Abstract/Summary

Currently, there is a significant interest in incorporating polyphenols into dairy products. However, critical information is required, not only on the impact of polyphenol addition on the technological quality of the resulting dairy products, but also as to how dairy processing operations impact on the incorporated polyphenols. Ultimately such information is required to enable processors to develop successful strategies for polyphenolic supplementation of dairy products. Therefore, the overall aim of this thesis was to determine the optimum processing point at which polyphenols could be added to fluid milk before it is subjected to further processing and ultimately if such additions impact on final product quality. Due to their high levels of consumption acidified dairy products were chosen as an appropriate model product for this research. Initially, this study determined the effect of addition of a range of phenolic sources, before and after heat treatment on the properties of milk-polyphenol mixtures. The milk-polyphenol mixtures were then used to produce acidified milk gels to determine the impact polyphenol addition on gelation kinetics and the rheological properties of the gels. Secondly, this study investigated the effect of 28 days of refrigerated storage on physicochemical properties of the acidified milk gels. Four sources of phenolic compounds (green tea, white grape, tannic acid, gallic acid) were used in the study and incorporated into pasteurized skim milk. A heat treatment (85ºC for 30 min) was applied to pasteurized-skim milk either before (MhP) or after polyphenols addition (MhPh). Heat treatment decreased the total phenolic content (TPC) and ferric ion reducing antioxidant power (FRAP) values of the samples. Although the stage of polyphenol addition had no significant effects on TPC and FRAP, the addition of gallic acid before heat treatment resulted in a significant increase in casein micelle size (CMS) due to the lower pH of this sample prior to heating. Acid gelation decreased the extractable total polyphenols, however there was no significant difference between the FRAP of the acid gel and MhPh milk samples. The stage of gallic acid addition had a significant effect on the rheological properties of the acidified milk gel sample. The addition of gallic acid before heat treatment (MhPh) resulted in the longest gelation time and significantly decreased gelation pH, final storage modulus (G′) and fracture stress. This was attributed to higher attachment of denatured whey proteins to casein micelles during heat treatment. The addition of polyphenols, with MhPh treatment, had an effect on physicochemical properties of acidified milk gels during 28 days of refrigerated storage, depending on source of polyphenols. The findings of this thesis demonstrated that polyphenols could sucessfully be incorporated into milk used for production of acidified dairy products. It was found that the nutritional profile of the products could be improved without a significant impact on final product stability and quality. However consideration as to the properties of the phenolic source used, as well as the stage at which it is added to the milk must be given in order to achieve this. In particular the pH of phenolic compounds should be taken into consideration when fortifying dairy products with polyphenols