High hydrostatic pressure processing on microstructure of probiotic low-fat yogurt

The effect of milk processing on the microstructure of probiotic low-fat yogurt was studied. Skim milk fortified with skim milk powder was subjected to three treatments prior to innoculation: thermal treatment at 85 °C for 30 min, high hydrostatic pressure at 676 MPa for 5 min, and combined treatments of high hydrostatic pressure (HHP) and heat. The processed milk was then fermented by using two different starter cultures containing Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus, and Bifidobacterium longum. The microstructure of heat-treated milk yogurt had fewer interconnected chains of irregularly shaped casein micelles, forming a network that enclosed the void spaces. On the other hand, microstructure of HHP yogurt had more interconnected clusters of densely aggregated protein of reduced particle size, with an appearance more spherical in shape, exhibiting a smoother more regular surface and presenting more uniform size distribution. The combined HHP and heat milk treatments led to compact yogurt gels with increasingly larger casein micelle clusters interspaced by void spaces, and exhibited a high degree of cross-linking. The rounded micelles tended to fuse and form small irregular aggregates in association with clumps of dense amorphous material, which resulted in improved gel texture and viscosity.     

EFFECT OF COLD STORAGE ON CULTURE VIABILITY AND SOME RHEOLOGICAL PROPERTIES OF FERMENTED MILK PREPARED WITH YOGURT AND PROBIOTIC BACTERIA

ABSTRACT

We examined the effect of storage time on culture viability and some rheological properties (yield stress, storage modulus, loss modulus, linear viscoelastic region, structural recuperation and firmness) of fermented milk made with Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus (LA) and Bifidobacterium animalis ssp. lactis in coculture with Streptococcus thermophilus (ST). Acidification profiles and factors that affect viability (postfermentation acidification, acidity and dissolved oxygen) were also studied during 35 days at 4C. Fermented milk prepared with a coculture of ST and Bifidobacterium lactis gave the most constant rheological behavior and the best cell viability during cold storage; it was superior to ST plus LA for probiotic fermented milk production.

PRACTICAL APPLICATIONS

Probiotic cultures should grow quickly in milk, provide adequate sensory and rheological properties to the product, and remain viable during storage. Commercially, it is very common to use yogurt starter culture (i.e. Streptococcus thermophilus[ST] and Lactobacillus delbrueckii ssp. bulgaricus) in combination with the probiotic bacteria in order to reduce fermentation time. However, LB tends to post acidify fermented milk, which reduces the viability of the probiotic bacteria; thus, it is recommended to use starter cultures devoid of this species. We found that the technological properties and the viability of the probiotic bacterium Bifidobacterium animalis ssp. lactis BL O4 in coculture with ST make it suitable for probiotic fermented milk production; it produces rheological characteristics similar to those of yogurt.     

An Influence of Fructan Containing Concentrate from Jerusalem Artichoke Tubers on the Development of Probiotic Dairy Starters on Milk and Oat-based Substrates

Supplementation of milk and oat hydrolysate containing medium with Jerusalem artichoke concentrate (JAC) and subsequent fermentation with probiotic dairy starters resulted in substantial stimulation of probiotics Bifidobacterium lactis and Lactobacillus acidophilus as well as yogurt starter culture Lactobacillus bulgaricus development and acidification rate. The strain-specific responses of the general yogurt cultures, as well as probiotics to the addition of JAC, should be considered to achieve optimal composition of probiotic strains and conformable fermentation conditions. JAC is suggested to be perspective prebiotic additive for fermented synbiotic milks or oat-hydrolysate-based products.     

Type of starter culture influences on structural and sensorial properties of low protein fermented gels

