On the digestibility of starch in wheat bread - studies in vitro and in vivo

The digestibility of starch in white wheat bread was studied. Raw wheat starch and raw wheat flour were used as reference materials. The extent of digestion and absorption in vivo was evaluated through balance experiments in rats given test diets with and without addition of the antibiotic, Nebacitin, to reduce the fermentation in the hind-gut. As judged from a high faecal recovery of dietary fibre constituents in rats fed a wheat flour diet with Nebacitin, this drug significantly reduced the microbial activity in the hind-gut. Only minute amounts of starch could be detected in faeces of rats whether Nebacitin was present or not, indicating that pure wheat starch as well as starch in wheat bread and raw wheat flour was almost completely digested and absorbed in the rat small intestine. However, during baking, a fraction of the starch (0·6%–0·9%, dry basis) was rendered resistant to enzyme digestion in vitro unless solubilised in KOH. This modified starch fraction also remained undigested in vivo, but was readily metabolised by the hind-gut microorganisms, thus having physiological properties similar to those of dietary fibre. It is therefore recommended that resistant starch formed during baking (i.e. that rendered resistant to enzymic digestion as measured in vitro) should be regarded as dietary fibre rather than as dietary starch