Effects Of Blending, Physical And Chemical Modifications On The Properties And Structure Of Corn (Zea Mays) Starch Based Films.

In recent years, synthetic polymer materials and its accessory ingredients in the food preservation have caused food safety issues and white pollution, world governments and the majority of scientists have given them great concerns. Use the abundant renewable resources starch instead of synthetic polymer material has become an important research subject, however, there are some inherent shortcomings in the performance of natural polymers, which need to be modified and then can be applied in food. Therefore, in the view of food safety and environmental-friendly, it is of great scientific significance and value that improve the performances of starch-based films to fit the purpose of food preservation.As we known,one of the functionalities of biomaterials mostly in need in the field of food science and processess is that related to the protection of food products to ensure food safety and improve shelf life. Biopolymers based films need however to respond to number of requirements, related principally to their physico-chemical and mechanical properties.In this thesis, the effect and efficiency of various methods of improving the protective ability and the properties of polysaccharides based films have been investigated. The major party of this work was principally carried out using corn (Zea mays) starch as the main polysaccharide. Using technologies such as blending and/or modifications (involving physical, chemical and double modifications), and with varying the reaction conditions, we aimed to overcome some drawbacks related to the intrinsic properties of starch granules that could influence the final quality of films. The changes in some aspects such as chemical structure, morphology, crystallinity, mechanical properties (tensile strength and elongation at break), water vapor permeability (WVP), transparency, hydrophobicity, mobility and distribution of water molecule and antimicrobial activities, in function to the technology used and to the reaction condition were analyzed. The following results were obtained:1. The rheological behaviors of starch mixtures and the properties and structure of their blend filmsIn order to determine their rheological behaviors and their films properties, suspension mixture of starch with Konjac glucomannan (KGM), chitosan, and KGM-chitosan at volumetric ratios of 1:1,1:1 and 1:0.5:0.5 respectively were prepared. Blending drastically reduced the solution viscosity, and the initial solid-like behavior of starch gel was inversed by the presence of non-starch polysaccharides, which decreased the values of the storage modulus (G’) and increased those for loss modulus (G"). Blend films were thinner, more transparent, and less resistant to water absorption, but more resistant to water vapor flux than the starch based film. The results of structure analysis showed that the semi-crystalline structure of starch dominated in the x-ray diffraction patterns, and a good miscibility between polysaccharides was observed by FT-IR. Nevertheless, the starch-KGM-chitosan films demonstrated the best properties with respect to their lower tensile strength and higher elongation at break.2. Effect of antimicrobial agents on the properties and antimicrobial activities of polysaccharides based film.Three types of antimicrobial agents Clove essential oil, nisin and EDTA were employed separately, in order to determine and to compare the impact of the effect of their incorporation on the physico-mechanical properties and antimicrobial activity of starch, konjac glucomannan and chitosan based films. It was observed that WVP of the different films decreased with the addition of essential oil, increased with the presence of EDTA and, except for the KGM film, the other films were strongly weakened with nisin.The increase of EDTA (from 5 to 10%) affected positively the expected tensile strength of films. EDTA however, tended to diminish the flexibility of both starch and KGM films, nevertheless brought a better flexibility to the chitosan film.The activity of these antimicrobial films against Gram (+) and Gram (-) bacteria showed that clove essential oil and EDTA displayed a good ability of inhibiting microbial growth. The incorporation of nisin by cons has showed a feeble activity both for gram (+) and Gram (-). To improve the protective ability of polysaccharide films without damaging the properties of the films, EDTA and then clove essential oil were according to our results, the more recommended for this purpose. However, the type of film and the agent concentration should be thoroughly controlled depending to the adequate physico-mechanical properties needed and the final application.3. Properties and structural characterization of physically modified corn starchWe studied the effect of milling time and ethanol as moisturizing agent on the physical modification (machinery activity) of starch. The results observed that the increase of the milling time during the physical modification tended to increase the amylose content in a more effective way than the variation of the starch to ethanol ratios (w:v). Physically modified starches were more transparent, more soluble in water, less crystalline, and presented damaged granules structure and an uneven surface. In all cases, the granule sizes were observed to be better distributed at a ratio of starch to ethanol of 1:0 and 1:3 (w:v) respectively, and the largest mean granule size was observed at higher ethanol content (ratio 1:3, w:v). The results above suggested that:a) Apparently, the hydrogen bonds which are more exposed were broken, and the ball milling would also slightly destroy the carbon-carbon bonds, leading to a disbranching of amylopectin. b) The better motilities of the starch particles observed at a ratios of 1:0 and 1:3(starch:ethanol, w:v) were crucial for the efficiency of the physical modification, and favored the collision and heat effect. c) The evolution of the transverse relaxation time (T22) associated to the more mobile phase varied with the temperature, and important variations occurred at 60℃. d) On the other hand, T22 augmented with the increase of the ethanol content, and the longer milling time, for which it showed higher value. e) The