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Note
1 ☆ Jéssica de Matos Fonseca and Betina Luiza Koop were contributed equally.
3 Nanostructures Based on Starch, Their Preparation, Processing, and Application in Packaging
Cristian C. Villa
Universidad del Quindio, Programa de Quimica, Facultad de Ciencias Básicas y Tecnologias, Cra 15 Calle 12 630004 Armenia, Colombia
3.1 Introduction
Starch is a polysaccharide used by plants as an energy reserve that is often considered the second most common biomass on Earth [1–4]. It is found as granules of different morphologies (depending on the botanical source) in plant tissues, mainly seed, roots, tubers, leaves, and fruits [5]. Starch structure is composed by α-D-glucopyranosyl units that can be linked in either α-D-(1–4) and/or α-D-(1–6) linkages. Likewise, these linkages give rise to two types of molecules: the linear amylose formed by approximately 1000 glucose units linked in α-D-(1–4) manner and the branched amylopectin, formed by approximately 4000 glucose units, branched through α-D-(1–6) linkages [2, 3, 6]. The union of both amylose and amylopectin forms a semi-crystalline structure arranged as small granules with diameters between 1 and 100 μm. Most unmodified starches have an amylose content around 20–30%, with amylopectin ranging from 70% to 80%. Some starches can have very low amylose contents, below 1%, such as waxy corn starches. The size and morphology of starch granules are dependent on their botanical source, as their shape can vary from spherical, oval, polygonal, lenticular, and kidney shapes and their size can range from <1 to 100 μm [7].
3.1.1 Starch Nanoparticles and Nanocrystals
Nanomaterials based on starch can be classified according to their nature in two main groups: starch nanoparticles (SNPs) and starch nanocrystals (SNCs). SNPs are almost completely amorphous particles that are commonly obtained by the controlled nanoprecipitation of gelatinized starch, while SNC synthetized through the hydrolysis of the amorphous phase of the starch granule, removing mostly amylose until nanosized particles are achieved [8, 9].
As mentioned before the synthesis of SNP materials is achieved mostly through nanoprecipitation of gelatinized starch, and most methods involve either hot or cold (addition of NaOH) gelatinization, which is followed by the addition of an anti-solvent, mostly ethanol, methanol, or acetone [10–13]. The anti-solvent methods involve the nucleation of the amylose molecules that precipitate once the anti-solvent is slowly added; this causes supersaturation in the solution and particle growth [14]. After the critical overlapping concentration is reached, nucleation starts and particles are formed. Furthermore, as nuclei are formed and growth supersaturation decreases, quickly a controlled process of growing nuclei is allowed [15]. In general, the anti-solvent method is considered simple, and the physicochemical properties of the SNP can be controlled by small changes in the process, such as the addition of surfactants [16] and changing the anti-solvent or the amylopectin/amylose ratios of the starch [16, 17]. Furthermore, aspects like the concentration of starch and process temperature are the important parameters affecting the yield of SNP [15, 18].
Extrusion has also been used as methods to synthetize SNP. In this method starch granules with limited amounts of water are subjected to high temperatures, pressures, and mechanical forces, thus undergoing changes such as melting, fragmentation, and incomplete gelatinization [15]. This process allows the disruption of the molecular bonds in the starch granules, forming smaller particles with less crystallinity [19]. Likewise, in reactive extrusion native starches are mixed with different reactants like plasticizers (glycerol, sorbitol, etc.) and cross-linkers and subjected to the extrusion process [20–22]. Starch fragments formed due to the extreme conditions are then cross-linked forming nanoparticles of different sizes. Furthermore, SNPs size can be reduced after addition of cross-linkers, as they increase the shear forces and torque, which facilitate size reduction [15].
Another mechanical technique used in SNPs formation is high-pressure homogenization [20]. This process, usually used in emulsion formation and microorganism inactivation,