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.     

Isolation and identification of exopolysaccharide producer lactic acid bacteria from Turkish yogurt

Lactic acid bacteria (LAB) were isolated from traditional yogurt samples and genotypic characterization of these isolates revealed the presence of 21 distinct LAB strains belonging to Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, Leuconostoc mesenteroides, and Lactobacillus plantarum as new LAB strains. Determination of the exopolysaccharide (EPS) production characteristics of the selected strains of each species revealed that all strains possessed at least one gene required for both homopolymeric‐ and heteropolymeric‐type EPS production. Structural analysis of the EPSs showed that L. delbrueckii subsp. bulgaricus Y39 and S. thermophilus Y102 produced heteropolymeric EPS containing glucose and galactose, whereas Leuc. mesenteroides Y35 and L. plantarum Y36 produced homopolymeric glucan‐type EPS. The level of EPS production in these strains was found to be in a similar range. These strains with EPS production characteristics are good candidates for future studies as new LAB for yogurt production.

Practical applications

Recent trends in yogurt production technology have led to an increased use of ropy starter cultures in yogurt production due to the technological roles of exopolysacharides (EPS) produced by these cultures. The main role of EPS in yogurt production is to improve the textural properties of yogurt as an in situ produced natural polymer. In addition to the yogurt starter cultures, use of adjunct cultures during production of yogurt is also of special interest to enhance the technological and nutritional characteristics of yogurt. Therefore, in this study, potential yogurt starter and adjunct cultures from traditional yogurt samples with EPS production characteristics were isolated. From these isolates, Lactobacillus delbrueckii subsp. bulgaricus Y39 and Streptococcus thermophilus Y102 produced heteropolymeric EPS containing glucose and galactose, whereas Leuconostoc mesenteroides Y35 and Lactobacillus plantarum Y36 produced homopolymeric glucan.     

Effects of yogurt starter cultures on the survival of Lactobacillus acidophilus

Recognized to confer health benefits to consumers, probiotics such as Lactobacillus acidophilus are commonly incorporated into fermented dairy products worldwide; among which yogurt is a popular delivery vehicle. To materialize most of the putative health benefits associated with probiotics, an adequate amount of viable cells must be delivered at the time of consumption. However, the loss in their viabilities during refrigerated storage has been demonstrated previously. This study focused on the effects of yogurt starter cultures on the survival of five strains of L. acidophilus, with emphases on low pH and acid production. Differential survival behavior between L. acidophilus strains was further analyzed. To this end, viable cell counts of L. acidophilus were determined weekly during 4 °C storage in various types of yogurts made with Streptococcus thermophilus alone, L. delbrueckii ssp. bulgaricus alone, both species of the starter cultures, or glucono-delta-lactone (GDL). All yogurt types, except for pasteurized yogurts, were co-fermented with L. acidophilus. Yogurt filtrate was analyzed for the presence of any inhibitory substance and for the amount of hydrogen peroxide. Multiplication of L. acidophilus was not affected by the starter cultures as all strains reached high level on day 0 of the storage period. Throughout the 28-day storage period, cell counts of L. acidophilus PIM703 and SBT2062 remained steady (~ 6 × 10⁷ CFU/g) in yogurts made with both starter cultures, whereas those of ATCC 700396 and NCFM were reduced by a maximum of 3 and 4.6 logs, respectively. When starter cultures were replaced by GDL, all strains survived well, suggesting that a low pH was not a critical factor dictating their survival. In addition, the filtrate collected from yogurts made with starter cultures appeared to have higher inhibitory activities against L. acidophilus than that made with GDL. The presence of viable starter cultures was necessary to adversely affect the survival of some strains, as pasteurized yogurts had no effect on their survival. In particular, the inhibitory effect exerted by L. delbrueckii ssp. bulgaricus on L. acidophilus NCFM was highly pronounced than by S. thermophilus, nevertheless, the same effect was not observed on SBT2062. The inhibition against stationary-phase NCFM cells might be caused by the elevated level of hydrogen peroxide produced by L. delbrueckii ssp. bulgaricus. Delineating factors driving the differences in survival trait among probiotic strains will lead to a more efficacious delivery of health benefits in fermented dairy products through targeted technological interventions.     

