Food processing industries and consumers require starches to have better behavior characteristics than that provided by native starches. Fortunately, shortcomings of native starches can be overcome by chemical, physical, or enzymatical modification.
- Introduction to Functional Starch
- Varieties of functional starches in the food industry
- Characterization of Functional Starch
- Functional Starch Applications
- Applications of functional starch in the food industry
Chemical Modification
Chemical modification is a widely used method that involves the polymer molecules of the starch granule in its native form and enables the enhancement and/or introduction of functionality into the modified starch. The common chemical modifications used in the food industry involve oxidation, cross-linking, stabilization, and depolymerization, as well as changes in the pasting properties, freeze-thaw stability, shear resistance, solubility, and digestibility. The chemical treatments used for starch modification can be classified as depolymerization or derivatization reactions based on the molecule weight of the modified starch. Acid and oxidative modifications are depolymerization procedures used to produce thinned starch with decreased paste viscosity, lower gelatinization and pasting temperature, improved emulsification, increased solubility, and improved film properties (i.e., stiffness and water vapor permeability). The chemical modifications of starch are generally achieved through derivatizations such as etherification, esterification, cross-linking, oxidation, cationization, grafting of starch, and chemical derivatization. In the US market, food starches are modified by using hydroxypropyl, acetate, phosphate, octenyl succinate, and adipate compounds. The consumer safety of chemically modified starch and the resulting pollution of the processing environment is not optimal and are therefore major drawbacks for chemical modification of starch. Thus, there is a considerable need to develop novel and cleaner methods for functional starch preparation with more emphasis on enzymatic, physical, and genetic modifications.
Physical Modification
Physical modification can be safely used in the preparation of functional starch because it usually does not involve any chemicals. Recently, novel physical modification methods have been introduced into this field.
Plasma Modification
Plasma is a medium that contains both negatively and positively charged particles with an overall electrical charge of zero. These include electrons, protons, negative and positive ions, free radicals, atoms, and non-excited or excited molecules. Plasma modification, which is a physical treatment that induces chemical changes, can be divided into two types, thermal and non-thermal, based on different generated conditions. Thermal plasma is generated at high pressure (up to 105 Pa) and/or high temperature (≥2 × 104 K). In contrast, nonthermal plasma processing is an emerging green technology that uses lower pressures with much less power input without generating industrial waste, resulting in a greater potential to improve the quality and microbial safety of various food materials. There has been increasing interest in utilizing plasma to modify the functionalities of starch through interactions with reactive molecules. A range of techniques and instruments, such as coronal discharge, a gliding arc, and a plasma jet, have been used to generate nonthermal plasma. Plasma modification induces various chemical changes to the starch, which include depolymerization, cross-linking, and the formation and addition of new functional groups. Plasma modification has been used to change the surface properties of various starch-based materials, including thermoplastic films and biocomposites, and the compatibility of starch-based biocomposites with proteins and cells. The desired surface properties of starch can be obtained through carefully manipulating these factors, including the type of plasma, treatment conditions, and starch/composite type. Plasma treatment coupled with multiple starch modification should be further explored to create a range of starch functionalities in the future.
Hydrothermal Treatment
Annealing (ANN) and heat-moisture treatment (HMT) are two hydrothermal treatment procedures, which alter the physicochemical properties of starch without destroying its granular structure. These two related processes differ in the amount of water and the temperatures used. ANN is carried out with excess water and relatively low temperatures (below the starch gelatinization temperature), while HMT uses heat at temperatures above the gelatinization temperatures (90–120 °C), but with insufficient water (10–30%) for gelatinization. Regardless of the origin, ANN improves the physicochemical properties of starch by improving the crystalline qualities and facilitating interactions between the starch chains to reorganize the starch molecules. HMT allows for the control of molecular mobility at high temperatures by limiting the amount of water used to promote a change in the structural arrangement of the starch chains within the amorphous and crystalline areas of the granules. The granular swelling, crystallinity, gelatinization, retrogradation, thermal stability, amylose leaching parameters, and paste properties are modulated by HMT. ANN- and HMT-modified starches are suitable for utilization in canned foods, noodle manufacturing, and frozen foods.
