Ultrasound (US) technology has become of great interest in the food industry, since it can be used as an alternative to conventional thermal methods. However, there is very limited information about the effect of high frequency US on inactivation of food enzymes responsible for food quality degradation, including peroxidase (POD) and polyphenoloxidase (PPO). Thus, the present thesis examines the effect of high frequency US on the activity of commercial horseradish peroxidase (HRP), PPO in mushroom extract, and both enzymes in strawberry puree. Structural alterations of these enzymes after US treatment were studied using time resolved fluorescence. The impact of US treatment on the antioxidant activity, polyphenols, and anthocyanins in strawberry puree samples after treatment and during storage, in comparison with untreated and thermally processed samples was also evaluated. The most effective conditions for complete inactivation of both enzymes were either 378 kHz or 583 kHz at 48 W for 30 min for HRP and 60 min for mushroom PPO. HRP inactivation kinetics upon sonication followed a first order model, whereas those of PPO fitted to the Weibull model. Fluorescence analysis of US treated samples of HRP revealed the removal of the haem from the active centre and the formation of di-tyrosine, possibly mediated by the production of hydrogen peroxide caused by cavitation, while for PPO it was speculated that US caused fragmentation of the tetramer. The POD and PPO activity after sonication of strawberry puree samples was significantly lower than in those untreated throughout storage. Pasteurisation at 90 oC of strawberry puree inactivated both enzymes however, this treatment had a detrimental effect on anthocyanins, total phenolic content and antioxidant activity of strawberry puree. On the contrary, the application of US enhanced the antioxidant activity, and anthocyanin, and total phenolic content, in strawberry puree samples, as compared to those untreated or pasteurised after treatment and during storage. This study provides a scientific and technological basis to further develop sonication as an alternative to conventional thermal processing of strawberry puree avoiding quality losses of the product.
|Date of Award||21 Nov 2018|
|Supervisor||Adilia Lemos (Supervisor) & Boon Chu (Supervisor)|
- Strawberry Puree