Manipulation of the heat stability of homogenized concentrated milk

An examination has been made of the means of improving the heat stability of homogenized concentrated milk. It was found that heat stability could be enhanced to the greatest extent by high temperature forewarming (145° C, 5 s), two-stage homogenization, and addition of sodium phosphates. It was shown that in the winter months homogenization per se greatly reduces the heat stability of subsequently concentrated milk. Consequently, in the production of sterilized milk products, it is essential to stabilize such milks before rather than after this destabilizing step.     

Changes in the surface protein of the fat globules during homogenization and heat treatment of concentrated milk

The changes in milk fat globules and fat globule surface proteins of both low-preheated and high-preheated concentrated milks, which were homogenized at low or high pressure, were examined. The average fat globule size decreased with increasing homogenization pressure. The total surface protein (mg m-2) of concentrated milk increased after homogenization, the extent of the increase being dependent on the temperature and the pressure of homogenization, as well as on the preheat treatment. The concentrates obtained from high-preheated milks had higher surface protein concentration than the concentrates obtained from low-preheated milks after homogenization. Concentrated milks heat treated at 79 degrees C either before or after homogenization had greater amounts of fat globule surface protein than concentrated milks heat treated at 50 or 65 degrees C. This was attributed to the association of whey protein with the native MFGM (milk fat globule membrane) proteins and the adsorbed skim milk proteins. Also, at the same homogenization temperature and pressure, the amount of whey protein on the fat globule surface of the concentrated milk that was heated after homogenization was greater than that of the concentrated milk that was heated before homogenization. The amounts of the major native MFGM proteins did not change during homogenization, indicating that the skim milk proteins did not displace the native MFGM proteins but adsorbed on to the newly formed surface.     

Determination of the heat stability profiles of concentrated milk and milk ingredients using high resolution ultrasonic spectroscopy

In the present work, we used high resolution ultrasonic spectroscopy for the analysis of heat coagulation process in milks. Two skim milks recombined from powder samples differing by their preheating treatments as well as dairy ingredients (phosphocasein, whey protein isolate, and a milk model) were monitored while submitted to a temperature of 120°C. Three stages in the precoagulation and coagulation processes can be distinguished in the ultrasonic velocity and attenuation profiles. The first stage shows a very sharp decrease in ultrasonic velocity and increase in ultrasonic attenuation. This could be attributed to fast denaturation and aggregation of whey proteins and precipitation of calcium phosphate. In the second stage, small changes in ultrasonic velocity are observed, the rate of which depends on the nature and pH of the samples. In the third stage, a sharp decrease in ultrasonic velocity and attenuation is recorded for all samples, which corresponds to the coagulation and formation of the gel network. After coagulation, a very small change in ultrasonic velocity and attenuation was observed. From the ultrasonic measurements, we determined the heat stability vs. pH profiles of the different samples studied. The observed ultrasonic attenuation profiles were also interpreted in terms of changes in the size of casein micelles and coagulation processes.     

Influence of added calcium chloride on the heat stability of unconcentrated and concentrated bovine milk

The influence of added calcium chloride (1–10 mmol/L) on the heat-induced coagulation of skim bovine milk was examined. Unconcentrated milk displayed a pH-heat coagulation time (HCT) profile with a maximum at pH 6.6 and minimum at pH 7.0. Adding calcium chloride to unconcentrated milk progressively reduced the HCT at the maximum, increased the pH at which the maximum occurred and reduced the HCT at pH > 7.0. For concentrated milk, the shape of the pH-HCT profile, that is, a maximum at pH 6.6, was not altered by added calcium chloride, but HCT was reduced progressively with increasing concentration of calcium chloride. Preheating (90°C for 10 min) shifted the maximum in the pH-HCT profile of unconcentrated milk to a more acidic pH, and addition of 5 mmol/L calcium chloride to preheated milk induced changes in heat stability similar to those noted for unheated milk. Addition of calcium chloride to milk prior to preheating strongly reduced the stability of milk against heat-induced coagulation. These data suggest that calcium has a strong destabilizing effect on the stability of milk systems against heat-induced coagulation.     

