Survival and Metabolic Activity of Microencapsulated Bifidobacterium longum in Stirred Yogurt

ABSTRACT: Live cells of Bifidobacterium longum, microencapsulated in K‐carrageenan, were added to stirred yogurt after fermentation (pH 4.6) and stored at 4.4 °C for 30 d. Cell enumeration indicated no decline of encapsulated cell number in yogurt samples, while there was significant reduction in nonencapsulated cell population (89.3% for B. longum B6 and 91.8% for B. longum ATCC 15708). Ion‐exchange high‐performance liquid chromatography showed comparable amounts of lactic and acetic acids in all samples, indicating little metabolic activity by bifidobacteria in experimental yogurts. Consumer sensory analysis of blackberry‐flavored yogurts revealed that samples containing encapsulated bifidobacteria had a grainy texture. Results suggested that microencapsulation protected bifidobacteria from the low pH of yogurt.     

Viability and activity of bifidobacteria in yoghurt containing fructooligosaccharide during refrigerated storage

The viability of yoghurt bacteria and two commercial strains of bifidobacteria was assessed in either yoghurt containing chicory fructooligosaccharide (FOS) or without any prebiotic, during 28 days storage at 4 °C. All the products showed a decrease in the viable count of yoghurt bacteria and bifidobacteria during storage. Numbers of Lactobacillus delbrueckii ssp. bulgaricus decreased faster than those for Streptococcus thermophilus. The viability of bifidobacteria in yoghurt was affected by the strain type and the presence of FOS. Bifidobacterium animalis exhibited better stability in the yoghurt than B. longum. The recommended level of 1 million cells was exceeded for B. animalis throughout storage. The highest viable number of bifidobacteria (3.59–2.25 × 10⁷ CFU g^?1) was obtained in the product containing B. animalis and FOS. Viability of B. longum in yoghurt containing FOS remained above 10⁶ CFU g^?1 for up to 21 days, whereas this level was maintained for only 7 days for that organism in yoghurt without any prebiotic.     

Physiochemical Properties,Microstructure, and Probiotic Survivability of Nonfat Goats’ Milk Yogurt Using Heat‐Treated Whey Protein Concentrate as Fat Replacer

There is a market demand for nonfat fermented goats’ milk products. A nonfat goats’ milk yogurt containing probiotics (Lactobacillus acidophilus, and Bifidobacterium spp.) was developed using heat‐treated whey protein concentrate (HWPC) as a fat replacer and pectin as a thickening agent. Yogurts containing untreated whey protein concentrate (WPC) and pectin, and the one with only pectin were also prepared. Skim cows’ milk yogurt with pectin was also made as a control. The yogurts were analyzed for chemical composition, water holding capacity (syneresis), microstructure, changes in pH and viscosity, mold, yeast and coliform counts, and probiotic survivability during storage at 4 °C for 10 wk. The results showed that the nonfat goats’ milk yogurt made with 1.2% HWPC (WPC solution heated at 85 °C for 30 min at pH 8.5) and 0.35% pectin had significantly higher viscosity (P < 0.01) than any of the other yogurts and lower syneresis than the goats’ yogurt with only pectin (P < 0.01). Viscosity and pH of all the yogurt samples did not change much throughout storage. Bifidobacterium spp. remained stable and was above 10⁶CFU g^‐1 during the 10‐wk storage. However, the population of Lactobacillus acidophilus dropped to below 10⁶CFU g^‐1 after 2 wk of storage. Microstructure analysis of the nonfat goats’ milk yogurt by scanning electron microscopy revealed that HWPC interacted with casein micelles to form a relatively compact network in the yogurt gel. The results indicated that HWPC could be used as a fat replacer for improving the consistency of nonfat goats’ milk yogurt and other similar products.     

Survivability and β‐galactosidase activity of bifidobacteria stored at low temperatures

Abstract

This work evaluated the ability of strains representing six species of Bifidobacterium with probiotic potential to survive and maintain β‐galactosidase activity through a two‐step, low‐temperature storage period. Cultures were also evaluated for their ability to ferment skim milk and retain viability during storage at 4°C. Bifidobacterium longum ATCC 15707, B. breve 15700, and B. bifidum 29521 maintained the greatest viabilities at > 1 x 10⁷ CFU/mL, and B. infantis 15702 maintained the highest β‐galactosidase activity at > 1 U/ml (with < 1 × 10⁵ CFU/mL) after ‐60 to 4°C storage. In fermented skim milk, B. breve 15700, B. bifidum 29521, and B. animalis 25527 tolerated a final product pH of 4.75 with > 1 × 10⁸ CFU/mL remaining after 14 days of storage at 4°C. Overall, it was found that highest levels of β‐galactosidase activities did not necessarily correlate to the highest plate‐count populations.     

