Factors that affect the digestibility of starch

There are many factors that affect the digestibility of starch, including the physical form of grains and seeds, plant genotype, associations between starch and other food components, and food processing methods (milling, cooking, annealing, high-pressure processing, autoclaving, irradiation, and extrusion and storage times).

The Inaccessibility of Starch

When starch-rich foods are not finely ground, the digestibility of starch may be significantly reduced. The amount of indigestibility depends on the varieties of food, structural tightness, and other matrices around the starch granule. For example, food made from barley flakes has a starch digestibility of 83.5%, while food made from the same barley in a milled form has a digestibility of 99.1%. The starch digestibility of whole canned beans is also unusually low at approximately 64.1% because of incomplete chewing, resulting in large particle sizes. This form of RS (RS1) is used in a variety of conventional foods because it is relatively stable to most cooking operations. However, treatments including fine milling, which can damage the integrity of the plant cell or tissue structure, will decrease the content of RS1.

Starch Granules and Their Crystallinity

The botanical origin, which determines the morphology and crystalline organization, is the most important factor that determines the rate and extent of amylolytic hydrolysis of granular starches. X-ray diffraction has been used to identify distinct crystalline forms of amylose and amylopectin in starch granules. Type A is characteristic of cereals, type B is more commonly present in potatoes and bananas, and type C is an intermediate form found in legumes.

In A-type polymorph starch, amylopectin has a larger percentage of branch chains of DP 6–12 than that of B-type polymorph starch, and the branching linkages of the amylopectin are scattered in both the amorphous and crystalline regions. This packing of the double helices in the A-type polymorph results in starch granules containing peripheral pores with weak points in the crystalline region. However, B-type polymorph starch possesses longer branch chains to form longer and more stable double helices, and the branching linkages of amylopectin are mostly in the amorphous region, which results in a compact structure of starch granules. The A-type polymorph starch granules are digested from the surface to the hilum because of their porous structure, which facilitates enzymatic hydrolysis. B-type polymorph starch granules are digested by erosion of starch granules starting from the surface, which is unfavorable to enzymatic hydrolysis of the starch molecules.

X-ray diffraction of starch granules showed that the chain fragments packed in a B-type crystalline structure with enlarged crystal lattices affect the formation of RS2. Gelatinization treatment, which eliminates starch crystallinity or damages the integrity of starch granules, reduces the RS2 content.

The Amylose/Amylopectin Ratio

The high amylose content of starch decreases its digestibility. Amylose molecules interact with amylopectin, which restricts starch swelling and reduces the accessibility of enzymes to hydrolyze starch molecules. Also, high-amylose maize starches with very long chains can be perfectly ordered into double helices to form B-type crystallinity.

A higher content of resistant starch was found in Hylon VII than in Hylon V (high-amylose genetically modified cornstarches), which might result from the higher-amylose content in Hylon VII. Some studies with high-amylose maize starches have reported that the RS content is negatively correlated with amylopectin content of the maize ae-mutant starch, indicating that amylopectin makes little or no contribution to the RS formation in the maize ae-mutant starches.

Retrogradation of Amylose

Starch granules are disrupted by heating in an excess of water in a process commonly known as gelatinization. Upon cooling, starch molecules reassociate and form tightly packed structures stabilized by hydrogen bonding, which is commonly termed retrogradation. Retrograded amylose, which is known as RS3, is highly resistant to digestion.

High-amylose starch is a rich source of RS2, which produces retrograded starch or RS3 in high yields after heating and cooling treatments. Amylopectin interferes with amylose retrogradation. The yield of retrograded amylose may increase if amylopectin is debranched by a debranching enzyme such as pullulanase. The degree of polymerization of amylose also affects the yield of RS3. It is generally accepted that glucose unit ranges of 10–100 favor the formation of a double helix. This observation explains why amylopectin is so unfavorable to the formation of thermally stable resistant starch. Not only are the branches hindered in movement, but the typical lengths of 20–40 glucose units are far from the optimum number of 100.

The Effect of Lipids on the Digestibility of Starch

Amylose has been reported to form a single helical complex with lipids such as free fatty acids, monoglycerides, phospholipids, and long-chain alcohols. The amylose-lipid complex (ALC) is classified as type I or type II depending on the dissociation temperature of the crystalline components. Type I generally has dissociation temperatures between 90 and 105 °C, whereas type II dissociates between 105 and 125 °C. Type I complexes consist of a partially ordered structure with no distinct crystalline regions, whereas type II complexes are composed of distinct crystalline or semicrystalline structures. Type II can be further subdivided into types IIa and IIb. They differ slightly in the degree of crystallinity or the perfection of the ordered domains, in which type IIb is the most stable form of the ALC.

