An original pH-reversible treatment of milk to improve rennet gelation

Abstract

The ability of an original pH-reversible treatment of milk to improve rennet gelation was evaluated by rheological measurements on reconstituted skim milk supplemented or not with casein. Samples were acidified at 4°C to pH 5.10 by injection of CO2 in a laboratory pilot plant (pressure less than 2MPa) and then depressurised under vacuum. Depressurisation induced the pH to return to its initial value (6.65±0.02) and restored the mineral and protein partition of milk as well as the water solvation, the size diameter and the zeta potential of the casein micelles. After rennet addition, CO 2-treated milk showed a decrease of the rennet clotting time and an increase of the firming rate compared with a non-treated sample. Moreover, this treatment had a positive effect on the release of glycomacropeptide during enzymatic gelation, suggesting that the rennet accessibility of κ-casein was greater in the CO2-treated than in the non-treated sample. The pH-reversible CO2 acidification greatly activated the rennet gelation of casein-supplemented milk. Indeed, without this treatment, no clotting was observed even 300min after rennet addition. Therefore, this CO 2 treatment might involve modification of the micellar surface properties in favour of rennet gelation. © 2003 Elsevier Ltd. All rights reserved.