乳中蛋白酶与UHT乳贮存中的胶凝现象

乳中的蛋白酶有2个主要的来源途径，乳中天然存在的蛋白酶和由某些微生物产生的蛋白酶，其中纤维蛋白溶酶和嗜冷菌产生的耐热性蛋白酶是存在于UHT乳中的主要蛋白酶，这些蛋白酶非常耐热，经UHT灭菌处理仍可存活。耐热性蛋白酶在UHT乳贮存中水解乳蛋白质从而导致了UHT乳的胶凝．简要介绍了纤维蛋白溶酶、嗜冷菌耐热性蛋白酶的性质、活性测定方法，论述了这些酶引起的UHT乳的胶凝性质、形成原因及影响因素，并提出了一些防止措施。     

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。     

UHT乳结块原因分析

总结了UHT乳产生结块问题的主要原因，并对发生结块的UHT乳进行了微生物指标与蛋白酶活性的检测。结果显示：市售UHT灭菌乳中残留蛋白酶是导致结块现象的主要原因。     

基于高通量测序解析UHT灭菌乳品质劣变关联微生物

针对超高温灭菌（ultra-high temperature treated,UHT）乳在货架期内出现的脂肪上浮、水乳分离、沉淀等品质劣变问题,利用高通量测序和生物信息学等技术方法,对大型乳企采集的9份在货架期内品质劣变的UHT乳和正常质量的UHT乳样品的理化指标、酶活性、微生物菌群进行了分析比较,并通过关联分析解析UHT灭菌乳品质劣变的关联微生物。结果表明：品质劣变UHT乳在理化指标、酶活性和微生物群分布方面与正常UHT乳有明显不同,假单胞杆菌、不动杆菌为品质劣变UHT乳中核心功能微生物且均属于常见的嗜冷菌,能够产生耐热酶,这可能是导致UHT乳腐败变质的主要原因。本研究为进一步解决UHT乳品质劣变问题提供了参考。     

嗜冷菌蛋白酶及其对乳制晶的影响

嗜冷菌蛋白酶是存在与乳中的主要的耐热性蛋白酶,通过水解乳蛋白对乳及乳制品的品质产生影响.介绍了这些蛋白酶的性质、活性测定方法、控制措施及其对UHT乳及干酪的影响.     

原料乳中产耐热蛋白酶细菌的鉴定及其酶学特性研究

为了研究UHT乳在货架期内发生水解、凝块、变苦等品质劣变现象的原因,从原料乳中分离得到一株产耐热蛋白酶细菌,通过细菌形态观察、生理生化试验、脂肪酸分析、16S rDNA基因序列测定及16-23S rRNA间区序列分析,对该菌株进行鉴定,并对其所产蛋白酶进行初步研究。结果表明:该菌株属于荧光假单孢杆菌。该菌株所产的耐热蛋白酶的最适温度为40℃,最适pH为6.8。该蛋白酶可以耐受140℃,4 s热处理。     

耐热性脂肪酶及其对UHT乳品质的影响

UHT(超高温)乳饮用方便、营养丰富、备受消费者欢迎,近年来在我国的乳品消费结构中增长迅速,但是UHT乳在整个贮存过程中常因其中残留的脂肪酶、磷脂酶等因素而导致脂肪上浮、脂肪分解等现象,使产品的风味、品质变差,主要从耐热性脂肪酶对UHT乳品质的影响出发,综述了国内外对牛乳中主要耐热脂肪酶的研究现状,并提出今后深入研究应关注的方向.     

纤维蛋白酶及其对乳制品的影响

纤维蛋白酶及其酶原是存在于乳中的一对主要碱性蛋白酶，通过水解乳蛋白对乳及制品的品质产生形响。简要介绍了该酶的性质、活性测定方法，最后论述了对干酪及UHT乳的影响。     

耐热蛋白酶对UHT乳蛋白水解作用

采用SDS-PAGE、Urea-PAGE 分析耐热蛋白酶对UHT 乳蛋白的水解作用。结果表明：荧光假单胞菌(PF)蛋白酶和纤溶酶(PL)作用底物不同,产物也不同;前者优先水解κ- 酪蛋白生成副κ- 酪蛋白,而PL 主要水解β- 酪蛋白与α- 酪蛋白生成γ- 酪蛋白及胨、肽等。分别经PL和细菌蛋白酶水解3h的UHT乳水解产物,在pH4.6和12g/100mL三氯乙酸(TCA)沉淀后经反相高效液相色潽仪(RP-HPLC)分析滤液,两者呈现不同的色谱图。     

UHT牛奶蛋白凝块产生原因及控制措施的研究

采用酶活性检测技术,系统地研究了储存5、6、7、8个月的UHT牛奶蛋白酶活性变化和中温加热方式对UHT奶耐热酶的灭活效果.结果表明,随着产品储存期的延长,产品中蛋白酶的活性越强;中温加热条件能灭残留耐热酶的活性,在中温加热温度50～60℃和加热时间15～25min的条件下,耐热性蛋白酶的灭活率达到最高,为46.67%.     

