豆腐凝固过程的研究进展

本文论述了豆腐凝固机理、凝固剂和凝固操作研究的进展。提出了盐凝豆腐的新理论,盐中的高价阳离子与豆乳中植酸盐、柠檬酸盐发生络合反应后,引起溶液pH 值下降,从而接近蛋白质等电点,形成豆腐胶。     

模拟溶液法对豆腐凝固机理的研究

用豆乳模拟溶液与豆乳溶液对比的方法，发现在豆腐凝固过程中豆乳的ｐＨ值随温度升高而下降，证明盐凝固剂中的二价金属离子与植酸盐、柠檬酸盐等发生络合反应是豆乳ｐＨ值下降的原因，阐明了酸和盐凝固豆腐本质上的一致性。     

影响豆乳凝固特性条件的研究

前言在豆腐及豆腐制品制作过程中,豆乳的凝固工程,严重的影响着豆制品出品率和品质,为此进行了豆乳凝固与一些因素关系的研究。豆乳凝固的形成除了来源于大豆种子内部蛋白质组分等因素影响外,在相当大的程度上要受到凝固工程的外界条件的影响,我们从中选择了豆乳浓度、pH值、脂肪、豆乳的温度、离子强度等作为影响因素进行了研究,现把结果整理如下。     

豆腐凝胶强度和保水性能的研究

研究了制作条件对ＧＤＬ豆腐凝胶强度和保水性的影响。研究表明,生豆乳９５℃加热５ｍ ｉｎ左右为宜;豆腐凝胶的保水性不受ＧＤＬ 浓度的影响;随着ＧＤＬ浓度增加,豆腐凝胶强度提高,但当ＧＤＬ浓度超过０．０８Ｍ 时,豆腐凝胶强度缓慢下降。豆乳浓度增加时,豆腐凝胶强度和保水性都增强,但豆乳浓度过高时,对提高豆腐凝胶强度反而不利。提高凝固温度、增加凝固时间都可显著提高豆腐凝胶的强度。本文还讨论了豆腐凝胶的形成机理,认为高浓度豆乳中大豆蛋白变性后热胶凝不利于豆腐凝胶网络的形成。     

高浓度煮浆对豆腐品质提升的作用及其机制

为明确高浓度煮浆对豆腐品质的影响，本研究通过采取高浓度煮浆（固形物质量分数9.2%～12.2%）低浓度稀释（统一至8.0%）法制备豆腐，对豆腐的品质进行比较，并在此基础上对豆乳和豆乳凝固状态进行比较，明确引起豆腐品质差异的原因。结果发现，与固形物质量分数为8.0%的豆乳组相比，随着煮浆浓度的上升，豆腐的硬度和咀嚼性逐渐降低，其中9.2%时最大，而弹性、得率、水分质量分数和保水性差异较小。进一步研究发现，当固形物质量分数为9.2%时，豆乳蛋白粒子比例最小（26.2%）、蛋白表面疏水性最大，而平均粒径、Zeta电位绝对值和B亚基比例随煮浆浓度增加而呈上升趋势，α与α’比例呈下降趋势。分析凝乳速度变化趋势发现，煮浆浓度越高，凝固起始时间越早，凝固速率越快。结果表明，高浓度煮浆可促使豆乳蛋白聚集程度增加，能够优化豆乳及其凝胶的加工特性，这一结果为实际生产提供了一定的理论基础。     

豆腐家族展新姿

1　蔬菜豆腐一般豆腐用天然卤水作凝固剂。天然卤水是从海水浓缩而成的粗盐中提取的 ,很难取得。为了提高豆乳的凝固效率 ,人们普遍使用海水高度精制分离而成的氯化镁和工业生产的硫酸钙、氯化钙。也就是说 ,目前制成的豆腐大多数都不能说是 1 0 0 %的天然食品。日本一家公司研制成功用无头甘蓝及洋白蔬菜汁作凝固剂的色拉豆腐 ,完全不用添加剂。钙能使豆乳凝固 ,而植物所含的水溶性钙盐 ,也具有凝固作用。作为豆乳凝固剂使用的食物有 :无头甘蓝、元白菜、芥菜、小松菜、辣椒叶、苏紫叶等。其中辣椒叶的水溶性钙盐含量最高 ,每 1 0 0ｇ豆乳…     

碱性蛋白酶Alcalase凝固豆乳过程的流变学特性变化

在Alcalase凝固豆乳过程中pH值下降程度小于0.2,SDS-PAGE电泳表明大豆蛋白质各亚基在凝固过程中均降解为8～14 Ku的肽段,说明大豆蛋白的部分降解可能是Alcalase凝固豆乳的根本原因.对Alcalase与盐类凝固剂氯化镁和氯化钙凝固豆乳的主要流变学性质比较发现,Alcalase凝固豆乳的硬度比盐类凝固剂大,凝胶强度比盐类凝固剂低,粘附性比盐类凝固剂高10倍以上,使凝胶较粘,乳清不易排出.     

