Moisture Sorption Isotherms of Dried Sea Cucumber Holothuria scabra FOR PREDICTION OF SHELF LIFE
Moisture sorption isotherms (MSI) for dried sea cucumber (Holothuria scabra) were generated using an isopiestic gravimetric method with closed-loop air circulation. Equilibrium moisture content (EMC) of pre-dried samples was determined at different levels of relative humidity (RH) at 30, 45 and 60°C. Constant RH was generated using saturated salt solutions, with a corresponding range of water activity (aw) of 0.036-0.970. Six mathematical models were fitted to EMC versus water activity at each temperature; these were the Guggenheim-Anderson-de Boer, Modified Halsey, Modified Henderson, Modified Oswin, Peleg and Smith models. The best model was selected based on values of the coefficient of determination (R2), standard error of estimate (SEE), and the mean relative percentage deviation modulus (Pe). Water vapor transmission rates (WVTR) of commercially available plastic film was measured at 30oC and aw = 0.75. Using the best MSI model and a mass balance analysis on water vapor, the predicted shelf life of dried product packed in plastic film at 30oC was determined.
The EMC vs aw plots for dried sea cucumber followed a Type III curve (Fig. 1); the Peleg model had the best fit (R2 = 0.9598 to 0.9991, Pe = 1.48 to 11.94, SEE = 0.005 to 0.028). The product was stable when dried to 10% moisture content (dry basis) and stored at aw < 0.40. For the range 0.40 < aw < 0.60, EMC starts to increase, while at aw > 0.60, the dried product rapidly absorbs moisture. Temperature had no significant effect on EMC.
WVTR of polyethylene (thickness = 44 μm) and polypropylene (thickness = 35 μm) film was measured as 1.47 and 0.45 g m-2 d-1, respectively. Shelf life was taken as the time required for dried sea cucumber to reabsorb moisture (initial EMC = 0.12 g g-1 dry matter) to a final EMC (0.18 g g-1 dry matter); the final EMC represented the point at which the absorption rate of water started to increase rapidly. Estimated shelf life for dried product packed in PE and PP film was 210 and 688 days, respectively.