Characterization of starch in apple juice and its degradation by amylases

Soluble and insoluble starches from apple juice (Granny Smith variety) at different degree of ripeness were separated and quantified by an iodometric method. Non pasteurised unripe apple juice had as much as 8 g/L starch, 2 weeks before usual harvest date. However, juices obtained from unripe apples stored for 3 weeks at room temperature, had undetectable starch contents. Scanning electron micrographs (SEM) showed apple starch granules are particles of regular shape (mean diameter=9.21 μm, standard deviation=2.74). SEM studies also revealed that after pasteurising apple juice (90 °C, 5 min) virtually all the starch granules lost their spherical structure and only gel-like starch fragments dispersed among the other components of haze (pectin, cell wall, etc.) were observed. Röhalase HT (RO) and Tyazyme L300 (TY) amylases normally used in fruit juices clarification, even in doses lower than those industrially recommended, quickly reduced starch contents in pasteurised juices to undetectable values. Microscopy studies also demonstrated that RO enzyme efficiently degraded raw apple starch granules, which showed evidence of centrifugal and centripetal hydrolysis.