| Sumario: | The development of different products that confer health benefits on the population is a challenge for thosewho 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 milkand kefir grains were used in preparing the products and their percentage composition was determined. C1, C2, W1 and W2were contaminated with five different logarithmic fractions (A = 8log to E = 4log CFU/ml) of Staphylococcus aureus (ATCC25923) and Escherichia coli (ATCC 11229), with antibacterial activity assessed over 0, 24, 48 and 72 hours of exposure. Theresults demonstrated the antibacterial activity of kefir cheese and whey, especially after 24 hours. Escherichia coli was themost sensitive of the bacteria, with maximum antibacterial activity seen in the cheese at population densities D and E, and inthe whey at densities B, C, D and E after 48 and 72 h, showing that the in situ antibacterial activity of foods produced withkefir grains tends to be lower when compared with studies in vitro. The greater the nutrient content of the food, the lowerthe antibacterial activity seen, probably due to the protective action that the nutrients confer on the microorganisms againstbacteriocins and the metabolites from fermentation.
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