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 .     

Effect of 'ropy' strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus on rheology of stirred yogurt

The TA-TX2 Texture Analyser and the Brookfield RVT Viscometer have been used to investigate the contribution of ropiness to the texture of stirred yogurts made using ropy strains of bacteria. Back extrusion and texture profile analysis, not commonly used to quantify rheological properties of semi-solid foods, have been found useful in distinguishing the contribution of exopolysaccharides to different texture attributes (Toba et al ., 1990). Thus ropiness, a characteristic which is imparted to the product as a result of fermentation with particular polysaccharide-producing strains, contributes to 'adhesiveness', while 'firmness' and 'elasticity' are likely to be influenced more by the protein matrix of the yogurt than by secretion of the polysaccharide by the ropy strains. Effects on viscosity and ability to recover viscosity after disruption were apparent, although the contribution of ropiness was not always positive. Ropy strains increased viscosity of stirred yogurts when compared to yogurt made with non-ropy cultures. But, whilst a ropy Lactobacillus delbrueckii ssp. bulgaricus (Lb r⁺) combined with a non-ropy Streptococcus thermophilus (St r⁻) produced a viscous product which recovered its viscosity well, a yogurt made by combining both ropy strains did not recover its viscosity as well as yogurt made by combining two non-ropy cultures and lost its structure more rapidly during the destructive testing. These results show therefore that inclusion of a ropy strain will not always lead to improved texture attributes, that while ropy strains may increase viscosity they may not influence 'firmness' and lend support to the view that this latter attribute is more influenced by protein–protein interactions.     

Effect of fermentation temperature on the properties of exopolysaccharides and the acid gelation behavior for milk fermented by Streptococcus thermophilus strains DGCC7785 and St-143

We investigated the effect of fermenting milk with 2 strains (DGCC7785 and St-143) of Streptococcus thermophilus, which are known to produce different types of exopolysaccharide (EPS) structures. The yields and physical properties of these ropy EPS were monitored during the fermentation of milk at different temperatures. We wanted to understand how these types of EPS properties affected yogurt gelation. Reconstituted skim milk was fermented at 33, 39, or 45°C until pH values reached 5.2, 4.9, 4.7, and 4.5. Molar mass of ropy EPS samples was determined using size exclusion chromatography coupled with multiangle laser light scattering. Rheological properties of fermented milk gels were analyzed using small-strain dynamic oscillatory measurements. In both strains, concentrations of ropy EPS increased during fermentation and at all temperatures. Fermentation times, by both strains, were shortest at 45°C and longest at 33°C. For both strains, molar mass of ropy EPS ranged from 2 to 4 × 10⁶ g/mol during fermentation. A major proteinaceous contaminant that was co-isolated with the ropy EPS fraction by our isolation method was identified as a milk-derived phosphoglycoprotein PP3. Increase in fermentation temperature from 33 to 45°C significantly decreased the storage modulus values (from 170 to 41 Pa) for milk gelled by strain DGCC7785, whereas the gels made with St-143 had very low storage modulus values (11–17 Pa) regardless of fermentation temperatures. For both strains, the values of maximum loss tangent in the milk gels increased with fermentation temperature; the maximum loss tangent occurred at higher pH values when milk was fermented by strain DGCC7785. The specific type of EPS produced appeared to be responsible for the differences in yogurt texture rather than the concentration or molar mass of the EPS.     

RHEOLOGICAL PROPERTIES OF PROTEIN/STARCH MIXED GELS

The influence on the viscoelastic behaviour of the heat transition temperatures of starch and protein in gelatinous systems prepared from the two components was studied. Four different starches were combined with Bovine Serum Albumin (BSA) in several protein-to-starch ratios. Both the transition temperature and the rates of gelation of the components were critical for the behaviour of the complex systems. In those cases where the starch gel was formed before the protein gel, the storage and loss moduli of the complex system could be predicted by a simple addition of the moduli of the components at corresponding concentrations. When the gelation occurred in the reverse order, the gels were considerably stronger than predicted by the additivity model. When the starches were combined with gelatin, the complex gels were all weaker than predicted. The gelation of gelatin was very slow compared to that of the starches and the BSA, and it is likely that the weakness of these gels was due to the aggregation of the amylose molecules or some other part of the starch retrogradation process.     