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.     

Physicochemical and sensory characteristics of soy yogurt fermented with Bifidobacterium breve K-110, Streptococcus thermophilus 3781, or Lactobacillus acidophilus Q509011

The physicochemical and sensory characteristics of soy yogurt fermented with Bifidobacterium breve K-110, Streptococcus thermophilus 3781, or Lactobacillus acidophilus Q509011 for 36 hr at 37°C were investigated and the soyasaponin metabolites of soy yogurt were measured. L. acidophilus reduced the pH of soy yogurt on day 0 to optimum level. After 36 hr, the Brix dropped to 8.48–9.12°Bx. The viable cell counts of soy yogurts on day 0 were 8.43 to 9.01 log CFU/mL. Among starters, B. breve alone produced soyasapogenol B, although it was not detected in non-fermented soybean. B. breve produced poor sensory scores in all sensory aspects. By mixing B. breve with S. thermophilus and L. acidophilus, the sensory scores of B. breve could be improved. These findings suggest that the mixed culture of S. thermopilus or L. acidophilus with B. breve may be an ideal starter for the preparation of soy yogurt.     

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.     

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.     

The Applications of Polysaccharides from Various Mushroom Wastes as Prebiotics in Different Systems

The bases or stipes of mushrooms are normally discarded as low‐economic value animal feed and compost. There are no known reports on deriving polysaccharides from these mushroom wastes for use as prebiotics. This study showed that the relatively low concentration (0.1% to 0.5%) of polysaccharides from Lentinula edodes stipe, Pleurotus eryngii base, and Flammulina velutipes base can enhance the survival rate of Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium longum subsp. longum during cold storage. The polysaccharides had synergistic effects with the peptides and amino acids from a yogurt culture to maintain probiotics above 10⁷ CFU/mL during cold storage, and they also had significant protective effects on these probiotics in simulated gastric and bile juice conditions to achieve beneficial effects in the host. These results showed that mushroom wastes, which are cheaper than other sources, could be an important, new, alternative source of prebiotics.     

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.     

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.     

Effect of packaging conditions and temperature on viability of microencapsulated bifidobacteria during storage

In this study, Bifidobacterium longum B6 and B infantis CCRC 14633 were microencapsulated in various wall materials, including skim milk, gum arabic, gelatin and soluble starch. The stability of these microencapsulated bifidobacteria held at 25 or 4 °C in glass or polyester bottles with or without deoxidant and desiccant was determined. Microencapsulated cells of B longum B6 were generally more stable than the corresponding microencapsulated cells of B infantis CCRC 14633 under the various storage conditions tested. The presence of deoxidant and desiccant, especially at 25 °C, increased the survival of microencapsulated cells. Furthermore, the survival of bifidobacteria was enhanced when they were stored at 4 °C in glass bottles. It was also found that the wall material affected the survival of microencapsulated bifidobacteria. The viability of B longum B6 and B infantis CCRC 14633 was best when they were encapsulated in skim milk and held at 4 °C in glass bottles. Skim milk‐encapsulated B longum B6 cells showed a relatively low viability reduction of only 0.15–0.20 log (colony‐forming units (cfu g^?1)) after 42 days of storage at 4 °C in glass bottles, regardless of the presence of deoxidant and desiccant. A reduction of 0.38–0.76 log (cfu g^?1) was noted for skim milk‐encapsulated cells of B infantis CCRC 14633 under similar storage conditions. Copyright © 2004 Society of Chemical Industry     

Viability of bifidobacteria in commercial dairy products during refrigerated storage

Commercial milk and two brands of yogurt containing bifidobacteria were obtained from retail outlets. All products were evaluated for viability of bifidobacteria and lactic acid bacteria during refrigerated storage at 4 degrees C. Milk was evaluated at 9, 6, and 3 days prior and past its expiration date. The yogurts were evaluated at 3, 2, and 1 week prior and past their expiration. Viability of bifidobacteria and lactic acid bacteria in milk and yogurt remained above 10(6) CFU/ml or g until the expiration date of the respective products. This microbial concentration is the recommended minimum dose to receive the health benefits of these organisms.     

