Development of composite film coating for freeze-dried fruit slices

Abstract

Edible film coating on food before freeze-drying process can be used for food preservation, especially in heat-sensitive food such as fruits and vegetables. Previous studies have been reported that trehalose had the cryoprotective effect and unique properties such as non-reducing sugar and low hygroscopic. However, no research has been conducted on the use of trehalose as film coatings. Therefore, this study aimed to develop a composite film consisting of alginate and trehalose and to evaluate physical properties of freeze-dried ​​fruit slices coated with a composite film. The experiment was designed to prepare a composite film consisting of various concentrations of trehalose, alginate and calcium chloride (3.0-9.0% w/v, 1.2-2.0% w/v and 0.2-0.6% w/v, respectively). The response surface methodology, Box-Behnken design, was used to optimize the factors of composite film preparation which were concentrations of trehalose, alginate and calcium chloride. The study showed that the optimized formula, which gave the least in thickness, opacity, moisture content, and water vapor permeability, could be prepared using 6.71% w/v trehalose, 2.0% w/v alginate and 0.4% w/v calcium chloride. Higher concentrations of trehalose seemed to increase thickness and water vapor permeability of a composite film whereas the moisture content of a composite film was decreased. The effect of composite film coating on physical properties of freeze-dried fruit (apple) slices was determined. After freeze drying, the mass loss of composite film-coated samples was statistically significant higher than the uncoated (control) samples (p<0.05; p=0.001). The water activity of composite film-coated samples was statistically significant lower than the uncoated samples (p<0.05; p=0.001). Moreover, the color h* values of composite film-coated freeze-dried samples were statistically significant higher than the uncoated samples (p<0.05; p=0.047), while the browning index was statistically significant lower than the uncoated group (p<0.05; p=0.034). For freeze-dried samples coated with a composite film stored in a desiccator at 25 ± 5 °C for 4 weeks, the rehydration capacity and firmness of film-coated samples were statistically significant higher than the uncoated samples (p<0.05; p=0.038 and p=0.012, respectively), while the moisture content of film-coated samples was statistically significant lower than the uncoated samples (p<0.05; p=0.002). The results were possibly due to ability of trehalose to form glassy state rather than crystal state during freeze drying and ability to inhibit ice crystal growth. Also, trehalose did not promote the Maillard reaction. In conclusion, the trehalose-alginate composite film could be used as film coating on fruit slices prior to freeze drying process in order to maintain appropriate food properties. Ongoing research should be done on the effectiveness of composite film coating on physical properties of a variety of fruits.