Rheological properties of milk gels formed by a combination of rennet and glucono-delta-lactone

The effects of heat treatment of milk, and a range of rennet and glucono-delta-lactone (GDL) concentrations on the rheological properties, at small and large deformation, of milk gels were investigated. Gels were made from reconstituted skim milk at 30 degrees C, with two levels each of rennet and GDL. Together with controls this gave a total of sixteen gelation conditions, eight for unheated and eight for heated milk. Acid gels made from unheated milks had low storage moduli (G') of < 20 Pa. Heating milks at 80 degrees C for 30 min resulted in a large increase in the G' value of acid gels. Rennet-induced gels made from unheated milk had G' values in the range approximately 80-190 Pa. However, heat treatment severely impaired rennet coagulation: no gel was formed at low rennet levels and only a very weak gel was formed at high levels. In gels made with a combination of rennet and GDL unusual rheological behaviour was observed. After gelation, G' initially increased rapidly but then remained steady or even decreased, and at long ageing times G' values increased moderately or remained low. The loss tangent (tan delta) of acid gels made from heated milk increased after gelation to attain a maximum at pH approximately 5.1 but no maximum was observed in gels made from unheated milk. Gels made by a combination of rennet and GDL also exhibited a maximum in tan delta, indicating increased relaxation behaviour of the protein-protein bonds. We suggest that this maximum in tan delta was caused by a loosening of the intermolecular forces in casein particles caused by solubilization of colloidal calcium phosphate. We also suggest that in combination gels made from unheated milk a low value for the fracture stress and a high tan delta during gelation indicated an increased susceptibility of the network to excessive large scale rearrangements. In contrast. combination gels made from heated milk formed firmer gels crosslinked by denatured whey proteins and underwent fewer large scale rearrangements.     

Rheological Properties and Microstructure of Acid Milk Gels as Affected by Fat Content and Heat Treatment

Gels were made from recombined milks containing 0, 1.5 or 3.5% fat that were heated at 75, 80 or 90°C for 30 min, followed by acidification with glucono-8-lactone at 30°C. The rheological and microstructural properties of acid gels were investigated using dynamic low-amplitude oscillatory rheology and confocal scanning laser microscopy (CSLM). Heating milks, at temperatures ≥80°C, increased the storage moduli (G′) and decreased the gelation time. Recombined milks containing high fat (3.5%) had higher G′ than gels made from low-fat or skim milk. Milk heat treatment resulted in gels with a cross-linked microstructure. Recombined fat globules appeared to be embedded in the protein matrix.     

Gelation properties of partially renneted milk

Acid- and rennet-induced gelation properties of milk with modified casein micelles, produced by partial renneting at 4^oC for 15 min, followed by inactivation of enzymes by heat at 60^oC/3 min (referred as low heat treatment milk) and 85^oC/30 min (high heat treatment milk), were investigated to provide a mechanistic understanding of gel formation from partially renneted milk. Acidification of low heat treatment milk gave firmer gel quality, this was reflected by its high elastic modulus (G′) and hardness. In addition, the high heating condition for enzyme inactivation of high heat treatment milk alone increased the elastic modulus of both the control and renneted milk samples. Gel development of the two milk types (low heat treatment and high heat treatment milks) was different. In contrast with acid gelation, rennet-induced gelation of partially pre-rennet treated milk had no impact on the elastic modulus of low heat treatment milk and the rennet gels were very weak. Similarly, the addition of rennet to pre-rennet treated high heat treatment milk did not produce “true gels,” most likely due to the effect of the heat treatment on impairing the rennet coagulation. The findings in this study confirmed that pre-rennet treated milk had positive effects on the end-product acid gels of low heat treatment and high heat treatment milk.     

