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Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Published online by Cambridge University Press:  19 October 2000

ERIC C. NEEDS
Affiliation:
Institute of Food Research, Reading Laboratory, Earley Gate, Reading RG6 6BZ, UK
MARTA CAPELLAS
Affiliation:
Unitat de Technologia del Aliments, Facultat de Veterinària, Universitat Autonoma de Barcelona, E-08193 Bellaterra (Barcelona), España
A. PATRICIA BLAND
Affiliation:
Institute of Food Research, Reading Laboratory, Earley Gate, Reading RG6 6BZ, UK
PRETIMA MANOJ
Affiliation:
Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich, NR4 7UA, UK
DOUGLAS MACDOUGAL
Affiliation:
Department of Food Science and Technology, University of Reading, Reading, RG6 6AP, UK
GOPAL PAUL
Affiliation:
School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Abstract

Heat (85 °C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90% denaturation of β-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of β-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 °C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.

Type
Research Article
Copyright
Proprietors of Journal of Dairy Research 2000

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Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure
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