Characterization of functional, biochemical and textural properties of synbiotic low-fat yogurts during refrigerated storage

This study examined the influence of exopolysaccharides (EPS) produced in situ on the viability of Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus and probiotics, the proteolytic and ACE-inhibitory activities, and textural and rheological properties of inulin-containing probiotic yogurt during refrigerated storage. Two types of yogurt were prepared using strains of S. thermophilus not producing EPS (NEPY) and producing EPS (EPY). The yield of crude EPS increased (by 2.4 times) until day 21 of storage. Presence of EPS showed a protective effect on the survival of L. delbrueckii ssp. bulgaricus and Lactobacillus acidophilus but not on S. thermophilus, Lactobacillus casei and Bifidobacterium longum. No changes in post-acidification, lactic acid content or the ACE-inhibition activity of the two types of yogurt were observed. Overall, EPS containing yogurts exhibited higher proteolysis in the presence of inulin and probiotics (0.698 units) than the corresponding control (0.563 units). The storage and loss moduli (Pa), yield stress (Pa), consistency index (Pa s) and thixotrophic behaviour (Pa s⁻¹) of both samples were similar at day 1 and the influence of EPS was observable only after day 7. Such a variation of the effect of EPS on the textural and rheological properties of low-fat yogurt appears to be partially due to the presence of probiotics.     

Probiotic Strains as Starter Cultures Improve Angiotensin-converting Enzyme Inhibitory Activity in Soy Yogurt

Suitability of soy yogurt as a system for delivering probiotics and other bioactive compounds was assessed by fermenting soy milk using starter culture containing Lactobacillus delbrueckii ssp. bulgaricus Lb1466, Streptococcus thermophilus St1342, and probiotic organisms (Lactobacillus acidophilus LAFTI® L10, Bifidobacterium lactis LAFTI® B94, and Lactobacillus paracasei LAFTI® L26). Fermentations were terminated at different pH of 4.50, 4.55, and 4.60 and metabolic patterns of cultures (viability, proteolytic activity, organic acids production, angiotensin‐converting enzyme (ACE) inhibitory activity) were investigated during 28 d of storage at 4 °C. The presence of probiotics enhanced the growth of L. delbrueckii ssp. bulgaricus Lb1466 and S. thermophilus St134 in soy yogurt in comparison to the control produced by sole yogurt culture. In general, different termination pH had no effect (P > 0.05) on the viability of probiotic organisms that maintained good viability in soy yogurt during cold storage. Higher levels of essential growth factors in the form of peptides and amino acids in soy yogurts may have promoted the growth of L. acidophilus LAFTI® L10, B. lactis LAFTI® B94, and L. paracasei LAFTI® L26. The use of probiotic strains as a part of starter culture in soy yogurt resulted in a substantial increase in in vitro ACE inhibitory activity compared with the control produced by yogurt culture only. This improvement of ACE inhibition in soy yogurt is partly due to higher proteolytic activity of probiotics.     

Impact of a plant extract on the viability of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in nonfat yogurt

The effect of a plant extract (prepared from olive, garlic, onion and citrus with sodium acetate as a carrier) on the viability of yogurt starter cultures was studied. Nonfat yogurt was prepared with various levels of supplements: plant extract (0, 0.5 or 1.0%, w/v) or l-cysteine HCl (0.014 or 0.028%, w/w). Microbial and physicochemical analyses were conducted weekly for 50 days. Fermentation time increased for supplemented yogurts compared with the non-supplemented yogurt. Lactobacillus bulgaricus counts in supplemented yogurts were >6 log cfu mL^?1 for a longer time (7–21 days) compared with the non-supplemented yogurt. Streptococcus thermophilus counts in all yogurts were > 6 log cfu mL^?1 throughout the storage. Overall, redox potential and titratable acidity of yogurts on day 50 were greater compared with day 1, but pH and syneresis were less. Plant extract at 0.5% enhanced L. bulgaricus viability in nonfat yogurt while least affecting the physicochemical characteristics.     

Effect of exopolysaccharides (EPSs) produced by Lactobacillus delbrueckii subsp. bulgaricus strains to bacteriophage and nisin sensitivity of the bacteria

Lactobacillus strains used in this study were isolated from village-type yogurt and raw milk. The isolates were identified as Lactobacillus delbrueckii subsp. bulgaricus by 16 s rDNA sequence analysis and API 50 CHL identification systems. The exopolysaccharide (EPS) production of the strains growth in skim milk were investigated. In addition sensitivity and insensitivity of these strains against domestic bacteriophages and nisin were examined. It was deduced that those strains which had relatively high EPS-producing capacity were insensitive against phages and nisin. Linear relationships were determined between EPS production of the bacteria and bacteriophage and nisin insensitivity of the bacteria.There was a negative correlation between EPS production quantity and phage and nisin sensitivity of the bacteria. Of all the strains, L. delbrueckii subsp bulgaricus B3 produced the highest EPS quantity, and it was insensitive against phages and nisin. Based on these results, it is suggested that L. delbrueckii subsp bulgaricus B3 can be used with the starter culture in dairy industry for stable and high-quality yogurt production.     