Influence of heat treatment of goat milk on casein micelle size,rheological and textural properties of acid gels and set type yoghurts

Acid gels and yoghurts were made from goat milk that was heated at 72°C/30 s, 85°C/5 min, and 95°C/5 min, followed by acidification with starter culture at 43C until pH 4.6. The rheological and textural properties of acid gels and yoghurts were analyzed using dynamic low amplitude oscillatory rheology and back extrusion texture analysis, respectively. The effect of goat milk heat treatment on the mean casein micelle diameter and protein profile was also determined by dynamic light scattering and SDS PAGE electrophoresis, respectively. The shortest gelation and fermentation time was recorded for yoghurt prepared from milk heated at 85°C/5 min. Also, the pH of gelation, the storage moduli (G′) and yield stress were higher for this yoghurt, compared with the other two. Textural properties of goat milk yoghurts such as firmness and consistency were strongly affected by milk heat treatment, and the highest values were recorded for yoghurt produced from milk preheated at 85°C/5 min, as well. The largest casein micelles were measured after 85°C/5 min treatment and their size decreased at higher temperature, despite higher denaturation of whey proteins at the most intense heat regime, indicating the structure changes that influence on the acid gelation.     

Serum Protein Aggregates in the High-Heated Milk and Their Gelation Properties in Rennet-Induced Milk Gel

The influence of different heat treatments on the protein aggregates and changes in gelation properties of rennet-induced milk gels were investigated. In the heated milk, a visible difference in milk serum proteins was found resulting from the formation of protein aggregates. Meanwhile, the size of protein aggregates increased from 25 to 170 nm with increasing the intensity of heat treatment. Furthermore, the differences in textural variables of rennet gels were found among the heat treatments using the principal component analysis. The water holding capacity and cheese curd yield of rennet gels obtained from the heated milk were significantly greater than those of raw milk (p < 0.05). It was also found heat treatments above 80°C could endow rennet-induced milk gels with novel textural properties.     

Short communication: extraction of beta-casein from goat milk

Peptides derived from milk β-casein have potential biological activities, such as antihypertensive and immunostimulating properties. These biological properties increase the demand for the production of specific bioactive peptides. β-Casein can be isolated directly from renneted skim milk, based on the preferential solubilization of β-casein at low temperature. This study was conducted to compare the recovery and purity of β-casein extracted from goat and cow milks. Rennet casein was prepared from both milks, heat treated, and dispersed in demineralized water at various temperatures. β-Casein recovery in the soluble phase increased with decreasing incubation temperature. Concentration of β-casein was 43% higher in goat milk than in cow milk, which had a direct effect on β-casein recovery. Furthermore, β-casein was extracted more efficiently from goat rennet casein. As a result, the extraction yield of β-casein was 53% higher in goat milk than in cow milk. The purity of β-casein extracted from both milks reached approximately 90% after incubation at 0°C.     

Effect of controlled kappa-casein hydrolysis on rheological properties of acid milk gels

An experimental method based on the controlled chymosin-induced kappa-casein hydrolysis of milk was proposed to modify micellar reactivity. Milk samples with a degree of kappa-casein hydrolysis of 19, 35, and 51% were obtained. The physicochemical properties of partially converted casein micelles were determined. The net negative charge of casein micelles was reduced with increasing degree of kappa-casein hydrolysis and a small but significant decrease in hydrodynamic diameter and micellar hydration were noted. Dynamic low amplitude oscillatory rheology was used to monitor the rheological properties of acid milk gels (GDL) made with partially chymosin-hydrolyzed milks in comparison with those of strictly acid and rennet gels. An increase in the gelation pH value was observed with increasing the degree of kappa-casein hydrolysis. The moduli values (G' and G') reached 2 h after the point of gel were, for all degrees of hydrolysis tested, significantly higher than those of strictly rennet and acid gels. Comparison of changes in delta G'/delta t with time indicated differences in gel formation that could be related to the increased values of G' obtained for acid gel made with chymosin-treated milk. At a given time after gelation (2 h), increasing the degree of kappa-casein hydrolysis in milk led also to an increase in the loss tangent and the serum holding capacity of acid milk gels suggesting a correlation between these two parameters.     

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.     

