acid from tank 206 to form an acidized mash the mash was preheated to 190 u00b0 f in preheater 250 and then heated to 340 u00b0 f at 150 psi in solar cooker 240 the solar heat input to the solar cooker was 4 860 btu hr the cooked mash from the solar cooker was cooled to about 210 u00b0 f in preheater 250 by indirect heat exchange with the acidized feed mash in line 217 the cooled mash passed from preheater 250 through line 255 and was mixed with 3 2 gal hr of a 5 calcium carbonate solution from line 219 to neutralize the mash the neutralized mash was cooled to about 90 u00b0 f in air cooler 230 and then mixed with 1 2 oz hr of 30 brewer 39 s yeast water solution in line 235 the mixture was passed to four tubular solar fermentors in series each having an inner metal tube diameter of about 4 inches and tube length of 40 feet the residence time of the fermenting mixture in the solar tubes was about 11 hours the heat input rate to the fermentor tubes augumented by parabolic reflectors was 3 360 btu hr and the temperature within the fermentor tubes was controlled to.