Effects of starter culture and its exopolysaccharides on the gelation of glucono-δ-lactone-acidified bovine and caprine milk

The effects of the starter culture biomass material and its exopolysaccharides (EPS) on the structure of bovine and caprine acid gels were investigated. Inactivated cell material and/or extracellular polysaccharide, which had been separated from yogurt starter cultures, were added to both types of milk. The milk systems were acidified using glucono-δ-lactone (GDL) and the gelation processes were monitored using dynamic (oscillatory) rheological testing. The addition of cell material had no effect on the bovine acid gel, whereas the addition of polysaccharide produced a weaker gel structure. The addition of cell material (> 5 × 10⁸ cells/mL) and polysaccharide (0.35% w/v) weakened the caprine acid gel system. The treated gels were some three to eight times weaker than the untreated samples, suggesting that the starter culture metabolites and especially the EPS may act by interfering with the gel structures, particularly in the cases of milk systems such as caprine yogurt or low protein content bovine yogurt.     

Effect of fermentation temperature on the rheology of set and stirred yogurt

The effect of fermentation temperature (over the range 37–46°C) on the formation and rheology of yogurt has been studied using reconstituted skimmed milk (13.9% solids) pre-heated over a period of 20 min to ~87.5°C and fermented with either a ropy or non-ropy starter culture. Fermentation was arrested by cooling to 5°C when the pH reached 4.5. Stirred samples were prepared at 4 l scale, using a carefully defined and reproducible stirring regime; set-style samples were produced by fermentation in retail cartons (~140 ml per carton); both were characterised after storage for 2 days at 5°C; the time-course of structure formation was monitored by in situ fermentation in an oscillatory rheometer. For both cultures, increasing fermentation temperature gave a progressive increase in: (i) the rate of pH reduction and structure formation; (ii) the pH at which the onset of gelation (abrupt increase in G′) occurred; (iii) gel strength from compression testing of set-style samples; (iv) viscosity of stirred samples, as characterised by the time taken for a fixed volume to flow out from a standard funnel; and (v) solid-like character (G′) of the stirred samples. The increased rate of fermentation is attributed to increased metabolic activity of the lactic acid bacteria; the onset of network formation at higher pH (i.e. with less suppression of electrostatic repulsion between casein micelles) and the enhanced rheology of the resulting yogurt, both before and after stirring, are attributed to a progressive increase in the extent and strength of hydrophobic association as fermentation temperature is raised. Plots of G′ and G″ vs time showed an abrupt decrease in slope in the early stages of gelation, and at longer times the moduli recorded using horizontal (cone-and-plate) geometry were much lower than those obtained using a vertical (cup-and-bob) configuration. We attribute these effects to slippage, triggered by network contraction, sedimentation and syneresis, and suggest that extreme caution should be exercised in quantitative interpretation of moduli obtained from measurements of casein networks under oscillatory or unidirectional shear.     

Structure and physical properties of yogurt gels: effect of inoculation rate and incubation temperature

The effects of inoculation rates and incubation temperatures on the physical properties and microstructure of yogurt gels were investigated. A 2-factor experimental design with 3 replicates was used for data analysis. Yogurt gels were made with 0.5, 1, 2, 3, or 4% (wt/wt) inoculation rates and incubated at 40 or approximately 46 degrees C. Dynamic low amplitude oscillatory rheology was performed to monitor the formation of yogurt gels. Gel permeability and the amount of surface whey were determined. Gel structure was examined by confocal scanning laser microscopy. Higher storage modulus values were observed in yogurt gels made at higher inoculation rates and lower incubation temperatures. Gels made at higher inoculation rates and incubation temperatures exhibited higher yield stress and maximum loss tangent values, respectively. Permeability, pore size, and whey separation of yogurt gels increased with decreased inoculation rate and increased incubation temperature. An increase in the inoculation rate resulted in a decrease in the pH where the maximum in the loss tangent occurs, presumably reflecting less efficient solubilization of colloidal calcium phosphate (which is a slow process) and the need to attain a lower pH to complete the solubilization. Significant positive correlations were observed between whey separation and the value of maximum in loss tangent (r = 0.94) and permeability (r = 0.89). Whey separation was negatively correlated with storage modulus (r = -0.48). It was concluded that rearrangements of casein particles in the gel network and the rate at which the solubilization of colloidal calcium phosphate occurred were important driving forces for whey separation and weak gel.     

