Yeast and Brewing Fermentations


The following paper wns read and discusscd :-

Yeast and Brewing Fermentations.


In speaking of yeast, I prefer to deal more with the animal nature of its life than vegetable, since many explanations become thus more simple.  C. G. Mathews refers to yeast life as being half animal and half plant life.  Bearing in mind that in a fair average sample of yeast, even if it amounts to only one ounce in weight, we are dealing with some five million cells, all definite changes observed on the larger scale in yeast must be of the nature of an average, and therefore true, and not of a haphazard nature.  Since, therefore, the generative period of yeast varies from 10 hours (at 56o F.) to 64 hours (at 82o F.), variation in a Darwinian sense is attained in a comparatively short period, owing to the enormous number of successive generations producible within, say, a brewing season.  Yeast multiplies as we all know by budding (germination) or by the formation of acrospores.  Acrospores are only formed however when the yeast dries up, and hence this means of multiplication is of little interest to English brewers; but in hot countries, such as India, it proves to be the only available method of preserving yeast during the non-brewing (summer) weather.  In this latter case yeast may be mixed with plaster of Paris and allowed to set and dry, the cakes being subsequently ground up and used for pitching.  It is perhaps preferable to press into the form of bricks with rice powder.  These methods remind us of the fact that such dry spores retain their vitality for at least 12 months.

In differentiating yeasts, we must not place too great reliance upon form, since this depends more or less upon such factors as food, aeration, &c.  In extreme cases (such as growth in pure cane-sugar solutions) the form may, in fact, vary very appreciably and become quite unrecognisable as the same organism.  The mean specific gravity of yeast is 1509, or half as heavy again as water and considerably heavier than our heaviest worts. This is interesting when we ask how it is that yeast rises in a liquid lighter bulk for bulk than itself, more especially when we remember that the “apparent” air-bubble or vacuole in the cell is filled with liquid, and therefore not a factor of buoyancy.

In no one organism, whether large or small, can we reasonably expect fulfillment of every good character from one narrow artificial standpoint of our own: hence to expect from a one-cell or absolutely pure yeast of only one species defined (ns best we know how) both a lively primary fermentation, satisfactory secondary fermentation, good multiplication, good and clear beer, which will keep – for that it must if free from false germs – is toask much, yet it has been stated to be attainable.  I cannot, however, endorse this from any experience of my own, although it is the desideratum of all brewers.  Jürgensen finds a one cell yeast in course of time to differentiate itself into a mixture of races of one species (with the above proviso).  This may and probably does happen to every species, and therefore to every mixture of species or composite or ordinary brewery yeast and must therefore be always gradually changing.

In buying mixed yeasts which may be assumed to be free from false ferments, and starting to develop from these one constantly varying brewers’ yeast, we shall be doing just the same if we gave one soil and one treatment to a large sowing of mixed–any grass–seeds.  Those to which the conditions are suitable will flourish, others will be stifled out or fail to “take”, hence we begin at once with “reselection.”

In this lies the secret of a good brewer’s success.  No plant, materials, water, and yeast are always perfect, and yet, unlike wine, we have to produce uniformity with variable materials and conditions (especially of temperature).  Until a brewer has found out practically, that treatment which, with his conditions, can give him the result he requires, he may with his materials turn out n good beer spasmodically but not regularly.  Hence the first thing to do is to find n yeast that can do this, and continue to do so, not relapse into something else (unsatisfactory re-selection), but maintain the same racial distinction without specially viable species dying out or becoming super-abundant.  It is impossible for any brewer to tell another where to get this, but once having found it, and proved that it will remain immutable, make the most of it; it is the most valuable possession any brewery can own (as every brewer who has lost it finds to his cost, perhaps for years afterwards), and very few possess it.  In other words, very few can do entirely without a change of yeast.  With a good yeast, materials may vary largely, yet the beers remain good, but by no means vice versa.

It is a well-known fact that with an inferior grape juice pitched with the best champagne yeast, a champagne flavor is imparted to the finished product.  Most subtle aromas of beer are due to specific races of yeast; when these are very pronounced, and thereby easily separated, we get to know the character of the pure race, such as that of Saccharomyces Feotidus, that is, the yeast yielding the “Burton stench”, so long erroneously put down to the reduction of the richly sulphated water to sulphides, and the consequent setting free of the sulphuretted hydrogen.

