(a) Nomenclature

For details on botanical nomenclature, see Newton (2004).

• Chem. Abstr. Serv. Reg. No.: 8001-97-6
• Chem. Abstr. Name: Aloe barbadensis
• Botanical name: Aloe vera (L.) Burm. f. (synonym, Aloe barbadensis, Aloe humilis Blanco, Aloe indica Royle, nomen nudum, Aloe perfoliata var. vera L., Aloe vulgaris Lam.) (GRIN, 2013).
• Family: Xanthorrhoeaceae
• Genus: Aloe
• Plant part: Leaf
• Common names: Aloe vera; Aloe vera Linné; True aloe; Aloe barbadensis; Barbados aloe; Curaçao aloe; Mediterranean aloe; Ghritakumari; Lu Hui; Luhui, etc.

From WHO (1999), Eur Ph (2008), O’Neil et al. (2006), SciFinder (2013), IASC (2013a), Boudreau et al. (2013a).

(b) Description

Aloes are perennial succulents or xerophytes; they can adapt to habitats with low or erratic water availability, are characterized by the capacity to store large volumes of water in their tissue, and are able to use crassulacean acid metabolism, an adaptation to the photosynthetic pathway that involves the formation of malic acid (Boudreau et al., 2013a). Aloe plants, such as Aloe vera (Fig. 1.1), all have green fleshy leaves covered by a thick cuticle or rind, under which is a thin vascular layer covering an inner clear pulp (Boudreau et al., 2013a; Fig. 1.2) The leaves are 30–50 cm in length and 10 cm in width at the base, pea-green in colour (when young spotted with white), and with bright yellow tubular flowers 25–35 cm in length arranged in a slender loose spike (WHO, 1999).

Fig. 1.1

Aloe vera (L.) Burm. F, plant and flower

Fig. 1.2

Schematic representation of the Aloe vera plant, showing a cross-section through a leaf

The vascular bundles, located within the leaf pulp, transport (i) water and minerals from the roots to the leaves; (ii) synthesized materials to the roots; and (iii) latex along the margins of the leaf for storage (Ni et al., 2004; Fig. 1.2). The number of vascular bundles varies depending on the size of the leaves and the age of the plant (Ni et al., 2004).

Aloe vera plants contain two major liquid materials (Fig. 1.2): first, a bitter yellow latex located under the strongly cutinized epidermis of the leaves in the vascular layer and containing a high concentration of anthraquinone compounds, which has been used throughout the centuries as a cathartic and for medicinal purges; and, second, a clear mucilaginous gel produced by the thin-walled tubular cells in the inner central zone (parenchyma) that has been used since ancient times to treat burns and other wounds, where it is thought to increase the rate of healing and reduce the risk of infection (Joseph & Raj, 2010). A third liquid may also be obtained by macerating the whole leaf.

[Both the scientific and the lay literature (e.g. on internet sites) are extremely inconsistent when referring to products obtained from Aloe vera. The problem starts with the fact that the three types of liquids that are obtained from Aloe vera leaves are interchangeably referred to as “Aloe juice,” which has caused confusion in the literature. For disambiguation reasons, the term “Aloe juice” should be restricted – if used at all – to the latex material of the pericycle, which is in accordance with the pharmacopoeial definitions (WHO, 1999; Eur Ph, 2008; JP XVI, 2011); and the inner leaf liquid material should be referred to as “gel” (WHO, 1999). Interchangable terms found in the literature for the “gel” are inner pulp, mucilage tissue, mucilaginous gel, mucilaginous jelly, inner gel, and leaf parenchyma tissue (Hamman, 2008).]

(a) Aloe vera gel extract

The inner leaf pulp of the Aloe vera plant contains large, thin-walled cells that produce gel, the clear, mucilaginous, and aqueous extract of the inner central area of the leaf pulp (Fig. 1.2). Aloe vera gel serves as the water and energy storage component of the plant. The mechanical extrusion of the mucilaginous gel from the inner leaf pulp gives a 70% yield with a water content of 99–99.5% (Femenia et al., 1999).

