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Impaired Rennetability of Heated Milk; Study of Enzymatic Hydrolysis and Gelation Kinetics

A. J. Vasbinder^*, H. S. Rollema^* and C. G. de Kruif^*,,1

^* NIZO Food Research, P.O. Box 20, 6710 BA, Ede, The Netherlands
Van’t Hoff Laboratory, Debye Research Institute, University of Utrecht, Padualaan 8, 3584 CH, Utrecht, The Netherlands

¹ Corresponding author: C. G. de Kruif; e-mail:
kees.de.kruif{at}nizo.nl.

Casein micelles in milk are stable colloidal particles with a stabilizing hairy brush of -casein. During cheese production rennet cleaves -casein into casein macropeptide and para--casein, thereby destabilizing the casein micelle and resulting in aggregation and gel formation of the micelles. Heat treatment of milk causes impaired clotting properties, which makes heated milk unsuitable for cheese production. In this paper we compared five different techniques, often described in the literature, for their suitability to quantify the enzymatic hydrolysis of -casein. It was found that the technique is crucial for the yield of casein macropeptide and this yield then affects the calculated enzymatic inhibition caused by heat treatment, ranging from 5 to 30%. The technique, which we found to be the most reliable, demonstrates that heat-induced calcium phosphate precipitation does not affect the enzymatic cleavage, while whey protein denaturation causes a very slight reduction of enzyme activity. By using diffusing wave spectroscopy, a very sensitive technique to monitor gelation processes, we demonstrated that heat-induced calcium phosphate precipitation does not affect the clotting. Whey protein denaturation does not affect the start of flocculation but has a clear effect on the clotting process. This work adds to a better understanding of the processes causing the impaired clotting properties of heated milk.

Key Words: renneting • heated milk • enzymatic hydrolysis • gelation kinetics

Abbreviation key: CMP = casein macropeptide, DWS = diffusing wave spectroscopy, HAC = acetic acid, NaAc = sodium acetate, RCT = rennet clotting time, RP = reverse phase

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