Physical and Chemical Modifications in Starch Structure and Reactivity

Starch is a naturally occurring glucose homo-polysaccharide of nutritional, pharmaceutical, and industrial importance. The complex polymeric structure and poor solubility of native starch in water limits their importance at pharmaceutical and industrial level. The structure, reactivity, and functionality of the native starch can be modified by physical, chemical, enzymatic, and biotechnological methods. Various physical modifications techniques, including the thermal, radio-thermal, freezing and thawing, annealing, high-pressure, ultrasonic, and pulsed electric field treatment, and chemical modifications, including oxidation, etherification, esterification, cationization, cross-linking, and graft polymerization, have been found to change the surface properties, polarity and linearity of the molecular chains, the degree of substitution, the polymeric, granular, and crystalline structure, amylose to amylopectin ratio, solubility, viscosity, pasting, gelatinization, swelling, water absorption, and emulsifying properties of starch. The structural changes have resulted in the improvement of thermal and freeze-thaw stability, viscosity, solubility, water binding capacity, swelling power, gelling ability, and enzymatic digestibility of starch. The exposure of reactive functional groups after physical or chemical modification modifies the reactivity of starch toward water, oil, acids, enzymes, and other chemical species. These modification techniques have led to some revolutionary changes in reactivity, functionality, and application of starch in various fields.


Introduction
Starch is the most important polysaccharide as well as storage polymer of plants abundantly found in leaves, stem, fruit, seed, tubers, and roots of various plants. Starch is produced in chloroplast and amyloplast of plant cells by photosynthesis, stored as a source of food and energy. It is stored in plant cell during tubers sprouting, germination of seeds and fruit maturation [1,2]. Major sources of starch included cereals grains such as wheat, barley, rice and corn, the seeds of the legumes such as beans, garden peas, chickpeas, and pulses, the tubers such as potato, sweet potato, ginger, turmeric and groundnut and immature fruits and vegetables [3][4][5].
Starch has great nutritional, pharmaceutical and industrial significance due to its unique physical, chemical and functional and nutritional properties. Starch is a good source of nutrition as it is hydrolyzed into glucose on digestion by α-amylase. The metabolic oxidation of glucose provides instant energy which is utilized in various metabolic and other cellular activities [6][7][8]. Due to the higher concentration of amylose, starch is used as an excipient to activate drugs and act as an encapsulating agent facilitating the deliver the drug to its target organ [9][10][11]. As a natural polymer, starch is used to replace plastic in the coating of food materials and production edible films in the food industry. It is usually mixed with food components to protect them from mechanical damage, to extend their half-life and to improve their appearance. It is also used as a recyclable component for molds production in the food industry. It is added as a bulking agent in food and pharmaceutical formulations to enhance handling and stability as well as preservation of components texture and to enhance their viscosity [12][13][14]. Moreover, due to water-resistant nature of amylose, it can form excellent films due to which it has great importance at industrial level [15].

Starch structure and composition
Chemically starch is a homopolymer of α-Glucopyranose units with the chemical formula (C 6 H 10 O 5 ) n . Starch is composed of two types of polymer chains known as amylose and amylopectin. Amylose possesses a linear structure with α1-4 glycosidic linkage while amylopectin possesses a branched structure with α1-4 as well as α1-6 glycosidic linkages ( Figure 1) [16,17]. Normally, starch consists of relatively lower amount of amylose (20-30%) than that of amylopectin (70-80%). The ratio of amylose and amylopectin affects the starch structure in terms of crystallinity, size of the granules and chemical nature and arrangement of polymers within the granule. The studies have shown that the fine structure of amylopectin plays an important role in the functionality of starch. It is the relative concentration of amylose and amylopectin which determines the physical and functional properties of starch. The starches containing low amylopectin have been found to show the quick onset of gelation as compared to low amylose starches. The starches containing relatively high amylose content have been found to form comparatively hard and rigid gels and strong films while high amylopectin starches are dispersed easily in water and form soft gels and weak films [15,[18][19][20][21]. The amylose to amylopectin ratio also influences the nutritional quality of starch that is assessed by its rate of digestion and glycemic index (GI) as an indicator to check the quality of carbohydrates [22].

Functional properties of starch
Based on its compact structure, starch possesses diverse functional properties and applications in biomedical and industrial fields. Due to polymeric and branching nature starch shows relatively less solubility in water and possesses relatively lower ability to absorb water and oil. Starch shows good iodine-binding ability. It also possesses a relatively high viscosity and good swelling power and gelatinization abilities. It also shows good pasting properties with consistency, smoothness, and clarity and can form thin films. Starch shows freeze-thaw and cold storage stabilities which make it a favorable candidate for various food and industrial formulations. Starch is resistant to moderate temperature and pressure but susceptible to acid and enzyme-catalyzed hydrolysis. However, the native starches show relatively lower values of enzymatic digestibility [18,[23][24][25][26]. To increase its nutritional, biomedical and industrial importance, the functional properties of starch can be improved under the influence of various physical and chemical factors.

