As already indicated, the starch does not itself undergo alcoholic fermentation; it is first converted into maltose and other products by the action of the enzyme diastase (amylase). This diastase is usually supplied by malt - or, in the "amylo" process (described further on) by certain moulds.

The proper quantity of malt, amounting to 1 1/2-3 per cent, of the weight of potato used, is ground finely and added to the mash-tun, where the starchy mass from the converter has been cooled down to a temperature of 50-55°. The mash has usually a specific gravity of about 1.110, or, in the amylo-process, about 1.070. The temperature mentioned is the one most favourable for diastatic action, and the mass soon becomes more fluid by reason of the liquefaction of the starch. When a portion of the liquid, tested with iodine solution, no longer gives the blue colour of starch iodide, the process of conversion is complete. The time required for this may vary from one to three hours or more, according to circumstances.

1 Brauer und Malzer Kalendar, 1880.

2 J. Amer. Chem. Soc, 1917, 39, 742.

Usually the mash at this stage is heated to a temperature of 70-75° for a few minutes in order to destroy injurious bacteria with which the wort may have become infected during the course of saccharification. This is accomplished by reserving a part of the hot, gelatinised, starchy material in the converter, and allowing it to pass into the main bulk as soon as the conversion of the latter is completed. By the time the temperature of the whole has been raised sufficiently to destroy the micro-organisms, the starch of the fresh portion is saccharified.

Whilst this raising of the temperature destroys harmful bacteria, it is liable greatly to impair the diastase. This is a disadvantage because as long as diastase is present there is a further slow conversion of dextrins into maltose during the subsequent period of fermentation, and therefore to that extent a better yield of alcohol. To allow of this further conversion, one device is to raise the temperature, but not beyond 68°. Diastase in solutions containing much sugar can withstand this temperature without its activity being greatly impaired, although in water alone it is much weakened even at 63°. Another plan, now frequently adopted, is not to heat the mash to these high temperatures at all, but to add a small quantity of hydrofluoric acid or of ammonium fluoride during the fermentation. This antiseptic, in proportion up to about 0'2 per cent., is not toxic to the enzymes of yeast which has been "acclimatised" to it, whereas bacteria are destroyed at a far smaller concentration. Hence if this procedure is adopted, the saccharification can be effected at the most favourable temperature (50-55°), and the diastase retained in full vigour to play its further part in the secondary fermentation.

Whichever process is used, the mash, after saccharification is complete, is cooled down to about 20°, and passed through a centrifugal or other form of de-husking apparatus to remove solid matters. It is then ready to be "pitched" with yeast for the fermentation.

In this country potatoes are rarely used for making alcohol. Malt alone is employed in many distilleries, chiefly pot still distilleries which make whisky; but a mixture of malt and unmalted grain is usual in distilleries using patent stills. (Molasses is largely employed in the manufacture of industrial alcohol.)

The unmalted grain used is chiefly maize, with a relatively large proportion of malt, ranging from 10 to 25 per cent. A part of the maize may be replaced by barley, oats, rice, rye, sago, ox wheat, in various proportions up to 40 per cent, of the whole.

A preliminary partial conversion of the raw maize o:c rice is generally effected by treating the ground material (grist) with either a small quantity of green malt, or with sulphuric (or hydrochloric) acid. In the former case, the grist is mixed with three or four times its weight of warm water in the converter or "maize tun," and steam is passed in until the liquid is nearly at the boiling point in order to gelatinise the starch. The temperature is then reduced to 40°, and a quantity of green malt, about 4 per cent. of the grist by weight, is suspended in water and mixed with the starchy materials in the maize tun. After the action of the diastase has proceeded for an hour or so, the maize or rice mash is run into the mash-tun, in which malt and water have already been placed, together with any barley or oats that may be required. To facilitate drainage, oat-husks are often included in the mash.

Fig. 18. - mash tun, with steel's mashing machine.

For effecting a preliminary mashing of the grist between hopper and tun (Haslam Foundry Co., Ltd., Derby).

Matters are so arranged that when the mixture is made the temperature is about the optimum for diastatic action (50-55°), and not above 57° or 58° in any case. By means of stirrers fitted in the tun the mash is kept well mixed, and saccharification proceeds apace. Warmer water is then admitted until the temperature has risen to 63-65°, for the purpose of inhibiting bacterial development. After settling, the wort is run off, and the residual grains are extracted again with hot water; in some cases a third or even a fourth extraction is made. The weaker worts thus obtained are, after separation from the grains, used for mashing the next charge.

The saccharified mash is then cooled down and passed to the "wash-backs" for fermentation. In this country distillers usually arrange the materials of the wort so that the specific gravity of the latter before fermentation is about 1.030 to 1.040, or else 1045 to 1.055. If too much saccharine matter is present, the large proportion of alcohol formed tends to check fermentation before all the sugar is decomposed, thus causing a loss of material. (Special races of yeast, however, can be "acclimatised ' to withstand relatively high concentrations of alcohol, and these are used on the Continent, where the " thick mash ' system is practised.) On the other hand, too small a proportion of alcohol is uneconomical, by reason of the comparatively large expenditure of time and fuel required for the distillation. In distilleries where yeast is made, a low specific gravity of the wort is the rule.

Fig. 19. - mash tun with rake stirring gear (Blair, Campbell, and McLean).

Thick potato-mashes are common in Germany, because there is a tax levied which depends upon the capacity of the mash-tun, and the inducement therefore is to obtain the largest yield from a given tun. Towards the end of the fermentation, when the effervescence has moderated, water is added to dilute the mash. This lowers the alcoholic concentration, and also that of the carbon dioxide, thus allowing the yeast to complete the fermentation better.