In addition to the surface-volume relationship,, there is evidence that the rate 

 of evaporation during vacuum-cooling is affected by the structure, tissue 

 density, thickness of skin, and waxiness of the different fruits and vegetables© 



Vacuum-Cooling o f Wet and Dry Produce Inasmuch as the temperature reduc- 

 tion during vacuum-cooling is the result of evaporation of water, some fruits 

 and vegetables have been moistened with water to determine whether lower- 

 temperatures would re suit o The wetting of celery, cranberries, grapes, and 

 mushrooms had little effect (table U), whereas the wetting of strawberries 

 caused a substantial cooling (figo !)• 



In tests to determine the effects of added moisture, celery, grapes^ and 

 strawberries were dry when removed from the vacuum chamber e We" mushrooms felt 

 slightly moister than dry mushrooms after vacuum-cooling, and in addition 

 developed a slight brownish discoloration « Friedman (?) found that wet spinach 

 remained wet after it had been vacuum-cools d for only "% minute s„ 



Vacuum-cooling of strawberries, and possibly other commodities, might be 

 practicable commercially if they were moist from rain, heavy dew, from being 

 washed, or from application of fungicides in water « Vacuum-cooling might serve 

 to dry moist commodities,, 



Wet- and Dry-bulb Temperature Readings During Vacuum Cycle , Vacuum-cooling 

 of fresh f raits and vegetables must be controlled accurately to avoid the 

 possibility of freezing „ Vacuum methods have been used commercially to freeze 

 shelled green peas (17) o Wet-bulb thermometers generally are used for controlling 

 the vacuum-cooling prccess« A dry-bulb thermometer alone would obviously be 

 unsatisfactory to indicate commodity temperature e When lettuce temperatures were 

 about 36° F and the wet-bulb air temperature 32®, the dry-bulb temperature was 

 Ii.9«5°o By means of wet- and dry-bulb thermocouples, air temperature readings 

 were taken during the vacuum-cooling of prepackaged vegetables (figo 2)o In 

 the first part of the cycle, when pressure apparently was still high and the air 

 was saturated with water vapor, wet- and dry-bulb temperatures were approximately 

 the same© When the main vacuum booster valve was opened 5 1/2 minutes after the 

 start of the vacuum cycle, the pressure dropped rapidly and the temperature 

 spread between the wet- and dry-bulb readings increased until by 12 minutes 

 after the start of the cycle there was a difference of 1? 1/2 degree So 



Relation of Amount of Vacuum and Tempe rattle o It was noted previously that 

 the temperature at which water boils is determined by pressure,, The lower the 

 pressure, the lower the boiling temperature e This principle was demonstrated 

 experimentally by vacuum-cooling spinach for 5 minutes at 2 different pressures, 

 namely at 3><»0 and U.£ mm„ of mercury o The pressures were measured by a McLeod 

 gage. At the start the temperature of the spinach was 80® Fo The temperature 

 of the spinach vacuum-cooled at 5o0 mm c was 39% while that at Iu£ mm was 32 r 



i ■ 



Effect of Initial Temperature on Cooling., To determine what effect the 

 initial temperature has upon the rate of cooling, warm and cool loads of celery 

 and lettuce were vacuum-cooled together „ The results (figs 3 and h) show that 



