Kemerovo, Kemerovo, Russian Federation
Kemerovo, Kemerovo, Russian Federation
A model of additional stabilization for the milk colloid system by means of the micelle electric charge arising owing to dissociation of micellar calcium caseinate is offered. The model allows comprehending the unique role of calcium in milk clotting and describing some features of coagulation temperature dependence, as well as explaining the nature of rennet, acid, heat-acid and heat-calcium coagulation within uniform concepts.
milk coagulation, sticky hard spheres, calcium caseinate, colloid calcium phosphate, ionized calcium.
INTRODUCTION
Milk clotting is one of important technological processes in the manufacturing of many foodstuffs, in particular, cheeses. This process is based on the coagulation of casein micelles, which may be caused by various factors, such as enzymes, acids, spirits, salts, or high temperature [1].
It is recognized now that the colloid stability of casein micelles in milk is ensured, basically, by the presence of the -casein macropeptide hairy layer on the casein micelle surface, sterically restricting the possible clinging together of micelles [2--5]. In essence, this layer represents a quasielastic polyelectric brush formed by negatively charged macropeptide residues [6, 7].
The loss of colloid stability by the casein micellar system may be attributed to different ways of the destruction of the hairy layer. Therefore, under rennet conditions, -casein macropeptide hairs are split off by chymosin, which leads to the destruction of the protective layer. During acid coagulation, additional hydrogen ions easily get into the polyelectric brush and shift ion equilibrium to the recombination of dissociated -casein macropeptide acid groups, thus reducing the electric charge of macropeptide hairs and finally collapsing the protective layer [8].
Certain distinctions in mechanisms of the destruction of the casein micelle protective layer, as well as a number of factors affecting the micellar casein system's colloid stability, make it very difficult to describe various kinds of milk coagulation with a uniform approach. For example, it is known that the lack of calcium in milk has no significant effect on acid milk coagulation, while it is impossible to coagulate this milk by adding chymosin even after it has completely cut off the protective hairy layer [9, 10].
This research is an attempt to work out a universal model of milk coagulation, which would correctly describe, at least qualitatively, observable features of the milk coagulation phenomenon under various conditions of casein colloid system destabilization.
The background of our model is represented by both well-known experimentally confirmed facts and somehow substantiated but still hypothetical assumptions. In particular, the basic hypothesis rests on the analysis of the outstanding role of calcium ions in the stabilization of the micellar colloid systems in milk.
We hope that this paper will become a stimulus for the direct experimental check of our hypotheses by interested experimentalists.
MATERIALS AND METHODS
Skim milk was reconstituted by mixing up 90 g of low-fat milk powder (Milk Factory, Kemerovo, Russia) with 910 ml of distilled water and 4 cm3 of 10% solution of calcium chloride. Then, after complete dissolution, the reconstituted milk was left for about 12 hours at 6 ± 2ºС.
Milk coagulation was carried out in a thermostatted 200 ml cell.
The chymosin under the trademark of Maxiren® (DSM, Netherlands) was used for rennet coagulation. To prepare the enzyme solution, 0.1 g of dry Maxiren® powder was dissolved in 100 cm3 of distilled water.
For simulating acid coagulation, a 10% lactic acid solution (Univerkhim, Chelyabinsk, Russia) was slowly brought to milk under careful mixing.
To increase the pH of some milk samples, a 0.5-mM sodium hydroxide solution (NaOH) (Univerkhim, Chelyabinsk, Russia) was used.
Soluble calcium was added to milk in the form of 10% CaCl2 medical solution (Shenlu Pharm, China).
To decrease calcium ion concentration in milk, in a number of experiments Trilon B® (Na2EDTA) (Khimservis, Ufa, Russia) was used as a chelating agent.
Calcium ion concentration and pH in milk were measured with ELIT (Niko-Analit, Moscow, Russia) ion selective electrodes.
The casein micelles -potential was measured by means of Zetasizer Nano Z - ZEN2600 (Malvern Instruments, Malvern, UK).
Milk coagulation was monitored with a computer-driven "thermometric" gauge of our own design [11]. This device measures temperature difference between two thermocouple junctions immersed in milk at a distance of about 3 cm from each other. One of the junctions is attached directly to a small resistor dissipating permanently about 0.5 W of heat. An increase in milk viscosity during coagulation leads to an increase in temperature difference. It is more correct to say that the temperature increase near the warmed-up junction is due not only to the viscosity increase but also to the formation of a gel net structure, which also restricts convection in milk. In a sense, our method is similar to the hot wire method [12]. Hereinafter, the curves obtained by means of the thermometric gauge are called thermograms (by analogy with rheograms).
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