Lipid and composite coatings
Lipid compounds, such as long-chain fatty acids and waxes, can be incorporated in the film or coating matrix because of their hydrorepellency. Waxes are the most efficient substances to reduce moisture permeability. Their high hydrophobicity is a consequence of a high content in esters of long-chain fatty alcohols and acids, as well as long-chain alkanes (Kester and Fennema 1986; Donhowe 1992;Hagenmeier and Shaw 1992).
Paper and paperboard, which are the most widely used materials in food and drink packaging, are frequently wax-coated to improve their water-resistance and increase the shelf life of the packaged products (Rodriguez and others 2007). Paraffin wax applied in a molten form was commonly used to produce a water vapor barrier. Recyclable packaging paper materials using autodispersible waxes have been reported (Back 1995).
Lipid coatings provide good moisture barrier, but they have certain disadvantages such as brittleness, lack of homogeneity, and presence of pinholes and cracks in the surface of the coating. Composite coatings or multilayer coatings, applied either in the form of an emulsion or in successive layers (multilayer coating), have been prepared to combine the good structural and gas-barrier properties of hydrocolloid coatings with the good moisture-barrier characteristics of lipids. The method of application affects the barrier properties of the coatings obtained.
Parris and others (1998) measured the water barrier and grease permeation properties of Kraft paper coated with a combination of zein and paraffin wax. Their data demonstrated that the zein layer of the bilayer coating contributes grease-proofing and the wax layer water resistance. In a previous study, WVP decreases have been documented for NaCAS-coated paper due to the addition of carnauba wax (Khwaldia and others 2005). The WVP of coated papers decreased as the amount of wax in the coating increased. The addition of hydrophobic substances (carnauba wax) to this hydrophilic matrix provides the moisture-barrier properties.
Khwaldia (2009) found that the greatest reduction in paper WVP is achieved by addition of a wax layer to the paper already coated with NaCAS, due to the high resistance to moisture transfer of the paraffin wax. Indeed, the barrier ability of bilayer coating against water vapor transfer is higher than that of an emulsion coating. Emulsion coatings have the advantage that they are easier to apply on paper materials than bilayer coatings and they need only one drying step. Moreover, no problem separation of the two layers occurs, and their both hydrophilic and lipophilic nature allows their good adhesion onto any support.
Despond and others (2005) processed a gas-barrier multilayer material with paper, chitosan, and carnauba wax. Because of the hydrophobic character of the external wax layer, the water sorption in the multilayer decreased greatly, and gas permeability values lower than 0.5 barrer were obtained in the hydrated state.