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Effect of somatic cell count on proteolysis and lipolysis in pasteurized fluid milk during shelf-life storage 总被引:11,自引:0,他引:11
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. 相似文献
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阐述了一种以三电极体系为基础,研究溶液体系特性的高性能传感器阵列的设计原理和方法。它涵括了循环伏安法扫描、差分脉冲伏安法扫描、常规脉冲伏安法扫描、多电位阶跃扫描等多种功能等,能够对溶液体系施加多种方式的激发信号,得到多种相应特性,从而实现对溶液的全面分析。对新鲜生乳、熟乳(巴氏乳)、酸败乳溶液进行循环伏安法、脉冲伏安法、多电位阶跃等扫描方法检测,并进行主成分分析表征。研究结果表明,传感器阵列对生乳、熟乳(巴氏乳)、酸败乳有明显的辨别能力,不同的工作电极通过不同的检测方法对乳样有不同的区分力。 相似文献
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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. 相似文献
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不同季节原料乳中主要微生物和理化指标分析 总被引:2,自引:0,他引:2
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。 相似文献
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目的通过初级模型和二级模型来描述巴氏杀菌奶中微生物的生长动力学模型。方法样品放置在4、8、16、25、30、37、40和43℃条件下培养,使用IPMP 2013软件进行拟合,初级模型采用Huang模型和Baranyi模型分析不同温度下的生长动力学特征,二级模型采用Ratkowsky平方根模型、Huang平方根模型和Cardinal模型描述温度对其生长速率的影响。结果在所有温度下都能观测到微生物的生长,综合比较2种初级模型的均方根误差(root mean squared error,RMSE)、均方误差(mean squared errorMSE)、赤池信息量准则(Akaike information criterion, AIC)和生长速率,得到2种初级模型具有同等拟合效果;而3种二级模型得到的最低生长温度和最高生长温度分别是-3.191、0.56、-4.962和47.309、45.277、44.408℃。结论 Baranyi模型和Huang模型均适合描述巴氏杀菌奶中微生物菌群的生长,3种二级模型都能用于评价温度对其生长速率的影响,但Ratkowsky平方模型覆盖的温度范围更广,可能更适合描述温度对巴氏杀菌奶中微生物菌群生长的影响。 相似文献
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阻抗法快速检测巴氏杀菌乳中菌落总数的研究 总被引:1,自引:0,他引:1
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. 相似文献
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Zhen Jia Lihan Huang Zhaoyi Wei Yukun Yao Ting Fang Changcheng Li 《Journal of dairy science》2021,104(3):2654-2667
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. 相似文献
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S. Vázquez-Román D. Escuder-Vieco M.D. Martín-Pelegrina B. Muñoz-Amat L. Fernández-Álvarez P. Brañas-García D. Lora-Pablos J. Beceiro-Mosquera C.R. Pallás-Alonso 《Journal of dairy science》2018,101(12):10714-10719
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. 相似文献
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《Journal of dairy science》2022,105(12):9439-9449
Bacterial spores from raw milk that survive the pasteurization process are responsible for half of all the spoilage of fluid milk. Bactofugation has received more attention as a nonthermal method that can reduce the presence of bacterial spores in milk and with it the spoilage of fluid milk. The objective of this work was to determine the effectiveness of bactofugation in removing spores from raw milk and estimate the effect the spore removal could have on shelf-life of fluid milk. The study was conducted in a commercial fluid milk processing facility where warm spore removal was performed using one-phase bactofuge followed by warm cream separation and high temperature, short time pasteurization. Samples from different stages of fluid milk processing with and without the use of bactofuge were tested for total plate count, mesophilic spore count, psychrotolerant spore count (PSC), and somatic cell count. Results were evaluated to determine the count reductions during different stages of fluid milk processing and compare counts in fluid milk processed with and without bactofugation. Bactofugation on average reduced the total plate count by 1.81 ± 0.72 log cfu/mL, mesophilic spore count by 1.08 ± 0.71 log cfu/mL, PSC by 0.86 ± 0.59 log cfu/mL, and somatic cell count by 135,881 ± 43,942 cells/mL. Psychrotolerant spore count in final pasteurized skim milk processed with and without bactofugation was used to predict the shelf-life of the pasteurized skim milk using the Monte Carlo simulation model. Although PSC in the initial raw milk was already low (?0.63 ± 0.47 log cfu/mL), the predicted values from the simulation model showed that bactofugation would extend the shelf-life of pasteurized skim milk by approximately 2 d. The results of this study will directly help fluid milk processors evaluate the benefits of using bactofugation as an intervention in their plants, and also demonstrate the benefits of using mathematical modeling in decision making. 相似文献
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《Journal of dairy science》2023,106(1):75-83
