毛细管电泳法测定不同体细胞数的原料乳的蛋白组成及含量

为明确不同体细胞数(Somatic Cell Count,SCC)数量的原料乳蛋白质组成的变化规律。本研究首先建立了乳蛋白的毛细管电泳分析方法,并采集了不同SCC数量原料乳,分析其乳蛋白成分及含量的差异。结果表明,随着SCC数量的增加,原料乳中酪蛋白的占比显著降低(p<0.05),乳清蛋白比率相对增加;酪蛋白中β-酪蛋白B和β A1-酪蛋白的含量显著下降(p<0.05),水解为新的小片段。毛细管电泳技术能够更准确的分析原料乳蛋白质的组成及变化规律,有利于乳品企业在原料乳收购时进行更深一层的品质分析。     

原料乳体细胞数对UHT乳贮藏期间品质的影响

为了探究体细胞数与蛋白质、脂肪水解的关系及其对凝胶和脂肪上浮等劣变的影响。试验采集30,80万个/mL两组不同体细胞数的原料乳,制成UHT乳,20℃下做180 d的贮藏期试验,每隔30 d测定酶活、蛋白含量、游离脂肪酸、脂肪球粒径等指标。结果显示,经UHT处理,两组产品中纤维溶酶和脂肪酶基本丧失活性,贮藏期间无显著性变化(P0.05)。各酪蛋白组分发生显著水解(P0.05),κ-酪蛋白和αS2-酪蛋白降幅高达90%以上,均未发生蛋白间的相互交联,体系的黏度并无显著变化(P0.05)。脂肪球粒径在第60天迅速增加,之后无明显变化;高体细胞数组脂肪球表面的蛋白膜崩解,脂肪球聚集,从贮藏期第120天开始出现脂肪上浮;而低体细胞数组脂肪球膜蛋白也发生水解,未出现脂肪上浮。本研究表明在现有工艺条件下UHT乳出现的主要问题是脂肪上浮而非蛋白凝胶,将原料乳体细胞数控制在30万个/mL的水平,可以有效防止产品贮藏期内发生脂肪上浮,为UHT乳的品质控制提供了理论依据。     

不同体细胞数原料乳对契达干酪成熟过程中蛋白水解的影响

为探讨原料乳中体细胞数(SCC)对契达干酪成熟过程中蛋白质水解的影响,选择SCC分别是5.6×104(LSCC)、48.8×104(MSCC)、476.1(HSCC)×104 个/mL的原料乳制作契达(cheddar)干酪,得到LSCC、MSCC、HSCC组干酪,并对各组干酪成熟过程中蛋白质水解的各项指标进行了测定.结果显示3组干酪成熟过程中蛋白的水解产物不同,LSCC组干酪品质更好.     

原料乳中体细胞数与UHT乳中酪蛋白成分的关系研究

研究了原料乳中体细胞数与15批次UHT乳样本中酪蛋白成分之间的关系。将原料乳巴氏杀菌后进行超高温处理。分别于8,30,60,90和120 d采集贮藏于室温条件下的UHT乳样本,并使用高效液相色谱法对酪蛋白成分进行分析。体细胞数范围1.97×105～8×105 mL-1。体细胞数与原料乳或UHT乳中的κ-酪蛋白质量浓度之间没有相关性(P<0.05)。原料乳中αs2-酪蛋白和β-酪蛋白与体细胞数呈负相关(P<0.05)。UHT乳中,αs1-酪蛋白(P<0.05)和β-酪蛋白(P<0.05)与体细胞数在贮藏第8天呈负相关,αs2-(P<0.01)与体细胞数在贮藏第60天呈负相关。结果表明,原料乳中体细胞数较高与β-酪蛋白和αs-酪蛋白的大量水解有关,并且可能导致UHT乳在贮藏期内出现质量问题。     

原料乳中蛋白质与脂肪比例对 Mozzarella干酪品质的影响

采用3×3拉丁方试验设计,3个奶酪槽中原料乳的蛋白质与脂肪质量比分别为1∶1,1.2∶1,1.3∶1(通过添加脱脂干奶粉调整蛋白质含量)。研究蛋白质与脂肪比例对Mozzarella干酪的品质的影响。结果表明,随着原料乳中蛋白质脂肪比例的增加,干酪的含水量、油脂析出性显著降低(P<0.05),干酪的弹性显著升高(P<0.05),蛋白质与脂肪比例对Mozzarella干酪的蛋白质水解没有显著的影响。     