Influence of added lyophilized butter serum on the heat stability of unconcentrated and concentrated bovine milk

The influence of added lyophilized butter serum on the heat stability of milk was investigated. The addition of lyophilized serum from salted butter to unconcentrated skimmed milk (SM) reduced the heat coagulation time (HCT) at, and increased the pH of, the maximum in the pH–HCT profile and caused disappearance of the minimum. NaCl had similar effects on the heat stability of SM as lyophilized salted butter serum, whereas lyophilized serum from unsalted butter had little effect. The addition of lyophilized salted butter serum to concentrated skimmed milk (CSM) also shifted the pH of maximum heat stability to a higher value and, at certain concentrations, increased the maximum HCT; similar effects were obtained on addition of NaCl, but lyophilized serum from unsalted butter had little effect. These results suggest that the effects of lyophilized serum from salted butter on the heat stability of SM or CSM are due primarily to the presence of a high level of NaCl in this serum.     

均质及杀菌条件对花生奶稳定性的影响

通过分析产品的稳定系数R，油脂析出率，离心沉淀率及粒度分布，研究均质及杀菌条件对花生奶稳定性的影响。结果表明，花生奶在温度65～70℃，压力25MPa下均质2次，再经121℃杀菌15min后产品的稳定性最佳。     

搅拌和均质对牛乳稳定性的影响

采用BECKMAN LS13-320激光粒度分析仪对不同搅拌速率和均质条件的牛乳脂肪球粒径进行检测分析.结果表明,高搅拌速率特别是在2 000 r/min时随搅拌时闻的延长对牛乳脂肪球粒径的影响较显著,转速为1 500 r/min时一直到搅拌12 h才开始出现脂肪球轻微聚集现象,搅拌速率为1 000 r/min时搅拌12 h后仍与原鲜奶的粒径分布图相似.不同的均质条件对牛乳粒径分布图有一定的影响,其中,75℃/25 MPa均质后牛乳的粒径分布图相对而言较圆滑、重叠最少,体系较稳定.经中试试验同样证明75℃/25 MPa的均质条件更适于UHT乳的生产,牛乳体系相对而言最稳定.     

Chemical changes in bovine milk fat globule membrane caused by heat treatment and homogenization of whole milk

The effects of heat treatment and homogenization of whole milk on chemical changes in the milk fat globule membrane (MFGM) were investigated. Heating at 80 degrees C for 3-18 min caused an incorporation of whey proteins, especially beta-lactoglobulin (beta-Ig), into MFGM, thus increasing the protein content of the membrane and decreasing the lipid. SDS-PAGE showed that membrane glycoproteins, such as PAS-6 and PAS-7, had disappeared or were weakly stained in the gel due to heating of the milk. Heating also decreased free sulphydryl (SH) groups in the MFGM and increased disulphide (SS) groups, suggesting that incorporation of beta-Ig might be due to association with membrane proteins via disulphide bonds. In contrast, homogenization caused an adsorption of caseins to the MFGM but no binding of whey proteins to the MFGM without heating. Binding of caseins and whey proteins and loss of membrane proteins were not significantly different between milk samples that were homogenized before and after heating. Viscosity of whole milk was increased when milk was treated with both homogenization and heating.     

Sensitivity analysis of a small‐volume objective heat stability evaluation test for recombined concentrated milk

To quantify the stability towards heat coagulation, an objective test method was developed. The combined use of emulsion preparation by microfluidisation and heat stress by immersion of capped samples in an oil bath enabled small‐volume heat stability evaluation of milk formulations. From experimental data, it became obvious that a heating period of about 9 min was necessary for the samples to acquire the requested temperature (i.e. 121 °C). Similarly, both viscosity and particle size analyses showed an increased aggregation tendency when samples were heated for longer than 10 min, whereby their positioning and the cooling medium seemed to have an insignificant effect.     

Proteomics and microstructure profiling of goat milk protein after homogenization

This study investigated the protein changes in goat milk during the homogenization process using label-free quantification. We quantified 310 and 315 proteins in the control group (CG) and homogenized group (HG), respectively, and 16 proteins were significantly different between the 2 groups. For HG, the goat milk protein particle sizes were smaller and more evenly distributed and exhibited an increase in the regular arrangement of the secondary structures. Proteomics analysis verified that xanthine dehydrogenase and asparaginase-like 1 expression in CG were higher than in HG, whereas the opposite was observed for fructose-bisphosphate aldolase, κ-casein, and β-casein. Significant changes were found in the homogenization-treated goat milk proteome that were related to goat milk glycolysis/gluconeogenesis metabolism. This work provides updated information on the current proteome characteristics of homogenized goat milk, which may be important for applying the protein component of goat milk to human nutrition and health.     

