Wetting behavior of chitosan solutions on blueberry epicarp with or without epicuticular waxes

The physical properties of fresh highbush blueberry epicarp were determined in order to increase the effectiveness of chitosan-based coatings. The measured roughness of the blueberry epicarp was 42 ± 10 μm. The surface free energy, γSV of blueberry epicarp with and without epicuticular wax was calculated using the "acid-base" theory. Blueberry epicarps with or without wax have mainly an apolar (Lifshitz-van der Waals) character with a small polarity (Lewis acid-base). In both cases, blueberry surface is a low-energy surface. With wax γSV = 29.1 ± 0.1 mJ m-2 while after the wax removal, γSV was increased significantly to 38.3 ± 2.8 mJ m-2. Various chitosan-based coatings were applied to the blueberry epicarp. The surface tension, the cohesive energy and the spreading coefficient were calculated for each solution. In particular, the effects of concentration of a plasticizer (glycerol) and a surfactant (Tween 20) on the wetting of blueberry epicarp were studied. A mathematical relationship between concentrations and spreading coefficient S was proposed. Finally, the optimal concentrations of chitosan-based solutions were determined according to S.