原料乳体细胞数与纤溶酶活性的相关性研究

随机检测了102个原料乳样品的体细胞数(SCC)与纤溶酶(PL)活性。结果显示,SCC与PL活性之间存在一定的正相关。以总体为样本,PL活性与SCC线性相关系数为0.7367;SCC3×105mL-1样本的相关系数达到0.7928,SCC5×105mL-1样本的相关系数达到0.8616。SCC在3×105mL-1时SCC与PL无相关性。人工调配的含不同SCC的11个原料乳样,采用荧光染色法进行SCC计数,其理论折算数和实际数配对两处理t检验,结果t=1.2787,P=0.2299,差异不显著。线性回归分析得:Y=1.0055X-3.4473(R2=0.9996)。11个人工调配SCC原料乳样品的PL活性随SCC数的增加而明显增加(R2=0.9619)。通过超声波处理模拟SCC自溶状态,原料乳中体细胞经超声波处理后,PL活性明显增加。     

不同体细胞数原料乳对UHT贮存期间蛋白和脂肪水解的影响

不同体细胞数(21.4×104mL-1,75.8×104mL-1,118.1×104mL-1和216.2×104mL-1)原料乳生产的4组UHT乳在37℃贮存84d,对其贮存期间的蛋白水解及脂肪水解进行研究。结果表明,4组UHT乳贮存期间的蛋白水解速率无显著性差异(P>0.05),原料乳体细胞数并未对蛋白水解造成影响;4组UHT乳贮存期间的脂肪水解速率具有显著性差异(P<0.005),原料乳体细胞数与脂肪水解速率间存在极明显的正相关(R=0.9886,P<0.05)。     

凝乳酶对牦牛乳硬质干酪成熟期间蛋白降解的影响

以新鲜牦牛乳为原料,采用小牛皱胃酶、木瓜蛋白酶和微生物凝乳酶制作硬质干酪,探讨凝乳酶种类对牦牛乳硬质干酪成熟期间蛋白质降解的影响。结果表明:三种凝乳酶牦牛乳硬质干酪成熟过程中,不同凝乳酶牦牛乳硬质干酪在成熟期间蛋白质降解能力存在较大差异,总氮（TN）、p H4.6水溶性氮（p H4.6-SN/TN）、12%的三氯乙酸氮（12%TCA-N/TN）、5%磷钨酸氮（5%PTA-N/TN）含量、游离氨基酸均随成熟时间延长不同程度的增加,蛋白氮和酪蛋白氮逐渐降低,多肽氮呈先升高后下降趋势,且微生物凝乳酶降解牦牛乳硬质干酪蛋白能力显著（p<0.05）高于木瓜蛋白酶和小牛皱胃酶。      

原料乳体细胞数对硬质干酪蛋白水解及风味与质构品质的影响

干酪成熟过程中蛋白质的水解是风味形成的重要途径。在中国自由放养和小规模奶牛养殖仍占有一定比例,这其中的手动挤奶方式会导致原料乳中含有一定数量具有蛋白酶活性的体细胞,影响干酪成熟及风味。目前关于体细胞通过蛋白水解作用对脱脂干酪挥发性风味物质的影响尚未明确。本研究选取3 种不同体细胞数的原料乳,解析体细胞的细胞组成并制作脱脂干酪,在90 d的干酪成熟期中测定干酪的蛋白酶活性、蛋白水解水平、成熟后干酪的挥发性风味物质组成及其质构特性,评价不同体细胞数、干酪的蛋白质水解程度及其对干酪风味和品质的影响。结果表明：体细胞数越高的干酪蛋白酶活性越高,αs2酪蛋白水解程度越高,对干酪风味和质构也有不同程度的影响;使用体细胞数适量增加的原料乳（10×104～30×104 个/mL）有利于干酪风味的形成;中等体细胞数组中干酪特征风味物质3-羟基-2-丁酮含量最高（34.57%）,是低体细胞数组含量（28.64%）的1.2 倍、高体细胞数组含量（20.72%）的1.6 倍;原料乳中过多体细胞（多于86×104 个/mL）则会导致干酪过度水解并产生不良风味,高体细胞数组中检测到会引起不良风味的风味物质如辛醛、壬醛、仲辛酮、己醛。     

