Effect of somatic cell count on proteolysis and lipolysis in pasteurized fluid milk during shelf-life storage

The general goal of this research was to provide fluid milk processors with data to enable them to estimate the economic benefits they might derive from longer fluid milk shelf-life or new marketing opportunities due to a reduction in raw milk SCC. The study objectives were: 1) to measure the time in days for pasteurized homogenized 2% milk to achieve a level of lipolysis and proteolysis caused by native milk enzymes present in milks of different somatic cell count (SCC) at 0.5 and 6 degrees C that would be sufficient to produce an off-flavor, 2) to determine whether milk fat content (i.e., 1, 2, and 3.25%) influences the level of proteolysis or lipolysis caused by native milk enzymes at 6 degrees C, and 3) to determine the time in days for milks containing 2% fat with different SCC to undergo sufficient lipolysis or proteolysis to produce an off-flavor due to the combination of the action of native milk enzymes and microbial growth at 0.5 and 6 degrees C. In experiment 1, pasteurized, homogenized milks, containing 2% fat were prepared from raw milk containing four different SCC levels from < 100,000 to > 1,000,000 cells/ml. Each of the four milks was stored at 0.5 and 6 degrees C for 61 d. In experiment 2, pasteurized, homogenized milks containing 1, 2, and 3.25% fat were prepared starting from two raw milks containing two different SCC levels, one < 100,000 and the other > 1,000,000 cells/ml. In experiment 3, pasteurized, homogenized 2% fat milks were prepared starting from raw milks containing two different SCC levels, one < 100,000 and the other > 1,000,000 cells/ml. For experiments 1 and 2, all milks were preserved with potassium dichromate to prevent microbial growth but to allow the activity of native milk proteases and lipases during storage. For experiment 3, one set of milk was preserved with potassium dichromate to prevent microbial growth but to allow the activity of native milk proteases and lipases, and a second set of milk was unpreserved during storage at 0.5 and 6 degrees C for 29 d. Based on previous work, an off-flavor due to proteolysis was detected by 50% of panelists when the decrease in casein as a percentage of true protein (CN/TP) was > 4.76%. Our data indicated (assuming 50% of consumers would detect an off-flavor when CN/TP decreases 5%) that pasteurized milk containing 2% fat would develop an off-flavor at a time long after 61 and at 54 d for the low SCC milk, and at about 54 and 19 d for the high SCC milk, at 0.5 and 6 degrees C, respectively. Previous research reported that 34% of panelists could detect an off-flavor in milk containing 2% fat due to lipolysis at a (free fatty acid) FFA concentration of 0.25 meq/kg of milk. Based on these results, it was estimated in the present study that 34% of panelists would detect an off-flavor in a 2% fat pasteurized milk with low SCC at a time long after 61 and just after 61 d at 0.5 and 6 degrees C, respectively, while for milk with high SCC, an off-flavor would be detected by 34% of panelists at slightly longer than 61 and 35 d at 0.5 and 6 degrees C, respectively. The combination of low SCC milk and low storage temperature when coupled with processing technology to produce very low initial bacteria count in fluid milk could produce fluid milk that will maintain flavor quality for more than 61 d of storage at temperatures < 6 degrees C.     

浅析巴氏杀菌奶在中国的发展前景

介绍了巴氏杀菌奶的生产加工工艺,阐述了制约我国巴氏杀菌奶发展的主要因素,通过对国内外乳业巴氏杀菌奶发展形势的对比,说明巴氏杀菌奶在我国有着广阔的发展空间。     

巴氏牛奶中蜡样芽孢杆菌的风险评估

为了了解巴氏牛奶中蜡样芽孢杆菌的风险状况，调查研究了北京地区乳品企业巴氏牛奶的运输和销售的总体情况以及北京地区居民牛奶消费的相关情况，结合预测微生物学的方法建立了巴氏牛奶中蜡样芽孢杆菌在流通领域的暴露评估模型。从整个评估过程来看，流通领域的温度和时间是影响巴氏奶安全性的主要因素。     

