Modified starches have been the subject of intensive research over many decades, resulting in many published studies on functional starch with newly developed functionalities. Preparing functional starch is an ongoing process as there are numerous possibilities resulting from an evolution of new processing technologies and market trends. Slowly digestible starch, resistant starch, porous starch, starch microemulsions, nanosized starch, and starch copolymers are the wide varieties of functional starches that can be used in the food industry.
- Introduction to Functional Starch
- Preparation of functional starch
- Characterization of Functional Starch
- Functional Starch Applications
- Applications of functional starch in the food industry
Slowly Digestible Starch (SDS)
Slowly digestible starch (SDS) is the starch fraction that is gradually digested throughout the small intestine, resulting in a low initial glycemia and, subsequently, a slow and prolonged release of glucose into the bloodstream, coupled with a low glycemic response in humans. Unlike most starch components, SDS is indigested by salivary α-amylase in the oral cavity and gastric acid in the stomach or through the vigorous grinding action resulting from gastric mobility. The healthy benefits of SDS are linked to stable postprandial glucose metabolism, reduced meal-associated hyperglycemia of diabetes, and better mental performance of the brain. There are two fundamental metabolic mechanisms for SDS benefits based on its starch structure: the physical structure of the starch granule that limits the accessibility of digestive enzymes and the chemical systems that restrict the rate of digestive enzyme actions. Native starch granules are ideal SDS due to their semicrystalline physical structure with slow digestibility. However, if added in thermally processed starch food, SDS will lose its slow digestible property. In this regard, amylopectin is another critical component of SDS, where the molecular structure, or the branching pattern, is key to forming starch molecules with slow digestibility. Controlled enzymatic starch treatments with debranching enzyme, amylosucrase, 4-α-glucanotransferase, α-amylase, and/or β-amylase are successful in producing unique molecular structures of starch with slow digestibility. However, all these tailor-made starches merely partially retain the slow digestibility after thermal processing. Therefore, the challenges for applications of SDS in the food industry are to improve the current stability issues by introducing new product designs or novel processing methods into the preparation of SDS.
Resistant Starch (RS)
Resistant starch (RS) is the fraction of starch that escapes digestion in the small digestion of healthy individuals and may be fermented in the colon. It has been classified into five general types depending on the structural characteristics and indigestible mechanisms of RS. RS1 is physically inaccessible starch in the food matrix, RS2 is native granular starch, RS3 is retrograded or crystalline non-granular starch formed through the reorganization of starch chains, RS4 is chemically modified or re-polymerized starch, and RS5 is amylose-lipid complexed starch. RS can be fermented by the probiotic bacteria in the colon into short-chain fatty acids (principally acetate, propionate, butyrate, and lactate), along with a small number of gasses such as carbon dioxide, methane, and hydrogen.
Its physiological effects include colonic cancer prevention, hypoglycemic/hypocholesterolemia effects, prebiotic functions, and the inhibition of fat accumulation and bile stone formation, which contribute to the health effects of RS. Thermal processing and retrogradation treatment, combined with enzymatic and/or chemical treatment, are frequently used in RS preparation. The unique properties of RS, such as its white color, bland flavor, fine particle size, and high resistance in processing, make it widely used in food products. In the future, novel processing methods should be developed to prepare new RS products from natural sources to help lower the production costs of RS.
Porous Starch
Porous starch is a novel functionally modified starch with large micro-sized pores extending from the surface to the interior of the starch granules. The pores covering the granule surface are approximately one μm in diameter, and their volume constitutes about half of the starch granule. Amylase can act on the surface of native starch granules at a temperature below that of starch gelatinization, can be used in porous starch preparation, where α-amylase and/or amyloglucosidase are the most often used enzymes. Porous starch can also be prepared when starch granules have been entirely destroyed by replacing ice crystals in a frozen starch gel with selected solvents using a solvent exchange technique. Porous starches effectively increase the specific surface area and provide excellent natural absorbents. In the food industry, porous starch can be used as the carrier of sweeteners, flavorings, and colorants and to protect sensitive elements such as phenolics, oils, minerals, vitamins, and probiotics. Chemical treatments are frequently introduced into porous starch preparations to improve adsorption, stability, and controllability. Jiang et al. developed an absorbent chitosan-modified starch to enhance the adsorption of procyanidins, which were fabricated by facile surface modification of chitosan on porous rice starch. Chemical modifications, such as using succinic ester, citric acid, and/or xanthate combined with the solvent exchange technique, have been successful in improving the absorption behavior and stability of porous starch in aqueous solutions. Further attempts should be encouraged to solve the processing stability of porous starch applicated in the food industry.
