酶解对牛乳酪蛋白抗原性的影响

采用5种常用酶对酪蛋白进行酶解,用间接竞争性ELISA方法测定酪蛋白水解物的残留抗原性.结果表明,采用酶解的方法能够有效降低酪蛋白的抗原性,但是不能够将抗原表位完全除去,酶解产物仍保留有一部分抗原性.不同的酶降低酪蛋白抗原性的程度不同,碱性蛋白酶降低酪蛋白抗原性的效果最佳,但是酶解物苦味较重,木瓜蛋白酶既能有效降低酪蛋白的抗原性,酶解物又具有较好风味.     

Effect of minerals on casein micelle stability of cows' milk

The effects of minerals on casein micelle stability of individual cows' milk, throughout a complete lactation, were investigated. Calcium and calcium ions, magnesium, phosphorus, sodium, potassium and citrate contents were analysed, together with the following physical properties of milk; pH, ethanol stability, rennet clotting time and coagulum firmness. There was an inverse non-linear relationship between free calcium ion concentration and ethanol stability (ES; r=0.84). Rennet coagulation time showed a weaker relationship with free calcium ion concentration (r=0.44) but a stronger relationship with pH (r=0.66). In addition, samples containing higher amounts of free calcium ions produced a firmer gel. Citrate in natural samples acts as a stabilizing factor, as it slightly improves milk stability. Potassium, on the other hand, exhibited a negative correlation, but only with rennet clotting time (r=-0.52). Throughout lactation the average values were; free Ca2+ concentration 1.88 mM, pH 6.63, ES 83.2% and clotting time 13.6 min. The equilibrium relationship between pH and free Ca2+ concentration was investigated by adjusting milk pH from 5.9 to 7.1, using acid and alkali. There was a good inverse linear relationship between pH and log (free Ca2+) for individual milk samples, with a gradient of -0.62 and a standard deviation of 0.042.     

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.     

羊奶酪蛋白热稳定性的研究

以莎能和关中羊奶为原料,通过从羊奶中提取酪蛋白,分别在不同的pH、温度以及添加不同浓度的Ca^2＋、柠檬酸钠、三聚磷酸钠、干酪素的条件下测定酪蛋白的热凝固时间（HCT）,研究其对羊奶酪蛋白热稳定性的影响。结果表明,pH在6．8时酪蛋白的热稳定性最好,高温会降低酪蛋白的热稳定性,钙离子可以降低羊奶酪蛋白的热稳定性,适量的柠檬酸钠或三聚磷酸钠可以有效提高羊奶酪蛋白的热稳定性,干酪素对酪蛋白稳定性影响不明显。     

大豆磷脂对牛乳蛋白乳状液热稳定性的影响

研究了大豆磷脂对牛乳蛋白乳状液热稳定性的影响,并对相关作用机理进行了探讨.测定了添加磷脂前后牛乳蛋白乳状液的热凝固时间(HCT)、粒径分布、表面疏水性、Zeta位和游离巯基含量的变化.结果表明:添加大豆磷脂能够提高牛乳蛋白乳状液的热稳定性.其中添加量为2.5 g/L时热稳定性最高,最大HCT为23.49 min.大豆磷脂可能通过与乳蛋白发生疏水相互作用改变蛋白的构象、增加乳状液的负Zeta电位、抑制巯基氧化或降低游离豌基之间的反应几率来提高乳状液的热稳定性.     

Effect of transglutaminase on the heat stability of milk: a possible mechanism

Treatment of milk with transglutaminase (TGase) affects its heat stability, but the manner in which it does so depends on whether or not the milk had been preheated before incubation and on the temperature of preheating. In raw milk, it appears that cross-link formation between the individual caseins is responsible for preventing the dissociation of kappa-casein from the micelles at pH values in the region of minimum stability. In milks preheated before incubation with TGase, denaturation of whey protein may have allowed the formation of cross-links by TGase between denatured whey proteins and the individual caseins which, in combination with cross-linking of the caseins, contributed to greatly improved heat stability at pH > 6.5. It appears from the results of this study that TGase has potential commercial applications as a food-grade additive capable of improving the heat stability of milk.     

