STUDY OF THE ENZYMATIC STAGE OF MILK GELATION: CHANGES IN VISCOSITY AND MICROSTRUCTURE

I T Smykov

doi:10.21323/2618-9771-2019-2-3-4-8

Abstract

The article presents the results of experimental joint studies of changes in the viscosity and microstructure of milk at the enzymatic stage of gelation. Based on the statistical processing of the array of research results, it was determined that the viscosity change at this stage is not monotonic, as it is usually stated, but two-stage in the middle part and S-shaped, preceding the gel point, at its end. It was found that the S-shaped change in viscosity at the end of the enzymatic stage of milk coagulation coincides with changes in the microstructure of casein micelles and reflects the existence of a cooperative conformational phase transition in casein molecules of micelle clusters. A description of the possible mechanism of this phase transition is proposed. It was noted that the moment of the S-shaped change in the milk viscosity at the enzymatic gelation stage and the corresponding cooperative phase transition in casein micelles are a physical reflection of the gel point. The research results provide a better understanding of the mechanism of enzymatic coagulation of milk in a cheesemaking tank.

Highlights

Milk gel formation is the key to the technological process of cheese production, one of the main dairy products
The milk viscosity can be represented as the sum of the viscosity of water and the increment of viscosities from the dispersed phase and structural bonds [15]: ηm0 = ηh + ∆ηd + ∆ηst, (1)
According to the technological documentation, the temperature of the milk in a cheesemaking tank at which it coagulates can fluctuate within two degrees, and the milk viscosity, like any other liquid, strongly depends on the milk properties and temperature

Summary

Introduction

Milk gel formation is the key to the technological process of cheese production, one of the main dairy products. The rennet coagulation process of milk, according to the generally accepted opinion, can be divided into two main phases: primary (enzymatic), including κ-casein hydrolysis to form paraκ-casein micelles and partial aggregation of destabilized micelles; secondary (non-enzymatic) phase of aggregation, including complete aggregation of micelles into a single spatial structure of the gel and its densifying. When a sufficient decrease in repulsive forces (electrostatic and steric) is achieved, the nearest or adjacent micelles begin to aggregate due to hydrophobic interactions. These conditions are achieved at the end of the first and beginning of the second stage of coagulation [1] with a degree of hydrolysis of κ-casein of about 80% and at a pH of 6.6 [2]. These two stages partially overlap in time, but are generally well defined

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