In the above we have referred generally to yeast simple and composite; let us now turn to the more interesting question still of yeast in health and disease.  With the latter I do not intend to link false ferments, except in so far as specific mention is concerned.  Taking yeast in rotation from its youngest stage we have, as exemplified in fermentations, the so-called ’‘heads” – the preliminary cap with its raised small solid particles, the curly head passing into bold rocky, the loose fluffy head and heavy yeasty head, the  top yeast (skimmed), the bottom (left in the squares), and that finally produced and left in the barrels.

This order of heads should be marked and distinct.  If the rocky falls or fails soon, it is a sign of weakness, and although thorough rousing will aid it, probably a poor pitching yeast (and little of it) will follow.

If the curly head fails, we shall, if our water be free from sulphate of lime, probably have a “boiler”, in which large blisters rise to the surface giving n boiling appearance, and bursting with considerable noise; little or no yeast will rise, and hence, no pitching yeast is obtained, but a large amount of bottom yeast be produced.  Rise of temperature with dressing of powdered gypsum, with much rousing, cures this at once, but this must be done in time before the beer is spoilt.  The remedy is obvious; put gypsum in the brewing water.

Sometimes yeast multiplies abnormally, 20 or 30 fold; such yeast soon breaks down and liquefies; the beer brightens at once, followed by a rapid after-fermentation, the resulting beer remaining flat and thin, accompanied by most curious flavonr at times: that of onions, &c.  I have experienced this with 7 lbs. pitched and 230 lbs. pressed got off.  More aeration and sugar (one-third) cures this in a few brews, and cures the “disease” the yeast itself when healthy, be it remembered, not being a fault.

During fermentation we have various stages to consider; that in which the past absorbs its nitrogeneous food, breaks down the sugars and multiplies as well as separates.  Hence we distinguish between the failure of an early head (which may be due merely to a cold draught overhead), or one later on, especially at skimming time. 

Yeast as obtained from n brew depends also much upon the amount used at pitching time. There is one correct amount only, which any known bulk of wort of its own particular condition requires, to produce the strongest output.  Here quantity does not come into play, since double the yeast and you halve its food and halve the work to be done, lessen it and more food remains in the wort capable of – and which will produce a larger generation of new cells; remembering, however, that the wort is most susceptible of bacterial attack before the yoast has “taken”, there is here the practical dangerous limit.  Curiously enough in point of time there is little or no difference in arriving at the same find result.

Now in regard to yeast, especially pitching yeast, besides the microscopical appearance, we have to take into account its appearance in mass, that is, macroscopically.

(1) It should possess a glutinous character.  A watery yeast which does not hang together well from the finger lacks constitution and power to resist liquefaction and auto-fermentation.

(2) It should not be frothy.

(3) Its smell should be agreeable and fresh, this depending much on the races present, and its freshness and food.

(4) Pure yeast is practically tasteless; bitterness is often merely due to hop resin.

The effect of good yeast in fermentation is to definitely rest after the primary fermentation, hence the final India rubber-like head should show no signs of working locally; if the latter we may expect the same restless fretting in the cask, and cloudy beer produced by constant movement.

One word as to bacterial contamination.  Here we have to protect a tender organism (yeast) against a hardier in admixture, which is obviously very difficult.  Bacteria, when present with yeast, may have to struggle for the same food; the former may alter the yeast food, or render it un-assimilable or deleterious.  The yeast cells, themselves may be attacked by the bacteria.

Variations in the constitution of the water used in brewing yield very different results in their effect upon the yeast.  Thus we have seen the effect of lack of lime and sulphuric acid, a very soft water producing in time upon any initially tenacious good yeast a variety which has very little resistance towards breaking down, this being especially noticeable when carbonate of soda is present in an otherwise nearly pure water, ie., where soluble mineral constituents are nearly absent, the pasty head in this case being very heavy, and apparently everything that could be desired, yet falling down to nothing in the storage tubs.   Again, excess of common salt (NaCl) requires very heavy amounts of pitching yeast, showing its retarding effect upon the fermenting powers of yeast.  Soft waters also give darker beers. Of course highly contaminated water would naturally be condemned, but, beyond bacterial effects, I have not observed any marked effect from the presence of comparatively large, although infinitesimal, amounts of so-called albuminoid nitrogen.