谈原料奶按质论价

通过阐述欧洲原料奶付款系统，介绍了欧洲一些国家决定原料奶价格的参数，包括乳脂、乳蛋白、细菌总数、体细胞数、抗生素、冰点、季节等，以及这些参数的奖罚措施，并从中分析了国内按质论价体系与国外的差别。     

Low pressure CO2 storage of raw milk: microbiological effects

The effects of holding raw milk under carbon dioxide pressures of 68 to 689 kPa at temperatures of 5, 6.1, 10, and 20°C on the indigenous microbiota were investigated. These pressure-temperature combinations did not cause precipitation of proteins from the milk. Standard plate counts from treated milks demonstrated significantly lower growth rate compared with untreated controls at all temperatures, and in some cases, the treatment was microcidal. Raw milk treated with CO₂ and held at 6.1°C for 4 d exhibited reduced bacterial growth rates at pressures of 68, 172, 344, and 516 kPa; and at 689 kPa, demonstrated a significant loss of viability in standard plate count assays. The 689-kPa treatment also reduced gram-negative bacteria and total Lactobacillus spp. The time required for raw milk treated at 689 kPa and held at 4°C to reach 4.30 log₁₀ cfu/mL increased by 4 d compared with untreated controls. Total coliform counts in the treated milk were maintained at 1.95 log₁₀ cfu/mL by d 9 of treatment, whereas counts in the control significantly increased to 2.61 log₁₀ cfu/mL by d 4 and 2.89 log₁₀ cfu/mL by d 9. At d 8, Escherichia coli counts had not significantly changed in treated milk, but significantly increased in the control milk. Thermoduric bacteria counts after 8 d were 1.32 log₁₀ cfu/mL in treated milk and 1.98 log₁₀ cfu/mL in control milk. These data indicated that holding raw milk at low CO₂ pressure reduces bacterial growth rates without causing milk protein precipitation. Combining low CO₂ pressure and refrigeration would improve the microbiological quality and safety of raw milk and may be an effective strategy for shipping raw single strength or concentrated milk over long distances.     

不同季节原料乳中主要微生物和理化指标分析

研究了冬季、春季和夏季原料乳的主要理化指标和微生物指标。3个季节内,蛋白质、乳脂、乳糖和干物质的变化范围分别是3.38%～3.52%,3.98%～4.26%,4.80%～4.85%和12.78%～13.19%。理化指标检测结果表明冬季牛乳的营养成分高于春夏两季,并且3个季节的乳成分均高于生鲜牛乳的收购标准;此外,对原料乳中主要的微生物:总菌数、乳酸菌、大肠菌群、沙门氏菌、蜡样芽孢杆菌、单增李斯特菌和嗜冷菌的菌数进行了检测。结果中未检测到沙门氏菌、蜡样芽孢杆菌和单增李斯特菌,其他微生物质量分数均在可接受范围内,但是大肠菌群的出现说明需要建立相关的卫生质量标准。     

阻抗法快速检测巴氏杀菌乳中菌落总数的研究

微生物检测技术对巴氏杀菌乳的质量安全、营养健康及疾病预防具有重要意义.国标规定的菌落总数检测方法需要48h,极大地影响产品货架期,又无法满足消费者对产品新鲜度的要求.采用阻抗法建立曲线方程Y=-1.0307X+9.0268,复相关系数R2=0.9761,与国标法相比,结果准确、可靠,而且检测周期缩短至8h～10h,该方法的应用将对乳品企业的生产和管理发挥重要作用.     

