PRESERVATION OF FRUITS AND VEGETABLES 7 



dried foods, but light-colored fruits and vegetables did not. The 

 attempt to overcome this difficulty led to the introduction of blanch- 

 ing, or processing, and sulfuring. 



Blanching, or Processing 



The blanching, or processing, agent is usually steam or hot 

 water, which helps the product to retain its natural color. In the 

 steam treatment the material is subjected to live steam for the 

 required period. In blanching by hot water, the temperature is 

 maintained at 190° F. to the boiling point, depending on the material 

 being treated. As a rule, steam blanching is preferred to blanching 

 in liquid, because the loss of soluble constituents of the food is less, 

 a better flavored product results, and the use of steam is ordinarily 

 more convenient. 



Sulfuring 



Light-colored fruits (apricots, peaches, pears, and at times grapes 

 and figs) are sulfur ed in order to prevent discoloration during and 

 after drying and to facilitate drying. Sulfuring plasmolyzes the 

 cells and makes permeable the semipermeable cell membrane, thus 

 facilitating the diffusion of water from the interior to the surface. 



When the general plan of operation makes it desirable, the fruit 

 on trays is sulfured in an enclosed chamber, provided with an entrance 

 for the sulfur gas and an exit for a draft. The chamber is usually 

 large enough to hold one to two loaded trucks. Preferably the sulfur 

 is burned in shallow pans stacked one above the other in zigzag 

 formation. This method gives a large quantity of sulfur dioxide in 

 a comparatively short time. Sometimes a sulfur stove is placed 

 outside the chamber and the sulfur fumes are carried into the chamber 

 by flues. Sulfuring should always be as light as possible to accom- 

 plish the desired results. 



DRYING 

 Types of Driers 



The type of equipment best adapted to any particular use depends 

 upon several factors. If the products are to be dried for home or 

 farm use, then the equipment should be as simple as possible. Such 

 equipment and suitable methods are described in Farmers' Bulletin 

 984 (3). 



If the dried material is to be prepared in large quantities for sale 

 to the public, then the type of equipment will depend to a great extent 

 upon the nature of the product desired. To meet these needs, many 

 devices have been originated and patented, so that all phases of drying 

 are well covered. 4 



VACUUM DRIERS 



Materials can be dried more rapidly and at lower temperatures in 

 vacuum than at atmospheric pressure. Such foods as are extremely 

 susceptible to damage by heat are more safely dried in vacuum. 



However, vacuum driers are seldom used in large-scale operations 

 on the usual commercial grades of dehydrated fruits and vegetables. 

 The apparatus is too expensive and usually the advantages gained 



4 See list of United States Patents, p. 37. 



