Effects of Drying Pretreatments on the Cell Wall Composition of Grape Tissues

Pretreatments by consecutive dipping in NaOH, citric acid, and K(2)S(2)O(5) solutions help to increase the drying rate of grapes and to reduce the darkening due to enzymic and nonenzymic browning during storage of raisins. However, such pretreatments have also an important effect on the cell wall composition of grape tissues. In both skin and pulp tissues the yield of cell wall material decreased substantially with processing, by 19.7 and 22.5%, respectively. Sodium hydroxide and citric acid solutions solubilized large amounts of pectic substances and xyloglucans, whereas potassium metabisulfite solution caused minor modifications to the composition of cell wall polysaccharides. Moreover, drying pretreatments promoted important changes in the amounts of Ca and Mg associated with cell wall components, which suggested possible structural rearrangements of polymers within the wall. All of these observations were in good agreement with the main results obtained after the application of a nondegradative technique such as FTIR spectroscopy to the cell wall preparations of fresh and processed tissues.