5. Barbados Aloes

Barbados Aloes. But very little aloes is now produced in the island of Barbados, and that which bears this name is in reality prepared in the Dutch islands of Curacao, Aruba, and Bonaire; it is therefore, often and more appropriately, termed Curacao aloes. The plant which yields it appears to be Aloe vera, Linne, or possibly A. chinensis, Baker.

Curacao or Barbados aloes arrives usually in boxes, sometimes in gourds. It occurs in the vitreous as well as in the livery variety; the former, commercially known as ' Capey ' Barbados, may become opaque on keeping, a change to be ascribed to the slow crystallisation of the aloin. It is also by no means uncommon to find packages that are partly filled with glassy, partly with opaque, aloes; such differences in the appearance are probably due to slight differences in evaporating the juice.

Livery Curacao aloes of good quality varies in colour from yellowish or reddish brown to chocolate brown, lower grades being black and occasionally burnt. It breaks with a dull, waxy, even fracture, small splinters exhibiting under the microscope numerous minute crystals of aloin. It closely resembles Zanzibar aloes, and, in fact, can be distinguished (apart from chemical tests) only by its distinctive odour.

Vitreous Curacao aloes, which is not a highly esteemed variety, is distinguished from the livery by its transparency. It has usually, in small fragments, a garnet-red colour; in other respects it resembles the foregoing.

The student should have little difficulty in identifying the foregoing varieties of aloes. Two only (Cape and Curacao) are commonly met with in the vitreous form, and they are easily distinguished both by their colour and by their very different odour. - Five (Socotrine, Zanzibar, Natal, Curacao, and Uganda) are hepatic. Of these the odour of the Natal distinctly recalls that of Cape, whilst the greyish green or pale yellowish brown powder is characteristic. Uganda aloes is readily identified by its bronze-gold colour. Both Natal and Uganda aloes are practically obsolete. Socotrine aloes is remarkable for its unpleasant odour and uneven, porous, fracture. Zanzibar and hepatic Curacao are very similar in appearance, but differ in odour; these are the only two likely to be confused, and if necessary a chemical test must be applied.

Chemical Reactions

{a) General Reactions. The following are the most important general reactions for aloes: -

Schonteten's Reaction

Dissolve 0.1 gramme of aloes in 10 c.c. of boiling water and add 0.5 gramme of powdered borax; a green fluorescence is produced. This reaction is due to barbaloin; all aloes except Natal (see below) respond to it.

Borntrager's Reaction

Dissolve 0.1 gramme of aloes in boiling water, cool, add 10 c.c. of benzene, shake vigorously, separate the benzene solution and shake it with solution of ammonia; the alkaline solution is coloured red. This reaction is due to aloe-emodin; all aloes except Natal, respond to it.

Chrysammic Acid Reaction

Heat 1.0 gramme of aloes with 20 c.c. of nitric acid in a dish on a waterbath for 2 hours replacing the liquid lost by evaporation; then dry on the waterbath; the resulting brown powder dissolves in ammoniacal water with violet coloration.

(b) Special Reactions: -

Natal Aloes, which is quite distinct from all other aloes, is readily identified by Histed's reaction: - Dissolve 0.1 gramme of aloes in a few drops of sulphuric acid on a porcelain slab and gently blow the vapour of fuming nitric acid over the surface; a green, then red, and finally deep blue colour is developed.

Curagao Aloes

To 10 c.c.of a freshly prepared, 0.1 per cent, aqueous solution of the aloes add 1 drop of a 5 per cent, solution of copper sulphate, a trace of sodium chloride and a few drops of alcohol; a fine wine-red colour permanent for 24 hours is produced. Of the other varieties of aloes, Cape aloes alone may give a pale colour which, however, rapidly fades. The reaction is due to isobarbaloin (see below).

Cape Aloes

Drop a few drops of nitric acid on a little of the crushed aloes; a permanent green colour is gradually produced.

Socotrine And Zanzibar Aloes

With the nitric acid test, a reddish brown colour is produced. Under the same conditions Curacao aloes gives a crimson colour.

Constituents

The principal constituent of all the foregoing varieties of aloes (with the exception of Natal aloes) is the pale yellow, crystalline, glucoside, barbaloin (formerly called socaloin, zanaloin, capaloin, etc, according to its origin). In Curacao and in true Barbados aloes the barbaloin is accompanied by isobarbaloin which is crystalline and isomeric with barbaloin but easily distinguished from barbaloin, as it yields the cupraloin test (see above). Socotrine and Zanzibar aloes contain no isobarbaloin, and Cape aloes traces only.

The crystalline aloin is accompanied by an amorphous aloin, β-barbaloin, which may be produced by heating barbaloin for about three hours to 160°-165°; it is isomeric with barbaloin, and constitutes part at least of the water-soluble substances other than barbaloin and isobarbaloin. β-barbaloin is particularly abundant in Cape and Uganda aloes.

Other constituents of aloes are resin and aloe-emodin, in addition to water-soluble substances (other than the aloins) of which nothing definite is known. The resin of Curacao aloes consists of barbalore-sinotannol combined with cinnamic acid; that of Cape, Uganda, Natal, and probably Zanzibar and Socotrine aloes consists of capa-loresinotannol combined with paracumaric acid.

Aloe-emodin is a decomposition product of barbaloin and occurs in small proportion only.

Various formulae have been proposed for barbaloin of which that of Leger, C21H10O9, is probably the most correct. It is sparingly soluble in water, more readily in alcohol and acetone. Its aqueous solution slowly acquires a green fluorescence when saturated with borax. It is a glucosidal methylanthra-quinone derivative, yielding by prolonged standing of an alcoholic solution, or by treatment with sodium peroxide (but not with mineral acids), aloe-emodin and d-arabinose. It is isomeric with frangulin (compare p. 245), but the latter is hydrolysed by mineral acids yielding rhamnose.

The proportion in which the aloins are present in the respective aloes is not accurately known. Tilden (1872) and Treumann (1880) obtained from 20 to 25 per cent, of aloin from Curacao aloes. Carr and Reynolds (1907) found in a number of specimens from 12.6 to 27.9 per cent. As no exact assay process is known, and as there appears to be considerable loss in its extraction, it may be assumed that good Curasao aloes may contain upwards of 30 per cent, of crystallisable aloins. Socotrine and Zanzibar aloes appear to contain less. Cape aloes has yielded over 9 per cent.

Natal aloes differs materially from other aloes in its composition. It contains nataloin, homonataloin, and a resin consisting of nataloresi-notannol (probably identical with capaloresinotannol) combined with paracumaric acid. Nataloin does not yield a fluorescent solution when dissolved in a solution of borax.

Uses

All the varieties of aloes have more or less powerful purgative action, Cape aloes being the strongest and Natal aloes the weakest, all of them acting with remarkable slowness. Aloes is one of the most valuable purgatives in certain forms of constipation, as it improves the digestion and does not lose in activity by repetition.

Varieties

In addition to the above-described varieties Mocha aloes is occasionally imported from Bombay in tin-lined cases; it is a black, brittle, glassy aloes of very strong odour and inferior quality, Jafferabad aloes, which is sent from that port to Bombay, is also nearly black; it does not enter into English commerce.