Polysaccharides in Aloe vera gel consist of linear chains of glucose and mannose molecules, and, because there is considerably more mannose present than glucose, the molecules are referred to as polymannans. These linear chains range in size from a few to several thousand monosaccharide molecules. The major polysaccharide, acetylated mannan, is composed of one or more polymers of various chain lengths with molecular weights ranging from 30 to 40 kDa or greater, and consisting of repeating units of glucose and mannose in a 1:3 ratio (Channe Gowda et al., 1979; Mandal & Das, 1980; Yaron, 1993; Femenia et al., 1999; Boudreau et al., 2013a; Fig. 1.3). Chemically preserved fresh Aloe vera gel stored at room temperature or incubated at 40 °C for 48 hours exhibited degradation in its rheological properties, a decrease in the content and composition of polysaccharides, and a substantial increase in the mannose:glucose ratio, from 2.9 in the fresh gel to 13.4 in the incubated gel (Yaron, 1993).

Fig. 1.3

Chemicals present in gel and latex prepared from Aloe vera

(b) Aloe vera whole leaf extract

The Aloe vera whole leaf extract (sometimes referred to as whole leaf Aloe vera juice, Aloe juice or nondecolorized whole leaf extract), is the aqueous extract of the whole leaf with lignified fibres removed. The whole leaf extract contains both the gel from the inner parenchyma leaf pulp and the latex. The restricted distribution of the bitter latex within the margins of the leaves of the Aloe vera plant suggests that this thin layer is the primary site of secondary metabolites biosynthesis: compounds that do not function directly in plant growth and development and serve as a plant defence strategy (Boudreau et al., 2013a). A wide variety of secondary compounds have been isolated from the Aloe vera latex (Reynolds, 2004). The isolated compounds are largely phenolic in nature, and many are anthraquinone C-glycosides, anthrones, and free anthraquinones (Park et al., 1998). The levels of anthraquinone C-glycosides in Aloe vera latex are quite variable; however, they may constitute up to 30% of the dry weight of the latex (Groom & Reynolds, 1987). Aloe vera latex contains four major C-glycosyl constituents: aloin A, aloin B, aloesin, and aloeresin A (Fig. 1.3; Saccù et al., 2001). Aloin A, a C-glycosyl anthrone, also referred to as barbaloin, is the major component of aloe latex. Aloin A and its epimer, aloin B, also referred to as isobarbaloin, have a 9-anthrone skeleton and a β-D-glucopyranosyl substituent. Aloesin, also known as aloeresin B, is a 5-methyl chromone with an 8-β-D-glucopyranosyl substituent, and aloeresin A is a 5-methyl chromone with an 8-β-D-glucopyranosyl-2-O-trans-p-coumarol substituent. Several other C-glycosyl-chromones and anthrones have been isolated from Aloe vera, including aloe-emodin, the anthraquinone of barbaloin and isobarbaloin (Boudreau et al., 2013a).

The occurrence in Aloe vera latex of endogenous free anthraquinones and anthrones results from oxidative processes acting on the glycosides rather than from metabolic synthesis (Boudreau et al., 2013a). In addition, the latex from Aloe vera contains several aromatic compounds, such as aldehydes and ketones (Saccù et al., 2001). The sugar moiety in aloins is D-glucose, and studies indicate that carbon atom 1 of the D-glucose moiety is linked directly to carbon atom 10 of the anthracene ring in a β-configuration (Fig. 1.3). The carbon–carbon bond is quite resistant to acid and alkaline conditions; however, the intestinal microflora of humans and animals have been shown to cleave the β-C-glucosyl bond, although considerable variation in response among animal species occurs. Cleavage of the β-C-glucosyl bond results in the formation of aloe-emodin, the cathartic principle of the latex, and other free anthraquinones and anthrones (Boudreau et al., 2013a; see Section 4.1.1b). In commercial products containing whole leaf extract, a rapid deterioration of aloin was detected during storage, especially at higher temperatures (Pellizzoni et al., 2011).

(c) Aloe vera decolorized whole leaf extract

Activated carbon treatment of the Aloe vera whole leaf extract is used to remove bitterness and colour caused by the anthraquinone components of the latex. This results in a product termed “decolorized whole leaf extract” that has quite different properties from the whole leaf extract. Aloe vera decolorized whole leaf extract is also referred to as “whole leaf Aloe vera gel” (Boudreau et al., 2013a). Dentali (2013) noted that an industry standard for aloin content of decolorized Aloe vera whole leaf extract is < 10 ppm. Sehgal et al. (2013) reported results of toxicological assessment of a commercial decolorized whole leaf extract that contained approximately Aloin A at 0.9 ppm, Aloin B at 1.3 ppm, and aloe-emodin at 0.2 ppm. A decolorized Aloe vera whole leaf extract assessed for safety by Shao et al. (2013) was reported to contain combined Aloin A and Aloin B at < 0.1 ppm.

