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. vera products are often stabilized by dehydration, a process which can have irreversible effects in the structural characteristics of polysaccharide, such as degree of acetylation, molecular weight, and removal of side chains. These modifications can promote changes in the biological activities and rheological behaviors of these polymers [20]. The techniques usually employed are hot-air drying, spray drying, and freeze drying [20, 23, 62].
Femenia et al. [20], assessing the effects of hot-air drying temperature on the physicochemical properties of polysaccharide, observed structural changes in acemannan when samples were dehydrated using temperatures ranging from 30 to 80 °C, particularly at 70–80 °C, comparing results with a sample produced by freeze drying. Of the different dehydration processes, freeze drying is described as one of the most efficient methods of dehydration, conserving the physicochemical properties of A. vera gel compared with other drying methods. The results found by Chang et al. [63] showed that the samples at 50 and 90 °C had greater polysaccharide losses. The decrease at 90 °C may have been due to thermal degradation of the polysaccharides from the A. vera gel, and particularly enzymatic hydrolysis at temperatures of 50 and 60 °C. Maximum stability of polysaccharide occurred at 70 °C.
Another widely used process is spray drying that have the ability to produce powders with specific particle sizes using continuous operation in short production time [64]. The process also provides high rates of retention of properties of the product such as flavor, color and nutrients [65]. However, the high temperatures applied in spray drying can negatively affect the properties of the resultant powders causing degradation. This phenomenon was found in studies conducted by Cervantes-Martínez et al. [23], who reported a reduction in the viscosity of reconstituted powder using the spray drying technique, to which the degradation of the sample was attributed. However, Sriariyakul et al. [30] and Minjares-Fuentes et al. [27] used alternative non-traditional drying techniques, such as far infrared (FIR) radiation, high-voltage electric field (HVEF), assisted hot-air drying, refractance window-drying and radiant zone-drying.
1.3.1 Obtaining Polysaccharide Fraction or Acemannan
A variety of methods are used for polysaccharide purification and separation, such as alcohol precipitation [4, 7, 9, 14, 20, 22, 37, 66, 67], ion-exchange chromatography [5, 34], gel permeation chromatography