Heat stability of plasmin (milk proteinase) and plasminogen

The effect of heating on plasmin activity in various media, including phosphate buffer pH 7.0, skim milk, blood plasma, solutions of casein and solutions of whey proteins were investigated. Plots of log residual activity v. heating time were linear at all temperatures from 63 to 143 degrees C. In buffer solutions the presence of casein led to substantial substrate protection, the Arrhenius plots being linear both in the presence and absence of casein. The activation energy, Ea, for the inactivation reaction, was 62.4 kJ/mol in buffer alone and 58.4 kJ/mol with casein present at 25 mg/ml. In skim milk, despite the presence of casein at a similar concentration, plasmin was no more stable to heat than in buffer alone, and a curved Arrhenius plot was obtained indicating a more complex inactivation mechanism. Heating in the presence of proteins having free -SH groups accelerated the inactivation of plasmin. The role of -SH groups was confirmed by experiments with added alpha-lactalbumin, in which no free -SH groups occur, and reduced carboxymethylated beta-lactoglobulin, both of which were without effect. In blood plasma, plasmin was less stable to heat than in buffer (pH 7.0) or in skim milk. Plasminogen behaved very similarly to plasmin either when activated to plasmin with urokinase before heating or when activated afterwards. A hypothesis is presented to describe the heat inactivation and denaturation of plasmin. Technologically important findings are that in skim milk plasmin was largely unaffected by pasteurization conditions and 30-40% of its activity remained even after ultra high temperature processing conditions.     

酪蛋白水解进程的研究

研究了牛乳酪蛋白的水解进程和水解产物的血管紧张素转换酶(angiotensin-I-convertingenzyme,ACE)抑制活性。研究结果表明,用胰蛋白酶对酪蛋白进行水解,在水解度相同时,所得的水解产物的ACE酶抑制活性也不一样;用高的酶/底物比水解酪蛋白,所得的水解物的ACE抑制活性较高,达88%～90%;水解时间和水解度与水解产物的ACE抑制活性之间没有相关性。     

葡萄糖对脱脂乳酪蛋白胶束稳定性的影响研究

研究了葡萄糖添加量、体系pH、热处理温度、热处理时间对脱脂牛乳酪蛋白胶束稳定性的影响。研究表明,在接近中性pH条件下,少量葡萄糖分子可降低脱脂乳酪蛋白胶束的稳定性,大量葡萄糖分子可增强其稳定性。添加葡萄糖后,在高温热处理的诱导下,酪蛋白沉淀的pH升高。随着热处理时间的延长,脱脂乳中酪蛋白胶束表现出聚集行为,致使粒径增大,浊度和沉淀率总体呈现上升趋势。该研究结论可为乳品加工提供参考依据。      

Dry heat stress stability evaluation of casein peptide mixture

During food processing, peptide-containing products often experience thermal stress, which can be voluntary heat treatments to prolong expiration date, or unwanted side-effects, e.g. local heating during powder compaction or milling. No information is currently available on the primary structure stability of peptides when heated in the dry state. Therefore, the short-term dry heat stress stability of casein hydrolysate was evaluated by exposure to temperatures of 100, 140 and 180 °C during 1, 3 and 5 min time intervals. Moreover, the impact of oxidising and reducing agents, as well as photolytic stability were assessed. Contrary to the general belief that peptides are heat-labile, based on degradation results in solution, all peptides remained stable up to 3 min at 180 °C. The influence of a reducing environment was found to be minimal, while the impact of an oxidising environment was significant. Our findings open perspectives for thermal peptide processing techniques.     

酪蛋白凝固与乳纤维蛋白溶酶活性的关系

采用分光光度计和SDS-PAGE电泳的方法,以鲜乳为原料,分别对酸沉,酶凝和钙凝的酪蛋白胶粒中的纤溶酶活性进行测定,对酪蛋白凝固与乳纤维蛋白溶酶活性的关系进行研究。结果表明,酸沉酪蛋白纤溶酶活性及总纤溶酶活性最高,分别为212.00 U/mL和250.00 U/mL,酶凝次之,Ca2+凝酪蛋白纤溶酶活性最低。     

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.     

The effect of dietary-induced changes in milk urea levels on the heat stability of milk

Two experiments were carried out to investigate the relationship between milk urea content and the heat stability of the protein in the skim milk. In experiment 1, four cows were offered a diet of grass silage with different amounts of hay and a protein concentrate. Although there were individual differences between the cows in the relationship between coagulation time and milk pH, there was a significant correlation between milk urea content and the maximum coagulation time. In experiment 2, two groups of five cows were given a basal diet of hay with a supplement of either barley or soya bean meal. These diets were exchanged weekly over a 3 week period. A significant correlation between milk urea content and maximum coagulation time was observed, and a close relationship between milk urea content and the mean urea content of the blood found. It is concluded that, although milk urea content is not the sole determinant of coagulation time, there is considerable potential for manipulating the urea content of milk in order to increase the heat stability.     

