SECTION XIII.. Preparation for Market--(_continued_).

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Parchment Coffee, when in an unseasoned state, is prone to enter into decomposition from the time at which it is withdrawn from the protection of the living organism until it is thoroughly seasoned by drying, after which it may be kept for any length of time in a dry place.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 9l.

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.