纤维素酶法测定巴氏杀菌乳中微晶纤维素的研究

应用透析处理、纤维素酶水解与还原糖测定的流程对巴氏杀菌乳中的微晶纤维素进行检测。结果表明,纤维素酶在巴氏杀菌乳中活性稳定,可有效地水解牛奶中的MCC产生还原糖,MCC的检测限达到0.5 mg/mL以下。本研究提供了牛奶中MCC检测的新方法,为乳制品质量控制探索了新的途径。     

不同贮存条件对生牛奶品质影响的研究

从对生牛奶品质影响较大的贮存温度和时间着手，探讨了不同贮存条件对生牛奶中细菌总数以及组成的影响，旨在为优质生牛奶的贮运提供参考。     

蛋黄免疫蛋白奶加工工艺的研究

开发了蛋黄免疫球蛋白消毒奶,研究了蛋黄免疫球蛋白消毒奶的加工工艺。得出最佳工艺条件为:IgY添加量为10%,采用低温巴氏杀菌65℃,30min。产品在4℃储藏条件下比普通巴氏乳的货架期延长8h。因此,蛋黄免疫蛋白消毒奶是一种既有营养,又具有生物活性功能的新型乳制品,且具有较长的货架期,有一定的发展前景。     

Dynamic kinetic analysis of growth of Listeria monocytogenes in pasteurized cow milk

The objective of this study was to develop a dynamic model for predicting the growth of Listeria monocytogenes in pasteurized cow milk under fluctuating temperature conditions during storage and temperature abuse. Six dynamic temperature profiles that simulated random fluctuation patterns were designed to change arbitrarily between 4 and 30°C. The growth data collected from 3 independent temperature profiles were used to determine the kinetic parameters and construct a growth model combining the primary and secondary models using a 1-step dynamic analysis method. The results showed that the estimated minimum growth temperature and maximum cell concentration were 0.6 ± 0.2°C and 7.8 ± 0.1 log cfu/mL (mean ± standard error), with the root mean square error (RMSE) only 0.3 log cfu/mL for model development. The model and the associated kinetic parameters were validated using the data collected under both dynamic and isothermal conditions, which were not used for model development, to verify the accuracy of prediction. The RMSE of prediction was approximately 0.3 log cfu/mL for fluctuating temperature profiles, and it was between 0.2 and 1.1 log cfu/mL under certain isothermal temperatures (2–30°C). The resulting model and kinetic parameters were further validated using 3 growth curves at 4, 7, and 10°C arbitrarily selected from ComBase (www.combase.cc). The RMSE of prediction was 0.8, 0.4, and 0.5 log cfu/mL, respectively, for these curves. The validation results indicated the predictive model was reasonably accurate, with relatively small RMSE. The model was then used to simulate the growth of L. monocytogenes under a variety of continuous and square-wave temperature profiles to demonstrate its potential application. The results of this study showed that the model developed in this study can be used to predict the growth of L. monocytogenes in contaminated milk during storage.     

盒装巴氏奶货架期的快速预测

采用阻抗法进行盒装巴氏奶货架期预测,结果表明,巴氏奶货架期与细菌数对数值密切相关,在30℃、6h预培养下加样300,400μL及37℃、6h预培养下加样100－400μL进行阻抗测定,建立回归方程式均可快速准确地进行盒装巴氏奶货架期预测,整个预测过程耗时14－20h.     

Results of milk samples submitted for microbiological examination in Wisconsin from 1994 to 2001

The objective of this study was to examine the characteristics of milk samples submitted for microbiological examination at the Wisconsin Veterinary Diagnostic Laboratory between 1994 and 2001. Results (n = 83,650) of microbiological testing of milk samples (n = 77,172) submitted to the Wisconsin Veterinary Diagnostic Laboratory from January 1994 until June 2001 were analyzed. Submissions included milk samples obtained from cases of clinical and subclinical mastitis as well as samples obtained for mastitis surveillance programs. Results were recorded as no growth, contaminated, or identified as specific bacterial pathogens. Statistical analysis was performed to determine trends in the isolation of mastitis pathogens. The proportion of samples identified as contaminated decreased from 20.6 (1997) to 9.5% (2001). The proportion of samples coded as no growth increased from 22.6 (1994) to 49.7% (2001). Isolation of Staphylococcus aureus decreased from 17.7% (1994) of isolates to 9.7% (2001), while isolation of Streptococcus agalactiae decreased from 8.1 (1994) to 3.0% (2001). Coagulase-negative Staphylococcus spp. were isolated from 12.7 to 17.5%, environmental Streptococcus spp. were isolated from 11.6 to 20.1%, and Escherichia coli were isolated from 3.1 to 6.7% of all isolates. No growth and contaminated samples comprised almost 50% of total submissions, and it is important that producers have proper expectations when submitting milk samples. The proportion of isolates identified as Staph. aureus and Strep. agalactiae decreased, suggesting the proportion of contagious bacteria causing mastitis has decreased. Environmental and contagious pathogens demonstrated characteristic differences by season.     