Organoleptic properties of skimmed milk fermented gels are progressively demanding to produce optimal quality yogurts. Chr‐Hansen trademark registered cultures were used to produce low‐protein (3.4%) gels to assess the ability to redesign the sensorial and textural properties with the choice of starter culture. Resulting gels were assessed for sensorial, textural, rheological, and microstructural properties and compared with a commercial control (4.5% protein). Mouth thickness, syneresis, firmness, elasticity, and consistency values were lower for polysaccharides‐producing cultures. Such cultures contributed to the higher creaminess and tended to give higher ropiness. Observed differences among microstructures of the gel were minute. Microstructural and rheological data corresponded and reflected the instrumental and sensory interpretations. Strong correlations were observed between sensory and instrumental data. Nonprobiotics cultures resulted in promising overall gel properties compared with probiotic cultures according to the principal component analysis. Yet probiotic cultures resulted in lower syneresis than nonprobiotic cultures. Thus, the choice of bacterial culture modifies the sensorial and textural properties of fermented gel with strong correlations, as a result of altered gel network formation with the production of polysaccharides. Inferior textural and sensorial quality aspects, particularly at low protein levels, have negative impact on consumer demand of low protein yogurts. Thus, we attempted to gain required gel textural and sensorial properties with a choice of starter culture with a low protein level. Resulting gel properties at lowered protein content with different starter cultures are not fully known. The present study compares the effect of probiotic and nonprobiotic starter cultures on gel properties, as gel texture and sensory properties are of great interest and thus not willing to be compromised. In addition, we examined the overall texture profile of studied cultures and correlate with sensory properties. Therefore, reducing protein level in milk and achieving required gel properties with the choice of appropriate starter culture is of great commercial interest as a cost‐cutting strategy to produce low‐cost optimum quality yogurt.     

Low-fat set yogurt made from milk subjected to combinations of high hydrostatic pressure and thermal processing

The combined use of high hydrostatic pressure (300 to 676 MPa, 5 min) and thermal treatment (85 degrees C, 30 min) in milk for the manufacture of low-fat yogurt was studied. The objective was to reduce syneresis and improve the rheological properties of yogurt, reducing the need for thickeners and stabilizers. The use of high hydrostatic pressure alone, or after thermal treatment, reduced the lightness and increased the viscosity of skim milk. However, milk recovered its initial lightness and viscosity when thermal treatment was applied after high hydrostatic pressure. The MALDI-TOF spectra of skim milk presented monomers of whey proteins after a treatment of 676 MPa for 5 min. Yogurts made from skim milk subjected to 400 to 500 MPa and thermal treatment showed increased yield stress, resistance to normal penetration, and elastic modulus, while having reduced syneresis when compared to yogurts from thermally treated or raw milks. The combined use of thermal treatment and high hydrostatic pressure assures extensive whey protein denaturation and casein micelle disruption, respectively. Although reaggregation of casein submicelles occurs during fermentation, the net effect of the combined HHP and thermal treatment is the improvement of yogurt yield stress and reduction of syneresis.     

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.     

Influence of lupin-based milk alternative heat treatment and exopolysaccharide-producing lactic acid bacteria on the physical characteristics of lupin-based yogurt alternatives

In the present study we investigated the influence of heat treatment of lupin-based (LB) milk alternatives and different exopolysaccharide (EPS)-producing lactic acid bacteria on the physical characteristics of set-type LB yogurt alternatives. LB milk alternatives, obtained from protein isolate of Lupinus angustifolius cv. Boregine, were either pasteurized at 80 °C for 60 s or ultra-high temperature (UHT) heated at 140 °C for 10 s and was fermented with Lactobacillus plantarum TMW 1.460 and 1.1468, Pediococcus pentosaceus BGT B34 and Lactobacillus brevis BGT L150. Fermentation duration was strongly affected by heat treatment: different strains needed between 25 to 35 h in UHT LB milk alternative to reach a pH of 4.5 compared to 14 to 24 h in pasteurized LB milk alternative. EPS extraction revealed slightly higher amounts of EPS for UHT LB yogurt alternatives (~ 0.5–0.9 g/l; pasteurized: ~ 0.4–0.7 g/l). The more intensive heat treatment (UHT) resulted also in better rheological (apparent viscosity, hysteresis loop area, flow point, elastic, viscous and complex modulus) and textural properties (firmness, consistency, cohesiveness and index of viscosity) of the investigated LB yogurt alternatives. Furthermore, LB yogurt alternatives out of UHT milk alternative revealed a lower tendency to syneresis, measured with siphon and centrifugation method. This work contributes to the fundamental knowledge of the textural properties of LB yogurt alternatives.     

Manufacture of high-protein yogurt without generating acid whey – Impact of the final pH and the application of power ultrasound on texture properties

The fermentation of preconcentrated milk is a challenging method to avoid acid whey during the manufacture of high-protein fermented milks like Greek yogurt. Milk concentrates (10% protein) were fermented to a final pH of 5.0, 4.8, or 4.6 and processed into stirred yogurt. Additionally, the potential of power ultrasound (US) as a post-processing tool was examined by sonicating the stirred yogurt with a sonotrode at 20 kHz. Set gels fermented to pH 4.8 and 5.0 were considerably softer than gels fermented to pH 4.6. Stirred yogurts fermented to pH 4.8 or 5.0 were less grainy and exhibited a reduced apparent viscosity and water-holding capacity. The application of US further decreased the visual graininess and product viscosity whereas the particle size was only slightly affected. The final pH and sonication are two powerful approaches to control the rheological properties of high-protein fermented milks, offering the potential for innovative processes and products.     

