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



20 40 60 80 100 



TIME (MINUTES) 



Chart 6. — Changes in Brix value, color, and pH in sap 

 during the evaporation period. A, Soon after evapora- 

 tion begins the sap becomes alkaline, reaching a pH of 

 8 to 9; it then decreases in alkalinity until at the end of 

 the period it is about neutral. Little color is produced 

 until after the sap reaches a pH of 8, at which point 

 color increases at a rapid rate. It increases further as 

 the concentration of the sap approaches that of fin- 

 ished sirup (30° Brix and above). B, Increase in Brix 

 value is slow at the beginning and becomes more rapid 

 as evaporation progresses. 



source is constant, the liquid in both the flue 

 and sirup pans is in a state of full boil, and the 

 sirup is being drawn off at a constant rate or at 

 regular intervals. The evaporation (holdup) 

 time can be lengthened by increasing the level 

 of liquid in the pans. The lowest depth of liquid 

 in the evaporator (both pans) will give the 

 shortest evaporation time. If the depth of liquid 

 is too low, the pans will bum, so this control is 

 limited. 



Liquid Level in Evaporator 



The depth of sap to maintain in the evapora- 

 tor is determined by a number of factors. Most 

 important is the minimum depth that must be 



maintained to keep the pans from burning. 

 Many sirupmakers find that a liquid level of 1 

 inch in the sirup pan is ideal. When the evapo- 

 rator is operating correctly with a steady 

 source of heat, there will be a slight gradient or 

 decline in the liquid level in the evaporator. The 

 highest level will be at the point of sap intake 

 and the lowest at the point of sirup drawoff. 

 With uneven firing, this gradient is upset. Dur- 

 ing periods of low heat, when the sap is merely 

 simmering, the gradient is lost. The depth of 

 the sap tends to become level, and there is an 

 intermixing of sap of different concentrations. 

 Intermixing, together with an increase in the 

 average depth of sap, results in a longer holdup 

 time and the production of darker sirup. The 

 lower the Brix value of the sap, the longer the 

 holdup time, since there must be greater gra- 

 dient in the sap levels. Since the minimum level 

 at the point of sirup drawoff is fixed to prevent 

 burning the pans, the level at the sap intake 



30 



60 90 



TIME (MINUTES) 



120 



150 



Chart 7. — The average time (time required to remove 50 

 percent of the water) that any lot of sap remains in the 

 sap pan (see dotted lines) is slightly less than 30 

 minutes. The time can be shortened or lengthened by 

 using sap of lower or higher solids concentration 

 (° Brix), by varying the depth of sap in the evaporator, 

 and by varying the intensity of the heat. 



