Resistant Starch Preparation

Many approaches can be used to increase the content of Resistant Starch (RS), including acid hydrolysis, debranching, autoclaving/cooling cycles, heat moisture treatment (HMT), chemical modification, and starch-lipid complexation.

There has been great interest in increasing the RS content in food. The content of RS formed during the processing of starchy foods is controlled by a number of factors such as water content, pH, heating temperature and time, the number of heating and cooling cycles, and freezing and drying.

Resistant Starch (RSs) from Natural Sources

Natural cereals, seeds, and heated starch foods contain high amounts of RS (RS1 and RS2). Among nonprocessed foods, unripe bananas are the richest source of RS (47–57%), and unripe banana flour is rich in RS. Tuber starches such as potato starch contain B-type crystallinity that is highly resistant to enzymatic digestion (RS2). Whole grains are rich sources of RS1, dietary fiber, and oligosaccharides. The amylomaize VII (ae-VII) hybrid of corn contains approximately 70% amylose. The transition associated with gelatinization of ae-VII occurs between 65 and 120°C. This is because of native RS2, which is thermally more stable than raw common cornstarch. The level of resistance in a RS2 product can be increased by using hybrids, which have amylose levels >70%. Starch granules may also be fractionated by size to obtain higher-amylose levels and higher levels of RS2 resistance.

RS3 from the Heating and Cooling Treatment of Starch

Heating and cooling are involved in the preparation of RS3. During gelatinized starch retrogradation, starch molecules reassociate and form tightly packed structures stabilized by hydrogen bonding. These structures are thermally very stable, with the melting of amylose double helices at 80–150 °C, depending upon the extent and nature of the retrogradation. The application of repeated heating and cooling cycles of starch in the presence of moisture has been reported to increase RS3 levels. The temperatures commonly used for heat moisture treatments are usually in the range of 100–150 °C for periods ranging from 30 min to 1 h. Heat treatment in the presence of sufficient water facilitates increased starch chain mobility and molecular rearrangement, which results in the retrogradation of gelatinized starch and increased levels of RS3. Higher temperatures are optimal. Thermal cycling may not be necessary at the extremely low temperature of 4 °C, but cycling to 140 °C is still advantageous for the formation of extremely stable RS3.

Retrogradation occurs very fast for the amylose moiety, because the linear structure facilitates cross-linkages via the formation of hydrogen bonds. The branched nature of amylopectin inhibits its recrystallization, which occurs over several days. The final level of RS is strongly dependent on the amylose content, and the retrogradation of amylose has been identified as the main mechanism for the formation of RS3 that can be generated in larger amounts by repeated autoclaving. It is therefore advantageous to start with native starch high in amylose. The naturally high level of amylose in the ae-VII hybrid of corn makes it particularly suitable for the production of RS3.

CrystaLean is a commercial, highly retrograded RS3 product based on the ae-VII hybrid. It is produced by first fully hydrating and disrupting the starch granules, followed by enzymatic debranching. The mixture is then treated by thermal cycling to produce a high level of retrogradation prior to drying. DSC analysis shows that the endothermic peak is located in the range of 105–145 °C, which enables it to withstand most normal cooking procedures.

Resistant Starch (RS) from the HMT of Starch

HMT is a physical modification of starch that uses controlled heat and moisture. The treatment does not involve gelatinization and thus does not cause any damage to the granular structure. The method involves treatment of starch granules at limited moisture levels [<35% (w/w/) moisture] for 15 min to 16 h at temperatures above the gelatinization temperature (84–120 °C). This treatment promotes retrogradation and formation of RS3. The mechanism involves initial disruption of the crystalline structure and dissociation of double-helix structures, followed by reassociation of the disrupted crystals. HMT also results in the formation of starch crystallites. It initially leads to incipient swelling and the resulting mobility of amorphous regions, which then favor ordering of the double helix.

However, a study showed that HMT led to a decrease of RS and RDS, with an increase of SDS. The effects of HMT on enzyme digestibility usually depend on the species of starch, moisture content during the HMT, temperature and duration of the HMT, amylose-lipid interactions, and starch chain-chain interactions.

Resistant Starch (RS) from Enzyme Treatment of Starch

Enzymatic debranching is a common method to produce linear chains, which results in a high content of RS. Pullulanase (EC is one of the notable debranching enzymes that cleaves the α-1→6 linkages in pullulan, amylopectin, and other polysaccharides. Debranching of amylopectin generates more short linear molecules to further realign a new crystalline structure that can undergo retrograde processing. Debranching is therefore commonly combined with autoclaving and cooling or HMT, which facilitate retrogradation of short linear molecules, to increase the yield of RS molecules.