Growth and survival of bifidobacteria and lactic acid bacteria during the fermentation and storage of cultured soymilk drinks

The behaviours of the bifidobacteria B. infantis CCRC 14633 and B. longum B6 and of the lactic acid bacteria Lactobacillus acidophilus CCRC 14079, L. bulgaricus CCRC 14009 and Streptococcus thermophilus CCRC 14085 during the fermentation and storage of cultured soymilk drinks were investigated. Soymilk was found to support the simultaneous growth of the bifidobacteria and L. acidophilus or S. thermophilus, but B. infantis and B. longum both had a deleterious effect on the growth of L. bulgaricus in soymilk. Values for pH, ranging from 6·13 to 4·19, and titratable acidity ranging from 0·09% to 0·25% were noted in soymilk after 48 h of fermentation with various combinations of bifidobacteria and lactic acid bacteria. The viable populations of bifidobacteria and lactic acid bacteria decreased rapidly in the fermented soymilk drinks held at 25°C. When sucrose was added, the reduction in numbers were more pronounced, especially for bifidobacteria. However, the numbers of bifidobacteria and lactic acid bacteria showed no marked change in drinks with or without sucrose during storage for 10 days at 5°C.     

Glucose oxidase: A potential option to decrease the oxidative stress in stirred probiotic yogurt

The effect of the addition of glucose oxidase as a technological option todecrease the oxidative stress in a stirred yogurt during 15 days of refrigerated storage was investigated. A significant increase in the viable population of Bifidobacteria longum (6.9–8.7 log CFU g^?1) was observed in all yogurts added with glucose oxidase and glucose. Additionally, the yogurts containing glucose oxidase and glucose presented and caused higher pH, proteolysis rate, and contents of organic acids and aroma compounds, and this effect was comparable to the control sample (without glucose oxidase and glucose). Overall, the combined effect of the glucose oxidase system was important to reduce the oxygen content during storage of the probiotic yogurt. These findings suggest that glucose oxidase is a potential option to decrease the oxidative stress in probiotic yogurts.     

Shelf life and quality of probiotic yogurt produced with Lactobacillus acidophilus and Gobdin

This study evaluated the effect of dry white mulberry and walnut paste (Gobdin, a traditional Turkish food) in probiotic yogurt on the survival of Lactobacillus acidophilus and yogurt properties. Six different yogurts were produced with 0%, 5% and 10% Gobdin using Lactobacillus bulgaricus + Streptococcus thermophilus and with 0%, 5% and 10% Gobdin using L. bulgaricus + S. thermophilus + L. acidophilus. The physical, chemical, microbiological and sensorial properties of the yogurts were evaluated based on storage at 4 ± 1 °C. Probiotic shelf life and the most suitable combinations were determined. The highest L. acidophilus count (8.65 log cfu g^?1) was found in the 5% Gobdin‐supplemented yogurt on the 7th day of storage, while the lowest count (8.11 log cfu g^?1) was found in the probiotic control yogurt on the 21st day. Although the L. acidophilus counts in the probiotic yogurts declined during storage, all values found throughout the 21‐day storage period were >8 log cfu g^?1. This is above the level necessary to provide the desired therapeutic effect in probiotic products (10⁶–10⁷ cfu g^?1). The highest overall acceptability score was obtained on the first day from the yogurt with 5% Gobdin. However, all yogurt samples had general acceptability scores between 7 and 8 points from a 9‐point maximum. Thus, this study determined that a new functional yogurt can be produced using L. acidophilus with 5% Gobdin.     

Production of oligosaccharides in yogurt containing bifidobacteria and yogurt cultures

