Gradient slow-release active composite film and preparation method thereof

A gradient slow-release active composite film, wherein the film, from inside to outside, is composed of an antioxidative hygroscopic internal layer, at least one gradient anti-microbial antioxidative intermediate layer and at least one waterproof external layer;

• wherein the antioxidative hygroscopic internal layer is prepared by an alcohol-soluble protein-water-soluble protein substrate and a water-soluble antioxidant;
• the gradient anti-microbial antioxidative intermediate layer is prepared by an alcohol-soluble protein-water-soluble protein substrate and a lipid-soluble plant essential oil and a water-soluble antioxidant, wherein a mass ratio of the alcohol-soluble protein to the water-soluble protein is (1-2):(1-2), a mass percentage of the lipid-soluble plant essential oil is 2-10 wt %, a mass percentage of the water-soluble antioxidant is 1-8 wt %;
• and the waterproof external layer is composed of a hydrophobic alcohol-soluble protein layer,
• wherein a content of the lipid-soluble plant essential oil progressively increases from inside to outside and layer by layer, a content of the water-soluble antioxidant progressively decreases from inside to outside and layer by layer,
• wherein a number of the gradient anti-microbial antioxidative intermediate layer is 3, and the gradient anti-microbial antioxidative intermediate layer includes a first intermediate layer, a second intermediate layer and a third intermediate layer,
• wherein the mass percentage of the lipid-soluble plant essential oil is 2-4 wt % in the first intermediate layer, the mass percentage of the lipid-soluble plant essential oil is 5-7 wt % in the second intermediate layer, and the mass percentage of the lipid-soluble plant essential oil is 8-10 wt % in the third intermediate layer,
• wherein the alcohol-soluble protein is one of or a mixture of two or more of zein, wheat gliadin, hordein, and proso millet prolamin.