Short communication: Effect of refrigerated storage on the pH and bacterial content of pasteurized human donor milk

Once pasteurized donor milk is thawed for its administration to a preterm or sick neonate, and until it is administered, it is kept refrigerated at 4 to 6°C for 24 h. After this time, unconsumed milk is discarded. This time has not been extended, primarily because of the concern of bacterial contamination. The aim of this study was to determine the changes in pH and bacterial count when pasteurized donor milk was kept under refrigeration for a prolonged period (14 d). In this prospective study, 30 samples of pasteurized donor milk from 18 donors were analyzed. Milk was handled following the regular operating protocols established in the neonatal unit and was kept refrigerated after thawing. pH measurements and bacteriology (on blood agar and MacConkey agar plates) were performed on each sample at time 0 (immediately after thawing) and then every day for 14 d. Changes in pH of samples over time were evaluated with linear mixed-effects regression models. A slow but gradual increase in milk pH was observed starting from the first day [mean (±SD) pH of 7.30 (±0.18) at time 0 and 7.69 (±0.2) on d 14]. No bacterial growth was observed in any of the samples throughout the complete trial except in one sample, in which Bacillus flexus was isolated. In conclusion, pasteurized human donor milk maintains its microbiological quality when properly handled and refrigerated (4–6°C). The slight and continuous increase in milk pH after the first day could be due to changes in the solubility of calcium and phosphate during refrigerated storage.     

原料乳生产中的HACCP体系研究

通过对原料乳生产线上的工序、设备微生物污染状况的检测，分析了微生物数量的变化规律，确定了奶牛饲养、乳房清洗、设备清洗、原料奶暂存运输4个工序为关键控制点，并提出了相应的标准操作规程。     

Effect of microbiological testing on subsequent mid-infrared milk component analysis of the same milk sample

Our objectives were to determine if mixing and sampling of a raw milk sample at 4°C for determination of total bacteria count (TBC) and if incubation at 14°C for 18 h and sampling for a preliminary incubation (PI) count influenced the accuracy of subsequent fat, protein, or lactose measurement by mid-infrared (IR) analysis of milk from the same sample container due to either nonrepresentative sampling or the presence of microbial metabolites produced by microbial growth in the milk from the incubation. Milks of 4 fat levels (2.2, 3, 4, and 5%) reflected the range of fat levels encountered in producer milks. If the portion of milk removed from a cold sample was not representative, then the effect on a milk component test would likely be larger as fat content increases. Within the milks at each fat level, 3 treatments were used: (1) 20 vials of the same milk sampled for testing TBC using a BactoScan FC and then used for a milk component test; (2) 20 vials for testing TBC plus PI count followed by component test; and (3) 20 vials to run for IR component test without a prior micro sampling and testing. This was repeated in 3 different weeks using a different batch of milk each week. No large effect on the accuracy of component milk testing [IR fat B (carbon hydrogen stretch) and fat A (carbonyl stretch)] due to the cold milk sample handling and mixing procedures used for TBC was detected, confirming the fact that the physical removal of milk from the vial by the BactoScan FC (Foss Electric, Hillerød, Denmark) was a representative portion of the milk. However, the representativeness of any other sampling procedure (manual or automated) of a cold milk sample before running milk component testing on the same container of milk should be demonstrated and verified periodically as a matter of routine laboratory quality assurance. Running TBC with a BactoScan FC first and then IR milk analysis after had a minimal effect on milk component tests by IR when milk bacteria counts were within pasteurized milk ordinance limits of <100,000 cfu/mL. Running raw milk PI counts (18 h of incubation at 13–14°C) with the BactoScan FC before milk component testing by IR milk analysis had an effect on component tests. The effect was largest on fat test results and would decrease the accuracy of milk payment testing on individual producer milks. The effect was most likely due to the absorption of light by bacterial metabolites resulting from microbial growth or other chemical degradation processes occurring in the milk during the PI count incubation, not by the sampling procedure of the BactoScan. The direction of the effect on component test results will vary depending on the bacteria count and the type of bacteria that grew in the milk, and this could be different in every individual producer milk sample.     

