Production process improvement and characterization of starch nanocrystals
Authors:   , Nzama Nkosingiphile Lucky 
Published: June 2023

Starch nanocrystals (SNCs) are promising biomaterials for novel applications in foods, cosmetics, and medicine. In general, acid hydrolysis below the gelatinization temperature of starch is the most common method used for nanocrystals production. Major drawbacks associated with this method are the extended hydrolysis time required (up to 5 days) and the low yield (4–15%) of SNCs. Different methods, including physical and enzymatic pretreatments of starch prior to acid hydrolysis, have been investigated. Among these methods, enzymatic hydrolysis can be regarded as a promising and green strategy for the creation of pores in starch to enhance acid diffusion into the inner regions during SNCs fabrication. Debranching enzymes such as pullulanase are gaining attention in the food industry due to their ability to modify the starch structure and properties through selective hydrolysis of the branched chain of α-1,6-glycosidic bonds. However, pullulanase has not yet been applied as a pretreatment method aiming at starch nanocrystal preparation. Therefore, the pretreatment of starch granules with pullulanase and β-amylase (i.e., to hydrolyze the linear α-1,4-linkages) concurrently could be a novel technique to modify starch surfaces for faster production of SNCs and improved yield. To improve the efficiency of starch nanocrystals production and properties, pullulanase (15 U/g starch) was used alone or together with β-amylase (50 and 100 U/g starch) to modify the starch before acid hydrolysis. The compound enzyme system of pullulanase:β-amylase (15 : 50 U/g starch) had the most pronounced effect on starch morphology compared to a single enzyme system by creating a dense and more porous structure on starch surfaces as evidenced by microscopy images, a high degree of oil absorption and extent of hydrolysis data. Nanocrystals were produced after 3 days with modified starches instead of 5 days. The yield of SNC was approx. 25 wt.%, which is 3 times greater than that of the conventional SNC preparation method. SNC derived from the modified starches were small in size (less than 50 nm) and appeared mostly as platelet and isolated round particle aggregates. Nanocrystals from modified starches showed the A-type crystalline structure similar to the native starch, but with a significant increase in the degree of crystallinity (from 32.85% to 45.28%.), and the short-range molecular order during the early stage of acid hydrolysis. Starch hydrolysis using compound enzymes consisting of pullulanase and βamylase hydrolysis seems to be the most effective and green to produce SNC in a shorter time and with increased yield and enhanced properties. SNCs were incorporated in different concentrations (0, 5, 10, 15, and 20 wt.% starch) together with stearic acid to improve cassava starch-based nanocomposite film properties using a solution casting method. The addition of SNCs from 5 to 15% in combined with stearic acid into starchbased nanocomposite films presented better water resistance, water vapor permeability, and tensile strength than native cassava starch film. Conversely, beyond 15% SNC content, nanocrystals seem to aggregate which impaired the tensile strength of the nanocomposite films. The surfaces of the nanocomposite films were relatively smooth and homogenous after the addition of nanocrystals at up to 15 wt.% concentration compared to native starch film as demonstrated by the atomic force microscopy (AFM). Furthermore, the opaqueness of the nanocomposite films progressively increased with the SNC content, which might be beneficial in the packaging of foods that are easily degraded when exposed to light and high moisture. XRD analysis revealed sharp peaks at approximately 2θ of 13.5° and 20.3°, which are characteristics of typical V-type crystalline pattern in starch films prepared with added steric acid. This further indicates the formation of amyloselipid complexes in films. The inclusion of SNC in films also enhanced their thermal stability. Therefore, the combined effect of SNC at different concentrations and stearic acid into cassava starch-based films was a successful approach to further improve the mechanical reinforcement and barrier properties of nanocomposite films.