Studies On The Drying Kinetics Of Sea Cucumber (Stichopus Japonicus)

Consumption demand of sea cucumber (Stichopus japonicus) is increasing, since its eminent nutritive value. Recently, the enlarging of sea cucumber aquiculture and the output are remarkable. It indicates an optimistic future of the sea cucumber market. Of all the global fresh sea cucumber, 90% is dewatered to different kinds of products. At the same time, the processing methods remain old-fashioned, coarse and random. Studies of this paper are based on the integral consideration of pre-treatments, drying dynamics of sea cucumber and the rehydration properties of dried sea cucumber, at different conditions. It is the trend of modern drying techniques, so will give a processing standard to sea cucumber industry.Firstly, after comparing the drying characteristics, shrinkage properties and the rehydration quality of sea cucumbers at different conditions, the pre-treatment methods are determined. The result indicates that the contraction coefficient increases with the ascending of drying temperatures, and the contraction coefficient of the brine salting sea cucumbers is greater than the untreated ones'. Varying pre-treatment method, drying of sea cucumber would experience different characteristics of falling rate period. Boiling sea cucumber in the supersaturated sodium chloride solution prolongs drying period and deteriorates the rehydration quality. So soaking in the supersaturated sodium chloride solution is advised instead of boiling in it when brine salting sea cucumber is processed. The soaking time should be no less than 24h. The determined pre-treatment methods for the untreated sea cucumber and the brine salting sea cucumber are boiling in the 3.5% sodium chloride solution for 15min, and soaking in supersaturated sodium chloride solution for 48h after boiling in the 3.5% sodium chloride solution for 15min, respectively.Secondly, the drying dynamics characteristics and drying model of brine salting sea cucumbers and untreated sea cucumbers are studied and constructed, respectively. It can be seen that the removal of moisture from the material is governed by diffusion phenomenon. Moreover, the temperature dependence of the diffusivity obeys the Arrhenius-type relationship. The drying behaviors of the present study are best fitted with the Two-term model. Brine salting sea cucumbers are more difficult to dry than the untreated. In studied temperature range, the effective diffusivities for drying untreated sea cucumbers vary from 3.553×10^-10m²/s to 8.056×10^-10m²/s, and the activation energy is found to be 11.772kJ/mol. While effective diffusivities for brine salting sea cucumbers vary from 3.349×10^-11m²/s to 34.404×10^-11m²/s, and the activation energy is found to be 35.455kJ/mol. Considering the rehydration quality of the dried sea cucumbers and the efficiency of drying, drying at 45℃is proper for the untreated sea cucumbers. For brine salting sea cucumbers, the temperature range is from 60℃to 75℃. However, by hot air drying the product quality could not satisfy both the drying efficiency and the rehydration quality.Thirdly, in order to choose the best drying method and parameters for the brine salting sea cucumber, its drying characteristics,shrinkage properties,and the rehydration quality under freeze drying, vacuum drying, microwave vacuum are studies and analyzed. It is indicated that sea cucumbers dewatered by freeze drying could maintain the primary shapes and the rehydration of them in room temperature water is excellent. However, the drying rate is small, thus more energy is consumed. At the same time, before freeze drying, sea cucumber must undergo a long-time pre-treatment process and be stored in refrigerator, so the advantages of this drying technology in sea cucumber is weakened. When sea cucumber is vacuum-dried at 85℃and 1.3kpa, the drying efficiency is high and the rehydration quality is good. The product even could be rehydrated in room temperature water. By and large, the sensory property is good, but it tastes hard. Microwave vacuum drying is a swift method for sea cucumber. The volume of sea cucumber gets expended and parts of the sea cucumber get burnt. By pulsed microwave input, the surface temperature could be lowered further more, but the drying time is prolonged and the energy consumption increases.Fourthly, on the base of the virtues of different drying methods, the drying characteristics,shrinkage properties,and the rehydration quality of the brine salting sea cucumbers by combined drying are studies and analyzed. It is shown that cross pile and temperature programming in heat pump-hot air drying could obviously cut short drying time, and reduce energy consumption. The rehydration quality is good, but the whole drying cycle is still too long. When hot air (90℃)-microwave vacuum (1.3kpa) drying is combined, the drying time is only 80% as that by hot air drying at 90℃. The volume expending phenomena eliminates, and the sensory quality of rehydrated sea cucumber is relatively good. When microwave vacuum (1.3kpa) - hot air (90℃) drying is used, the drying time is only 50% as that by hot air drying at 90℃. The contraction is uniform, and the volume expending phenomena does not appear any more. Using combined drying technology, the long drying time at low temperatures in conventional drying process could be shortened and the energy concentration could be cut down.Lastly, the sorption isotherm is determined, to find out the storage condition. It is shown that dried sea cucumbers should be stored in the environmental relative humidity smaller than 52%, when the moisture content of is lowered to safety range.In a word, the drying techniques are discussed theoretically and practically in this paper. It will improve the producers'ability to use modern drying methods for sea cucumber processing, so the old-fashioned way will be renewed. It will be acceleration to the sea cucumber drying industry, so it will benefit both the economy and the society.