Studies On Processing And Mechanism Of Mircrowave-Freeze Hybrid Drying Sea Cucumbers

Sea cucumber (Stichopus japonicus) is always looked on as a delicious and expense dish for its high nutrition and tonic function, and has very high commercial value. It is one of the most expensive marine products. Since the sea cucumber can autolyze after leaving sea water, it is difficult to preserve and transport. As a result, more than 80% of the fresh sea cucumber all over the world is processed to dehydrated product. Because the main component of the body wall of sea cucumber is collagen, dehydration is difficult and deterioration can occur during process. Besides, the dried product rehydration needs about 3 days for incrustation of body wall during drying process.Up to now, most of sea cucumbers are dehydrated by traditional techniques called salt-dehydration which need to add salt, boil again and again and expose to solar radiation for 2-3 days, and the product quality is poor. Another used technique is freeze drying which can ensure very well product quality, but needs more than 20 hours and the energy consumption was very high. As a resut, a new hybrid drying technology based on freeze drying was introduced to replace the traditional freeze drying. The new drying technology need not only ensure good product quality but also cost much lower energy consumption.In order to reduce drying time of freeze drying, a two-stage combined drying technology,freeze drying-vaccum microwave dyring was used. It was found that the new technology could reduce about 40% of drying time of traditional freeze drying and its product quality was close to freeze drying. Conversion moisture content of sea cucumber was the most important parameter of the combined drying process, and the optimal drying parameters were gotten by a reponse surface analysis method. So the combined drying technology can replace traditional freeze drying, and it also can be used to upgrade the exixting freeze drying productline.In order to reduce drying time of freeze drying and improve product quality, microwave was used as heating source of freeze drying, which could be called one-stage microwave freeze drying. Electric field distribution of the drying cavity was simulated by finite element method, and thus uniform heating could be ensured. Relationship between discharge power density and drying pressure was investigated, and the rule of low air pressure discharge was found. The rules of microwave loading were brought forward, which could lead to good product quality and short drying time. Sterilization characteristic of microwave freeze drying was found, and nanoscale silver coating combined with microwave freeze drying method was introduced to manufacture active sea cucumber. There was no significant difference between product quality of microwave freze drying and that of traditional freeze drying, wherease microwave frreze drying could reduce almost half of freeze drying time.In order to reduce more drying time of microwave freeze drying process, dielectric properties of sea cucumber were studied. The change rules of dielectric properties of sea cucumber as a function of temperature and moisture content was investigated, and it was found that the loss factor of frozen sea cucumber rapidly decreasing led to low microwave absorption capability. Some methods were brought forward to avoid thermal run away and material burning. In addion, nanoscale calcium vaccum impregnation and slat impregnation treatments were introduced to improve the dielectric properties of sea cucumber, and the product qulity also could be ensured. In fact, nanoscale calcium vaccum impregnation treatment could improve sublimation rate, and slat impregnation treatment could improve atmospheric microwave drying rate.In order to reduce more energy consumption, the osmotic pretreatment technology of microwave freeze drying sea cucumber was studied. Considering traditional sea cucumber processing, salt was used as osmotic agent to pre-dehydrate sea cucumber. It was found that ultrasound assisting osmotic pretreatment could lead to relative high water loss rate and solute gain rate increase was not very signifficalt. So ultrasound assisting osmotic pretreatment could had no significant affect on product quality, wherease it could reduce about 20% of microwave freeze drying energy consunmption. The optimal osmotic parameters also were worked out by a response surface analysis method.