Dynamic in vitro gastric digestion behavior of goat milk: Effects of homogenization and heat treatments

The gastric digestion behavior of differently processed goat milks was investigated using a dynamic in vitro gastric digestion model, the human gastric simulator. Homogenization and heat treatment of goat milk resulted in gastric clots with highly fragmented structures. They also delayed the pH reduction during digestion, altered the chemical composition of the clots and the emptied digesta, promoted the release of calcium from the clots, and accelerated the hydrolysis and the emptying of milk proteins. The apparent density of the protein particles and the location of the homogenized fat globules changed during the digestion process, as shown in the emptied digesta of the homogenized goat milks. The effects of processing on the digestion behavior of goat milk were broadly similar to those previously reported for cow milk. However, the overall gastric digestion process of goat milk was more affected by homogenization than by heat treatments.     

Effect of buttermilk made from creams with different heat treatment histories on properties of rennet gels and model cheeses

Although many studies have reported negative effects on cheese properties resulting from the use of buttermilk in cheese milk, the cause of these effects has not been determined. In this study, buttermilk was manufactured from raw cream and pasteurized cream, as well as from a cream derived from pasteurized whole milk. Skim milks with the same heat treatments were also manufactured to be used as controls. Compositional analysis of the buttermilks revealed a pH 4.6-insoluble protein content approximately 10% lower than that of the skim milk counterparts. Milk fat globule membrane (MFGM) proteins remained soluble at pH 4.6 in raw cream buttermilk; however, when heat was applied to cream or whole milk before butter making, MFGM proteins precipitated with the caseins. Rennet gel characterization showed that MFGM material in the buttermilks decreased the firmness and increased the set-to-cut time of rennet gels, but this effect was amplified when pasteurized cream buttermilk was added to cheese milk. The microstructure of gels was studied, and it was observed that gel appearance was very different when pasteurized cream buttermilk was used, as opposed to raw cream buttermilk. Model cheeses manufactured with buttermilks tended to have a higher moisture content than cheeses made with skim milks, explaining the higher yields obtained with buttermilk. Superior retention of MFGM particles was observed in model cheeses made from pasteurized cream buttermilk compared with raw cream buttermilk. The results from this study show that pasteurization of cream and of whole milk modifies the surface of MFGM particles, and this may explain why buttermilk has poor coagulation properties and therefore yields rennet gels with texture defects.     

热处理影响牛乳蛋白凝胶特性的研究进展

热处理是各种乳制品生产过程中不可或缺的工艺手段,热处理过程中乳体系的蛋白质会发生变化,影响制品的凝胶特性.介绍了牛乳酸凝胶的模型和酶凝胶的形成阶段,分析了加热过程中蛋白质可能发生的变化,简要阐述了热处理对凝乳效果的影响,指出热诱乳蛋白聚合物与凝乳的机制密切相关,旨在为乳制品加工中热处理的潜在影响提供理论参考.     

Rheological and textural properties of goat and cow milk set type yoghurts

Yoghurts were made from pasteurised and homogenised goat milk (2.5% protein), goat milk fortified with a milk protein isolate (5% protein) and cow milk (3% protein), by acidification with a starter culture at 43 °C until a pH of 4.6 was reached. The rheological and textural properties of gels and yoghurts were analysed using dynamic low amplitude oscillatory rheology and back extrusion texture analysis. Gelation and fermentation times of goat milk were longer, while gelation pH, storage moduli (G′) and yield stress values were lower, compared with those of cow milk. Textural properties of goat milk yoghurts such as firmness, consistency, cohesiveness and viscosity index were very poor. Consequently, the products could not be classified as set type yoghurts. Fortification of goat milk with a milk protein isolate contributed to a significant improvement of the rheological and textural properties of yoghurt.     

