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Alcohol-mediated temperature-induced reversible dissociation of the casein micelle in milk

Authors: J.G. Zadow

Abstract:

As a part of a study on the influence of alcohol on casein micelle stability, the L*, a* and b* values of skim milk/ethyl alcohol mixtures were determined at temperatures ranging from 20°C to 80°C. At 20° there was little effect of alcohol concentration in the range of 0% to 75% on the values of L*, a* and b* (note that the figures of alcohol content herein refer to the alcohol content of the water/alcohol mix which was mixed 1:1 by mass with milk to evaluate alcohol stability). Coagulation of the sample occurred at an alcohol level of 70% at 20°C. At and above 40°C, there was no evidence of coagulation at any of the alcohol contents examined (0 to 100% alcohol). Further, above 40°C, samples of higher alcohol content (above about 40%) became progressively less turbid as the alcohol content was increased. At higher temperatures (above about 60°C) and alcohol levels (above about 50%), the samples became almost clear and colourless - any remaining turbidity was presumed to be due to the presence of residual lipid. These changes in appearance were completely reversible. On cooling, samples became fully milky again and returned to their original L*, a* and b* values. These results suggest that the casein micelle undergoes a reversible temperature-induced alcohol-mediated dissociation under these conditions. There was little effect of temperature on the value of a* below about 30°C. Above this temperature a* increased. The extent of this increase was greatest for samples treated at the highest temperatures. Below about 50°C, there was little effect of temperature on b*. Above this temperature, b* increased, reaching a maximum at about 40% alcohol content, and then decreased to its original level. This alcohol-mediated temperature-induced reversible disassociation of the casein micelle has applications in enabling the ready modification to studies on micelle composition and properties. The procedure is simple and rapid, and does not alter the solids components of the milk. After dissociation, addition of modifiers (if desired) and reassociation, the alcohol may be removed by evaporation or membrane techniques, allowing examination of the modified reformed micelle in an aqueous medium of unchanged solids composition.

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