Effect of pH at heat treatment on the hydrolysis of κ-casein and the gelation of skim milk by chymosin

Skim milk was adjusted to pH values between 6.5 and 7.1 and heated at 90 °C for times from 0 to 30 min. After heat treatment, the samples were re-adjusted to the natural pH (pH 6.67) and allowed to re-equilibrate. High levels of denatured whey proteins associated with the casein micelles during heating at pH 6.5 (about 70-80% of the total after 30 min of heating). This level decreased as the pH at heating was increased, so that about 30%, 20% and 10% of the denatured whey protein was associated with the casein micelles after 30 min of heating at pH 6.7, 6.9 and 7.1, respectively. Increasing levels of κ-casein were transferred to the serum as the pH at heating was increased. The loss of κ-casein and the formation of para-κ-casein with time as a consequence of the chymosin treatment of the milk samples were monitored by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The loss of κ-casein and the formation of para-κ-casein were similar for the unheated and heated samples, regardless of the pH at heating or the heat treatment applied. Monitoring the gelation properties with time for the chymosin-treated milk samples indicated that the heat treatment of the milk markedly increased the gelation time and decreased the firmness (G^′) of the gels formed, regardless of whether the denatured whey proteins were associated with the casein micelles or in the milk serum. There was no effect of pH at heat treatment. These results suggest that the heat treatment of milk has only a small effect on the primary stage of the chymosin reaction (enzymatic phase). However, heat treatment has a marked effect on the secondary stage of this reaction (aggregation phase), and the effect is similar regardless of whether the denatured whey proteins are associated with the casein micelles or in the milk serum as nonsedimentable aggregates.     

Effect of micellar structure of casein and its modification on plasmin-induced hydrolysis

Plasmin-induced hydrolysis of casein in milk can lead to many defects including proteolysis, age gelation, and bitterness. The susceptibility of casein to plasmin can be affected by micellar structure and modification of the lysine residues on caseins. Different levels of casein modification and dissociation of the casein micelle structure were achieved through succinylation. Succinylation occurred at residues Lys7, Lys34, Lys36, Lys42, Lys83 and Lys124 in α_S1-casein; Lys80, Lys150, Lys152, Lys158 and Lys165 in α_S2-casein; Lys28, Lys29, Lys32, Lys99, Lys105, Lys107 and Lys113 in β-casein, as identified using liquid chromatography–tandem mass spectrometry. The dissociation of caseins from the casein micelle reduced steric hindrance and made the protein more readily susceptible to hydrolysis by plasmin. However, the formation of succinyl-lysine rendered β-casein unrecognisable to the substrate-binding pocket of plasmin, resulting in a non-linear decrease in level of hydrolysis because of the competitive effect of micelle dissociation.     

Addition of sodium caseinate to skim milk increases nonsedimentable casein and causes significant changes in rennet-induced gelation,heat stability,and ethanol stability

The protein content of skim milk was increased from 3.3 to 4.1% (wt/wt) by the addition of a blend of skim milk powder and sodium caseinate (NaCas), in which the weight ratio of skim milk powder to NaCas was varied from 0.8:0.0 to 0.0:0.8. Addition of NaCas increased the levels of nonsedimentable casein (from ～6 to 18% of total casein) and calcium (from ～36 to 43% of total calcium) and reduced the turbidity of the fortified milk, to a degree depending on level of NaCas added. Rennet gelation was adversely affected by the addition of NaCas at 0.2% (wt/wt) and completely inhibited at NaCas ≥0.4% (wt/wt). Rennet-induced hydrolysis was not affected by added NaCas. The proportion of total casein that was nonsedimentable on centrifugation (3,000 × g, 1 h, 25°C) of the rennet-treated milk after incubation for 1 h at 31°C increased significantly on addition of NaCas at ≥0.4% (wt/wt). Heat stability in the pH range 6.7 to 7.2 and ethanol stability at pH 6.4 were enhanced by the addition of NaCas. It is suggested that the negative effect of NaCas on rennet gelation is due to the increase in nonsedimentable casein, which upon hydrolysis by chymosin forms into small nonsedimentable particles that physically come between, and impede the aggregation of, rennet-altered para-casein micelles, and thereby inhibit the development of a gel network.     

酶解工艺对燕麦浆稳定性和糖组分的影响

淀粉酶解是解决燕麦浆稳定性下降的一种有效手段,研究了α-淀粉酶和糖化酶酶解工艺对燕麦浆稳定性的影响,确定了最优的酶解工艺条件,并对比分析了2种酶酶解后燕麦浆中糖组分的变化情况。结果表明,使用0.5%α-淀粉酶70℃酶解1.0 h和使用0.8%糖化酶40℃、p H 4.0~5.0酶解4.0 h时的燕麦浆稳定性最好,α-淀粉酶酶解的浆液酶解效果和稳定性均优于使用糖化酶的浆液。α-淀粉酶酶解后的燕麦浆中糖组分以葡萄糖和麦芽低聚糖为主;糖化酶酶解的燕麦浆中糖组分几乎全部为葡萄糖。α-淀粉酶酶解是较好的一种酶解工艺。