Characterization of rennet coagulation of milk concentrated by vacuum condensing and ultrafiltration



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Dairy Science and Technology








A comparison of rennet coagulation of milk supplemented with vacuum-condensed or ultrafiltered milk was undertaken. Five treatments with two levels of protein (45 and 60 g·kg−1) from vacuum-condensed (CM1 and CM2) and ultrafiltered milk (UF1 and UF2) along with a 32 g·kg−1 protein control were compared. A Formagraph was used to characterize rennet clotting time (RCT), amplitude at 40 min (a 40), and time to achieve amplitude of 20 mm (k 20). Rennet was added at 192 μL·kg−1 milk, without prior addition of starter. RCT was lower for CM than for UF, and lower for higher protein milks. The k 20values were lower for higher protein samples, indicating a higher rate of curd firming; and were not affected by the method of concentration. The a 40 values (i.e., curd firmness) were higher for samples with higher protein content. Statistically there was no effect of method of concentration on a 40; however subjective analysis of curds during cheese making indicated that CM curds were firmer than UF curds. T 0 further evaluate the discrepancy in coagulum characteristics, viscosity during coagulation was monitored. Samples of milk were inoculated with starter culture; each treatment was inoculated with two levels of rennet (132 and 192 μL·kg−1); and changes in viscosity were determined throughout coagulation using Brookfield viscometer. Calculated coagulum strength varied from 2.72 to 9.22 N·m; and increased as protein level increased. Also, CM curd was firmer than its UF counterpart. A higher rate of inoculation of rennet can reduce coagulum strength; however the rate should be optimized to avoid decreasing coagulation time to an unacceptable level.