increase of the starch concentration in the aqueous solution (from 1:5 to 1:1 w:v) tended to decrease the transverse relaxation time in function of the temperature, f) In addition, we remarked that the modified starch based gel seemed more stable compared to the native starch in the temperature range of 30 to 100℃, suggesting a lower gelatinization temperature. The distribution of the different protons populations (1H T2) of water in starch was confirmed.’H T2 was closely dependant to the water content and to the temperature, however the majority of the protons was concentrated in the water involved in the interaction with starch (’H T22a),1H T22b associated to the free water was the less significant and was particularly perceive at temperatures< 60℃.4. Effect of the molar ratios of acetic acid and acetic anhydride on structure and attributes of starch acetatesThe effects of the change in molar ratios of acetic acid and acetic anhydride on the acetylation of starch showed that starch acetate with high degree of substitution (DS,2.93) was obtained at a molar ratio (acetic acid/acetic anhydride) of 1:1. However, the DS tended to decrease with the change of this ratio. Therefore, the ratio of acetic acid to acetic anhydride complies with the relationship of chemometrics and exhibit a critical point. The FT-IR analysis indicated characteristic absorption peaks with increasing DS materialized by an increase of the carbonyl C=O group and a decrease of the hydroxyl OH group at about 1750 cm-1 and 3450 cm-1 respectively. The X-ray diffraction patterns of acetylated starch showed an amorphous structure. Degree of crystallinity, surface morphology, water solubility and water absorption index of corn starch were also affected by the changes in reagent ratios. The glass transition (Tg) and melting (Tm) temperatures of acetylated starches also decreased after acetylation.5. Effect of the catalytic activity of iodine on the properties and structure of acetylated corn (Zea mays) starch product.Under microwave assistance, the effect of various amounts (from 0.5 to 1.75 mmol) of iodine catalyst on properties and structure of acetylated corn starch was investigated. The study of Fourier Transform Infrared (FT-IR) spectra, X-Ray diffraction pattern, thermal properties, granule morphology, water absorption and solubility, revealed important changes on the physical properties of the obtained products related to the increase of the iodine content. As shown by the scanning electron microscope (SEM), a specific structure on the granule surface was discerned for each amount of iodine. By its exo-corrosive action on starch, iodine indirectly induced loss of crystallinity, decrease of the glass transition and melting temperatures, and a progressive reduction in the water absorption and water solubility index of the modified products. Starch esters with better hydrophobicity and good processability for bio-based material applications were synthesized. Alternatively, the variation in the concentration of the iodine could also be of interest in special purposes, in which starch granules with controlled surface porosity are needed.6. Structure and Properties of double physical and chemical modified corn starchThe effect of double ball milled and chemical modifications on the structure and Properties of corn starch were investigated. The scanning electron microscopy and X-Ray diffraction analysis of ball milled starch (BMS) granules showed a fragmented aspect with a total loss of their initial crystallinity and an increase of their potential hydroxyl substitution sites. This morphology facilitated the access of the esterifying solvents into the starch molecule as corroborated by the spectacular increase of the water solubility and water absorption index. Following the acetylation, the degree of substitution (DS) increased up to 2.83 with the augmentation of the iodine catalyst content (p=0.011). However, under the same condition of esterification, the DS of the BMS was slightly lower in comparison to the control, due to an increase in the number of free hydroxyl groups. Consequently to the modification of starch, a reduction in the glass transition temperature (Tg), strong hydrophobicity and important exo-corrosion affecting the granules were noticed.7. Preparation and performances of free organic solvent starch acetate based filmsThe chemically modified starches with different DS (1.7,2 and 2.7) were dissolved in a dilute acetic acid solution (90%, v:v) at a ratio of 1:10 (w:v), in order to prepare their related films. It was observed that the obtained SA based films were compact, pliable with various degree of transparency. Their permeability to water was diminished with the increase of the DS, and SEM image showed that microporosity appeared at low DS on films surface, but disappeared progressively at higher degree of substitution. However, a recrystallization was noticed on the film structure at a DS of 2.7. As expected, the wettability of SA film decreased slightly with the increase of the DS, however contact angle measurement revealed that due to their structures, the films were not totally hydrophobic. In the contrary, owing to the amylopectin like structure of starch acetates, their molecular bonding led to films with low tensile strength and elongation at break in comparison to native starch.8. Properties and structural characterization of SA/chitosan blend films.In order to improve some properties related to the brittleness of their films, starch acetate with a degree of substitution of 2.7 was blended with chitosan. Acetic acid was used as solvent to dissolve both polymers. It was observed that after starch was blended with chitosan the crystallinity was strongly reduced, and the composites films were more transparent and showed improved tensile strength and elongation at break in function of the SA/chitosan ratios employed. At a ratio of 7:3 (v:v) SEM micrographs showed that 56% of the film surface was lined with micropores of uneven diameters and distributions and a structural variation was noticed in their cross sections. A different morphology characterized by a fibrous like structure was observed at lower ratios (1:1; v:v). The wavenumbers at which peaks were recorded in the FT-IR spectra suggested a good miscibility between SA and chitosan at different ratios.