EFFECT OF FERRIC-CHLORIDE ON CHYMOSIN HYDROLYSIS AND RENNET CLOTTING TIME OF MILK

The influence of FeCl3 on the renneting properties of milk in the preparation of iron-fortified cheeses were evaluated. The FeCl3 (0 to .86 mM) was added to milk at various stages of its thermal history: 1) raw milk not cooled below 30-degrees-C and FeCl3 added at 30-degrees-C; 2) raw milk cooled to 4-degrees-C and FeCl3 added at 4-degrees-C; 3) FeCl3 added to raw cooled milk at 4-degrees-C and pasteurized at 64-degrees-C for 30 min; and 4) raw cooled milk pasteurized at 64-degrees-C for 30 min, cooled to 30-degrees-C, and FeCl3 added at 30-degrees-C. The pH of milk decreased with FeCl3 addition. Rate of chymosin hydrolysis of micellar kappa-casein, as assessed by release of macropeptide, and Formagraph rennet clotting time were determined at 30-degrees-C without pH correction. Addition of FeCl3 to raw uncooled milk increased the rate of chymosin hydrolysis but did not affect the enzymic reaction when it was added either to raw cooled milk or to milk before or after pasteurization. Ferric chloride decreased rennet clotting time when it was added to raw uncooled milk and to milk before pasteurization and increased rennet clotting time when it was added to raw cooled milk and to milk after pasteurization. Ferric chloride at concentrations (about .2 mM) normally employed in iron fortification of Cheddar cheese did not affect the rennet clotting time of whole milk whether it was added to raw cooled milk or to milk before or after pasteurization. However, at high concentrations (>.43 mM), addition of FeCl3 to milk before pasteurization is beneficial when making iron-fortified cheese.