Effects of processing parameters on colour stability of strawberry nectar from puree Original Research Article

The effect of processing steps on the colour stability and the anthocyanin content of nectars made from strawberry puree was investigated, including (i) the use of frozen strawberries, (ii) processing temperature (10 °C vs. 20 °C), (iii) sieving, (iv) pH reduction and (v) storage temperature (20 °C vs. 4 °C). The use of frozen strawberries significantly improved the colour stability of the nectar even for long storage periods, and allowed the production of strawberry nectars with a shelf-life of up to 12 months without any additives. Moreover, half-life of anthocyanin monomers increased significantly. A negative effect of a higher processing temperature on the content of anthocyanins after pasteurisation could be observed as well as the reduction of the pH value during processing on the colour and the content of anthocyanin monomers. Sieving had no significant effect on colour stability and the content of anthocyanin monomers. Storage temperature had a strong impact on colour stability and degradation of anthocyanin monomers. At 4 °C, the nectar colour remained acceptable over even more than 12 months. Compared to frozen strawberries stored at −80 °C, frozen strawberries stored at −18 °C had a lower activity of polyphenoloxidase (PPO) and peroxidase (POD) of about 53% and 22%, respectively. No complete inactivation of PPO and POD could be achieved during the different processing steps. POD activity was more affected by pH treatment than PPO activity.