Quality characteristics of yogurts fermented with short-chain fatty acid-producing probiotics and their effects on mucin production and probiotic adhesion onto human colon epithelial cells

Probiotics can ferment nondigestible carbohydrates and produce short-chain fatty acids (SCFA; acetate, propionate, and butyrate) in the human colon. In this study, the levels of SCFA were determined in the following yogurts fermented with different combinations of probiotics: (1) cocultures of Streptococcus thermophilus and Lactobacillus bulgaricus (control, C); (2) S. thermophilus, L. bulgaricus, and Bifidobacterium bifidum (C-Bb); (3) S. thermophilus, L. bulgaricus, and Lactobacillus acidophilus (C-La); and (4) S. thermophilus, L. bulgaricus, and Lactobacillus gasseri (C-Lg). Results showed that the acetate levels were significantly higher in C-Bb, C-La, and C-Lg yogurts than in C yogurt. Fermentation and physicochemical characteristics of all yogurts were identical. Treatment of mucus-secreting colon epithelial cells (HT29-MTX) with C-Bb, C-La, and C-Lg yogurt supernatants resulted in an increase in the expression of MUC2 and CDX2 and the production of mucin proteins. The adhesion of probiotics onto HT29-MTX cells increased following treatment with C-Bb, C-La, and C-Lg yogurt supernatants. Our data suggest that a yogurt diet rich in acetate improves the protective function of the intestinal epithelium.     

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.     

The effect of using an exopolysaccharide‐producing culture on the physicochemical properties of low‐fat and reduced‐fat Kasar cheeses

A mixed starter culture containing exopolysaccharide (EPS)‐producing strains of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus was combined with Lactobacillus helveticus LH301 and used in the manufacture of low‐fat and reduced‐fat Kasar cheeses. For comparison, low‐fat (C10) and reduced‐fat (C20) cheeses were made using EPS‐producing (EPS⁺) starter strain and EPS‐non‐producing (EPS^?) starter strain. The physicochemical properties of the cheeses were assessed in terms of chemical composition, texture, microstructure and microbial content over 90 days. Cheeses made with EPS‐producing culture (EPS10 and EPS20) had lower protein contents than control cheeses with 10% and 20% fat in dry basis (C10 and C20). Scanning electron microscopy images showed that using EPS‐producing culture resulted in a less compact protein matrix and sponge‐like structure in the cheese samples. In general, cheeses made using EPS‐producing culture had lower total viable counts. This could be related to the reduced survivability of EPS‐producing cells in the cheese matrix during ripening due to autolysis ability.     

Gelation and resistance to shearing of fermented milk: Role of exopolysaccharides

Milk was fermented with the exopolysaccharide-producing (EPS⁺) strains Lactococcus lactis subsp. cremoris, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus and with the non-EPS-producing strain (EPS⁻) L. lactis subsp. cremoris. The kinetics of gelation and the behaviour of set fermented milk during and after shearing were studied using rheometry and confocal scanning laser microscopy. The time of gelation of milk depended on the kinetics of acidification of strains whereas the pH of gelation depended mostly on the presence of exopolysaccharides (EPS). In set fermented milk with EPS⁺ strains, bacteria were observed in protein-free areas likely filled with EPS. Phase-separated EPS and caseins contributed to induce the gelation of fermented milk at pH 5.6. The high resistance to shearing of milk fermented with the EPS⁺ strain L. lactis subsp. cremoris might be due to the negative charge of the exopolysaccharide allowing an attractive interaction with caseins.     

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.     

Quality attributes of yogurt with Lactobacillus casei and various prebiotics

The objective was to determine the effect of chain length of inulins on the characteristics of fat-free plain yogurt manufactured with Lactobacillus casei. Probiotic fat-free plain yogurts were manufactured using Streptococcus thermophilus, Lactobacillus bulgaricus and L. casei. The treatments were inulins of short (P95), medium (GR) and long (HP) chain lengths. The inulins were incorporated at a concentration of 1.5 g/100 g yogurt mix. Inulins of various chain lengths did not affect viscosity, L*, a*, b* and appearance of yogurts manufactured with L. casei. Yogurt with HP had less syneresis compared to the control, while yogurt with P95 had syneresis comparable to the control. Yogurt with P95 had a significantly lower pH than the control, while the pH of the yogurts with other treatments was not different from the control. Flavor scores of the control were comparable to yogurt with P95. The flavor scores for yogurts with P95 were significantly higher than for yogurts with HP. The yogurts with HP had better body and texture compared to the control and P95. Chain length of prebiotics affected some characteristics of the yogurts.     

