Effect of hydrolysis of casein by plasmin on the heat stability of milk |
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Affiliation: | 1. Institute of Nutrition and Functional Foods (INAF), Centre de recherche en sciences et technologie du lait (STELA), Université Laval, Québec, QC GIV 0A6, Canada;2. Department of Food Science, Pavillon Paul-Comtois, Université Laval, Québec, QC GIV 0A6, Canada;3. Agriculture and Agri-food Canada, Food Research and Development Centre, St-Hyacinthe, QC J2S 8E3, Canada;4. School of Nutrition, Université Laval, Québec G1V 0A6, Canada |
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Abstract: | This study examined the effect of hydrolysis of casein by added plasmin (6 mg L?1) on the heat stability of raw, pre-heated, serum protein-free or concentrated skim milk. Plasmin activity markedly affected the heat stability–pH profile of skim milk and serum protein-free milk, apparently by altering the properties of the casein micelles. It is probable that changes in the surface charge of the micelles, as a result of the hydrolysis of caseins, contributed to this effect. Hydrolysis by plasmin reduced the zeta-potential of the casein micelles from ~?19 to ~?16 mV. The effect of hydrolysis of casein by plasmin on the heat stability of pre-heated milk was less pronounced, shifting the heat stability–pH profile to more alkaline values; the heat stability of concentrated milk was unaffected by plasmin. A very high (50 mg L?1) level of added plasmin resulted in clearing of the skim milk; the L* value decreased from ~75 to ~35 after 24 h incubation at 37 °C. Clearing was correlated with a change in casein micelle diameter from an initial value of ~175 to ~250 nm. It is suggested that plasmin-induced changes in zeta-potential may promote micellar aggregation or changes in micelle stucture. |
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