Scanning electron microscopic and texture studies on characteristic consistency of Nordic ropy sour milk

The characteristic consistency of Nordic ropy sour milk was studied. Skim milk, reconstituted from non-fat dry milk, was fermented at 20 degrees C for 24 h by addition of 5% (v/v) inoculum of slime-producing (ropy) strain of Lactococcus lactis ssp. cremoris SBT 0495, isolated from Finnish ropy sour milk 'viili' starter culture, and its non-ropy variant SBT 1275. Measurements of texture showed that milk gel prepared by the ropy strain exhibited remarkably increased adhesiveness as compared to that by the non-ropy variant. Milk gel prepared by the ropy strain also exhibited decreased syneresis (wheying-off) as compared to that by the non-ropy variant. Scanning electron micrographs of milk gel prepared by the ropy strain showed that slime was in the form of a network attaching the bacterial cells to the protein matrix. A thick network of slime attached the casein micelle clusters to each other to make casein conglomerates, which is likely to result in the characteristic consistency of 'viili'.     

Rheological Changes During Gelation of High-Methoxyl Pectin/Fructose Dispersions: Effect of Temperature and Aging

Gels made with 0.5% and 1% pectin in 60% fructose, and a commercial sample exhibited macromolecular solution/weak gel behavior. Cooling the gels from 50–10°C increased storage (G') and loss moduli (G') of the gels. As the rate of cooling (10, 5.7, and 2.5°C/hr) a 1% pectin, 60% fructose, dispersion increased, the elasticity of the formed gel decreased; however, G' was not affected much by rate of cooling. During the first 24 hr, the changes in G'and G' were small. After 14 days storage at ambient temperature, G’values of gels increased substantially, while G' values increased slightly.     

不同胞外多糖产生特性的乳酸菌菌种对酸乳品质的影响

不同EPS产生量的两种乳酸菌，用于酸乳的生产，在4种不同菌株组合形成的发酵乳中，球、杆菌比例和乳糖消耗基本一致，但是在粘度、EPS产生、EPS的单糖组成、对乳清析出的影响，以及感官等方面，均有区别。实验结果表明，在酸乳生产中，以产EPS的德氏乳杆菌保加利亚亚种不产EPS的唾液链球菌嗜热亚种组合较好。     

RHEOLOGY OF ACID-INDUCED SODIUM CASEINATE STABILIZED EMULSION GELS

The storage modulus G', loss modulus G", and phase angle δ of acidinduced sodium caseinate emulsion gels were measured at 25C for a certain period of time after the addition of glucono-δ-lactone (GDL). Comparison between pure protein gels and emulsion gels revealed that the presence of emulsion droplets greatly enhanced the gel strength. Acidification by mixing an emulsion with a GDL solution caused immediate gelation but the emulsion gel had similar mechanical properties to the gel formed by direct addition of GDL granules. The viscoelasticity of the gel was strongly related to the pH value of the system. There was no evidence for a three-dimensional network when the pH value was higher than 5.8 or lower than 3.2. The largest storage and loss moduli were observed for gels formed at pH values near the isoelectric point of sodium caseinate (pH 4.6). Rheological differences for gels made at different pH values became distinguishable at low frequencies, where a much smaller phase angle was determined for a gel made at a pH value below the isoelectric point. Partial recovery of the three-dimensional gel network was observed for disrupted gels formed at a pH near the isoelectric point.     