超声处理改善羊乳理化性质及其酶凝胶流变学特性

在超声频率20 kHz,超声功率800 W,超声时间15、20、25、30 min的条件下处理生羊乳,分析超声处理对羊乳粒径、Zeta-电位、表面疏水性和二级结构的影响,并通过流变学分析和扫描电子显微镜观察进一步研究添加凝乳酶对羊乳酶凝胶的流变学特性和微观结构的影响。结果表明,超声处理降低了羊乳样品的粒径,使蛋白质的二级结构发生明显变化,β-折叠和无规卷曲相对含量增加,α-螺旋和β-转角相对含量降低,β-折叠结构相对含量与表面疏水性呈正相关（r＝0.74）。与对照组（未处理的生羊乳和巴氏杀菌羊乳）相比,较长时间（超过20 min）超声处理能显著提高最大储能模量（G’max）（P＜0.05）,但超声处理25、30 min的羊乳凝乳时间缩短。扫描电子显微镜结果显示超声处理羊乳样品的凝胶微观结构更加致密。综上,超声处理可以有效改善羊乳酶凝胶品质,在实际生产中具有潜在的应用价值。     

Vitamin profiles of kefirs made from milks of different species

Kefirs were produced under laboratory conditions using 10 different kefir grain cultures from different households, and reconstituted cow, ewe, goat and mare milk as substrates. The base milks and corresponding kefirs were examined for their content of water-soluble vitamins and orotic acid. On average, enrichment of vitamin concentration of>20% was observed with thiamine (only ewe milk kefirs), pyridoxine (kefirs from all milk species except cow milk), and folic acid (kefirs from all milk species except mare milk). Orotic acid content was reduced during fermentation throughout.     

Heat and/or high-pressure treatment of skim milk: changes to the casein micelle size, whey proteins and the acid gelation properties of the milk

Skim milk was subjected to heat, pressure or combined processes. In general, higher levels of whey protein denaturation were observed for milk subjected to combined processes than those heat- or pressure-treated only. Heat treatment caused small changes to the casein micelle size. Pressure treatment decreased the casein micelle size; however, the effect was less marked when heat and pressure treatments were combined. Acidification of the skim milks produced gels with a range of firmness, yield stresses and yield strains depending on the treatments applied. These changes in acid gel properties were not related only to whey protein denaturation levels in the milks.     

不同乳基婴幼儿配方乳粉的理化性质和表面组成

为探究不同乳基对婴幼儿配方乳粉稳定性的影响,本研究对以牛乳和羊乳为基料制备的婴幼儿配方乳粉的水分质量分数和水分活度(water activity,a_w)、玻璃化转变温度(glass transition temperature,T_g)、乳糖结晶度、溶解度、色度、蛋白组成、总脂肪酸和表面游离脂肪酸组成等理化性质进行分析,用X射线光电子能谱(X-ray photoelectron spectroscopy,XPS)仪对乳粉表面成分进行测定,并通过扫描电子显微镜(scanning electron microscopy,SEM)观察乳粉表面形貌。结果表明：羊乳婴幼儿配方乳粉具有较好的理化性质,与牛乳婴儿配方乳粉相比具有较低的水分质量分数、a_w、色度和T_g,而结晶度和溶解度接近,通过气相色谱-质谱(gas chromatography-mass spectrometry,GC-MS)对脂肪酸含量进行测定发现,牛乳和羊乳婴幼儿配方乳粉均表现出总脂肪酸中不饱和脂肪酸含量较高,表面游离脂肪酸中饱和脂肪酸含量较高的...     

Effect of Goat and Cow Milk Ratios on the Physicochemical,Rheological, and Sensory Properties of a Fresh Panela Cheese

Fresh cheeses, Panela type, were manufactured from cow milk and with goat milk incorporation, constituting 4 blends of milks (G10:C90, G20:C80, G30:C70, G40:C60, v/v). The cheeses were analyzed to determine the effect of the different goat milk ratios on the physicochemical, textural, rheological, and sensory properties over 15 d of storage. Significant differences in protein (14.6% to 18.5%), fat (13.0% to 19.4%), and moisture contents (51.7% to 61.3%), pH (6.38 to 6.67), color (L_h > 64.4, a_h > 1.06, b_h > 5.14), textural (σ_f > 14.8 kPa, ε_C: 0.77 to 0.79, elasticity modulus > 13.5 kPa), and rheological parameters (G′ > G′′, G′: 10.6 to 31.9 kPa, G′′: 2.39 to 7.31 kPa, tan δ: 0.21 to 0.24) were detected as a function of the milks ratio, as well as a function of the storage time. The incorporation of goat milk improved the overall quality in the formulation of Panela cheese, enhancing the texture, flavor and aroma, commonly associated with hand‐crafted cheeses when they are used in the proper ratio. Furthermore, the nutritional value of the cheese is increased with the incorporation of goat milk, which can contribute to a better consumer health.     