碱性蛋白酶Alcalase凝固豆乳的物性特征

在Alcalase凝固豆乳过程中pH下降程度小于0.2,SDS-PAGE电泳表明,大豆蛋白质各亚基在凝固过程中均降解为8～14kDa的肽段,说明大豆蛋白的部分降解是Alcalase凝固豆乳的根本原因。对Alcalase与盐类凝固剂氯化镁和氯化钙凝固豆乳的主要流变学性质比较发现,Alcalase凝固豆乳的硬度比盐类凝固剂大,凝胶强度比盐类凝固剂低,粘附性比盐类凝固剂高10倍以上,使凝胶较粘,乳清不易排出。     

蛋白酶对豆乳的凝固作用研究

以凝固时间和凝固强度为指标,考察了13种蛋白酶的豆乳凝固能力,发现胃蛋白酶、凝乳酶和风味蛋白酶没有豆乳凝固活性,Alcalase、木瓜蛋白酶和菠萝蛋白酶具有较强的豆乳凝固能力。各种蛋白酶的最适豆乳凝固温度范围比最适水解温度范围高20℃左右,豆乳凝固能力随pH的降低而增强。40℃和70℃下用蛋白酶作凝固剂得到的豆乳凝块破断强度最大为105.2Pa和148.2Pa,小于用氯化钙作凝固剂的豆乳凝块的破断强度(40℃,142.4Pa和70℃,198.4Pa),而且用酶凝固的豆乳凝块黏度较大,乳清不易排出。Alcalase具有较强的豆乳凝固能力,而且形成的豆乳凝块质地细腻,无苦味,具有一定的实际应用价值。     

点浆条件对酸浆豆腐品质的影响

为探究点浆时豆乳温度、点浆后蹲脑温度和蹲脑时间对酸浆豆腐品质的影响,提高酸浆豆腐品质,本研究利用质构分析、SDS-PAGE和低场核磁等技术分析了豆腐品质、豆乳中蛋白凝固过程、大豆蛋白各亚基含量和水分分布的变化规律。结果表明,点浆时豆乳温度对豆乳沉淀中蛋白和大豆蛋白各亚基含量无显著影响,但对豆腐品质和水分分布有较大影响。当点浆时豆乳温度为75℃时,豆腐的硬度最小,而其弹性、得率、含水量、保水性、感官得分和T₂₁弛豫时间最大。蹲脑温度和蹲脑时间对豆腐品质、豆乳沉淀中蛋白含量和大豆蛋白各亚基含量影响显著,当蹲脑温度75℃和蹲脑时间20 min时,豆腐硬度最小为15.80 N,而弹性、得率、含水量、保水性和T₂₁弛豫时间最佳,分别为5.15 mm、241.3%、76.36%、65.68%、43.29 ms,且豆腐品质豆乳沉淀中蛋白含量最佳。当点浆时豆乳温度为75℃、蹲脑温度为75℃和蹲脑时间20 min时,豆腐品质和感官得分最优。     

Effects of pH, calcium chloride, and chymosin concentration on coagulation properties of abnormal and normal milk

Individual Holstein cow milk samples were selected for good and poor chymosin-coagulation characteristics. The effect of pH adjustment, addition of .02% calcium chloride, and variation in chymosin concentration on coagulation properties of good and poor coagulating samples was evaluated. Pooling 50% good and 50% poor coagulating samples did not improve the average coagulation properties of the poor samples. Reducing milk pH to 6.3 caused a significant decrease in coagulation time but a less marked increase in curd firmness. The greatest increase in curd firmness was obtained by a combination of reducing milk pH, addition of .02% calcium chloride, and reducing chymosin concentration. High-chymosin concentration at reduced pH decreased coagulation time without substantially increasing curd firmness. Curd disintegration was more apparent at high-chymosin concentration in the poor coagulating samples.     

Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins

Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7–5.3) and pepsin concentration (0.110–2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ～6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ～73% to ～33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.     

