Vernon L.S. Charley
(from the Long Ashton Report for 1939)
One of the major effects of the War on the cider industry will be the difficulty in obtaining supplies of sugar, and stringent precautions will naturally be necessary to conserve the stocks of sugar apportioned to the industry. The process known as "checked fermentation", in which an appreciable percentage of the natural fruit sugar is retained in the cider, will obviously ease the sugar position and, in cases where juices with satisfactory original gravities are fermented and checked at a medium sweet gravity, this process will call for the use of only small amounts of sugar.
Various reports on the retention of sweetness have been published in
the Annual Report of the Long Ashton Research Station, but these have dealt
chiefly with the use of the super-centrifuge and a comparison of its effectiveness
with that of the filter. The commencement of War, however, in September
1939, brought a large number of enquiries requesting precise details of
the most efficient way of dealing with the checked fermentation process.
Short explanatory statements were broadcast and published in the daily
press in October 1939 and leaflets were prepared for distribution. The
following article is an expansion of the various leaflets, the fresh material
being largely concerned with an explanation of general principles involved
and allusions to the experiments carried out on this subject in the Cider-house
programme in the winter of 1939. The results of this work are not
yet complete, but it is hoped to give publicity to any new recommendations
that may appear to be desirable before next cider-making season.
Active fermentation will take place in apple juice or cider when a supply of live yeast cells is accompanied by a sufficient quantity of the essential mineral or organic chemicals necessary for the nutrition of the yeasts. Provided the supplies of nutrients are adequate, a very few yeast cells can rapidly develop into enormous numbers and carry out a proportionately greater amount of fermentation. On the other hand, a very large number of live yeast cells will remain dormant in a medium which is "starved" with respect to the essential nutrients for yeast development. These facts suggest that it is of paramount importance to remove yeast food from the juice if natural sugar is to be retained and this is the objective in all the processes mentioned later.
The writer has often been asked whether it would not be helpful to centrifuge or filter the freshly expressed juice and thus retard later fermentation. Such a treatment certainly postpones the onset of fermentation but, as the yeasts in fresh juice contain a very small proportion of the mineral foods, their removal by the centrifuge or filter does not appreciably reduce the "food level" in the juice. The comparatively few yeast cells remaining in the clarified juice gradually develop and propagate, and in due course the fermentation becomes as rapid as if no preliminary treatment had been undertaken.
The position is quite otherwise, however, in a medium sweet cider which is passed through the centrifuge or filter or even merely racked from its deposit. During the course of the fermentation the yeasts assimilate mineral materials, such as phosphates and organic compounds containing nitrogen, depriving the juice to an equivalent extent of these nutrients. The removal of the yeasts consequently results in the direct removal of a substantial proportion of nitrogenous and inorganic compounds, and the cider issuing from the centrifuge, filter or racking tube does not contain sufficient nutrients for the further development of the yeasts.
The above principles, interpreted into practical procedure, indicate
that it is essential to allow the growth of a heavy crop of yeast material,
and then to remove this yeast, together with its associated, assimilated
nutrients from the cider. In actual practice, it has been found inadvisable
to attempt the separation of the yeast until the cider has fermented 10°-12°
from the original gravity.
Methods of Fermentation Control.
This process is in wide use by farm cider-makers and, in certain factories where adequate facilities exist for the convenient racking of large bulks of cider from one vessel to another. The process suffers by reason of the difficulty of achieving a thoroughly efficient separation of yeast material without leaving a considerable volume of lees which need separate treatment. When racking a cider which has fermented 10°-12° in gravity in a 100-gallon cask, it is necessary to place the racking tube about 4" from the bottom of the cask, thus leaving a large volume of cider in the cask with the lees. Again, unless stringent precautions are taken to avoid access of air to the juice, racking may actually increase the rate of fermentation. In experiments on large bulks of cider in the Long Ashton programme of research work in October-December 1939, the ordinary racking process was found to be an unsatisfactory means of retaining sweetness in large bulks of cider. Certain ciders responded admirably to the treatment but others were practically unaffected. If racking is to be entrusted with the duty of maintaining sweetness in a considerable bulk of cider, it should be combined with the use of a chemical preservative added after racking.
A quick series of rackings on an individual cider should be avoided. The efficacy of a racking treatment on a cider depends on the amount of yeast removed by the treatment and a suitable period must be allowed for an appreciable crop of yeasts to develop before effecting its removal.
Racking of a cider in full fermentation will usually have little effect
as the yeast will be in suspension in the cider and not in the form of
a heavy deposit. In France, racking is recommended to be carried out only
on fine days of equable atmospheric conditions and pressure, which conditions
conduce to the formation of a heavy deposit of yeasts.
(b) Sulphur Dioxide (SO2).
This is the only preservative allowed by law for use with cider. The maximum permitted dose is 200 parts per million (p.p.m.), which is equivalent to 14 grains per gallon. The use of SO2 is complicated by the following considerations:-
(a) It is oxidised by atmospheric oxygen to a compound which is not similarly toxic to yeasts.
(b) It combines with sugars and other compounds to form loosely combined chemical substances which have little or no toxic action.
With reference to (a), little can be done by the cider maker to minimise this loss, except to avoid undue aeration in processing, especially racking. With regard to (b), there are two important points which must be considered. The tendency of SO2 to combine with sugars and similar compounds suggests that SO2 should not be added to fresh juices, or juices and ciders which still retain considerable amounts of carbohydrates. Further, when active fermentation is taking place certain intermediate compounds, such as aldehydes, are presumed to have a momentary existence in the fermenting medium and these compounds, in the actual presence of SO2, can form quite stable products which involve the removal of a proportion of the SO2 from active participation in yeast suppression. These two considerations indicate that SO2 should not be added to fresh juices or to ciders in such a way or at such a period that active fermentation will ensue in the presence of the SO2.
