Studies on Drying of Agricultural Products (III)

Characteristics on Moisture Transfer of Vegetables and some other Agricultural Products

Abstract

The authors studied on the characteristics of of moisture transfer of the dehydration process of some agriculutural products such as vegetables and grass which contained much moisture.
1) In the case of drying agricultural products like vegetables and grass which had comparatively high moisture content, and their drying characteristics having the period of constant drying rate, the relationship among initial moisture content (M_i, %d. b.), first critical moist. cant. (M_f1, %d. b.) and absolute temp. of drying air (T, °K) was expressed as follows;
I-M_f1/M_i=A·exp(B·T) where, A, B=constants.
2) Though definite functional relationship between second critical moisture content (M_f2, %d. b.) and T was not shown, the following ranges of M_f2 were obtained.
KANPYO 16-20
carrot 12.5-20 (G=341-471)
carrot 11-46 (G=1310-1640)
Onion 11-17
spinage (raw) 3-7
spinage (blanched) 2-49
italian rye grass 6-8
horse radish 10-16 where, G=mass flow rate of drying air per dry matter (kg/h·kg-d. m.)
3) Equilibrium moisture content (M_E) on drying agricultural products which had much moist. cont. was substituted for M_E in the equation of Henderson (I-r. h.=exp(-a·T·M_Eⁿ), the constants such as a and n of every agricultural products experimented on were as follows;
4) The equation expressing the relationship between the heat transfer coefficient of film (h_c, kcal/m²·h·c) on the drying material and mass flow rate per drying air per dry matter (G) was as follows;
h_c=C·G^d where, C, D=constants.
5) The relationship between the first falling rate of drying constant K₁ of each agricultural product experimented on and the absolute temp. of drying air was shown in the equation of Arrhenius (K₁=E·exp(f/T)), where E. f were as follows;
6) There were three kinds of characteristics in the relationship between the second falling rate of drying constant K₂ and absolute temp. of drying air T.
(1) The first group of agricultural products experimented on such as onions, spinage (blanched) and Italian rye grass which had thin leaves showed positive relationship between K₂ and T.
(2) The second group included carrots, KANPYO, spinage (raw) and other which had thick leaves and their plant organizations were not mollified. This group showed negative relationship between K₂ and T.
(3) The last group such as horse radish which contained volatile materials, had K₂ as constant on T.
7) In the case of these agricultural products experimented on except horseradish, the relationship between the moisture decreasing ratio (1-M/M_i) and the volume decreasing ratio (1-V/V_i) was about 1:1 in the period of constant drying rate. The relationship became 1-M/M_i>1-V/V_i due to the period of first falling rate drying, and then, the shrinkage phenomena stopped when the moist. cont. reached near the second critical moist. cont. M_f2.