Utilization of soy yoghurt in tarhana production

Tarhana, a traditional fermented food made from a mixture of white wheat flour and yoghurt, is widely consumed in Turkey. Tarhana samples with cow's milk yoghurt, soy milk yoghurt, and a blend of cow's and soy milk yoghurt were produced and evaluated for pH, viscosity, color, and sensory properties. Acid production was slower in soy milk-added samples, and increasing yoghurt amount in tarhana significantly contributed to acidity of the samples (P<0.05). Soy milk-added samples had higher viscosity compared with cow's milk yoghurt-added samples (P<0.05). Soy milk yoghurt bleached the wheat flour and therefore these samples appeared whiter than the cow's milk yoghurt-added samples (P<0.05). Panelists could not detect any beany-off flavor in soy milk yoghurt-added samples and gave similar scores to soy milk yoghurt-added tarhanas for all sensory properties     

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.     

Short communication: Prediction of milk coagulation and acidity traits in Mediterranean buffalo milk using Fourier-transform mid-infrared spectroscopy

Milk coagulation and acidity traits are important factors to inform the cheesemaking process. Those traits have been deeply studied in bovine milk, whereas scarce information is available for buffalo milk. However, the dairy industry is interested in a method to determine milk coagulation and acidity features quickly and in a cost-effective manner, which could be provided by Fourier-transform mid-infrared (FT-MIR) spectroscopy. The aim of this study was to evaluate the potential of FT-MIR to predict coagulation and acidity traits of Mediterranean buffalo milk. A total of 654 records from 36 herds located in central Italy with information on milk yield, somatic cell score, milk chemical composition, milk acidity [pH, titratable acidity (TA)], and milk coagulation properties (rennet coagulation time, curd firming time, and curd firmness) were available for statistical analysis. Reference measures of milk acidity and coagulation properties were matched with milk spectral information, and FT-MIR prediction models were built using partial least squares regression. The data set was divided into a calibration set (75%) and a validation set (25%). The capacity of FT-MIR spectroscopy to correctly classify milk samples based on their renneting ability was evaluated by a canonical discriminant analysis. Average values for milk coagulation traits were 13.32 min, 3.24 min, and 39.27 mm for rennet coagulation time, curd firming time, and curd firmness, respectively. Milk acidity traits averaged 6.66 (pH) and 7.22 Soxhlet-Henkel degrees/100 mL (TA). All milk coagulation and acidity traits, except for pH, had high variability (17 to 46%). Prediction models of coagulation traits were moderately to scarcely accurate, whereas the coefficients of determination of external validation were 0.76 and 0.66 for pH and TA, respectively. Canonical discriminant analysis indicated that information on milk coagulating ability is present in the MIR spectra, and the model correctly classified as noncoagulating the 91.57 and 67.86% of milk samples in the calibration and validation sets, respectively. In conclusion, our results can be relevant to the dairy industry to classify buffalo milk samples before processing.     

Short communication: Influence of composite casein genotypes on additive genetic variation of milk production traits and coagulation properties in Holstein-Friesian cows

The aim of the study was to quantify the effects of composite β- and κ-casein (CN) genotypes on genetic variation of milk coagulation properties (MCP); milk yield; fat, protein, and CN contents; somatic cell score; pH; and titratable acidity (TA) in 1,042 Italian Holstein-Friesian cows. Milk coagulation properties were defined as rennet coagulation time (RCT) and curd firmness (a₃₀). Variance components were estimated using 2 animal models: model 1 included herd, days in milk, and parity as fixed effects and animal and residual as random effects, and model 2 was model 1 with the addition of composite β- and κ-CN genotype as a fixed effect. Genetic correlations between RCT and a₃₀ and between these traits and milk production traits were obtained with bivariate analyses, based on the same models. The inclusion of casein genotypes led to a decrease of 47, 68, 18, and 23% in the genetic variance for RCT, a₃₀, pH, and TA, respectively, and less than 6% for other traits. Heritability of RCT and a₃₀ decreased from 0.248 to 0.143 and from 0.123 to 0.043, respectively. A moderate reduction was found for pH and TA, whereas negligible changes were detected for other milk traits. Estimates of genetic correlations were comparable between the 2 models. Results show that composite β- and κ-CN genotypes are important for RCT and a₃₀ but cannot replace the recording of MCP themselves.     

Calcium Fortification of Soy Milk with Calcium-Lecithin Liposome System

Calcium ion was enveloped with a membrane system before addition to soy protein to prevent soy protein from being coagulated and precipitated by calcium ion. Soy lecithin was first sonicated in calcium salt solution to envelop the calcium ion with a liposomal structure composed of lecithin. Then, the calcium-lecithin liposomes were added to soy protein solution. Precipitation and coagulation were not observed in this soy protein-lecithin liposome system containing 60 mM Ca²⁺. By this method, it was possible to prepare calcium fortified soy milk containing more calcium (120 mg/100g) than in cow's milk. These results suggest that this calcium-lecithin liposome system is useful for calcium fortification of soy milk.     