As a result of these conditions, it is clear that SO2 will be most effective when it is added to a medium sweet cider (sp. gr. 1.035 - 1.025) in which the cider has been clarified and freed from the bulk of the yeast material by racking, centrifuging or filtering.
It is not possible to give any definite figure for the loss of SO2 by oxidation or removal with the CO2 evolved during fermentation, as this percentage loss varies with different varieties and with many other factors. The only accurate way is to determine the SO2 content by the method given at the end of this article, and the amount in the cider may then be raised, if necessary, in accordance with the figures obtained. In the event of the analysis not being possible at the cider factory or farm, samples may be sent to Long Ashton for determination.
The maximum dose of SO2 allowed by law in a cider offered for sale, i.e. 200 parts per million, is equivalent to 6¼ oz. potassium metabisulphite (K.M.S.) per 100 gallons of cider. The K.M.S. in powdered form is most convenient for general use. The SO2 content of K.M.S. which has been in stock for some time should be carefully checked, and the solution of the powder should be effected with great care in a small volume of luke-warm water.
In general. it is advisable to restrict the use of SO2 to
the final stabilization of the cider at a gravity of 1.030-1.025. For perfect
safety, centrifuged cider should be treated with the full dose of SO2
(equivalent to 6¼ oz. potassium metabisulphite per 100 gallons).
If the cider is to be stored for a period of months before consumption,
there will probably be a 25% loss of total SO2 in this period
under normal conditions of storage in casks or wooden vats.
This method of retaining sweetness in cider has been discussed in full in various technical reports published by the Long Ashton Research Station. The centrifuge in its various forms has the following definite advantages:-
(i) It can be used effectively on thick, cloudy ciders of high specific gravity which show excessive rates of fermentation, where a filter would be practically useless. The actual period of the main fermentation can thus be lengthened, a very desirable result in the case of cooking and dessert apple juices or normal cider juices in periods of excessive warmth which causes turbulent fermentation.
(ii) It effectively removes the yeast cells and simultaneously clarifies the cider to a remarkable extent.
(iii) It results, in general, in a check to fermentation even more substantial than that caused by the filter.
(iv) It can deal with the entire bulk of cider to be treated, and reduces the lees to a hard, coherent paste of yeast and other materials. Where disposal of liquid lees constitutes a problem the centrifuge can generally provide a solution.
(v) It needs an absolute minimum of attention and repair, and runs for many hours without interruption
The principles governing the most effective use of the centrifuge are similar to those for racking. The greater the growth of yeast prior to its removal in the centrifuge the greater will be the effect of the treatment in retarding fermentation. It is hardly ever worth while using the centrifuge before the cider has fallen 10-12 points in gravity and the best effect is found when the machine is used at this gravity and again at about 1.030. In many cases of normal rates of fermentation it is not necessary to centrifuge at all until the cider reaches a gravity of 1.030 ; treatment at this gravity will often cheek the fermentation to such an extent that the cider becomes "dead". In cases where excessive rates of fermentation are expected the double treatment, with or without a threequarter or full dose of SO2 after the initial centrifuging, should be adopted.
The drawbacks to the use of the centrifuge by farm cider-makers in the past have been the expensive nature of the machine and the large output of the standard commercial types. During the recent cider-making season, extensive and remarkably efficient use has been made of a small Alfa-Laval centrifuge with an output of 15-30 gallons per hour. Practically clear ciders were obtained and the machine should prove an attractive investment for small makers. The whole question of small scale centrifugation is being actively pursued and the possibility of using other forms of apparatus (e.g. milk centrifuges) is being examined, especially with regard to the centrifugal power of the machine and the metals used for the parts coming into contact with the juice or cider.
Both factory and farm cider-makers. who use the centrifuge method should
take adequate precautions against the incidence of acetification in centrifuged
ciders. The absence of active evolution of gas may allow the development
of acetic bacteria and low-acid, centrifuged ciders are likely to be attacked
by sickness bacteria when the warm weather commences. A careful blending
programme should be arranged to obviate this latter danger.
The filter was the first machine introduced for the purpose of retaining sugar in cider and it is in wide use amongst farmers and small makers of cider. Filtration suffers from the disadvantage of being a time-consuming process, occupying much time over the packing and unpacking with pulp and the associated process of washing and beating of the filter pulp. These manipulative inconveniences assume greater importance when it is desired to make use of the filter when the active making season is still in progress and, in circumstances like these, the advantages are all on the side of the centrifuge. However, from the point of view of brilliance, the filter gives perfect clarity which is definitely superior to that imparted by the centrifuge and for this reason alone the filter is still to be regarded as an essential piece of cider-house equipment.
It is generally impossible to use the filter for ciders with gravities
greatly in excess of 1.035 and consequently the filter finds its chief
use in effecting the final stabilization of the medium sweet cider rather
than in being applied to any process of lengthening the fermentation of
quickly fermenting ciders. With this one difference in mind, the use of
the filter is similar to that of the centrifuge and the same principles
are involved in each case.
All vessels used for the reception of racked, filtered, or centrifuged
ciders should be steamed or otherwise rendered as sterile as possible.
Regular inspection of the gravity of the cider and the ullage in the casks
should be made, and the necessary action taken either to cheek the fermentation
once again or to fill up the casks to full capacity. The specific gravity
hydrometer should be in constant use throughout the making season and until
the cider is stabilized.
Determination of SO2 in Cider.
The recognized method for determining the SO2 in cider is that of Monier-Williams, but special apparatus is needed and an individual determination takes about two hours. An alternative method is available, however, which affords reliable results of sufficient accuracy for the present purpose.
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