Microbiological, chemical, physical, and proteolytic activities of raw milk after thermal processing

ABSTRACT The aim was to evaluate the microbiological, chemical- physical, and shelf-life quality of milk samples after pasteurization (HTST) for 10 days or ultra-high temperature (UHT) treatment for 120 days. Raw milk counts of mesophilic aerobic microorganisms, Staphylococcus spp. and thermotolerant coliforms before HTST and UHT processing were 6.73 and 7.77; 2.84 and 4.30, and 4.68 and 4.37log10, respectively. Pseudomonas spp. were found in raw milk samples. No presence of any other microorganisms studied was detected and no microbial inhibitor was found. Processed samples met microbiological legal requirements. However, aerobic mesophilic counts for HTST pasteurized milk samples stored for 5 and 10 days increased to values comparable to those in raw milk. Composition chemical- physical of all samples were within legal limits. These results demonstrate that, although HTST and UHT processed milk comply with the microbiological standards required by Brazilian law, high microbial counts in raw milk are an issue, possibly due to failures in the early stages of the production chain. Increase in casein macropeptide (CMP), probably because of proteases psychrotrophic bacteria. It is concluded that the quality of raw milk directly influences the progressive increase of the CMP values.摘要 本研究旨在评估经高温短时（HTST, High-Temperature Short Time）巴氏灭菌法处理后储存10天，以及超高温（UHT, Ultra-High Temperature）灭菌法处理后储存120天的牛乳样品的微生物学、理化及货架期品质。经HTST与UHT加工前，生乳中嗜温需氧微生物、葡萄球菌属（Staphylococcus spp.）及耐热大肠菌群的菌落数分别为6.73 log₁₀与7.77 log₁₀、2.84 log₁₀与4.30 log₁₀、4.68 log₁₀与4.37 log₁₀。生乳样品中检出假单胞菌属（Pseudomonas spp.），未检出其余研究涉及的微生物，亦未发现微生物抑制剂。经加工后的样品符合微生物学法定要求，但经HTST巴氏灭菌的牛乳样品在储存5天及10天后，嗜温需氧菌菌落数升至与生乳相当的水平。所有样品的理化成分均处于法定限值范围内。上述结果表明，尽管经HTST与UHT加工的牛乳均符合巴西法规规定的微生物学标准，但生乳中较高的微生物菌落数仍是一个不容忽视的问题，其成因可能与生产链早期环节的操作失误有关。酪蛋白大肽（CMP, casein macropeptide）水平升高，推测与嗜冷菌产生的蛋白酶相关。综上，生乳品质直接影响酪蛋白大肽数值的逐步升高。