Populations, types and biochemical activities of aerobic bacteria and lactic acid bacteria from the air of cheese factories

This investigation was carried out in order to identify the airborne bacteria in the processing rooms of two creameries that make, mainly, feta cheese and study some of their biochemical activities. Our results suggest that the numbers of airborne micro-organisms may differ between plants, and the same was observed with the predominant types. The bacteria isolated were primarily Gram-positive, catalase-negative cocci, micrococci and bacilli. The predominant lactic microflora were lactococci and mesophilic lactobacilli, commonly found in feta and other traditional Greek cheeses. A significant number of aerobic bacteria could grow at 4°C and in 6.5% NaCl. In addition, both aerobic and lactic acid bacteria (LAB) exhibited acidifying and proteolytic activities. Slow- and medium-acidification ability of LAB strains were found. There was also a tendency of the strains to preferentially degrade αs-casein. The amounts of amino acids accumulated in the milk were strain dependent. Therefore, there is a possibility that airborne bacteria contaminating cheese milk in the open vats may multiply and contribute to the ripening changes in the cheese.     

Dairy products: technology

The paper deals with methods of prolonging the storage life of milk at creameries and then with technologies that have been mooted as possible alternatives to conventional heat exchangers. Postpasteurization contamination is dealt with briefly. In the case of cheese and spray drying, it is pointed out that the technology may have outreached the supporting science. The use of the microfluidizer, as opposed to the conventional valve homogenizer, is discussed. Finally, the current state of research aimed at decreasing the cholesterol level of milk is summarized.     

Protective influence of several packaging materials on light oxidation of milk

Light-induced degradation reactions in milk create a serious problem for the dairy industry because of the development of off-flavors, the decrease in nutritional quality, and the severity and speed by which these phenomena develop. Packaging materials are essential to avoid this particular deterioration of milk. Therefore, efforts are being made to design protective polyethylene terephthalate (PET) packages. In the present study, a number of PET bottles were compared for their ability to avoid photo-oxidation in UHT semi-skimmed milk. The milk was packed in 3 types of PET bottles: one transparent bottle provided with an active oxygen-binding inner layer, one bottle with perfect light barrier, and one transparent bottle provided with a UV-absorbing additive. During 2 storage experiments, running parallel to each other for 2 mo, chemical milk quality parameters such as fat oxidation, vitamin and protein degradation, oxygen consumption, and color change were monitored. A trained taste panel compared the sensory quality of the illuminated milk stored in these bottles, with milk perfectly protected against light and oxygen. In the first study, milk was continuously illuminated at room temperature. A comparison was made for milk under storage conditions that simulated those expected during display in retail and supermarkets. The results of the 2 shelf-life studies showed that an adequate light barrier was apparently sufficient to avoid the light-induced oxidation of milk during extended storage. Oxygen barriers, on the other hand, did not provide a significant protection, nor did bottles with UV filter. If wavelengths detrimental to riboflavin were not completely excluded by the packaging material, incoming light could still give rise to photo degradation of milk. Accordingly, riboflavin and vitamin A were gradually degraded, milk fat was photo-oxidized, oxygen dissolved in the milk was consumed, and the sensorial quality decreased significantly.     

UHT Milk Processing and Effect of Plasmin Activity on Shelf Life: A Review

Abstract: The demand for ultra‐high‐temperature (UHT) processed and aseptically packaged milk is increasing worldwide. A rise of 47% from 187 billion in 2008 to 265 billon in 2013 in pack numbers is expected. Selection of UHT and aseptic packaging systems reflect customer preferences and the processes are designed to ensure commercial sterility and acceptable sensory attributes throughout shelf life. Advantages of UHT processing include extended shelf life, lower energy costs, and the elimination of required refrigeration during storage and distribution. Desirable changes taking place during UHT processing of milk such as destruction of microorganisms and inactivation of enzymes occur, while undesirable effects such as browning, loss of nutrients, sedimentation, fat separation, cooked flavor also take place. Gelation of UHT milk during storage (age gelation) is a major factor limiting its shelf life. Significant factors that influence the onset of gelation include the nature of the heat treatment, proteolysis during storage, milk composition and quality, seasonal milk production factors, and storage temperature. This review is focused on the types of age gelation and the effect of plasmin activity on enzymatic gelation in UHT milk during a prolonged storage period. Measuring enzyme activity is a major concern to commercial producers, and many techniques, such as enzyme‐linked immunosorbent assay, spectrophotometery, high‐performance liquid chromatography, and so on, are available. Extension of shelf life of UHT milk can be achieved by deactivation of enzymes, by deploying low‐temperature inactivation at 55 °C for 60 min, innovative steam injection heating, membrane processing, and high‐pressure treatments.     

