2.3 Contents of sugars, organic acid and water analysis
To extract reducing sugar and non-reducing sugar, 80% ethanol was added to 1.0 g fresh weight of peel discs and heated 90 C for 30 min in a water bath. The mixture was filtered, and the residue extracted two times. The three extracts ware combined and evaporated to dryness under vacuum. The dried extract was dissolved in distilled water, diluted to 100 ml and stored at -20 C until analyzed. Colorimetric technique was used to determine the concentration of sugars (Dubois et al., 1956; Somogyi, 1952). Malic acid in the discs of apple peel was extracted with distilled water and analyzed by acidilyzer (Model 3, Fujihira Industry Co., Tokyo, Japan)
The water contant in apple skin was also estimated. Fifteen peel discs were immediately weighed, and then dried at 70 C for 48 h before dry weight was measured, with FW as the fresh weight, and DW the dry weight, water content (WC) = (FW-DW)/FM.