Top Fermentation


by L. R. Bishop, M.A., Ph.D.  (1938)

The chemical reactions which go on in fermentation by yeast have been followed out in detail by many workers and have proved of extraordinary interest to science, but the discoveries made have had little influence on brewing practice. While, on the other hand, the physical behaviour of yeast in the fermentation vessel has been neglected, perhaps for the reason that it is too simple.

However, this study of the physical behavior of yeast may prove of interest in providing an explanation of a number of difficulties encountered in top-fermentation breweries.

As suggested in the introduction, the key to much in yeast behaviour was found to be the coarse flocculum or “break” formed in wort cooling. The first observation was that yeast itself is almost white in colour (unless it has been allowed to dry) and consequently the degree of brownness of a sample of yeast is an index of the proportion of brown flocculum present. It is easy to show this by stirring the yeast sample with water in a tall cylinder and allowing it to settle, when the yeast and flocculum separate themselves. Consequently in these papers the colour of a sample of top or of sedimentary yeast has been used as a rough index of the proportion of wort flocculum or turbidity present.

The first studies were made on fermentations in the usual type of tall “Thermos” vessel as described in Part II. The particular vessels here differed in that vertical strips of silvering had been removed to provide observation windows. By daily observation it was shown that the coarse turbidity settles to the bottom of the vessel and that, when this is large in amount, then the sediment is almost free from yeast (judged by colour) and a large top-yeast crop is obtained.

Thus at first sight there is “action at a distance”—the turbidity settled at the bottom causes a high top-yeast yield. This is clearly demonstrated also in the experiments, involving malt dust and other additions. In these, figures are given showing the high yields of top yeast obtained; while the greater part of malt dust or other additive could be recovered from the sediment at the bottom by stirring it with water and allowing it to settle out from the small amount of yeast present.

In experiments where the fermentation behaviour was observed, there was the further apparent anomaly that those worts with a heavy flocculum at the bottom were rapidly evolving CO2 there. While the clear worts had, even at an early stage, much more yeast at the bottom and appeared to be fermenting only slowly: yet it was these worts which attenuated much further. Some qualitative observations show this—for instance in one experiment, the two worts cooled quickly, that portion without coarse sediment attenuated from 1046° original gravity to a racking gravity of 1017·0° and during the third, fourth and fifth days the average number of bubbles liberated from the bottom, as observed through the “window,” was 15 per minute, while the half with coarse sediment attenuated only to 1020·20 and yet the corresponding number of bubbles was estimated at 600 per minute. Other experiments gave similar results.

The second anomaly (more bubbles and less fermentation) was found to explain the first (sediment at the bottom increasing yeast at the top) but first it was necessary to study the parts played by both sediment and bubbles. What apparently happens is that the yeast, when it passes out the CO2 formed in fermentation, does so in solution, and there is no evolution of gaseous carbon dioxide at the surface of the yeast cell. Perhaps the most important evidence for this view is the simple fact that it is possible, by fermentation with yeast, to obtain solutions which are supersaturated with carbon dioxide. This is supported by direct observation of the yeast under the microscope during fermentation, when no bubbles appear to come from the cells themselves.

On the other hand the wort flocculum particles appear to encourage the formation of gas bubbles so that the supersaturated solutions rapidly give up their surplus of carbon dioxide.

This behaviour was illustrated by yeast fermenting wort in a hanging drop under the microscope, but the evolution of carbon dioxide gas is seen to commence at a part where there is no yeast. The bubbles appear to represent a sequence in size and so presumably in order of formation. The smallest ones are deformed owing to the particles present which, it is suggested, are acting as nuclei for bubble formation. The diffraction of light by the larger bubbles hides any possible source.

The effect of this evolution of gas from the particles of wort flocculum or “break” is very striking. For, in a turbid wort fermenting in a glass cell the particles of coarse flocculum or sediment could be observed lying among the yeast at the bottom. Here the particles acted as centres of evolution of carbon dioxide gas and produced a series of “volcanoes” in the sedimentary yeast, so that the surface of this looked like a region of active volcanic eruption.

The most direct evidence of the action of sediment particles was supplied by an examination, which showed the individual sediment particles with a stream of bubbles rising from each to form the volcano.

In this experiment, the particle is buried in the sedimentary yeast, and the evolution of bubbles in a stream makes the volcano-like crater. This is the usual type. Also seen is a layer of yeast at the bottom and a small sediment particle above it, with a bubble forming on its side. This sediment particle was observed for some time. During the most active stage of fermentation, as the bubble formed, it carried the particle rapidly up; then the bubble disengaged and the particle slowly sank until it reached the supersaturated layer of wort above the yeast; here another bubble formed and the cycle was repeated. Using the particle as an indicator it was possible to deduce that, at the most active stage of fermentation, the wort was supersaturated for about an inch above the sedimentary yeast. Tests on brewery vessels, by adding pumice powder, showed that the wort was supersaturated with carbon dioxide at a distance of several feet from the bottom.

