The effect of thermosonication of milk on selected physicochemical and microstructural properties of yoghurt gels during fermentation

Yoghurt cultures (0.1, 1.5 and 3.5% fat) were prepared from milk which was preheated to 45 °C and subjected to thermosonication (TS) for 10 min at an ultrasound amplitude of 24 kHz. Compared to conventional yoghurts prepared from preheated (90 °C, 10 min) milk, cultures from TS milk at similar fat contents had higher gelation pH values, greater viscosities and higher water holding capacities (WHC). On average, yoghurts from TS treated milks with 1.5 or 3.5% fat had almost 2 fold greater WHC and 25% higher G’ values than conventionally produced yoghurt. Electron microscopy showed differences in the microstructure, with TS yoghurt having a honeycomb like network and exhibiting a more porous nature. These characteristics are absent in conventional yoghurts. In addition, the average particle size in TS yoghurts was smaller (<1 μm) than in conventional yoghurts.     

Physical characteristics of set yoghurt made with altered casein to whey protein ratios and EPS-producing starter cultures at 9 and 14% total solids

This study investigated firmness and syneresis of set yoghurts made at the CN to WP ratios of 4:1, 3:1, 2:1 and 1:1 using non-EPS-, capsular EPS- and ropy EPS-producing starter cultures. The yoghurts were made at 9 and 14% (w/w) total solids. The total solids, total protein, the concentration of lactose and the ratios of CN to WP as well as the protein's profiles (native- polyacrylamide gel electrophoresis) in heated and unheated milk blends were investigated. The level of soluble denatured whey protein aggregates in heated milk was also determined. The concentration of EPS, firmness and level of spontaneous syneresis in set yoghurt were monitored weekly throughout 28 days of storage. The microstructure of the set yoghurt made with milk blends at the CN to WP ratio of 4:1 and using three types of starter cultures was carried out after 1 day of storage. There was no difference in the total solids, total protein and lactose concentration of liquid milk blends, except the CN to WP ratios. There was no difference in whey protein denaturation between milk blends. The level of soluble denatured whey protein aggregates in heated milk blends decreased with reducing CN to WP ratio. The firmness and the level of spontaneous syneresis in set yoghurts decreased as the CN to WP ratios were reduced. The use of EPS-producing starter cultures reduced firmness and syneresis and changed the protein matrix in the microstructure of set yoghurts made at 9% (w/w) total solids compared to the control products. These were not observed in set yoghurts made at 14% (w/w) total solids.     

Rheological characterization of set yoghurt produced with additives of native whey proteins

The influence of native whey protein isolate (WPI) addition on the consistency and microstructure of set yoghurt was studied. Native WPI with nutraceutical properties was added (3–12%) to the yoghurt milk immediately before fermentation. The consistency of the yoghurts was analysed and compared using two rheological methods: the penetration resistance of a cylinder and an oscillation method for which a vane was used. The effects of the WPI on the rheologically determined values were quantified whereby a remarkably good correlation between the two methods was achieved. With both methods the texture characteristics of the different set yoghurts could be well characterized and enabled good comparisons. The addition of WPI gave rise to a weaker texture of the set yoghurt. Microstructure was observed by confocal laser scanning microscopy. The addition of WPI led to a less-coherent microstructure, which was in agreement with the rheological observations.     

RELATION BETWEEN SENSORY TEXTURE PROPERTIES AND EXOPOLYSACCHARIDE DISTRIBUTION IN SET AND IN STIRRED YOGHURTS PRODUCED WITH DIFFERENT STARTER CULTURES

Set yoghurts produced with seven different exopolysaccharide‐producing starter cultures were subjected to sensory texture profiling including texture evaluation before and after a stirring procedure carried out by trained assessors. The microstructure of four selected yoghurts was investigated by confocal laser scanning microscopy with special focus on the distribution of exopolysaccharides (EPS) in the protein network. Two different types of microstructures were observed, one in which EPS were associated with the protein network and another where EPS appeared to be incompatible with the protein; as a consequence, the EPS were situated in the void spaces in the protein network. The latter yoghurts were less ropy and had higher serum separation than the yoghurts where EPS were associated with the protein, and they exhibited a significant increase in mouth thickness as a result of stirring. Yoghurts in which the EPS were associated with protein had high ropiness, low serum separation and appeared more resistant to stirring. The nature of EPS‐protein interactions is extremely important for the textural effects of EPS in fermented milk products, and such interactions depend on the specific starter culture used.     

