Figure 13.3 The isolation and preparation of RS from EFY (updated from Reddy et al. (2014).
13.5.2 Processing EFY (Konjac) Flour for Gum and Gel
Konjac flour, a water soluble dietary fibre, has been extensively used for thickening and gelling in various food products. This flour produces a high viscosity and forms a gel by alkaline coagulant such as calcium hydroxide or by combining with secondary gum or co-gelated gum such as K-carrageenan and xanthan gum (Huang and Lin, 2004). The gel can be applied as a fat analogue in low or reduced fat food products such as sausages, bolognaise, burgers, cakes and mousses (Jimenez-Colmenero et al., 2010,2013). Regarding its linkage structure and health benefits, including triglycerides and cholesterol reduction, weight control and constipation alleviation, it is popular in health food products (Delgado-Pando et al., 2010).
When konjac flour is dissolved in alkaline coagulants (i.e. calcium hydroxide, sodium or potassium carbonate), deacetylation occurs and a thermally stable gel is formed (Thomas, 1997). The rate of gel formation is dependent upon the pH and processing temperature. Usually gel formation occurs over a pH range from 9-10 and gels are thermally stable to temperatures greater than 200 °C (Akesowan et al., 2014; Thomas, 1997).
A thermally stable konjac gel can be formed by deacetylation in a mild alkali (calcium hydroxide) solution, but the resulting gel is hard and brittle (Hsu and Chung, 2000). The sensory quality of Chinese-style sausage, containing 10–20 % konjac gels was acceptable, but had a harder gel texture when compared with regular sausage with 30 % fat (Huang et al., 1998).
13.6 A. konjac K. Koch as Industrial Crop
One of the most valuable components found in the Amorphophallus tuber is glu-comannan (GM). GM is a water-soluble dietary fibre derived from idioblast cells inside the Amorphophallus tuber matrix. The GM content is about 5–9% (w/w, db) of the original Amorphophallus matrix (An et al., 2010). GM is a non-ionic (neutral), high molecular weight, natural polysaccharide. It is a β-1, 4 linked polysaccharides composed of a D-glucose and D-mannoses backbone lightly branched, with branches through β-1,6-glucosyl units (Figure 13.4).
Figure 13.4 The chemical structure of a selection of GM; G, glucose or acetylated glucose at 6th position; M, mannose.
Not all EFY species are rich in GM. GM content (% of total dry weight) in the corm of the Amorphophallus species varies from 5–6%. Only the species, A. konjac K. Koch, is gifted with this highly valuable biomaterial. Konjac glucomannan (KGM), one of the richest natural polysaccharides, is derived from this species (Huang and Lin, 2004). It is the main constituent in konjac flour (KF) and has been recognized as GRAS (Generally Regarded as Safe) since 1994. As a kind of non-caloric food, KGM had been applied in many kinds of food as a high-quality dietary fibre, thickening agent and fat replacer (Charoenrein et al., 2011; Fang and Wu, 2004; Jimenez-Colmenero et al., 2013; Khanna and Tester, 2006; Xu et al., 2014). In recent decades, methods for the extraction and purification of KGM have been studied and developed. KGM is extracted either by mechanical means (dry processing) or by wet (chemical) processing methods.
13.6.1 Extraction and Purification of KGM
There are several studies describing methodologies for the extraction and purification of KGM. The general procedure is as follows: Konjac flour (KF) is dissolved in 10 % NaOH solution to give 1 % solution and the GM is precipitated by adding Fehling solution. The precipitate is decomposed with 1 % HCl solution in ethanol, filtered, successively washed with 50 % ethanol, ethanol and ether, and dried in a vacuum. Due to strong alkali treatments, the purified GM does not accompany acetyl groups, which are present in native KGM (Chua et al., 2012; Maekaji, 1978).
Dimethyl sulfoxide (DMSO) is a good starch solvent used for the purification of KGM. Ye et al. (2014) purified KGM by using DSMO. In this, 20.00 g KF was added into 120 mL DMSO or its aqueous solutions. The mixtures were refluxed at 80 °C for 4 h under constant stirring and then cooled down to room temperature. The product was collected by vacuum filtration and rinsed with 95 % ethanol 3 times. Finally, the purified KF was dried in a vacuum oven at 80 °C for 2 h. In another study, Xu et al. (2014) developed a simple, straightforward extraction route in the ethanol/water system for the preparation of purified KGM via the temperature controlled method. KF was dissolved in 40 % (v/v) ethanol solution with feed liquid ratio of 1: 6. While stirring, the mixture was heated at a series of temperatures (28, 38, 48, 58, 68 and 78 °C) and refluxed for 4 h. After purifying, filtering and washing with 40 % ethanol solution at the pre-temperature, the precipitates were vacuum dried at 8 °C. By controlling the temperature, a series of KGM products with different characteristics can be obtained.
13.6.2 Konjac Glucomannan (KGM)-based Food Products
The KGM flour is prepared by washing, slicing, drying and milling the mature A. Konjac tubers. It contains significant amounts of KGM ranging from 51.3-96.9 % (dB) (Xiao et al., 1999). The KGM consists of mannose and glucose in a molar ratio of 1.6: 1, with β 1–4 linkage (Chin et al., 2000, 2009). It has a strong water-binding ability and when combined with other polysaccharides (e.g. carrageenan and starch), it exhibits a synergistic effect on protein gelation and water-binding in comminuted meat products. When konjac flour is dissolved in alkaline coagulant (i.e. calcium hydroxide, sodium or potassium carbonate), deacetylation occurs and a thermally stable gel is formed (Thomas, 1997). So it is used as a gelling agent in many meat products (Chin et al., 2000; Kao and Lin, 2006). It has high solubility and can be used in hot or cold liquids. The konjac flour is also used as a thickener in sauces, gravies, puddings and pie fillings without affecting the taste. These konjac-based foods are called moyu or juruo in China, and konnyaku or shirataki noodles in Japan (Thomas, 1997). KGM is also a source of soluble dietary fibre (Fang and Wu, 2004). It has wide applications in food (Cheng et al., 2002; Huang and Lin, 2004) and biomedical (Alonso-Sande et al., 2006; Alvarez-Mancenido et al., 2006; Chen et al., 2005; Lu et al., 2008; Yu and Xiao, 2008) domains (Table 13.2).