There is a long list of substances which have been used in experimenting with yeast from a feeding point of view, and which it is not my intention to recapitulate: I shall limit my remarks to the most markedly useful knowledge we possess upon the subject, with such points as can be made use of practically in brewing.  Asparagin (naturally present in wort) is accredited with being the best of that class of foods (termed amides) for which yeast shows the most marked preference.  Its presence in large quantities gives rise to a special and peculiar stench during the fermentation (Stern).  Although I have added this body in a pure state to one square, I have been unable to detect any practical brewery improvement. Another peculiar flavour given is that of dough (by alloxan).  Ammonium salts are absorbed readily, especially such as contain organic acids.  Amides disappear before peptones: amides are absorbed at the higher temperatures preferably, the peptones at a lower.  Amides are credited with yielding a lighter yeast, more difficult of settlement, peptones a heavy variety.  Glycerin can be absorbed as a food by yeast: this is very curious, since the excreta of all animals are un-absorbable, and glycerin is always stated to be a product of fermentation.  This fact might point perhaps to its being a side issue of some other chemical reaction.  The higher the percentage of nitrogen the less glycerin is produced.  When we place yeast in the copper and boil it up with our wort the substances extracted form but an indifferent food: note that for plants a decoction of their juices in no case forms a food, the mineral constituents which are soluble would of course give their own quota to the result.

The lower the temperature at which we ferment the higher the percentage of nitrogen, if we aerate much, we lower this.  Yeast besides fermenting really also feeds on maltose, producing the body corresponding to our fat or reserve material, in this case a body like dextrin called glycogen.  Again salicin is assimilated; it puzzles me rather to know here what becomes of its strongly antiseptic radical, salicyl.  It is, from a food point of view, most important to pitch with such a quantity as to offer the proper amount of food for the pitching.  We may add such an excess of yeast as to have only enough food to keep this going, we then get little or no multiplication, but the yeast may raise its percentage of nitrogen by 2 per cent.  This brings me to the fact that yeast containing high percentages of nitrogen is sluggish, a large excess of sugar will lower this again and improve the quality of the yeast.  Sugar means having to do work; too much nitrogen means over-feeding, with ‘ humanly” analogous results.  Again non-multiplying yeast loses weight in fermentation.

Aeration produces movement, new unexhausted surroundings, and fresh food for every cell.  It increases multiplication and diminishes fermentative power, rapid fermentation is brought to a close by aeration, the final being higher in yeast liable to fret.  The limits of multiplication are enormous, Pasteur’s limits being 4 and 176.  More sugar is fermented in presence of less oxygen (Duclaux), and more alcohol produced, eg., old beer and wine in full casks (lower finals from want of rousing), .in contact with air much sugar gets broken-down directly to carbonic acid and water (that is loss!) (Duclaux).  This seems to me to be one of the most remarkable statements in brewing; it means that there exist conditions under which we may lose a certain (unknown) proportion of our sugars absolutely and get nothing for it, not even (preservative) alcohol, treatment of which would enable us to attain the same end with a loss gravity of original wort:  just at this time this is a most important question, viz, How much do we lose now?  How can we prevent it?

Again too little aeration produces bitterness (lasting and unpleasant) and bitterness in the yeast.

The higher the temperature of fermentation the more air we require.  In the manufacture of “press-hefe”, a current of air is driven over the yeast at between 80o and 90o F.

If we have two squares of similar wort and one pitched with more yeast than the other, we find in practice the former starts faster, slackens sooner, and  both finish together, at approximately the same final weight; evidently in both cases leaving a similar liquid immune to future attacks following directly after with its own produced yeast; but the former will require more aeration for good separation.

