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Title: Production process improvement and characterization of starch nanocrystals
Authors: Nzama, Nkosingiphile Lucky 
Issue Date: May-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.
Submitted in fulfillment of the academic requirement for the degree Masters of Applied Sciences in Food Science and Technology, Durban University of Technology, Durban, South Africa, 2023.
Appears in Collections:Theses and dissertations (Applied Sciences)

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