Modifying the microstructure of low-fat yoghurt by microfluidisation of milk at different pressures to enhance rheological and sensory properties

The effects of microfluidisation of milk at different pressures, prior to heat treatment, on structural and sensory properties of low-fat stirred yoghurt, were investigated. Low-fat yoghurts prepared from microfluidised milk were compared with low-fat (1.5%) and full-fat (3.5%) control yoghurts made with homogenised (20/5 MPa) milk. The microstructure of low-fat yoghurts prepared with microfluidised milk consisted of smaller and more uniform fat globules, well incorporated into more interconnected fat-protein gel networks, compared with those of control yoghurts. This modification in microstructure caused significant changes in gel particle size, sensory profile and rheological behaviour. Microfluidisation increased the gel particle size, gel strength and viscosity; marked beneficial effects were found at higher pressures (50–150 MPa). Microfluidising milk at 50–150 MPa increased the gel strength by 171–195% and viscosity by 98–103%, creating low-fat yoghurts with creaminess and desirable texture properties similar to, or better than, full-fat conventional yoghurt.     

Effect of exopolysaccharides from lactic acid bacteria on the texture and microstructure of buffalo yoghurt

To examine the effect of exopolysaccharides (EPS) on the microstructure and textural and rheological properties of yoghurt, purified EPS were added to buffalo milk to make yoghurts. The results showed that the viscosity, water-holding capacity, hardness, and microstructure of yoghurt were distinctly affected by the level of addition of EPS. With the increasing amount of supplemented EPS, the viscosity of buffalo yoghurt showed a downward trend with the increasing shear velocity, and an increasing degree of recovery of viscosity with the decrease in shear velocity, indicating that the steric barrier formed between the EPS and protein enhanced the viscoelastic properties. The microstructure showed more dense interspaced voids with EPS addition.     

Effect of potato starch addition on the acid gelation of milk

Potato starch was added to skim milk at levels of 0–1.5%. The milks were heated and then acidified to form acid milk gels. The properties of the milks during acidification and the final properties of the acid gels were studied. The addition of starch resulted in a higher storage modulus, G′, in the final acid gels, and increasing the level of starch caused a linear increase in the final G′. Compared with acid gels prepared with no starch, the gelation time was reduced and the gelation pH was increased. However, the temperature and frequency dependences of the acid gels were not affected by the addition of starch. Furthermore, the breaking strain of the acid gels was not markedly affected by the addition of starch, whereas the breaking stress was dependent on the level of starch added. Confocal microscopy showed that the acid gels contained swollen starch granules embedded in a protein network, and that the protein network increased in density as the level of starch added increased.     

Synthesis and utilisation of folate by yoghurt starter cultures and probiotic bacteria

Thirty-two bacterial isolates from species commonly used in yoghurts and fermented milks were examined for their ability to synthesise or utilise folate during fermentation of skim milk. The organisms examined included the traditional yoghurt starter cultures, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, and probiotic lactobacilli, bifidobacteria, and Enterococcus faecium. Folate was synthesised by S. thermophilus, bifidobacteria, and E. faecium. S. thermophilus was the dominant producer, elevating folate levels in skim milk from 11.5 ng g⁻¹ to between 40 and 50 ng g⁻¹. Generally, lactobacilli depleted the available folate in the skim milk. Fermentations with mixed cultures showed that folate production and utilisation by the cultures was additive. Fermentations using a combination of Bifidobacterium animalis and S. thermophilus resulted in a six-fold increase in folate concentration. Although increased folate levels in yoghurts and fermented milks are possible through judicious selection of inoculum species, the folate levels remain relatively low in terms of recommended daily allowance.     

