Effect of Ferric Chloride on Chymosin Hydrolysis and Rennet Clotting Time of Milk

¹ Western Center for Dairy Proteins Research and Technology, Department of Nutrition and Food Sciences, Utah State University, Logan 84322-8700

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°C and FeCl₃ added at 30°C; 2) raw milk cooled to 4°C and FeCl₃ added at 4°C; 3) FeCl₃ added to raw cooled milk at 4°C and pasteurized at 64°C for 30 min; and 4) raw cooled milk pasteurized at 64°C for 30 min, cooled to 30°C, and FeCl₃ added at 30°C. The pH of milk decreased with FeCl₃ addition. Rate of chymosin hydrolysis of micellar -casein, as assessed by release of macropeptide, and Formagraph rennet clotting time were determined at 30°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.

Key Words: milk • chymosin hydrolysis • rennet clotting time • ferric chloride

Submitted on January 31, 1992
Accepted on May 11, 1992