The Study On Vacuum Drying Characteristics And Model Of Konjac

Konjac,a sort of perennial herb and important plant resources in our country, is Amorphophallus and belongs to Araceae, which has a large number of glucomannan. Nowadays, Konjac has been widely applied in medicine, food, textile, printing and dyeing industry, cosmetics, ceramic, fire control, environmental protection, war industry, oil drilling, etc. In the last nearly 30 years, the konjac has become an emerging sunrise industry in our country. The initial moisture content of the konjac freshly harvested is very high, generally Amorphophallus albus’ s moisture content is80%~85%, and the Amorphophallus konjac’s moisture content is 91%.This phenomenon of konjac becoming putrid is serious if we can’t dry it timely. Therefore, the konjac must be dried in a short time to reduce the moisture content to a safe moisture level which is about 15% so that the dried konjac could be packaged, transported and stored easily.Vacuum drying technology was applied in konjac drying in this paper. This study selected the vacuum degree, temperature and slice thickness as the factors, and each factor chose three levels to carry out the single factor of vacuum drying experiment, orthogonal of vacuum drying and air-vacuum combined drying test, and the gulcomannan content detection experiments of dried konjac; drew the konjac’s drying characteristic curve, analyzed the rule of konjac drying moisture variation; chose nine mathematical models to simulate the experimental data, with average relative error, determination coefficient, chi-square and root mean square error as the four index to evaluate the result of the fitting, got the mathematical equations that was the most suitable to describe the konjac vacuum drying characteristics; Analysis the vacuum and drying characteristics of konjac through the method range analysis, analysis of variance(ANOVA), comprehensive equilibrium method, reached the optimum technological conditions of konjac in different drying methods;compared the difference between the konjac vacuum drying and air-vacuum drying on the drying rate and drying quality.The main conclusions from the study are as follows:(1) According to the test results of the single factor experiments on konjac vacuum drying, the vacuum drying characteristics curves of konjac was drawn, the tests show that the drying rate increased with the increase of the vacuum degree and the temperature and decrease of the thickness of konjac slices. The Cubic model was a much more adequate model for describing the drying characteristics of konjac, the average relative error, determination coefficient, chi-square and root mean square error respectively are:0.999、0.997、0.60% and 5.20%;the effective moisture diffusion coefficient of konjac increased with the increase of vacuum degree, temperature and thickness of konjac.(2) Through the orthogonal vacuum test of konjac, the significance that each factor impacted on konjac vacuum drying rate was ordered successively as: thickness >temperature >vacuum degree;and the significance that each factor impacted on the quality of konjac vacuum drying was ordered successively as: vacuum degree>thickness>temperature. The optimum technological conditions forkonjac vacuum drying through integrated evaluation method based on weighting factor is: vacuum temperature is 70 ℃, vacuum degree is 0.04 Mpa and the thickness is 2 mm,the drying time under the condition is 5.33 h and the glucanmannan content is 52.64%.(3) In the air-vacuum orthogonal test of konjac, the best model obtained for describing the air-vacuum drying of konjac was Modified Henderson and Pabis, and the average relative error,determination coefficient, chi-square and root mean square error respectively were: 8.42%, 0.996,0.03% and 1.40%; the significance that each factor impacted on konjac combined drying rate was drawn orderly as: thickness >temperature >vacuum degree; and the significance that each factor impacted on the quality of konjac combined drying was drawn orderly as: temperature >vacuum >thickness. The optimum technological conditions for konjac vacuum drying was: konjac’s thickness of 2mm drying 15 min firstly in the conditions that temperature was 70 ℃ and air velocity was 1.95m/s, then put it in the conditions that temperature was 70℃ and vacuum degree was 0.06 Mpa to dry until the end, the drying time under the condition was 3.25 h, glucanmannan content was 47.91%.(4) Comparison of the drying characteristics of konjac hot air drying and vacuum drying.Under the same temperature and the thickness, konjac hot air drying moisture evaporation rate was greater than the vacuum drying, the minimum time required to dry the vacuum to the same conditions was about 4.67 times of the shortest time needed for hot air drying; while the other conditions being the same, such as the same thickness and temperature of konjac hot air drying rate were greater than konjac vacuum drying rate.(5) In temperature, thickness and vacuum degree which were under the same conditions, the average time needed for konjac vacuum drying was about 1.98 times longer than that of combined drying time, the glucanmannan content of konjac vacuum drying was at utmost 1.40 times more than the glucanmannan content of konjac combined drying.