High temperature, short time pasteurization temperatures inversely affect bacterial numbers during refrigerated storage of pasteurized fluid milk

The grade A Pasteurized Milk Ordinance specifies minimum processing conditions of 72°C for at least 15 s for high temperature, short time (HTST) pasteurized milk products. Currently, many US milk-processing plants exceed these minimum requirements for fluid milk products. To test the effect of pasteurization temperatures on bacterial numbers in HTST pasteurized milk, 2% fat raw milk was heated to 60°C, homogenized, and treated for 25 s at 1 of 4 different temperatures (72.9, 77.2, 79.9, or 85.2°C) and then held at 6°C for 21 d. Aerobic plate counts were monitored in pasteurized milk samples at d 1, 7, 14, and 21 postprocessing. Bacterial numbers in milk processed at 72.9°C were lower than in milk processed at 85.2°C on each sampling day, indicating that HTST fluid milk-processing temperatures significantly affected bacterial numbers in fluid milk. To assess the microbial ecology of the different milk samples during refrigerated storage, a total of 490 psychrotolerant endospore-forming bacteria were identified using DNA sequence-based subtyping methods. Regardless of processing temperature, >85% of the isolates characterized at d 0, 1, and 7 postprocessing were of the genus Bacillus, whereas more than 92% of isolates characterized at d 14 and 21 postprocessing were of the genus Paenibacillus, indicating that the predominant genera present in HTST-processed milk shifted from Bacillus spp. to Paenibacillus spp. during refrigerated storage. In summary, 1) HTST processing temperatures affected bacterial numbers in refrigerated milk, with higher bacterial numbers in milk processed at higher temperatures; 2) no significant association was observed between genus isolated and pasteurization temperature, suggesting that the genera were not differentially affected by the different processing temperatures; and 3) although typically present at low numbers in raw milk, Paenibacillus spp. are capable of growing to numbers that can exceed Pasteurized Milk Ordinance limits in pasteurized, refrigerated milk.     

Quality and Emulsion Stability of Milk from Ettawah Crossed Bred Goat During Frozen Storage

Goat milk has specific chemical composition and physical properties. The purpose of this study was to investigate the change of milk quality and emulsion stability during 60 d frozen storage. Milk sample was taken from three Ettawah Crossedbred goats that were divided into three groups. Samples were frozen and stored for 0, 30 and 60 days. At the end of each storage periode, milk sample was thawed in the refrigerator and analyzed for total number of bacteria, chemical (acidity, pH, free fatty acids), physical (alcohol test, clot on boiling test) quality, and emulsion stability. The microbiological and chemical data were analyzed statistically using one way ANOVA, whereas physical quality and emulsion stabilty were expressed qualitatively. There were no change in the total bacteria, acidity, pH and free fatty acid (FFA) of milk during storage, whereas the assessment by 70% alcohol showed positive since 0 d. Emulsions stability changed after 30 d of storage. While, the clot on boiling (COB) test of milk was positive at 60 d of storage. Recommendation is frozen storage of goat milk should not longer than 30 d.     

