VACUUM-COOLING FRESH VEGETABLES AND FRUITS 



By 



B. A. Friedman, senior plant pathologist,, and 

 W. A. Radspinner, associate horticulturist 

 Biological Sciences Branch 

 Agricultural Marketing Service 

 New York, N. Y. 



Vacuum-cooling for agricultural produce was used commercially for the 

 first time in 19U8 when 3U carloads of lettuce were cooled by this method at 

 Salinas, Calif. Since then development has been so rapid that it was estimated 

 that about lj.0 to 85 percent of the lettuce crop of California and Arizona were 

 vacuum-cooled in 195>U (h, lk) ±' Vacuum-coolers for lettuce have been built in 

 Texas, Wisconsin, and Canada. A similar installation for the precooling of 

 prepackaged vegetables has been in use since 1°50 in New Jersey (£)• 



The purpose of this report is to summarize results of tests on vacuum- 

 cooling of vegetables and fruits to date* 



PRINCIPLES OF VACUUM-COOLING 



Evaporative cooling was utilized early in the history of man to cool water 

 as in earthenware jugs. Cooling towers and spray ponds are later applications 

 of the same principle. At normal atmospheric pressure (29.92 inches or 760 mm. 

 of mercury) water boils at 212 B F. As the pressure is reduced, however, the 

 boiling point of water is reduced until at a partial pressure of 0.18 inches 

 (U.6 mm<>) of mercury, water boils at 32° (15). The water which evaporates from 

 fresh fruits and vegetables cools the commodity to a temperature corresponding 

 to the evaporating temperature of water 'at the reduced pressure or vacuum 

 attained (16 ) • 



In commercial practice, air is pumped out of the vacuum chamber either by 

 a multi-stage steam ejector system -and barometric condenser, or by electrically 

 driven mechanical pumps connected with refrigerated condensers (18)« 



TEMPERATURE CHANGES DURING VACUUM-COOLING 



Relation of Ratio of Surface to Volume of .Produce . The results obtained 

 by the vacuum-cooling of a number of fruits and vegetables show that leafy 

 vegetables, which have a large surface area for evaporation in proportion to 

 their volume, are rapidly cooled (table l). Commodities which have a somewhat 

 smaller ratio of surface area to volume are generally less readily vacuum-cooled 

 (table 2)j fruits and vegetables which have a small evaporating surface in 

 proportion to their volume are usually poorly cooled by vacuum methods (table 3)« 



l/ Underscored figures in parentheses refer to Literature Cited, page 11. 



