The superiority of the vacuum pan is not universally admitted, and we are told that in France it is superseded by open pans, similar to those called in America "Mape's Evaporators." However this may be, I cannot help believing that the vacuum pan has many decided advantages over all others. One is manifest; the sugar may be grained in the pan, and the granulation is completely under the control of the operator. He may accelerate or retard it at pleasure; he may carry it so far that sugar will not run from the pan, and will have to be taken out of it; he may so conduct the operation as to increase, almost at will, the size and hardness of the crystals. This last is an indispensable requisite if the practice of draining sugar in pneumatic pans should be adopted.
The atmospheric pressure is made too powerful for sugars boiled in any other manner; it breaks and destroys the crystals, and in a very few days sets the sugar to fermenting.
The pneumatic draining of sugar has many things to recommend it—the usual loss by drainage is avoided, sugar is got ready for market day by day, as it is made, and it may be bleached by pouring white syrup over it and forcing it through the mass. It is said that the process is attended with considerable loss in weight, but as all that drains from the pan may be boiled over once or twice, it is not easy to conceive how the loss can occur.
Cane juice contains many ingredients besides sugar, the principal of which are albumen, gluten, gum, starch, resin, wax, coloring matter, and certain salts, all of which, either collectively or individually, have the power of preventing granulation, as may be proved by their addition to a syrup of pure sugar, which will then defy all attempts to make it crystallise. If, therefore, we want to make good sugar, we must endeavour to free our cane juice as much as possible from those substances.
Now, cane juice is no more the sap of the cane, than apple juice is that of the apple tree; it is the natural product of the cane, and, in all probability, would contain but a small proportion of these foreign matters if it could be expressed without being accompanied by the sap, they being the natural constituents of the last-named fluid. A patent has, I believe, been lately taken out for separating the cane juice without the sap. However, in the absence of such an improvement, much may be done by care and attention at the mill; the green bands and trash which usually accompany the canes from the field, should, therefore, be carefully removed before they are passed through, as they contain no saccharine matter, abound in the deleterious substances already mentioned, and communicate a bad color to the juice; therefore, the ripe cane only should pass through the mill. There are but few planters who have not had to contend with sour juice, and they attribute the difficulty they experience in making sugar therefrom, to the presence of acetic acid, or vinegar; but this is quite an erroneous idea, as the acetic acid is very volatile, and evaporates quickly on the application of heat, which may be proved by throwing a gallon of strong vinegar into a pan of liquor; it will do no harm, provided it be boiled before tempering; on the contrary, the effect, if it be properly done, will be beneficial, as it will promote the coagulation of the albumen; it is the gum which is always formed during the acetous fermentation of sugar that prevents granulation; hence, then, acidity is strictly to be guarded against, as fermentation once commenced, it will be impossible to make good sugar, it will continue throughout the process, and even in the hogshead; so that canes should be ground as soon as possible after they are cut, and all rat-eaten and broken ones carefully excluded. Canes may, however, be kept some days without fermenting, provided they be not broken or damaged, it being, as we said before, the mixture of the sap and the cane juice that makes the liquid so prone to fermentation; and the mill, gutters, and everything with which the juice is likely to come in contact, should be kept carefully clean, and whitewashed immediately after, and the whitewash removed before use, as acetate of lime being an exceedingly soluble and deliquescent salt, will not improve the quality of the sugar; whilst the gutter should be short, and sheltered from the sun's rays, they having the effect of greatly expediting chemical action.
