46 MISC. PUBLICATION 54 0, U. S. DEPT. OF AGRICULTURE 



the difficulty of traying the softened product. It is possible to apply 

 the sulfite as a spray, followed by a fresh-water spray on trayed 

 blanched material. This procedure, however, is not wholy satisfac- 

 tory and cannot be recommended for cabbage because of draining 

 complications. 



Laboratory and pilot -plant experiments have demonstrated that 

 both potatoes and carrots can be successfully treated by application 

 of sulfite solutions as a spray during blanching in much the same 

 manner as for cabbage. If future specifications require sulfiting of 

 these commodities, the required solution concentrations will of course 

 be influenced by the specified content of sulfite in the dry product. 



PRINCIPLES INVOLVED IN THE DRYING PROCESS 



The drying operation is a major step in the manufacture of dehy- 

 drated foods and may be defined as controlled evaporation of nearly 

 all of the water present in the fresh product. The evaporation of water 

 under these conditions is, however, a complex process, and its princi- 

 ples are sufficiently important to justify discussion here. 



The Vaporization of Water 



If a container is partly filled with water at a temperature of 100° F. 

 and the air is removed from the space above the water by means of a 

 vacuum pump, and then the container is tightly closed, a sensitive 

 manometer connected to the vapor space will indicate the gradual de- 

 velopment of a pressure within the "empty" space. If the temperature 

 of the water is maintained at 100° this pressure will increase up to 

 1.93 inches of mercury. The higher the temperature of the water, 

 the higher the pressure of its vapor will be ; at 212° the vapor pressure 

 of water is 29.92 inches of mercury — the same as average barometric 

 pressure at sea level. Table 7 presents values for vapor pressure of 

 water over a range of temperatures (39, p. 960) . 



Suppose that the container of water at 100° F. is left open to the 

 air, so that the space above the water must remain at atmospheric 

 pressure — say 30 inches of mercury. Water evaporates from the sur- 

 face of the liquid, and the water vapor pushes out some of the air, 

 since the pressure cannot rise above 30 inches. When the system comes 

 to equilibrium, the space above the water will be filled with a mixture 

 of air and water vapor. Each of these components is contributing 

 a part of the total pressure of 30 inches ; at a temperature of 100° F. 

 the water vapor is contributing 1.93 inches, the air 28.07 inches. 



These two quantities are termed "partial pressures." Before the 

 system reached equilibrium, the space was not yet saturated with 

 water vapor. It became saturated when the partial pressure of wateV 

 vapor in the space rose to equality with the vapor pressure of water 

 at that temperature. 



The fluid content of the cells of vegetables and fruits does not consist 

 of pure water, but is a complex solution of salts, sugars, and other 

 substances. The vapor pressure of such a solution is always somewhat 

 lower at any given temperature than the vapor pressure of pure water ; 

 the more concentrated the solution, the greater the difference. The 

 liquid in a fresh vegetable is dilute, but as dehydration progresses the 

 liquid left behind becomes more and more concentrated and its vapor 

 pressure becomes lower and lower. This is one of the major reasons 