Factors affecting the clotting properties of sheep milk

A Formagraph was used to test the effects of some of the exogenous factors that can affect the processing properties of milk (pH, soluble calcium, rennet concentration, coagulation temperature), and two of the endogenous factors (protein and fat concentration), on the comparative clotting properties of sheep and cows' milk, namely renneting time (r), rate of firming (k20) and curd consistency (A30). A lower pH decreased r and k20 and increased A30 in both sheep and cows' milk. The addition of calcium chloride did not affect the clotting properties of sheep milk, but in cows' milk it decreased r and k20 and increased A30. Increasing the concentration of rennet decreased r and k20 and increased A30 for both sheep and cows' milk. Increasing the coagulation temperature from 30 to 38 °C decreased r for both sheep and cows' milk, but it decreased k20 and increased A30 only in cows' milk. Increasing the protein concentration decreased r in both sheep and cows' milk; it did not affect k20 of sheep milk, but it decreased that of cows' milk and increased A30 in both milks. Increasing the fat concentration had little effect on r and k20 in either sheep cows' milk, but it decreased A30 in both milks. In general, sheep milk had faster renneting times and rates of firming and greater curd consistency than cows' milk, and its clotting properties tended to be less affected by changes in the clotting conditions. © 2002 Society of Chemical Industry     

Changes in texture,composition and sensory characteristics of Camembert cheese made from a mixture of goat milk and cow milk during ripening

The study was conducted to evaluate the nutritional, textural and sensory characteristics of Camembert cheese made from goat milk, cow milk and their blends. The type of milk used affected yield, physicochemical properties and fatty acid profile of the cheeses. The percentages of C6:0, C8:0 and C10:0 acids were higher in the cheeses containing ≥ 75% goat milk. The sensory evaluation results showed differences in texture, colour, aroma and flavour among the cheeses. Cheeses made with the mixture of cow and goat milk particularly 50% goat milk mixed with 50% cow milk (50G:50C) maintained particular positive nutritional characteristics especially with respect to the mineral contents, fatty acids and consumer acceptability.     

Structural changes in cow,goat, and sheep skim milk during dynamic in vitro gastric digestion

Coagulation of milk in the stomach is the first crucial step in its digestion. Using a human gastric simulator, the dynamic gastric digestion of goat and sheep skim milk were compared with that of cow skim milk, focusing particularly on their physical characteristics. The gastric contents were analyzed for changes in dry matter and microstructure, and the extent of protein digestion. The study revealed that the skim milk from all species formed a curd within the first 15 min of gastric digestion, at which time the pH was ~6.1 to 6.3. Compared with cow skim milk, the dry matter contents of the clots formed from goat and sheep skim milk were lower and higher, respectively, which was due to the differences in their total solids and protein contents. Microstructural analysis showed that, as digestion progressed, the clot structure became more cohesive, along with a decrease in moisture content, which in turn affected the breakdown and hydrolysis of caseins by pepsin; this phenomenon was similar for milk from all species. However, the extent of moisture retained in the sheep skim milk clot appeared to be lower than those of the cow and goat skim milk clots. In addition, the relative firmness of the sheep milk clot was higher than those of the cow and goat milk clots at the end of gastric digestion. The pattern of protein hydrolysis by pepsin was similar for the milk of all species, despite the differences in the proportions of different proteins. The study provided insight into the coagulation kinetics of goat and sheep skim milk under in vitro gastric digestion conditions.