Twenty-three strains of lactic acid enterococci isolated from dromedary raw milks in Southern Algeria were investigated for their proteolytic activity and ability to produce biogenic amines by decarboxylating amino acids: histidine, tyrosine and ornithine. All strains tested displayed proteolytic activities, but none of them were able to produce detectable biogenic amines in the usual adequate laboratory media, namely, Joosten and Northolt medium and Leuschner medium. Polymerase chain reaction experiments indicate a positive correlation between the absence of the hdc and odc genes and the lack of histamine and putrescine production for all tested strains. By contrast, all strains appeared to possess tdc gene, but no correlation was found with a negative expression of tyrosine decarboxylase. PRACTICAL APPLICATIONSIn foods, biogenic amines are mainly generated by decarboxylation of the corresponding amino acids through substrate-specific enzymes of the microorganisms present in this environment, particularly histidine, tyrosine, lysine, ornithine and arginine, and the capability of enterococci and some other lactic acid bacteria to produce biogenic amines has been described. For this purpose, several qualitative and quantitative methods to determine biogenic amines production by these microorganisms have been developed. There is great interest to identify and to characterize the bacteria strains intended for an industrial use and to verify if they are potentially able to produce biogenic amines in order to prevent it's synthesis and limit the poisoning risk.

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In this work, the individual and interactive effects of temperature and pH on the proteolytic activity of Lactococcus lactis LCL strain in milk were studied by quadratic Response Surface Methodology. The effects of the selected variables have been analysed using a Central Composite Design and the maximum response (proteolysis rate) under the optimum values of variables and of a satisfactory fit model was realized. The mathematical relationship of the proteolytic activity on the two significant independent variables can be approximated by a nonlinear polynomial model. Predicted values were found to be in a good agreement with experimental values (R2 = 0.937) demonstrating the good significance of the model. The result of optimization predicted by the model has shown that the maximal result for the proteolysis rate revolved around of 43.7 ± 1.4 % at the optimal condition (30°C and pH 7).