A Rapid Electrophoresis Method on Agarose Gel to Characterise Dairy Protein Aggregates

In the dairy industry, heat treatments—such as pasteurization—are widely used to stabilize the microbial evolution of the milk and increase the shelf-life of dairy products, or simply to cook the product. But meanwhile, the milk undergoes physico-chemical and biochemical changes: the proteins, especially, can be modified to a greater or lesser extent. In fact, heat treatment of milk may cause whey proteins and caseins to form aggregates. These soluble and micellar aggregates and their other properties (size, composition, shape, etc.) can affect the techno-functionalities to the milk, conferring interesting or negative features depending on the application in dairy industries.

In this study, the authors proposed a new approach to characterise those protein aggregates. The authors prepared the samples by using skim milk and whey powders. pH and acidity were measured in triplicate before the heat treatment of the milk. Then the heat treatment of milk was carried out. The two extracts of pre-heated milk (supernatant and pellets) were analyzed by agarose electrophoresis and compared with the corresponding control milk and reference whey. SDS-agarose electrophoresis was followed by the calculation of a retention factor (Rf) for each protein spot. Rf allows milk aggregates to be compared qualitatively under the same conditions. Each sample was analyzed ten successive times in order to determine a 95% confidence interval for each retention factor.

The results indicated that the whey proteins with low (between 14 and 18 kDa, spots d and f) and high (between 66 and 83 kDa, spots b and e) molecular mass were both identified in the control milk sample and in the reference whey sample. Caseins were also spotted between 19 and 25 Da. As expected, aggregates only appeared in the pre-heated milk, in the supernatant as regards soluble aggregates and in the pellets for the micellar aggregates. On the gel, a protein aggregate was observed both in the control milk sample and in the pellets of the pre-heated milk samples.

In conclusion, under the same analytical conditions, micellar aggregates appeared bigger than soluble aggregates. This methodology could be helpful in dairy research in order to study the presence of protein aggregates in dairy ingredients or subsequent to a heat treatment even though we are aware of the necessity to go on improving the methodology, especially for the detection of smaller-sized aggregates.

Article by Laetitia Gemelas, et al, from France.

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