A notable feature of yeast is the large proportion of nitrogenous substances which it contains. The quantity varies a good deal, apparently according to the conditions of nutrition under which the yeast has been grown; but in general more than one-half of the dry substance consists of proteins and other nitrogenous bodies. The proximate constituents other than nitrogen compounds include glycogen, gum, mucilage, fat, resinous matters, and cellulose, together with a good proportion of mineral ingredients. As showing the average composition of dry yeast the following analysis may be given: -

Per cent.

Proteins, peptones, amides, etc. ....................................................

51.8

Gum and other carbohydrates...................................................

29.5

Fat.............................................................................................

1.0

Mineral matters.........................................................................

110

Cellulose and other constituents, by difference.........................

6.7

100.0

In pressed yeast - that is, yeast practically free from extraneous moisture - there is from 70 to 80 per cent, of water present, mainly in the cells. The following analysis of brewery yeast gives particulars of the soluble nitrogenous bodies present.2

1 H. Will, Zeitsch. qes. Brautv., 1913, 36, 576.

2 F. P. Siebel, J. Inst. Brewing. (Abst.), 1916, 22, 398.

Brewery yeast.

Per cent.

Soluble nitrogenous bodies.

Water...............................................................

76.41

Per cent.

=

Albumins ...........

0.16

Soluble nitrogenous bodies

1.12

Albumoses........

0.05

Peptones ..........

0.11

Insoluble ,, ,, . ...........

11.35

Amides...............

0.80

Mineral matters............................................

1.65

--------

Resinous matters.....................................

0.08

112

Fat, cellulose, etc. (difference)......

9.39

100.00

A number of ultimate analyses of the organic portion of dry yeast have been published: the average of several of these is subjoined: -

Per cent.

Carbon ................................................................

48.3

Hydrogen............................................................

6.0

Nitrogen...............................................................

10.6

Oxygen (including a little sulphur and phosphorus).....................................................

34.5

100.0

The inorganic constituents of yeast consist almost wholly of phosphates. Of these, potassium phosphate forms by far the largest part, the remainder being made up of magnesium and calcium phosphates, which, with a little sulphate and silicate and a small quantity of iron compounds, practically complete the tale of mineral ingredients. According to Lintner, the average composition of the ash left on incinerating yeast is as follows: -

Per cent.

Potassium oxide (K2O) (including a little sodium oxide)...................

33.5

Magnesia (MgO) .........................................................................

61

Lime (CaO)........................................................................

5.5

Iron oxide (Fe2O3)..........................................................................

0.5

Phosphoric acid (P2O5)........................................................................

50.6

Sulphuric acid (SO3)........................................................................

0.6

Silica (SiO2) .................................................................... ................

1.3

Ingredients not determined....................................................................

1.9

100.0

It may be gathered from these analyses that yeast requires for its growth a supply of both organic and inorganic nutriment, especially nitrogen- and potassium-compounds, and phosphorus. Moreover, as already seen, it contains no chlorophyll, and therefore must obtain its carbon in some soluble and assimilable form, since it cannot get it from carbon dioxide like green plants do. As regards the nitrogen, this can be supplied by ammonium sulphate, fluoride, or other ammonium salt; but it is found that yeast grows more actively if its nitrogenous nutriment is furnished in an organic form - e.g., as amino-compounds, amides, and peptones. Now these are precisely the substances which, as we have seen in connection with the germination of malt, are produced by the action of proteolytic enzymes upon the insoluble proteins of malt and other grain. In fact, malt wort, containing as it does these assimilable nitrogen compounds, soluble carbon nutriment in the form of sugar, and phosphates, with potassium, magnesium, and calcium salts, is a very excellent medium for the nourishment of yeast. Much the same can be said of grape juice also. Occasionally, however, and especially when a large proportion of brewing-sugar or of unmalted grain is used in making the wort, there is a deficiency of suitable nutriment; and this deficiency is corrected by adding special preparations of "yeast foods."

Yeast can be "acclimatised" to the presence of antiseptics such as sulphurous acid and hydrofluoric acid, and can be cultivated to withstand the effects of these up to a certain degree. Advantage is taken of this property in helping to keep distillery "wash" free from bacteria and wild yeasts, which are much more susceptible to the action of antiseptics. Distillery yeasts can be accustomed to grow actively in the presence of hydrofluoric acid or ammonium fluoride to the extent of 02 per cent, of the fermenting liquid - a proportion which is quite effective in destroying bacteria. Also a slightly acid "wash" encourages the development of distillery yeasts, but is unfavourable to bacteria and brewery yeasts; hence it is common in distillery practice, even where hydrofluoric acid is not used, to acidify the wash with lactic acid or sulphuric acid. Further, distillery yeasts develop best at a temperature (24-27°) much higher than that most favourable to brewery yeasts (about 156°).

By successive culture in worts of gradually increasing strength yeast can, up to a certain point, be accustomed to tolerate more and more alcohol, and its ability to ferment more concentrated worts is thus developed. This property is applied in the method of fermentation known as the "Molhant" process, used abroad in the distillation of molasses.

The zymase activity of yeast, whereby the hexose sugars (dextrose, lævulose) are converted into alcohol, can be completely suppressed by means of toluene, without preventing the action of the maltase. It is thus possible to investigate the maltase activity of yeast without the complications arising from simultaneous fermentation produced by the zymase. Schonfeld and Krumhaar found in the course of such studies that by adding toluene in the proportion of 8 per cent, of the volume of the liquid, the complete suppression of fermentation could be ensured.1

Antiseptics in general will prevent fermentation when present in relatively large amount; but many, on the other hand, have a stimulating effect if present in only minute proportion. Thus mercuric chloride at a concentration of more than 1 in 20,000 inhibits fermentative action, but at 1 in 300,000 or less it has a favourable influence. Salicylic acid in the proportion of more than 0 1 per cent, has a deterrent effect, whereas less than one-sixth of this amount acts as a stimulant. Phenol is inhibitive at a concentration of more than 0.5 per cent., and stimulative at less than 0 1 per cent. Minute amounts of other toxic substances, such as strychnine, nicotine, and carbon disulphide, have also been found to favour fermentation.

1 Wochensch. Brau., 1917, 34, 157; J. Inst. Brewing, 1918, 24, 36..