Date of Award:

1992

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Nutrition, Dietetics, and Food Sciences

Advisor/Chair:

Donald J. McMahon

Abstract

The purpose of this research was to relate lactose reactivity and age gelation of UHT processed concentrated milk. Skim milk was pasteurized, diafiltered batchwise to reduce lactose concentration to less than 0.05%, and UF concentrated to 3X (one-third volume reduction). Lactose and sucrose were then each added at 3% or 6% w/v to part of the concentrate. The five samples, control ( < 0.05% lactose), 3% w/v lactose, 6% w/v lactose, 3% w/v sucrose, and 6% w/v sucrose, were UHT processed at 140°C for 4 s using the indirect heating method. Samples were collected aseptically in presterilized plastic containers and stored at 4°C, 20°C, and 35°C for periodic analysis.

All samples stored at 4°C and 20°C gelled after 21 weeks of storage. The viscosity changed slightly during the first 19 weeks of storage but increased suddenly (> 100 cPs) just before gelation. Samples stored at 35°C did not gel but showed sedimentation. Samples stored at 4°C or 20°C underwent little browning; but samples containing 3% and 6% lactose, stored at 35°C, browned considerably. The SOS-PAGE patterns of gelled samples showed new bands because of proteolysis whereas samples stored at 35°C showed bands due to proteolysis and protein crosslinking and a streaking pattern. Electron micrographs of gelled samples showed various casein particles connected together by hairy appendages protruding from the surface of casein particles, to form a continuous three-dimensional network. In non-gelled samples, the micelles were not joined into a continuous network and few hairy appendages protruded from their surfaces. Hairy appendages were not a result of Maillard reaction occurring during storage.

Maillard reaction neither provided protection against nor promoted age gelation. Proteolysis was not the only cause for gelation. Protein modifications prevented gelation in samples stored at 35°C. Age gelation was probably a two-step process involving dissociation of proteins from the casein micelles that reformed onto the micelle surface as hairy appendages. Aggregation of the protein particles occurred through these appendages rather than through the original micelle surface.

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