VEGETABLE AND FRUIT DEHYDRATION 145 



be removed but some other property of the material such as electrical 

 conductivity, dielectric loss, or vapor pressure is used as a measure of 

 moisture content. A vapor-pressure method has been described by 

 Makower and Myers (30a) . 



VACUUM-OVEN METHOD 



When applied to dehydrated products, the vacuum-oven method 

 effects substantial completion of the drying process begun in the cle- 

 hydrator. The sample of material is weighed before and after drying 

 and the moisture content is calculated from the loss in weight. In 

 actual practice the drying may not always be wholly completed in 

 the time allotted to it because of the slow rate of removal of water. 

 In some cases the loss in weight represents not only the loss of moisture 

 but also loss of other volatile materials either present before or formed 

 during the drying operation. 



For those reasons the vacuum-oven method is empirical and the re- 

 sults obtained with it are reproducible only when the adopted proced- 

 ure is strictly adhered to. The results are reproducible to within 

 ± 0.2 percent. 



Equipment : Analytical balance, vacuum oven, vacuum pump, closed-type 

 grinding equipment for dehydrated materials, such as a coffee mill, Waring or 

 Universal blender or a Wiley mill, hand meat grinder for raw undried materials, 

 one or more pyrex desiccators, a number of standard metal drying dishes with 

 tight-fitting lids (diameter 2 to 3 inches, depth % to 1 inch), and a 20-mesh sieve. 



Procedure : The carefully selected sample of dehydrated material (25 gm. or 

 more) is quickly transferred to the closed grinder and reduced to a powder 

 which will pass readily through a 20-mesh sieve. From the well-mixed powder, 

 duplicate samples of about 2 gm. each are accurately weighed into tared metal 

 dishes which are promptly placed in the vacuum oven in direct contact with the 

 metal shelf. With the lids "cocked" or removed the samples are dried for 

 exactly 6 hours at 70° C. under a pressure held below 100 mm. of mercury. A 

 stream of air, dried in a sulfuric acid tower (about 2 bubbles per second), is al- 

 lowed to pass through the oven during the whole period of drying. After 6 hours 

 the evacuation is stopped, the oven is brought to atmospheric pressure with dry 

 air, the lids are put on the dishes in the oven, and the dishes are then transferred 

 to desiccators charged with calcium chloride and are weighed after they have 

 reached room temperature. The net weight is taken to be the weight of the 

 moisture-free sample and the loss in weight as the moisture content of the sample. 



Undried samples are handled in the same manner except that grinding is best 

 carried out in a meat grinder, with care taken to avoid separation of expressed 

 juice from the solids. Gentle but thorough stirring immediately before weighing 

 to insure homogeneity of the sample is good practice. 



Careful attention to the following factors will make it easier to obtain re- 

 producible results : 



Size of particles : Grinding the whole sample to pass a 20-mesh sieve favors 

 reproducibility because small particles are more rapidly dried than large. 

 Thorough mixing of the ground material before the samples are weighed is im- 

 portant, since the drier pieces of vegetable tend to break up into smaller particles 

 than more moist pieces, and a separation into layers of coarser and finer par- 

 ticles may result in samples that vary considerably in moisture content. Most 

 if not all fruits and vegetables dried to low moisture levels are very hygroscopic 

 and must not be exposed to room air longer than is absolutely essential. Grind- 

 ing must therefore be carried out quickly in a closed mill. 



Temperature of drying: The temperature has been set at 70° C. At higher 

 temperatures the drying will be more rapid but with some materials results 

 will be erroneous because of decomposition and charring. 



Air pressure : Drying in vacuum is more rapid than at atmospheric pressure 

 and also protects the material from oxidation. However, in order to sweep out 

 the moisture and to supply heat to the sample, some air is allowed to circulate 

 through the oven. This air is first dried in the sulfuric acid towers to insure 

 efficient removal of water from the sanrples. 

 569074 — 44 X0 



