PREPARATION FOR MARKET--(_Continued_).
Parchment Coffee, when in an unseasoned state, is prone to enter into
decomposition from the time at which it is withdrawn from the protection
of the living organism until it is thoroughly seasoned by drying, after
which it may be kept for any length of time in a dry place.
The worth of coffee as an article of commerce is lessened in proportion
to the extent to which these progressive changes are allowed to go on.
If heating has taken place, the bean can never afterwards acquire the
pellucid colour which is indicative of well-dried coffee, but partakes
more or less of a dingy appearance. If mouldiness ensues, the aromatic
properties, like those of tea, give place to an insipid flavour; and
finally, if the bean undergoes putrefaction, it assumes a dull black
colour, and becomes totally destitute of every valuable property.
When the crops ripen, they must be gathered and cured under all
circumstances of weather; and as it generally happens that this has to
be done during the prevalence of the periodical rains, the difficulties
to be contended with are so much the greater. The extensive nature of
the operations has also to be taken into account in forming an estimate
of the difficulties to be provided for. During the busy season of crop
upwards of 1000 bushels of cherries are daily gathered from some
plantations, yielding an increase of 500 bushels of parchment coffee to
be daily added to that which has already accumulated in the store.
Past experience having shown that coffee was most easily preserved in a
sweet state when spread thinly on the floor, large and commodious
buildings were called into use, notwithstanding the unusually heavy
expense which attended their erection in situations remote from town,
where sufficiently skilful labour was only to be had at the time, and
with great difficulty. On this account inadequate accommodation was
provided on many plantations, and the coffee accumulating to a
considerable depth, no amount of hand-turning could keep it from
contracting a musty smell, its proneness to decomposition increasing
greatly in proportion to the extent of the accumulation.
Some years ago it occurred to Mr. Clerihew that it was possible, by
means of fanners, working on the exhausting principle, so to withdraw
air from an enclosed space as to establish a current of air through
masses of coffee spread on perforated floors forming the top and bottom
of that space. This plan he carried into execution at Rathoongodde
plantation, and it has since been adopted by many planters.
The following is a detailed description of Mr. Clerihew’s invention, a
model of which was shown at the International Exhibition of 1851:
The water-wheel is an overshot one, nine feet in diameter, and is of
much smaller dimensions than any wheel that has hitherto been employed
for pulping. It is, however, sufficient in power to work the fans and
pulpers simultaneously, the excess of its power over that of other
wheels being gained by the diminution of friction consequent on there
being no intervening shafting and gearing. The entire wheel is
constructed of wood, with the exception of the shaft, which is unusually
light, as it has merely to serve as a support to the wheel. By means of
a double band rim-bolted to the arms on each side, motion is given to
the pulpers from the one and to the fans from the other.
The floors of the curing-house are laid with laths 1¼ inch square and 2
inches apart; these are covered with open coir matting; being rather
cheap and durable, this material answers the purpose remarkably well.
The side walls of the curing-house are constructed in the manner of the
country, viz. of wattled work filled in with clay and smoothed over so
as to be air-tight.
To derive the full benefit of natural heat, the roof is covered with
felt or with sheet-iron, so that in fine weather the temperature of the
air in the upper floor is raised considerably by contact with the hot
roof, and its capacity for absorbing moisture much increased,
preparatory to its being drawn down through the mass of coffee in the
upper floor. Even in the cool climate of the district of Upper
Hewahette, at an elevation of 4500 feet, in a fine day the temperature
of the air under a felt roof is 120° when the fans are not working, so
that a great drying power is thus made available at no expense.
The lower floor, on the other hand, is adapted for the application of
artificial heat for the purpose of evaporating the surface water from
each separate batch of coffee as it is taken from the washing cisterns
preparatory to its being deposited in the upper floor. In wet weather
this is essential, for the atmospheric air being then saturated with
moisture, no drying can take place until its capacity for absorbing
moisture is increased by an increase of temperature. One other reason
for adopting this arrangement is, that when the coffee is first taken
wet from the washing cisterns the interstices of the beans are more or
less occupied with water, and thus present a medium less pervious to air
than is the case when the surface water has been dried off. Consequently
it is desirable that, until this has been done, the depth of the coffee
should not exceed six inches, and, to be equal to every emergency, the
heating power ought to be sufficient in the wettest weather to evaporate
the surface water from the produce of a day’s picking (within
twenty-four hours), so as to allow of its being removed into the upper
floor. The daily number of bushels picked from any, or the same
plantation, is of course a variable quantity: depending on the extent of
the cultivation, the quality of the trees, the number of hands employed,
and the elevation of the land; the latter, when considerable, having the
effect of prolonging the picking season. The stove is more than
sufficient for a daily picking of 400 bushels of cherries.
The heating stove is square, has a waggon head with a semicircular
opening in the centre for the passage of air, and is constructed of
stout sheet-iron. It is placed within an arch, with a clearance of nine
inches all round also for the passage of air, the guiding principle in
its construction being to adapt it to the burning of wood, and to expose
as much heating surface as possible to the air which flows past it into
the air-chamber beneath the ground-floor. The stove opening is the only
one which admits air to the coffee on the ground-floor. Consequently,
when it is more or less closed by a damper, the power of the fans is
exerted either in part or altogether on the mass of coffee in the upper
floor.
