58 



MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



Equilibrium Moisture Content 



If a piece of cut vegetable is exposed to a current of air of constant 

 temperature and constant humidity for a long period of time, it will 

 lose weight by evaporation of moisture, rapidly at first, then slower 

 and slower, and at last the weight will remain substantially constant, 

 even if the air circulation is continued for an indefinite lengtjh of 

 time. If the piece is then transferred to a current of "bone-dry" air, 

 or to a vacuum oven, it will lose more weight and eventually reach an- 

 other lower, constant weight. Equilibrium moisture content is a term 

 used to designate the amount of moisture held by a piece in equilibrium 

 with air at a given temperature and humidity. It is the amount of 

 moisture that will give the piece a vapor pressure equal to the partial 

 pressure of water vapor in the surrounding air. 



Since vapor pressure in a piece of vegetable becomes lower and lower 

 as the piece approaches dryness, the partial pressure of water vapor 

 in air which is in equilibrium with the piece must also fall. But the 

 partial pressure is proportional to the relative humidity at any given 

 temperature. The higher the relative humidity of the air, therefore, 

 the higher the equilibrium moisture content. If the air is "bone dry," 

 the vegetable also will be "bone dry" at equilibrium. 



Equilibriums between the common vegetables and air of varying 

 temperatures and humidities have been determined recently. (See 

 table 9, p. 118.) A way of presenting the data which is especially 

 convenient for use in analyzing dehydrator operation is to plot 

 equilibrium moisture against temperature of air at several different 

 levels of wet-bulb temperature. Figure 25 represents a kind of 



0.30 



>• 



z Q 



CO CO 



1.3 



,10 











\ \ 100 



\90 \ 



\llO \[20 



\ 130 Wet- 

 \ Tempe 



Bulb 

 rature 





\^^ 



00 1 

 DR 



20 l< 



f-BULB TE 



10 I 

 -.MPERATUF 



50 180 

 *E (°F.) 



Figure 25. — Approximate equilibrium moisture content of common vegetables. 



general average of these equilibriums for the vegetables which are 

 commonly dehydrated. Such an average is sufficiently precise for 

 many dehydrator calculations. 



Knowledge of these equilibrium values is important both in de- 

 hydrator design and in analyzing the pick-up of moisture into de- 

 hydrated vegetables through sheet packaging materials. The finish- 

 ing end of a dehydrator cannot dry the material to any lower mois- 

 ture content than the value corresponding to equilibrium with the 

 air at that point. If the finishing air is high in humidity, a very 

 dry product cannot be made. Furthermore, the closer the approach 

 to equilibrium, the slower becomes any further removal of moisture. 



