Microstructure of Indirectly and Directly Heated Ultra-High Temperature (UHT) Processed Milk Examined Using Transmission Electron Microscopy and Immunogold Labeling

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Food Sci. Technol.



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A layer of cream (or fat) that forms on ultra-high-temperature processed milk during storage can make it unacceptable to the consumer. Milk homogenized after ultra-high-temperature treatment is apparently more susceptible to forming a cream layer than milk homogenized before the heat treatment. However, this study shows that milk homogenized after indirect ultra-high-temperature treatment has both a microstructure, as determined by transmission electron microscopy and immunogold labelling, and a cream layer thickness similar to that of milk homogenized before indirect ultra-high-temperature treatment, suggesting that it is feasible to homogenize milk before ultra-high-temperature treatment. Also, directly heated milk had a microstructure similar to that of indirectly heated milk. The locations of the caseins (kappa-casein, alpha(S1)-casein and beta-cnsein) and whey proteins (beta-lactoglobulin and alpha-lactalbumin) were localized with the immunolabelling procedure. beta-lactoglobulin and beta-casein were found to be important proteins in forming the fat globule membrane of homogenized ultra-high-temperature processed milk and were localized on the fat globule membrane. The micrographs confirm that kappa-casein dissociates from casein micelles on heating.