Pasteurization is carried out in dairy industries to kill harmful bacteria present in raw milk. However, endospore-forming bacteria, such as Bacillus, cannot be completely eliminated by pasteurization. In this study, a total of 114 Bacillus strains were isolated from 133 pasteurized milk samples. Antibiotic susceptibility tests showed that the percentage of Bacillus with intrinsic resistance to ampicillin and penicillin were 80 and 86%, respectively. Meanwhile, some Bacillus isolates had acquired resistance, including trimethoprim-sulfamethoxazole resistance (10 isolates), clindamycin resistance (8 isolates), erythromycin resistance (2 isolates), and tetracycline resistance (1 isolate). To further locate these acquired resistance genes, the plasmids were investigated in these 16 Bacillus strains. The plasmid profile indicated that Bacillus cereus BA008, BA117, and BA119 harbored plasmids, respectively. Subsequently, the Illumina Novaseq PE150 was applied for the genomic and plasmid DNA sequencing. Notably, the gene tetL encoding tetracycline efflux protein was found to be located on plasmid pBC46-TL of B. cereus BA117. In vitro conjugative transfer indicated that pBC46-TL can be transferred into Bacillus invictae BA142, Bacillus safensis BA143, and Bacillus licheniformis BA130. The frequencies were of 1.5 × 10?7 to 1.7 × 10?5 transconjugants per donor cells. Therefore, Bacillus strains with acquired antibiotic resistance may represent a potential risk for the spread of antibiotic resistance between Bacillus and other clinical pathogens via horizontal gene transfer. 相似文献
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Results of milk samples submitted for microbiological examination in Wisconsin from 1994 to 2001 总被引:1,自引:0,他引:1
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. 相似文献
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《Journal of dairy science》2019,102(8):6928-6942
Innovative processing technologies, such as ultrasonication, can change the properties of milk, allowing for the improvement or development of dairy foods. Yet taking bench-scale equipment to pilot plant scale has been challenging. Raw milk, standardized to 3% fat and warmed to inlet temperatures of 42 or 54°C, was exposed to continuous, high-intensity, low-frequency ultrasonication (16/20 kHz, 1.36 kW/pass) at flow rates of 0.15, 0.30, and 0.45 L/min that resulted in resident times within the reaction cell of 6, 3, and 2 min per pass, respectively. Multiple passes (3, 5, and 7, respectively) were required to obtain a total exposure time of 14 to 18 min. Evaluation of fat droplet sizes, enzyme coagulation properties, and microstructure of milk and milk gels, as well as determining compositional and lipid properties, were conducted to determine the potential of the ultrasound system to effectively modify milk. Laser scanning particle sizing and confocal microscopy showed that the largest droplets (2.26 ± 0.13 µm) found in raw milk were selectively reduced in size with a concomitant increase in the number of submicron droplets (0.37 ± 0.06 µm), which occurred sooner when exposed to shorter bursts of ultrasonication (0.45 L/min flow rates) and at an inlet temperature of 54°C. Ultrasound processing with milk entering at 42°C resulted in faster gelling times and firmer curds at 30 min; however, extended processing at inlet temperature of 54°C reduced curd firmness and lengthened coagulation time. This showed that ultrasonication altered protein-protein and protein-lipid interactions, thus the strength of the enzyme-set curds. Scanning electron microscopy revealed a denser curd matrix with less continuous and more irregular shaped and clustered strands, whereas transmission electron microscopy showed submicron lipid droplets embedded within the protein strands of the curd matrix. Processing at inlet temperature of 54°C with flow rates of 0.30 and 0.45 L/min also reduced the total aerobic bacterial count by more than 1 log cfu/mL, and the number of psychrophiles below the limit of detection (10 cfu/mL) for this study. Ultrasonication exposures of 14 to 18 min had minimal effect on the milk composition, fatty acid profiles, and lipid heat capacity and enthalpy. The findings show that this continuous ultrasound system, which is conducive to commercial scale-up, modifies the physical and functional properties of milk under the parameters used in this study and has potential use in dairy processing. 相似文献