reduce the high amounts of hydrolyzing agent [14, 32]. Among this methods, enzymatic hydrolysis followed by acid hydrolysis has become one of the most important alternatives. Most examples use both α and β amylases as the hydrolyzing agent; the first one is a type of endo-amylase that hydrolyses internal α-1,4 bonds, while β-amylase is an exo-amylase that causes cleavage of alternated α-1,4 bonds from nonreducing ends [14]. Although amylases are an important part in starch digestion, it seems that using them by themselves is not enough for obtaining nanosized particles. Studies have shown that using only α-amylose as the hydrolyzing agent for waxy maize and rice starch results in starch particles of 11 and 3.6 μm, respectively [33, 34]. On the other hand, it has been shown that combined treatments of both enzymatic and acid hydrolysis can reduce synthesis time from seven to three days [6, 35]. Finally some physical methods have been used in combination with acid hydrolysis, such as ball milling and ultrasonication [36, 37].
3.1.2 Starch Nanomaterials in Food Packaging
Over the last decades, starch-based nanomaterials have been proposed as fillers in composite polymeric films, as they have shown the capability to improve mechanical, barrier, and electrical properties of the films [6]. They have been used in order to improve properties of biodegradables films made from biodegradables polymers, such as starch, and other carbohydrate polymers, proteins, and lipids; furthermore, some examples can be found of their use in nonbiodegradables composite polymers [38]. Both SNCs and SNPs have been used in order to reinforce mechanical and barrier properties of polymeric films.
According to Le Corre and Angellier-Coussy [6], two types of nanocomposite formation can be distinguished. In the first case aqueous systems and hydrosoluble and hydro-dispersable polymers are grouped. The second group is formed by nonaqueous systems that use organic solvents.
Examples in the first group include the inclusion of SNCs in films formed by natural rubber and latex [39–41]. Likewise, one of the main applications of starch nanomaterials as fillers in polymeric packaging has been in the development of starch-based films. Biodegradable (sometimes edible) films made from starch have been of great interest as they are odorless, tasteless, colorless, nontoxic, and semipermeable to moisture, gases (carbon dioxide and oxygen), and flavor components [42]. However, they have shown high water solubility and poor water vapor barrier due to their hydrophilicity; furthermore, their mechanical properties are poor, with low tensile strength and elongation values [43–45]. Studies have shown that due to their small size, SNCs can interact with the polymeric matrix by forming strong hydrogen bonds. This interaction allows the transfer of stress from the matrix to the nanoparticles that carries the load and enhances the film's strength. Furthermore, water vapor barrier values decreased due to the tortuous path created by the SNCs on the polymeric film, stopping water vapor transmission though the film [26, 31, 46–48]. These behaviors have also been observed in protein films reinforced with starch-based nanomaterials. Soy protein films reinforced with citric acid modified SNPs showed an increase on their tensile strength and water resistance, as nanoparticles created a hydrophobic surface [49, 50]. This was also observed for amaranth protein films [51–53].
3.1.3 Starch Nanomaterials as Carriers of Bioactive Molecules
One of the most interesting applications of starch-based nanomaterials is in nanocarriers of bioactive molecules, as they are biodegradable, nontoxic, and biocompatible [54]. Most studies use SNPs as carriers, as they have shown an increase in both solubility and bioavailability of the bioactive molecules [54].
A study by Farrag et al. [55] presented results of the use of SNPs made from potato, pea, and corn starch to encapsulate quercetin. This molecule is a polyphenol found in many leaves, fruits, and vegetables, which is well-known for its antioxidant and anticancer activities, as well as for its low water solubility [56, 57]. It was reported that starch's amylopectin content has a significant effect on the molecule encapsulation. SNPs obtained from potato starch (highest amylopectin content) encapsulated higher quercetin molecules due to their higher amylopectin than SNPs obtained from corn starch; this as the branched regions produced by the amylopectin molecules allowed a better “accommodation” of the quercetin molecules [55]. Furthermore, they reported that quercetin antioxidant activity is preserved by encapsulation in the SNPs and is related to the SNPs loading capacity; thus, the higher loading percentage of quercetin leads to higher radical scavenging activity [55].