I shall say no more on this subject, but will proceed to consider the mode of tempering and clarifying cane juice, and the action of lime on the various substances contained therein. The expression "tempering" has, I presume, been, adopted in consequence of the use of tempered lime for the purpose of precipitating the feculencies, held in solution in the cane juice, into a state of suspension; and clarification is the process by which we afterwards clear the liquor of these and other foreign matter. Now, as I before observed, "fermentation should be most strictly guarded against;" our first efforts should be directed to free the cane juice from those substances most conducive to that process; and on inquiry we find these to be albumen and gluten; so far, however, from getting rid of them in cold tempering, we adopt a course which retains them permanently in solution, as lime has the power of rendering them permanently soluble, and of forming soapy compounds with resin, wax, and chlorophyle, or the green coloring matter of leaves, forming an insoluble compound with and precipitating only the starch, and converting at the same time the green color of the chlorophyle (which is, in all probability, attached to the resin), into a dark brown, of a greater or less intensity, according to the composition of the cane juice, and, consequently, the quantity of lime required; it follows, therefore, as a matter of course, that if juice be tempered before these substances have been removed, they must be permanently retained, and they have all the power of preventing granulation.
Albumen, and gluten are both coagulable by heat; if, therefore, we raise the liquor to the boiling point prior to applying the lime, taking care to remove the scum as soon as it shows signs of breaking, and continuing the boiling until the scum thrown to the surface becomes inconsiderable, we shall find that the albumen and gluten, in coagulating and rising, have carried with them the small particles of woody fibre, the wax, and a large proportion of the coloring matter, and that the lime will now throw down the starch, and any other little impurities remaining in suspension in the liquor, leaving it perfectly clear and bright. Tempering is an exceedingly delicate chemical operation, and I have no hesitation in saying, that on its proper performance depends the quality of the produce. The following simple experiments, which all have it in their power to try, will, if they give themselves the trouble, fully satisfy them of two important points—the superiority of the hot over the cold mode, and the necessity for great attention to the operation of tempering. Let them take a tumbler of cane-juice and a bottle containing lime water, add the latter to the former by drops, pausing and stirring between each, and they will find that, after the addition of a certain quantity, the opaque gummy appearance of the liquor undergoes a change, and the impurities contained in it separate into flakes, which increase in size with each drop of lime added, until they become extinct, and the supernatant liquor perfectly transparent; this is the precise point at which the liquor is tempered, and each drop of lime added after this, causes the flakes to diminish rapidly in size, at last entirely to disappear (being re-dissolved), and the liquor to resume its former gummy appearance; it is, therefore, evident that there should be no such expressions as tempering high or low.
The reason why some liquor is so difficult to clean is, that it is either tempered high or low; if it be exactly tempered, the impurities contained in it being entirely separated and thrown out of solution, rise to the surface immediately on the application of heat, and are easily removed; but if there be too little lime, a great portion remains in solution, and if too much, a proportional quantity is re-dissolved; and in either case cannot be removed by any mechanical means. It is, therefore, necessary to have some precise test for the application of lime.
As regards the superiority of the hot over the cold tempering, let any one take, in separate vessels, two gallons of cane-juice, and temper one, adding the lime in small quantities—say, of three grains at a time—and keeping an account of the quantity used; he will find that the first portions produce no effect whatever, and that it is only after the addition of a considerable quantity that the desired precipitation of the impurities manifest itself. Why is this? Because albumen, gluten, resin, and chlorophyle, being soluble in lime, lime is equally so in them, and they must first be saturated before it will produce any other effect. Let the liquor thus tempered, be then placed on one side. Put the other gallon over a fire, and boil it, removing the scum just before, and during, ebullition; let it then be taken off the fire, and tempered in the same way as the other. The very first quantity of lime added causes the appearance of the floccy precipitate; and if the addition of the lime be continued until it be precisely tempered, it will be found that the hot possesses the following advantages over the cold-tempered liquor:—In a quarter of an hour its impurities will have subsided to a sixteenth of its bulk, leaving the supernatant liquor as bright and clear as pale brandy; while those in the other have only sunk to one-quarter of its bulk. The color of the former clear liquor will not be less than one-half the intensity of that of the latter. The lime used in the hot has been less by one-third than the quantity used in the cold tempering.
Of course, on level estates there is little difficulty in tempering liquor, but on hilly properties scarcely two pans will require the same quantity.