Changes in Size and Composition of Protein Aggregates on Heating Reconstituted Concentrated Skim Milk at 120oC

Treatment at 120^oC caused initial transfer of protein from the serum and small micelle fraction to the medium-sized micelle fraction. More extensive heating caused transfer of protein from the medium fraction to the large-sized micelle fraction, and the serum fraction as 'soluble’casein. Protein composition of large-sized micelle fraction remained essentially constant, but for the other fractions it changed throughout the heating period. Experiments with heated milks that had been made free of colloidal calcium phosphate (CCP) showed that a proportion of the calcium phosphate in milk, whether indigenous or precipitated by heating, became less soluble. This‘insoluble’calcium phosphate was possibly involved in joining the smaller protein particles into larger-sized aggregates.     

Use of Electrodialysis to Improve the Protein Stability of Frozen Skim Milks and Milk Concentrates

Removal of calcium from skim milk by electrodialysis was logarithmically related to the extent of demineralization (log% calcium removed = 0.18% ash removed + 0.53). This removal of calcium caused a logarithmic dissociation of micellar to serum (sub-micellar) casein (log% total casein in serum = 0.020% calcium removed + 0.70). Increases in protein stability for skim milk and concentrated skim milk were related to this dissociation of micellar casein, and the initial degree of dissociation persisted throughout frozen storage. Protein stability was > 53 wk at –8°C when 80% of the casein in unconcentrated skim milk was dissociated. Concentrated skim milk was stable for > 30 wk at –8°C when 45% of the casein was dissociated. Control samples (7–9% casein dissociated) were stable for only 4–8 wk at –8°C.     

In vitro Digestibility of Whey Protein/K-Casein Complexes Isolated from Heated Concentrated Milk

The disulfide-linked complex of _K-casein and whey proteins that forms when concentrated milk is heated was isolated by centrifugation and column chromatography on Sephacryl S-1000. The rate of hydrolysis of β-lactoglobulin and _K-casein in the complex and the reduced and carboxymethylated components of the complex were measured by polyacrylamide gel electrophoresis. The rates of hydrolysis at pH 2.0 (pepsin) and pH 8.0 (trypsin and chymotrypsin) were similar for _k-casein in the complex and its reduced form. β-Lactoglobulin hydrolysis was faster for the reduced complex than for the complex which was much faster than for the native protein for all three enzymes. The results suggest that heating milk increases the digestibility of whey proteins, despite the formation of large protein complexes between the whey proteins and _K-casein.     

Breaking Stress and Syneresis of Rennin Curds from Reconstituted Skim Milk Frozen Concentrate

Skim milk was concentrated to 17–18% solid non-fat (SNF) by reverse osmosis and then stored at ?20 ± 2°C. Samples were thawed, reconstituted, and manufactured into rennin curds. Breaking stress and syneresis of curds increased with calcium or magnesium, and decreased as pH values increased. Breaking stress and syneresis of curds prepared from skim milk reconstituted from a frozen concentrate (SMRFC) were lower than of curds from raw or heated skim milk. The formation of curds results from a three-dimensional network of protein chain and clusters, and the addition of calcium chloride resulted in partial disintegration of the micelle, producing larger particles. Curds from raw milk showed more protein aggregatiom curds from SMRFC, and also had a more open matrix.     

乳脂乳球菌CH5生长特性的研究

本文以农家干酪生产使用的乳脂乳球菌(Lactococcus lactis ssp．cremoris)为研究对象，从细菌总数(CFU)，pH值和滴定酸度三个指标对本实验室分离得到的乳脂乳球菌CH5在脱脂乳中的生长特性和4℃下保存过程中的活力变化进行了研究。结果表明乳脂乳球菌CH5在脱脂乳中的延滞期短，能较快进入对数生长期，6h时进入稳定期，活菌数达109CFU／ml，Tdg约为1．03h，Tda约为2．87h，这表明乳脂乳球菌CH5细胞增长速度快于产酸速率。乳脂乳球菌CH5在4℃下保存时，稳定性较好，前11d活菌数和最大产酸速率(μacid)基本保持不变，此后逐渐下降，表明CH5发酵剂在前11d的活力比较稳定，可以确定生产农家干酪的CH5母发酵剂的最佳传代时间为11d，冷却保藏第3d为最佳生产使用时间。     

Extraction and Purification of Short-chain Fatty Acids from Fermented Reconstituted Skim Milk Supplemented with Inulin

Short-chain fatty acids (SCFAs) acetate, propionate, butyrate and lactate were determined in 12 % reconstituted skim milk (RSM) and RSM supplemented with inulin (RSMI). The fermentation was performed with Bifidobacterium animalis subsp. lactis (BB 12) and Lactobacillus rhamnosus LGG ATCC 53013. Fermentation culture activities produced substantial amounts of SCFAs, which were detected and quantitated using a HPLC-UV technique. Using HPLC-UV, we were able to detect low concentrations of lactate and SCFAs from fermented samples; lactate, acetate, propionate and butyrate were detected at 10.10, 12.06, 14.80 and 18.06 μg/mL, respectively. The retention time of all SCFAs and lactic acid were similar to the standard quality control (±0.05), and average recovery ranged between 89.73 and 91.03 %. The experimental conditions and sample preparation were applied to preparative HPLC to isolate and purify SCFAs with concentration range between 0.09 and 2.86 mg/mL. The purity of extracted SCFAs was confirmed using atmospheric pressure chemical ionization/mass spectrometry by determining the molecular masses of target purified compounds. The scaled up validated analytical HPLC-UV method will further enhance and improve the use of this approach to produce purified large-scale SCFAs.     

