FERMENTATION 



213 



taining oxygen or in one in which none is present. Experiments and practical 

 experience have shown that in presence of a limited amount of oxygen yeast 

 increases so vigorously and fermentation is so little retarded that the maximum 

 amount of alcohol may be formed. If a very small quantity of yeast be added to 

 a suitable nutritive solution from which air has been fully excluded the organism 

 at first greedily absorbs the oxygen dissolved in the solution ; it even makes 

 use of combined oxygen, such as the oxygen united with haemoglobin, although 

 it is unable to decolourize indigo-carmine. As the yeast increases and the 

 alcoholic fermentation begins, bubbles of carbon-dioxide make themselves 

 apparent in the fluid ; these, however, become rapidly smaller and finally cease. 

 The addition of a minute quantity of air at once increases the intensity of the 

 fermentation in a remarkable manner, and bubbles of carbon-dioxide once more 

 make their appearance (DUCLAUX, 1900). If oxygen be permanently excluded, 

 the yeast in the end dies, even though food-materials be still present (BEIJERINCK, 

 1894). In the long run, alcoholic fermentation proper is also promoted by the 

 presence of small quantities of oxygen. The statements in the literature on 

 this whole question are, however, more contradictory than in any other depart- 

 ment of physiology. 



It will perhaps help us to reach an accurate conception of the behaviour of 

 yeasts if we consider here two terms suggested by PASTEUR (1861 and 1863), and 

 generally employed since his time. Such forms as could go through their normal 

 development only in presence of oxygen he termed aerobic, and anaerobic such 

 as could get on without oxygen, or such as were injured by that gas. In 

 studying typical aerobic plants we saw that a certain partial pressure of oxygen 

 was injurious to them also. This oxygen pressure lies in their case far above 

 that of normal atmospheric air, whilst that affecting typical anaerobes lies far 

 below it. The extremes are connected by transitions which, on the one hand, 

 from their increasing liability to injury from oxygen, and on the other in their 

 increasing need for that gas, shade quite gradually into each other. Since 

 nothing is known as to the injury done to yeast by oxygen, we can only 

 characterize it by its oxygen requirements. It differs obviously in this point 

 both from typical aerobes and from anaerobes, since it can live and multiply 

 for a long, though not unlimited, time without respiration. 



We have not as yet solved the chief question before us, viz. why yeast 

 forms alcohol not merely when its respiration is interfered with but under all 

 conditions ? From the point of view of energy we have here to deal with a loss, 

 and the question is whether or not gain in another direction does not counter- 

 balance this loss ? To WORTMANN (1902) we owe a very probable hypothesis ; 

 he considers the alcohol as a protection employed by the yeast against associated 

 micro-organisms, since yeast itself can tolerate 10-18 per cent, of alcohol while 

 all other organisms occurring in saccharine fluids are injured by 4-10 per cent, 

 of alcohol. This theory reminds us of the formation of acids by Fungi, not with 

 the object of gaining energy but for certain biological reasons. The difference 

 between the formation of acids by Aspergillus and the formation of alcohol 

 by yeast lies in this, that Aspergillus aims at acidifying the substratum, 

 only in order that it may be made unfavourable for other organisms ; it ceases 

 forming acids before it is itself injured by this product of its own metabolism, 

 while yeast, on the other hand, goes on producing alcohol until it is itself killed 

 by the alcohol, and apparently almost all fermentative actions are similarly 

 brought at last to a standstill by the products of fermentation. This fact does 

 not seem to us to be quite in accord with WORTMANN'S hypothesis. If our 

 definition is to be maintained and we are to speak of fermentation only where 

 destructive metabolism precedes a gain in energy, then alcoholic fermentation as 

 well as acid ferment at ion by Fungi must be excluded from this category of actions. 

 We must also inquire what other fermentative processes exist, since the poisonous 

 effect of the chief product has apparently a definite significance apart from the 



