Antibacterial activity of different formulations of cheese and whey produced with kefir grains

ABSTRACT The development of different products that confer health benefits on the population is a challenge for those who work with food. The aim of this study was to elaborate two formulations of kefir cheese (C1 and C2) and whey (W1, W2), and to evaluate their in situ antibacterial activity against microorganisms of interest in food. Pasteurized milk, powdered milk and kefir grains were used in preparing the products and their percentage composition was determined. C1, C2, W1 and W2 were contaminated with five different logarithmic fractions (A = 8log to E = 4log CFU/ml) of Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 11229), with antibacterial activity assessed over 0, 24, 48 and 72 hours of exposure. The results demonstrated the antibacterial activity of kefir cheese and whey, especially after 24 hours. Escherichia coli was the most sensitive of the bacteria, with maximum antibacterial activity seen in the cheese at population densities D and E, and in the whey at densities B, C, D and E after 48 and 72 h, showing that the in situ antibacterial activity of foods produced with kefir grains tends to be lower when compared with studies in vitro. The greater the nutrient content of the food, the lower the antibacterial activity seen, probably due to the protective action that the nutrients confer on the microorganisms against bacteriocins and the metabolites from fermentation.

摘要
开发对人群具有健康益处的各类食品，是食品领域从业者面临的一项挑战。本研究旨在制备两种开菲尔奶酪（kefir cheese，C1、C2）与乳清（whey，W1、W2）配方，并评估其对食品相关目标微生物的原位抗菌活性。实验采用巴氏杀菌乳、乳粉与开菲尔粒（kefir grains）制备样品，并测定其百分组成。将C1、C2、W1、W2分别接种五组不同对数梯度（A组为8log至E组为4log 菌落形成单位每毫升（CFU/ml））的金黄色葡萄球菌（Staphylococcus aureus，ATCC 25923）与大肠杆菌（Escherichia coli，ATCC 11229），并在暴露0、24、48、72小时后评估其抗菌活性。结果表明，开菲尔奶酪与乳清均展现出抗菌活性，尤其在24小时后效果显著。大肠杆菌对该抗菌作用最为敏感：在48与72小时检测节点，奶酪中的抗菌活性峰值出现在D、E密度组，乳清中则见于B、C、D、E密度组。本研究发现，以开菲尔粒制备的食品，其原位抗菌活性往往低于体外实验报道结果。食品营养成分含量越高，抗菌活性越低，这一现象可能源于营养物质为微生物提供了抵御细菌素（bacteriocins）与发酵代谢物的保护作用。