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ß-Lactoglobulin is a Thermal Marker in Processed Milk as Studied by Electrophoresis and Circular Dichroic Spectra

¹ Research Institute of Biochemical Engineering, Department of Biological Science and Technology, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
² Department of Chemical Engineering, Ta Hwa Institute of Technology Hsinchu, Republic of China
³ Department of Bioengineering, Tatung University, Taipei, Taiwan, Republic of China

Corresponding author: S. J. T. Mao; E-mail: mao1010{at}ms7.hinet.net.

As much of the sterilization process involves heat treatment during the preparation of milk on an industrial scale, the unpredictable measures of the process are an essential issue in determining the quality of the milk. The purpose of the present study was to investigate the major protein change(s) of whey proteins in processed milk and extend the knowledge for future reference in the dairy industry. Using a native polyacrylamide gel electrophoresis, we showed almost a 90% loss and denaturation of ß-lactoglobulin (LG), but not -lactalbumin (LA), in some brands of the processed and dry milks. Immunochemical analysis using Western blotting revealed that part of the loss was attributed to the formation of large multiple forms of LG in the processed product. Such denaturation was presumably associated with the heating procedure used in the process. Essentially, LG was the only major fraction converted to aggregates in milk heated at 95°C for 30 min on 2-dimensional PAGE. The detailed thermal denaturation of purified LG and LA at various temperatures (50 to 95°C) and time (5 to 960 s) were investigated using a circular dichroic analysis. The maximal changes of ellipticity at 205 nm (converting ß-structure to disordered structure) were correlated to heating temperature and time. There were no significant conformational changes of LG at temperatures below 70°C for as long as 480 s. Pronounced and rapid changes occurred between 80 to 95°C in a time-dependent manner. Fifty percent of the maximal changes could be reached within 15 s. In conclusion, the unique chemical and immunochemical loss and conformational changes made LG a superior marker for evaluating the thermal processing of milk. The detailed thermal denaturation curves of LG constructed with its time and temperature in this study provide a valuable reference for the dairy industry. We postulate that heat treatment over 80°C in 15 s may induce a significant denaturation of milk LG.

Key Words: ß-lactoglobulin structure • thermal denaturation • processed milk • -lactalbumin

Abbreviation key: CD = circular dichroism, CM = carboxymethylated

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