16 BULLETIN 1158, U. S. DEPARTMENT OF AGRICULTURE. 
cooking was given the potatoes. This softened them so that they 
could be easily mashed into a uniform pulp by the stirrer, thus 
breaking down the cells sufficiently to secure a good yield of sirup. 
Immediately after cooking, water of the same weight as the pota- 
toes was dumped into the tank and the stock was reduced to a pulp 
by stirring. With the stirrers still going, the temperature of the 
mash was raised to 140° F. by running in live steam. The tempera- 
ture was carefully determined by withdrawing large samples of the 
mash through the gate valve and testing by thermometer. Readings 
of the tank thermometers screwed into the sides of the mashing vat 
were not reliable. 8 When a temperature of 140° F. (the point at 
which the saccharifying action of malt is most active) had been 
reached a quantity of ground malt, equivalent to one-fifth of 1 per 
cent of the weight of potatoes taken, made into a thin mush with 
cold water, was added. The stirring was continued during the addi- 
tion of the malt and for a few minutes thereafter so that it would be 
thoroughly incorporated. 
The laboratory work had shown that one-fifth of 1 per cent of 
malt was sufficient for the conversion of the undigested starch still 
present after heating the sweet potatoes for 1J hours, in the case of 
several standard varieties, including Big Stem Jersey, and that one- 
tenth of 1 per cent was sufficient for the Porto Rico and Nancy Hall 
varieties. In the field work one-fifth of 1 per cent was used. Diges- 
tion of the starch was complete in from one-half to three-quarters 
of an hour, the end-point taken being when no color was given when 
a drop of a portion of the filtered liquid was added to a dilute solu- 
tion of iodin in potassium iodid. The mash was then ready for 
pressing. 
PRESSING. 
In pressing the hot pulp was made up in the form of layers from 
2 to 3 inches thick between racks and cloths. The press cloth was 
just large enough to inclose the pulp completely. 
The procedure in laying up the pulp was as follows: The press 
truck was wheeled under the quick-opening valve of the mash tank 
and a rack was placed on the pressure platform. On this the form 
was placed and over it was spread the press cloth, with its edges 
parallel to the adjacent sides of the form. The center portion of the 
cloth was then filled with hot pulp so as to fill the form, and the edges 
of the cloth were folded over, forming the first press cake. The 
form was then removed, a rack was placed on the press cake, and 
the form was placed on the rack. Another layer was then built. 
From 10 to 14 layers constituted a cheese. 
When loaded the truck was pushed into the press. Cooked sweet- 
potato pulp drains so readily that a large proportion of the liquid 
(technically termed " wort ") flowed out of the cheese before the appli- 
cation of any pressure except that caused by the weight of the layers 
of the cheese. As it flowed off the wort was collected in a galvanized 
iron tank on the floor between the mash tub and the press, from which 
8 This was due, in part at least, to the effect of the temperature of the tank walls on 
the armored thermometer bulb. The tank walls were thicker than was anticipated when 
the thermometers were ordered, and the bulb did not project far enough into the tank to 
eliminate the influence of the tank-wall temperatures. 