热处理与均质压力对扣碗酪凝乳质构的影响

热处理与均质是牛奶处理的必须工艺,这两个因素对凝乳形成、蛋白变性等方面都有影响,这些因素也影响到扣碗酪的凝乳质构性质。本文应用物性测定的方法,对热和均质扣碗酪的凝乳质构中的硬度、粘着性和凝聚性的影响进行了分析,发现牛奶的均质压力与热处理温度之间对扣碗酪的凝乳质构性质没有交互作用,选择低温热处理以及低均质压力能够得到比较好的凝乳质构。硬度和粘着性对整体饱满度的影响比较大,相关系数分别达到0.679和0.743。凝聚性对扣碗酪的整体饱满度影响不大。     

Modelling of heat stability and heat‐induced aggregation of casein micelles in concentrated skim milk using a Weibullian model

Heat stability is known to be limited in concentrated skim milk leading to severe coagulation and sediment formation during storage. It has often been found to be in conflict with the extension of the shelf life by thermal treatment. A Weibullian model was used to describe the course of coagulation of casein micelles in concentrated skim milk of different total solids and at different heating temperatures. A maximum heating temperature–time–total solids relationship was calculated based on a certain maximum allowance of sediment formation. Optimal heat treatment conditions for concentrated skim milk of different total solids content could be defined.     

均质对全脂核桃营养乳稳定性的影响

本文就全脂核桃营养乳均质目的，原理及其操作进行阐述，并确定均质分二次进行。第一次均质压力为４５ＭＰａ；第二次均质压力为２０ＭＰａ。均质时料液温度为７５℃；ＰＨ为６．８－７．０。     

Influence of storage, heat, and homogenization upon xanthine oxidase activity of milk

Xanthine oxidase activity in milk was determined by measuring the rate of formation of vanillic acid from vanillin. Raw milk received at a dairy plant had .208 units xanthine oxidase activity per ml and after 24-h storage at 4 C, .228 units per ml. Upon further storage activity decreased. Heating the fresh raw milk in a water bath to 55 C increased xanthine oxidase activity to .236 units per ml. Partial inactivation of the enzyme occurred when milk was heated at 60, 65, or 70 C for 5 min, and destruction was almost complete with heat at 75 C for 5 min. Raw milk heated at 48 C for 5 min and homogenized at pressures between 70.3 and 281.2 kg/cm2 had xanthine oxidase activities which were a linear function of pressure and showed that each additional kg/cm2 pressure resulted in additional xanthine oxidase activity of .16 milliunits per ml of milk.     

Effect of diacetyl on the heat stability of concentrated milks

Very low levels of diacetyl (1–5 mM) markedly increased the heat stability of concentrated milks (20% total solids) reconstituted from skim milk powders. Diacetyl caused only slight stabilization of the concentrates (following dilution to 20% total solids) used in the manufacture of these powders but its efficacy was increased by heating the concentrates (70°C for 10 min) prior to diacetyl addition and especially by freeze drying. Laboratory concentrated milks were destabilized by diacetyl and these samples were also destabilized by freeze drying. However, addition of diacetyl prior to preheating and concentration increased stability.     

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.     

Use of reconstituted concentrated nonfat milk for the production of UHT milk: physical and chemical effects and stability

The objective of this work was to evaluate the use of reconstituted concentrated nonfat milk for the production of UHT milk and to evaluate its physical and chemical effects and stability. The study considered three treatments related to the raw material and processing parameters. Increasing the intensity of thermal treatments resulted in increased hydroxymethylfurfural (HMF) content as well as the denaturation of whey protein. During storage, the milk that was produced from reconstituted concentrate displayed a lesser degree of proteolysis and higher pH value, sedimentation index and HMF, compared to natural milk. Under higher storage temperatures, the degree of proteolysis, HMF and pH decreased and the sedimentation index increased.     

Effect of starches on the heat stability of milk

Starches from different botanical sources have been added (0.5–1.5% w/w) to skim milk and pregelatinized by heating at 70°C for 30 min. the heat coagulation time (HCT) of the milk-starch mixtures was then evaluated at 140°C. the addition of native starches to milk had a destabilizing effect. Increasing the concentration of added starch caused a decrease in HCT's. the reduction in the heat stability of milk proteins was not related to the amylose/amylopectin composition or to the granular structure of the starches. Modification of the molecular properties of the starch components by acid hydrolysis or cross-linking resulted in an enhancement of the heat stability of the mixtures.