瑞士乳杆菌对契达干酪成熟期间所产ACE抑制肽的影响及其消化稳定性

为明确瑞士乳杆菌对契达干酪中血管紧张素转换酶（angiotensin-converting enzyme,ACE）抑制肽活性的影响,以蛋白质水解度和ACE抑制率为指标,与干酪乳杆菌组、鼠李糖乳杆菌组和空白组干酪进行对照,研究瑞士乳杆菌对干酪成熟期间蛋白质水解及ACE抑制活性的影响,并对ACE抑制活性最高时期的干酪进行消化稳定性研究。结果表明：成熟期间,3 组益生菌干酪的活菌数无明显差异（P＞0.05）,但均高于空白组;益生菌干酪的蛋白质水解程度和ACE抑制活性显著高于空白组（P＜0.05）,其中瑞士乳杆菌干酪的蛋白质水解程度最强,活性最高（79.71%）。模拟消化后,瑞士乳杆菌干酪活菌数降低14.30%,ACE抑制活性显著增加（P＜0.05）,达到86.06%,多肽质量浓度增加至2.81 mg/mL;研究不同分子质量超滤组分消化后的ACE抑制活性发现,其中大于10 kDa的多肽活性升高,小于10 kDa的活性下降。此外,添加瑞士乳杆菌不影响干酪的整体可接受性。因此,瑞士乳杆菌能促进干酪ACE抑制肽的产生并提高其活性,消化后活性的升高主要与大分子肽的降解有关。     

Chemical and electrophoretic properties of Holstein cow milk as affected by somatic cell count

Chemical and electrophoretic properties of raw milk samples with different somatic cell counts (SCCs)(<200 000, 200 000–800 000 and >800 000 cells/mL) were determined. Milk was analysed for fat, lactose, total nitrogen (TN), non‐casein nitrogen (NCN), ratio of casein nitrogen to total nitrogen (CN/TN), pH, titratable acidity (TA), solid‐non‐fat (SNF), total solid (TS), the amount of α _s ‐casein and β‐casein. Milk SCC had no significant effect (P ≥ 0.05) on fat and TN. An increase in the SCC resulted in an increase in pH and NCN content, and a significant decrease in lactose, TA, SNF, CN/TN, α_s‐casein and β‐casein. Milk quality of Holstein dairy cattle deteriorate due to higher proteolytic activity associated with high SCC.     

不同品种牛乳冰点及其与乳理化指标相关性分析

测定中甸牦牛(n=38)、犏牛(n=85)、迪庆黄牛(n=18)和西门塔尔牛(n=20)原乳的冰点、体细胞数、尿素氮以及脂肪、蛋白、乳糖含量等理化指标,并分析冰点与其他理化指标的相关性。结果表明:中甸牦牛、犏牛和迪庆黄牛乳的冰点分别为-0.589、-0.587℃和-0.582℃,极显著低于西门塔尔牛的-0.555℃(P0.01);中甸牦牛、犏牛、迪庆黄牛和西门塔尔牛的乳中体细胞数分别为52.21×10~4、56.06×10~4、48.67×10~4个/m L和45.45×10~4个/m L,相互间比较差异不显著(P0.05);中甸牦牛的乳尿素氮含量为11.70 mg/100 m L,显著高于犏牛的7.63 mg/100 m L(P0.05),极显著高于西门塔尔牛的4.81 mg/100 m L(P0.01),高于迪庆黄牛的8.51 mg/100 m L,但差异不显著(P0.05)。Pearson相关分析结果表明:中甸牦牛和迪庆黄牛乳的冰点与其他理化指标间无明显的相关性(P0.05),犏牛乳的冰点与乳脂肪含量、乳脂蛋白比和体细胞数呈极显著正相关(P0.01),西门塔尔牛乳的冰点与乳糖和乳非脂固形物含量呈极显著正相关(P0.01),与乳总固形物呈显著正相关(P0.05);对全部乳样(n=161)测定数据相关性分析结果,冰点与乳脂肪、乳蛋白、乳总固形物含量和乳脂蛋白比均呈极显著正相关(P0.01),与体细胞数呈显著正相关(P0.05)。本研究完成对中甸牦牛、犏牛和迪庆黄牛原乳的冰点、体细胞数、尿素氮的测定,可为今后制定生鲜乳收购按质论价标准提供参考。     

