Substrate processing alters starch digestibility.

Resistant starch is a dietary starch that resists digestion in the small intestine. But it can undergo bacterial fermentation in the large intestine. Resistant starch, in addition to traditional dietary fibers, may contribute significantly to the health benefits of those who are on plant-based diets.

There exist four main groups of resistant starches: RS1, RS2, RS3 and RS4. RS1 is physically inaccessible starch--trapped in seeds. RS2 starch is granular starch. Examples include banana and high amylose starches. RS3 starch is a highly retrograded starch: extruded cereals. RS4 is a chemically modified starch. Each has different properties and offers positive effects on food texture, processing and colonic health. The starches analyze as total dietary fiber under the AOAC method.

Besides resistant starch, two other classes of dietary starch exist: rapidly and slowly digested starch. Starch fractions can vary widely both in their concentration and composition in food ingredients. These starch fractions vary in their rate or extent of digestibility and may alter the digestibility of other nutrients. In addition, chemically modified starches are unique types of resistant starch that are gaining interest as food ingredients because they can be tailored for specific food applications.

Researchers at the University of Illinois have determined concentrations of rapidly and slowly digested starch and of resistant starch in numerous foods and ingredients. While starch concentration and starch fraction distribution can vary due to the type of substrate, the results imply that substrate processing alters starch digestibility, increasing the concentrations of rapidly digested starch and decreasing the concentrations of resistant starch.

Investigators also compared the in vitro hydrolytic starch digestibility of chemically modified and unmodified starch sources. While the extent of starch digestibility decreased, the relative rate of starch digestion was not significantly affected. Although it is widely recognized that dietary starches are not completely digested in the small intestine, little information is available on the concentrations and relative proportions of resistant starch, or rapidly and slowly digested starch, in foods and ingredients.

Future research should focus on increasing the information available on starch fractions used in foods. Scientists also should look into the effects that processing and chemical modification have on the concentrations and relative digestibility of starch fractions.

Further information. George Fahey, Department of Animal Sciences, University of Illinois, Urbana-Champaign, 132 Animal Sciences Laboratory, 1207 W. Gregory Dr., Urbana, IL 61801; phone: [phone]; fax: [phone]; email: [e-mail].