Fast real-time polymerase chain reaction for quantitative detection of Lactobacillus delbrueckii bacteriophages in milk

One of the main microbiological problems of the dairy industry is the susceptibility of starter bacteria to virus infections. Lactobacillus delbrueckii, a component of thermophilic starter cultures used in the manufacture of several fermented dairy products, including yogurt, is also sensitive to bacteriophage attacks. To avoid the problems associated with these viruses, quick and sensitive detection methods are necessary. In the present study, a fast real-time quantitative polymerase chain reaction assay for the direct detection and quantification of L. delbrueckii phages in milk was developed. A set of primers and a TaqMan MGB probe was designed, based on the lysin gene sequence of different L. delbrueckii phages. The results show the proposed method to be a rapid (total processing time 30 min), specific and highly sensitive technique for detecting L. delbrueckii phages in milk.     

Effect of Aqueous Extract of the Seaweed Gracilaria domingensis on the Physicochemical,Microbiological, and Textural Features of Fermented Milks

The effects of the Gracilaria domingensis seaweed aqueous extract in comparison with gelatin on the physicochemical, microbial, and textural characteristics of fermented milks processed with the mixed culture SAB 440 A, composed of Streptococcus thermophilus, Lactobacillus acidophilus, and Bifidobacterium animalis ssp. lactis, were investigated. The addition of G. domingensis aqueous extract did not affect pH, titratable acidity, and microbial viability of fermented milks when compared with the control (with no texture modifier) and the products with added gelatin. Fermented milk with added the seaweed aqueous extract showed firmness, consistency, cohesiveness, and viscosity index at least 10% higher than those observed for the control product (P < 0.05). At 4 h of fermentation, the fermented milks with only G. domingensis extract showed a texture comparable to that observed for products containing only gelatin. At 5 h of fermentation, firmness and consistency increased significantly (P < 0.05) in products with only seaweed extract added, a behavior not observed in products with the full amount of gelatin, probably due to the differences between the interactions of these ingredients with casein during the development of the gel network throughout the acidification of milk. The G. domingensis aqueous extract appears as a promising gelatin alternative to be used as texture modifier in fermented milks and related dairy products.     

Effect of the addition of pulse ingredients to milk on acid production by probiotic and yoghurt starter cultures

Pulses contain carbohydrates, proteins, minerals and vitamins which are essential requirements in the human diet and which could also serve as growth nutrients for probiotic and yogurt starter cultures. In this study, milk supplementation with pulse ingredients is examined as a means to increase the nutritional properties of yogurt and probiotic type beverages. The acid production rate of two yogurt starters (A and B) and two probiotic cultures (Lactobacillus rhamnosus and Lactobacillus acidophilus) was followed in milk supplemented with the following soy and pulse ingredients: pea protein, chickpea flour, lentil flour, pea fibre, soy protein concentrate and soy flour. The pulse ingredients had no negative effect on the acidification trends of the fermented milks. On the contrary, with yogurt culture B, pea fibre, pea protein and lentil flour significantly enhanced the acidification rate. All ingredients used for supplementation improved the acidification rate of probiotic cultures, and the highest effects were obtained with lentil and soy flour. Lentil flour had the lowest pH after 12 h which was significantly lower than the product enriched with the same quantity of skim milk powder. The effect of ingredient supplementation on the microbial composition (ratio of cocci to bacilli) of the yoghurt products was also examined. The ratio of cocci to bacilli was between 1.8 and 2.5 for all supplemented yogurt samples obtained with culture A, and these variations were not judged to be statistically significant (p < 0.05). With yogurt products obtained from culture B, however, there was a higher proportional level of lactobacilli in all supplemented samples, as compared to the milk control; the enhanced growth of the lactobacilli was particularly noted when lentil flour was added to milk.     

