Effect of whey protein isolate on the oxidative stability of Vitamin A : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Science in Nutrition Science, Institute of Food, Nutrition and Human Health,Massey University, Palmerston North
The purpose of this study was to investigate the possible protective effect of whey protein isolate (WPI) on the oxidative stability of vitamin A in an aqueous phase. The first part of the study focused on the development of a reliable method to extract retinol from the samples for analysis. Direct solvent extraction and saponification were both tested, with saponification, which converts retinol acetate into retinol, chosen for the experimental work. Extracted retinol was quantified using reversed phase HPLC, to obtain the degradation trends of the samples that had been subjected to various conditions. During the second part of the study, the degradation trends were obtained for the samples of retinol acetate in the presence and absence of WPI, when subjected to fluorescent light, pasteurisation, UHT treatment or storage at 5 or 40 °C. Samples exposed to fluorescent light at 4 °C showed exponential degradation of retinol acetate. Within 48 hours of light exposure, almost 60% of the retinol acetate had degraded regardless of the initial concentration. However, samples containing WPI retained slightly more retinol acetate at initial retinol acetate concentrations >25 μg/ml. The presence of WPI had a protective effect on retinol acetate during pasteurisation at 72 °C for 15 seconds. This protective effect appeared to be associated with the WPI concentration. However, there was no difference between samples with or without WPI after UHT treatment at 144 °C for 3-4 seconds, presumably due to the denaturation of the whey proteins. The effect of WPI on retinol acetate during storage was minimal at 40 °C, where total degradation of retinol occurred within 48 hours of storage regardless of the initial retinol acetate concentration. In contrast, WPI showed a significant protective effect on retinol acetate at 5 °C, especially when the initial retinol acetate concentration was >25mg/ml. Overall, the presence of WPI and higher initial retinol acetate concentrations showed better stability than the control samples when exposed to light, stored at 5 °C or during pasteurisation.