PHYSICAL-CHEMICAL AND MICROBIOLOGICAL QUALITY OF BOVINE MILK IN DIFFERENT PRODUCTION SYSTEMS AND SEASONS

Abstract The objective of this work was to evaluate the effect of the interaction between production systems and seasons on physical-chemical and microbiological qualities of bovine milk. Three farms were assessed monthly, along the seasons, for four specialization levels of the production systems: highly specialized, specialized, partially specialized, and non-specialized. In terms of milk quality, the levels of fat, protein, total solids, somatic cell count, total bacterial count, titratable acidity in degrees Dornic, density, cryoscopic index, alizarol stability, potential of hydrogen (pH) and methylene blue reduction test. In addition, due to their importance for the industrial yield of protein-based dairy products, total milk protein fractionation was performed in protein equivalent of non-protein nitrogen, true protein and casein.The milk Dornic acidity was higher in spring and winter, while the highest somatic cell count values were in the summer. Total and true protein levels were lower during the summer and total solids increased in the autumn. With the decrease in the levels of specialization of farms, there was an increase in total bacterial count and decrease in non-protein nitrogen. The density presented an inverse behavior to that of the fat while the hydrogen potential (pH) was expressed in a way antagonistic to the acidity. In the multivariate analysis of the data it was possible to identify the combinations between production strata and stations that presented the best milk protein quality. In this sense, all the arrangements of production systems with autumn, winter highly specialized, summer partially specialized, spring specialized had positive protein quality, being highlighted by the smaller of somatic cells counts in the spring partially specialized combination.摘要 本研究旨在评估生产系统与季节的交互作用对牛乳理化及微生物品质的影响。本研究按月对3个牧场开展四季周期内的监测评估，覆盖4类生产系统专业化等级：高度专业化、专业化、部分专业化与非专业化。针对牛乳品质，本研究测定了脂肪、蛋白质、总固体含量、体细胞数、细菌总数、多恩尼克度滴定酸度、密度、冰点指数、茜素红稳定性、亚甲蓝还原试验以及pH值等指标。此外，鉴于其对蛋白类乳制品工业得率的重要意义，本研究还对牛乳总蛋白组分进行分级分析，涵盖非蛋白氮当量蛋白、真蛋白与酪蛋白三类组分。研究结果显示：春季与冬季的牛乳多恩尼克度滴定酸度更高，夏季的体细胞数最高；夏季的总蛋白与真蛋白水平较低，秋季的总固体含量则有所升高。随着牧场专业化水平降低，细菌总数呈上升趋势，而非蛋白氮含量则出现下降。密度与脂肪含量呈负相关关系，而pH值与酸度表现为拮抗效应。通过对数据进行多变量分析，可确定生产层级与季节的最优组合，以获得最佳牛乳蛋白品质。据此，所有与秋季、冬季高度专业化、夏季部分专业化、春季专业化配置的生产系统组合均表现出优良的蛋白品质，其中春季部分专业化组合的体细胞数最低，最为突出。