不同分子量降解壳聚糖和降解卡拉胶复合凝胶体系及其流变性质

以降解壳聚糖与降解卡拉胶为主要原料,将不同分子量级别的降解多糖在一定条件下共混制得不同凝胶体系,通过选取3个代表性的体系考察分子量对凝胶体系流变特性的影响。在特定条件下,降解壳聚糖和降解卡拉胶复配可形成凝胶。通过流变测试可见,G'、G'随着分子量的不同有差异,通过考察凝胶触变环可得到复合凝胶体系受力后恢复能力也随分子量的不同有差异,可见分子量对复合凝胶性能有显著影响;通过幂律模型表征该凝胶体系剪切速率随黏度变化趋势,表明该复合凝胶体系为假塑性流体。     

Characterisation of the exopolysaccharide (EPS)-producing Lactobacillus paraplantarum BGCG11 and its non-EPS producing derivative strains as potential probiotics

Traditional fermented foods are the best source for the isolation of strains with specific traits to act as functional starters and to keep the biodiversity of the culture collections. Besides, these strains could be used in the formulation of foods claimed to promote health benefits, i.e. those containing probiotic microorganisms. For the rational selection of strains acting as probiotics, several in vitro tests have been proposed. In the current study, we have characterized the probiotic potential of the strain Lactobacillus paraplantarum BGCG11, isolated from a Serbian soft, white, homemade cheese, which is able to produce a "ropy" exopolysaccharide (EPS). Three novobiocin derivative strains, which have lost the ropy phenotype, were characterized as well in order to determine the putative role of the EPS in the probiotic potential. Under chemically gastrointestinal conditions, all strains were able to survive around 1-2% (10(6)-10(7)cfu/ml cultivable bacteria) only when they were included in a food matrix (1% skimmed milk). The strains were more resistant to acid conditions than to bile salts and gastric or pancreatic enzymes, which could be due to a pre-adaptation of the parental strain to acidic conditions in the cheese habitat. The ropy EPS did not improve the survival of the producing strain. On the contrary, the presence of an EPS layer surrounding the strain BGCG11 hindered its adhesion to the three epithelial intestinal cell lines tested, since the adhesion of the three non-ropy derivatives was higher than the parental one and also than that of the reference strain Lactobacillus rhamnosus GG. Aiming to propose a potential target application of these strains as probiotics, the cytokine production of peripheral blood mononuclear cells (PBMC) was analyzed. The EPS-producing L. paraplantarum BGCG11 strain showed an anti-inflammatory or immunosuppressor profile whereas the non-ropy derivative strains induced higher pro-inflammatory response. In addition, when PBMC were stimulated with increasing concentrations of the purified ropy EPS (1, 10 and 100μg/ml) the cytokine profile was similar to that obtained with the EPS-producing lactobacilli, therefore pointing to a putative role of this biopolymer in its immune response.     

Proteolytic degradation of ewe milk proteins during fermentation of yoghurts and storage

Yoghurts are mostly produced from cow milk and to a very limited extent from ewe milk. The evolution of caseins and whey proteins in ovine milk submitted to different thermal treatments (63 degrees C/30 min; 73 degrees C/15 min; 85 degrees C/10 min or 96 degrees C/5 min) was followed during fermentation of yoghurts and during their storage up to 14 days, using two different sets of starters. One set of starter LAB was a "ropy" culture (YC-191), which is a well-defined mixed strain culture containing Streptococcus thermophilus ST-143 and Lactobacillus delbrueckii subsp. bulgaricus (LB-18 and LB-CH2). The other set of starter bacteria (YC-460) was a standard yoghurt culture("non-ropy") containing mixed strain culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Contents of free amino groups in produced yoghurts increased gradually during the fermentation, up to a maximal value obtained after 4 h fermentation, then they did not change significantly during storage of yoghurt produced with YC-191 starter. In contrary, a large drop in the amount of free amino groups was observed in the first 24 h of storage in the case of yoghurt made with YC-460 indicating that microorganisms continue still to grow in low temperatures. During fermentation and storage of both yoghurt types, alpha-lactalbumin was hydrolyzed to a slightly bigger extent than beta-lactoglobulin. During fermentation, beta-casein was slightly more degraded than alpha(s)-caseins; however, the opposite was observed during storage up to 14 days. Generally, a more intense heat pretreatment led to a higher degradation of whey proteins and caseins during fermentation and storage. Differences in proteolytic activity between the two starters used (whey proteins more degraded by YC-191; caseins more degraded by YC-460) may lead to improvement in production and formulation of yoghurts differing in their physicochemical and rheological properties.     

