CAJ | 학술논문

为了充分了解原料乳中纤溶酶活性，以便从源头上提高原料乳品质，该文针对中国原料乳的生产现状，分析了奶牛品种、胎次、养殖模式及挤奶时间对原料乳中纤溶酶活性的影响，并对纤溶酶的热稳定性进行了研究。结果表明，养殖模式、奶牛品种及胎次对原料乳中纤溶酶活性影响显著（P＜0.05），牧场养殖、1、4胎和娟珊牛原料乳中纤溶酶活性显著低于养殖小区、2、3胎及荷斯坦牛乳中的纤溶酶活性；挤奶时间对原料乳中纤溶酶活性影响不显著（P＞0.05）；原料乳中纤溶酶活性随热处理温度的升高而逐渐下降，巴氏杀菌（75℃、15 s）仅可使原料乳中纤溶酶活性下降25%；半胱氨酸对乳中纤溶酶活性具有一定的抑制作用。研究结果对根据纤溶酶活性对原料乳进行品质评价及分级具有一定借鉴作用。
위료충분료해원료유중섬용매활성，이편종원두상제고원료유품질，해문침대중국원료유적생산현상，분석료내우품충、태차、양식모식급제내시간대원료유중섬용매활성적영향，병대섬용매적열은정성진행료연구。결과표명，양식모식、내우품충급태차대원료유중섬용매활성영향현저（P＜0.05），목장양식、1、4태화연산우원료유중섬용매활성현저저우양식소구、2、3태급하사탄우유중적섬용매활성；제내시간대원료유중섬용매활성영향불현저（P＞0.05）；원료유중섬용매활성수열처리온도적승고이축점하강，파씨살균（75℃、15 s）부가사원료유중섬용매활성하강25%；반광안산대유중섬용매활성구유일정적억제작용。연구결과대근거섬용매활성대원료유진행품질평개급분급구유일정차감작용。
Plasmin(PL) is by far the predominant and most completely studied endogenous protease in bovine milk. PL activity level is an important factor affecting dairy product quality and its processing characteristics. Raw milk with a high PL level can have detrimental effects on the quality of pasteurized milk and UHT milk, causing undesirable flavor, precipitation and gelation, and shortening the shelf life of the product. Considerable research has focused on minimizing the activity of PL in fluid milk. Many factors, such as pH, mineral content, whey proteins, and storage temperature can influence the kinetics of the PL-induced hydrolysis. However, according to China’s current raw milk production situatioin, the objective of this study is to determine how such factors such as cattle breed, parity, dairy farming mode, and milking time affect PL activity in milk; thermal stability of plasmin is also studied. The results show that the PL activity between dairy farming mode, parity, and cow breeds are significant (P<0.05). Although there are effects of dairy farming mode, parity, and cow breeds on plasmin activity, they are lower in large scale farming, parity (1,4), and Jersey cows. However, the effect of the milking time on PL activity is not significant (P>0.05). PL activity of raw milk gradually decreases with increasing thermal treatment temperature. However, the thermal treatment associated with pasteurization of milk (75℃, 15 sec) has been shown to increase PL levels. This increase in PL activity has long been attributed to inactivation of plasmin inhibitors(PI) and plasminogen activator inhibitors(PAI), but plasminogen activators (PA)is even more heat stable than PIand PAIduring pasteurization and UHT. In this temperature range, plasminogen (PG) loses its naturally occurring tertiary structure, but is not yet inhibited. On the contrary, PG becomes more accessible to the action of PA by unfolding of its kringles. Activation of PG is enhanced upon pasteurization heating. Further heat treatment has been shown to reduce PL levels. PL activity only decreases 25 percent after pasteurized (75℃, 15 sec) treatment. The effects of cysteine addition and heat treatment on PL activity is also studied. The results show that cysteine has some inhibited effect on PL activity in milk. When milk was stored after heat treatment (75℃), there is a decrease in PL activity. PL activity is significantly affected due to interactions with cysteine, which contains free SH groups that cause irreversible denaturation of PL by S-S/S-H interaction. When milk is heated to higher temperatures of 85℃ or 90℃, a larger decrease in PG activity and no further increase in PL activity is observed. This is due to PG interaction with cysteine. PG activity is tested in the presence of cysteine, which causes a marked decrease in PG activity. Two-dimensional electrophoresis confirms that heating of PG in the presence of cysteine causes polymerization of PG into dimers, trimers, and polymers through disulfide interchange. Formation of polymers hinders the activation of PG into PL. This result suggests that the addition of cysteine prior to the pasteurization significantly decreases the potential for proteolysis caused by the PL system during storage. This study has certain reference value to the quality evaluation and grading of raw milk.