Theses and dissertations (Applied Sciences)
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Item Production process improvement and characterization of starch nanocrystals(2023-05) Nzama, Nkosingiphile Lucky; Amonsou, Eric OscarStarch 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.Item Characterization and application of amadumbe starch nanocrystals in biocomposite films(2017) Mukurumbira, Agnes R.; Amonsou, Eric Oscar; Mellem, John JasonAmadumbe (Colocasia Esculenta) commonly known as Taro is an underutilized tuber crop that produces underground corms. It is a promising tropical tuber grown in various parts of the world including South Africa, where it is regarded as a traditional food. It is a significant subsistence crop, mostly cultivated in rural areas and by small scale farmers. Amadumbe is adapted to growing in warm and moist conditions. The tubers are characterised by a high moisture content and consequently high post-harvest losses. The losses can be minimized through the utilization of various preservation techniques such as flour and starch production. Amadumbe corms may contain up to 70-80% starch. The starch granules are characterised by a small size and relatively low amylose content. The combination of high starch content, low amylose and small starch granules thus make amadumbe a potentially good candidate for nanocrystal production. In this study two amadumbe varieties were utilized to extract starch. Amadumbe starch nanocrystals (SNC) were produced using an optimized hydrolysis method. The physicochemical properties (morphology, crystallinity, thermal properties) of the resulting SNC were investigated. The SNC were then applied as fillers in three different matrices namely, amadumbe starch, potato starch and soy protein. The influence of the SNC at varying concentrations (2.5, 5 and 10%) on the physicochemical properties of bio-composite films was examined. Amadumbe starch produced a substantially high yield (25%) of SNCs. The nanocrystals appeared as aggregated as well as individual particles. The individual nanocrystals exhibited a square-like platelet morphology with sizes ranging from 50-100 nm. FTIR revealed high peak intensities corresponding to O-H stretch, C-H stretch and H2O bending vibrations for SNCs compared to their native starch counterparts. Both the native starch and SNC exhibited the A–type crystalline pattern. However, amadumbe SNCs showed a higher degree of crystallinity possibly due to the removal of the armorphous material during acid hydrolysis to produce SNCs. Amadumbe SNC showed slightly reduced melting temperatures compared to their native starches. The SNC presented similar thermal decomposition properties as compared to their native starches. In general, the inclusion of SNCs significantly decreased water vapour permeability (WVP) of composite films whilst thermal stability and tensile strength were increased. The degree of improvement in the physicochemical properties of the films varied with the type of matrix as well as the concentration of the nanocrystals. It generally seemed that the enhancement of the physicochemical properties of starch matrices occurred at a lower SNC concentration in comparison to that of soy protein films. Amadumbe SNC can indeed potentially be used as a filler to improve the properties of biodegradable starch and protein films