Near the epidermis or outer skin the leaves of aloes contain a row of fibrovascular bundles, the cells of which are much enlarged and filled with a yellow latex. Aloe latex is obtained by cutting the leaf transversely close to the stem and inclining the leaf so that the latex flows out in about six hours (Figure 9.2).
No pressure must be applied otherwise the product will be contaminated with the mucilage present in the inner part of the leaf. The preparation obtained is bitter and yellow; it is concentrated to dryness by evaporation in open kettles, rarely by boiling in a vacuum, until it becomes a shiny mass, like broken glass with a yellow greenish to red-black colour (Capasso and Gaginella, 1997). A slow evaporation carried out either by inappropriate temperature or by spontaneous evaporation gives an opaque mass with a wax-like fracture. The taste is nauseating and bitter and the odour sour, recalling that of rhubarb, apple-tart or iodoform.
Aloes require two or three years standing before they yield their latex. In Africa the latex is collected from the wild plants; in the case of aloe plantations the drug is collected in April/July. Aloe latex (=aloe) contains mainly anthraquinones, cathartic compounds useful in constipation. It is totally different from the aloe gel (=aloe vera), a colourless gelatin obtained from the central portion of aloe leaf. The mucilaginous parenchyma tissue is excised from fresh leaves and immediately utilised for pharmaceutical preparations or lyophilized and kept dry until use. During extraction of the gel it is practically impossible to prevent contamination by the latex as the leaves are cut. On the other hand in intact leaves, anthraquinones may diffuse into gel from the bundle sheath cells. To reduce such contamination the starting material must be from varieties of Aloe with a reduced anthraquinone content. Aloe gel is sensitive to heat and light and can quickly deteriorate when exposed at high temperature. It contains mainly mucilage and it is now a familiar ingredient in cosmetics and ointments for skin ailments (Henry, 1979; Reynolds and Dweck, 1999). Some recent observations have proven that the rind and the outer leaf, normally thrown away, contain greater healing components.
The fresh whole leaf is also cut into small pieces and whipped in order to obtain a homogenous yellowish or reddish material. The preparation (total leaf extract = aloe extract) is bitter and has a very characteristic odour. Finally, it has been reported that the leaf epidermis of A. arborescens contains lectin, a product that is able to inhibit the growth of a fibrosarcoma in animals through a host-mediated effect (Imanishi etal., 1981). This could stimulate further research and give rise to a new medicinal preparation.
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