Distribution of phosphorus and hydroxypropyl groups within granules of modified sweet potato starches as determined after chemical peeling

The distributions of phosphorus and hydroxypropyl groups within granules of cross-linked and hydroxypropylated sweet potato starches were investigated. Chemical surface peeling of starch granules was performed after sieving of native and modified starches into large-size (diameter ≥ 20 μm) and small-size (diameter < 20 μm) fractions. Starch granules were surface gelatinized in a 4 M calcium chloride solution at different levels. After the peeling step, the remaining starch granules were analysed for the content of phosphorus and hydroxypropyl groups. The phosphorus level of the parental starch gradually decreased from periphery to core of the granules. The increase in phosphorus content after cross-linking in periphery was higher than that in core. The subsequent hydroxypropylation reaction resulted in lower phosphate levels. Hydroxypropylation resulted in a gradient of hydroxypropyl group concentration from periphery to core. Cross-linking prior to the hydroxypropylation resulted in lower levels of hydroxypropyl groups and less pronounced differences between periphery and core.