Starch Modification Methods
Starch modifications can be performed to address their shortcomings. This can lead to a decrease in retrogradation, an improvement in filmogenic capacity, and a decrease in hydrophobicity. Modifications can be achieved through chemical, physical, enzymatic, and genetic means. Chemical modification involves introducing functional groups into the starch molecule through reactions of derivatization or decomposition.
The molecular structure of starch can be tailored through chemical reaction to modify its properties. This can involve derivatization reactions (such as etherification, esterification, crosslinking, and grafting) or decomposition reactions (such as acid or enzymatic hydrolysis and oxidation). The use of chemically modified starches to make films has been extensively studied in the literature, with examples including the work of López, García, and Zaritzky (2008) and Woggum, Sirivongpaisal, and Wittaya (2015).
Chemically modified starches can be used to make films with improved properties such as transparency, flexibility, and lower solubilization temperature. These modifications include acetylation, hydroxypropylation, and acid modification. Researchers have developed biodegradable films by casting using different chemical modified corn starches and selecting the most suitable modified starch and its concentration.
Starch can be improved for different uses by changing its structure through chemical modification. One way to do this is crosslinking, which can improve mechanical properties and water stability. However, some crosslinking agents can be toxic or expensive. Citric acid is a safer and cheaper alternative that can crosslink starch and improve its properties. Researchers have found that crosslinked starch films have 150% higher strength than non-crosslinked films. They also maintain their biodegradability and decrease water vapor permeability. The highest temperature reached during the gelatinization stage affects the final properties of the material, and films processed with citric acid at 75°C show the best results. These films remained unchanged over time and fully degraded in compost after six days.
Various chemical and physical treatments have been combined to produce modified starch with specific functional properties in shorter times and increased production. Some examples include microwave radiation with lipase as a catalyst, hydrophobic reaction of starch and alkenyl ketene dimer, esterification of starch nanoparticles with lipase, dual modified crosslinked-phosphorylated, crosslinking coupled with osmotic pressure, starch-based hydrogels prepared by UV photopolymerization, starch esterified with ferulic acid, microwave-assisted synthesis of starch maleate and starch succinates, hydroxypropylation and enzymatic hydrolysis, and ozone-oxidized starch.
Different physical modification methods for starches have been developed that are environmentally friendly and do not require chemicals. However, chemical treatments still need careful consideration of the reagents used and reaction products generated.
Starch crystalline structures and properties can be altered through methods like deep freezing and thawing, mechanical activation, extrusion heating, and fluidized bed heating. Each method produces different effects on the crystallinity and granule integrity.
Physical treatments can modify gelatinization transition temperatures and enzymatic hydrolysis of starches. One such treatment is the instantaneous controlled pressure drop, which uses saturated steam to increase these characteristics.
Gamma radiation can be used to break down starch molecules into smaller fragments, which can be used to create starch nanoparticles. Lamanna et al. (2013) used gamma radiation to produce SNPs from cassava and waxy maize starch, achieving sizes of around 20-30 nm.
Enzymes have been used to modify starch in various ways, such as using glycogen branching enzyme (SmGBE) to produce more branched structure in starch, and cyclomaltodextrinase to create low-amylose starch products. These modifications have the benefit of slowing down the retrogradation rate compared to unmodified starch.
To avoid the environmental harm caused by chemical and enzymatic modifications, genetic modification of starch can be performed in plants. This can be done through traditional plant breeding techniques or biotechnology. (Davis et al., 2003; Johnson et al., 1999).
Gamma radiation can be used to break down starch molecules into smaller fragments, which can be used to create starch nanoparticles. Lamanna et al. (2013) used gamma radiation to produce SNPs from cassava and waxy maize starch, achieving sizes of around 20-30 nm.
Enzymes have been used to modify starch in various ways, such as using glycogen branching enzyme (SmGBE) to produce more branched structure in starch, and cyclomaltodextrinase to create low-amylose starch products. These modifications have the benefit of slowing down the retrogradation rate compared to unmodified starch.
