By L. R. Bishop, M.A., Ph.D. Feb 1938
INTRODUCTION
The experiments described in this series of papers have been made in an attempt to elucidate the causes of yeast weakness, which is often a source of considerable trouble in breweries. But, as soon as enquiries were made, it was found that the term yeast weakness is ill-defined and used in very different ways in different parts of the country. Perhaps the only characteristic to be found in common among the various forms is that, if the symptoms become serious, a change of yeast is required. But the reason for needing a change in one brewery may be the opposite of that in another. Consequently it was found necessary to abandon the term as there appeared no prospect that it could be narrowed down to one set of symptoms, so as to have scientific precision. Therefore, in these papers we have studied, in the main, three of the things which can be precisely defined and measured and which are connected with yeast weakness in many breweries; these are the degree of attenuation at racking, the size of the yeast outcrop and the amount of yeast in suspension at racking.
Taking the first of these, attenuation, this has not in the past proved entirely amenable to scientific control, and yeast weakness is often associated with deficient attenuation, that is with a high racking gravity; but there are a number of breweries where extreme attenuation of the wort is associated with yeast weakness.
Many of the causes of incomplete attenuation have long been recognised: it arises for instance from incorrect mashing temperatures, from faulty materials or from in judicious use of the attemperators. Since these causes are known they can be recognised and the difficulty overcome and it is not proposed to deal with them here.
There remain many cases in brewing of variations in the extent of the primary fermentation which appear inexplicable or only partly explicable, and it is hoped that the present work will help to explain some of these cases and to suggest possible means of overcoming them. As views stand at present, if an incomplete attenuation is shown not to result from shortage of fermentable sugars, then suspicion falls on shortage of some essential nutrient such as phosphates, assimilable nitrogen or bios; but a number of cases of incomplete attenuation are given in this series of papers where these causes are not operative. In such cases suspicion may fall on the yeast as weakly attenuative or as unable to ferment some of the malto-dextrins which are presumed to be fermented by the more attenuative yeasts. In turn this explanation fails when tested, for the yeasts giving incomplete attenuation in the brewery may give satisfactory fermentation in laboratory tests; and in one of the following papers it is shown that a number of brewery yeasts, giving widely different racking gravities, all attenuate a wort to the same extent in an attenuation limit test; so that these yeasts all have the same ability to ferment malto-dextrins.
The key to these difficulties and apparent contradictions between brewery and laboratory results, lies, we suggest, in the simple fact that in laboratory tests the yeasts are shaken or stirred to keep them in suspension; whereas in the brewery the yeasts are not stirred artificially, except by rousing, and their behaviour is governed by the physical factors controlling the amount of “natural stirring.” This stirring is caused by the evolution of carbon dioxide bubbles from the bottom of the fermenting vessel and we find in turn that the evolution takes place from the sedimentary matter or “break” of the wort. Consequently a good deal of attention has been given to establishing this finding, and to following out its implications under different brewing conditions. It was found impossible to deal with the other factors governing attenuation, without including a study of rousing. It is often held that the supply of oxygen to the fermenting wort is the function of rousing but we can find no evidence in favour of this view, and prefer to regard rousing as stirring by air which is similar in action to the natural stirring by carbon dioxide bubbles.
The second factor studied, the amount of the yeast outcrop, is again a subject of much discussion; whether it should be high or not too high and whether, if low, the growth has been checked by some shortage such as those mentioned earlier. We are able to demonstrate some of the factors which determine the amount of yeast coming to the top of the vessel. Among these, sediment again plays a part together with the well-known effect of the gravity of the wort. In our experiments, as in breweries, the yeast outcrop may vary through a wide range, but we find that the total crop is less variable for a given gravity; so that if the expected crop has not appeared in the head, it is found in suspension or at the bottom of the vessel. In fact, the yeasts studied appeared to tend to one or other of two extreme types. At one of these the yeast comes to the top early, leaving a bright but incompletely attenuated wort; and at the other extreme the yeast stays in suspension longer, so that the wort becomes completely attenuated and a greater amount of yeast remains in suspension or deposits on the bottom of the vessel. The British yeasts were found to be of a mixed character and the yeast which deposited on the bottom, the sedimentary yeast, was found to be more attenuative than the top yeast.
Thus the two extremes each offered an advantage and a disadvantage; and only one way could be found for combining the advantage of complete attenuation with that of freedom from excess yeast at racking. This combination was effected by a new design of fermentation vessel which, it is considered, will give a more complete control of the process of fermentation.
This series of papers includes the following parts:—
I. “The Natural History of a Brewery Top Fermentation.”
II. .”On the Function of Wort Flocculum in Affecting Attenuation and Yeast Behaviour.”
III. “The Effect of Rousing on Attenuation and Yeast Behaviour.”
IV. ” The Effect of Yeast Type on Attenuation and Yeast Behaviour.”
V. “The Effect of Fermentation Vessel Design on Attenuation and Yeast Behaviour.”
Crescent City Brew Talk 