Ultrasonic Treatment
Ultrasonic treatment is a physical method employing high-frequency ultrasound (>15–20 kHz), which creates strong shear force, high temperatures and free radicals, and may result in changes in the structures and functions of starch in a starchwater system. Ultrasound causes many pores and fissures in the starch granules, inducing either degradation or having little effect on the starch chain on the molecular level. New pores in the range of 1.7–300 nm may be existed during the ultrasonic treatment of starch. Ultrasound could significantly change the thermal stability, retrogradation, and gel properties of starch, causing depressions on the surface of the starch granules. The resulting cracks and pores in the starch granules greatly enhance the reaction efficiency of other chemical, physical, or enzymatic agents.
High-Pressure Treatment/Homogenization
High-pressure treatments (HPTs), such as ultrahigh-pressure and high hydrostatic pressure treatment, are nonthermal processing technologies suitable for the production of minimally processed foods. In the food industry, HPT is carried out at 100– 1000 MPa and room temperature to modify and sterilize food packed materials in vessels. At present, HPT has been successfully used for gelatinization of various types of starches while maintaining the integrity of the granules. Compared with the conventional heat-gelatinized starches, HPT-gelatinized starches show dissimilar properties, such as lower swelling indices, lower rates of retrogradation, lower susceptibilities to amylolytic enzymes, and increased creaming stabilities.
High-pressure homogenization (HPH) is different from static HPT, where liquid samples undergo cavitation, high shear, turbulence, and velocity gradients induced by a rapid change in pressure. In industry, homogenization pressure usually occurs between 20 and 50 MPa, and for ultra-HPH, the pressure can be above 100 MPa. In the food industry, HPH is extensively used to disperse, emulsify, mix, and process the food substances. Starches are frequently processed by HPH with the purpose of imparting novel functional properties, producing new products, and/or isolating starch from other constituents. HPH induces an inside-out disruption of starch granules, occurring close to the linkage among blocklets. There have been increased applications of HPH in the field of starchlipid complex preparation. HPH facilitated the rapid dispersion of lipids with lower water solubility and enhanced the chances of reactions between lipids and amylose. In addition, HPH has been used recently as an efficient emulsification technique to provide mechanical energy to produce miniemulsions for preparing nanoparticles.
Extrusion Processing
Extrusion cooking is a commercially promising technique used to process and produce large numbers of starch-based foods of varying shapes, sizes, tastes, and textures; it is a barothermal treatment that involves shear energy, pressure, and heating. The main advantages of extrusion cooking are the ability to process highly viscous polymers in the presence of plasticizers and the promotion of significant structural qualities in the starch molecule. Changes for starch during extrusion include gelatinization, melting, degradation, and fragmentation. The disruption of molecular, supramolecular, and granular structure of starch depends on the barrel temperature, plasticizer content, screw speed, feeding rate, die size, and screw configuration used during the extrusion cooking process. The intense shear force used during extrusion can cleave both α-1→6 and α-1→4 linkages of starch molecules and the starch ordered structures such as the double helical and crystalline structures. Amylopectin with short branch lengths is associated with higher susceptibility to shear degradation than the linear and smaller molecular weight amylose. Extrusion degradation prefers cleavage of longer branches when the starch polymers are in a semicrystalline granular formation. Moreover, the rigid crystal structure in amylopectin is more sensitive to shear treatments than the flexible amorphous structure of amylose. Extrusion processing lowers the retrogradation degree, improves the freeze-thaw stability, and decreases the digestibility of starches. Extrusion processing techniques have been successful in producing starch-based films, resistant starch, thermoplastic starch, starch-lipid complexes, and noodles as well as preparing chemically modified starch as an initial reactant.
Microwave Irradiation
Microwaves are electromagnetic waves of 300 MHz to 300 GHz. For domestic food applications, the microwave usually operates at a frequency of 2.45 GHz, and industrial applications operate at frequencies of 915 MHz and 2.45 GHz. Microwave irradiation has many applications in the food processing field, including drying, pasteurization, sterilization, thawing, tempering, and baking of food materials. Microwaves create heat deep inside the materials being processed as a result of rapid alterations in the electromagnetic field at high frequency, thus achieving shorter processing time, higher yields, and better quality when compared with conventional processing techniques. Microwave irradiation induces the rearrangement of crystalline structures within the starch granule leading to changes in the molecular structure and physicochemical properties, such as water absorption ability, swelling power, and viscosity. In addition, the advantages of microwave technology in promoting chemical reactions include lower energy use, improved reaction rates, simplification of procedures, and significantly reduced waste by-products from the chemical reaction production process. Based on these properties, microwave irradiation was used to prepare insoluble starch composite foams, starch ester films, starch acetate, and resistant starch.
