44 



AGRICULTURE HANDBOOK 134, U.S. DEPT. OF AGRICULTURE 



Figure 77. — Multiple-kettle method of making maple sir- 

 up. In this method, the sap was partly evaporated in 

 the first kettle, then transferred to the second and 

 third kettles, and finally to the fourth kettle, where 

 evaporation was completed. (Courtesy of W. W. Si- 

 monds, Pennsylvania State University.) 



l'N-1774 



Figure 7H. — The flat pan was the forerunner of the 

 modern flue pan. 



evaporation time, improved the quality of the 

 sirup, and lowered the cost of production. 



Design of Evaporator 



The modern flue-type evaporator, which oper- 

 ates under atmospheric pressure, consists basi- 

 cally of two sections: (1) The sap pan, in which 

 the flues are located, and (2) the sirup pan. The 

 sections are separated to facilitate their re- 

 moval from the arch for cleaning and repair. A 

 semirigid pipe or tubing connects the pans. The 

 connections tend to restrict the free movement 

 of sap as it travels through the evaporator and 

 minimize the intermixing of the dilute sap in 

 one pan with the more concentrated sap in the 

 adjacent pan. 



So that the evaporators can be operated in a 

 continuous or semicontinuous manner, baffles 

 or partitions are built in the pans to form 

 channels through which the sap flows as it is 

 being concentrated. The location of these parti- 

 tions and the size and shape of the channels 

 differ with different manufacturers. 



The sap pan can be made with narrow, deep 

 channels because the sap, while in this pan, is 

 never concentrated enough to become viscous; 

 it flows readily. Use of narrow flues increases 

 the heating surface and thereby increases 

 transfer of heat. Fresh sap is admitted to the 

 sap pan through a float valve that can be 

 adjusted to maintain the desired depth of liquid 

 in the evaporator (fig. 79). 



The sirup pan, often called the fi-ont pan, is 

 usually located over the firebox. Concentration 

 of the sap to sirup is completed in this pan. It 

 has a flat bottom to facilitate cleaning and to 

 permit evaporation of shallow layers of sirup 

 without danger of burning. 



Changes in Sap During Its Evaixji-ation 

 to Sirup 



Development of the desired maple flavor and 

 color is the result of chemical reactions that 

 occur while the sap is boiling in the evaporator. 

 (See p. 67.) The extent of these reactions is 

 determined in part by the length of time the 

 sap is boiled (HI). 



Chart 5 shows the effect of length of boiling 

 period on amount of color {150) produced in sap 

 of different solids concentrations (" Brix). At low 