Resistant Starch (RS) from Chemically Modified Starches

Chemical modifications have long been used to reduce the in vitro digestibility of starch. Cross-linked starches are obtained by the reaction of starch with bi- or polyfunctional reagents such as phosphorus oxychloride, sodium trimetaphosphate (STMP), or mixed anhydrides of acetic acid and dicarboxylic acids like adipic acid. Cross-linking of starches from rice, wheat, corn, potato, tapioca, oat, and mung bean using STMP, sodium tripolyphosphate (STPP), epichlorohydrin, or phosphoryl chloride (POCl3) produces RS (RS4). The levels of RS in wheat starch cross-linked with 2% POCl3, 12% STMP/STPP, and 2% epichlorohydrin were 85.6, 75.6, and 75.8 g/100 g starch, respectively. A previous study prepared acetylated bean starch with high levels of RS (44%). Acetylation of starch increased the RS content because of acetyl groups that blocked the action of digestive enzymes. Modification of starch with octenyl succinic anhydride is known to increase levels of SDS and RS more than other modifications such as acetylation, hydroxypropylation, or cross-linking.

The RS5 Starch-Lipid Complex

The ALC can be produced by classical, enzymatic, and thermomechanical methods. The classical method involves solubilizing amylose in solvents such as dimethylsulfoxide or KOH (followed by neutralizing with HCl) by heating to 121°C, and then dissolved lipids in absolute ethanol are added to the cooled starch solution and centrifuged. The precipitates are successively washed with hot water to remove free starch and fatty acids. This method yielded type I and type II ALCs at manufacturing temperatures of 60°C and 90°C, respectively. In the enzymatic method, glucose-1-phosphate is usually used as a primer, with a polymerization enzyme (i.e., phosphorylase) added to promote glycosidic bonds. During this process, the primer is first polymerized to produce amylose chains of sufficient length to accommodate the first lipid, with further chain extension occurring to form insoluble ALCs. ALCs formed by this method were short chain ALCs and mostly type I.

Alternative methods to produce ALCs use thermal processing technologies including steam jet cooking, homogenization, pasting, and extrusion. These thermal processing methods are more “greener” than the classical and enzymatic methods, which are suitable for the laboratory scale production of pure ALC.

During steam-jet cooking, water dispersions of granular starch are continuously pumped through a hydroheater, where they are instantly heated with steam under high-temperature and high-shear conditions. Mm-sized spherulites of helical inclusion complexes of amylose and the native lipid material are formed during the steam-jet cooking of high-amylose maize starch with palmitic acid. The steam-jet cooking conditions that influence the formation of spherulites involved the starch concentration, presence of lipid, and cooling conditions. Particles <1  μm were obtained with rapid cooling on ice to 25 °C.

High-pressure homogenization during heating enhanced the formation of ALC between cornstarch and fatty acids. Homogenization disintegrates starch granules to release amylose and improves the fatty acid dispersion in the starch, increasing its contact with amylose in the starch granules. ALC formation increased with homogenization, and the enthalpy of the unhomogenized starch-fatty acid complex was lower than that of the homogenized complex.

Wet heat processing is continuous heating of the starch-lipid mixture in water. A biphasic pasting phenomena is often observed during the heating process. This biphasic pasting was characterized by a commonly observed paste peak viscosity after pasting for a short time (<15 min of heating) and a second, higher peak paste viscosity after pasting for a longer time (> 30 min of heating). The observed biphasic phenomenon was due to the occurrence of ALCs. Using DSC, Type I ALCs were observed after short pasting times, and type II ALCs were observed after long pasting times.

Extrusion cooking is a high temperature and short time process, which can result in the formation of ALCs due to the high temperature and high moisture combined with pressure and mechanical shear. High-amylose (45%) corn extrudates higher ALCs compared to the corresponding native cornstarch extrudates during extrusion cooking with a twin screw extruder. Taken together, the results show that the actual degree of lipid binding depends on the amylose content.

Resistant Dextrins from Partial Acid Hydrolysis or Pyroconversion

When starch granules are exposed to mineral acids, the hydroxonium ion attacks the oxygen in the glycosidic bond and later hydrolyzes the linkage. At temperatures below the gelatinization temperature, the acid works on the starch granule surface prior to entering the inner region of starch and alters physicochemical properties but maintains the granule structure intact. Acid hydrolysis is a common method to generate linear chains with desirable lengths for optimal formation of resistant starch. For example, consecutive treatments of mild acid hydrolysis and freeze thawing enabled more amylose to aggregate and thus enhanced the possibility of retrogradation. It was reported that a combination of acid hydrolysis and three cycles of autoclaving and cooling in arrowroot starch resulted in a fourfold increase in resistant starches.

When starches are heated with partial acid hydrolysis, resistant maltodextrin is produced. The complex process, called pyroconversion of starch, includes depolymerization, transglucosidation, and repolymerization reactions. The formation of new 1→2- and 1→3-glucosidic bonds makes dextrins less susceptible to the activity of digestive enzymes. Pyrodextrins are commercially produced by heating dry acidified starch in a reactor with agitation.

Some resistant dextrins are produced by microwave heating of potato starch acidified with hydrochloric and citric acids. The water solubility of samples increased with an increase in microwave power and heat exposure time. The selected dextrins with the largest low molecular weight fraction and the highest water solubility were characterized by increased (up to 25%) dietary fiber (DF) content. These enzyme-resistant dextrins did not paste, even at 20% concentration, and were characterized by a low retrogradation tendency. These features make the tested enzyme-resistant potato starch dextrins suitable for use in the soft drink industry.

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