Effect of processing on slowly digestible starch and resistant starch in potato

The effect of a number of laboratory-scale pretreatments on the proportions of rapidly digested (RDS), slowly digested (SDS) and resistant starch (RS) in raw and cooked potato has been examined using an in vitro digestion procedure. Potatoes of the variety Frisia were prepared in three states: raw, cooked, and cooked followed by a cold treatment (4 degrees C, two days). Each preparation was then subjected in triplicate to freeze-drying, coarsely mincing, pasting, freezing, dry-milling after freeze-drying, in 22 different combinations, before digesting. In raw potato, very little RDS and SDS (< 5% total starch (TS)) were present, and the mechanical treatments of the potato did not affect the amounts of RDS and SDS. Cooking resulted in an almost complete conversion to RDS (> 95% TS) in freshly-cooked potato, but after post-cooking cold treatment much of the RDS transformed to SIDS, which reached a maximum of about 45% TS. SDS formation was independent of the degree of tissue disruption after cooking, and was generally associated with formation of RS, however, freezing after cooking allowed SIDS formation without prolonged cold treatment and with very little associated RS (SDS 35% and RS 4% of TS). Freeze-drying caused an increase in RS in most treatments of the cooked potatoes. The observed effects provided guidance for sample handling in potato research, but also suggested several approaches to the enrichment of SDS and/or RS, with a concurrent reduction in RDS, that could be used to improve the nutritional profile of potato products by decreasing RDS (lowered glycaemic impact), and increasing SIDS (more sustained energy availability) and RS (prebiotic benefits).