In general, the dissociation temperature of the ALC increases with increasing chain length of the fatty acids and decreases with the increasing number of double bonds. Among the unsaturated fatty acids, the fatty acids with trans double bonds form ALC with a higher dissociation temperature than those with cis double bonds. After heating at a temperature above the dissociation temperature, amorphous Type I ALC rearranges into lamellar crystallites (Type II), which display a V-type X-ray diffraction pattern and show a higher dissociation temperature (>100 °C).

Single helix complex formation with lipids and the further development of lamellar crystallites protect amylose from enzymatic hydrolysis. The ALC also restricts the swelling of starch granules, to further reduce the susceptibility of starch molecules to amylolysis. Thus, the ALC has been called RS5. A study involving the effect of endogenous lipids on wheat starch showed that defatting of the starch samples resulted in a decrease of the RS content. When SDS was added to defatted wheat or amylomaize starch, the resistant starch yield decreased significantly. However, regarding the preparation of RS by adding lipids to starch, the yields of RS5 from complexed lipids are lower compared to that of retrograded RS3, and the complex formation of amylose with lipids has an adverse effect on amylose recrystallization, which is important in RS formation.

The Interaction of Starch with Other Components in Foods

Proteins, sugars, lipids, and polyphenols, when mixed with starch in foods, significantly affect starch retrogradation by preventing hydrogen bond formation between amylopectin and amylose chains and thereby reducing the content of resistant starch.

When a mixture of potato starch and albumin was autoclaved and then cooled to −20 °C, the added albumin reduced the content of RS. The addition of soluble sugars such as glucose, maltose, and sucrose reduced the level of crystallization and subsequently reduced the yields of RS. The mechanism of retrogradation inhibition involved the interaction between sugar molecules and the starch molecular chains, which changed the matrix of gelatinized starch. The yields of RS in potato starch gels decreased in the presence of calcium and potassium ions compared with those with no added constituents, presumably because of the prevention of retrogradation.

Enzyme inhibitors such as phytic acid, polyphenols, and lectins, which are present in leguminous seeds, have been found to inhibit in vitro digestion and the glycemic index of starch. Both amylase and intestinal maltase activities were inhibited by tannic acid. Because phytic acid inhibits amylolysis, an increase in phytate content decreased the digestibility of starch.

Processing Conditions

In whole grains, starch is encapsulated in plant structures and therefore contains more RS (RS1) than flour. Milling is a high-shear process. When starch granules are milled, their crystalline regions are damaged, and the susceptibility to enzyme degradation increases. The extent of milling therefore affects starch digestion in cereals and legumes.

Starchy foods are usually subjected to heat treatments in the presence of water before consumption. The heating and cooling increases the nutritional value and generates desirable flavors and textures and can also affect both the gelatinization and retrogradation processes and thus destroy RS1 and RS2 but form RS3. The two types of starch granules involve those with either nonelevated or high-amylose starches. The former lose their RS on cooking, whereas the latter retain some granular integrity after heat processing.

Different types of thermal processing affect the degrees of gelatinization and retrogradation, depending on the moisture, temperature, and duration of heating and subsequent cooling. Numerous studies have reported that thermal treatments such as steam cooking, autoclaving, and baking increase the production of RS. Starches isolated from several steam-heated legumes were rich in indigestible RS (19–31%), which was not observed in raw beans, suggesting that retrogradation was mainly responsible for the reduction in digestibility. White flour subjected to repeated autoclaving and cooling cycles showed a threefold increase in total RS in bread flours and a fourfold increase in pastry flours. During the tempering (holding) process, cooked grains undergo time-dependent changes involving the equilibration of either the temperature or the grain moisture. This process may further result in an improvement in the textural properties of grains and a decrease in starch digestibility. Retrogradation of starch is greatly affected by storage temperature, and the storage of starch gels at lower temperatures generally increases retrogradation.

Extrusion is a thermal process involving the application of high heat, high pressure, and shear forces to cooked substances such as cereal foods. Extrusion of cereals is performed using both single and twin screw extruders. The distinguishing characteristic of these cooker units is the high temperature and short time of cooking (i.e., 0.5–5 min). Mechanical energy input is the primary mechanism of cooking, although intensive thermal input via barrel heating and steam injection is also used. The results of extrusion on the RS contents of starches are controversial. Numerous studies have reported a decrease in the RS content after extrusion and attributed this decrease to the destruction of granular structure resulting from thermal treatment, high pressure, and shear forces, but there are some studies that reported a significant increase of RS3 content in high-amylose starches after extrusion cooking.

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