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.
Full access: http://mrw.so/4NHiNd
Image by Tamerlana, from Flickr-cc.
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