– Color measurement
(–) samples are not subjected to hydrolysis
Chokboribal et al. [14] also used the acid hydrolysis method to determine the composition of monosaccharides, structure and molecular mass of the alcohol precipitated polysaccharide performing these analysis using liquid chromatography with reflective index detector, Nuclear Magnetic Resonance Spectroscopy (1H NMR), Fourier Transform Infrared Spectroscopy (FT-IR), and size-exclusion chromatography, respectively. The data obtained confirmed the polysaccharide was acemannan.
The presence of uronic acids together with trace levels of arabinose and ramnose is indicative of residual amounts of pectinic polysaccharides in the polysaccharide fraction, as previously observed in the hydrosoluble extracts from A. vera leaf pulp [4, 9].
1.4.2 Analytical Techniques
HPLC is a technique used for fractionating, purification and determining the size of molecules. When separation is based on molecule size, these techniques are referred to as High Performance Size Exclusion Chromatography (HPSEC), using specific chromatographic columns and known size standards (dextran or pullulan). The detectors employed were refraction index (RI) [14, 21, 75, 78] and Multi-angle Laser Light Scattering (MALLS) [21, 75, 78]. HPAE-PAD was also reported, consisting of a separation technique based on the charge of molecules and useful for separating neutral molecules.
Gas Chromatography is another widely employed technique suitable for analyzing volatile compounds. Other commonly used analytical techniques include Infrared Spectroscopy (IR), Nuclear Magnetic Resonance (NMR), and Mass Spectrometry (MS). These methods can be used to investigate the structure of a compound or to identify a compound based on the specific signatures of a chemical bond or group present.
UV/Vis studies were also found as a complementary technique for characterizing compounds, in which a marker such as chromophore was used. Recently, using a microarray-based technique, it was possible to study the relative abundance, and interactions between hundreds or thousands of molecules simultaneously, using very small volumes of A. vera extracts of different species (Table 1.2).
1.4.2.1 Chromatography Analysis
Femenia et al. [9] used a HPLC method with a RI detector to quantify the free sugars in different parts of A. vera leaf and found that glucose accounted for 95% of the total monosaccharides analyzed. For the composition of carbohydrates of the polysaccharides hydrolyzed with sulphuric acid, the predominant sugars were mannose and glucose in all fractions, representing 55–75% of the total monosaccharides determined. Mannose residues in the fillet and gel were probably from acemannan, the main component of A. vera, where large amounts of acemannan were found in these tissues [8]. After elution on a specific size-exclusion column using HPLC-RI, Medina-Torres et al. [72] reported that the molecular weight of dry mucilage in a spray dryer was determined as 4.18 × 104 Da and for fresh mucilage as 5.96 × 104 Da. The reduction in molecular weight is expected owing to the heat treatment and high shear forces present in the spray drier chamber.
Chokboribal et al. [14] reported that the mean molecular weight of the acemannan isolated, calculated based on its retention time, after analysis by HPLC using a size-exclusion column with RI detectors was 190–220 kDa. Campestrini et al. [13], also using size-exclusion chromatography calibrated against dextran standards with a MALLS detector, observed a large peak at approximately 38 min for both the samples from the crude extract of A. vera and for the polysaccharide fraction, corresponding to a high molecular weight compound, identified as a partially acetylated glucomannan whose molecular weight was determined at 1.2 MDa (1,200 kDa).
A marked increase in molecular weight of purified acemannan
