Apparently it appears that due to low pH one of the the thickening agents added viz. the modified starch is getting hydrolyzed resulting in reduction in viscosity. You can replace this with another thickening agent which is acid resistant under the conditions of acidity used and expected shelf life for the product. You may also confirm that there is no microbial degradation of the product.

Dear Arun,

The pH of product is 4.02 and using Ingredion company modified starch ULTREX 4 which stable at this pH. we have already tested producr for mold and yesat and found nil.

Please let me know mixing speed i-RPM and time most desirable to make fine droplets? 

Dear Muhammad,

Now it is clear that there is no acid degradation of starch and also microbial degradation is  also ruled out. As you have rightly said you can optimise shearing rate by lowering rpm but you have to adapt trial and error approach to get good emulsion with small oil droplets to be uniformly distributed in an aqueous phase. You may prepare a microscopic slide and look for the desired results in terms of uniform size of oil droplets ( low or medium magnification in the range of 100x to 450x can be useful.)

Dear Muhammad,

With microscopic observation you will also come to about emulsion type. Ideally if it is like butter it has to be water in oil type rather than oil in water emulsion. You May also choose a proper emulsifying agent based on the HLB value to get maximum stability to a particular type of emulsion under consideration. It is observed that phase reversion makes lot of changes in the theological property of a food system.

What is the aw after 30 day compared to initial? It might be that there is moisture migration into the product. This might result into fat hydrolysis. Try adding another emulsifier and see what happens

Thanks for Good suggestion , would you please let me know max value of acceptable aw.

Muhammad

I can not really specify the correct water activity of your product. However I can suggest that while in storage make sure the product does not gain moisture and you can do this by equilibrating water activity of your product with the air trapped withing the packaging. This should be able to avoid moisture migration and solve the problem if it results from moisture uptake.