Analysis of film formation of the hottest Chitosan

2022-08-24
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Film formation analysis of Chitosan Edible fruit and vegetable fresh-keeping film

edible fruit and vegetable fresh-keeping film is a thin, uniform and transparent fresh-keeping film with multi microporous channels formed by using edible materials such as natural sugars, starch, protein and oil as the main raw materials, adding film-forming additives, controlling film-forming conditions, coating or directly forming a film on the surface of fruits and vegetables. By adjusting the components of the film and the film forming process conditions, a certain performance of oxygen, carbon dioxide and steam permeability can be obtained, so as to control the respiratory environment of fruits and vegetables and achieve the purpose of spontaneous controlled atmosphere preservation

there are two ways to form edible film: one is coating method, that is, melt soluble lipids are coated on the formed polysaccharide or protein film to form a double-layer composite film; The second method is emulsification, that is, lipids are miscible with polysaccharides and proteins in ethanol solution. When drying and forming a film, it volatilizes with ethanol, and the lipid phase is separated from the polysaccharide phase, forming a double-layer membrane. The research shows that the water resistance of the emulsion composite membrane is better than that of the coating composite membrane, and the emulsification method is simple and easy, and the mechanical properties of the membrane are also better

oxygen is an important factor to maintain the respiration of fruits and vegetables. The research shows that when the oxygen concentration is less than 3%, the normal respiration of fruits and vegetables will be inhibited, so the oxygen concentration in the packaging system should not be less than 3%. The concentration of carbon dioxide should not be too high, and the ratio of carbon dioxide to oxygen transmittance is about 4:1. The water vapor transmission rate of the coated film cannot be ignored, too large transmission rate will cause the product to lose water and wilt, and the high relative humidity in the film will cause bacteria to breed and rot. Therefore, the main consideration in choosing fruit and vegetable fresh-keeping film is the permeability of the film to carbon dioxide and water vapor. The permeability of edible films to oxygen is lower than that of plastic films, and the permeability ratio of carbon dioxide to oxygen is also higher than that of plastic films. The permeability ratio of carbon dioxide/oxygen plays an important role in designing suitable edible fruit and vegetable coatings. The larger the permeability ratio, the less carbon dioxide is accumulated, which can not only effectively prevent anaerobic respiration caused by too much carbon dioxide accumulation in the film, but also slow down the respiration rate of fruits, Therefore, the ideal fresh-keeping effect can be achieved

although the gas regulation performance of edible films is generally better than that of plastic films, edible films often fail to achieve the required mechanical strength, and most edible films are very sensitive to water, so their performance must be improved. Compared with edible films such as cellulose film, starch film and protein film, chitosan film has the advantages of good film-forming performance and high mechanical strength. In particular, chitosan can form different semi permeable films on the surface of fruits and vegetables, have selective permeability to gases, and can adjust the physiological metabolism of fruits and vegetables after picking. Moreover, because chitosan and its derivatives have antiseptic and antibacterial effects, they can induce fruits to produce their own resistance, It includes activating the protein on the plant cell membrane to stimulate the enzyme, making the enzyme in the cell produce phosphorylation reaction, improving the enzyme activity, starting the plant defense system and producing plant interferon, phenolic compounds and other disease resistant substances, which can inhibit the pathogenic bacteria. Therefore, the coating preservative with chitosan as the main raw material shows a good application prospect in food preservation

preparation of chitosan fresh-keeping film

the film fresh-keeping technology of food mainly includes two methods: coating and film fresh-keeping. For chitosan, the method of coating preservation is to spray the low acid solution of Chitosan on the surface of fruits and vegetables or fresh meat products. After drying, a layer of polymer fresh-keeping film can be formed on the surface of fruits and vegetables. Chitosan can be made into a single film or a composite film of chitosan and other substances. Dissolve a certain amount of chitosan in 1% acetic acid aqueous solution by volume, heat and stir to make it fully dissolved to prepare chitosan solution. After degassing the chitosan solution by ultrasonic, the chitosan film can be prepared by quantitatively coating it on the glass plate covered with polytetrafluoroethylene, which is widely used in many industries, and drying it at a certain temperature. For high concentration chitosan solution, the chitosan solution can be extruded through the slit machine head under pressure and formed on a rotating smooth substrate. Generally, films with good gas permeability can be obtained by evaporation

the film making process is:

chitosan is dissolved in enoic acid solution → stirring → suction filtration → defoaming → coating → air at room temperature → put into a constant temperature and humidity box for thorough drying → uncover the film

performance of chitosan fresh-keeping film

first, mechanical performance

chitosan film has certain mechanical properties. For example, chitosan starch film improves the low strength of starch film, making it suitable for packaging semi-solid or liquid food. Domestic researchers use chitosan and glycerol to make edible film, and its tensile strength can exceed the level of LDPE (low density polyethylene) and slightly lower than HDPE (high density polyethylene)

the tensile strength of the film is closely related to the concentration of chitosan. Under certain conditions, the tensile strength of the film increases with the increase of chitosan concentration. Because the relevant factors affecting the tensile properties are the structure, average molecular weight and molecular arrangement of the polymer. When the molecular weight is fixed, the greater the concentration of chitosan, that is, the more the number of molecules per unit volume, the stronger the interaction between polymer chains and the stronger the intermolecular hydrogen bond, the stronger the strength of the membrane. When the concentration of chitosan is too small, due to the large fluidity of the film-forming solution, the film formed is very thin, so it is inconvenient to uncover the film; When the concentration of chitosan is too high, because the film-forming liquid is viscous, it is not easy to degass, and bubbles are easy to form during film making, making the film not 4 Each length, pneumatic unit and display digit of the safety rope tension testing machine are leveled by dynamic exchange

