Conventional pectin extraction involves treatment of the raw material in acid solution (pH 1.5) at about 90 °C for at least 1 h (Iglesias, and Lozano, 2004; Pagan et al, 2001); however, the pectin is readily degraded under these conditions. It has been reported that high methoxyl pectins can be extracted from raw materials using water or mineral acids or bases. However, low methoxyl pectins are not easily removed with the above-mentioned solvents, because certain low methoxyl pectins are bound to polyvalent metal ions via salt bridges. The addition of salt solution, which readily binds these polyvalent metal cations via a replacement reaction, results in the formation of soluble pectin salts (Turakhozhaev and Khodzhaev, 1993; Turquois et al., 1999). The extraction solvents consist mainly of polyphosphates, oxalates, monovalent metals of hydrochloric, nitric, sulfuric and phosphoric acids (Turakhozhaev and Khodzhaev, 1993). A production line for pectin from sweet potato is shown in Figure 11.3.19.
Figure 11.3.19 Production line for sweet potato pectin.
The extraction procedure of sweet potato pectin is explained in Figure 11.3.20.
Figure 11.3.20 Extraction procedure of sweet potato pectin.
11.3.10.4 Sweet Potato Anthocyanins
Anthocyanins can be extracted from purple sweet potato by the ultrasonic-assisted ethanol solvent method or the ethanol-ammonium sulfate aqueous two-phase extraction method. Compared to the anthocyanins from purple grape, basil, purple rice, black bean and other resources, sweet potato anthocyanins have similar thermal stability against purple rice, which is better than other pigments and has the strongest light stability (Pu and Fu, 2010). Moreover, the purple sweet potato is rich in source, the price is reasonable, and it also has high productivity. Animal experiments and clinical experimental results have demonstrated that when sweet potato anthocyanins are taken up by the human body, they can be absorbed into serum and thus increase serum antioxidant capacity as an intact molecular form, so that the observed physiological functions (from the in vitro experiments) can be achieved when anthocyanins reach the predetermined places in vivo. Therefore, sweet potato anthocyanins are attracting increasing attention.
It is well known that anthocyanins are soluble in polar solvents and commonly extracted by aqueous mixtures of organic solvents such as ethanol, methanol or acetone (Kano et al., 2005). The addition of a small amount of hydrochloric acid or formic acid is recommended to prevent the degradation of the non-acylated compounds. Besides the conventional solvent extraction, new methods based on more advanced extraction techniques were reported, such as microwaves (Sun et al., 2007; Yang et al., 2008; Yang and Zhai, 2010) and ultrasonic methods (Ghafoor et al., 2009; 2011). However, these methods have drawbacks due to the higher cost, special equipments and stringent operating conditions.
Aqueous two-phase extraction (ATPE) has been widely applied to the separation of biomacromolecules, such as proteins (Klomklao et al., 2005) and antibiotics (Paula, 2007), because of its mild conditions and high capacity. Up to now, most ATPE was based either on polyethylene glycol (PEG)/salt or polymer/polymer (e.g. PEG/dextran) systems. However, because of the high cost of the polymers and difficulty in isolating the extracted molecules from the polymer phase by back extraction, these systems cannot be used for large-scale production (Ozlem et al., 2011).
Recently, short-chain alcohol/inorganic salt systems have been used as a novel ATPE system to purify natural compounds (Jiang et al., 2009). This ATPE system has many advantages such as low cost, low interfacial tension, good resolution, high yield, high capacity and simple scale-up (Rito-Palomares, 2004). Moreover, because of its structure, these are suitable for hydrophilic compounds. Short-chain alcohols (ethanol, methanol and 2-propanol) can form stable and adjustable ATPE system with inorganic salts (e.g. phosphate and sulfate) (Gunduz, 2000). This might be because of the salting-out effect and the low solubility of inorganic salt in alcohols. When an ATPE system is formed, the top phase is rich in alcohol and the bottom phase is rich in inorganic salt. The water content of the two phases are both 80 % or more, and show very low surface tension (Yuan et al., 2011). Ethanol-ammonium sulfate is a common and economic ATPE system, which has been applied to extraction of anthocyanins from mulberry (Wu et al., 2011) piceid, emodin and resveratrol (Wang et al., 2008). Purple sweet potato anthocyanins under different pH conditions are shown in Figure 11.3.21. The extraction procedure of sweet potato anthocyanins is shown in Figure 11.3.22.
Figure 11.3.21 Purple sweet potato anthocyanins under different pH conditions.
Figure 11.3.22 Extraction procedure of sweet potato anthocyanins.
11.3.10.5 Sweet Potato Polyphenols
Research has shown that there are large amounts of polyphenol substances contained in sweet potato stems and leaves, and 70 % of the polyphenols is chlorogenic acid and its derivatives. Prepared by the ultrasonic-assisted ethanol solvent method, sweet potato polyphenols have biological activities such as free radical scavenging, anti-bacterial and anti-inflammatory properties, tumor inhibition, hepato-protection and gall bladder benefits, promoting blood circulation and lowering blood pressure (Huang et al., 2010; Kurata et al., 2007, 2011), Therefore, these may have broader prospects in food, medical care and chemical industries.
The purification methods for plant polyphenols are mainly organic solvent extraction, the membrane separation method and supercritical fluid extraction (Dai and Mumper, 2010; Farias et al., 2013; Nawaz et al., 2006; Turkmen et al., 2006). However, these methods have some disadvantages, such as long production cycles and high cost, which make them unsuitable for use on an industrial production scale. Macroporous resins are durable polar, non-polar or slightly hydrophilic polymers with high adsorption capacities for organic molecules (Fu et al., 2006). They can selectively adsorb the targeted constituents from aqueous and non-aqueous systems through electrostatic force, hydrogen bonding interactions, complexation and size sieving (Gao et al., 2007). Therefore, macroporous resins have been widely used in the separation and purification of biologically active substances due to their physicochemical stability, high adsorption selectivity and easy recycling (Wan et al, 2014). AB-8 macroporous resins are weak polar resins and have been widely used in the purification of plant polyphenols because of their appropriate surface area and nuclear pore size (Gao et al., 2013; Zhao et al., 2013).