脱脂奶粉勾兑牛奶酒工艺中pH值的影响

探讨了以脱脂奶粉为原料勾兑奶酒的工艺条件,主要研究了牛奶蛋白在乙醇存在下影响其稳定性的一个重要因素——pH值.实验表明,在以脱脂奶粉为原料勾兑奶酒时,酒精应该先进行适量的稀释再加入到复配奶中;随着复配奶pH值的升高,其牛奶蛋白的乙醇稳定性也随之升高.复配奶的pH值决定着勾兑奶酒的pH值,其最适pH值为6.5～7.3,但是酒精的pH对于勾兑奶酒的pH并没有决定性的影响.实验还研究了勾兑奶酒在储存过程中其体系的pH值的变化.     

Accelerated Tests for Evaluating Protein Stability in Frozen Skim Milk

Skim milk was diafiltered (DF) to remove about 90% of the lactose, and major salts and vitamins were restored to normal levels. Protein stability at –9°C was monitored using two accelerated test procedures. One involved DF skim milk containing a normal level of carbohydrate and 3.9 times the normal level of calcium, and the other involved DF skim milk containing a normal level of calcium and one-half the normal level of carbohydrate. Both approaches resulted in casein instability in about 1 wk at -9°C. These test procedures should be useful for rapidly assessing the effects of additives or processing on the stability of casein in frozen skim milk.     

超滤在生产浓缩乳蛋白类产品中的应用

超滤技术是一种先进的膜分离技术,其作为一种分离和浓缩的工具已经被广泛应用于乳品工业。本文对超滤技术进行了概述,综述了其在生产乳蛋白浓缩物及浓缩乳清蛋白产品中的研究及应用现状,同时对如何通过超滤过程中条件的选择以提高膜过滤性能进行了阐述,并且对该领域的未来发展趋势做了展望。     

Skim Milk Whey Cryoconcentration and Impact on the Composition of the Concentrated and Ice Fractions

The present work is a continuation of our previous study on the cryoconcentration of whole milk whey. The aim of the present work was to study a cryoconcentration procedure of skim milk whey and to compare the results with those obtained using whole milk whey. In the present study, skim whey was cryoconcentrated at four stages. It was found that by increasing the cryoconcentration stage, total dry matter content of the concentrated fraction increased, while it decreased in the ice fraction only until the third stage. Total dry matter content in the concentrated fraction reached an average value of 35% (w/v) at the fourth stage. The ratio of total protein to total dry matter remained constant in both fractions independently of the cryoconcentration stage. Results showed that the fat matter is an important factor for protein and lactose distribution between the concentrated and the ice fractions. Mathematical equations were developed to help the optimization of the process. Results showed that three cryoconcentration stages are optimal.     

Date Bars Fortified with Soy Protein Isolate and Dry Skim Milk

Fortification of plain and chocolate coated date bars with soy protein isolate (SPI) and dry skim milk (DSM) in different proportions (1.5% SPI + 10.5% DSM; 3.0% SPI + 7.0% DSM and 4.5% SPI + 3.5% DSM) resulted in an increase of protein, fat, fiber, ash, Na, K, Ca, Mg, P, Zn, and all the essential amino acids. The chemical score increased from 48 to a range of 83–95. The in vitro protein digestibility (IVPD) values (77.8–81.8%) and calculated protein efficiency ratios (C-PERs) of the fortified bars (2.40–2.47) were close to the IVPD (90.0%) and C-PER (2.50) of ANRC-casein. In general, sensory evaluation indicated no difference in the control and fortified date bars, which were stored up to six months. 1.5% SPI + 10.5% DSM was the optimum level of supplementation in terms of nutritive benefit.     

Microporous Ultrafiltration of Skim Milk

Membranes with porosities of 100 and 200 nm were used to obtain a 4:1 milk volume reduction. Average micelle diameters determined from electron micrographs were 46 nm (permeate) and 52 nm (permeate) for the 100-nm-pore fractions and 46 and 55 nm for the 200-nm-pore fractions. The calculated average micellar volumes of the retentate fractions were about twice those of the corresponding permeate fractions. Casein-whey ratios were 0.7-0.9 in the permeates and 5.0-7.7 in the retentates. Higher α_s2- and lower β-casein contents were found in the permeate micelles than in the retentates.     