To avoid the environmental harm caused by chemical and enzymatic modifications, genetic modification of starch can be performed in plants. This can be done through traditional plant breeding techniques or biotechnology. (Davis et al., 2003; Johnson et al., 1999).
Wischmann et al. (2005) used Escherichia coli glg B to obtain starch with more amylopectin branches and less phosphate content. They did this by adding a SmGBE from a patatin promoter onto potato lines. Jobling et al. (1999) also altered the composition and structure of potato starch by completely removing SBE A of potato plants, which resulted in longer amylopectin chains, higher apparent amylose content, and increased phosphorous levels.
Plasticizer
Native starch has strong hydrogen bonding, making it not a thermoplastic polymer, but it can melt and be processed at high temperatures (90-180°C) with plasticizers like water, glycerin, and sorbitol. Plasticizers lower the melting temperature below its decomposition temperature (230°C).
The ideal plasticizer for starch processing is a small, polar, hydrophilic molecule that is compatible with starch. Water and glycerol are the most effective plasticizers because of their small size and ease of insertion into the starch networks. Other possibilities include urea, formamide, ethanolamine, and ethylene bisformamide, but their use as food-related material is discouraged due to their potential toxicity. A minimal amount of plasticizer (at least 20%) is required to extrude flexible starch films.
Many molecules have been proposed as potential plasticizers for starch processing, including water, glycerol, sucrose, fructose, glucose, glycols, urea, amides, and amino acids. Water and glycerol are the most effective plasticizers, followed by molecules containing amide groups, but their use as food-related material is discouraged due to their potential toxicity. A minimal amount of plasticizer (at least 20%) is required to extrude flexible starch films.
Composites
A composite material is created by combining two or more materials that have different properties. To improve the properties of starch, many composite materials have been suggested. Two main types of starch composites exist: one where fillers are added into starch, and the other where films are made by blending two or more polymers.
Fillers
Fillers, such as starch, cellulose, and layered silicate, can be added to starch matrices to improve their mechanical and barrier properties. These fillers come in different shapes, including fibers, flakes, platelets, and particles. Nanofillers are more promising than micron-sized ones because they have a higher interface-to-volume ratio and do not scatter light, which maintains material transparency.
The size of the filler and the interfacial adhesion between the filler and matrix are crucial factors that determine the physicochemical properties of the composite. Nanofillers are more effective than larger ones because they have a higher interface-to-volume ratio. Different types of bonding, such as mechanical, electrostatic, and chemical bonding, improve the interaction between the filler and matrix, depending on the compatibility of their chemical groups.
The bonding by interdiffusion is another type of bonding that happens when chains of the reinforcement and matrix spread between them. The bonding mechanism plays a vital role in improving the tensile and shear properties of the composite.
To make a good nanocomposite, it’s important for the filler to be well dispersed in the matrix. This creates a network that can improve the composite’s properties. Carbon nanotubes are an example of a filler with great potential, but they often clump together due to strong forces between them. To fix this, different methods have been proposed, like functionalizing the nanotubes or using surfactants. Water-stable nanohybrids composed of nanotubes and inorganic particles have also been developed.
Blends
One way to improve the limitations of starch-based materials is to mix starch with other polymers. This has been done with petroleum-based plastics like PE, PP, and PS as well as biodegradable polymers like PLA, PCL, PVA, and poly3-hydroxybutyrate. Blending can result in a new material with optimal properties, but processability and interfacial adhesion are important factors to consider.
Processability is an important factor when assessing the potential of a blend. Starch/PVA blends, for example, cannot be melt-processed and solution processing is not economically viable. Interfacial adhesion is another important feature to consider. Simple mixtures of starch and PCL have poorer mechanical properties than pure PCL because they lack significant interfacial adhesion. Modifications to PCL and the use of compatibilizers can enhance interfacial adhesion between blend components.
In order to improve the properties of starch-based materials, they are often mixed with other polymers. However, this process requires considering factors such as processability and interfacial adhesion. Starch/PVA blends cannot be melt-processed and simple mixtures of starch and PCL have poor mechanical properties due to a lack of interfacial adhesion. Modifications to PCL and the use of compatibilizers can enhance interfacial adhesion between blend components.