A comparison of selected quality characteristics of yoghurts prepared from thermosonicated and conventionally heated milks

Thermosonication (TS) of preheated (45 °C) milk (0.1%, 1.5% and 3.5% fat) for 10 min at an ultrasound frequency of 24 kHz allowed the preparation of yoghurts with rheological properties superior to those of control yoghurts produced from conventionally heated milk (90 °C for 10 min). Texture profile analysis and flow curves showed that yoghurts from the TS milks had stronger gel structures which displayed higher water-holding capacities (WHC) and lower syneresis. Based on averaged data from a sensory panel (n = 30), TS yoghurts showed superior texture and colour properties and samples with a fat content of 0.1% scored best in terms of overall acceptability. Retentions of water-soluble (thiamine and riboflavin) and fat-soluble (retinol and tocopherol) vitamins were similar in TS and conventionally prepared yoghurts.     

Physicochemical properties of probiotic frozen yoghurt made from camel milk

The purpose of this research was to determine and compare the physicochemical properties of probiotic frozen yoghurts made from camel and cow milks. The results showed that the viscosity of frozen yoghurt made from camel milk was significantly higher (P < 0.05) than that from cow milk. Less fat was destabilised in frozen yoghurt made from camel milk. The frozen yoghurt made from camel milk required more time for melting than that from cow milk. Flavour scores of the frozen yoghurt made from camel milk constituents were significantly lower (P < 0.05) than those made from cow milk.     

Effects of various whey proteins on the physicochemical and textural properties of set type nonfat yoghurt

In this study, the changes during storage in the physicochemical, textural and sensory properties of nonfat yoghurts fortified with whey proteins, namely whey protein concentrates (WPC), whey protein isolates and whey protein hydrolysates, were investigated. Enrichment of nonfat yoghurt with the whey protein additives (1% w/v) had a noticeable effect on pH, titratable acidity, syneresis, water‐holding capacity, protein contents and colour values on the 14th day of storage (P < 0.01). The addition of whey proteins to the yoghurt milk led to increases in the hardness, cohesiveness and elasticity values, resulting in improved textural properties. The addition of WPC improved the texture of set‐type nonfat yoghurt with greater sizes in the gel network as well as lower syneresis and higher water holding capacity. This study suggests that the addition of whey protein additives used for fortification of yoghurt gave the best textural and sensory properties that were maintained constant during the shelf life.     

Effect of exopolysaccharide-producing starter cultures and post-fermentation mechanical treatment on textural properties and microstructure of low fat yoghurt

The microstructure and texture of yoghurts produced by four different exopolysaccharide (EPS)-producing starter cultures and mechanically treated post-fermentation at four levels of intensity (applied back-pressure) were studied. Two Lactobacillus delbrueckii ssp. bulgaricus (LB) strains were used in combination with two Streptococcus thermophilus (ST) strains and yoghurts were formulated by pairwise combining one LB and one ST strain. The choice of ST strain was the major determinant for the rheological properties of the yoghurts, since one of the ST strains conferred a ropy texture and resulted in yoghurts with decreased water holding capacity and an open microstructure. In addition, one of the LB strains used produced both aggregated and threadlike EPS and improved water holding. When combined with an ST strain that produced neglible amounts of EPS this LB strain resulted in yoghurt where a moderate mechanical treatment post-fermentation was able to further improve the water holding capacity.     

Comparison of the effects of high-pressure microfluidization and conventional homogenization of milk on particle size,water retention and texture of non-fat and low-fat yoghurts

The effect of high-pressure homogenization using a Microfluidizer^® on texture, water-holding capacity, and extent of syneresis on stirred yoghurts was compared with that of conventional homogenization. The effect of homogenization condition on particle size was also assessed in milk and in yoghurt. Stirred yoghurts were prepared from recombined milk samples (0 and 1.5% fat) heat-treated (95 °C, 2 min) and then treated by conventional valve homogenization at 25 MPa or microfluidization at 150 MPa. Homogenization conditions influenced the particle size in milk, gel particle size, and textural quality of stirred yoghurts in a manner dependent upon fat content. Milk microfluidized at 150 MPa had smaller particle size than homogenized milk, but resulted in larger particles in yoghurt. Microfluidization of low-fat milk modified the microstructure of yoghurt, giving more interconnectivity in the protein networks with embedded fat globules, but with similar texture profiles and water retention compared with yoghurt made from conventionally homogenized milk.     

