of L. bulgaricus 2038 and S. thermophilus 1131 was significantly increased in lactose-hydrolyzed milk compared with that in unhydrolyzed milk. Although the cell number of S. thermophilus 1131 showed no difference, production of formic acid and reduction of dissolved oxygen were enhanced in lactose-hydrolyzed milk. Further, in lactose-hydrolyzed milk, S. thermophilus 1131 was found to have increased the expression of NADH oxidase, which is responsible for oxygen reduction. These results indicated that glucose utilization promoted S. thermophilus 1131 to rapidly reduce the dissolved oxygen amount and produce a high concentration of formic acid, presumably resulting in the increased cell number of L. bulgaricus 2038 in the coculture. Our study provides basic information on the metabolic changes in starter strains in lactose-hydrolyzed milk, and demonstrates that lactose-free yogurt with increased cell number of L. bulgaricus can be prepared without delay in fermentation and decrease in the cell number of S. thermophilus.     

Potential of LAB starter culture isolated from Chinese traditional fermented foods for yoghurt production

A total of 32 indigenous lactic acid bacteria (LAB) isolates were previously obtained from 7 samples of traditional dairy products in specific ecological niches throughout the northwestern area of China. Among the 32 isolates, 9 strains of LAB were selected for their potential in fermented dairy products and identified by 16S rDNA sequence analysis. In this study, these 9 isolates were combined each other and 8 combinations were selected according to their fermentation characteristics in yoghurt production. The effects of these 8 combinations on acidification, post acidification, texture and sensory quality of yoghurt were analysed. Results indicated that Streptococcus thermophilus SP1.1 combined with Lactobacillus delbrueckii ssp. bulgaricus 3 4.5 was the best combination and better than the commercial starter culture in terms of yoghurt fermentation time and sensory quality, suggesting that the combination has potential as a commercial starter to improve the quality of yoghurt.     

Selective enumeration and identification of mixed cultures of Streptococcus thermophilus,Lactobacillus delbrueckii subsp. bulgaricus,L. acidophilus,L. paracasei subsp. paracasei and Bifidobacterium lactis in fermented milk

This study describes selective plating methodologies for enumeration of mixed cultures of Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, L. acidophilus, L. paracasei subsp. paracasei and Bifidobacterium lactis in fermented milk based on selective antibiotic-free media. Enumeration of S. thermophilus was performed using M17-lactose. MRS-fructose was suitable for enumeration of L. bulgaricus and MRS-maltose for differentiation between L. acidophilus and L. paracasei. The selective enumeration of B. lactis was obtained using MRS-raffinose containing 0.05% LiCl. The bacterial counts obtained using selective methods were equivalent to those under optimum culture conditions at a probability level of 95%. Performance of the methods was verified in fermented milk products where identification of the enumerated species was confirmed by species-specific polymerase chain reaction. This study shows that combination of species-specific polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis has great detection and identification potential for verification of accurate species labelling in fermented milk without prior isolation of the bacteria.     

Effect of epigallocatechin gallate on the fermentative and physicochemical properties of fermented milk

Yogurt, a traditional fermented dairy product, is made with a starter that contains Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. The aim of this study was to investigate the effects of different concentrations of epigallocatechin gallate (EGCG; 0, 0.5, 1.0, 3.0, and 5.0 mg/mL) on the growth, metabolism, and acid production of lactic acid bacteria, as well as the texture, stability, and antioxidant activity of fermented milk (yogurt). The results showed that a low concentration of EGCG had no significant effect on the acid production capacity of the starter or on the water-holding capacity of the yogurt but did increase its viscosity. A high concentration (5.0 mg/mL) of EGCG delayed the acid production rate of the starter and decreased the water-holding capacity, but significantly increased the antioxidant activity of yogurt. The addition of EGCG significantly increased the hardness of yogurt. Therefore, EGCG can improve the texture of fermented milk and enhance its antioxidant activity and stability, thus improving the overall quality of yogurt.     

Influence of culture conditions and preconditioning on survival of Lactobacillus delbrueckii subspecies bulgaricus ND02 during lyophilization

The cryotolerance of Lactobacillus delbrueckii ssp. bulgaricus is weak during vacuum freeze-drying. Many factors affect cryoresistance of these bacteria, such as cryoprotectant composition, the lyophilization technology used, and the intrinsic characteristics of the bacteria. In this research, we explored the fermentation technology and other preconditioning treatments of cells in improving the cryoresistance of Lactobacillus delbrueckii ssp. bulgaricus strains during lyophilization. The addition of yeast extract in the propagation medium exerted a negative effect on the cryotolerance of these bacteria and decreased survival during lyophilization. The count of the freeze-dried cells from medium containing a high level (4%) of yeast extract was only 4.1 × 10⁹ cfu/g, indicating a death rate as high as 88%, compared with the culture medium without yeast extract, with a lower death rate of 44.7%. When Lactobacillus delbrueckii ssp. bulgaricus ND02 was propagated in yeast extract-free de Man, Rogosa, and Sharpe broth at a set pH value of 5.1, the cells showed unexpectedly higher survival after freeze-drying. Viable counts of the lyophilized cell of strain ND02 cultivated at pH 5.1 could reach 1.05 × 10¹¹ cfu/g and survival of the freeze-drying process was 68.3%, whereas at pH 5.7, survival was only 51.2%. We also examined the effects of pretreatment of cells on survival of the bacteria after vacuum freeze-drying. By analyzing the effect of pretreatment conditions on the expression of cold- and heat-shock genes, we established 2 pretreatments that improved survival of cells after lyophilization. Optimal fermentation conditions and pretreatment of the cell-cryoprotectant mixture at 10°C for 2 h or 37°C for 30 min improved the cryoresistance of 4 strains of Lactobacillus delbrueckii ssp. bulgaricus to varying degrees. Cells of IMAU20269 and IMAU20291 that were pretreated showed enhanced survival of 16.06 and 16.82%, respectively, after lyophilization. Expression of cold- and heat-shock genes for pretreated strains ND02, IMAU80423, IMAU20269, and IMAU20291 was analyzed by using quantitative PCR. From the expression of 2 cold shock-induced genes (cspA and cspB) and 6 heat shock-induced genes (groES, hsp, hsp20, hsp40, hsp60, and hsp70), strain ND02 showed a higher relative quantity of gene expression and displayed superior resistance to cold-induced stress during the freeze-drying process.     