Although Aloe vera gel and the decolorized whole leaf extract are similar in that each contain little or no latex anthraquinones, carbon adsorption changes the physical and chemical properties of the whole leaf extract. Aloe vera decolorized whole leaf extract differs from the gel in that it exhibits a degradation in rheological properties and a loss of approximately 19–23% of the complex polysaccharide content (Pelley et al., 1998).

(d) Dried Aloe vera latex (pharmaceutical material)

The dried Aloe vera latex is the solidified liquid originating in the cells of the pericycle and adjacent leaf parenchyma, and flowing spontaneously from the cut leaf, allowed to dry with or without the aid of heat (WHO, 1999). The material is used for medicinal purposes and its composition is specified in several official pharmacopoeias (see Section 1.6).

(a) Medicinal use

The Aloe vera plant has been used in folk medicine for more than 2000 years, and it remains an important component of traditional medicine in many contemporary cultures, such as China, India, the Caribbean, and Japan (Grindlay & Reynolds, 1986). Aloe vera first gained popularity in the USA in the 1930s with reports of successful use of freshly cut leaves in treating X-ray burns (Ulbricht et al., 2007). Both classes of Aloe vera leaf products, gel and latex, are reported to possess a wide range of pharmaceutical activities.

WHO lists the short-term treatment of occasional constipation as a use for Aloe vera latex that is supported by clinical data (WHO, 1999). The well established cathartic properties of anthraquinone glycosides provide strong evidence in support of the laxative properties of Aloe vera (Ulbricht et al., 2007). The European Medicines Agency also found that the therapeutic indication as an “herbal product for short-term use in cases of occasional constipation” is a well established use of Aloe vera latex (EMA, 2006).

For the gel, WHO identified no uses supported by clinical data. Traditional uses include the external treatment of minor wounds and inflammatory skin disorders. The gel may be used in the treatment of minor skin irritations, including burns, bruises, and abrasions (WHO, 1999).

In recent times, the oral consumption of Aloe vera has been promoted as prophylaxis and therapy for a variety of unrelated systemic conditions. The scientific literature yields little to substantiate claims of usefulness for systemic conditions by the ingestion of Aloe vera (Boudreau et al., 2013a).

Aloe vera may be used in veterinary medicine as laxative or in topical applications, e.g. in udder disinfectants (Leon, 2003).

(b) Food use

Aloe vera extracts may be used in beverages as bitter flavouring agent (O’Neil et al., 2006). Food products include health and soft drinks, yoghurts, jams, instant tea granules, candies, alcoholic beverages, and ice cream (Ahlawat & Khatkar, 2011). Aloe vera may also be used in food supplements (Steenkamp & Stewart, 2007). The Dietary Supplements Label Database lists 43 products that contain Aloe vera as an active ingredient in amounts of 0.33 to 750 mg per capsule (NLM, 2012). Aloe vera whole leaf extract (which combines both the gel and latex) and Aloe vera decolorized whole leaf extract (from which most of the latex components have been removed) are popular as dietary supplements for various systemic ailments. The anthraquinone components of these products appear to vary significantly in their content of aloe-emodin and aloin A, the major anthraquinone constituent of Aloe vera latex. (Elsohly et al., 2007) evaluated 53 liquid and 30 semisolid and solid aloe-based commercial products. The liquid samples all contained either aloe-emodin or aloin A at ≤ 10 ppm, with many having no detectable levels of either of the two compounds. Unlike liquid products, many solid and semisolid products (11 out of 30) contained one or both of the compounds, aloe-emodin and aloin A, at ≥ 10 ppm.

(c) Cosmetic use

The gel may be used as emollient and moisturizer in cosmetics and personal care products (O’Neil et al., 2006). The gel is used in the cosmetics industry as a hydrating ingredient in liquids, creams, sun lotions, shaving creams, lip balms, healing ointments, and face packs (WHO, 1999). Other products containing Aloe vera include after-shave gel, mouthwash, hair tonic, shampoo, and skin-moistening gel (Newton, 2004).

