mentioned properties, recent research on the use A. vera gel investigate its use conjugated to natural and synthetic polymers in order to produce 2D and 3D matrices, such as hydrogels, microspheres, sponges, nanofibers and functional films [47–50]. A large part of these biomaterials has been prepared by combining A. vera gel with cellulose, chitosan, alginates, gellan gum, tragacanth gum, poly(e-caprolactone), jelly, collagen and glucan [50–59].
1.3 Aloe vera Gel Processing
The process of obtaining the mucilaginous gel of A. vera entails essentially the same procedure, with one or two variations. The standard procedure includes: harvesting the leaves, washing, removal of the rind (skin), elimination of the yellow sap (exudate), filleting, homogenization (grinding, pressing or extrusion), followed by filtration or centrifugation to remove fibrous material, and stabilization of the gel, through concentration by rotary evaporation, drying and sterilization (Table 1.1).
Table 1.1 Process for obtaining mucilaginous gel, polysaccharide fraction, oligosaccharides, |and acemannan.
| Raw material | Techniques applied | References |
|---|---|---|
| Mucilaginous gel | Homogenization | |
| Filleting | [7, 9, 20, 22, 30] | |
| Pressing | [13, 26–28, 63] | |
| Grinding | [4, 7, 14, 24–26, 30, 34, 37, 60, 61, 72–75] | |
| Extrusion | [9] | |
| Separation of insoluble fibers | ||
| Filtration | [26, 27, 34, 66, 75] | |
| Centrifugation | [4, 13, 14, 37, 63, 72, 75] | |
| Sterilization | ||
| Pasteurization | [7, 26] | |
| High hydrostatic pressure treatment | [60, 61] | |
| Drying | ||
| Concentration by rotary evaporation | [26–28, 34] | |
| Freeze drying | [13, 20–22, 24–28, 75] | |
| Spray drying | [27, 28, 72] | |
| Hot-air drying | [20, 22, 26] | |
| Refractance window-drying and radiant zone-drying | [27, 28] | |
| FIR radiation and HVEF assisted hot air drying | [30] | |
| Polysaccharide Fraction | Alcohol precipitation | [4, 7, 67, 9, 13, 20, 22, 27, 34, 37, 66] |
| Dialysis | [13, 68, 70] | |
| Freeze drying | [4, 22, 37, 68] | |
| Hot air drying | [13, 34] | |
| Grinding | [5, 13] | |
| Ammonium sulfate precipitation | [67] | |
| Purified acemannan | Alcohol precipitation | [5, 14, 30, 69] |
| Dialysis | [5, 7, 20, 27, 30, 34, 69] | |
| Freeze drying | [14, 20, 30, 34, 69] | |
| Anion-exchange chromatography | [5, 34] | |
| Gel-permeation chromatography | [7, 20, 27, 34] | |
| Ionic liquid-based aqueous two-phase system | [69] | |
| Aqueous two-phase system | [71] | |
| Membrane separation | [71] | |
| Oligosaccharides | Homogenization | [76] |
| Centrifugation | [76] | |
| Dialysis | [40] | |
| Freeze drying | [40] | |
| Membrane fractionation | [76] | |
| Size-exclusion chromatography | [11] | |
| Gel-permeation chromatography | [40, 76] | |
A. vera rapidly degrades and therefore suitable techniques should be applied after harvesting to prolong the half-life of the product, such as preserving and transporting the leaves at low temperatures [24, 25]. Better results are achieved when the plant is processed immediately after harvesting. This is because the steady decomposition of the gel commences with natural enzymatic reactions, as well as microbial growth, due to the presence of oxygen [17].
The filtering process can involve filtration with activated charcoal filters, intended to decolorize the gel and remove anthraquinones, especially if the gel is intended for internal use, owing to the laxative properties of these substances [17, 34]. Several filtration steps ensue which may involve course screening filters (400–800 µm) and finer filters (25–100 µm) [27] to remove insoluble fibrous material.
Sterilization and stabilization techniques are used [17, 29] to prevent oxidation, decomposition, and loss of active substances. Sterilization can be performed by means of cold processes, using enzymes, UV light and microfiltration, or hot processes, such as pasteurization [17].
Pasteurization is a widely used technique in the manufacture of food products. Besides the intended effects on pathogenic microorganisms and deactivation of enzymes, thermal processing can also negatively impact the quality of foods. In fact, pasteurization is the processing technique most commonly used by the A. vera industry [7]. As an alternative to pasteurization techniques, Vega-Gálvez et al. [60, 61] applied Hydrostatic High Pressure (HHP) techniques or High Pressure Processes to extend the shelf life of the products, as an alternative to conventional food production techniques. HHP consists of a non-thermal technique for preserving foods involving enzymatic and microbiologic inactivation with less effect on the quality and nutritional parameters compared with heat treatments.
A.
