| Natural biodegradable polymer materials are an emerging alternative to conventional packaging because of their excellent biodegradability,biocompatibility,and applications.Among them,the starch-based film has become a research hotspot in food packaging.At present,the research on starch film mainly focuses on improving the properties of the starch film by adding a plasticizer and using modified starches.The relationship between starch structure and the properties of starch-based films is not clear.Therefore,it is necessary to understand the correlation between the structure of starch and the properties of the starch film.In this study,corn starch was selected as raw material.The first step was to investigate the structural changes of corn starch particles under different gelatinization temperatures.The chain structure was selected for a follow-up based on the experimental results.In the second step,the corn starch-based film was prepared by the casting method,and the optimum amount of plasticizer was determined.The corn starch-based film was prepared according to the results in the follow-up experiment.In the third step,based on the chain structure selected from the previous test results and the finished corn starch film,the relationship between the chain structure and the properties of the corn starch-based film was further discussed.The fourth step is to analyze further the chain structure changes during the aging and retrogradation process of corn starch film and get the relationship between the chain structure changes and the properties of corn starch film in the aging process.The main results of this study are as follows:(1)The changes of different structural levels of corn starch during film formation.Corn starch was gelatinized at different gelatinization temperatures(50,60,70,80,and90?),and the structural changes of corn starch during gelatinization were studied.The results showed no gelatinization reaction of cornstarch at 50-60?,and each hierarchical structure could be preserved entirely without destruction.The gelatinization reaction occurred gradually at 70-90?.The granule structure,growth ring structure,crystal region,and lamellar structure of corn starch were destroyed and disappeared,and the chain structure(amylose and amylopectin structure)could still be detected.Therefore,in the subsequent study,the chain structure was taken as the primary research object to explore the effect of chain structure on the physical and chemical properties of corn starch-based films.(2)Effects of different concentrations of glycerol on properties of starch-based films.Because pure corn starch film is easy to break and cannot form a complete corn starch film,glycerin needs to be added in the production process of corn starch film.In this experiment,the physical and chemical properties of corn starch film with different glycerol concentrations(15%,20%,25%,30%and 35%)were studied.It was found that corn starch film with 25%glycerol content had good physical and chemical properties(low solubility:32.49±0.53%,low water content:16.34±0.45%,low water vapor transmittance:3.79±0.12 g·cm/cm2·s·Pa),and the corn starch film needed for subsequent experiments was made according to the results.(3)Changes of chain structure of corn starch during film formation.Corn starch film was prepared by using corn starch at different gelatinization temperatures(70,80 and 90?),and the changes of chain structure of corn starch at different temperatures were studied.The main changes of corn starch chain structure are as follows:With the increase of treatment temperature,the chain length distribution of corn starch changed.From the longer B2(16.03±0.20-15.19±0.09%)and B3(5.64±0.17-5.01±0.19%)chains to the shorter A chain(20.75±0.57-22.64±1.08%).At the same time,the molecular weight polydispersion coefficient of corn starch increased with the increase of temperature(3.345-5.853).DD(1.47±0.01-1.17±0.01)and DO(0.88±0.01-0.71±0.01)of corn starch decreased with the increasing of treatment temperature,on the contrary,the amylose content(23.73±0.26-26.89±0.23%)increased with the increasing of treatment temperature.The crystallinity of corn starch(13.17±0.12-4.14±0.57%)decreased gradually under high temperature,and the characteristic birefringence disappeared with the increase of temperature.(4)Effects of chain structure changes on properties of corn starch film during film formation.The thickness of maize starch film(194.00±2.31-169.67±3.46?m)decreased with the increase of temperature because of the increase of A chain content and amylose content in the chain length distribution,which made maize starch film form A more compact structure.With the increase of treatment temperature,the free hydroxyl groups of amylose and amylopectin increased continuously,resulting in more hydroxyl groups bound to water molecules,and the intermolecular force was strong and not easy to damage.Therefore,the solubility of water content(13.21±0.09-16.10±0.16%)decreased with the increase of treatment temperature.Water vapor transmittance(3.09±0.03-3.39±0.05 g mm-2·s-1Pa)and surface roughness(Rq:111.15-170.50 nm,Ra:83.60-139.00 nm)increased with the increase of treatment temperature.With the increasing of corn starch treatment temperature,the production of amylose and small molecular weight substances gradually increased.Because small molecular substances can fill into the amorphous region,the tensile strength of corn starch film(14.32±0.59-22.03±0.45mpa)increased continuously.The elongation at break(21.34±1.78-14.18±1.89%)decreased continuously.In addition,the degradation temperature and maximum degradation rate of corn starch film decreased due to the destruction of chain structure.The relationship between the properties of corn starch film and chain structure was established.(5)Changes of chain structure of corn starch during aging and regeneration.The molecular weight polydispersion coefficient of corn starch decreased with the aging and regeneration time of corn starch film prolongation,and the a-chain of chain length distribution,B1 chain,but the changes of B2 and B3 chains were small.Amylose content decreased with the aging and rejuvenation time of corn starch,mainly due to the formation of amylose crystal structure during the aging and rejuvenation process.Crystallinity of corn starch,DD and DO increased with the prolongation of aging recovery time.(6)Effects of chain structure changes on properties of corn starch film during aging and regeneration.The thickness of corn starch film and solubility gradually decreased.At the same time,as amylose and amylopectin crystallize successively in the aging and regeneration process,the free hydroxyl group in corn starch film decreases,resulting in water content decreased in the process of aging and rejuvenation.Due to the continuous increase of crystallization zone,the density of corn starch film increases,so it is difficult for water vapor to pass through corn starch film,so the water vapor transmittance decreased continuously.In addition,the formation of crystallization zone and the reformation of three-dimensional network of corn starch film lead to roughness and elongation at break,tensile strength,maximum degradation rate and maximum degradation temperature increased continuously.The relationship between the changes of chain structure and properties of corn starch film during aging and regeneration was established. |
PDF Full Text Request