COMPARING EXPRESSION BEHAVIOR OF FROZEN/THAWED FRUITSCONTAINING SOLUBLE AND INSOLUBLE FIBERS

The effect of freezing and thawing pre-treatments and storage time on expression of apple and pineapple were studied using compression permeability cell. In this study, expression experiments of pineapple (representative of fruit rich in insoluble fiber) and apple (representative of fruit rich in soluble fiber) were done at three levels of pre-treatments, which were control (fresh fruit), slow freezing, and cryogenic freezing. For the slow freezing pre-treatments prepared samples were frozen at -18ºC in the freezer for 24 hours, while cryogenic freezing pre-treatment was achieved by freezing with liquid nitrogen until the temperature of fruit decreased to -90ºC. Both of freezing pre-treatment methods gave more expression efficiency than those obtained from control pre-treatment method. However, slow freezing pre-treated samples gave higher de-liquoring efficiency than those obtained from cryogenic freezing pre-treatment samples in the initial period of expression. Slow freezing pre-treatment method was chosen as an experiment condition for determination of storage effect. It was found that Terzaghi-Voight combined model fitted better than Terzaghi’s model but there were slight differences between 3, 6, and 9 weeks. The fitting of normalized cake thickness (ratio of the cake thickness at any expression time to initial cake thickness) versus time was also done. It was clearly found out that the empirical equation can evaluate the changes of cake thickness during expression time (r2 ≥ 0.99). Soluble solid, pH and sugar acid ratio were significantly higher in juices obtained from frozen samples than those obtained from control samples for both fruits, except reversible observed was found out in soluble solid of pineapple (p≤0.05). There is no significant difference for Vitamin C in juices obtained from neither frozen samples nor control sample. (p>0.05). With longer storage time, sugar acid ratio was clearly found out higher in both apple and pineapple (p≤0.05).