苦味UHT乳原因分析

通过对发生苦味的UHT乳进行分析，将引起UHT乳贮存中出现苦味的原因分为两大类：一是受残留的微生物污染导致UHT乳产生苦味和其它异味，这类微生物包括耐热性强的解淀粉芽孢杆菌和青霉菌等；二是由于牛乳中存在的水解酶分解蛋白质和脂肪．生成肽、氨基酸和脂肪酸类，导致苦味的出现。针对这些影响因素，可以通过提高原料乳的卫生质量、严格控制生产加工过程和环境卫生，以避免UHTSL出现苦味的质量问题。     

Proteolytic activity among psychrotrophic bacteria isolated from refrigerated raw milk

Psychrotrophic bacteria were isolated from refrigerated raw milk from a processing plant in Southern Brazil. Psychrotrophic counts were between 4.9 and 7.8 log cfu/mL, and 5.3 to 7.2 log cfu/mL, for samples collected at the truck and the milk storage silo, respectively. Among the bacterial isolates, 90% were Gram-negative. Most strains presented low proteolytic activity, but strains of Burkholderia cepacia, Klebsiella oxytoca and Aeromonas sp. showed higher than 20 U/mL on azocasein as substrate. Crude proteases from selected strains were resistant to conventional heat treatments and caused coagulation of UHT milk after 5 days storage at room temperature.     

Enhanced inactivation of bacterial lipases and proteinases in whole milk by a modified ultra high temperature treatment

Cultures of Pseudomonas spp. strains P10, P12 and P15 grown in whole milk which contained approximately 1 x 10(8) viable bacteria ml-1 demonstrated near linear increases in the concentration of short-chain free fatty acids and trichloroacetic acid soluble free amino groups at 20 degrees C, following either ultra high temperature (UHT) treatment (140 degrees C for 5 s) or dual heat treatments (140 degrees C followed by either 57, 60 or 65 degrees C). The dual heat treatments reduced the rates of lipolysis and proteolysis compared to the UHT treatment by up to 25-fold. The dual heat treatment utilizing 60 degrees C for 5 min also effectively limited both lipase and proteinase activities in raw milk culture samples which had contained either 6 x 10(6), 5 x 10(7) or 1 x 10(8) viable bacteria ml-1. In this system enzyme activities were reduced by up to 10-fold following dual heat treatment compared to UHT treatment alone.     

Effects of different calcium salts on properties of milk related to heat stability

Insoluble calcium salts were added to milk to increase total calcium by 30 mM, without changing properties influencing heat stability, such as pH and ionic calcium. There were no major signs of instability associated with coagulation, sediment formation or fouling when subjected to ultra high temperature (UHT) and in‐container sterilisation. The buffering capacity was also unaltered. On the other hand, addition of soluble calcium salts reduced pH, increased ionic calcium and caused coagulation to occur. Calcium chloride showed the largest destabilising effect, followed by calcium lactate and calcium gluconate. Milk became unstable to UHT processing at lower calcium additions compared to in‐container sterilisation.     

The Extracellular Protease AprX from Pseudomonas and its Spoilage Potential for UHT Milk: A Review

The negative effects of proteases produced by psychrotrophic bacteria on dairy products, especially ultra‐high‐temperature (UHT) milk, are drawing increasing attention worldwide. These proteases are especially problematic, because it is difficult to control psychrotrophic bacteria during cold storage and to inactivate their heat‐resistant proteases during dairy processing. The predominant psychrotrophic species with spoilage potential in raw milk, Pseudomonas, can produce a thermostable extracellular protease, AprX. A comprehensive understanding of AprX on the aspects of its biological properties, regulation, proteolytic potential, and its impact on UHT milk can contribute to finding effective approaches to minimize, detect, and inactivate AprX. AprX also deserves attention as a representative of all extracellular metalloproteases produced by psychrotrophic bacteria in milk. The progress of current research on AprX is summarized in this review, including a view on the gap in current understanding of this enzyme. Reducing the production and activity of AprX has considerable potential for alleviating the problems that arise from the instability of UHT milk during shelf‐life.     

Review of UHT processing methods

The flavour and nutritional quality of UHT processed milk is discussed and suggestions are made for ways of minimizing undesirable changes during heat treatment. UHT treatment of milk using an infusion heater is described, and it is concluded that whole milk processed in this way is a typical directly heated UHT milk.     

Proteolysis in ultra‐heat‐treated skim milk after exposure to multispecies biofilms under conditions modelling a milk tanker

Bacteria from milk tankers can form multispecies biofilms and produce heat‐stable enzymes. In this study, milk was exposed to multispecies biofilms in stainless steel vessels and was then used to produce ultra‐heat‐treated (UHT) milk, which was stored for 5 months. The UHT milks were assessed for microbial counts, free peptide concentration and pH. The free peptide concentration, which indicated proteolysis, was higher in UHT milk that had been exposed to multispecies biofilms than in untreated UHT milk. Biofilm formation may be promoted in milk tankers that are not properly cleaned, which may compromise the quality of the final dairy product.     

Heat-induced coagulation of concentrated skim milk heated by direct steam injection

Direct steam injection heat treatment on pilot scale was applied to investigate heat stability of concentrated skim milk across a broad range of temperatures from 117 °C to 153 °C and from 0.5 to 13 s holding time, assessing options for heat treat treatment without a significant amount of protein sediment formation. The relationship between total solids content of concentrated skim milk and temperature–time combinations of heat treatment could be established using minimal heat-induced coagulation as a criterion. Coagulation of destabilised casein micelles was shown to proceed non-linear over heating temperature. Transition of critical temperature–time combinations resulted in a marked increase in sediment formation indicating that preceding reactions, noticeable as the formation of dissociated material, need to take place to some extent to induce coagulum formation. UHT pre-heat treatment of skim milk prior to concentration was shown to increase heat stability in terms of possible temperature–time combinations without coagulation.