Production of Glycosidases by Psychrotrophic Bacteria

Six of 19 psychrotrophic bacteria common in milk and meat hydrolyzed large amounts of p-nitrophenyl-glycosides during 48 hr in minimal growth broth at 20°C. Four of the 19 were inactive against the four glycosides used in the screening tests, and nine were weakly active. In tests of the six active cultures against 11 p-nitrophenyl-glycosides, strains of Enterobacter had more glycosidase activity than strains of Pseudomonas, β-anomers were hydrolyzed more rapidly than α-anomers, more p-nitrophenol was released from the β-D-galactoside than from other substrates, there was activity against the D form but not the L form of fucoside, and p-nitrophenyl-β-D-glucoside was hydrolyzed at about the same rate by each culture.     

原料乳中嗜冷菌计数及产脂肪酶特性的研究

本文主要对乳中的嗜冷菌数的检测方法进行了初步研究。同时,考虑到嗜冷菌能在乳中产生大量的耐热性脂肪酶,力求在脂肪酶活与菌数之间建立联系。即对不同时间测得的酶活与菌数之间的关系进行初步探讨。     

嗜冷茵对UHT奶品质影响的研究

嗜冷菌产生耐热蛋白酶和脂肪酶是UHT奶在长期存放过程中产生劣变的主要原因之一。原料乳中蛋白酶、脂肪酶的活性与产品长期保存时的品质呈负相关。通过控制原料奶乳中嗜冷菌数,测定蛋白酶和脂肪酶活力,对原料奶进行分级和评价是确保UHT奶品质的一种有效手段,本实验为乳品企业嗜冷菌的检测提供了理论依据。     

Impedimetric Determinaton of Total, Mesophilic and Psychrotrophic Zounts in Raw Milk

A rapid impedimetric determination for total, mesophilic and psychrotrophic counts in raw milk showed correlations (between impedance detection times and standard plate counts) of -0.96, -0.95, and 0.96, respectively. Mesophiles were most often seen as the predominant population, the impedimetric method allowed for these samples containing above 1.0 × 10⁵ CFU/ml to be screened out within 4 hr. Psychrotrophic levels of 1 × 10⁵ CFU/ml and above were screened within 21 hr, while total concentration of samples containing above 1 × 10⁵ CFU/ml were screened within 16 hr.     

Analysis of Raw Milk Quality at Reception and During Cold Storage: Combined Effects of Somatic Cell Counts and Psychrotrophic Bacteria on Lipolysis

The objective of the article was to analyze the influence of psychrotrophic bacteria counts (PBCs) and somatic cell counts (SCCs) on the extent of lipolysis in bulk samples of cow's milk at reception and during cold storage. Samples of milk were analyzed on the day of sampling and subsequently during cold storage. The acidity, fat, density, chloride content, electrical conductivity (EC), bulk milk SCCs (BMSCC), and PBC values were analyzed on the day of sampling and the levels of acidity, EC, SCC, and PBC were analyzed during cold storage at 4 °C for 72 h. The SCC value 869 × 10³ mL^?1 was higher than the recommended threshold. Lipolysis level at sampling day was related more closely with SCC than with PBC. There was no significant correlation between milk acidity and PBC among others parameters, while the milk mean density was only significant (P < 0.01) correlated with the fat content. The EC and chloride content were consistently correlated (P < 0.001) with BMSCC that allowed them to be used as indicators of mammary gland infection. The milk acidity, EC, PBC, and lipolysis levels increased in relation to the storage time at 4 °C. The lipolysis level during storage was in closer relation to the SCC, but not relation to the PBC. Effects of SCC and PBC on lipolysis decreased throughout the chilling period. It was concluded that initial lipolysis level and intrinsic milk lipoprotein lipase appear more effective than SCC and PBC on the development of lipolysis during storage.     

