Investigating rennet coagulation properties of recombined highly concentrated micellar casein concentrate and cream for use in cheese making

摘要

Highly concentrated micellar casein concentrate (HC-MCC) contains ～18% casein with ～70% of whey proteins removed by microfiltration and diafiltration of skim milk, followed by vacuum evaporation for further concentration. When blended with cream, HC-MCC forms recombined concentrated milk (RCM), which could be used as a starting material in cheese making. Our objective was to investigate the rennet coagulation properties of RCM while varying parameters such as casein level, pH, rennet level, and coagulation temperature. The HC-MCC was mixed with cream using low shear at 50℃ for 10 min, followed by cooling to 31, 28, or 25℃ and adding rennet, and rheological properties were determined. Rennet coagulation time [RCT, the time at which storage modulus (G') = loss modulus (G")] decreased from 8.7 to 7.4 min as casein level increased from 3.2 to 5.7%, without a significant additional difference in RCT at casein levels >5.7%. The initial G" (G"_0) increased about 10-fold when casein levels were increased from 3.2 to 10.9%, whereas no change in initial G' (G'_0) was observed. When G' was measured relative to RCT (i.e., 1, 1.5, or 2 times RCT after RCT was reached, and expressed as G'_1, G'_1(1.5), and G'_2), log relationship was found between relative G' and casein level (R~2 > 0.94). Lowering coagulation temperature from 31 to 25℃ increased G"_0 by 6 fold and extended RCT from 7.4 to 9.5 min. After coagulation, relative G' was initially higher at the lower temperature with G'_1 of 3.6 Pa at 25℃ and 2.0 Pa at 31℃, but delayed in further development with G'_2 of 0.8 kPa at 25℃ and 1.1 kPa at 31℃. Lowering pH of RCM from 6.6 to 6.2 resulted in reduced RCT from 11.9 to 6.5 min with increased relative G' after coagulation. When less rennet was used, RCT increased in a linear inverse relationship without changes in relative G' or G". The microstructure of RCM coagulum (～11% casein), observed using transmission electron microscopy, confirmed that RCM curd had a rigid protein matrix containing extensively cross-linked protein strands.