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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, uses a continuous flow of liquid, which is subjected to external pressure sources and passed through microfabricated channels [23]. The mechanical forces reduce the size of particles breaking molecular bonds, obtaining very narrow size distributions [24]. Finally, gamma irradiation has also been used as a technique to reduce the starch granule size. In this technique a homogeneous starch paste is irradiated with gamma rays inducing breaking in the bonds of the amorphous regions, thus reducing particle size while maintaining crystallinity [25, 26]. Other methods include successive ultrasound treatments [27] and ball milling [28].
The methods for SNCs synthesis involve the hydrolysis of the amorphous, reducing particle size while increasing crystallinity. Most methods use acid hydrolysis, either with HCl or H2SO4; the later allows faster hydrolysis rates and higher yields, while HCl hydrolysis is more time consuming [29]. Reports have shown that SNCs can be obtained after 5–7 days using H2SO4, while using HCl this can take up to 15 days [11, 15, 30]. However, one of the main drawbacks of H2SO4 hydrolysis is that sulfate groups are incorporated into the SNCs surface; this can lead to increase stability in aqueous solution but can limit their applications in several fields [31].
In recent years, there has been an increasing need for green methods
