40 



MISC. PUBLICATION" 540, U. S. DEPT. OF AGRICULTURE 



The batch-type steam blancher is usually a chamber in which trays 

 of cut vegetables are placed. After the door is closed, steam is ad- 

 mitted at various points along the box so as to fill the chamber com- 

 pletely, thus exposing the product to steam in motion. The resulting 

 washing action by the steam increases the rate of heat transfer from 

 the steam to the product. Entrapped air, being heavier than steam, 

 is discharged through ports located at or near the bottom of the 

 cabinet. Without these ports, air locks may form, which will prevent 

 effective and uniform blanching. 



The continuous atmospheric blancher usually consists of a tunnel 

 or large cabinet containing a wire or perforated-plate belt on which 

 the product is loaded. The tunnel is fitted with steam sprays both 

 above and below the belt throughout its entire length. Reinforcing 

 the steam supply at the initial phases of the blanching cycle is some- 

 times practiced, since the additional steam is useful in rapidly heat- 

 ing up the product to the blanching temperature. 



A modification of this form of blancher consists of a similar tunnel 

 fitted with conveyor chains upon which product-laden trays are placed 

 and conveyed through the tunnel (fig. 3). Usually the same trays 

 are used for both drying and blanching. This type of blancher is 

 particularly suitable for blanching cabbage and other leafy vegetables 

 which cannot be easily handled, once blanched. Inasmuch as tray 

 loading in this case is determined by drying rather than blanching 

 limitations, blanchers of this type must be of considerably greater 

 proportions than simple belt blanchers of equivalent capacity. The 

 repeated exposure of wood trays to blanching will result in rapid 

 deterioration. Trays that have absorbed moisture during the blanch- 

 ing operation naturally increase the drying load. 



It is desirable that the center or active section of all tunnel blanchers 

 be located at a higher elevation than either the entrance or discharge 

 ends so as to form a heat lock. Heat lock is caused by the difference 

 in density between steam and air at ordinary temperatures ; thus the 

 steam is trapped in the upper portion of the blancher (fig. 19). It 



Curtain 



"0 D 1 



Figube 19. — Hump-back blancher showing heat lock. 



is desirable to minimize the clearance between the tunnel walls and the 

 belt so that all of the steam admitted to the chamber comes into inti- 

 mate contact with the product being blanched. 



It is important that all air be kept out of a blancher. To prevent 

 suction of air into the blancher, steam jets should be positioned so 

 as to neutralize the kinetic energy of the jets. This is accomplished 

 by dividing the branch lines into pairs and drilling the steam orifices 

 so that the jets oppose each other. 



If steam is allowed to impinge directly on the product, furrows 

 will be cut in the bed, thereby increasing the depth of the bed between 

 furrows. This will retard the rate of blanching, inasmuch as blanch- 

 ing can be considered complete only when the innermost product in 

 the center of the bed is heated up to or above 190° F. 