In these applications of natural and artificial heat to the curing of
coffee, the heat is conveyed by the air through the whole depth of
coffee in such a manner that each bean feels its influence, whilst the
watery products elicited by the heat are at the same time, and by the
same means, carried off. It cannot be doubted that these applications
are far more effectual than any of the modes hitherto in use; in some
cases stoves were employed in the apartment containing the coffee, but
it is obvious that their influence could not extend beyond the surface
of the mass, and that, if the apartment was closed, there was no
provision for carrying off the air that had become loaded with moisture
due to its temperature; whilst, if the apartment was open, so as to
afford a free draught of air, the greater portion of the heat given out
by the stove was carried out before the heated air could act on the
coffee. In other cases, heating pipes of various kinds were used below
the floors on which the coffee was placed. This arrangement, however,
has the effect of injuring the coffee, by steaming it; no provision
being made for carrying off the excess of hot watery vapour which
accumulates within the mass, but, on the other hand, the natural
processes of decomposition are assisted and promoted.
The construction of Mr. Clerihew’s heating apparatus is simple, and a
moderate supply of fuel has a considerable effect in raising the
temperature of cold damp air before it is brought into contact with the
coffee through which it is drawn by the aid of the fans. This heated air
becomes diffused throughout the whole of the chamber, which extends
beneath the ground-floor in such a manner that no portion of the coffee
which is on that floor can be free from its influence.
The fans at Rathoongodde are much more powerful than those in common
use, the peculiarity in the shape of the blade giving them a great
advantage as air-moving machines, in so far as the indraught is
concerned, whilst one-half of the periphery being open a ready exit is
afforded for the discharge of air. In the ordinary fan, if a smoking
match is applied to any part of the indraught opening, the air will be
seen to flow towards a neutral point in the centre of the fan, following
a spiral direction, and thence in the periphery of the fan.
In Mr. Clerihew’s modification of the blade each film of air, so to
speak, flows into the fan directly, until it impinges on the curvilinear
part of the blade, and from that point is thrown at a right angle
towards the periphery. The column of air being thus less distorted in
its progress, there is not only a greater quantity discharged, but much
less power is consumed in effecting that discharge--in the common fan
it is evident, from the circumstance of the air flowing to an apex,
that a great amount of power is wasted in producing the increased
velocity with which a column of air equal in volume to the two ingress
openings of the fan must pass so contracted an area before it is
discharged; hence it is that the fan, as an air-moving machine, has been
considered unequal to the screw.
The enclosed space of the coffee-curing house at Rathoongodde has an
area in the cross section of 100 superficial feet, it is 70 feet long,
and a pair of fans are placed at one end. Repeated experiments have
shown that, when the fans make 100 revolutions per minute, a cloud of
smoke travels to them from the centre of the enclosed space (a distance
of 35 feet) in precisely 15 seconds, hence we have 100 × 35 = 3500 cubic
feet of air discharged in a quarter of a minute, or 14,000 cubic feet
per minute; a screw of nearly seven feet in diameter would be required
to discharge the same amount of air, and the cost of it in England is 84
guineas, whilst the pair of fans made and fitted up at Rathoongodde cost
under 9_l._
In the centre of the enclosed space, with a depth of four feet of coffee
in the upper floor, the flame of a candle is blown to a right angle when
the whole power of the fans is put on that floor; near to the fans it is
extinguished, the air moving forward with a uniformly accelerated
velocity from the farther end towards the fans, owing to the constant
accessions made by the air entering the enclosed space throughout its
whole length.
It has already been mentioned that the only entrance of air into the
air-chamber beneath the ground-floor is by the opening in which the
stove is placed, consequently, when this opening is closed by a damper,
it is obvious that the whole power of the fans is exerted on the mass of
coffee which is being cured on the upper floor, and that the division
of this power may be regulated at will by more or less obstructing the
entrance of air to the air-chamber by the damper. The upper floor is not
supposed to be an air-tight apartment, but as the chief entrance of air
is by the two doors in the end, its influx may be so far obstructed by
closing them as to throw the greater part of the power of the fans on
the coffee which is on the ground-floor, when this is required. Again,
since it is obvious that, in wet weather, when the atmosphere is fully
saturated with moisture proportionate to its temperature, it becomes a
desideratum to introduce a portion of the artificially heated air into
the vacant space which is over the coffee in the upper floor, so that
the air which passes down through that coffee may have an absorbing
tendency; this is accomplished by shutting the doors of the upper floor
and throwing open the top-covering of the fan. By this means one-half of
the air which is drawn from the stove is thrown in above the coffee in
the upper floor, whilst the other half is discharged altogether. This
infusion of heated air would on many occasions be attended with benefit,
but the advantage will naturally depend on the comparative state of
dryness of the coffee on the two floors.
