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 °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 °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.     

Effect of activating lactoperoxidase system in cheese milk on the quality of Saint‐Paulin cheese

Saint‐Paulin cheese was made from cow’s milk refrigerated at 4 °C for 72 h and preserved by the lactoperoxidase (LP) system. The effect of the LP system on the microbiological, physicochemical and biochemical properties of cheese over a ripening period of 23 days was investigated, using a control (C0), refrigerated LP‐inactivated cow’s milk (C1) and refrigerated LP‐activated cow’s milk (LPA). The LPA treatment showed the least contamination in flora count, particularly salt‐tolerant bacteria at the end of the ripening period. LPA cheese had significantly lower coliform, yeasts and mould counts (P < 0.05) than the other cheeses; this demonstrated the bacteriostatic effect of the LP system. The proteolysis results showed the least value for LPA cheese as compared with the two other samples, as determined by using sodium dodecyl sulphate‐polyacrylamide gel electrophoresis of casein fractions extracted from the three samples. The findings indicated that the preservation of cow’s cheese milk by the LP system can be used to improve the microbiological quality, inhibit psychotropic germs, correct the losses of soluble nitrogen fractions in the whey and conserve the cheese yield affected by refrigeration.     

Effect of thiocyanate and hydrogen peroxide on the keeping quality of ovine,bovine and caprine raw milk

The preservation of raw ovine, bovine and caprine milks by the activation of their natural lactoperoxidase (LP) systems was investigated. The LP system of the samples was activated by adding different amounts of sodium thiocyanate and sodium percarbonate to give three different concentrations of thiocyanate and hydrogen peroxide: 7, 14 and 28 mg/L and 15, 30 and 60 mg/L, respectively. Each type of raw milk, ovine, bovine and caprine, was analysed after being treated as follows: Control (C), LP inactivated (T0), LP activated with different concentrations of thiocyanate (7, 14 and 28 mg/L) and hydrogen peroxide (15, 30 and 60 mg/L) and stored at 4°C for 72 h. The results indicated that concentrations of 28 mg/L (SCN^?) and 60 mg/L (H₂O₂) would be adequate for preserving milks of different mammals at 4°C, but we should take into consideration the international norm – 14 mg/L of SCN^? and 30 mg/L of H₂O₂, respectively. Overall, the results have shown that, by activation of the LP system in raw milk, it was possible to store ovine, bovine and caprine milks at 4°C for several days. Changes in titratable acidity, total colony counts, psychrotrophic bacteria, coliforms, moulds and yeasts of the milk samples were followed during storage at low temperature with increasing dose. The effect of doses was greater in bovine milk than in caprine milk and finally in ovine milk.