Conditions of pressure may vary from a few inches, as in the Pfaudler system, to 300 lb. per square inch or over. I have, through the kindness of my firm, been able to do the latter in a special copper apparatus, and I had to case the pressure, at 200 lbs., showing under what strain a delicate organism such as yeast can work.  There was no aeration beyond the air in the apparatus naturally.  This we are accustomed to in the secondary fermentation in barrels, the power requisite to blow out the ends being very great.

Exposure to too low a temperature for a length of time spoils the fermentative power of yeast (say 30—40o F.); its absolute vitality at really low temperatures is marvelous.

I have drawn attention to a peculiar fermentation by yeast, that when mixed with dry powdered cane-sugar in which at first liquefaction occurs and we can readily get a filtrate, later on most violent fermentation takes place, with great rise of temperature but no filtrate; the same occurs in a dry-glycerin solution of either cane-sugar or glucose (both being very soluble at high temperatures), showing that glycerin (an excreta?) does not hinder fermentation, nor is the presence of extraneous water required.

In all the above we must not forget that we vary our food (wort), according to our requirements, for stout, pale ale, stock or running ales, &c.

Some chemical notes on yeast of especially great interest and practical importance I append, for the more scientifically-inclined minds.  Yeast respires carbonic acid from its surface when not fermenting.  This may be looked upon partly as a non-quiescence of action—a residual effect; when violent we look upon it as a self-fermentation and breaking down of its tissues.

The oxygen absorbed by yeast cells is put down to a reducing substance which reduces iodine and sulphur (Wroblowsky).  De Rey Pailbade recognises a body—“philothion”–which can hydrogenise sulphur itself; which can easily be proved by adding flowers of sulphur to yeast and covering with filter paper soaked in acetate of lead, which soon blackens (PbS).

The browning of yeast by air has been ascribed to an oxygen absorbing enzyme culled oxydase (as in cut apples exposed to. the air). 

The bitter principles of hops only slacken, but do not limit, the action of yeast. 

Wroblowsky finds two enzymes in yeast, one hydrolysing glycogen, the other starch (the latter being very weak in action); we should expect the former to act preliminarily to the self-fermentation of yeast, and which with Buchner’s zymase would explain this.  A third hydrolyses maltose and is called maltase, forming the glucose for our yeast to ferment. Formic acid (Nature’s honey preservative) is also present in yeast.  Buchner found that zymase appeared to be destroyed in its action by a peptonising ferment; this requires explanation and good proof in face of the fact that pancreas stimulates fermentation (zymase being the enzyme which in solution produces fermentation directly, and is obtained by mere pressure and filtration from the yeast-cell juice.)  We must bear in mind that there seems to exist proof that vegetable albuminoids require “vegetable pancreas or pepsins” to convert them–such as that of the action ascribed to peptone.  In our blood we possess white corpuscles which can attack bacteria in the blood directly, and thus form our “body-guard”: yeast contains nuclein (in combination), nuclein solution itself possesses similar strong germicidal powers, hence it is very analogous and “animal like” in a similar way.

That shall we have to be very careful in distinguishing separate entities from altered reactions among enzymes, is seen in the fact that although whole yeast does not reduce maltose to glucose, its dry extract does; again, dry yeast liquefies starch paste, producing dextrose from a starch conversion in which all soluble bodies have been separated by 80 per cont. alcohol (ie., sugars).

In employing yeast in large quantities, I find large quantities of alcohol and fusel oils given off on boiling.

All multiplication of yeast ceases after four or five days, and when 3*75—3*85 per cont. extract has fermented, after which the cells increase in amount from 10*7 or 12*2 to 37*8 or 40.

In giving the above very short resume of a few chemical facts, I have done so for those interested in puzzling out the highly interesting points of the most complex chemistry, rather than to trouble you at length with abstruse matter.


The CHAIRMAN said, in his opinion, the paper was one which would readily lend itself to discussion by practical men.  If he might be allowed to indicate the points likely to prove fruitful in this respect, he would draw their attention to what the lecturer had said regarding stout yeast being purer than ale yeast, as he thought practical brewers would have something to say on that point.  His remarks on “Boiling Fermentations’’ were also of special interest, and he thought that all present would have some experience of these which would be of interest to the meeting.  The author had also referred to the disagreeable smell derived from alloxan; that is a point upon which they could, very well do with some little enlightenment.  Another matter was the effect of cold on the storage of yeast; he took it that, from the author’s point of view, there were disadvantages to keeping yeast as a low temperature in a water-jacketed vessel.  And lastly, there was the failure of an all-malt brew to invigorate the yeast; almost every brewer would have tried the effect of that, it also was a point upon which discussion might very well be invited.

