The effect of refrigerated storage of raw milk on the physicochemical and microbiological quality of Tunisian semihard Gouda‐type cheese during ripening

A Tunisian semihard Gouda-type cheese made from milk kept at 4 degrees C for 24, 48, 72 and 96 h was monitored during 45 days of ripening. The effect of milk refrigeration on the evolution of physicochemical parameters in relation to the quantitative variation of the microbial population during ripening of Gouda-type cheese was investigated. Microbiological and physicochemical analyses were performed on raw milk and cheese samples after curding, 2, 9, 16, 23, 30, 37 and 45 days of ripening time. The raw milk kept under refrigeration at 4 degrees C for 96 h showed the highest microbial count and proteolysis level. The duration of storage significantly reduced the cheese yield as a result of important solubilisation casein in proteoses-peptones. Results of different nitrogenous fractions by Kjeldahl method showed enzymatic hydrolysis products of casein whose intensity depended on the maturing stage as well as the refrigeration time. Besides the evident action of the plasmin, original milk protease, on the hydrolysis of casein in soluble fractions, the proteolysis of cheese caseins is also initiated by proteolytic action of the chymosin and extracellular heat-resistant proteases notably produced by the same psychrotrophic microflora. Lactic acid bacteria starters that constitute the dominant microflora of this type of cheese are also considered as aroma precursors.