Abstract
The functional properties of biopolymer-based film packaging materials are susceptible to external storage conditions. The effects of different storage temperature, relative humidity (RH) and duration on the apparent form, barrier properties, mechanical properties and microstructure of corn–wheat starch/zein bilayer films were studied. From 0 to 150 days, storage temperature and RH, but not storage time, affected the appearance and colour of the bilayer films. The increase in haze of the bilayer films stored at 25°C was much greater than that at low temperatures. With increased storage time, the moisture content first increased and then decreased, while the water resistance and oxygen barrier properties of the bilayer films worsened. After 150 days, the bilayer film stored at 25°C with 54% RH had better water resistance properties. The oxygen barrier properties of the bilayer film stored at 25°C with 43% RH were preferable to those of other groups because the peroxide value of vegetable oil packed in the former bilayer film was the lowest. The tensile strength of bilayer films stored at 25°C with RH of 43, 54 and 65% decreased, but was still better than those stored at low temperatures (−17°C, 4°C), which were tough due to their high elongation at break. Scanning electron microscopy results showed tight bonds between the bilayer films, and the network structure inside the films disappeared and reappeared during storage. The cross-sectional compactness changed, and there was no film separation after 150 days.
Highlights
Food packaging materials play an important role in extending shelf life and ensuring food safety and quality
Under storage conditions of 25°C with 65% relative humidity (RH), the aà values reached the maximum values of −1.50 (30 days), −1.66 (60 days), −1.81 (90 days), −1.69 (120 days) and −2.05 (150 days), and the green colour of the bilayer films decreased in intensity
The increase in aà values and decrease in bà values of the bilayer films may be due to the increases in storage temperature and RH and the loosening of the internal structure of the bilayer films, which allowed a part of the Z film to enter the C–W film
Summary
Food packaging materials play an important role in extending shelf life and ensuring food safety and quality. Synthetic petroleum-based polymers are widely used in the food packaging industry because of their variety, low price and stable nature. With the increasing pollution from petroleum-based polymer packaging materials and the depletion of petroleum resources, new degradable and edible environmentally friendly food packaging materials prepared from biopolymer resources, including polysaccharides, proteins and lipids, have gradually become a popular research topic [2]. A natural low-cost macromolecular polymer with the ability to biodegrade, has been widely used in edible packaging films [3]. Due to the interactions of the disulfide bond (S–S), covalent bond, hydrogen bond and the electrostatic force and hydrophobic interaction between protein chains, Z-based films usually have excellent film-forming ability, water resistance, toughness, oxygen barrier properties and biocompatibility [7,8]. The poor tensile strength and high price of Z-based films limit their application in the packaging industry [9]. Due to the distinct advantages of starch and Z films, a layer of Z can be added to starch films to make full use of these advantages, which complement each material to prepare edible bilayer films with excellent properties and expanded applications in practical production
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