Another polyphenol that has nanoencapsulated in starch-based nanomaterials is curcumin, a molecule present in the rhizomes of turmeric, and it is well-known for its anticancer, antioxidant, anti-inflammatory, antimicrobial, and antiviral activity. Even curcumin has shown great potential in several biological applications, its use has been limited by its low water solubility and low bioavailability [58–66]. Chin et al. [67] used SNPs made from sago as carriers of curcumin, reporting particle sizes around 50–80 nm and loading capacity reaching around 78% [16].
In general, encapsulation of the bioactive molecules in SNP is mostly done using the anti-solvent synthesis method. In this method, an organic solvent containing the bioactive molecules is slowly added to gelatinized starch at room temperature. The molecules are then encapsulated in interior of the SNPs, due to its hydrophobic nature. In this method, it is possible to increase molecule encapsulation by reducing starch polarity through chemical modifications like acetylation or cross-linking with other molecules [10, 68]. This was considered by Pang et al. [69] in order to increase loading efficiency of curcumin into SNPs made from sago starch. They modified the SNPs by using maleate ester modified sago starch, reaching a loading capacity of 85%. Increasing loading capacity by chemical modification of the SNPs was also observed by Acevedo-Guevara et al. [10], using native and acetylated banana starch SNPs with higher encapsulation capacity and particle size in the modified SNPs than in their native counterpart. In both cases, it was explained that modifications such as acetylation and esterification lead to higher hydrogen bond interactions between the bioactive molecule and ester groups incorporated into the SNPs structure, leading to higher incorporation into the nanoparticle. Finally, Sadeghi et al. [17] studied the effect of the amylose/amylopectin ratios in different corn starch SNPs in curcumin encapsulation. It was reported that high amylose content leads to increasing SNPs sizes; furthermore, they observed that nanoencapsulation can be used to protect curcumin from photodegradation, as more than 83% of the encapsulated polyphenol remained after 10 days of storage.
Other molecules with high antioxidant activity has been encapsulated in SNPs. Report by de Oliveira et al. [70] showed that chemical such as acetylation increases loading capacities of both molecules. This was explained as the formation of specific interactions (hydrogen bonds) between the SNPs and the antioxidant molecules drives more molecules into the starch-based nanomaterial. Ahmad et al. [71] used SNPs made from horse chestnut, water chestnut, and lotus stem starch as carriers of catechin, while Shabana et al. [72] used SNCs obtained from potato starch to encapsulated antioxidant molecules (ascorbic and oxalic acid). Results showed that mechanical methods, such as ultrasound, lead to smaller particle sizes while increasing antioxidant loading capacity
Starch-based nanomaterials have also been used in order to protect vitamins, like vitamin A and vitamin D, due to their high lipophilic nature and easy degradation by light and oxygen [73–75]. Firstly, Hasanvand et al. [76] reported that SNPs made from high amylose maize can be used as carriers of vitamin D. They reported encapsulation values up to 78% and through the use of several techniques (DSC, FTIR, and XRD) observed the formation of hydrogen bonds between vitamin D molecules and the starch-based nanovehicle as the encapsulation main drive. More recently, Santoyo-Aleman et al. [68] used citric acid modified SNPs made from banana starch to encapsulate vitamin A. They reported that hydrogen bond formation between the citric acid molecules in the modified vehicle and the vitamin allowed higher encapsulation values.
Due to the high biocompatibility of starch and their edible nature, SNPs can be used as vehicles for controlled release of molecules in the gastrointestinal track. SNPs can be used to shield bioactive molecules from the harsh environment of the stomach (low pH and enzymatic attacks) while increasing