姜汁凝乳影响因素的研究

生姜经清洗、打浆榨汁后可制得姜汁,将姜汁以2%的比例添加到经杀菌和冷却处理的鲜乳中可制成具有凝乳状态的姜汁牛乳。为了探讨姜汁凝乳的影响因素,文中研究了温度、pH值、牛乳蛋白质和盐类等对姜汁凝乳的影响。结果表明,温度和pH值是影响姜汁凝乳的关键外部因素。牛乳蛋白质、CaCl2、NaCl是凝乳促进物。     

Short communication: Fourier-transform mid-infrared spectroscopy to predict coagulation and acidity traits of sheep bulk milk

Sheep milk is mainly transformed into cheese; thus, the dairy industry seeks more rapid and cost-effective methods of analysis to determine milk coagulation and acidity traits. This study aimed to assess the feasibility of Fourier-transform mid-infrared spectroscopy to determine milk coagulation and acidity traits of sheep bulk milk and to classify milk samples according to their renneting capacity. A total of 465 bulk milk samples collected in 140 single-breed flocks of Comisana (84 samples, 24 flocks) and Sarda (381 samples, 116 flocks) breeds located in Central Italy were analyzed for coagulation properties (rennet coagulation time, curd firming time, and curd firmness) and acidity traits (pH and titratable acidity) using standard laboratory procedures. Fourier-transform mid-infrared spectroscopy prediction models for these traits were built using partial least squares regression analysis and were externally validated by randomly dividing the full data set into a calibration set (75%) and a validation set (25%). The discriminant capacity of the rennet coagulation time prediction model was determined using partial least squares discriminant analysis. Prediction models were more accurate for acidity traits than for milk coagulation properties, and the ratio of prediction to deviation ranged from 1.01 (curd firmness) to 2.14 (pH). Moreover, the discriminant analysis led to an overall accuracy of 74 and 66% for the calibration and validation sets, respectively, with greater sensitivity for samples that coagulated between 10 and 20 min and greater specificity to detect early-coagulating (<10 min) and late-coagulating (20–30 min) samples. Results suggest that Fourier-transform mid-infrared spectroscopy has the potential to help the dairy sheep industry identify milk with better coagulation ability for cheese production and thus improve milk transformation efficiency. However, further research is needed before this information can be exploited at the industry level.     

Genetic parameters for milk coagulation properties in Estonian Holstein cows

The objective of this study was to estimate heritabilities and repeatabilities for milk coagulation traits [milk coagulation time (RCT) and curd firmness (E₃₀)] and genetic and phenotypic correlations between milk yield and composition traits (milk fat percentage and protein percentage, urea, somatic cell count, pH) in first-lactation Estonian Holstein dairy cattle. A total of 17,577 test-day records from 4,191 Estonian Holstein cows in 73 herds across the country were collected during routine milk recordings. Measurements of RCT and E₃₀ determined with the Optigraph (Ysebaert, Frepillon, France) are based on an optical signal in the near-infrared region. The cows had at least 3 measurements taken during the period from April 2005 to January 2009. Data were analyzed using a repeatability animal model. There was substantial variation in milk coagulation traits with a coefficient of variation of 27% for E₃₀ and 9% for the log-transformed RCT. The percentage of variation explained by herd was 3% for E₃₀ and 4% for RCT, suggesting that milk coagulation traits are not strongly affected by herd conditions (e.g., feeding). Heritability was 0.28 for RCT and 0.41 for E₃₀, and repeatability estimates were 0.45 and 0.50, respectively. Genetic correlation between both milk coagulation traits was negligible, suggesting that RCT and E₃₀ have genetically different foundations. Milk coagulation time had a moderately high positive genetic (0.69) and phenotypic (0.61) correlation with milk pH indicating that a high pH is related to a less favorable RCT. Curd firmness had a moderate positive genetic (0.48) and phenotypic (0.45) correlation with the protein percentage. Therefore, a high protein percentage is associated with favorable curd firmness. All reported genetic parameters were statistically significantly different from zero. Additional univariate random regression analysis for milk coagulation traits yielded slightly higher average heritabilities of 0.38 and 0.47 for RCT and E₃₀ compared with the heritabilities of the repeatability model.