Influence of casein proteolysis by starter bacteria, rennet and plasmin on the growth of propionibacteria in Swiss-type cheese

Abstract

Summary - The low proteinase activity of propionibacteria renders their growth dependent on the primary hydrolysis of casein by starter bacteria. The purpose of this work was to develop a simple method which would allow us to measure the effect of casein hydrolysates by starter bacteria on the growth of propionibacteria in simulated cheese manufacture. The growth of lactic acid bacteria of starter was performed in UHT milk containing calcium carbonate to maintain the pH constant. Alter simulating the various steps (temperatures, duration) of cheese manufacture for 24 h, growth of propionibacteria was performed in a minimum medium to which casein hydrolysates produced by starter bacteria, rennet or plasmin were added. Growth of propionibacteria was followed photometrically and proteolysis of casein determined by the quantitative reaction of free amino acids with a cadmium ninhydrin reagent. The results have shown that growth of propionibacteria was very poor on medium containing milk alone, but amino acids released by the starter bacteria in milk influenced the growth of the microorganisms. Plasmin had no effect on case in on the liberation of free amino acids, but peptides produced by the enzyme were used by the propionibacteria. Rennet added to milk alone or together with plasmin in the absence of starter bacteria did not provoke the growth of propionibacteria; the proteolytic activity of rennet on casein did not modify starter bacteria growth, but produced a two- to threefold increase in free amino acids, depending on the starter used, and influenced the kinetics of the growth of propionibacteria. Il was demonstrated that rennet may have a significant effect on the growth of propionibacteria in Emmental cheese, whereas it is generally admitted that after milk coagulation, the thermo denaturated enzyme no longer plays a role during the ripening of hard cheeses. Hydrolysis of Cl.srcasein to Cl.sl I-casein by the rennet, before heating the milk to 53°C for Emmental manufacture, produces substrates which are necessary for the Iiberation of amino acids by peptidases. Analyses of experimental cheeses manufactured with starter bacteria of high or low proteolytic activity have shown that a too high concentration of free ami no acids in cheese may inhibit growth of propionibacteria.