Botanical sources provide native starches for many purposes, but these starches may not work well for certain industrial applications due to their natural limitations. Starch modification can greatly improve its properties through physical, chemical, or enzymatic processes.
Starch can be modified by reacting its hydroxyl groups to create materials with different properties. Different strategies have been studied to improve its mechanical and barrier performance and reduce its hydrophilic nature, including adding crosslinking agents, blending with thermoplastic polymers, chemical modification, and developing multilayer structures. However, chemical modification has limitations due to toxicity and the diversity of by-products.
Blending starch with other degradable polymers has been proposed by many authors to enhance microstructures, such as co-continuous structures formed by melt-blending. Biodegradable polymers such as chitin/-chitosan, castor oil-based polyurethane, and synthetic-derived polyester have been used to blend with TPS, along with cellulose nanoparticles.
Castor oil-based waterborne PU was blended with wheat TPS to make a biodegradable plastic through an extrusion process. The hydrogen bonding between urethane groups of PU and hydroxyls on starch improved physical properties, such as Young’s modulus, tensile strength, elongation at break, and toughness. Even after 30 weeks, the blends maintained high values of elongation at break and also showed increased hydrophobicity and water resistance.
Starch nanoparticles can be used as fillers in polymeric matrices to enhance mechanical and barrier properties. Various processes have been tried out to produce starch nanoparticles, including precipitation of amorphous starch, complex formation and enzymatic hydrolysis, and micro fluidization. Each process results in different properties, crystallinity, and shape of the nanoparticles. Starch nanocrystals are particularly studied as nanoreinforcements in nanocomposites.
Ma et al. (2008) made starch nanoparticles from pea using ethanol precipitation and citric acid. They used these nanoparticles to reinforce plasticized pea starch composites through casting. The modified pea starch nanoparticles had a strong reinforcing effect on the composites’ tensile strength and Young’s modulus due to the chemical similarities between the filler and matrix. The resulting films had higher storage modulus and water vapor barrier than pure plasticized pea starch films. The authors suggested that these starch nanocomposites could be used in medical, agriculture, drug-release, and packaging applications.
García et al. (2011) prepared waxy maize starch nanocrystals using acid hydrolysis and used them to reinforce nanocomposites made from plasticized waxy maize starch through casting. The starch nanocrystals were observed to be closely associated with glycerol-rich domains in the composites, as seen in SEM micrographs. However, the reinforced film unexpectedly had a higher water vapor permeability due to the formation of threads with high concentration of -OH groups that acted as preferential paths for water vapor diffusion.
Water vapor and oxygen permeability are important factors to consider when designing bio-based food packaging materials. Incorporating starch nanocrystals in polymer matrices like thermoplastic starch can significantly reduce the permeability of these materials. This is because the platelet-like nanoparticles create a tortuous diffusive pathway. However, the level of permeability reduction and optimal filler content vary depending on the tested matrices.
Crosslinking is a method of modifying starch that can strengthen the intermolecular binding by adding covalent bonds to supplement the natural hydrogen bonds. This improves the mechanical properties and water resistance of the films produced.
Ghanbarzadeh et al. (2010) used different levels of carboxymethyl cellulose (CMC) to create blend films based on modified cornstarch. The starch was modified through chemical crosslinking using citric acid (CA). The films exhibited improved crystallinity due to the modification, which was attributed to the formation of covalent bonds within the starch polymer backbone.
Gutiérrez et al. (2015) modified native starch from cassava and dark purple cush-cush yam through crosslinking with sodium trimetaphosphate and used the resulting derivatives to make edible films. The crosslinking resulted in thicker and less grainy films, indicating stronger glycerol-starch interaction. This also led to an increase in film flexibility due to an increase in hydrogen bonds between the modified starches and glycerol. The authors found that films made from cassava had stronger glycerol-starch interaction compared to those made from cush-cush yam and that this interaction could be further strengthened through crosslinking.
Maleic anhydride esterified starch can improve the performance of composites, but traditional methods for producing it consume energy and cause environmental pollution. Zuo et al. (2015) proposed a new, dry method that produces maleic anhydride esterified cornstarch and creates a biodegradable composite with polylactic acid (PLA) polymer using melt extrusion. Esterification improves the compatibility between starch and PLA, enhances the interfacial bonding force, and effectively resists penetration by both water and microorganisms during soil burial degradation over 120 days.
Bio-based materials, including starch, can be used to produce packaging such as trays, cups, bottles, and films, using the same equipment and technology used for conventional materials. However, when using polysaccharides, the sensitivity towards relative humidity must be overcome. Designing packaging to meet the specific demands of foodstuffs is necessary, much like with synthetic materials used today. The more diverse side chains and functional groups of bio-based materials offer more possibilities to tailor the properties of the finished packaging, leading to further research and development for materials with even better properties.