Effect of somatic cell count on Prato cheese composition

The objective of this research was to evaluate the effect of 2 levels of somatic cell counts (SCC) in raw milk on Prato cheese composition, protein and fat recovery, cheese yield, and ripening. A 2 × 6 factorial design with 3 replications was performed in this study: 2 levels of SCC and 6 levels of storage time. Initially, 2 groups of dairy cows were selected to obtain low (<200,000 cells/ mL) and high (>600,000 cells/mL) SCC in milks that were used to manufacture 2 vats of cheese: 1) low SCC and 2) high SCC. Milk, whey, and cheese compositions were evaluated; clotting time was measured; and cheese yield, protein recovery, and fat recovery were calculated. The cheeses were evaluated after 5, 12, 19, 26, 33, and 40 d of ripening according to pH, moisture, pH 4.6 soluble nitrogen, 12% trichloroacetic acid soluble nitrogen as a percentage of total nitrogen, and firmness. High-SCC milk presented significantly higher total protein and nonprotein nitrogen and lower true protein and casein concentrations than did low-SCC milk, indicating an increased whey protein content and a higher level of proteolysis. Although the pH of the milk was not affected by the somatic cell level, the cheese obtained from high-SCC milk presented significantly higher pH values during manufacture and a higher clotting time. No significant differences in cheese yield and protein recovery were observed for these levels of milk somatic cells. The cheese from high-SCC milk was higher in moisture and had a higher level of proteolysis during ripening, which could compromise the typical sensory quality of the product.     

Effects of somatic cell count and stage of lactation on the plasmin activity and cheese-making properties of ewe milk

The experiment was conducted from March to July 2002 using 5 intensively managed flocks of Southern Italy. In each flock, 2 groups of 50 ewes were created. The groups were designated LSCC (low somatic cell count [SCC]) when their milk SCC was lower than 500,000/mL and HSCC (high SCC) when their milk SCC was higher than 1,000,000/mL. Bulk milk and whey samples were analyzed for fat, total protein, lactose, casein, and whey protein contents. Renneting properties of milk were also determined. Moisture, NaCl, and nitrogen fractions were determined in fresh cheese curds. In addition, plasmin (PL) and plasminogen (PG) activities in milk and cheese were monitored. The proteolytic activity of plasmin by urea-polyacrylamide gel electrophoresis and the white blood cell (WBC) differentials were determined. The HSCC resulted in higher pH values in milk and in higher moisture and lower fat contents in fresh cheese curds. Moreover, a lower recovery of fat and whey proteins was obtained from the HSCC than from the LSCC raw milk. The crude protein and casein contents were higher in the HSCC than in the LSCC curds during early and midlactation; an opposite trend was observed in late lactation. Plasmin and PG activities underwent more marked fluctuations in the LSCC than in the HSCC curds through lactation. The results of this experiment demonstrate that the PL activity in ewe milk is markedly influenced by the SCC, although SCC is not the only parameter for predicting PL and PG evolution in ewe milk. The LSCC milk resulted in a higher proteolytic potential of Canestrato pugliese cheese curds.     

Effect of somatic cell count in goat milk on yield, sensory quality, and fatty acid profile of semisoft cheese

This study investigated the effect of somatic cell count (SCC) in goat milk on yield, free fatty acid (FFA) profile, and sensory quality of semisoft cheese. Sixty Alpine goats without evidence of clinical mastitis were assigned to 3 groups with milk SCC level of <500,000 (low), 500,000 to 1,000,000 (medium), and 1,000,000 to 1,500,000 (high) cells/mL. Thirty kilograms of goat milk with mean SCC levels of 410,000 (low), 770,000 (medium), and 1,250,000 (high) cells/mL was obtained for the manufacture of semisoft cheese for 2 consecutive weeks in 3 lactation stages. The composition of milk was analyzed and cheese yield was recorded on d 1. Cheese samples on d 1, 60, and 120 were analyzed for total sensory scores, flavor, and body and texture by a panel of 3 expert judges and were also analyzed for FFA. Results indicated that milk composition did not change when milk SCC varied from 214,000 to 1,450,000 cells/mL. Milk with higher SCC had a lower standard plate count, whereas coliform count and psychrotrophic bacteria count were not affected. However, milk components (fat, protein, lactose, casein, and total solids) among the 3 groups were similar. As a result, no significant differences in the yield of semisoft goat cheeses were detected. However, total sensory scores and body and texture scores for cheeses made from the high SCC milk were lower than those for cheeses made from the low and medium SCC milks. The difference in milk SCC levels also resulted in diverse changes in cheese texture (hardness, springiness, and so on) and FFA profiles. Individual and total FFA increased significantly during ripening, regardless the SCC levels. It is concluded that SCC in goat milk did not affect the yield of semisoft cheese but did result in inferior sensory quality of aged cheeses.     

