Thermal inactivation of strawberry polyphenoloxidase and its impact on anthocyanin and color retention in strawberry (Fragaria x ananassa Duch.) purees.

摘要

Strawberry (Fragaria x ananassa Duch.) cultivars were screened for their polyphenoloxidase (PPO) activities, and thermal stability of PPO was evaluated in vitro for three cultivars under different time-temperature regimes (60, 75 and 90 degrees C for 3 and 5 min, respectively). Heating strawberry purees should further elucidate the impact of thermal treatments on strawberry PPO in its natural matrix ('in situ' activity). To evaluate the consequences of PPO inactivation on anthocyanin and color stability, the purees were stored for 28 days at +20 degrees C monitoring the contents of monomeric, polymeric (spectrophotometrically) and individual anthocyanins (HPLC-DAD-MSn) as well as color properties (CIE L*a*b*). Antioxidant activities (FRAP), total phenolic (Folin-Ciocalteu) and ascorbic acid contents of freshly prepared and stored purees, respectively, were determined spectrophotometrically. PPO activities varied considerably among the cultivars investigated. Accordingly, different time-temperature regimes were required for their complete in vitro and in situ inactivation. Unexpectedly, thermal inactivation of PPO was disadvantageous regarding pigment and color retention of strawberry purees, which was ascribed to partial regeneration of PPO. Hence, protection of antioxidants, total phenolics and ascorbic acid from oxidative degradation could not be achieved by heating the purees prior to storage.