Physicochemical properties and molecular structure of kiwifruit starch

Abstract

Kiwifruit is a popular fruit crop worldwide due to its palatable taste and desirable nutritional properties. Kiwifruit is a starch-storing fruit: it accumulates abundant starch by the time commercial harvesting occurs, and most of the starch is gradually hydrolyzed into simple sugars during ripening. Starch plays an important role in kiwifruit development, but basic knowledge on kiwifruit starch (e.g., its physicochemical properties and molecular structure) has remained very limited. In addition, the starch properties and contents may vary across the tissues of kiwifruit, and the starch structure may change with the growth stage of the fruit. This study aimed to investigate the dynamics of starch properties and structure among different tissues and growth stages of kiwifruit and explore a potential use for kiwifruit starch. During kiwifruit growth, the starch content (dry weight basis) was observed to increase from ~ 2% to 46% and 58% for green and gold kiwifruit, respectively, while the starch granules enlarged from 4 to 10 μm (mass moment mean diameter). The polymorph type of the starches remained B-type, and the amylose contents leveled off (~ 10–12%) in green kiwifruit starch and increased (~ 15 to 19%) in gold kiwifruit starch throughout fruit development. The external/internal structures of amylopectins remained similar as the starch granules enlarged in growing kiwifruit, indicating homogeneous molecular structure from the inside to the outside of a kiwifruit starch granule. The dynamics of amylose content and molecular structure of starch in growing kiwifruit shared some similarities to those of starches from other growing crops (e.g., kidney bean and various cereals). The starch content, properties and structure from the core and outer pericarp of a gold kiwifruit were significantly different, suggesting the different biosynthetic properties of these two starches. The starch content in the outer pericarp was ~ 5–15% higher than that in the core, a significant difference. The enzyme susceptibility and amylose content of the core starches were higher, and the granule size, degree of crystallinity and gelatinization parameters of core starches were somewhat lower than those of the outer pericarp starches. Compared with other starches, the enthalpy changes associated with the gelatinization of outer pericarp starches were exceptionally high (~21 J/g). The flow properties of the outer pericarp starches also showed high yield stress. The internal structure of outer pericarp amylopectin was composed of higher amounts of shorter chains than that of core amylopectin, while the structure of core amylopectin consisted of higher amounts of longer chains than that of outer pericarp amylopectin. The internal chain length of core amylopectin was longer than that of outer pericarp amylopectin. These results suggested that outer pericarp amylopectin had a more branched and tightly arranged structure than that of core amylopectin. Comparing to the outer pericarp, soluble starch synthase in the core can be expected to exhibit a higher activity level than that in the outer pericarp, while starch branching enzymes in the core are less active. Although starch granules from potato were many times larger than those of kiwifruit starch, the amylose leaching, swelling power, and dynamic oscillatory properties exhibited by the two starches were similar, which might be due to their sharing the same or similar polymorph type, amylose content and molecular structure. The total dietary fiber, free phenolic content and in vitro antioxidant capacities of kiwifruit flours were ~ 3, ~ 6–10 and ~ 5–20 times higher than those of potato, maize and wheat flours, respectively, while the starch contents of the kiwifruit flours were ~40–50%, which were significantly lower than those of the traditional flours. Pasting, gel texture and dynamic oscillatory analysis showed that the starchy kiwifruit flours had some similar characteristics to those of traditional flours and some differences. It might be feasible to incorporate starchy kiwifruit flour into food to achieve desired physical and nutritional properties for “novel” and “healthy” food formulations. The information presented by this study contributes to the fundamental knowledge for developing kiwifruit as a sustainable crop and provides new insights into the relationship between the architectural/molecular structures of a starch granule.