Effect of storage and separation of milk at udder quarter level on milk composition, proteolysis, and coagulation properties in relation to somatic cell count

Coagulation properties of milk are altered by elevated somatic cell count (SCC), partly due to increased proteolytic and lipolytic activity in the milk and, thereby, degradation of protein and fat during storage. Milk is commonly stored on the farm at cooling conditions for up to 2 d before transport to the dairy for processing. This study evaluated the effects of storage on milk with altered composition due to high SCC and the effects of exclusion of milk from individual udder quarters with high SCC on milk composition, proteolysis, and coagulation properties. Udder-quarter milk and cow-composite milk samples from 13 cows having at least 1 quarter with SCC above 100,000 cells/mL were collected on 1 occasion. In addition, commingled milk from only healthy quarters (<100,000 cells/mL) of each cow was collected, representing a cow sample where milk with elevated SCC was excluded. The milk samples were analyzed for total protein content; protein content in the whey fraction; casein, fat, and lactose contents; SCC; proteolysis; curd yield; coagulation time; and total bacterial count, on the day of sampling and after 2 and 5 d of storage at +4°C. In addition to SCC, duration of storage and total bacterial count had an effect on milk quality. The content of total protein, fat and protein contents in the whey fraction, and curd yield were found to have different storage characteristics depending on the level of SCC at udder-quarter level. The exclusion of milk from udder quarters with elevated SCC decreased the content of total protein and protein content in the whey fraction and increased the content of lactose at cow level. However, the effect of separating milk at udder-quarter level needs to be further studied at bulk tank level to evaluate the effect on overall total milk quality.     

Relationship between haptoglobin and serum amyloid A in milk and milk quality

The objective of this study was to evaluate relationships between the presence in milk of the major bovine acute phase proteins, haptoglobin (Hp) and serum amyloid A (SAA), and milk quality parameters. Composite milk samples were collected from 89 clinically healthy dairy cows and analysed for Hp and SAA, total protein, casein, and whey protein levels, casein number, proteolysis, total fat and lactose levels, and somatic cell count (SCC). Milk samples with detectable levels of Hp showed lower total protein and casein levels than those samples without Hp, whereas milk samples with detectable levels of SAA had lower casein number and lactose level than samples without detectable SAA. Samples with detectable levels of acute phase proteins also showed an elevated SCC. The results suggest that the presence of Hp and SAA in milk might indicate unfavourable changes in milk composition, especially in relation to protein quality.     

Transmission Electron Microscopy Study of Casein Micelle in Raw Milk with Different Somatic Cell Count Levels

The relationship between elevated raw milk somatic cell count (SCC) and casein micelle dimension was investigated by transmission electron microscopy (TEM). Milk samples collected from the dairy cattle with three different levels of SCC (<200,000, 200,000 to 800,000, and >800,000 cells/ml) were studied by TEM. The results indicated that an increase in SCC resulted in a decrease in the casein micelle size with an increase in their aggregation. The present research supported the hypothesis that elevated proteolytic activity, reduced secretary ability of the mammary glands, lower electrostatic and steric repulsion as well as different mineral contents of mastitic milk could affect casein micelle properties.     

