DRYING BANANAS 145 



waste ground adapted to banana culture in the Philippines 

 to-day to supply a large portion of the food of 25,000,000 

 people, if the new methods of handling that wonderful 

 crop were put into operation." 



The system of drying in a vacuum deserves consideration. 



Exhaust steam of low pressure is usually employed as 

 the medium for conveying heat to the vacuum apparatus. 

 The total heat contained in 1 Ib. of steam at 212 F. is 

 about 5J times more than that contained in 1 Ib. of water 

 at the same temperature, and it is owing to this large 

 amount of latent heat in steam that makes it so suitable 

 for conveying the heat to drying apparatus. After the 

 material has been placed in the vacuum-drying apparatus, 

 the air is removed by a pump, until a vacuum of about 

 28 in., or more, is maintained. When the moisture in 

 ithe material has been warmed up to only about 100 F., 

 the boiling-point is reached, and the moisture is quickly 

 evaporated. The greater the heat, the quicker is the 

 evaporation ; but the temperature of the moisture, and 

 that of the material being dried, is not increased, so long 

 as the vacuum is maintained and some moisture remains. 

 By observation glasses and other means it is known when 

 all the moisture has been evaporated, and the material 

 is then removed from the drying apparatus. 



Comparing vacuum and air drying apparatus, the saving 

 of heat is considerable in the vacuum apparatus. In the 

 case of the vacuum- drying apparatus, the wet material 

 and its moisture absorb the whole of the heat supplied. 

 The only additional heat required is that lost by radiation 

 and the warming up of the material and moisture to the 

 boiling evaporating temperature. Owing to the short 

 time required for drying under vacuum, these driers are 

 much smaller than air driers working under atmospheric 

 pressure, and therefore the surfaces exposed to cooling are 

 much less, and the loss by radiation is comparatively 

 small. 



Emil Passburg, of Berlin, was the manufacturer of the 

 first vacuum drier for bananas used in Jamaica, as long 



K 