Evaluation of tilapia skin gelatin as a mammalian gelatin replacer in acid milk gels and low-fat stirred yogurt

Tilapia skin gelatin (TSG) was studied in a 3-stage process (cooling, annealing, and heating) for pure gelatin gels and in a 4-stage process (acidification, cooling, annealing, and heating) for acid milk gels and cultured yogurt. The aim was to evaluate the use of TSG as a replacement for mammalian gelatin in yogurt. In pure TSG gels, stronger gels with higher melting temperatures were formed with increasing TSG concentrations. Compared with bovine gelatin (BG), which gelled at a concentration of 2.5%, TSG gels had lower gelling (14.1°C) and melting (24°C) temperatures but comparable storage moduli during annealing. In acid milk gels, addition of TSG increased the firmness of the gels with increasing concentration. Gelling and melting points of TSG in milk gels were observed at sufficient concentrations during cooling and heating. Strands and sheets were observed in the electron micrographs of milk gels with 1% TSG and a very dense structure was observed with 2.5% TSG. Yogurt with 0.4% TSG had similar viscosity, consistency, pseudoplasticity, and thixotropy as yogurt containing 0.4% BG; no difference was perceived by sensory panelists according to a triangle test. Addition of 0.4% TSG completely prevented whey separation from the acid milk gel and yogurt. The results suggest that TSG could be a suitable replacement for mammalian gelatin in low-fat stirred yogurt.     

RHEOLOGICAL PROPERTIES, WHEY SEPARATION, AND MICROSTRUCTURE IN SET-STYLE YOGURT: EFFECTS OF HEATING TEMPERATURE AND INCUBATION TEMPERATURE

The effects of heat treatment and incubation temperature on the rheological and microstructural properties of yogurt were studied. A central composite experimental design and response surface methodology were used for data analysis. The rheological properties were determined by dynamic low amplitude oscillation and the amount of spontaneous whey separation was quantified by the volumetric flask test. Confocal scanning laser microscopy was used to examine the gel structure. The storage moduli of yogurts increased with an increase in heating temperature and a decrease in incubation temperature. The maximum loss tangent value during gelation, permeability, and amount of spontaneous whey separation of yogurts increased with a decrease in heating temperature and with an increase in incubation temperature. These parameters indicated an increased possibility for rearrangements, which was confirmed by presence of large pores in the gel network. Second order polynomial models successfully predicted the effects of heating temperature and incubation temperature on the rheological properties, permeability, and whey separation of yogurts. Whey separation was negatively correlated with storage modulus ( r = -0.66), and was positively correlated with the maximum loss tangent ( r = 0.63) and permeability ( r = 0.78). This study demonstrates that weak yogurt gels, which have high loss tangent values, favor rearrangements in the network and the resulting network has larger pores (high permeability) and exhibits greater whey separation.     

超声协同TG酶处理乳清蛋白/马铃薯蛋白水解物复合凝胶的特性

为开发乳清蛋白/马铃薯蛋白水解物复合凝胶，采用质构仪、流变仪、低场核磁和扫描电镜表征超声与TG酶处理对凝胶性质的影响。结果表明：超声协同TG酶处理凝胶的凝胶强度，胶黏性，回复性和咀嚼性低于对照组（P<0.05），但高于单独TG酶处理组（P<0.05）。在超声协同TG酶处理凝胶样品中，超声时间的增加，温度循环终点时凝胶储能模量与损耗模量增加；超声15和30 min协同TG酶处理样品的溶胀力相较于TG酶单独交联的样品提高了11.55%和55.02%，自由水含量相较TG酶单独交联样品分别提高了62.22%和111.11%。SEM微观结构表明，超声处理使凝胶中蛋白分子聚集体变大，蛋白网络致密但均一性不高；超声协同TG酶处理凝胶的聚集体较小，蛋白网络结构较为均一。研究表明超声协同TG酶交联会显著改变乳清蛋白/马铃薯蛋白水解物复合凝胶的性能及结构，为复合蛋白凝胶性质的调控提供了参考。     

