EFFECTS OF DRYING PRETREATMENTS ON THE CELL-WALL COMPOSITION OF GRAPETISSUES

Pretreatments by consecutive dipping in NaOH, citric acid, and K2S2O5 solutions help to increase the drying rate of grapes and to reduce the darkening due to enzymic and nonenzymic browning during storage of ra isins. However, such pretreatments have also an important effect on th e cell wall composition of grape tissues. In both skin and pulp tissue s the yield of cell wall material decreased substantially with process ing, by 19.7 and 22.5%, respectively. Sodium hydroxide and citric acid solutions solubilized large amounts of pectic substances and xylogluc ans, whereas potassium metabisulfite solution caused minor modificatio ns to the composition of cell wall polysaccharides. Moreover, drying p retreatments promoted important changes in the amounts of Ca and Mg as sociated with cell wall components, which suggested possible structura l rearrangements of polymers within the wall. All of these observation s were in good agreement with the main results obtained after the appl ication of a nondegradative technique such as FTIR spectroscopy to the cell wall preparations of fresh and processed tissues.