The effect of nisin on the keeping quality of reduced heat-treated milks

Milk was subjected to a combination process involving reduced heat treatment (RHT) of 117 degrees C for 2 s and nisin (75 and 150 IU ml(-1)). The microbial activity and other quality aspects were compared with a RHT control (without nisin) and with a ultrahigh temperature (UHT) milk processed at 142 degrees C for 2 s. Nisin was found to inhibit microbial growth for products stored without refrigeration, and RHT-nisin samples stored at 30 degrees C showed very low spoilage rates during 150 days, although not low enough to satisfy requirements for commercial sterility. RHT-nisin samples could be distinguished from and were preferred to the UHT control. Significant browning occurred during storage at 30 degrees C and above but was less in the RHT-nisin milk samples compared with the UHT milk. In RHT-nisin milk samples stored at 20 and 10 degrees C, no microbial activity could be detected in most samples after storage for 1 year. The effectiveness of this combination of RHT, nisin, and low storage temperatures against gram-positive spore-forming bacteria suggests potential for use of nisin in extended shelf life products.     

The development of heat-resistant phosphatase activity in raw milk

The detection of phosphatase activity is used as a legal test to determine whether milk has been adequately pasteurized or whether it has been contaminated with raw milk. Occasional failures of the Milk Regulations phosphatase test were experienced by a processing dairy from stored silo milk. Trials demonstrated that the phosphatase was heat resistant and associated with a pasteurization-sensitive, psychrotrophic organism isolated from one supply. The standard total viable count at 30°C (TVC) of the supply was satisfactory at 4.59 log cfu/ml; however, the psychrotrophic count at 7°C was much higher at 5.61 log cfu/ml. The test milk after storage produced sufficient heat-resistant phosphatase activity to give a test failure when the psychrotrophic count reached about 7.09 log cfu/ml or greater. The occurrence of a failure was dependent on the initial numbers of psychrotophic bacteria, the amount of dilution with other milks and the storage time before processing. The psychrotrophic count of the test milk and the count of the phosphatase-producing isolate were found to increase by approximately one log cfu/ml each day on storage at 4°C. This investigation has shown that a phosphatase failure may indicate the development of microbial phosphatase rather than a process failure. A retest after laboratory pasteurization of any sample failing the test will assist in identifying any microbially produced heat-resistant phosphatase activity. While a TVC at 30°C will normally be expected to count most types of psychrotrophic organisms, this investigation has shown that on this occasion it did not detect specific psvchrotophic organisms which had contaminated the milk.     

THE EFFECT OF STORAGE TEMPERATURE ON BACTERIAL GROWTH AND LIPOLYSIS IN RAW MILK

The temperature at which raw milk was stored, within the range 4°-8°C, affected the rate of growth of bacteria and the release of free fatty acids. The effects were of both statistical and practical significance and it was shown that, by maintaining milk temperatures at 4°C, a useful extension of the storage time of raw milk could be achieved. Lipolysis in stored milk was not closely related to the total concentration of psychrotrophic bacteria, but lipolytic rancidity was not observed when the psychrotroph count was below 5 × 10⁶ colony forming units/ml.     

豆浆饮料的微波杀菌特性初步研究

本文在微波加热的条件下,对豆浆饮料的微波杀菌特性进行了初步探讨,研究了加热时间、温度和菌落总数之间的关系,以及功率对豆浆中菌落总数的影响。比较在不同贮藏温度下,微波和传统水浴杀菌豆浆在不同温度贮藏过程中菌落总数的变化情况,同时测量了不同处理样品的总糖、蛋白质和折光率。结果表明:经本文方法制备的豆浆,样品体积为10ml时,其最佳微波杀菌条件为微波功率560W、加热时间22s,使豆浆温度达到75℃;在低温贮藏过程中,微波杀菌样品比传统水浴杀菌样品中的菌落总数生长缓慢,可以延长豆浆饮料的保质期;微波杀菌可以更好地保留豆浆中的营养成分。     