One type of particle was small and crystalline in nature, but others were brownish and opaque. However, after search through a number of fermentations, a large transparent crystalline fragment was found which afforded a clear illustration of the nature of “sediment action.” The fragment was shaped like a short hollow cone. Continual evolution of bubbles took place from it throughout the fermentation; but the evolution was from one part only—from inside the hollow of the cone. It would seem that in this crevice a permanent tiny bubble was retained and supplied sufficient of the gaseous phase to act as an evolving centre. This would explain why porous materials, such as pumice and malt dust, have been found to show “sediment action.” It is suggested that calcium phosphate is the crystalline substance chiefly concerned in the natural “sediment action” in wort. If this is, so then it is understandable that regular crystals formed by slow growth would not afford the necessary crevices, but irregular crystals, e.g., those formed by quick chilling, would be ideal for this purpose.

Thus a study of the behaviour of wort sediment has explained part of the original anomalies and the behaviour of yeast in relation to carbon dioxide bubbles explains the rest. This is dealt with in the following sections.

Process of Formation of the Yeasty Head.—
As bubbles shot up into the fermenting liquid from the “volcanoes” each carried up a streamer of yeast like a rocket. This gradually dropped down again but the action maintained a certain amount of yeast in suspension. Watching a single yeast cell suspended in fermenting wort, it could be seen slowly falling; then, as a bubble shot up near it, it too kicked quickly upwards; after which the slow fall began again, only to be reversed in a short time. Thus the bubbles from the bottom tended to keep yeast in suspension, and so actively fermenting.

At the same time, however, the bubbles tended to have an opposite effect. For when the bubbles arrived at the top, during the active stage of fermentation, each carried with it a single layer of yeast cells over the surface of part of a single bubble.

The bubbles were naturally heterogeneous in size, and smaller bubbles clustered round the bigger ones, each having its layer of yeast. It is a well-known physical fact that small bubbles have a higher internal pressure and so tend to “pump” their contents into larger ones and this is what happened—the carbon dioxide passing by solution through the walls. The result was that each small bubble collapsed, leaving its skin of yeast as a patch on the wall of the large bubble which is a point higher in the yeasty head, i.e., where the bubble has spent a longer time.

The final result of the collapse of smaller bubbles into bigger ones is large bubbles, each “armoured” by a thick skin of yeast, and the whole structure, being lighter than beer, floats on the surface. But the lightness is a result of the structure and if this is destroyed, e.g., by agitation, the yeast sinks and the gas escapes.

Returning to the accumulation of bubbles in the yeasty head, it must be remembered that the yeast cells which can be observed under the microscope are large-scale models of what happens to proteins and other substances which lower surface tension and so form a layer over the surface of the bubble, just as the yeast does. When the bubble collapses, the yeast is enclosed in a fine sac of this protein and other material, which does not readily redissolve in beer; so that, by examining yeast masses suspended in beer, the protein films and enmeshed masses of yeast.

If the yeast mass is treated with water then the proteins redissolve and the yeast is liberated as individual cells or bud groups.

Fine wort flocculum carried to the top is enmeshed at the same time, and so is seen associated with yeast; but, as we show later, it is the coarse sediment which affects attenuation and outcrop; so that the association of fine flocculum and yeast has no significant relation to these. This association can be dissolved in the same way by using water instead of beer.

A Possible Explanation of “Clumping.”—
Certain yeasts are well known to “clump” easily and ferment less fully; while others, the powdery yeasts, remain in suspension longer and ferment more completely. We have, however, in this series of papers preferred to call the types of yeast “top” and “sedimentary,” since, as far as our evidence goes, this is the primary thing. Our suggested explanation of clumping is that the top yeasts are readily carried to the top where, in the process of head formation, they become sealed together in large aggregates, many of which drop back into the fermenting liquid.

Consequently on examining an ordinary fermentation by mounting the top, sedimentary and suspended yeast in beer, large clumps can be seen in all. But one fermentation was carried out in a “Thermos” vessel with a fine copper-gauze sieve mounted just below the top of the wort, so that individual cells and bud groups could pass up or down, but clumps could not pass through. An examination of the top yeast and the deposit on the sieve (both mounted in beer) showed large clumps, but the suspended yeast and bottom yeast examined in the same way showed no large clumps. In other words, the clumps had been formed and retained above the sieve.

Consequently, we suggest that clumping is the result of the yeast being easily carried to the top and not that yeast is carried to the top as a result of clumping.

I should like to acknowledge the help of Mr. R. S. W. Thorne, B.Sc.

A study of fermentation has been made by eye and under the microscope and the following conclusions have been reached:—

(1) The-action of sediment particles on yeast behaviour is explained as follows:—
Yeast in fermentation liberates carbon dioxide in solution, oven if the liquid is already supersaturated. It is the sediment particles which act as evolving centres of carbon dioxide gas. The effect of this evolution is the formation of “volcanoes” in the yeast at the bottom of the vessel and the stream of bubbles from these carries the yeast up.

(2) The primary effect of the evolution of bubbles is to increase the amount of yeast in suspension and so actively fermenting. The secondary effect is to remove yeast from suspension by carrying it up on the surface of the bubbles to the yeasty head. It will depend on conditions which of these effects predominates.

(3) The process is described by which the stable yeasty head is formed on a top fermentation. The small bubbles each have a layer of yeast and, collapsing to form larger bubbles, eventually produce large bubbles, each “armoured” with a thick wall of yeast, which gives a stable foam lighter than water.

(4) A possible relation is suggested between the tendency of yeasts to carry to the top and
their tendency to “clump.”

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