The survival of foot-and-mouth disease virus in raw and pasteurized milk and milk products

The Foot-and-Mouth Disease virus (FMDV) is not a public health threat, but it is highly contagious to cloven-footed animals. The virus is shed into milk up to 33 h before there are apparent signs of the disease in dairy cows, and, in extreme cases, signs of disease may not appear for up to 14 d. During this time, raw milk can serve as a vector for spread of the disease both at the farm and during transport to the processing plant by milk tanker. Raw milk and milk products fed to animals have the potential to cause infection, but the potential for pasteurized milk products to cause infection is largely unknown. Current minimum pasteurization standards may not be adequate to eliminate FMDV in milk completely. The purpose of this paper is to assess the literature on the thermal resistance of FMDV in milk and milk products, to identify the risks associated with ingestion of pasteurized products by animals, and to lay a strategy to prevent the spread of FMDV from contaminated milk.     

高效液相色谱法分析原料乳和奶粉中三聚氰胺

建立了二极管阵列检测器-高效液相色谱测定原料乳及奶粉中三聚氰胺的方法。奶粉样品经三氯乙酸超声提取,乙酸铅沉淀蛋白,三氯甲烷除去脂肪和非极性杂质,PROELUTPXCSPE阳离子交换固相萃取小柱进一步除去样品基质干扰,氨化甲醇洗脱。实验对色谱分离条件进行了优化,三聚氰胺的质量浓度在1.0～100.0μg/mL范围内与色谱峰面积呈良好的线性关系,回归方程为y=89.763x-113.72,相关系数为R2=0.9988。加标回收率为90.5%～105.0%,相对标准偏差(RSD)小于4.0%。该方法简单、快速,结果准确可靠。     

室温下化学法激活乳过氧化物酶体系对原料乳品质的影响

本文对化学法激活乳过氧化物酶体系用于室温下原料乳保鲜进行了研究。研究结果表明,在25℃贮藏温度下,原料奶的货架期随着SCN-和H2O2的浓度增加而增加。对照组和实验组0.1mmol/L,0.25mmol/L,0.5mmol/L的保鲜期分别为19h,19.5h,21.5h和23.5h。在贮藏期间,所有的实验组的微生物数量均低于对照组。11.5h时,激活剂浓度为0.25mmol/L和0.5mmol/L的原料乳的菌落总数比对照组提高了3个卫生等级。在14h时,激活剂浓度为0.25mmol/L和0.5mmol/L的实验组比对照组的大肠菌群数分别低1.2和1.75个对数周期。     

原料奶分级收购标准的研究

以乳品企业所辖区域收购奶户的牛奶为研究对象,对原料奶理化指标、菌落总数进行检测,并对2009年原料奶的理化平均指标、菌落总数平均指标进行汇总分析.结果表明,原料奶的各理化指标在一年12个月平均指标不等,各地区之间也存在差异,河北和徐州地区原料奶的脂肪、蛋白质、全脂乳固体、非脂乳固体呈现规律性变化.该研究为企业原料奶分级提供了理论依据,建立了企业不同月份的分级收购标准.