枸杞牛乳肽酒制备工艺优化及其抗氧化性分析

为克服传统白酒功能性、营养性不足的缺陷,以脱脂牛乳和枸杞为材料,采用对比及正交试验,对乳蛋白酶解条件、枸杞牛乳肽酒的制备工艺和抗氧化功能进行了研究。结果表明:1)胰蛋白酶是水解牛乳蛋白的最佳蛋白酶;2)胰蛋白酶的最佳酶解条件是在pH 7.0、温度40℃、加酶量4 500 U/g的条件下酶解120 min;3)枸杞牛乳肽酒的最佳发酵工艺:以牛乳蛋白酶解液为载体,添加体积分数14%枸杞汁、7.5%的活化酵母菌液,28℃发酵9 d。制备的枸杞牛乳肽酒具有较高的体外抗氧化活性,且香味协调,口感良好,其感官指标和理化指标都符合并高于国家发酵型甜奶酒的标准。     

Effect of milk base and starter culture on acidification,texture, and probiotic cell counts in fermented milk processing

In the present work, the compared effect of milk base and starter culture on acidification, texture, growth, and stability of probiotic bacteria in fermented milk processing, was studied. Two strains of probiotic bacteria were used, Lactobacillus acidophilus LA5 and L. rhamnosus LR35, with two starter cultures. One starter culture consisted only of Streptococcus thermophilus ST7 (single starter culture); the other was a yogurt mixed culture with S. thermophilus ST7 and L. bulgaricus LB12 (mixed starter culture). For the milk base preparation, four commercial dairy ingredients were tested (two milk protein concentrates and two casein hydrolysates). The resulting fermented milks were compared to those obtained with control milk (without enrichment) and milk added with skim milk powder. The performance of the two probiotic strains were opposite. L. acidophilus LA5 grew well on milk but showed a poor stability during storage. L. rhamnosus LR35 grew weakly on milk but was remarkably stable during storage. With the strains tested in this study, the use of the single starter culture and the addition of casein hydrolysate gave the best probiotic cell counts. The fermentation time was of about 11 h, and the probiotic level after five weeks of storage was greater than 106 cfu/ml for L. acidophilus LA5 and 10(7) cfu/ml for L. rhamnosus LR35. However, an optimization of the level of casein hydrolysate added to milk base has to be done, in order to improve texture and flavor when using this dairy ingredient.     

Influence of skimmed milk concentrate replacement by dry dairy products in a low fat set‐type yoghurt model system. I: Use of whey protein concentrates,milk protein concentrates and skimmed milk powder

Ten commercial samples of dry dairy products used for protein fortification in a low fat yoghurt model system at industrial scale were studied. The products employed were whey protein concentratres, milk protein concentrates, skimmed milk concentrates and skimmed milk powder which originated from different countries. The gross chemical composition of these dried products were determined, including polyacrylamide gel electrophoresis (SDS‐PAGE) and isoelectric focusing of the proteins, and minerals such as Na, Ca, K and Mg. Yoghurts were formulated using a skim milk concentrated as a milk base enriched with different dry dairy products up to a 43 g kg⁻¹ protein content. Replacement percentage of skim milk concentrated by dry dairy products in the mix was between 1.49 and 3.77%. Yoghurts enriched with milk protein concentrates did not show significantly different viscosity (35.12 Pa s) and syneresis index (591.4 g kg⁻¹) than the two control yoghurts obtained only from skimmed milk concentrates (35.6 Pa s and 565.7 g kg⁻¹) and skimmed milk powder (32.77 Pa s and 551.5 g kg⁻¹), respectively. Yoghurt fortified with the whey protein concentrates, however, was less firm (22.59 Pa s) and had less syneresis index (216 g kg⁻¹) than control yoghurts. Therefore, whey protein concentrates may be useful for drinking yoghurt production. © 1999 Society of Chemical Industry     

