3.6. Enzyme susceptibility
Core starch of a golden kiwifruit was more susceptible to PPA than the outer pericarp starches (Fig. 2). There is a great variation in the enzyme susceptibility of kiwifruit starches among different varieties (Gold9 > Gold3 > Hort16A) (Fig. 2). The enzyme susceptibility of kiwifruit starches was lower than that of maize starch and higher than that of potato starch. Enzyme susceptibility of starch is affected by various factors including granule size, polymorph type and degree of crystallinity, degree of granule damage, molecular structures of amylose and amylopectin, and presence of other minor components (e.g., lipids) (Srichuwong and Jane, 2007 and Dreher et al., 1984). The size of core starch was smaller than that of outer pericarp starch of kiwifruit (Supplementary Fig. 3), and the outer pericarp starches had higher degree of crystallinity as well as the double helical order (ΔH). Also, amylose molecules tend to be amorphous in the granules and core starches had higher amylose content than the outer pericarp starches ( Table 1). These factors resulted in the lower enzyme susceptibility of the outer pericarp starch. Gold3 had the lower enzyme susceptibility to PPA than Gold9 and Hort16A. Gold3 is the newly introduced variety for commercial production, but it tends to have a relatively short shelf life during storage partially due to fast degradation of starch. Therefore, physiological factors other than the starch play important roles in the storability of the kiwifruit (e.g., fruit size). Starch of A-type polymorph tends to be more susceptible than B-type starch ( Srichuwong and Jane, 2007 and Jane et al., 2003). Indeed, maize starch had higher enzyme susceptibility than kiwifruit and potato starches.