Composition and properties of bovine milk: A study from dairy farms in northern Sweden; Part II. Effect of monthly variation

This study investigated the influence of monthly variation on the composition and properties of raw farm milk collected as part of a full-scale cheese-making trial in a region in northern Sweden. In our companion paper, the contribution of on-farm factors to the variation in milk quality attributes is described. In total, 42 dairy farms were recruited for the study, and farm milk samples were collected monthly over 1 yr and characterized for quality attributes of importance for cheese making. Principal component analysis suggested that milk samples collected during the outdoor period (June–September) were different from milk samples collected during the indoor period. Despite the interaction with the milking system, the results showed that fat and protein concentrations were lower in milk collected during May through August, and lactose concentration was higher in milk collected during April through July than for the other months. Concentrations of free fatty acids were generally low, with the highest value (0.86 mmol/100 g of fat) observed in February and the lowest (0.70 mmol/100 g of fat) observed in June. Plasmin and plasminogen-derived activities varied with sampling month without a clear seasonal pattern. The pH of farm tank milk ranged from 6.60 to 6.82, with the lowest and highest values in September and February, respectively. The highest somatic cell count was observed in August (201 × 10³ cells/mL) and the lowest in April (143 × 10³ cells/mL). The highest value of gel strength, was recorded in December (88 Pa) and the lowest in July (64 Pa). Rennet coagulation time and gel strength were inversely correlated, with the lowest rennet coagulation time value observed in December. Orthogonal projections to latent structures (OPLS) and discriminant analysis adaptation of OPLS identified casein micelle size and total proteolysis as the milk quality attributes with major responses to sampling month, with smaller casein micelle size and higher total proteolysis associated with the outdoor months. Using discriminant analysis adaptation of OPLS to further investigate causes behind the variation in milk traits revealed that there were factors in addition to feeding on pasture that differed between outdoor and indoor months. Because fresh grass was seldom the primary feed in the region during the outdoor period, grazing was not considered the sole reason for the observed difference between outdoor and indoor periods in raw milk quality attributes.     

Milk production and somatic cell counts: a cow-level analysis

The objectives of this study were to quantify the relationship between 24-h milk loss and lactation milk loss due to mastitis at the cow level. For the year 2009, individual cow test-day production records from 2,835 Ontario dairy herds were examined. Each record consisted of 24-h milk and component yields, stage of lactation (days in milk, DIM), somatic cell count (SCC, ×10(3) cells/mL) and parity. The modeling was completed in 2 stages. In stage 1, for each animal in the study, the estimated slope from a linear regression of 24-h milk yield (kg), adjusted for DIM, the quadratic effect of DIM, and the 24-h fat yield (kg) on ln(SCC) was determined. In stage 2, the estimated slope were modeled using a mixed model with a random component due to herd. The fixed effects included season (warm: May to September, cool: October to April), milk quartile class [MQ, determined by the rank of the 24-h average milk yield (kg) over a lactation within the herd] and parity. The estimated slopes from the mixed model analysis were used to estimate 24-h milk loss (kg) by comparing to a referent healthy animal with an SCC value of 100 (×10(3) cells/mL) or less. Lactation milk loss (kg) was then estimated by using estimated 24-h milk loss within lactation by means of a test-day interval method. Lactation average milk loss (kg) and SCC were also estimated. Lastly, lactation milk loss (kg) was modeled on the log scale using a mixed model, which included the random effect of herd and fixed effects, parity, and the linear and quadratic effect of the number of 24-h test days within a lactation where SCC exceeded 100 (×10(3) cells/mL; S100). The effect of SCC was significant with respect to 24-h milk loss (kg), increasing across parity and MQ. In general, first-parity animals in the first MQ (lower milk yield animals) were estimated to have 45% less milk loss than later parity animals. Milk losses were estimated to be 33% less for animals in first parity and MQ 2 through 4 than later parity animals in comparable MQ. Therefore, the relative level of milk production was found to be a significant risk factor for milk loss due to mastitis. For animals with 24-h SCC, values of 200 (×10(3) cells/mL), 24-h milk loss ranged from 0.35 to 1.09 kg; with 24-h SCC values of 2,000 (×10(3) cells/mL), milk loss ranged from 1.49 to 4.70 kg. Lactation milk loss (kg) increased significantly as lactation average SCC increased, ranging from 165 to 919 kg. The linear and quadratic effect of S100 was a significant risk factor for lactation milk loss (kg), where greatest losses occurred in lactations with 5 or more 24-h test days where SCC exceeded 100 (×10(3) cells/mL).