The AUTHOR, replying to a question by Mr. Lewis, said that in his opinion the only practical way of working under adverse circumstances was to keep a pure-yeast plant on the premises, so that directly the yeast in use became contaminated, another pitching might be taken from the pure-yeast supply, and the whole of the contaminated yeast thrown away.  Pure yeast had of course a much greater resisting power than impure yeast.  In the case of using a pure yeast, it would be possible to get the very best flavoured beer to commence with, but if hop of indifferent quality were used in hopping down, bacteria might be introduced thereby.  The only thing to be would be to add a sufficient quantity of antiseptic to keep the beer long until used.  Although “hopping down” was useful from n practical point of view, it seemed absurd, theoretically, to put in bacteria after the whole process had been worked with a view of keeping them out.  If yeast were kept in an ice store or at, say, a temperature of 40o F., its fermentative capacity would suffer.   Ordinary water or a temperature of above 50o F. would not harm yeast in the least.  Since yeast was not a very good non-conductor of heat, it was impossible to cool more than 4 inches thick of yeast, and if it was desired to cool a large body of yeast, it was therefore necessary to stir it continually, otherwise the outside layer would get quite cold, and the inside retain its original temperature.

Mr. LEWIS said that he got a better result by pitching with a small proportion of stout yeast mixed with the ordinary yeast.  He thought that it might be possible that the higher temperatures that were used in fermenting stout were detrimental to the yeast.

The AUTHOR replied that if the stout was at a considerably higher temperature toward the latter end of the fermentation, it would certainly weaken the yeast, although it might be pure; but after two or three consecutive brews, the yeast would acquire its original strength.  It was possible to ferment stout as a much higher temperature than he himself used, as he never went above 70o F.   It would make a great difference if it was allowed to go above that, as every degree above 70o  was much more decisive in its action than any one degree under 70o; the higher the temperature the more effect would one degree have upon the yeast.

Mr. LAURIE said that his experience coincided with that of the author in many respects.  His brewing had always been most successful when he had kept the temperature of the stout down, and he believed that in that way the full strength of the yeast could be preserved.  He had brewed with bad water, water which was impure in many respects at certain seasons of the year, owing to the fact of the watershed being contaminated by decomposed vegetable matter during the autumn, and the only thing to do under such circumstances was to change with the stout.  He thought that the advice given was very valuable, especially the remarks with reference to waiting too long when the yeast began to deteriorate, instead of endeavoring to improve matters before the effect was shown in the beers.  His own practice had been to use the new yeast, which he had always worked up in the best beers, and the deteriorating yeast in the smaller beers.  He had used yeast that was very inferior, brewing a large proportion of 14 lb. beer, and the boiling fermentation had taken place with bells of three or four inches appearing all over the top, the small beers in this case had to be bottled and consumed within a week.  With regard to the storage of yeast, he should like to know the author’s opinion as to whether plaster of Paris was preferable to Heron’s patented method of preserving yeast.  His own experience when he was abroad did not coincide with that of other brewers, with whom he was acquainted; they were of opinion that plaster of Paris was the best, but he had found Heron’s method just as good, and it kept yeast sound in a warmer temperature than might be thought possible for yeast to stand.  The only system that could be adopted in a very warm country was to preserve the yeast in one of these ways, and have it worked up every month, throwing it away at longer or shorter periods, according to the state of the water and the season of the year.  Mr. Overbeck had spoken of losing n very valuable yeast.  Ho (the speaker) was under the impression that the exact flavour and typo of yeast might have been preserved in plaster of Paris.

