Type 2 RS in native granules
It is well known that native starch granules from potato or banana are highly resistant to digestion [11]. There is little information available about how the specific cultivars of these species might be selected for high or low RS levels. In any event, RS from these species is something of a curiosity, since most potato products are eaten cooked, and ripe bananas contain little or no starch. (Bjorck has pointed out that plantain bananas will contain appreciable starch [4].) Little information about the level of type 2 RS in other sources is available, although it is often assumed that normal maize and wheat starch have little RS. We have suggested recently that RS in both starches may be substantial, although it is readily lost on cooking [10].
For some time it has been known that HAMS from a variety of different mutant maize genotypes are not well digested 21 and 22. HAMS is the result of one or more endosperm mutations that alter the nature and proportion of the amylose and amylopectin fractions [23]. While the adjective ‘high-amylose’ implies that the amylose fraction is enriched, the longer chains of the constituent amylopectin also contribute to a higher apparent amylose content [23]. Most HAMS are from endosperm containing the amylose-extender (ae) gene, and it has been known for some time that for ae-type maize starches the level of amylose varies considerably with the genetic background [24]. Selection of a starch cultivar for an altered amylose content may be a means of modulating the RS level. One company has been marketing a maize starch with an exceptionally high amylose content as a high-RS ingredient [14]. It is clear that judicious choice of genetic make-up of the plant is a strategy for manipulating type 2 RS.