西藏灵菇菌粒、瑞士乳杆菌LZ-R-5与普通乳酸菌的发酵乳特性对比及低场核磁共振在其储藏期间水分的在线监测应用

分别以西藏灵菇菌粒、瑞士乳杆菌LZ-R-5(分离自西藏灵菇菌粒)和"川秀"牌乳酸菌酸奶发酵粉发酵纯牛奶得到三种发酵乳。研究三种发酵乳发酵和储藏期间的生长特性,并利用低场核磁共振(low-field nuclear magnetic resonance,LF-NMR)技术研究三种发酵乳在发酵和储藏期间的水分分布和迁移规律,结合持水力和流变学特性,探究发酵乳水分迁移变化与流变学特性是否存在相关性。结果显示,发酵期间,三种发酵乳pH逐渐降低,乳酸酸活菌数先增多后减少。三种发酵乳在储藏期间,总水分含量和自由水含量都呈逐渐减少趋势,横向弛豫时间T₂₂先减小后增大;另外,三种发酵乳持水力变化、表观粘度、G'和G″变化一致,呈现先增大后减小的趋势。由此可知,LF-NMR技术可以应用于发酵乳储藏期间水分的在线监测,为发酵乳品质快速判定提供了理论依据。     

乳及乳制品中嗜冷菌的多样性及其腐败危害研究进展

乳是一种营养丰富的物质, 同时也是微生物生长繁殖的理想培养基。生乳的质量是影响乳制品产业链的关键因素, 随着低温储存和冷链运输技术的发展, 生乳中大部分细菌的生长受到抑制, 但嗜冷菌的生长并未受到抑制, 并逐步成为生乳中的优势菌。生乳在冷藏运输或储存期间, 嗜冷菌依旧可以生长繁殖, 其分泌的蛋白酶和脂肪酶可耐高温, 经过巴氏杀菌或超高温灭菌处理后依旧保持活性, 因此, 了解嗜冷菌的多样性及其产生的酶对提高乳及乳制品质量、减少腐败和浪费具有重要作用。本文通过介绍乳及乳制品中嗜冷菌污染现状及腐败危害研究, 旨在为乳及乳制品行业的风险评估提供背景信息, 从源头控制嗜冷菌对生乳的浪费, 保证乳及乳制品的品质。     

Shelf-life storage temperature has a considerably larger effect than high-temperature,short-time pasteurization temperature on the growth of spore-forming bacteria in fluid milk

In the absence of postpasteurization contamination, psychrotolerant, aerobic spore-forming bacteria that survive high-temperature, short-time (HTST) pasteurization, limit the ability to achieve HTST extended shelf-life milk. Therefore, the goal of the current study was to evaluate bacterial outgrowth in milk pasteurized at different temperatures (75, 85, or 90°C, each for 20 s) and subsequently stored at 3, 6.5, or 10°C. An initial ANOVA of bacterial concentrations over 14 d of storage revealed a highly significant effect of storage temperatures, but no significant effect of HTST. At d 14, average bacterial counts for milk stored at 3, 6.5, and 10°C were 1.82, 3.55, and 6.86 log₁₀ cfu/mL, respectively. Time to reach 1,000,000 cfu/mL (a bacterial concentration where consumers begin to notice microbially induced sensory defects in fluid milk) was estimated to be 68, 27, and 10 d for milk stored at 3, 6.5, and 10°C, respectively. Out of 95 isolates characterized with rpoB allelic typing, 6 unique genera, 15 unique species, and 44 unique rpoB allelic types were represented. The most common genera identified were Paenibacillus, Bacillus, and Lysinibacillus. Nonmetric multidimensional scaling identified that Bacillus was significantly associated with 3 and 10°C, whereas Paenibacillus was consistently found across all storage temperatures. Overall, our data show that storage temperature has a substantially larger effect on fluid milk shelf life than HTST and suggests that abuse temperatures (e.g., storage at 10°C) allow for growth of Bacillus species (including Bacillus cereus genomospecies) that do not grow at lower temperatures. This indicates that stringent control of storage and distribution temperatures is critical for producing extended shelf-life HTST milk, particularly concerning new distribution pathways for HTST pasteurized milk (e.g., electronic commerce), and when enhanced control of spores in raw milk is not feasible.     

STORAGE STUDY OF COMMERCIALLY PASTEURIZED WHOLE MILK1

Pasteurized whole milk was obtained from retail outlets and from all the processing plants in Maryland. During storage at 7.0°C the milk was evaluated by a consumer-type panel and analyzed for microbial quality. Over 90% of the milk obtained from the processing plants was judged acceptable by the panel following seven days of storage at 7.0°C, while 78% of the milk obtained at the retail level was judged as acceptable under the same conditions. Approximately 65% of all the milk surveyed at the retail level was less than two days old. Variations in the quality of milk among dairy processors and among retail outlets were noted. Total microbial numbers did not correlate well with organoleptic data.     

