Agriculture Reference
In-Depth Information
length with a relatively high content of α-1,6 branching chains. The linear structure
of amylose molecules allows its aggregation in crystalline structures inside the gran-
ules termed A domains, while the branched structure of amylopectin produces its
disorganized aggregation, forming the noncrystalline structure of B domains of the
granules. Lipids and proteins are also found in small but variable proportion in the
starch granule; however, these components have a relatively limited contribution to
the physicochemical and organoleptic properties of starch. The proportion of amy-
lose versus amylopectin, the average length of α-1,4 linear chains, and frequency of
α-1,6 branching are variable between starches derived from different species or vari-
eties of plants, affecting the degree of crystallization of the respective starch gran-
ules (Lee 1983; Robertson 1988; Gray 1992; Englyst et al. 1999). Highly crystalline
granules with high amylose content require more severe conditions to attain their
hydration and enzymatic hydrolysis. Corn, wheat, potato, tapioca, and rice are the
major grain sources of commercial starch. However, hundreds of different botani-
cal species have been used as sources of starch that differ in their physicochemical,
organoleptic, or nutritional properties.
Effective total amounts of glucose and rate of release produced during starch
digestion are therefore affected by its botanical origin, industrial processing, and
form of preparation and presentation in foods. Thus, diverse procedures have been
employed to measure starch product digestibility based on their rates and potential
amounts of glucose release (α-glucogenesis). The most widely used procedure is the
Englyst test based on the rates of glucose release during the enzymatic hydrolysis by
preparations of fungal and animal glucosidases and the measurement of the propor-
tions of starch that are rapidly (RD) or slowly (SD) digested, as well as the undigested
or resistant starch (RS) fraction after 2 hours of enzymatic treatment (Englyst and
Cummings 1990; Englyst et al. 1999; Englyst and Englyst 2005). Alternative meth-
ods attempting to measure the potential physiologic effects of starches, the glycemic
index (Ludwig 2000; Brand-Miller et al. 2002; Leeds 2002; Englyst et al. 2003;
Han et al. 2006) and insulinemic index (timed blood glucose or insulin concentra-
tions, respectively) are measured in human subjects after ingestion of a standardized
amount of starchy foods. However, these procedures are limited by the type and
number of products that can be administered, the number of human subjects who
can be studied, and the inherent physiologic variations among individuals, mak-
ing difficult the acquisition of reproducible or comparable data sets. More recently,
the use of recombinant human and mouse intestinal α-glucosidic enzymes has been
introduced, but the convenience of these procedures awaits standardization with a
wider collection of starch products and the formal comparison with more common
procedures.
In humans, the digestion of starch occurs by enzymatic hydrolysis during its
transit through the gastrointestinal tract and requires the participation of six dif-
ferent α-glucosidase enzyme activities (Corring 1980; Jones et al. 1983; Lee 1983;
Fujita and Fuwa 1984; Gray 1992). Given the structural complexity and variability of
starches as well as the diversity of enzymes involved in their hydrolysis, the detailed
mechanisms for the process have not been determined. The most accepted models
describe the process only superficially, without taking into account the complexity in
