Dear Jasim,

There are basically two ways of thinking that comes to mind when it comes to your observation.

One is that during the heat treatment of milk possible complexes are formed between for example К-casein and β-LG that results in changes in for example the casein micelle and affects thus the way in which casein provides stabilization. Prolonged heating of milk might result in β-LG being denatured since this protein unfolds and forms aggregates. So the protein thus not disappear but forms (large) aggregates (as can be seen in your figure: at least the decrease of all proteins in this gel).

The other way of thinking is that there is protease activity (or more general enzymatic activity). Either direct degradation of lactoglobulin proteins or indirectly by degrading casein proteins might explain your observation.

Personally I find the following reference rather nice (it contains a lot of valuable references):

http://scholar.ufs.ac.za:8080/xmlui/bitstream/handle/11660/6466/HattinghA.pdf;jsessionid=FA7E9694848CC0E903BA8C9BF19B6F43?sequence=1

PS. I am quite familiar with the process of protein translocation and the pattern looks somewhat similar as a precursor (larger MW) that decreases and a mature part (slightly lower MW) that increases. So it looks a bit like processing of precursor protein (the beta-LG contains a signal sequence). Not sure whether this is related to your observation though. More likely it is an indication that at least the protease activity argument is involved to some extent.

Hope this is of any help to you.

Dear Bob

I agree with you that the decrease in proteins band with increment of heat treatment degree is related to the formation of S-S bonds between casein and whey proteins , but the thing which surprise me was that with decrements of

β-Lg band intensity the intensity of the new bond formed below β-Lg was higher .

I do not think that the hydrolysis enzymes play this role because we used high temp. for long time .

i agree with both above answers but basically proteins are heat sensitive. When you rise the temperature naturally all protein get denature. But some are denature at low temperature, few get denature at high temperature. During denaturation process protein get loses its Mol .wt also.

Thank you Shivayogeeswar Neelagund . As I know denaturation change the secondary and tertiary structures and not the primary structure of protein .