EXPRESSION BEHAVIORS OF FROZEN/THAWED FRUITS CONTAINING SOLUBLE AND INSOLUBLE FIBERS
チュラーロンコーン大学 理学部 食品技術学科(タイ)
Department of Food Technology, Faculty of Science, Chulalongkorn University(Thailand)
The effect of freezing and thawing pre-treatments on the expression of apple and pineapple were studied using compression permeability cell under a constant pressure of 1.42 MPa. The expression experiments on pineapple (a representative for insoluble-fiber-rich fruit) and apple (a representative for soluble-fiber-rich fruit) were done at three levels of pre-treatments, which were control (fresh fruit), slow freezing, and cryogenic freezing. For the slow freezing pre-treatment the prepared samples were frozen at -18_C in the freezer for 24 hours, while the cryogenic freezing pre-treatment was achieved by freezing with liquid nitrogen until the temperature of fruit decreased to -90_C. Both freezing pre-treatment methods gave more expression efficiency than those obtained from control pre-treatment method. When either of the freezing pre-treatment methods was employed, higher fruit yields, i.e. more than 80% were obtained regardless of the type of the fruit. However, as the juice yields of the apple control sample was very low (33%) when compared to that of the pineapple control sample (65%), the freezing pre-treatments thus had more effect on soluble-fiber-rich fruit than the insoluble-fiber-rich one. 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 for both samples. As the effects of freezing can rupture the cell wall due to moisture migration and growth of ice crystal and it significantly increased the juice yield. The change in consolidation ratio (Uc) with time in all three cases fit well withhe Terzaghi-Voight combined model. This may be because this model is a combination of the spring model and the dashpot model that also considers the effect of a creep. An equation for this Terzaghi-Voight combined model is shown below.where, Ce = consolidation coefficient (m2/s) , B = creep constant (-), η = creep constant (s-1), L = thickness of cake at any time (m), L1= initial thickness of cake (m), L∞= final thickness of cake (m), i = number of drainage surfaces (-), _C = consolidation time (s), _0 = total solid volume per unit sectional area (m3/m2).