Comparison of the functionality of exopolysaccharides produced in situ or added as bioingredients on yogurt properties

The effect on yogurt properties of in situ production of exopolysaccharides (EPS) and addition of 2 EPS powders (crude and purified EPS from Lactobacillus rhamnosus RW-9595M fermentation in whey-based medium) at different concentrations was studied. No effect of purified powder addition for EPS concentrations up to 500 mg/L was observed on acidification rate to the difference of milks supplemented with crude EPS, which exhibited longer acidification times. The addition of EPS from 125 to 500 mg/L or the use of EPS-producing cultures resulted in yogurts with lower yield stress and viscoelastic moduli compared with control yogurts without EPS, with no apparent effect of EPS concentration. However, the consistency index was higher for yogurts produced with the commercial EPS-producing culture, and to a lesser extent with the mixed culture containing Lb. rhamnosus RW-9595M, compared with yogurts supplemented with EPS powders, which were not different from that for control yogurts. Our study showed that the mode of EPS incorporation in yogurts has a major effect on the rheological properties of the final product.     

Effect of using propionic acid bacteria as an adjunct culture in yogurt production

Propionibacteria are able to produce a wide variety of food components beneficial to human health. In this study, yogurt was produced by using the adjunct starter cultures Propionibacterium jensenii B1264 and Propionibacterium thoenii (jensenii) P126. Although the total solids and protein contents of the yogurts did not show any significant differences, titratable acidity of the control sample (YC-380) remained lower than that of Propionibacterium spp.-supplemented yogurts during 15 d of storage. The yogurts produced by YC-380 + P126 cultures had the firmest structure (0.26 N). The highest acetaldehyde (29.35 mg/kg) content was obtained with yogurt made with YC-380 + P126 + B1264 on d 1. The addition of propionibacteria to yogurt did not have any negative effect on the counts of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus in yogurt. During the first week of storage, propionibacteria counts remained high, suggesting that yogurt provided a good environment for these organisms. This new product would provide not only beneficial health effects, but also a new alternative product to plain set-type yogurt.     

Effects of Lactobacillus fermentum HY01 on the quality characteristics and storage stability of yak yogurt

Lactobacillus fermentum HY01 is a probiotic strain screened from traditional yak yogurt, which can effectively relieve enteritis and constipation. This study aimed to evaluate the effects of HY01 as an adjunct starter on the quality and storage of yak yogurt. A total of 36 main volatile flavor substances were detected in all samples. In particular, more aldehydes, esters, and alcohols were detected in yak yogurt prepared by mixed fermentation of L. fermentum HY01 and starter MY105 (including Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus). The rheological results showed that the yak yogurt prepared by mixed fermentation of L. fermentum HY01 and starter MY105 had higher apparent viscosity and lower tan δ value compared with compared with traditional yak yogurt, yak yogurt with only L. fermentum HY01, and cow yogurt with L. fermentum HY01 and starter MY105. Meanwhile, the conjugated linoleic acid in the yak yogurt prepared by mixed fermentation of L. fermentum HY01 and starter was significantly higher than those in the HY01 group or the yogurt starter group alone. After 28 d of storage at 4°C, the number of HY01 in the yak yogurt prepared by mixed fermentation of L. fermentum HY01 and starter was still higher than 10⁷ cfu/mL, its acidity was lower than 110°T, and its syneresis was the lowest. The results indicated that L. fermentum HY01 could improve the flavor, texture, and storage properties of yak yogurt.     

Effect of exopolysaccharides on the proteolytic and angiotensin-I converting enzyme-inhibitory activities and textural and rheological properties of low-fat yogurt during refrigerated storage

The aim of this study was to examine the influence of using exopolysaccharide (EPS) producing strain of Streptococcus thermophilus on the viability of yogurt starters, their proteolytic and angiotensin-I converting enzyme-inhibitory activities, and on the textural and rheological properties of the low-fat yogurt during storage at 4°C for 28 d. The use of an EPS-producing strain of S. thermophilus did not have influence on pH, lactic acid content, or the angiotensin-I converting enzyme-inhibition activity of low-fat yogurt. However, EPS showed a protective effect on the survival of Lactobacillus delbrueckii ssp. bulgaricus. Presence of EPS reduced the firmness, spontaneous whey separation, yield stress, and hysteresis loop area but not the consistency and flow behavior index of low-fat yogurt.     

