This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
It has been mentioned above that the preliminary conversion of maize or rice may be made with acid, as an alternative to using green malt. For this purpose, the ground grain is mixed with three to four times its weight of hot water, and dilute sulphuric acid added. The quantity of acid used is equivalent to 1 or 1 1/4 per cent, of strong sulphuric acid, calculated on the weight of the grist; or a corresponding amount of hydrochloric acid may be used. Steam is passed in until gelatinisation is effected, the mash being kept stirred during the process. The acid is then largely neutralised with milk of lime, and the liquid brought to the actual neutral point, or near it, by means of calcium carbonate. After being cooled down to about 63° with water, the gelatinised starchy product is run off into the mash tun, and the process finished as already described.
1 Trans. Chem. Soc, 1914,105, 1529.
In some Continental distilleries an essentially similar process is used, but the grain is heated under pressure, so as to give a more complete conversion, and in a shorter time. The maize is mixed with two and a half times its weight of hot water and steamed under a pressure of three atmospheres for two to three hours to effect gelatinisation. Strong hydrochloric acid is then added, in proportion equivalent to 2 1/2 per cent, of the weight of maize. After completion of the saccharification, which takes about half-an-hour, the wort is neutralised, cooled, strained, and run into the fermenting vats.
The main chemical reaction occurring in this process of acid saccharification is the hydrolysis of the starch, first to dextrins and maltose, and then to dextrose as the final product. It may be summarised in the equation: -
In practice, however, more or less of unchanged dextrins and maltose remain with the dextrose produced.
Solid "glucose" and glucose-syrups intended for fermentation purposes are also manufactured in a similar manner, namely, by converting the starch of maize, rice, etc., into sugar through the action of acids. The cereal substances are heated with water under a pressure of about three atmospheres, until the starchy material is converted into a paste. This is then mixed with the requisite quantity of sulphuric acid (about 3 per cent. of the weight of grain treated), and the heating continued until the conversion is complete. The acid is neutralised with chalk, and after settling, the clear liquid is removed from the precipitated calcium sulphate. The liquid, after being filtered through animal charcoal to decolorise it, is concentrated in vacuo, either far enough to solidify on cooling, or only sufficiently to yield a thick syrup, according to whether solid glucose or a glucose-syrup is required.
There is a good deal of variation in the composition of the products. Solid glucose generally contains about 60 to 80 per cent, of dextrose, a little maltose, and dextrins up to 10 per cent, or more; the syrups may contain about 50 to 60 per cent. of dextrose.
Fig. 20. - digester.
For saccharifying grain with acid under pressure (Egrot and Grang6, Paris).
Fig. 21. - battery of four diffusion vessels for extraction of sugar.