Yogurts were prepared by using yogurt cultures combined to mixed cultures of bifidobacteria (Bifidobacterium animalis, Bifidobacterium bifidum, Bifidobacterium breve, Bifidobacterium infantis, and Bifidobacterium longum) and by adding a preincubation step (1.5 h at 50 degrees C) with bifidobacteria to the conventional method of manufacture in order to produce oligosaccharides. The survival of bifidobacteria was drastically affected during storage of yogurts, except for products containing B. animalis, in which viable counts remained at >10(6) cfu/g after 28 d of storage at 4 degrees C. Oligosaccharides with a degree of polymerization of 3 were produced during the preincubation step (0.31 to 0.68%), and the amount in the final products varied according to the species of bifidobacteria inoculated during the preincubation step or the concentration of bifidobacteria used as second inoculum during the fermentation process. In fact, the higher concentration of oligosaccharides measured at the end of the fermentation process (0.72%) and the 28 d-storage period (0.67%) was obtained for yogurts containing B. infantis. However, yogurts containing B. breve showed higher beta-galactosidase activities and had lower lactose concentrations after the fermentation process and the storage period than the other yogurts. The use of a mixed cultures of bifidobacteria (B. animalis, B. infantis, or B. breve) thus allows the production of yogurts in which bifidobacteria can survive in relatively high cell numbers and contain appreciable amount of oligosaccharides.     

Antiviral activity of yogurt against enterovirus 71 in vero cells

Five yogurts were fermented with each bacteria strain. The viability and pH of each yogurt during fermentation or storage were evaluated, and then the cytotoxicity and antiviral activity against enterovirus (EV) 71 of cell-free supernatants (CFS) of the metabolites of each yogurt were compared with those of de Man, Rogosa, and Sharpe (MRS) broth fermented with the same bacteria. As the results, the number of viable bacteria for each yogurt after fermentation or during storage always remained higher than 5 log CFU/mL and the pH of those ranged from 4 to 6. The CFS of all yogurts showed antiviral activity over 45% against EV71, while it didn’t exhibit cytotoxicity in Vero cells. Specially, the CFS of yogurt fermented with Lactobacillus plantarum and Bifidobacterium bifidum exhibited high anti-EV71 activity of 92.74 and 90.44%, respectively. In contrast, the CFS of each MRS broth fermented with the same bacteria showed low antiviral activity of less than 30%.     

Accumulation of Iron in Lactic Acid Bacteria and Bifidobacteria

Lactobacillus acidophilus, L. delbrueckii var. bulgaricus, L. plantarum and Streptococcus thermophilum, all used extensively in the food industry, were tested for their ability to internalize and/or oxidize ferrous iron (Fe²+). For comparison some experiments were performed with bifidobacteria, B. thermophilum and B. breve. All organisms except L. bulgaricus could transport Fe²⁺ into the cell, where it was partially oxidized to the ferric form (Fe(III)). In addition, L. acidophilus and L. bulgaricus could oxidize Fe²⁺ to Fe(III) extracellularly through the elaboration of H₂O₂ into the medium when the experiments were carried out in air. L. bulgaricus elaborated H₂O₂ only in the presence of glucose, whereas L. acidophilus released H₂O₂ in absence of glucose. We concluded that lactic acid bacteria, like bifidobacteria, can exert some beneficial effects in animal organisms, or in food processing and storage, by making Fe²⁺ unavailable to harmful microorganisms.     

Survival of Listeria monocytogenes in vanilla-flavored soy and dairy products stored at 8 degrees C

The survival of Listeria monocytogenes V37 in vanilla-flavored yogurt (low-fat and nonfat) and soy milk (low-fat and Plus) stored at 8 degrees C for 31 days was investigated. Commercial samples of yogurt and soy milk were used. These samples were inoculated with either 10(4) or 10(7) CFU of L. monocytogenes per ml. Sampling was carried out every 3 to 4 days initially and was then carried out weekly, for a total storage time of 31 days. Each time a sample was collected, the pH of the sample was measured. After 31 days, low-fat plain, low-fat vanilla, and nonfat plain yogurt samples inoculated with 10(4) CFU/ml showed 2.5-log reductions in viable cell populations, and nonfat vanilla yogurt showed a 3.5-log reduction. For yogurt inoculated with 10(7) CFU/ml, reductions of 2.5 log CFU/ml were observed for plain low-fat and nonfat yogurts, and reductions of 5 log CFU/ml were observed for vanilla-flavored low-fat and nonfat yogurts. In vanilla-flavored and plain low-fat and Plus soy milk samples, cell counts increased from 10(4) and 10(7) CFU/ml to 10(9) CFU/ml at 7 and 3 days of storage, respectively, at 8 degrees C. Coagulation in soy milk samples was observed when the cell population reached 10(9) CFU/ml. In soy milk, the L. monocytogenes population did not change for up to 31 days. Vanillin had an inhibitory effect on L. monocytogenes in yogurt but not in soy milk.