Effect of extended storage on microbiological quality, somatic cell count, and composition of raw goat milk on a farm

Dairy goat herds in the United States generally are small, widely scattered, and distant from processing facilities. Unlike the situation for cow milk, it is not cost-effective to collect goat milk everyday or every other day. In some areas, goat milk is collected only once each week, which is in violation of regulations specified in the Pasteurized Milk Ordinance for grade A milk. This study was conducted to determine the effect of up to 7 days of refrigerated bulk tank storage on composition, somatic cell count (SCC), pH, and microbiological quality of goat milk. Duplicate farm bulk tank samples were taken daily after the morning milking for seven consecutive days each month during the lactation season. Samples were analyzed immediately for all variables except free fatty acids. There were no significant changes (P > 0.05) detected in milk fat, protein, lactose, nonfat solids, SCC, or pH during extended storage, although significant effects of stage of lactation (P < 0.05) were observed. The mean standard plate count (SPC) increased to 1.8 x 10(5) CFU/ml after 6 days of storage, exceeding the grade A limit (i.e., 1.0 x 10(5) CFU/ml). The mean psychrotrophic bacteria count increased steadily to 1.5 x 10(4) CFU/ml after 6 days of storage, whereas the mean coliform count was approximately 500 CFU/ml for the first 3 days and less than 2500 CFU/ml throughout the 7 days of storage. No significant changes (P > 0.05) in the concentrations of free fatty acids, except for butyric and caprylic acids, were observed during milk storage. When stored under refrigerated and sanitary conditions, goat milk in farm bulk tanks met the grade A criteria for both SPC and SCC during 5 days of storage but was of low quality thereafter because of the growth of psychrotrophic bacteria.     

电子舌联合微生物测序技术分析贮运温度对巴氏杀菌乳品质的影响

采用电子舌联合高通量测序两种快速现代检测技术,对5 个不同温度下贮藏不同时间的巴氏杀菌乳样品进行感官品质和细菌多样性测定,进一步利用统计学软件分别对各样品味感值及细菌种类作主成分和关联性分析,揭示贮运温度对巴氏杀菌乳感官品质和微生物的影响规律,以及主要残留微生物与巴氏杀菌乳贮运期间感官品质之间的关系。结果表明：巴氏杀菌乳在0、4、10 ℃贮藏3 d内皆能保持良好的乳香味,在15、25 ℃条件下贮藏会导致甜味的显著下降。而随着贮藏温度升高及时间延长,微生物的生长也导致了巴氏杀菌乳的发酵腐败,细菌多样性及群落结构与巴氏杀菌乳感官品质显著相关。研究发现,气单胞菌属（Aeromonas）、阪崎肠杆菌（Cronobacter）、沙雷氏菌（Serratia）、梭状芽孢杆菌（Costridium）菌属对鲜味味感的影响最小,同时与巴氏杀菌乳的苦味、咸味、甜味呈现负相关性。除此以外,其他属水平物种皆对苦味呈现显著的正相关性。因此,这些菌属可能是致使乳品发生腐败变质的关键因素,这为更好地分析微生物组成与品质变化的相互影响性,快速、精确判定乳制品的品质变化提供了理论基础。     

Microbiological changes throughout ripening of goat cheese made from raw,pasteurized and high-pressure-treated milk

The bacteriological quality during ripening of raw (RA), pasteurized (PA; 72°C, 15 s) and pressure-treated (PR; 500 MPa, 20°C, 15 min) goat milk assessed by enumeration of total bacteria, psychrotrophic bacteria, Enterobacteriaceae, lactobacilli, enterococci, Micrococcaceae and lactococci was evaluated. The high pressure treatment applied was as efficient as pasteurization in reducing the bacterial population of milk. Experimental cheeses were made from RA, PA and PR milks to study the microbial population during ripening. Lactobacilli and lactococci were the predominant microbiota present during ripening in all the cheeses. There were no differences in numbers of starter bacteria during ripening. However, lactobacilli counts for RA milk cheese were significantly higher than for PA and PR cheeses in all the ripening stages studied. Micrococcaceae and enterococci remained at a secondary level, and no differences were observed between cheeses at the end of ripening. On the other hand, the number of Enterobacteriaceae decreased during ripening, but faster in PR milk cheese than in PA and RA milk cheeses. The results of this study suggest that goat cheese made from PR milk had similar microbiological characteristics to PA milk cheeses.     