凝固型绵羊奶酸奶的发酵特性及活菌数变化

研究凝固型的绵羊奶与牛奶酸奶发酵特性和贮存期特性,利用pH计、酸度滴定、质构仪、电子舌、活菌计数及感官评价对其pH值、滴定酸度、质构特性、味觉特点、活菌数和感官评分进行分析。结果表明：在发酵过程中绵羊奶与牛奶有相同的pH值变化规律,即前2.5 h下降非常缓慢,之后迅速下降,在凝乳前0.5 h下降变缓直到凝乳结束,达到发酵终点的时间分别为5 h和4.5 h;在4 ℃贮存0～24 d期间,酸奶的pH值都在逐渐下降,牛奶酸奶比绵羊奶酸奶下降更大;滴定酸度方面起始时绵羊奶比牛奶高,达到发酵终点时也明显高于牛奶;质构测定结果为YSM（添加益生菌干酪-张发酵的凝固型绵羊奶酸奶）、JDM（嗜热链球菌与保加利亚乳杆菌共同发酵的经典凝固型绵羊奶酸奶）具有更高的硬度和稠度,黏聚性和黏性指数也显著高于JDN（嗜热链球菌与保加利亚乳杆菌共同发酵的经典凝固型牛奶酸奶）,奶的种类为影响酸奶硬度、稠度、黏聚性和黏性指数的主要因素,益生菌干酪-张的添加对这些指标的影响很小;电子舌测定结果为丰富性味感值绵羊奶酸奶明显高于牛奶酸奶,酸感值则低于牛奶酸奶,甜、苦、咸、涩、鲜感值差异不明显;感官评价方面绵羊奶酸奶综合评分高于牛奶酸奶,差异性主要体现在滋味、组织状态和喜爱程度上,在色泽和气味方差异不明显;在4 ℃、21 d贮存期间酸奶的活菌数都在逐渐降低,添加益生菌干酪-张的绵羊奶酸奶活菌数最多,第21天时活菌数分别是普通绵羊奶酸奶和牛奶酸奶的10 倍和100 倍以上,干酪-张对绵羊奶酸奶4 ℃贮存期间活菌数的维持有较好作用。同时,实验结果表明：益生菌干酪-张可以作为发酵剂之一与经典的酸奶发酵剂嗜热链球菌和保加利亚乳杆菌搭配用于凝固型绵羊奶酸奶的制作,以增加其功能价值。     

Conjugated linoleic acid content in fermented goat milk as affected by the starter culture and the presence of free linoleic acid

The effect of yoghurt starter cultures and probiotic Lactobacillus casei on the formation of conjugated linoleic acid (CLA) and microbial populations of fermented goat milk was investigated during 35 days of cold storage. The addition of hydrolysed sunflower oil as a source of free linoleic acid was investigated. The fermentation process enhanced the content of the cis9, trans11‐CLA isomer in milk, whereas the trans10, cis12‐CLA isomer was not detected in goat milk or control fermented milks. The use of both starters generated trans10, cis12‐CLA only when hydrolysed sunflower oil was supplemented. Populations of streptococci and lactobacilli were affected by the presence of hydrolysed sunflower oil.     

Use of homogenisation pressure to improve quality and functionality of probiotic fermented milks containing Lactobacillus rhamnosus BFE 5264

The effects of 60‐MPa milk homogenisation treatment were investigated on the viability of the probiotic strain Lactobacillus rhamnosus BFE5264, when used as yoghurt co‐starter, as well as on the aroma profile, texture and microstructure of the resulting set‐type fermented milks. The results demonstrated that the combined use of homogenisation pressure and co‐inoculation of the probiotic strain with the traditional yoghurt starters allowed the reduction in the product coagulation time, the increase of the probiotic strain viability and the improvement of the product volatile molecule profiles. The rheological indices and the microstructure results indicated the positive effects of the milk homogenisation treatment on the product viscosity index and exopolysaccharide production.     

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 milk supplementation with skim milk powder, whey protein concentrate and sodium caseinate on acidification kinetics, rheological properties and structure of nonfat stirred yogurt

Milk supplementation with milk proteins in four different levels was used to investigate the effect on acidification and textural properties of yogurt. Commercial skim milk powder was diluted in distilled water, and the supplements were added to give different enriched-milk bases; these were heat treated at 90 °C for 5 min. These mixtures were incubated with the bacterial cultures for fermentation in a water bath, at 42 °C, until pH 4.50 was reached. Chemical changes during fermentation were followed by measuring the pH. Protein concentration measurements, microbial counts of Lactobacillus bulgaricus and Streptococcus thermophilus, and textural properties (G′, G″, yield stress and firmness) were determined after 24 h of storage at 4 °C. Yogurt made with milk supplemented with sodium caseinate resulted in significant properties changes, which were decrease in fermentation time, and increase in yield stress, storage modulus, and firmness, with increases in supplement level. Microstructure also differed from that of yogurt produced with milk supplemented with skim milk powder or sodium caseinate.     