实时光电微生物法快速检测生鲜乳中的嗜冷菌

对50份生鲜牛乳样品中的嗜冷菌同时采用实时光电微生物快速检测方法和平板计数法进行检测,分析比较两种检测方法所得的嗜冷菌检测数据。结果表明:采用嗜冷菌检测标准曲线:Lg CFU/mL=-0.417*DT+7.91的实时光电微生物法检测的嗜冷菌含量在2.3~3.7×106CFU/mL,检出时间在3.2~18.1 h。采用平板计数法的检出结果范围在4.0~3.0×106CFU/mL。当实时光电微生物检测方法的检出时间大于18 h时,样品检测结果均为未检出,此时平板计数法的检测结果均为<1 CFU/mL。实时光电微生物法检测结果对数值与平板计数法检测结果对数值的差值范围在-0.9~0.8,两者差值的绝对值均<1。采用实时光电微生物法和平板计数法两种检测方法的检测结果相当吻合,实时光电微生物法检测生鲜牛乳中嗜冷菌具有良好的适用性,可大大缩短检测时间,且方法准确、简便易行。     

Antibiotic Resistance Patterns of Gram‐Negative Psychrotrophic Bacteria from Bulk Tank Milk

Bacterial resistance to antibiotics is a major global health problem and resistance of Pseudomonadaceae and Enterobacteriaceae is a serious concern. We investigated the prevalence of drug‐resistance in a total of 80 psychrotrophic strains from bulk milk belonging to Pseudomonas genus (n. 63) and Enterobacteriaceae group (n. 17). All the strains were tested against 16 antibiotics. Pseudomonas were further investigated for their sensitivity against 12 additional antibiotics. Pseudomonas showed a high susceptibility toward fluoroquinolones, aminoglycosides, and piperacillin and, to a lesser extent, to imipenem, ceftazidime, cefepime. Thirty‐five out of 63 Pseudomonas strains were susceptible to meropenem, while among antibiotics for which recommended breakpoints are not yet available, 55% of Pseudomonas strains had no inhibition halo in presence of nitrofurantoin, highlighting a resistance toward this drug. The results obtained in this study indicate a high efficiency of fluoroquinolones, chloramphenicol (94%), and kanamycin (76%) for Enterobacteriaceae while a high prevalence of resistant strains was found to ampicillin (13/17). Serratia marcescens is highly susceptible to fluoroquinolones, chloramphenicol, and kanamycin. Moreover, mupirocin seems to be the new antibiotic with the less efficacy for Enterobacteriaceae, with 41% of strains without halo, pointing out an important resistance. Further knowledge on resistance to known and new antibiotics among Pseudomonas species and Enterobacteriaceae of milk origin was acquired.     

Characterization of Gram‐Negative Psychrotrophic Bacteria isolated from Italian Bulk Tank Milk

Eighty psychrotrophic bacterial strains, isolated from different northwest Italian bulk tank milks destined for Grana Padano cheese production, were identified by 16S rRNA gene amplification and partial sequence analysis of the rpoB gene. Pseudomonas spp. were the most commonly occurring contaminants, P. fluorescens being the predominant isolated species, along with Enterobacteriaceae, primarily Serratia marcescens. RAPD‐PCR was used to study genetic variability and distinguish closely related strains; a high degree of genetic heterogeneity among the strains was highlighted. All the strains were characterized for their ability to produce proteases, lipases and lecithinases at different temperatures (7, 22, and 30 °C). Forty‐one of the psychrotrophic strains were positive for all the enzymatic activities. The highest number of positive strains for all the incubation temperatures was found for lipolytic activity (59), followed by proteolytic (31) and lecithinase (28) activities, and the enzymatic traits varied among the Pseudomonas and Enterobacteriaceae strains. The proteolytic psychrotrophic strains were screened for the presence of the aprX gene, coding for a heat‐resistant metalloprotease in Pseudomonas spp. The aprX gene was detected in 19 of 63 Pseudomonas strains, and was widespread in the P. fluorescens strains (14/19).     

原料奶嗜冷菌快速检测方法研究进展

嗜冷菌作为冷藏原料奶中生长的优势菌群,在原料奶贮存过程中会大量繁殖,并产生及其耐热的脂肪酶和蛋白酶,这些酶类在高温处理后仍会有残留,并在奶制品储藏过程中继续分解其中的脂肪和蛋白质,导致产品的风味和质地产生变化。因此,研发出一种即快速又实用的嗜冷菌计数技术成为乳品行业关注的焦点。本文主要介绍了国内外对原料奶中嗜冷菌快速检测的一些方法,并将各个方法在工业应用上的优缺点进行了比较。     

Effect of Psychrotrophic Growth on the Milk Fat Fraction at Different Temperatures of Storage

The use of cooling, without using adequate hygienic practices in primary milk production, allows for the growth of psychrotrophic microorganisms that produce the thermoresistant lipases that give milk a rancid flavor. This study aimed to verify how the variation in temperature influences the lipolytic metabolism of the psychrotrophic organisms. Samples of raw milk were collected and submitted to laboratorial analysis as follows: psychrotrophic bacteria count, lipolytic bacteria count, and free fatty acids dosage. Each sample was divided into 3 aliquots and then incubated at 4, 8, and 12 °C, respectively. For each temperature, analyses were repeated after 12, 24, and 48 h of storage. Despite the psychrotrophs growth increase, according to temperature rise, the lipolytic metabolism was not consistent and presented the lower index at 8 °C, suggesting an intensification of the proteolytic compensatory activity at this temperature.     