Some low homogenization pressures improve certain probiotic characteristics of yogurt culture bacteria and Lactobacillus acidophilus LA-K

Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus salivarius ssp. thermophilus, and Lactobacillus acidophilus are dairy cultures widely used in the manufacture of cultured dairy products. Commonly used homogenization pressures in the dairy industry are 13.80 MPa or less. It is not known whether low homogenization pressures can stimulate bacteria to improve their probiotic characteristics. Objectives were to determine the effect of homogenization at 0, 3.45, 6.90, 10.34, and 13.80 MPa on acid tolerance, bile tolerance, protease activity, and growth of L. delbrueckii ssp. bulgaricus LB-12, S. salivarius ssp. thermophilus ST-M5, and L. acidophilus LA-K. The cultures were individually inoculated in cool autoclaved skim milk (4°C) and homogenized for 5 continuous passes. Growth and bile tolerance of samples were determined hourly for 10 h of incubation. Acid tolerance was determined every 20 min for 120 min of incubation. Protease activity was determined at 0, 12, and 24 h of incubation. All homogenization pressures studied improved acid tolerance of L. delbrueckii ssp. bulgaricus LB-12 but had no beneficial effect on protease activity and had negative effects on growth and bile tolerance. A pressure of 6.90 MPa improved acid tolerance, bile tolerance, and protease activity of S. salivarius ssp. thermophilus ST-M5, but none of the homogenization pressures studied had an effect on its growth. Homogenization pressures of 13.80 and 6.90 MPa improved acid tolerance and bile tolerance, respectively, of L. acidophilus LA-K but had no effect on protease activity and its growth. Some low homogenization pressures positively influenced some characteristics of yogurt culture bacteria and L. acidophilus LA-K. Culture pretreatment with some low homogenization pressures can be recommended for improvement of certain probiotic characteristics.     

The nutrient requirements of Lactobacillus acidophilus LA-5 and their application to fermented milk

Lactobacillus acidophilus LA-5 is a suitable probiotic for food application, but because of its slow growth in milk, an increase in its efficiency is desired. To shorten the time required for fermentation, the nutrient requirements of L. acidophilus LA-5 were analyzed, including the patterns of consumption of amino acids, purines, pyrimidines, vitamins, and metal ions. The nutrients required by L. acidophilus LA-5 were Asn, Asp, Cys, Leu, Met, riboflavin, guanine, uracil, and Mn²⁺, and when they were added to milk, the fermentation time of fermented milk prepared by L. acidophilus LA-5 alone was shortened by 9 h, with high viable cell counts that were maintained during storage of nutrient-supplemented fermented milk compared with the control. For fermented milk prepared by fermentation with Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, and L. acidophilus LA-5, viable cell counts of L. acidophilus LA-5 increased 1.3-fold and were maintained during storage of nutrient-supplemented fermented milk compared with the control. Adding nutrients had no negative effect on the quality of the fermented milk. The results indicated that suitable nutrients enhanced the growth of L. acidophilus LA-5 and increased its viable cell counts in fermented milk prepared by L. acidophilus LA-5 alone and mixed starter culture, respectively.     

Selection of starter cultures forthe fermentation of soya milk

The growth of four cultures selected on the basis of their ability to produce lactic acid in soya milk and/or utilize oligosaccharides, namelyStreptococcus thermophilus, Lactobacillus delbrueckiisubspbulgaricus, Lactobacillus fermentiandLactobacillus fermentum, was examined in reconstituted, low fat, spray-dried soya milk powder (12% total solids). The single culture ofS. thermophilusproduced a drop in pH from 6.5 to 4.7 over a 10-h period, and reduced the level of stachyose from 8.5 mg ml^-;1in the original milk to 3.2 mg ml^-;1; after 24 h fermentation, pH fell to 4.5, and the stachyose concentration to 3.0 mg ml^-;1. The paired culture ofS. thermophilusandL. fermentumbehaved in a similar fashion, but with only a slight improvement in stachyose utilization. When yeast extract (0.3%) and glucose (1.0%) were added to the soya milk, acid production by all the cultures increased dramatically, andL. delbrueckiisubspbulgaricusalone or in combination withS. thermophiluslowered the pH of the milk to 4.3 over 10 h. This combination ofS. thermophilusandL. delbrueckiisubspbulgaricuswas considered to be a likely combination for the production of a fermented product from soya milk or modified soya milk, as neitherL. fermentinorL. fermentumwere appreciably more effective in lowering the concentrations of oligosaccharides.