Improvement of anthocyanin stability in fluid, pasty and particulate fruit products

Strawberries belong to the most important berry fruits used for industrial fruit processing, therefore being of high economic relevance. Besides texture and taste, consumers are primarily attracted by their red color being considered as indicator for high product quality and freshness. Therefore, the present study aimed at developing strategies to improve anthocyanin stability in various strawberry products. Pectin has frequently been reported to exert stabilizing effects on anthocyanins. Therefore, the initial part of this thesis (CHAPTER 1) consisted of a systematic study on the impact of enzymatic mash maceration on anthocyanin and color stability of strawberry purées using pectin-degrading and -modifying enzymes, respectively. It was demonstrated that this process step may be beneficial for anthocyanin and color retention of pasteurized strawberry purées. However, by far best color and pigment retentions were achieved when the purées were stored at lower temperatures. In a further study (CHAPTER 2), the influence of added, non-native pectins on anthocyanin and color stability of gelled strawberry products has been assessed demonstrating anthocyanin stability of strawberry jams and spreads to depend on various factors. Strawberry spreads generally showed lower pigment stability than the jams which can be attributed to their higher water activities. Furthermore, pigment storage stability strongly depended on the pectin type used for strawberry jam and spread preparation. Freezing and thawing of strawberries are inevitably accompanied by cell decompartmentalization allowing genuine enzyme activities to react with their corresponding substrates. Accordingly, anthocyanins and ascorbic acid may already be degraded during thawing. Thawing at +20 °C and in a microwave oven, respectively, were found to be favorable with respect to anthocyanin retention. Ascorbic acid was also best retained, when strawberries were thawed in a microwave oven. Interestingly, thawing the fruits at 4 °C, which is common practice, caused most pronounced pigment and ascorbic acid losses. Therefore, thawing regime was found to be a key factor in pigment and ascorbic acid retention in strawberries (CHAPTER 3). Apart from deleterious effects of heat, light, and oxygen, polyphenoloxidases (PPO) are assumed to play a prominent role in anthocyanin and color degradation during processing and storage of strawberries. Therefore, the effects of various food-safe PPO inhibitors on strawberry PPO and their impact on anthocyanin stability were evaluated. The results presented in CHAPTER 4 clearly demonstrate that anthocyanin and color stability of strawberry purées may be significantly enhanced by the addition of PPO inhibitors such as L- cysteine*HCl, L-cysteine, citric acid, and selected proteases, respectively. In contrast, thermal inactivation of PPO was disadvantageous regarding pigment stabilities of strawberry purées, which was ascribed to partial regeneration of PPO (CHAPTER 5).