Xiaoshan Li

Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences


Marie K. Walsh


Health consciousness drives people to drink skim milk. Yet, improving the blue-white appearance and watery texture of skim milk is necessary to make consumers happy with skim milk. In this study, the influence of limited proteolysis with soluble or immobilized proteases, heat treatment, and pH on the whiteness of skim milk were examined to meet this goal.

Limited proteolysis with milk-clotting enzymes increased the whiteness of skim milk. the proteases porcine pepsin and chymosin were immobilized onto nonporous ceramic, glass, and controlled pore glass (CPG) beads. The enzymes were coupled to beads either directly or via crosslinker proteins. Pepsin, immobilized onto CPG beads via crosslinker proteins, exhibited the best properties with respect to enzymatic activity, stability, and whitening efficiency. The L value (whiteness) of this immobilized enzyme-treated skim milk was 79.5, which approached the whiteness of 1% fat milk. Immobilized proteases whitened skim milk more effectively than did soluble proteases.

The whiteness of skim milk was determined at various temperatures from 4 to 90°C. The L value increased with increasing temperature throughout the range tested. For samples not heated above 50°C, the increases in L values were completely reversible on cooling. Partial reversibility was observed with samples heated at 70°C and above.

Milk whiteness was also determined at different pH values ranging from 5.0 to 8.2 at temperatures of 4, 20, and 30°C. The L value increased with decreasing pH and increasing temperature. A maximum L value of 80.0 was obtained at pH 5.0 and 30°C, which is higher than the L value of skim milk at its natural pH at room temperature.

The temperature-dependent dissociation of major caseins was investigated by size exclusion chromatography at temperatures from 10 to 40°C. Free soluble β-casein and κ-casein were found only at 10°C.



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