Preparation, characterization, stabilization and cross-linking of nano/submicron starch particles
Date Issued
2010
Date
2010
Author(s)
Chen, Chung-Jen
Abstract
Starch is an abundant and renewable plant resource but it is quite inert to reaction due to its high molecular weight and granular crystallinity. Thus, starch films exhibit poor mechanical properties that limit the utilization of starch as biodegradable packaging. Nanonization would be beneficial for increasing reactivity of starch with increasing surface area and surface properties modification and thus enhancing the cross-linkages of starch molecules to improve the mechanical properties of starch films.
In order to prove the hypothesis, the first part of the research was to explore the feasibility for preparing nano/submicron starch particles and understanding the change of functionality after size reduction. Media milling was employed to prepare the sample with different operating parameters. The effects of particle size and molecular weight on the related physicochemical properties of milled starch were also studied. HPSEC-MALLS-RI, DSC and dynamic rheometer were utilized to determine the molecular weight, thermal and rheological properties of milled products. After 30-min milling, the number average diameter of starch particle was reduced from 9.61 to 0.26 μm. SEM/TEM observation confirmed the presence of nano/submicron starch particles. As starch granules being disintegrated, the mechanical energy imparted by media milling resulted in more than 43.7% starch damage and 55% degree of gelatinization. The weight average molecular weight was reduced from 9.98 ×106 to 7.63 ×106 g/mole. The milled starch exhibited lower gelatinization temperature and heat for gelatinization probably were due to an increase in degree of gelatinization. The reduction of molecular weight let the rheological properties of milled samples become more shearing and thermal stable, and it also affected the swelling power and water soluble index by increasing the degree of starch damage. The result showed that it was possible to manipulate the molecular by media milling and to modify the functional properties of starch.
The second part of the research was to study the surface properties and morphology of nano/submicron starch particles made by media milling. ESCA, Zetasizer, NMR and XRD were utilized to determine the surface properties and microstructure of milled starch. Particle size distribution and morphology were examined by SLS and SEM in order to explore the size reduction mechanism of starch products prepared by media milling. The volume average diameter was reduced to 0.7 μm with a surface charge of -12 mV after being milled for 120 min. The milling resulted in an increasing of surface area around 25 folds and number of particles about 15,000 times. On the surface, the oxygen content was increased 60% and the ratio of carbon to oxygen was decreased from 1.04 to 0.84. The milling also resulted in loss 70% of double helix content and 60% of relative crystallinity along with the reduction of intrinsic viscosity from 181 to 98 mL/g due to mechanical degradation. The mechanism of milling was associated with surface erosion/shattering and appeared to follow the concept of amylopectin cluster. Size reduction and starch damage lead to molecular dislocation would increase the hydroxyl group on surface that could enhance the reactivity of starch.
The third part of the research was to stabilize the nano/submicron starch particles. After screen the suitable emulsifier according to the average particle size, surface charge, centrifugation and turbidity stability, the addition of 5% w/w monoglyceride was found to give the best result. The volume average diameter was reduced to about 0.3 μm with a surface charge of -16 mV after being milled for 180 min. The change of transmission electron micrograph, XRD diffraction and DSC thermogram showed that the stabilized mechanism probabily was due to the formation of inclusion complex with electrosteric effect. The addition of emulsifier also can increase the efficiency of size reduction and the yield of nano particles without affecting the milling mechanism by examined the PSD. The starch damage was decreasing when compared with the milled starch without emulsifier addition (33.8% at 30 min) but the thermal and rheological properties changed with the same tendency, and the swelling powder and water soluble index didn’t show significantly difference between them.
The final part of the research was to evaluate the cross-linking and mechanical properties of starch films made of nano/submicron starch particles. The thermal and rheological of cross-linked starch, morphology and mechanical properties of starch films were studied with DSC, dynamic rheometer, SEM and texture analyzer. The results showed that the degree of cross-linkage was increased with the milling and reaction time by using SMTP as reagent (the degree of cross-linking for 90-min milling sample increased about 3.7 times when compared with native starch after 300-min reaction) that confirmed nanoization can enhance the reactivity, particularly significant for the first 30 min of milling. The increasing of cross-linkage will increase the peak temperature and enthalpy for thermal property but decrease the gelatinized viscosity and setback for flow and paste properties. Cross-linkage could improve the tensile strength and the elong at break, with the addition of glycerol can further increase the elong at break but would decrease the tensile strength.
The study confirmed that physical modification can be done by media milling with controlling of molecular weight and crystallinity. The reactivity of starch can be increasing by increasing the surface area and functional group to improve the mechanical properties of starch films.
Subjects
starch
media milling
nano/submicron
physicochemical properties
stabilization
cross-linking
SDGs
Type
thesis
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