乳和乳制品中嗜冷菌及其耐热酶的危害与控制研究

嗜冷菌是乳低温贮藏中的主要腐败菌,嗜冷菌及其代谢酶的存在直接影响原料乳的质量,阐述了嗜冷菌及其耐热酶类对乳及乳制品的危害,并通过分析嗜冷菌的适冷机理和影响酶活的因素,探讨控制嗜冷菌及其代谢酶的方法,以保障原料乳质量。     

原料奶质量对UHT奶制品的影响及控制

主要论述了原料奶的理化指标及微生物指标对ＵＨＴ奶质量的影响，以及控制方法。     

生浆法制作豆腐的工艺优化

为确定生浆法生产豆腐的最优参数,该试验基于豆清发酵液点浆凝固大豆蛋白形成凝胶制作豆腐原理,采用Box-Behnken中心组合设计方案,研究豆水比、煮浆温度、煮浆时间和豆清发酵液添加量对豆腐品质特性（得率、蛋白质含量、水分含量和保水性）的影响。在单因素试验基础上,以豆腐得率和蛋白质含量为响应值设置响应面试验,进行方差分析和交互作用分析,对生浆法制作豆腐工艺进行优化。结果表明：生浆法中不同豆水比、煮浆温度、煮浆时间和豆清发酵液添加量下豆腐品质特性存在显著性差异（p<0.05）,最佳工艺参数为：豆水比1:4（kg:kg）,煮浆时间6.0 min,煮浆温度为105 ℃,豆清发酵液添加量26.50%。在此工艺参数下制作豆腐的得率和蛋白质含量分别为224.09 g/100 g、8.68%,比未优化加工工艺制作的豆腐得率提高33.15 g/100 g,蛋白质含量提高3.29%,表明优化后生浆法工艺条件具有可行性,可制作出高产率、高蛋白的豆腐。     

原料乳离心除菌对UHT乳品质的改善作用

离心除菌可以改善原料乳的质量,本文研究了常规UHT(N-UHT)乳和离心除菌UHT乳(C-UHT)在贮藏过程中理化性质、蛋白分布变化.结果表明:UHT乳贮藏期颜色变深、粒度增加、Zeta电位绝对值降低、黏度增加、酸度增加和纤溶蛋白酶活性增加,乳胶体稳定性降低;离心除菌UHT乳(C-UHT)的物理化学稳定性优于普通UHT...     

Inhibition of Psychrotrophic Bacterial Growth in Refrigerated Milk by Addition of Carbon Dioxide

Treatment of refrigerated milk with 20 to 30 mM CO₂ was evaluated as a method for extending storage-life by inhibiting growth of psychrotrophic bacteria. Generation times for each of five psychrotrophic pseudomonads were significantly longer when grown at 7°C in sterile milk treated with CO₂ than when the same bacteria were grown in ungassed sterile milk.When raw milks were stored at 7°C and treated with CO₂, the time required for aerobic plate counts to increase 10-fold was at least 24 h longer than in the same milks left untreated. Numbers of coliforms, psychrotrophs, and anaerobes (facultative and obligate) were significantly lower in raw milks treated with CO₂ than in untreated milks incubated at 7°C for 6 d.     

PCR法检测原料乳中金黄色葡萄球菌肠毒素A基因型

通过建立PCR方法来检测原料乳中金黄色葡萄球菌肠毒素A基因型,优化其退火温度并验证了其敏感性、特异性.结果表明此方法在退火温度为58.7℃时扩增效果较为理想;灵敏度为1.357 pg/μL;以大肠杆菌、枯草芽孢杆菌、嗜热链球菌、志贺氏菌、沙门氏菌的基因组DNA为模板作为特异性检测对照组并分别进行PCR扩增反应,结果为阴性.同时对采集的乳样进行检测,乳房炎乳样中检测出了金葡菌肠毒素A基因,正常乳中没有检测出金葡菌,初步判断乳房炎是由金葡菌引起的.     