Factors affecting the structure and properties of starch
The native starches possess a complex granular and crystalline structures which differ in size in various plants [2,16]. Several factors have been reported which affect the structural, physical, chemical, and functional properties of starch. Starch is sensitive to very high and very low pH, high temperature, high pressure, and osmotic pressure, light, radiation, mechanical stress, and ultrasound waves. Heating treatment of starch in aqueous medium has been found to cause transition of amorphous form to crystalline starch resulting in gelatinization of starch. The treatment with microwave radiation has been found to affect the crystalline structure and functional properties of starch which is linked with the loss of birefringence and crystallinity due to deformation during modification [27][28][29][30][31][32][33][34][35][36]. Interaction of starch with water and oil also affects the properties of starch. The absorption of water results in the breakdown of amylose-amylopectin linkages, loss of 3 Structure and Reactivity DOI: http://dx.doi.org/10.5772/intechopen.88870 crystallinity and swelling of starch granules. The swelling of starch granules is reversible at the initial stage but irreversible after a certain period [37]. Freezing at low temperature after gelatinization results in recrystallization of starch granules and increases the resistance and hardness of starch [38].

Physical and Chemical Modifications in Starch
Along with these physical factors, some chemical factors have been also reported to affect the structure and functions of starch. Various oxidizing agents, hydroxy or carboxy derivatives of hydrocarbons, some carboxylic acids, phosphates, different acid, and base cross-linkers and synthetic polymers, and some cationic molecules are the major chemical factors which have been reported to modify the structure and properties of native starches [39][40][41][42][43][44][45][46]. Starch is also susceptible to acids and enzymatic hydrolysis which results in degradation of amylose and amylopectin and alter the morphology and surface properties of granules leading to the change in functional value of starch [47][48][49]. These physical and chemical factors have helped improve the functional quality of starch to obtain better results while used in various food and industrial formulations.

Starch modification
Any changes in the structure of starch molecule caused by various environmental, operational and processing factors are termed as modifications. These modifications may exert either positive or negative effects on the structure and functionality of starch molecules. The native starches obtained from various plants are diverse in their structure and functions. To enhance the structural and functional quality of these starches and achieve better results in various formulations, the researchers suggest some modification in their structure. Several studies have been reported on the improvement of functional quality of starch by physical, chemical or mechanical modifications [23,43,45,48,[50][51][52][53][54][55][56][57][58]. However, some environmental and processing factors may reduce the functional quality of starch by various modifications during storage and processing [57]. The physical modifications are comparatively safe and preferable over chemical modifications as the later involve the changes in the chemical structure of the molecule which limit its use in most of the formulations.

Physical modification of starch
Physical modifications involve the changes in the morphology and threedimensional structure of starch under the influence of some physical factors such as milling, moisture, temperature, pressure, pH, radiation, pulse-electric field, ultrasonic waves, etc. Physical modifications result in the variation in particle size, surface properties, solubility index and functional properties such as water absorption, swelling capacity, pasting and gelation ability of starch. These modifications directly influence the functional quality and selectivity and suitability of the modified starch for various nutritional, pharmaceutical and industrial formulations. Several studies have been reported on the physical modification of starch using different techniques. The commonly used methods of physical modification include superheating of starch, thermal inhibition treatment, UV and gamma irradiation, microwave treatment, high pressure, osmotic pressure and instantaneous controlled pressure treatment, mechanical activation by stirring ball mill, treatment by pulsed electric field, micronization in vacuum ball mill, annealing and freeze-thaw treatment [28,29,31,33,51,53,55,56,[59][60][61][62][63][64][65][66][67][68]. The most frequently used and the most effective methods of physical modification are presented in Figure 2 and their effects on the structure and properties of various starches are summarized in Table 1.

Chemical modification of starch
The chemical modification involves the alteration of physiochemical properties of starch by introducing new chemical or functional groups in starch without any physical alteration in the shape and size of the molecule. Each of the glucose units in amylose and amylopectin has three reactive hydroxyl groups which are the major sites for chemical modification in starch. The chemical modification alters the physical behavior of starch including retrogradation, salting, and gelatinization that work by stabilizing the intermolecular and intramolecular bonding of starch granules. The commonly used methods of chemical modification of starch include oxidation by different oxidizing agents, etherification by addition of some hydroxyethyl, hydroxypropyl or carboxymethyl moieties on hydroxyl groups of starch, esterification by condensation of some fatty acids, other carboxylic acids and phosphates with active hydroxyl groups of starch, cationization by introducing some cationic molecules, cross-linking by addition of various cross-linkers and graft-polymerization of starch with synthetic polymers [40-43, 46, 68-72]. Cationic modifications involve the reaction of starch molecules that contain tertiary and secondary ammonium, imino, amino, sulfuric and phosphate groups which react with hydroxyl groups of starch. It improves the dielectric constant of starch granules. It has great importance in the textile industry as an additive, in paper and cosmetic industry due to low cost, rapid degradation and bioavailability [73].