INFLUENCE OF COAGULATING ENZYMES AND PLAS MIN ON THE GROWTH OF STARTER BACTERIA AT THE START OF EMMENTALER CHEESE MANUFACTURE - PRELIMINARY-REPORT

The growth conditions of S thermophilus and L delbrueckii subsp lactis in Swiss-type cheese have not been fully studied. Since these bacteri a show a low proteinase activity, it was assumed that their growth in cheese could depend on the preliminary hydrolysis of caseins by coagul ating enzymes and plasmin. The purpose of this work was to develop a s imple method which would allow us to measure the effect of casein hydr olysates produced by the coagulating enzymes of rennet, E parasitica a nd M miehei, as well as by plasmin, on the growth of starter bacteria during simulated cheese manufacture. The growth of one starter used fo r Emmental manufacture was studied in defatted UHT milk, containing ca lcium carbonate to maintain a constant pH similar to that of cheese. T he various steps (temperature, duration) of cheese manufacture for 24 or 36 h were then simulated; samples were taken at various times for a nalysis. Proteolysis of caseins was assessed by the quantitative react ion of free amino acids with a cadmium-ninhydrin reagent. In order to study the specific influence of each of the casein types on the growth of bacteria, a growth medium was developed by 'exhaustion' of the pro tein fraction from a normal medium. The kinetics of the bacterial grow th were followed photometrically. The results showed that: 1) alpha(sl )-I casein, produced by the proteolytic activity of rennet, was alread y present in whey after curd formation; its concentration increased du ring ripening, thus demonstrating the presence of residual proteolytic activity of rennet in Emmental, in spite of the high cooking temperat ure; 2) the growth of bacteria did not depend on the amount of amino a cids; 3) the production of free amino acids as well as the growth of l actobacilli increased at the same rate when four times more rennet was added to the milk; no such influence was observed with streptococci; 4) the growth of lactobacilli on caseins was shown to follow the follo wing preferential order: beta-casein > alpha(sl)-casein + rennet > alp ha(sl)-casein; no growth could be observed on kappa-casein, even after hydrolysis by rennet; 5) the addition of four times more plasmin than the average concentration encountered in milk did not increase either the liberation of amino acids or the growth of bacteria in the presen ce of rennet or M miehei, but in the presence of E parasitica enforced the proteolytic activity by the bacteria. Nevertheless, an increase i n the rate of bacterial growth could be observed when the milk was pre -incubated with plasmin before its transformation to cheese.