Physicochemical properties and application of pullulan edible films and coatings in fruit preservation

The effects of water, sorbitol and a sucrose fatty acid ester (SE) on the w ater sorption behaviour and thermal and mechanical properties of pullulan-b ased edible films as well as the physiological responses of fruit coated wi th pullulan have been studied. Incorporation of sorbitol or SE in pullulan films resulted in lower equilibrium moisture contents at low to intermediat e water activities (a(w)), but much higher moisture contents at a(w)>0.75; estimates of monolayer values (within 4.1-5.9gH(2)Okg(-1) solids) were give n by application of the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderso n-DeBoer (GAB) models. A single glass-rubber transition (T-g), attributed t o the polysaccharide component, was detected by calorimetry and dynamic mec hanical thermal analysis (DMTA) at a sorbitol level of 15-30% DM. With both tests the strong plasticising action of water and polyol was evident in th e thermal curves, and the T-g vs moisture content data were successfully fi tted to the Gordon-Taylor empirical model. Multifrequency DMTA measurements provided estimates for the apparent activation energy of the glass transit ion in the range of(similar to) 300-488kJmol(-1). With large-deformation me chanical testing, large decreases in Young's moduli (tensile and three-poin t bend tests) were observed as a result of water- andlor polyol-mediated gl ass-to-rubber transition of the polymeric films. In the moisture content ra nge of 2-8%, increases in flexural modulus (E) and maximum stress (sigma (m ax)) with small increases in moisture content were found for films made of pullulan or pullulan mixed with 15% DM sorbitol; a strong softening effect was observed when the water content exceeded this range. Addition of sorbit ol increased the water vapour transmission rate of the films, whereas addit ion of SE had the opposite effect. Application of a pullulan/sorbitol/SE co ating on strawberries resulted in large changes in internal fruit atmospher e composition which were beneficial for extending the shelf-life of this fr uit; the coated fruit showed much higher levels of CO2, a large reduction i n internal O-2, better firmness and colour retention and a reduced rate of weight loss. In contrast, similar studies on whole kiwifruits showed increa sed levels of internal ethylene, which caused acceleration of fruit ripenin g during storage. (C) 2001 Society of Chemical Industry.