71. The Heat Denaturation of Albumin and Globulin in Milk

Abstract

IT has been shown by Hardy(i), Chick and Martin(2), Lepeschkin(3), Wu and Wu(4) and Lewis (5), using solutions of egg albumin and oxyhaemoglobin, that heat coagulation consists of two distinct processes: (a) denaturation, or the alteration of the protein under the influence of heat, which must precede (b) flocculation, the rapid separation of the denatured protein in the presence of a suitable electrolyte. The denaturation causes loss of solubility of the protein in water and in dilute salt solutions at its isoelectric point, and is irreversible. The rate of change is influenced by temperature, the concentration of hydrogen or hydroxyl ions, and the neutral salt content. Water is necessary for heat denaturation, but the chemical change in the protein molecules is still obscure. Wichmann(6) and Chick and Martin (2) demonstrated that no change occurs when egg albumin is heated to high temperatures in the dry state.Denaturation and coagulation should not be confused, since coagulation includes the second process of flocculation by neutralisation of the particle charge, and denaturation may take place without being followed by flocculation. Salt-free albumin in solution will not coagulate on boiling, but will be denatured, after which precipitation will occur if a small amount of electrolyte be added. Similarly the term denaturation and not coagulation is applied to the change which takes place in the albumin and globulin when milk is heated. There is no visible coagulation, but the conditions determining the dispersion of the albumin and globulin are changed. The larger associated particles of denatured albumin and globulin are readily precipitated by acid and by salt solutions. The denaturation which takes place on heating milk is made evident, and its extent measured, by the precipitation of albumin and globulin in addition to casein when the heated milk is treated with those reagents which precipitate casein alone from unheated milk.The heat coagulation of isolated pure proteins in solution has been studied extensively, but there is little knowledge of the effects on the lactalbumin and lactoglobulin when milk is heated, and it will readily be seen that the data available are not in agreement. Sebelien(7) observed that an aqueous solution of lactalbumin was coagulated by heating in salt solution at 72° C, while on heating milk itself Orla-Jensen and Plattner (8) found only slight coagulation

A 100-Year Review: Yogurt and other cultured dairy products

“…Heating milk to a sufficiently high temperature denatures whey proteins, and the denatured whey proteins interact with casein micelles. Rowland (1934) determined the amount of denaturation of albumin and globulin upon heating milk as a function of temperature ranging from 63 to 80°C with variable heating times and reported that 83.4% of the total albumin and globulin became denatured after 30 min at 80°C. More recently, Dannenberg and Kessler (1988a) made a graph containing curves of equal degrees of β-lactoglobulin B denaturation in milk at various heating time and temperature combinations.…”

The age-thickening of sweetened condensed milk: II. Effects of temperature and of storage

“…We had hoped to draw some conclusions as to the nature of the age-thickening processes from the magnitude of the respective temperature coefficients. The coefficient of about 5 for the ' initial' viscosity seems to indicate neither a homogeneous chemical reaction for which a coefficient of 2-3 would be expected, nor a process similar to protein denaturation, for which a short-range coefficient of as much as 60 has been reported by Rowland (1933). Similar considerations apply to the temperature coefficient, of about 7, for the 'permanent' viscosity.…”

“…The temperature coefficient of age-thickening Temperature coefficients could not be directly calculated from the rates of agethickening at two different temperatures because the 'permanent' viscosity did not show a uniform rate of increase, and even with the 'initial' viscosity there was some doubt in assigning a value. A method used by Rowland (1933) and by Webb & Hufnagel (1948) was therefore employed. It consisted of substituting for the rate of age-thickening, the times necessary at various temperatures to give the same viscosity increase.…”

Enhancement of shelf life of paneer by surface salt treatment

“…The paneer samples were evaluated organoleptically for different quality attributes like Flavor (F), Body & Texture (BT), Color & Appearance (CA) and Overall Acceptability (OA) by a panel of six judges using 9 point Hedonic scale which is given below. , 1989), Non-protein nitrogen (Rowland, 1938), Lactose (BIS, 1989), Ash (BIS, 1989), Thiobarbituric acid (Strange et al 1977), Free fatty acids (Koniecko, 1979). The data obtained during the present investigation were suitably analyzed by employing statistical designs (Snedecor and Cochran, 1967).…”