Production of volatile aroma compounds by kefir starter cultures

Production of carbonyl compounds by single-strain cultures, kefir starter (Lactobacillus delbrueckii subsp. bulgaricus HP1+Lb. helveticus MP12+Lactococcus lactis subsp. lactis C15+Streptococcus thermophilus T15+Saccharomyces cerevisiae A13) and kefir grains during fermentation and storage of kefir was studied. The content of carbonyl compounds produced by kefir starter was greater than that produced by kefir grains. The maximum acetaldehyde concentration (18.3 μg g⁻¹) in kefir with starter culture was mainly due to the metabolic activity of Lb. delbrueckii subsp. bulgaricus HP1 isolated from kefir grains. The highest diacetyl production activity was recorded in the starter culture (1.87 μg g⁻¹) and the single-strain culture St. thermophilus T15 (1.62 μg g⁻¹), followed by Lb. helveticus MP12 (0.85 μg g⁻¹) and Lc. lactis subsp. lactis C15 (0.42 μg g⁻¹). The lactobacilli Lb. delbrueckii subsp. bulgaricus HP1 and Lb. helveticus MP12 produced acetone, which was not found in the cocci cultures. The presence of 2-butanone was related to the production ability of Lb. helveticus MP12. In comparison, Lc. lactis subsp. lactis C15 synthesized ethyl acetate more actively than the other single-strain cultures included in the starter. S. cerevisiae A13 produced ethanol and CO₂ in amounts (3975 μg g⁻¹; 1.80 g L⁻¹) that lent cultured kefir distinctive flavour and aroma characteristic of authentic kefir.     

Short communication: Influence of an aqueous myrrh suspension on yogurt culture bacteria over yogurt shelf life

Myrrh is an essential oil and natural flavoring approved by the US Food and Drug Administration, and it has antibacterial and antifungal activity against pathogens. Our objective was to determine the effect of an aqueous myrrh suspension on Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus counts in peptone solution and yogurt, as well as pH and titratable acidity of yogurt during 5 wk of storage at 1 to 4°C. The myrrh suspension (10% wt/vol) was prepared and incorporated into a pure culture dilution in peptone and into yogurt mix at a 1% (vol/vol) level. A control with no myrrh was also prepared, and 3 replications were conducted. Streptococcus thermophilus were enumerated using Streptococcus thermophilus agar with aerobic incubation at 37°C for 24 h, and Lactobacillus delbrueckii ssp. bulgaricus were enumerated using de Man, Rogosa, and Sharpe agar adjusted to pH 5.2, with anaerobic incubation at 43°C for 72 h. During the 8-h period after inoculation, S. thermophilus and L. delbrueckii ssp. bulgaricus counts in peptone solution at 37°C and 43°C, respectively, were not significantly different in the presence or absence of the aqueous myrrh suspension. Counts of S. thermophilus in yogurt containing myrrh (mean ± SD; 4.96 ± 0.58 log cfu/mL) were not significantly different from those in the control yogurt (4.87 ± 0.39 log cfu/mL). The log counts for L. delbrueckii ssp. bulgaricus in yogurt containing myrrh (5.04 ± 1.44 log cfu/mL) and those of the control (5.52 ± 1.81 log cfu/mL) did not differ, and the counts remained within 1 log of each other throughout 5 wk of storage. The pH of the yogurts containing the aqueous myrrh suspension was not significantly different from that of the control yogurts, and their pH values were within 0.1 pH unit of each other in any given week. Titratable acidity values remained steady around 1.1 to 1.2% lactic acid for both yogurt types throughout the storage period, with no significant differences between them. Yogurt culture bacteria can survive in the presence of a myrrh suspension in yogurt with no significant change in pH or titratable acidity. Therefore, it may be beneficial to add an aqueous myrrh suspension to yogurt.     

Biogenic amines formation in Streptococcus thermophilus isolated from home-made natural yogurt

Twelve different biogenic amines formation in 58 isolates of Streptococcus thermophilus from home-made natural yogurt were investigated in histidine (HDB) and lysine decarboxylase broth (LDB). All S. thermophilus isolates had an ability to produce twelve different biogenic amines in HDB and LDB. Most of the S. thermophilus isolates formed low amounts of histamine (1–50 mg/L) from histidine. Apart from one isolate, S. thermophilus produced tyramine at low (47 isolates) and medium (10 isolates) levels. The amount of each specific biogenic amine produced by S. thermophilus was generally lower than 100 mg L⁻¹. Also, the presence of hdcA gene was investigated using PCR technique and relation between gene and histamine production was conducted in S. thermophilus isolates. This study showed that most of the S. thermophilus isolates have the ability to form biogenic amines, especially histamine, and tyramine, which is an important consideration when selecting strains as starter cultures.