Enzymatic Modification
Enzymatic modification has been increasingly explored as an alternative technique to modify the granular and/or molecular structure to produce functional starches. The main advantages are the mild reaction conditions, high selectivity, and less undesirable by-products than that of chemical processes that require harsh conditions and highly reactive compounds.
Acylation
Acylation reactions are performed by lipases and proteases to increase the hydrophobicity, reduce the viscosity, and decrease the digestibility of starch. Unlike chemical esterification modification, the enzymatic technique is an environmentally friendly method, which is operated under milder conditions with less by-products. Regiospecific and stereospecific esterification of starch can be easily performed using enzymes. The esterification of starch palmitic acid was catalyzed by Novozyme 435 lipase in a micro-solvent or solvent-free system to produce functional starches with emulsifying properties. The esterification of starch catalyzed by lipase was also feasible on starch nanoparticles and in aqueous gel systems.
Copolymerization
Copolymer preparation with specific structural and functional properties can be catalyzed by specific enzymes to achieve the coupling of starch with another polymer. Starch is rather difficult to substitute with aromatic or phenolic compounds by conventional chemical modification methods, because there are few reactive functional groups such as the hydroxyl groups. Waxy maize starch is produced by sodium lignosulfonate using laccase, resulting in starch-sodium lignosulfonate copolymers with good antioxidant activity and cation binding properties.
Debranching Reaction
Pullulanase and isoamylase are two debranching enzymes that selectively hydrolyze the α-1→6 glycosidic bonds of starch in granular and gelatinized forms. Debranching reactions are normally performed by treating starch paste with pullulanase or isoamylase, leading to the formation of linear short chains and recrystallization under different storage conditions. The modification of starch through debranching reactions introduces new properties and functionalities into the starch molecules, providing promising applications in the food industry attributed to the recrystallization and gel-forming properties of the debranched starches. Debranched starches have shown great potential in the food industry as hypoglycemic foods, fat replacers, coating materials in ready-to-eat cereals, and tableting excipients, due mainly to their gelling properties. The molecular inclusion properties of debranched starch drive the formation of inclusion complexes, self-assembling spheroids, and nanoparticles.
Branching Reaction
The α-glucan branching enzymes are the only enzymes that catalyze transglycosylation reactions to form α-1→6 glucosidic bonds in amylopectin or glycogen, by breaking an α-1→4 glucosidic bond in starch and creating an α-1→6 glucosidic bond within a linear α-1→4 segment. Branching enzymes can catalyze various substrates, thus activating different action mechanisms and subsequently producing different products, such as highly branched cyclic α-glucan and cluster dextrin. The transfer activities of branching enzymes are activated and are higher in amylose-containing starch compared with that of amylose-free starch. Branching reactions are widely used in tailoring starch into highly branched structures with slow digestion properties and improved three-dimensional embedding.
Hydrolyzing Reaction
Enzymatic modification systems have also been used (hydrolyzing enzymes) for the modification of starch to prepare starches with various functional qualities. The α-amylases and/or amyloglucosidases have the ability to attack the surface of raw granular starch at a temperature below the gelatinization temperature of starch and have been widely used in the preparation of granular starch. There are more hydrolyzing enzymes now being tested for use in the modification of starch in gelatinized form. The use of amylomaltase to modify starch is expected to help to form thermo-reversible gels, fat replacers, and enhancers of creaminess in yogurt. Further applications include its use in the food industry as a plant and chemical-free alternative to gelatin. Amylosucrase is used to modify waxy cornstarch, to significantly elongate the chain length of the starches, resulting in the formation of precipitates. In addition, several amylases, such as β-amylase, maltogenic α-amylase, cyclodextrin glycosyltransferase, and maltotriohydrolase, are used to improve the branch density of starches to produce functional starches with slow digestibility.
Multiple Modifications
For specific applications in the food industry, functional starches are always tailormade with a combination of two or more modifications. Dual enzymatic modifications such as those catalyzed by branching enzymes and amylosucrase are employed for modification of starch to promote the formation of SDS and RS in sweet potato starch. Cassava starch prepared by carboxymethyl modification and thermal α-amylase hydrolysis is an ideal fat replacer in low-fat and energy sausages. Maize starches with different amylose contents are modified by acid-ethanol treatment, followed by enzymatic debranching and then recrystallization through temperature cycling to produce resistant starch. Moreover, rice starches are subjected to acid and heat-moisture treatments to produce resistant starch. Studies on the optimal combinations of modification procedures used in preparing functional starch are ongoing and merit further research using new technologies.