菊粉对Labneh浓缩酸乳微观结构和质构的影响

研究了添加菊粉对脱脂乳制备的Labneh(一种浓缩酸乳)物理和感官性质的影响。将市售菊粉按不同比例( 1%、3%、5 %和7% )加入至复配脱脂乳粉(reconstitutedskimmilkpowder,RSMP)制作Labneh样品。通过对酸乳宏观和微观结构的分析发现,添加菊粉后,Labneh凝胶结构和凝胶强度都有所改善。实验表明,与未加菊粉的酸乳相比,含有脂肪替代品的酸乳的微观结构比较均匀、疏松且开放度较高。感官评定的结果表明,5 %菊粉复配脱脂乳粉制作的酸乳与全脂牛乳制作的Labneh相比具有相似的特征,对全脂牛乳制作的样品和添加3%、5 %、7%菊粉的样品的统计分析结果没有显著的差别     

热处理对生鲜乳及复原乳蛋白质体外消化特性的影响

通过体外模拟消化的方法对不同热处理的牛乳样品进行检测,研究热处理对鲜牛乳及复原乳营养价值的影响。通过聚丙烯酰氨凝胶电泳、尺寸排阻高效液相色谱、扫描电镜及质谱分析等多种方法进行检测。结果表明:经过胃液消化后,牛乳中的部分蛋白被消化,其中酪蛋白消化最为明显,鲜牛乳中酪蛋白在胃液中的消化水平高于其他热处理样品;而经过肠液消化后,牛乳中蛋白消化完全,生成游离氨基酸及小肽。乳中蛋白质经消化主要生成分子质量低于1 500 Da的肽段,易于人体消化吸收。通过扫描电镜可以看出,热处理程度越大,乳蛋白变性,发生聚集现象,粒径增大,其中加热复原乳的粒径较其他乳制品大;通过质谱检测及高效液相色谱分析,热处理程度越大,蛋白质美拉德反应程度升高。     

全脂乳与脱脂乳挥发性风味成分对比分析

HS-SPME结合GC/MS对比分析全脂乳(WM)和脱脂乳(SM)的挥发性风味成分。结果表明,50/30μm CAR/PDMS/DVB萃取纤维萃取得到的全脂乳、脱脂乳的挥发性风味物质的种类与65μm PDMS/DVB相当,即:全脂乳11种,脱脂乳8种,只是在萃取含量上有差异。因此,相对于65μm PDMS/DVB而言,50/30μm CAR/PDMS/DVB较适合乳风味挥发性物质的萃取。HS-SPME结合GC-O对比分析全脂乳和脱脂乳的呈香组分结果显示,脱脂乳中未发现香气强度较高的呈香成分。而在全脂乳中辛酸(清香)、癸酸(油脂味)香气强度较高。这些结果能为全脂乳和脱脂乳的风味物质组成提供有用信息。     

蛋白酶水解脱脂乳培养基的优化

为减少进口发酵剂的使用,降低国内乳制品的生产成本,国内需要自主研发发酵剂,水解脱脂乳培养基的优化是发酵剂制备过程中的一个重要环节。为制备有利于乳酸菌生长的水解脱脂乳培养基,本研究首先以发酵乳杆菌V9为试验菌种,筛选出水解脱脂乳最适的蛋白酶,再通过单因素与正交实验对制备脱脂乳水解液的温度、时间、pH值、加酶量进行优化。结果表明,最适水解条件为:在p H为8.5的12%脱脂乳中按25 U/mL接入碱性蛋白酶,55℃水解1 h,以此水解脱脂乳为培养基培养乳酸菌活菌数达到2.5×109CFU/mL,较脱脂乳培养基中的活菌数提高了1.74倍。利用此优化培养基培养鼠李糖乳杆菌grx19和植物乳杆菌S7,得到的活菌数分别较脱脂乳培养基提高了1.50和1.62倍。     

Enzyme-catalyzed Production of Oligopeptides from Skim Milk

With the aim of preparing high nutritional dietary supplements containing oligopeptides from skim milk, an Aspergillus protease, papain and pepsin, separately or in association, were used in different enzyme:substract ratios (1%, 2% and 4%). The fractionation of peptides according to their size was used to evaluate the nutritional quality of protein hydrolysates. The hydrolysates were fractionated by high-performance liquid chromatography in size-exclusion mode and the rapid method of Correct Fraction Area was used for the quantification. In general, the isolated action of the enzymes produced better peptide profiles than their simultaneous use, especially in terms of a higher amount of oligopeptides, which reached 56%.     

Enzyme-catalyzed Production of Oligopeptides from Skim Milk

With the aim of preparing high nutritional dietary supplements containing oligopeptides from skim milk, an Aspergillus protease, papain and pepsin, separately or in association, were used in different enzyme:substract ratios (1%, 2% and 4%). The fractionation of peptides according to their size was used to evaluate the nutritional quality of protein hydrolysates. The hydrolysates were fractionated by high-performance liquid chromatography in size-exclusion mode and the rapid method of Correct Fraction Area was used for the quantification. In general, the isolated action of the enzymes produced better peptide profiles than their simultaneous use, especially in terms of a higher amount of oligopeptides, which reached 56%.