Application of whey protein emulsion gel microparticles as fat replacers in low-fat yogurt: Applicability of vegetable oil as the oil phase

Low-fat, healthy yogurt is becoming increasingly favored by consumers. In the present study, whey protein emulsion gel microparticles were used to improve the quality of low-fat yogurt, and the effects of vegetable oil emulsion gel as a fat substitute on the qualities of low-fat yogurt were investigated, expecting to obtain healthier and even more excellent quality low-fat yogurt by applying a new method. First, emulsion gel microparticles were prepared, and then particle size distribution of emulsion gel and water holding capacity (WHC), textural properties, rheological properties, microstructure, storage stability, and sensory evaluation of yogurt were carried out. The results showed that yogurt with emulsion gel had significantly superior qualities than yogurt made with skim milk powder, with better WHC, textural properties, rheological properties, and storage stability. The average particle size of whey protein-vegetable oil emulsion gel microparticles was significantly larger than that of whey protein-milk fat emulsion gel microparticles, and the larger particle size affected the structural stability of yogurt. The WHC of yogurt made with whey protein-vegetable oil emulsion gel microparticles (V-EY) was lower (40.41%) than that of yogurt made with whey protein-milk fat emulsion gel microparticles (M-EY; 42.81%), and the texture results also showed that the hardness, consistency, and viscosity index of V-EY were inferior to these of M-EY, whereas no significant differences were found in the cohesiveness. Interestingly, the microstructure of V-EY was relatively flatter, with more and finer network branching. The whey separation between V-EY and M-EY also did not show significant differences during the 14 d of storage. Compared with yogurt made with whey protein, vegetable oil, and skim milk powder, the structure of V-EY remained relatively stable and had no cracks after 14 d of storage. The sensory evaluation results found that the total score of V-EY (62) was only lower than M-EY (65) and significantly higher than that of yogurt made with skim milk powder. The emulsion gel addition improved the sensory qualities of yogurt. Whey protein emulsion gel microparticles prepared from vegetable oil can be applied to low-fat yogurt to replace fat and improve texture and sensory defects associated with fat reduction.     

Rheological and physical characteristics of non-fat set yogurt prepared with EPS-producing Streptococcus thermophilus and an H+-ATPase-defective mutant Lactobacillus delbrueckii subsp. bulgaricus

The rheological and physical characteristics of non-fat set yogurt produced using co-cultures of ropy EPS-producing Streptococcus thermophilus S3.3 and Lactobacillus delbrueckii subsp. bulgaricus LTM and of Streptococcus thermophilus S3.3 and Lactobacillus delbrueckii subsp. bulgaricus H+-ATPase-defective mutant L6 under different conditions were studied. Yogurts produced with mutant L6 had a higher pH and water-holding capacity and better textural properties compared to those produced with the LTM strain. The highest storage modulus (G′) and thixotropic loops were found for yogurt made with L6 at 42°C on day 21 of cold storage. This yogurt contained a network of thick, continuous protein aggregates and large void spaces.     

Use of isolated autochthonous starter cultures in yogurt production

The aim of this study was to produce yogurt from Iranian autochthonous starters. Strains of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus were isolated from autochthonous yogurts. The strains were used as starter for yogurt production. Chemical, microbial and organoleptic characteristics of products were examined. There were significant increases in acidity and acetaldehyde content, and significant decreases in pH and microbial population of samples during cold storage. There was no significant difference in organoleptic characteristics (taste, smell and texture) during cold storage. We conclude that autochthonous starters in production of yogurt can be satisfactorily used and the starters can introduce to national microbial collection.