Starch Microemulsion
Microemulsions are clear, isotropic, or translucent, thermodynamically stable dispersions comprised of oil, water, surfactant, cosurfactant, and ethanol. The microemulsion possesses special properties such as ultralow interfacial tension and large interfacial tension with the capability to stabilize and solubilize otherwise immiscible liquids. The most important use of starch microemulsion is as a medium for polymerization reactions. The starch microemulsion is prepared by the dropwise addition of a starch solution into an oil phase under mechanical stirring. There are several procedures developed to prepare starch microemulsions, such as solvent evaporation, spray drying, precipitation, and emulsion cross-linking techniques. The starch microemulsion is normally designed to prepare starch microspheres and nanoparticles in the food industry.
Nanosized Starch
Nanosized starch, also called starch nanoparticles (SNs), is a kind of organic nanoparticle of 50–200 nm prepared from starch using chemical, enzymatic, and/or physical methods. During the previous decade, studies have been focused on nanosized starch, due to its availability in nature, low cost, renewability, biocompatibility, biodegradability, and nontoxicity. The native starch granules are micro-sized and can be broken down into nanosized particles using different nanotechnology methods, which are grouped into “top-down” and “bottom-top” processes. In the “top-down” process, nanosized starch is prepared from structure and size refinement by the breakdown of larger starch particles. In the “bottom-top” approach, nanosized starch is produced from a buildup of atoms or molecules in a controlled manner, such as by self-assembly and nanoprecipitation from dropwise starch paste solutions. Currently, the “top-down” approach is the most employed method in nanosized starch preparation. Acid hydrolysis, regeneration, and mechanical treatments are widely used for the preparation of nanosized starch, and acid hydrolysis is the most commonly used protocol. It is still difficult for scientists and food producers to obtain uniform nanosized starch using a simple, economic, and high-yielding method at a commercial/industrial level. In the food industries, nanosized starch can be used as a unique component for the regulation of physicochemical and rheological properties of processed foods, which are also found to have applications as emulsion stabilizers, fat replacers, and nanocomposite agents in the food packaging industry. Innovative processing/modification approaches should be developed in order to solve the aggregation problem of nanosized starch, thus greatly improving its application in the future.
Starch-Lipid and Starch-Protein Systems
Molecular inclusion is a superior technique for the effective protection of sensitive substances, compared to conventional encapsulation procedures such as freeze drying, spray drying, conservation, and liposome entrapment because of the difficulty of these methods in achieving the ideal process conditions to ensure effective coating of the compound to be protected. Starch is nonallergenic, safe, and cheap; it provides a highly attractive alternative for use as a delivery system for the protection of sensitive and unstable substances. There has been increasing interest in starch used in native and modified forms to encapsulate food ingredients such as flavones, lipids, polyphenols, carotenoids, vitamins, enzymes, and probiotics. Starch in granular or amorphous forms is treated using chemical, physical, and/or enzymatic methods to confer the required properties for targeted encapsulation. The starch-based encapsulation of food ingredients is developed using various methods such as freeze drying, physical trapping, precipitation, spray drying, extrusion, complexation, electrospinning, emulsification, and electrostatic interaction. Amylose inclusion is usually carried out at rather high temperatures of over 100 °C, but Vasiliadou et al. discovered that the production of crystallites of amylose complexed with lipids can be quantitatively achieved at temperature as low as 75 °C. Therefore, it is possible to produce complexes under milder processing conditions where the substances will not suffer significant losses during complexation. The linear component of starch, especially amylose, can develop a helical structure with a hydrophobic cavity that can form inclusion complexes with hydrophobic ligands such as lipids, phenolics, and flavors. By forming an inclusion complex with amylose or starch, these active ingredients can be protected from the acidic environment of the stomach for targeting absorption in the small intestine, and in this manner, their bioavailability may be increased. Briefly, two methods, including coprecipitation and acidification of an alkaline solution, are commonly employed in starch-lipid complexation, while electrospinning-based complexation was recently reported. Amylose-omega-3 fatty acid complexes can be incorporated into bread formulations and can then be reduced by lipid oxidation and the formation of nonanal and hexanal during the baking.
Proteins and starch interact with each other in foods when brought together, and the interactions may be repulsive (repel each other) or associative (attract one another). It has also been reported that protein-starch interactions are driven by the attraction of opposite changes between the macromolecules, but there may be significant influences of the gelatinization parameters (increase in onset and peak temperatures and decrease in the delta H) and water evaporation of the starch. Starch and protein interactions have also been known to significantly alter macroscopic properties, such as flow, stability, texture, and “mouthfeel” of food products. The development of starch-protein complexes is complicated; therefore, the reaction processes and end products are determined by many internal and external factors. The starch-protein complexes function to improve the solubility, viscosity, gelation, emulsification, foam-forming ability, and inoxidizability of the starch and/or protein. The starch-protein complexes can be used in starch gels, in snacks, for a clean food contact surface, in yogurt, and as an emulsifier. Shah A. et al. have developed a soluble self-assembled complex from starch, proteins, and free fatty acids for healthy nutrient delivery as a nanoparticle carrier for hydrophobic small molecules.