Aloe vera may be used in cosmetics for marketing reasons (i.e. to impart a touch of “nature” to the product) rather than for actual effects, and the content may be normally kept at a low level (Committee of Experts on Cosmetic Products, 2008).

A study on skin hydration found that a single application of a cosmetic formulation containing > 0.25% of a commercial freeze-dried Aloe vera gel 200:1 concentrate improved the water content of the stratum corneum (Dal’Belo et al., 2006). However, the concentrations of Aloe vera raw materials in cosmetics vary widely from 0.1% or less up to 20% (Cosmetic Ingredient Review Expert Panel, 2007).

Anthraquinone-rich Aloe vera extracts may function as absorbers of ultraviolet radiation in suncreens, because anthraquinones absorb ultraviolet radiation (Committee of Experts on Cosmetic Products, 2008). Regulatory authorities in Germany have proposed that cosmetic products for which claims are made regarding Aloe vera should contain at least 5 g of Aloe vera per 100 g of product (Kratz, 2009).

(a) Production process

Aloe vera grows best in dry chalky soil or in a sandy loam (Grindlay & Reynolds, 1986). While the plant needs warm semi-tropical conditions, overexposure to sun results in stunted plants with low gel yield. Therefore, Aloe vera is commonly interplanted with other crops, such as fruit trees. The quality of Aloe vera plant products varies considerably due to differences in growing, harvesting, processing, and storage techniques (Boudreau et al., 2013a), and may also depend on the regulatory regime under which the product is sold (see Section 1.6).

Mexico, followed by the rest of Latin America, China, Thailand, and the USA were described as main producing countries (Rodríguez et al., 2010). Aloe vera has become an important plant crop in Arizona and in the Rio Grande valley of southern Texas (Boudreau et al., 2013a).

The production processes for Aloe vera products include various steps such as crushing, grinding or pressing, filtration, decolorization, stabilization, heat processing, and may be followed by addition of preservatives and stabilizers. A complete overview of production was provided by Ahlawat & Khatkar (2011). The technology for processing of Aloe vera gel was reviewed by Ramachandra & Rao (2008).

Harvesting of the leaves of the Aloe vera plant is generally performed by hand, with the leaves cut from the base of the plant (Grindlay & Reynolds, 1986). Individual leaves are wrapped, crated, and transported to processing plants. Ideally, the leaves are processed within a few hours after harvesting, as temperature, light, air, and humidity can affect the stability of the plant components (Paez et al., 2000). At the processing step, the leaves may be cleaned with water and a mild chlorine solution (Grindlay & Reynolds, 1986).

Aloe vera gel from the fillet of the inner leaf pulp is obtained either by manual removal of the outer layers of the leaf with a knife or by machine. Either method can be flawed and has the potential to contaminate the gel with latex (Grindlay & Reynolds, 1986). This process yields crude Aloe vera gel. High quality gel appears opaque, slightly off-white in colour, and is viscous (Vogler & Ernst, 1999).

Aloe vera whole leaf extract is obtained by grinding the whole fresh leaves, without removal of the rind. Extraneous material and lignified fibres are then removed by homogenizing and filtering the crude gel or whole leaf extracts (Yaron, 1993). Since various amounts of latex and rind may be present in the whole leaf extracts, the extracts may appear yellow to yellow-green in colour.

Activated carbon adsorption to produce Aloe vera decolorized whole leaf extract is the first processing step where an extract is intentionally subjected to chemical alteration. Aloe vera decolorized whole leaf has lower rheological values than the gel and has a lower content of complex carbohydrates than either gel or whole leaf extracts (Pelley et al., 1998).

The processed extracts are difficult to keep stable, a problem that may cause differences in product potency; therefore, the gel or whole leaf extracts can undergo a stabilization process before being bottled. This process may involve pasteurization, ultraviolet stabilization, chemical oxidation with hydrogen peroxide, addition of chemical preservatives and additives, or concentration, and/or drying (Boudreau et al., 2013a).

(b) Production volume

In the cosmetic industry, Aloe vera ingredients hold a prominent position at the top of the list showing the relative frequency of use of plant ingredients within formulations filed with the United States Food and Drug Administration (FDA) (Committee of Experts on Cosmetic Products, 2008).