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Rennet-Induced Gelation of Calcium and Phosphate Supplemented Skim Milk Subjected to CO₂ Treatment

Joint Research Unit for Agropolymer Engineering, and Emerging Technologies, University Montpellier II, 34095 Montpellier Cedex 5, France

Corresponding author: C. Guillaume; e-mail: c-guillaume{at}univ-montp2.fr.

A Doehlert design was performed to study the effect of calcium and phosphate supplementation at 0 to 25 mmol/kg and 0 to 16 mmol/kg, respectively, on the rennet gelation of reconstituted skim milk subjected to pH-reversible CO₂ acidification. Supplemented reconstituted skim milk samples were acidified to pH 5.80 by the addition of CO₂ under pressure and depressurized under vacuum to restore the initial pH value. The second-order polynomial models satisfactorily predicted the effect of salt addition on the micellar molar Ca:P ratio and the average diameter of the casein micelles, whereas only trends were used in the analysis of the rennet-clotting behavior of salt-supplemented, CO₂-treated milk. Whether added Ca was the most determinant factor on the micellar molar Ca:P ratio, added Pi (a mixture of Na₂HPO₄ and NaH₂PO₄) was the most determinant factor on the other responses studied, and its effect was most pronounced when Ca was simultaneously added. By comparison with control samples, changes observed in this study were essentially due to salt supplementation and not to the CO₂ treatment. Therefore, this CO₂ treatment could be considered as an entirely reversible treatment rather than only pH-reversible, and predictions might be applied to untreated milk. In the case of Ca-supplemented milk, the micellar molar Ca:P ratio increased, the average micellar diameter decreased, and the rennet-clotting properties were improved, whereas opposite effects were observed upon Pi supplementation. Since modification of the micellar molar ratio is the result of change in the chemical composition of micellar calcium phosphate, the effect of calcium and phosphate supplementation on the rennet clotting of milk was found to be also dependent on the nature of the interaction between caseins and colloidal calcium phosphate.

Key Words: salt addition • colloidal calcium phosphate • rennet gelation • carbon dioxide

Abbreviation key: CCP = colloidal calcium phosphate, FR = firming rate, IMCU = international milk clotting units, MCP = micellar calcium phosphate, MSD = mean standard deviation, RCT = rennet-clotting time, RMSE = root mean square error, RSM = response surface methodology.