The AUTHOR said he might have tried that, but it would have been very awkward to work out for the first brew after a long stoppage.  It would have been necessary to work up a great mass of plaster of Paris in order to get a sufficient quantity of yeast.  After the stoppage he had to begin brewing, and to start 45 vessels in one week and have them all filled.  On the first occasion he had kept a small brewery going for the purpose of preserving his yeast, but he could not do that on the second occasion; objection was raised, and of course it was a very expensive experiment.  With regard to Heron’s method of preserving yeast, he was not in n position to say anything, as he had no experience of its efficiency in foreign countries.

Mr. LAURIE thought it would be possible to get a yeast from one brewery and work it up, and by means of plaster of Paris to retain the flavour for almost any period.  He had found the effect of an all-malt brew to be very good and a decided change when brewing with a lot of sugar.  He had, however, used all foreign malt, which was, of course, the soundest of all malt, as it was thoroughly ripened in the autumn, and English barley was not.  Ho thought it was of no use to brew with all malt made from local barley to improve the yeast.

The CHAIRMAN said that on the question of pure yeast he himself had been brought seriously to book for advocating it, but he thought that he had been misunderstood; he saw no early prospect of it being used in this country–at any rate, in the sense in which it was used on the Continent, for what was the use of introducing pure yeast into the brewery when all around in the atmosphere and in the materials foreign germs were being introduced with the pure yeast?  When he (the Chairman) advocated pure yeast it was in a modified form, suited to the conditions of our working, and mainly for the purpose of preserving and duplicating a favourite yeast at will.  Mr. Overbeck had stated that he deeply regretted having lost a certain yeast; many other brewers were in exactly the same position.  They had great difficulty in replacing a good yeast once it was lost, and Mr. Overbeck admitted that even after some years’ efforts he had not succeeded in getting the same type back.  He (the Chairman) thought there was a way out of that difficulty; every brewer would have a period when his ales were particularly good and his tuns looking extremely well, and that was the correct moment, in his opinion, to think about preserving the type of yeast in use.  A specimen of yeast should be submitted to analysis, in the small quantity necessary for this purpose would be found representative of each type of yeast forming the whole; the various cells could be isolated, and amongst them would be found, by the process of elimination, that particular cell which had been mainly responsible for the observed result.  Having got it, it was a perfectly easy matter in a well equipped laboratory, and with some experience, to make a small culture and preserve it in 10 per cent  cane sugar. He had various types of yeast preserved in this way in his laboratory eight, nine, and ten years old, any one of which could be reproduced whenever required, and from which a change of yeast always of the same original character could be provided every year, every month, or every week if necessary.  In this way it was obvious that much of the difficulty and uncertainty attending changes of yeast was removed, quite irrespectively of the length of time the store lasted or its impure surroundings.  It was, of course, a rather expensive method, yet if they had once lost a valuable store of yeast, they would be ready to make almost any effort in order to prevent the recurrence of such loss.

The AUTHOR said that he quite agreed with Mr. Murphy as to a good yeast being a valuable commodity, but he thought that the plan suggested, of growing the different species of which this valuable yeast was composed in Pasteur flasks, was much easier described than done.  It would be very difficult to keep the different group separate, and know how many different species and in what proportion they were which made up the particular yeast.  It would mean, in the first instance,  a very interesting but at the same time a very intricate and long research.  On the other hand, from a single-cell yeast it was now proved that after a certain number of generations that single-cell split into various varieties, and on that account it would be very difficult to know whether a dozen different varieties were being grown together with four varieties of another species, which might hark back again to the old type from which one of them had come.  It was a very difficult thing indeed to get a degenerated yeast back again to its former sound constitution.

A vote of thanks to the author was proposed by Mr. HOWISON, seconded by Mr. LAURIE and carried unanimously.

The AUTHOR in acknowledging the vote of thanks, said that he felt sorry that he was in a county where there were very few large brewers, and on that account he had not an opportunity of exchanging ideas.  That was the only way they could get on to a solid basis. In brewing, the personal factor was very prominent, and every brewer could speak from his own experience, which might, of course, be entirely different from that of any other brewer.  Practice had shown many things to be perfectly correct, but in past times everything was done by rule of thumb, and brewers knew what best, but they did not know why.  Research was bringing them to know why they did it, and of course, every man was delighted when something was discovered which was as he thought, and when his ideas were verified.

%d bloggers like this: