The Nutrition of Yeast

ON THURSDAY, APRIL lira, 1907.

Mr. A. L. Lees in the Chair.

A paper was read by Mr. C. H. Field, of which the following is an abstract, discussion ensuing.

The Nutrition of Yeast
by C. H. Field, F.I.C.

It having been established in practice that if a yeast consist of several varieties, the one or other will predominate according to the composition of the wort or other nutrient in which it is cultivated, the author points out the possibility that yeast may from the same cause undergo definite changes in type.  A high grade well-manufactured malt, in which the carbohydrates bear the correct ratio to the nitrogenous matter, contains the requisite food for the yeast.  But since few malts fulfil these requirements, some auxiliary food is, not a rule, necessary, and the author maintains that this latter should be specially chosen according to the particular race which it is desired to cultivate.  He points out that a yeast food which may be used by distillers can be obtained by treating protein matter at a blood heat with acid, whereas similar treatment under pressure is necessary in order to produce a yeast food for brewers’ purposes.  In the former case he contends that a product is obtained suitable for the nutrition of various wild yeasts, which, according to him, unlike brewery yeasts, have the power of breaking down comparatively complex nitrogenous compounds; consequently, he states, brewery yeasts require substances intermediate between peptones and amides.  Further, he contends that provided the yeast be properly fed it does not follow that a beer produced from it will develop acidity even if the yeast is contaminated with lactic acid and acetic acid bacteria.  Indeed, even when a pure culture of yeast is employed and all precautions are taken to sterilize the plant and filtered air only is allowed to come in contact with the wort or beer, if materials below average quality are employed the beer may be defective.

A good yeast food for brewers’ purposes may, according to the author, be made by “peptonising” malt combs or bean flour.

Dealing with the occurrence of certain enzymes in the Saccharomycetes and their specific functions, the author discusses the discovery of zymase by E. Buchner, and the researches of Hayduck, as well as the more recent ones of Delbrück and his collaborators.

He points out the necessity of securing an ample amount of assimilable nitrogenous matter in the wort in order to produce a healthy yeast which, when sufficient carbohydrate matter of the right kind is present, will so purge the wort that the resulting beer contains the minimal quantity of putrescible matter.  He adds that too large a proportion of sugar to nitrogenous nutrient leads to an increase in the number of yeast cells, accompanied, however, by a degeneration of the yeast.  The author further states that any aeration at the beginning of the primary fermentation is beneficial up to the time of the yeasty head formation, as it furnishes the necessary oxygen and dispels the carbonic acid gas, which checks the attenuation.  Also any substances, such as brewers’ grains, by their buoyancy agitating the fluid, will dispel carbonic acid gas and so increase fermentation.  He remarks that some brewers have noticed a lower attenuation in a copper-lined vessel than in a wooden vessel.

The author quotes some experiments made to discover why frequent changes of yeast were required in a certain brewery:—

(1) The yeast was first stimulated by mixing un-boiled sweet wort at a temperature of 70″ F. to produce excessive budding and in consequence a lower percentage of nitrogenous contents of the cell.  The percentage of nitrogenous matter in the original yeast was 9•888 and after fermentation 9•262.  The yeast in this beer did not deposit readily.

(2) The same experiment was conducted, but aeration was resorted to with a result that the yeast readily deposited with spontaneous clarification of the wort; this increased the outcrop considerably and in consequence the nitrogenous contents of the cell.

(3) A further experiment was tried by stimulating with raw sugar, and the addition of acid phosphate of potassium and magnesium sulphate.  The reduction of the nitrogenous matter in this case was very great.  The resulting beer was excellent and the break all that could be desired.  He deduces from this particular instance that the reason which necessitated frequent changes of yeast was the excessive accumulation of nitrogenous matter, which latter was not of a sufficiently assimilable character, combined with insufficient aeration.  The washing of yeast with water at 34°—38° F. which is adopted on the Continent, is not much practiced in this country.  The author states, however, that he finds from his experiments that it is of benefit as it eliminates the weak cells, bacteria, etc.  Further, the storage of yeast at similar low temperatures is to be recommended.

Some brewers consider that unless they obtain a low attenuation, the beers will not keep, but on this point the author remarks that if a healthy strong yeast, well fed, be employed, attenuation is not of such importance provided it be got a little below one-third.  However, he points out that with a weak yeast, although the ratio of maltose to non-maltose may be correct, this will vary considerably in the finished beer, in addition to which such a yeast will disperse its contents throughout the liquid, conducing to fret, acidity, etc.


Mr. T. Hyde asked if it was not a fact that the amount of yeast added for pitching purposes was a great factor in producing a healthy outcrop.  That had been his experience.  If too large a quantity was used, the effect would be to deteriorate the yeast through lack of nourishment.  He also asked if the use of hardening liquor had not something to do with the deterioration of yeast?  In the Manchester District they were accustomed to the old type of yeast, which had always been used with a soft water.  He would also like to know if Mr. Field could explain the old custom that still existed in these parts of boiling yeast in the copper for a yeast food.

Mr. Field replied that with regard to the quantity of yeast used for pitching, the outcrop would depend a great deal upon this, providing they know they had to use a certain quantity to perform a certain work in a certain period.  If that given quantity was not used, his experience was that provided there was sufficient nutriment in the wort they would achieve the same thing, although a larger time would be occupied.  The quantity would have to depend upon the quality of the yeast used in pitching.  Some brewers were only using 2 oz. per barrel, while other brewers were using 1½ lb. to keep their attenuation down.  The brewery using the latter quantity was a large one, using high grade malts, and a large amount of yeast food besides.  The yeast itself was always the best that could be obtained.  The yeast could always be depended upon, and a large profit was derived from it.  As he had said, the quantity of yeast used for pitching must depend upon its quality, or rather the strength and power of the yeast.  In the case referred to the yeast was highly fed, and did not attenuate very low.  As to the use of hardenings, this was a big subject, and, without an analysis of the water, it was almost impossible to give the explanation asked by Mr. Hyde.  His experience was that with a soft water, soda and potassium salts of that class did not assist the yeast.  But he preferred a hard water, and to boil it to get rid of the lime, and then use the yeast food. The practice of boiling yeast in the copper was adopted by many.  If they boiled the yeast first and then peptonised it they got better results than by using the yeast simply boiled.

Mr. Glendinning considered that the keynote of Mr. Field’s paper lay in the distinction he drew between digestible and non-digestible forms of nitrogenous matter.  At present they could only say that certain forms of nitrogenous matter were digested by yeast, and that others were not.  He thought Mr. Field recognised that point.  In connection with the separation of these nitrogenous substances, a step forward had been taken by the Guinness’ Research Laboratory, where some very able work had been done, but even this only touched the fringe of the subject.  He was rather struck with the remark that potassium salts had a tendency to produce ropiness.  Ropiness was one of the things that was puzzling to brewers, but he was not aware that potassium could be associated with it.

Mr. Field replying, said as to the nitrogenous matter required by the yeast, chemistry taught them that when nitrogenous matter was peptonised in the ordinary way it gave excellent results.  With regard to potassium salts ho did not say that potassium caused ropiness, but that it was invariably associated with it.  If the brewery were thoroughly cleansed out, the yeast changed, and the use of potassium salts reduced, he thought that ropiness might be got rid of.

Dr. A. K. Miller asked how Mr. Field proposed to get rid of the potassium salts which were necessarily present in malt?  As to ropiness, they did not suffer much from this trouble in the Manchester District, but  it did occur occasionally, and in all cases that he had come across, the cause had been fairly easily traceable to external contamination.

The Author said that the potassium salts were gathered in the brewing process from malts, and from certain very low-class sugars.  Some brewers used kainit, and it was in a reduction of sugar and kainit that they would have to get rid of the salts.  He knew of no process of treating them, except very expensive ones.

A Member asked Mr. Field’s opinion as to the part that sulphur played in the nutrition of yeast?

Mr. Field said that magnesia and sulphites seemed to be beneficial to the yeast within certain limits, but they purged the yeast somewhat, and must not be used in excess. Much of these acids was not necessary.  Lime and phosphates seemed as beneficial as anything; and, generally, sufficient sulphate was obtained from the water without any addition.  If there had to any addition only a small quantity was needed in the form of hardening.  All the salts seemed to have a tendency to let the yeast run sloppy, but, of course, a certain quantity of them must be used. The Author observed that if yeast was fed, and made strong and healthy, the fermentations would not be attenuated so low as would be the case with a strong fat yeast.  An important point in brewing was to get the right proportion of maltose to non-maltose in the mash-tun.  If they used a weak yeast this proportion would vary considerably; the attenuations would run down, and they would have a drifting residual extract.  With regard to the strength of yeast, he would point out that the distillers’ yeast was of a hardier type than that of the brewers, and did not contain so much nitrogenous matter as the brewers’ yeast, nor would it give off so much carbonic-acid gas in the same period; but it would digest and ferment sugars and nitrogenous matters that brewers’ yeast would not do.

Mr. A. E. Taylor, referring to the question of ropiness, observed that this was attributed to the presence of potassium salts.  He had used a sugar for priming purposes, which after being boiled and diluted with water in the ordinary way to a gravity of 1150, was found to be distinctly ropy within 24 hours.  Presuming it to be pure cane sugar, would Mr. Field attribute the ropiness to an excess of potassium salts or to the presence of a mixture of cane and beet sugar?

Mr. Field replied that he had known of many cases whore beers had turned ropy, and he had always attributed this to salts present in the sugar used.  The more malt was grown the more food it contained, and the better would the yeast be.  Answering another question, he said overfed yeast would not, as a rule, run the fermentations down so low as would a weak yeast.  In reply to a Member as to a remedy for overfed yeast, Mr. Field recommended the use of more sugar.  To produce a greater iteration they would produce a greater outcrop of yeast, and the consequence would be that the yeast would be proportionately leaner.

At the conclusion of the discussion the Chairman proposed a vote of thanks to Mr. Field for his paper, which was seconded by Mr. T. Hyde, and carried unanimously.

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