Properties of aseptically-packed UHT milk: casein modification during storage and studies with model systems

Abstract

SummaryStorage of aseptically packed UHT milk produced changes in the electrophoretic pattern of the milk caseins when the milk was stored at ambient or higher temperatures. Lower temperature storage at 4°C did not give rise to these changes.The alterations in electrophoretic properties of the caseins appeared to be due to the action of carbonyl compounds, produced by a Maillard type of reaction, which led to changes in the charge of the protein together with some degree of polymerization. These conclusions have been drawn from results obtained on model systems of casein-lactose subjected to various heat treatments, and on casein and milk treated with acetaldehyde.Changes in the sensitivity to calcium ions of individual caseins, whole casein and milk that had been subjected to various heat treatments or to treatment with acetaldehyde showed that all these different treatments gave rise to modified casein which, in general, became less sensitive to calcium. κ-Casein when treated alone rapidly lost its ability to protect αs1-casein from precipitation by calcium ions, while αs1-casein treated alone only gradually became more soluble in the presence of calcium. Thus, on treating whole casein there was evidence for a stability minimum when the protective ability of the κ-casein has been destroyed without a compensating gain in the stability of the αs1-casein.The importance of these changes in relation to the stability of UHT milks has not yet been elucidated but the results indicate that cross linking between protein chains and changes in calcium sensitivity occur during long-term storage.

Age gelation in UHT‐processed reconstituted concentrated skim milk

“…The electrophoretic patterns of samples stored at 40°C became 'blurred' after storage for 3 months and therefore did not clearly show protein breakdown products. Previous studies have shown that the polymerization of casein and whey protein due to Maillard-type reactions occur during extended storage of unconcentrated and concentrated milks (Andrews & Cheesman, 1971;Andrews, 1975). In the present study the 'blurred' patterns observed in the samples stored at 40°C probably resulted from alterations in the molecular charges and molecular weights of the protein components due to these types of reactions.…”

Determination of the Heat Treatment Undergone by Milk by Following the Denaturation of α-Lactalbumin with a Biosensor

“…This hypothesis is supported by the fact that during storage at 25 or 35°C of UHT or sterilized milk, the furosine concentration of the milk samples significantly increased (17).…”

Milk protein analysis

“…A high Mr component, which could not penetrate the gel,appeared. It may correspond to a polyrner produced by Maillard reaction between milk proteins and lactose (Andrews & Cheeseman, 1971). …”