2. Polysaccharides from Different Sources
2.1. Natural Polysaccharides
Natural polysaccharides can be derived from four main sources: algae, plants, animals, and microorganisms (
Figure 2). Demand for polysaccharides from natural origins is tremendously because they provide several benefits, including natural abundance, ease of isolation, and ability to be chemically modified in order to fit technological demand. Furthermore, these polymers may be enzymatically hydrolyzed, and they produce a noncarcinogenic, degradable product [
30,
31].
Figure 2. Four main sources of natural polysaccharides.
Natural polysaccharides consist of many monosaccharide residues joined by O-glycosidic bonds. When hydrolyzed, polysaccharides produce simple sugars such as glucose, galactose, mannose, arabinose, xylose, uronic acid, etc. Among numerous sources of naturally occurring substances, plants are considered a potentially renewable resource since they can provide a certain amount of natural polymers of plant origin [
32,
33].
Figure 3 shows some essential steps in the isolation and purification of polysaccharides from plants.
Figure 3. Isolation and purification of polysaccharides from plants.
2.2. Petrochemical Synthesis
Structurally, polysaccharides are composed of several monosaccharides joined together by O-glycosidic linkages. O-glycosidic linkages are formed by dehydration of the hemiacetal hydroxyl group of one sugar (a glycosyl donor) with a hydroxyl group on the anomeric carbon of another sugar (a glycosyl acceptor). Due to the presence of multiple hydroxyl groups, one glycosyl acceptor residue can be connected to more than one glycosyl donor via different O-glycosidic linkages. Consequently, polysaccharides may be linear or branched, and branching may occur at different positions of sugar units in the polysaccharide backbone with different branching densities [
34,
35]. Generally, three methods are used to synthesize polysaccharides: (1) stepwise glycosylation [
36,
37]; (2) condensation polymerization [
38]; and (3) ring opening polymerization [
39,
40].
3. Structure and Physicochemical Properties of GM
GM is a natural heteropolysaccharide with a linear chain consisting of D-glucose and/or D-mannose in various proportions linked by β-1,4 glycosidic bonds. It also has multiple branching at β-1,3 glycosidic bonds to mannose units as shown in
Figure 4 [
41].
Figure 4. Structure of glucomannan.
The molecular weight varies from 200,000 to 2,000,000 Daltons, giving it incredibly higher viscosity than any known dietary fiber such as guar or locust bean gum [
42,
43]. When GM sol concentration is below 0.55%, it is only slightly affected by shear rate, indicating Newtonian fluid flow characteristics. However, at higher concentrations, shear rate can affect viscosity, leading to shear thinning and indicating non-Newtonian pseudoplasticity [
44]. Based on previous reports, the viscosity of konjac glucomannan solution (1.0 g/100 g) can reach ~30,000 cps [
45].
GM is a hydrophilic polymer due to the abundance of hydroxyl and carbonyl groups in its molecular chain. The hydrogen bonds between each molecule affect its solubility; hence, the stronger the bonds, the lower the solubility in water. In contrast, low acetyl group branching (5–10% at the C-6 position, i.e., one branch per approximately 19 sugar residues) reduces hydrogen bonding, thereby increasing solubility; this causes high water absorption of 105.4 g/g (water/GM) [
46]. Water absorption is also affected by granule size and surface morphology—a reduction in particle size will increase surface wrinkle density, which culminates in higher hydration rates [
47].
The formation of gel is by hydration of water; this can be accelerated by heating and vigorous stirring. GM also forms synergistic gels in a thermally reversible reaction with other polysaccharides, such as xanthan gum [
48], κ-carrageenan [
49], and gum tragacanth [
50], which increase the mechanical strength and decrease syneresis. This is presumably due to agglomeration or physical entanglement and dynamic hydrogen bonds with other polysaccharides [
48,
49,
50].
In recent years, GM has attracted special attention from researchers and the food industry due to its bioactive, biodegradable, and hydrophilicity properties. This high-molecular-weight polymer is known as a hydrocolloid and interacts strongly with water [
51]. Hydrocolloids are used in the food industry because of their thickening, gelling, stabilizing, texture-modifying, and film-forming properties.
4. Extraction Optimization
For extraction, the organic solvents usually used to obtain GM from flour are hot water [
52], ethanol [
7], or isopropanol sol [
53]. As the majority of the glucomannan is present in the cell walls (intracellular polysaccharides) of higher plants, extraction begins with crushing plants to release intracellular polysaccharides. Furthermore, plant cell walls are mainly enclosed by lipids, which are removed by organic solvents, allowing extraction of glucomannan. An illustration of GM’s separation from impurities is depicted in
Figure 5. The flour swells in organic solvents, and the aqueous part dissolves impurities trapped in the konjac particle, such as soluble sugars and partial proteins. During heating, gelatinization occurs, which irreversibly dissolves the starch in water, while ash is simultaneously removed with increasing temperature. Therefore, heat affects GM purity, while purified products are precipitated in the presence of an antisolvent [
54].
Figure 5. Separation of GM from impurities.
Among other techniques, extraction by ethanol precipitation is the most effective method to obtain GM; this is because its huge particles are difficult to destroy, and the incredibly tough shape is soluble in water but not in ethanol. Based on these characteristics, Yanuriati et al. found an easier and faster method to isolate GM from
Amorphophallus by repeated milling of the fresh tuber slices using ethanol, followed by filtration without further purification. The results showed exceptionally high purity (90.98%), viscosity (27.940 cps), and transparency (57.74%), and significantly reduced ash and protein concentrations without starch content [
7]. Furthermore, several studies used ethanol–water solutions in different concentration gradients to control the solubility of GM [
55,
56]. Isopropanol works excellently to remove impurities, including starch and carotene [
53], while ethanol is also effective for obtaining GM but does not dissolve carotene thoroughly.
Previous statistical studies applying response surface methodology (RSM) proposed that precipitation efficiency is correlated to several factors, namely, harvest time [
7], processing temperature [
53,
54], and solvent concentration [
53,
57]. First, the best harvest time to get GM from plants such as
Amorphophallus muelleri tubers is during dormancy. Plant dormancy is a period of arrested growth. Because GM is one of the energy sources for leaf growth, during dormancy it is not used for metabolic processes and accumulates in the tubers [
58]. Second, temperature correlates positively with GM content according to Xu et al., who reported an optimum temperature of 68 °C using 40% ethanol increased purity from 74.13% to 90.63% [
53], while temperatures > 78 °C are not recommended because they are higher than GM’s exothermic transition temperature and disrupt the molecular chain [
54]. Third, the optimum concentration of ethanol solvent is 50%; this is because it is difficult to remove water-soluble impurities from the flour with concentrated ethanol. Meanwhile, in diluted ethanol, more water is absorbed, which leads to greater swelling, making it difficult to obtain the GM [
53].
Several other sources of GM apart from Amorphophallus are shown in Table 1 below.
Table 1. Sources and extraction processes of GM from different crops.