II. Wettability

when chitosan coating is used to preserve fruits, the film must be evenly formed, that is, the wettability of chitosan solution is required to be good. The wettability of chitosan solution can be measured by contact angle method. The contact angle of the solution is different on different media. The larger the contact angle is, the worse the wettability is. Drop a drop of 2% chitosan solution on the clean glass slide, rough glass slide and waxed glass slide respectively. Later, project it onto the paper with a penetrant and measure the contact angle. The results showed that the contact angle of chitosan solution on Waxed glass was 105 °, and the wettability was very poor. Because most of the peel has a waxy layer, it is not easy to evenly wet the surface when used to treat fruits, which will inevitably affect the effect of fresh-keeping. Therefore, it is necessary to add an appropriate amount of wetting agent to improve the wetting performance

through the comparison of the performance of several surfactants to speed up the improvement of innovative system construction, insiders found that adding surfactant Tween20 to chitosan solution can effectively improve the poor wettability of chitosan coating when preserving fruits, and the interface contact angle is reduced from 105 ° to 52 °

III. antibacterial property

as early as 1979, Allan et al. Proposed that chitosan has broad-spectrum antibacterial property. Since then, many scholars have studied the antibacterial mechanism of chitosan. According to the different action targets of Chitosan on cells, the academic circles speculate its antibacterial mechanism into two categories: one is the mechanism that takes the negatively charged cell membrane of bacteria as the action target proposed by young et al.: under acidic conditions, the protonated ammonium -nh3+ in chitosan molecule is positively charged, attracting negatively charged bacteria, The negative charges on the bacterial cell wall and cell membrane are unevenly distributed, interfering with the synthesis of the cell wall, breaking the balance between the synthesis and dissolution of the cell wall in the natural state, making the cell wall tend to dissolve, the cell membrane deforms and breaks because it cannot withstand osmotic pressure, and the contents of the cell such as water and protein exude, causing bacterial dissolution and death; The other is the antibacterial mechanism proposed by hadwiger, which takes DNA in bacterial molecules as the target: after attracting bacteria, chitosan oligomer (mw-8000) on the model passes through the porous cell wall of E. coli and enters the bacterial cells, which may form a stable complex with DNA, interfere with the action of DNA polymerase or RNA polymerase, hinder the synthesis of DNA or RNA, and thus inhibit the reproduction of bacteria

the antibacterial property of chitosan fresh-keeping film is related to chitosan concentration, degree of deacetylation of chitosan, molecular weight of chitosan and environmental pH value

IV. edible and safety

based on the consideration of human safety and ecological environmental protection, antibacterial agents with high toxicity have been gradually eliminated. Chitosan has attracted great attention because of its good physical safety and broad-spectrum antibacterial properties. High molecular weight chitosan binds the freedom of microorganisms through the mutual attraction of -nh3+ and anions such as sialic acid phospholipids in the microbial cell wall, thus hindering microbial metabolism and reproduction. Chitosan derivatives with good human safety and antibacterial properties can be synthesized by chemically modifying chitosan and increasing the density of structural unit antibacterial factor nh3+. Chitosan has good affinity and compatibility with organisms, and has certain physiological and health care functions for human body. It is an edible substance

v. gas selective permeability and moisture resistance

the ratio of oxygen permeability and carbon dioxide permeability of chitosan film is less than 1, so this film can be used to preserve fruits and vegetables. Fruit and vegetable respiration will consume oxygen and release carbon dioxide. When fruits and vegetables are packed with this film, due to the poor permeability of the film to carbon dioxide, the carbon dioxide emitted by fruits and vegetables penetrates less, resulting in a high concentration of carbon dioxide inside the film, which controls the gas concentration in the microenvironment around fruits and vegetables, reducing the respiratory intensity of fruits and vegetables, and achieving the purpose of fresh preservation. This film also has a good barrier to water vapor. Therefore, it can prevent the transfer of water on the surface of food, prevent the evaporation of water on fruits and vegetables, and delay wilting. Therefore, packaging fruits and vegetables with chitosan film can have the effect of moisturizing, color protection and prolonging the storage period

the permeability of chitosan membrane to carbon dioxide and oxygen is small, and it is easy to operate with the increase of thickness. With the increase of film forming temperature, the permeability of the film increases. With the increase of film-forming temperature, the evaporation speed of solvent is accelerated, the molecular motion speed is fast, the hydrogen bonding between polymers is not dense, and the structure of the film is relatively loose, so the permeability coefficient increases

applied research

a preservative made of chitosan as the main component was used for the preservation of pengcitrus in the form of film. The results showed that chitosan preservative film could effectively control and adjust the respiration, water loss and chemical content changes of pengcitrus, and the preservation period was up to 6 months. In addition, different chitosan concentration, different film thickness, different preservation effect. When the concentration of chitosan is 2%, the fresh-keeping effect is the best. Less than 2%, the formed film thickness is small, and the fresh-keeping effect is not obvious; If the concentration is too high and the film is too thick, it may almost completely block the entry of oxygen. Anaerobic respiration is formed inside pengcitrus, which makes it difficult to discharge the metabolites carbon dioxide and alcohol, thus causing the internal fermentation, corruption and deterioration of pengcitrus

different concentrations of chitosan solution were used to film and preserve the pepper at different temperatures, and compared with sodium benzoate, a common preservative, to seek the best preservation technology. The results showed that when the concentration of chitosan was 0.5%, the effect of one-time coating was the best

chitosan was used as the fresh-keeping film agent of pepper. The coating method, film-forming conditions and the screening of anti mildew agents were tested. The results showed that the effect of twice dipping was the best; The best film forming condition: chitosan dosage 2.44% - 2.9

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