Effect of Inoculation Level of Lactobacillus rhamnosus and Yogurt Cultures on Conjugated Linoleic Acid Content and Quality Attributes of Fermented Milk Products

ABSTRACT:  The effect of inoculation concentration of Lactobacillus rhamnosus and yogurt cultures and storage time on conjugated linoleic acid (CLA) content and quality attributes of fermented milk products was determined. Yogurt culture ( Lactobacillus bulgaricus and Streptococcus thermophilus, 1:1 ratio, YC ) , L. rhamnosus (LB), and LB co-cultured with yogurt culture, were inoculated at 10⁶, 10⁷, 10⁸ CFU/mL into a milk with hydrolyzed soy oil as the lipid source. CLA content, microbial counts, acidity, texture, and volatile flavor profile of the fermented milk products were stable during storage at 4 °C for 14 d. Total CLA contents ranged from 0.51 to 1.00 mg CLA/g lipid following 14 d of storage. Inoculation level of L. rhamnosus and yogurt cultures had no significant effect on CLA content and texture, but affected acidity and the volatile flavor profile of the fermented milk products. The fermented milk products produced by L. rhamnosus co-cultured with yogurt culture with 10⁷ CFU/mL total inoculation level resulted in a high CLA content and desirable quality characteristics. This research demonstrated that the optimal inoculation concentration and the combination of L. rhamnosus and yogurt cultures were important factors to produce fermented milk products with CLA content and acceptable quality attributes.     

Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90 % denaturation of beta-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of beta-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 degrees C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.     

Rheological, sensorial, and chemopreventive properties of milk fermented with exopolysaccharide-producing lactic cultures

The objective of this research was to evaluate the rheological, sensorial, and chemopreventive properties of milk fermented with different exopolysaccharide (EPS)-producing lactic cultures. Reconstituted skim milk (11% wt/vol) was fermented with single strains of EPS-producing and non-EPS-producing cultures. Whey that collected on the surface of undisturbed fermented milks and after cutting was measured. All EPS-producing cultures reduced the amount of whey present on the surface of the undisturbed samples, whereas only 3 out of 5 strains reduced syneresis measured after cutting. All EPS-producing cultures except a strain of Lactobacillus delbrueckii ssp. bulgaricus reduced viscoelastic moduli in fermented milk. There was a linear correlation between ropiness and smoothness. In the chemoprevention study, 140 male Fisher rats were divided into 7 groups of 20 each. Rats in 6 groups were fed diets supplemented with fermented milks each made with a single strain of EPS-producing or non-producing cultures, whereas rats in group 7 (control) were fed a diet supplemented with milk acidified with glucono-δ-lactone (GDL). All rats were injected with azoxymethane (15 mg/kg, subcutaneous) at wk 7 and 8 of age to induce tumors and fed their respective diets ad libitum throughout the study. After 30 wk of initiation, all rats were anesthetized with ether, and their intestinal tissues were isolated and washed with cold normal saline. The number and size of tumors in the colon and small intestine were recorded. Rats fed diets supplemented with fermented milk made with 2 EPS-positive and 1 EPS-negative strains had significantly lowered incidence of colon tumor and colon tumor multiplicity. Cyclooxygenase-2 enzyme activity (the enzyme implicated in colon tumor development) was significantly lower in the colon tissue of rats fed diets containing milk fermented with 4 EPS-producing and 1 non-producing cultures than that in rats fed diets supplemented with GDL-acidified milk. Different EPS-positive cultures produced fermented milks with distinct rheological characteristics and levels of ropiness. No relationship was found between rheological properties or level of ropiness of fermented milk and its chemopreventive effect.     

Preliminary studies on the gelation processes of fermented and GDL-acidified bovine and caprine milk systems

Bovine and caprine milk gels were made by GDL-acidification and yogurt fermentation (using 'ropy' and 'non-ropy' starter cultures). The respective gelation processes were monitored rheologically using dynamic oscillatory testing. The bovine fermented systems produced gel structures with about half the strength of the equivalent chemically acidified gels. The fermented caprine milk systems produced gel structures some eight to 10 times weaker than the equivalent acidified systems. In all cases the caprine systems were weaker than the bovine gels despite having higher protein contents. In all cases the 'ropy' milk systems followed somewhat different gelation patterns and formed weaker gels than the equivalent 'non-ropy' and GDL-acidified systems. These data suggested that the starter culture material (biomass and extracellular polysaccharides) may have interfered with the protein–protein interactions during yogurt fermentation. This produced weaker gel structures, possibly by a modified gelation mechanism .