From the early years of emergence of nutritional science, it has
been recognized that the ingested nutrients in the diet are not
completely utilized in the body. An increasing volume of evidence
suggests that with very few exceptions, only a proportion of
the total ingested nutrients in a diet or food is available, and the
term “availability” has come into use for this proportion (Southgate
1989). The nutrients measured by chemical analysis may not
always be fully utilizable, mainly due to the indigestible cell walls,
a bulky or dense structure, a low solubility, the presence of some
compounds inhibiting the digestion, as well as components
abundantly present in plant foods such as dietary fiber, phytic
acid, and tannic acid, which may significantly reduce the absorption
and utilization of some nutrients (Rosado and others 1987).
During food processing, derivatization of nutrients and formation
of cross linkages occur, thereby making the food inaccessible for
digestion or/and metabolism. Such parts of nutrients are also “unavailable”
(Erbersdobler 1989).
Starch, which is the major dietary source of carbohydrates, is
the most abundant storage polysaccharide in plants, and occurs
as granules in the chloroplast of green leaves and the amyloplast
of seeds, pulses, and tubers (Ellis and others 1998). The relatively
recent recognition of incomplete digestion and absorption of
starch in the small intestine as a normal phenomenon has raised
interest in nondigestible starch fractions (Cummings and Englyst
1991; Englyst and others 1992). These are called “resistant starches,”
and extensive studies have shown them to have physiological
functions similar to those of dietary fiber (Asp 1994; Eerlingen
and Delcour 1995). The diversity of the modern food industry and
the enormous variety of food products it produces require starches
that can tolerate a wide range of processing techniques and
preparation conditions (Visser and others 1997). These demands
are met by modifying native starches with chemical, physical, and
enzymatic methods (Betancur and Chel 1997), which may lead to
the formation of indigestible residues. The availability of such
starches therefore deserves consideration. This review therefore
focuses on the availability of the major nutrient, that is, the starch,
with special reference to RS.