Resistant starch: Formation and measurement of starch that survives exhaustive digestion with amylolytic enzymes during the determination of dietary fibre

Heat-processed foods can contain appreciable amounts of resistant starch (RS) that has the ability to survive prolonged incubation with a-amylase and other amylolytic enzymes. The occurrence of RS has important implications for dietary fibre (DF) determination and, possibly, for human bowel physiology also. Studies using cereal and potato starches have identified three key factors that influence yields of RS after heat-processing, i.e. amylose content, processing temperature and water content. The highest yields of RS (20–34% of total dry weight) were obtained from amylomaize starches, either raw or processed, and from amylopectin starches (32–46% RS) after incubation with α-(1→6)-debranching enzyme (pullulanase) followed by heat-processing. In contrast, the lowest yields of RS (0⁗2–4⁗md2%) were obtained from intact (i.e. non-debranched) amylopectin starches, with or without heat-processing. Yields of RS from wheat starch were affected primarily by processing temperature, reaching levels of about 9% in a single cycle of autoc1aving at 134°C with excess water and subsequent cooling (cf. levels of less than 1% in uncooked wheat starch) and higher levels still (about 15%) after five repeated cycles of autoclaving and cooling. A similar increase in yields of RS was seen in dilute (1%) starch suspensions that were subjected to repeated cycles of heating to 100°C, followed by cooling and storage. The time of storage after gelatinisation was only important in these dilute systems: levels of RS in freshly prepared concentrated starch gels (typically 57–67% H2O) or in white bread did not alter significantly on storage.