The results of the work may be briefly summarised as follows:(a) The rate of fermentation of ciders and perries made from different varieties of vintage fruit varies considerably.(b) There is probably a relation between the rate of fermentation and the variety of fruit from which the cider or perry is made. Certain varieties, for instance, as a rule yield juices which ferment slowly, while others give juices which generally ferment at a rapid rate.(c) The main factor in determining the rate of fermentation appears to be the nitrogenous matter present in the juice, which is assimilable by the yeast. The quantity of such substances present is generally insufficient for satisfactory nutrition of the yeast, and consequently the rate of fermentation is normally slower than would be the case if sufficient nitrogenous food were present to meet the full requirements of the yeast. The relative rates of fermentation of ciders and perries fermented under similar conditions are probably an index of the relative amounts of assimilable nitrogenous matter present.(d) The state of ripeness of the fruit at the time of milling affects the rate of fermentation of the juice. During the course of ripening the rate becomes slower until a certain point is reached, which probably represents the period of perfect maturity of the fruit. Afterwards the rate increases in proportion as ripeness proceeds to decay. The degree of exposure of the fruit to direct sunlight also affects the rate of fermentation, the more exposed the fruit the slower the rate. In each case the result appears to be due to the quantity of assimilable nitrogenous matter in the juice being influenced.(e) Direct influence on the rate of fermentation of the juice by any of the chemical constituents other than the nitrogenous substances has not been observed. If any have an influence, it is masked by other factors of greater importance. An indirect influence on the rate by the mucilaginous elements is occasionally met with, due to the formation of a clot which mechanically impedes the action of the organisms of fermentation.(f) The rate of fermentation in practical cider-making does not appear to be materially affected by the fermentative powers of the kinds of yeast present in the juice. Normally there are present varieties which are capable of maintaining the fermentation at practically the maximum rate allowed by the nitrogenous constitution of the juice. “Dominant” fermentation with selected yeasts of high or low fermentative powers had comparatively little effect upon the rate.(g) The aeration of the juice has a marked effect upon the rate of fermentation, the admission of air to the juice producing a decided increase in the rate.(h) The temperature at which the fermentations are conducted affects the rate in the customary manner.It is clear, therefore, that apart from the use of purely practical methods, e.g. filtration,—which it is not intended to consider here— a certain measure of control over the rate of fermentation of ciders and perries can be exercised by the cider maker, and that therefore the production of sweet and dry types of these beverages need not be more or less haphazard, as is commonly the case. By careful selection of the varieties of fruit used and by suitable blending of various types, combined with attention to the condition of ripeness of the fruit at the time of making, it should be possible to obtain a juice possessing the desired rate of fermentation, although some allowance for seasonal influences is necessary. These have not been considered above, as the work has not been extended over a sufficiently long period to allow of definite conclusions being drawn. At the same time it is fairly established that in some seasons the average rate of fermentation is much faster than in others. It would appear from the results as to the effect of direct sunlight as though the amount of sunshine during the period of ripening of the fruit upon the trees played an important part in seasonal influence. During the course of fermentation of the liquors the rate may be controlled to some extent by aeration and temperature.Although the subject has been considered almost entirely from the point of view of the rate of fermentation, it should be mentioned that not only the rate but also the degree to which fermentation proceeds is involved. Although perhaps not invariably the case, as a rule fermentation can proceed to a further point in rapidly than in slowly fermenting juices. Accordingly no distinction has been made between them. In some cases the latter feature would more correctly express the facts than the former.In conclusion I take this opportunity of expressing my indebtedness to Mr James Watts for his kindness in placing his factory at my disposal for experiments with selected yeasts; to the many cider makers and others, who have rendered considerable assistance in the direction of obtaining different varieties of vintage fruit required for the work; and to the various members of the staff of the Institute, who have carried out the practical work in the cider house.
Reality in a few thermodynamic reference frames: Statistical thermodynamics from Boltzmann via Gibbs to Einstein