In having recourse to these practical modifications some little judgment
and observation are of more service than precept. It will be found, for
instance, that if the coffee in the upper floor approaches the dry
stage, it is better in wet weather to shut the doors of that floor as
well as the tops of the fans, so that only a small flittering of air
sufficient to ward off the first stages of decomposition may pass
through that coffee, whilst the wet coffee below has the full benefit of
a more rapid circulation of absorbent air.
Attention may now be directed to the practical results which these
arrangements have afforded in the curing of coffee.
The coffee in the upper floor, as the crop advanced, gradually
increased in depth until it stood at four feet all over the floor. When
at this depth, with the fans making 100 revolutions per minute, the flow
of air was quite sensible to the hand placed on the surface of the
coffee, and was rendered apparent by the smoke from a match following
the direction of the air; at the same time the rarefaction of the air
within the enclosed space was so very slight as barely to be appreciable
by a very delicate mountain barometer, though it had the effect of
causing the door to shut with a slam; thus showing that a slight
rarefaction of the air is sufficient to disturb the balance of
atmospheric pressure, even when acting through a medium of coffee of
considerable depth. The current of air thus established continued to
flow without interruption until the fans were stopped.
A cold glass tumbler taken into the store in a warm day, when the fans
were not in motion, became instantly dimmed and wet by the precipitation
of moisture from the internal air. When another glass was taken into the
store, one minute after the fans were put in motion, it remained clear,
without a trace of moisture.
A very satisfactory result soon showed itself, viz. that whilst the
temperature of the air as it entered the moist coffee was 80° in a warm
day, the temperature of the coffee itself, as indicated by an immersed
thermometer, was only 58°; the wet coffee being invariably coldest when
the air that was made to pass through it was warmest. This paradox
admitted of easy explanation, when it was considered that each bean of
undried coffee was under similar circumstances to evaporating vessels of
water placed in a draught of warm air for the purpose of cooling the
water. The cold thus produced was, therefore, the necessary concomitant
of the evaporation that was going on, and the difference between these
temperatures afforded a measure of the drying power in different states
of the weather. The circumstance of the hot air lowering the
temperature of the coffee was also favourable in another point of view,
seeing that it has been shown that heat is one of the conditions which
promotes mouldiness, or the germination of fungi.
Every bushel of parchment coffee contains half a cubic foot of air, a
fact ascertained by a bushel which took thirty-three measures of water
to fill it. When full of newly-washed coffee it took thirteen of these
measures of the water to displace the air from the interstices of the
beans without overflowing, so that we have 13--33 of air in a bushel of
coffee; in other words, half a cubic foot. Hence, the fans in use are
capable of giving a fresh atmosphere to 28,000 bushels of coffee every
minute, or in the same time four fresh atmospheres to 7000 bushels.
During a continuance of nearly three months of wet weather which
occurred at one crop time, the coffee in the curing-house dried very
slowly, but was kept in a perfectly fresh and sweet state without the
intervention of any manual labour, further than in depositing each day’s
increase in the lower floor to dry off the surface water, and removing
that of each previous day to the upper floor, where it was spread on the
top of all the coffee that had previously accumulated. These three
months of wet weather were succeeded by a fortnight of very dry weather,
and, on examining the coffee at the end of that period, it was found to
have reached the dry horny stage at which it is usual to despatch it
from the estate to Colombo, for the purpose of being peeled and shipped.
On examining the beans they were found to be of that clear colour which
distinguishes coffee carefully cured in small quantities, with the
advantage of the most favourable weather. Under like circumstances, viz.
during such a continuation of wet weather, it would have been impossible
to preserve the coffee free from more or less mustiness of smell, by
manual labour employed in the usual way, whilst at the same time the
expense of storework would have been more than fourfold. The whole
expense of the storework, viz. pulping, washing, curing, and storing the
Rathoongodde coffee, amounted to 2¼d. per cwt., and when it is
considered that during crop time the value of every man’s labour is
greatly increased, it is an object, as far as possible, to substitute
mechanical contrivance for manual labour, so that all hands may be
employed in gathering the crop as it ripens.
Every planter knows that when coffee is spread out in a single layer on
the floor of his store, it becomes dry after a time, and is well cured
without any further attention on his part; but it is impossible to
devote sufficient space for this purpose without incurring an expense
which would be quite incompatible with his circumstances. When, however,
coffee is thus spread out in a single layer, it is obvious that the
reason why it requires no attention is, because the beans being freely
exposed to the atmosphere, there is naturally a constant change of the
air by which they are surrounded; the same air is not sufficiently long
in contact with the beans to excite the first action of decomposition,
and the absorption of oxygen is not accomplished. Presuming, however,
that it were so, the subsequent actions could not take place, for the
products of the first action, viz. carbonic acid, heat and watery
vapour, would immediately make their escape and be dissipated by the
atmosphere, which is precisely what takes place when, by mechanical
means, a draught of air is carried through a _mass_ of coffee. Hence, it
is evident that the requirements of space are overcome by the adoption
of this plan, and that a great mass of coffee is placed under conditions
similar to those by which a single layer is influenced when exposed to a
natural draught of atmospheric air.