传统乳制品中乳酸菌的筛选及发酵性能研究

为获得优良发酵性能的乳酸菌,采用传统培养方法从传统发酵乳样品中分离乳酸菌,并将分离菌株制备发酵乳。通过考察发酵乳的感官品质筛选优良菌株,采用形态学观察及16S rDNA序列分析对筛选菌株进行鉴定;并对其生长性能、产酸性能及其制备发酵乳贮藏期内品质变化进行测定。结果显示,共分离得到67株乳酸菌,筛选出5株性能较佳的菌株（编号为C-1、Q164、Q165、Q166和Q167）,均被鉴定为嗜热链球菌（Streptococcus thermophilus）。其中,菌株C-1生长性能和产酸性能最佳,该菌株制备的发酵乳奶香明显,口感细腻顺滑,感官评分最高（93.5分）,30 ℃贮藏21 d后,酸度为86.0 °T,活菌数为7.2×103 CFU/g。综上,嗜热链球菌C-1具备优良发酵性能菌株的应用潜力。     

Potential for broad applications of flow cytometry and fluorescence techniques in microbiological and somatic cell analyses of milk

Monitoring the quality and safety of milk requires careful analysis of microbial and somatic cell loading. Our aim was to demonstrate proof of the principle that flow cytometry (FCM), coupled with fluorescence techniques for distinguishing between cell types, could potentially be employed in a wide variety of biological assays relevant to the dairy industry. To this end, we studied raw milk samples and ultraheat-treated milk, into which known numbers of bacteria or mouse cells were inoculated. For bacterial analyses, protein and lipids were removed, whereas only centrifugal lipid clearing was needed for somatic cell analyses. Cleared samples were stained with fluorescent dyes or with bacterial-specific fluorescent-labeled oligonucleotides and analyzed by FCM. A fluoresceinated peptide nucleic acid probe enabled efficient enumeration of bacteria in milk. Dual staining of samples with fluorescent dyes that indicate live (5-cyanol-2,3-ditolyl tetrazolium chloride, CTC or SYTO 9) or damaged cells (oxonol or propidium iodide, PI) enabled determination of viable bacteria in milk. Gram-positive and -negative bacteria were distinguished using hexidium iodide and SYTO 13 in dual staining of cleared milk samples. An FCM-based method gave a good correlation (r=0.88) with total microscopic counts of somatic cells in raw milk. The FCM method also correlated strongly (r=0.98) with the standard Fossomatic method for somatic cell detection. We conclude that FCM, coupled with fluorescence staining techniques, offers potentially diverse and rapid approaches to biological safety and quality testing in the dairy industry. Potential application of flow cytometers to a broad range of assays for milk biological quality should make this instrumentation more attractive and cost effective to the dairy industry and indeed the broader food industry.     

Spoilage patterns of skim and whole milks

The reason for the reported difference in spoilage behaviour of skim and whole pasteurised milks was investigated. The rates of growth of psychrotrophic bacteria were not significantly different in the two milks and the bacterial types, all pseudomonads, present at spoilage were also similar. However, when representative spoilage organisms were cultured into freshly pasteurised skim and whole milks, skim milks exhibited predominantly bitter flavours while whole milk showed mostly sour flavours. The different spoilage behaviours can be largely explained by greater proteolvsis in skim milk than in whole milk, caused by higher production of protease and greater susceptibility of the protein to protease attack. In addition, lipolysis in whole milk, caused by the substantial quantities of lipase produced by spoilage pseudomonads in this milk, also contributes to the different flavours produced during cold storage of these milk types.