Improving the structure and rheology of high protein,low fat yoghurt with undenatured whey proteins

The objective of this study was to investigate the effects of whey protein denaturation and whey protein:casein-ratio on the structural, rheological and sensory properties of high protein (8% true protein), low fat (<0.5% fat) yoghurt. Yoghurt milk bases were made by adding undenatured whey proteins from native whey protein concentrate (NWPC) to casein concentrate in different whey protein:casein-ratios. The degree of whey protein denaturation was then controlled by the temperature treatment of the yoghurt milk bases. Addition of NWPC in low (whey protein:casein-ratio 25:75) or medium levels (whey protein:casein-ratio 35:65) in combination with heat treatment at 75 °C for 5 min gave yoghurts with significantly lower firmness, lower storage modulus (G′), and better sensory properties (less coarse and granular and more smooth), compared with corresponding yoghurts produced from yoghurt milk bases heat-treated at 95 °C for 5 min or with control yoghurts (no addition of NWPC).     

EFFECT OF HIGH HYDROSTATIC PRESSURE PROCESSING ON RHEOLOGICAL AND TEXTURAL PROPERTIES OF PROBIOTIC LOW-FAT YOGURT FERMENTED BY DIFFERENT STARTER CULTURES

The effect of milk processing on rheological and textural properties of probiotic low‐fat yogurt (fermented by two different starter cultures) was studied. Skim milk fortified with skim milk powder was subjected to three treatments: (1) thermal treatment at 85C for 30 min; (2) high hydrostatic pressure (HHP) at 676 MPa for 5 min; and (3) combined treatments of HHP (676 MPa for 5 min) and heat (85C for 30 min). The processed milk was fermented using two different starter cultures containing Streptococcus thermophilus, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus and Bifidobacterium longum at inoculation rates of 0.1 and 0.2%. Rheological parameters were determined and a texture profile analysis was carried out. Yogurts presented different rheological behaviors according to the treatment used, which could be attributed to structural phenomena. The combined HHP and heat treatment of milks resulted in yogurt gels with higher consistency index values than gels obtained from thermally treated milk. The type of starter culture and inoculation rate, providing different fermentation pathways, also affected the consistency index and textural properties significantly. The combined HHP and heat treatment of milks before fermentation, and an inoculation rate of 0.1% (for both cultures), led to desirable rheological and textural properties in yogurt, which presented a creamy and thick consistency that does not require the addition of stabilizers.     

酪蛋白水解产物对含益生菌酸奶中益生菌数量的影响

文研究了牛奶蛋白水解产物对发酵牛奶中益生菌生长的促进作用,并对在两周的贮存期内活性益生菌菌数的稳定性进行了比较研究。实验以益生菌菌株-嗜酸乳杆菌作为研究用菌种。利用novo型碱性蛋白酶水解酪蛋白获得水解度分别为4.7%、7.1%、8.8%、10.1%、12.6%的酪蛋白水解产物,研究以0.5、1、2和4g/L四个添加水平加入发酵酸奶中,分析其对益生菌生长情况的影响。结果证明,酪蛋白水解产物的添加普遍促进了嗜酸乳杆菌在酸奶中的生长。随着添加量的增加,嗜酸乳杆菌在发酵牛奶中的稳定性得到了同步提高。但综合分析,水解度为8.8%的水解产物,添加量为2g/L时,可使益生菌的生长活性和稳定性状态最佳。研究结果证实,选择优化的酪蛋白水解产物对发酵牛奶中益生菌的生长及维持其生存能力均会产生积极地促进作用,也对发酵乳行业生产符合国际标准的益生菌产品具有重要的理论和实践的价值。