原料乳中脂肪含量对搅拌型酸奶品质的影响

以原料乳中脂肪含量的差异,对搅拌型酸奶的粘度、滴定酸度及pH值、感官指标、乳清析出率等指标进行检测分析,试验结果表明,原料乳中脂肪含量低于0.8%,酸奶粘度低于0.2Pa.s,且伴有乳清的析出、口感粗糙、缺乏乳香味。原料乳中脂肪含量的增加,酸奶粘度呈现上升趋势,且酸奶的品质得到较好地改善。     

Microbiological Quality of Raw Milk and Effect on Quality by Implementing Good Management Practices

Raw milk is a complete food which contains protein, fat, sugars, vitamins and minerals. Even though, raw milk is sterile at secretion, contamination of milk by microorganisms can take place during milk handling, storage and other pre-processing activities. The objectives of this study were to assess the initial microbial load and to determine correlation between Methylene blue dye reduction test with standard plate count method and find possible methods to improve the quality and reduce the wastage of raw milk. As per the records obtain from the chilling centers, about twenty per cent (per month) of raw milk collected to the centers in each district has to be discarded due to spoilage. Out of 13 chilling centers in Kurunagala district, five chilling centers namely Badalgama, Elabadagama, Kuliyapitiya, Dambadeniya and Minuwangoda chilling centers were randomly selected and the initial microbial loads of milk from chilling tanks were tested using Standard Plate Count and Methylene blue dye reduction test. Further, Self- filled questionnaire was used to gather data related to practices in milk supply chain and swab samples were taken from the containers used for milk collection process. Though the microbial population should be below log 6.00 according to the factory standards in Sri Lanka to accept milk for further processing, standard plate count of chilling centers were log 7.08, 6.76, 6.56, 6.70, and 6.88 (CFU/ml) respectively. Good management practices were introduced to rectify the main cause for high microbial counts. After the improvements low microbial counts of above chilling centers were achieved as log 5.91, 5.85, 5.85, 5.86 and 5.94 CFU/ml respectively. Standard Plate Count showed significant difference (p < 0.05) after practicing good management practices. Strong correlation (r² 0.91) was observed between Methylene blue dye reduction test and Standard Plate Counts (log CFU/ml). Therefore, Methylene blue dye reduction test can be used effectively, economically and efficiently, to detect the quality of raw milk and can use as an alternative method for costly and tedious microbiological analysis methods.     

原料乳及牧场环境中的微生物多样性及嗜冷菌污染源分析

原料乳中嗜冷菌超标会产生较高水平的耐热蛋白酶和脂肪酶,这两种酶是导致产品进入货架期后出现质量问题的重要原因。为探究牧场奶牛生活环境以及原料乳在流转过程中不同环节的微生物多样性,找出牧场中嗜冷菌的污染来源,本研究选取河北地区某规模化奶牛养殖场,采集TMR饲料、牛舍垫土、头三把奶、头三把奶之后的常规乳以及储奶罐中的原料乳等样品,利用宏基因组测序技术对所有样品的菌群结构、嗜冷菌种类及丰度进行分析。结果表明:在门水平上,主要微生物为变形菌门、厚壁菌门、放线菌门,在属水平上主要是无形体属、链球菌属、肠球菌属、短螺菌属、芽孢杆菌属、假单胞菌。此外对样品中的关键嗜冷菌(假单胞菌)进行分析,牛舍垫土的多样性最丰富的,包含了牧场中常规乳及储奶罐中乳假单胞菌种类的99.63%和96.39%,说明牛舍垫土是各类假单胞菌潜在的污染源。     

Effect of Carbonation on Microbiologic Parameters of Refrigerated Bovine Raw Milk

This study evaluated the effect of carbon dioxide addition on microbiological quality during refrigerated storage of raw milk collected in Curitiba city, Brazil. A three factor-two level full factorial design was used to investigate the effect of the pH （5.8-6.4）, storage time （0-10 days） and storage temperature （5-10 ℃）, on the responses, namely, mesophiles, psychrotrophs, lipolytic psychrotrophs and proteolytic psychrotrophs counts. Results showed that increase in pH and storage time had significant effect on the microbial count. No significant effect of storage temperature was observed for all the microorganisms studied. All responses were well predicted by selected mathematical models, as denoted by coefficient of determination above 0.95.