Sciences, Federal University of Paraíba, João Pessoa, Brazil
3Visiting scholar at MIT Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, USA
4Laboratory of Drug Development, Department of Pharmacy, Federal University of Rio Grande do Norte, Natal, Brazil
5Department of Chemistry, Federal Rural University of Pernambuco, Recife, Brazil
6University of Tiradentes (Unit), Aracaju, Brazil
7Institute of Technology and Research (ITP), Nanomedicine and Nanotechnology Laboratory (LNMed), Aracaju, Brazil
Abstract
Aloe vera L. (Aloe barbadensis Miller) from Asphodelaceae (Liliaceae) family, is a medicinal plant frequently used in medicines, cosmetics, and food products. Seventy-five potentially active compounds have already been identified from A. vera extract, among them: enzymes, minerals, lignin, saponins, salicylic acids, amino acids, sugars, and vitamins. In this chapter, the authors highlight the mucopolysaccharide acemannan (an acetylated glucomannan), extracted from the mucilaginous gel leaves. Acemannan has a backbone of β(1–4)-linked mannose units, partially acetylated, interspersed by glucose units, and some galactose side-chains. However, several of its structural characteristics, such as degree of acetylation, presence of glucose monomers, and its molecular weight, are always quantified because they present inconsistencies and disparities. Growing conditions of the plant and harvesting period, different processes for obtaining the gel, and its derivatives, besides the many different analytical methods applied, contribute to the lack of standardization of A. vera gel and its derivatives. Therefore, this chapter aimed to present the different techniques used for processing mucilaginous gel along with the various analytic methods applied in recent years.
Keywords: Acemannan, acetylated glucomannan, Aloe vera, polysaccharides, polymer
1.1 Introduction
Aloe vera L. (Aloe barbadensis Miller) is a medicinal plant belonging to the Liliaceae family, currently defined as Asphodelaceae by the Angiosperm Phylogeny Group III System—APG III of 2009 [1]. Of the over 300 species of Aloe, A. vera is the most widely used in medicines, cosmetics and food products. Numerous therapeutic activities, including antiviral, antibacterial, radiation protection, antioxidant, anti-inflammatory, anticancer, antidiabetic, antiallergic, immunostimulant, and ultraviolet (UV) protection have been attributed to the plant, particularly to its polysaccharides [1–7].
The leaves of A. vera can be divided into two main fractions, a thick epidermis, or outer green rind, including the vascular bundles, and an inner colorless pulp called A. vera gel. The bitter yellow exudate from the cells surrounding the vascular bundles is rich in derivatives of 1,8-dihydroxyanthraquinone and its glycosides, whereas the pulp contains proteins, lipids, amino acids, vitamins, enzymes, inorganic compounds, small organic compounds, besides different carbohydrates (soluble sugar and polysaccharides) [6].
The inner most part of the leaves (leaf pulp) of A. vera contains parenchymatous cells which produce a mucilaginous liquid referred to as A. vera pulp, among other terms such as inner gel, leaf parenchyma, mucilaginous gel and/or simply A. vera gel [4]. Water is the main constituent, ranging from 98.5 to 99.5% in the fresh plant, while around 60% of the remaining solid material is made up of polysaccharides [8, 9].
Several reports have identified acemannan as the major polysaccharide in the mucilaginous gel of A. vera. It is composed of large quantities (>60%) of partially acetylated mannose units, followed by glucose (approximately 20%) and, in lower amounts, galactose (<10%) [3, 9–11]. The acetyl groups are the only ones that are not functional groups present in sugars and appear to play a fundamental role not only in terms of physicochemical properties but also in the biological activity of A. vera [12–14].
Acemannan is highly unstable and readily degraded by different physicochemical factors including high temperatures, pH changes, microbiological factors like bacterial contamination, or by enzymatic action, such as from the mannases present in the gel [15, 16]. Studies have shown that the effects of deacetylation, i.e., removal of the acetyl group, reduce the bioactivity of the polysaccharides. Thus, the acetyl group may have functional control of acemannan affecting, at least in part, its physical properties and biological activity [14].
Controlling the chemical, functional, and physical properties of A. vera during its processing remains a significant challenge. This is due to the microbial, mechanical, enzymatic, and structural changes which take place under different climatic and processing conditions [9, 17, 18]. Thus, the choice of method for obtaining the gel is an essential factor, given that A. vera gel is often sold in concentrated powder forms [19].
Given the high activity of water in A. vera gel and its major carbohydrate composition, its shelf life is only 3–4 days at room temperature, requiring the use of stabilization processes to preserve most of the active ingredients and extend its usable life. Thus, stabilization is done to reduce the amount of water in the gel by concentrating and/or drying it, where freeze drying is the most used process [13, 20–28].
However, the conditions applied during the drying process can irreversibly modify A. vera polysaccharides, particularly acemannan, affecting its chemical structure and promoting changes in physiochemical, physiological and pharmacological properties attributed to the plant [20, 29]. A number of studies report that the drying process promotes deacetylation of the acemannan polymer [28, 30, 31], leading to a considerable reduction in its biological effect [14].
A. vera gel obtained from poor quality raw materials using non-standardized processing techniques are used to make commercial A. vera products [24, 25]. This situation creates a need for establishing standardized and validated analytical methods to guarantee the quality of products containing A. vera gel.
Bozzi et al. [19] emphasized that contamination represents a significant concern for the A. vera market, where historically, the most commonly used substance to adulterate A. vera gel is maltodextrin. Consequently, many methods have been developed for detecting contamination and establish the authenticity of A. vera gel powders, including the analyses of carbohydrates. However, in this case, the only adulteration with sugars (glucose, saccharose) or polysaccharides (i.e., maltodextrin) can be revealed [32].
Numerous different analytical methods for characterizing and quantifying A. vera gel components, such as acemannan, have been reported. Techniques used include high-performance liquid or size-exclusion chromatography, spectroscopic techniques, or colorimetric assays. However, some studies have used a more comprehensive analytical approach involving several methods concomitantly to create a profile of the material to overcome some difficulties and establish specifications.
1.1.1 Gel Composition from A. vera
Knowledge on the characteristics of A. vera allows a better understanding of the biochemical changes which take place during plant cultivation, processing and storage of its products. The gel consists predominantly of water (>98%) and polysaccharides, such as glucomannan, acemannan (acetylated glucomannan), mannose derivatives, pectins, cellulose, hemicellulose, of which acemannan is considered the main component [19]. Of the soluble sugars present in the gel, glucose accounts for over 95% [9].
According to Ni et al. [4], the sugar composition profile of the cell wall fibers, the micro-particles (degenerated organelles) and liquid gel was distinct. The mucilaginous gel, or simply gel, cannot be considered a homogenous entity, because it has three different structural components each with different chemical compositions. Galacturonic acid has been detected in the cell wall, suggesting the presence of high levels of pectin and pectic substances, the microparticles contained galactose-rich polysaccharide, and liquid gel containing mannose.
Two main types of polysaccharides present in A. vera