Inulins are long-chain storage carbohydrates that occur naturally in small amounts in various edible vegetables, fruits, cereals, and plants, such as asparagus, chicory, onion, wheat, banana, shallot, artichokes, leek, garlic, rye, tomatoes, topinambuco, and honey (Van et al. 1995). Inulin and its polyfructans consists of GFn molecules
(2 < n < 60) with linear β-2 → 1-linked polyfructose chains with glucose unit at its terminal (Waterhouse and Chatterton 1993). These compounds were discovered by Rose, German scientist, from hot water extract of Elecampane (Inula helenium) and the term was coined by Thomson in 1818. Oligofructose (inulin hydrolysate) is composed of GFn and Fm molecules (with 2 ≤ n and m ≤ 10) with DP (degree of polymerization) ≤10. It has more solubility and 30–50 % sweetness as compared to table sugar (Niness 1999).
Method of Production
Inulin hydrolysates can occur naturally or can be produced by enzymatic methods.
Inulinases produce inulin hydrolysates by partial enzymatic hydrolysis of inulin (Franck 2002). There are two types of inulinases:
1. Exoinulinases (β-D-fructanfructohydrolase; EC 18.104.22.168) act on both β 2 → 1 and
β 2 → 6 linkages at the nonreducing end, thereby cleaving terminal fructose
2. Endoinulinases (2,1-β-D-fructanohydrolase; EC 22.214.171.124) act on specific β 2 → 1
internal linkages yielding inulotriose, inulotetraose, and inulopentaose.
Inulin can be obtained from natural plant sources by extraction using hot water diffusion followed by purification and then drying of inulin extract to obtain pure powder (Angus et al. 2005). In spite of their natural occurrence, fructans can also be produced using microbial sources like bacteria and fungi. Inulin has been used as substrate for the production of its hydrolysates using inulinase and short-chain oligosaccharides, such as inulo-oligosaccharide, oligofructose, and fructo-oligosaccharide.
Inulinases can be produced from various bacterial strains (Streptomyces sp., Pseudomonas sp., Bacillus sp.), yeast strains (Kluyveromyces sp., Pichia sp., Candida sp.), and fungal sources (Aspergillus sp., Penicillium sp.) (Mazutti et al. 2006; Neagu and Bahrim 2011). Inulinase production has also been carried out using different agro-industrial waste, such as cassava flour, corncob, oat meal, rice straw, sugar cane bagasse, and wheat bran using Aspergillus ochraceus (Guimaraes et al. 2007). Sugarcane bagasse and corn steep liquor have been used for the production of inulinases using Kluyveromyces marxianus NRRL Y-7571 under SSF (Mazutti et al. 2006). Three exoinulinases and two endoinulinases were purified from Aspergillus ficuum JNSP5-06 (Chen et al. 2009).
Thermostable extracellular immobilized inulinases from Aspergillus fumigates have been used for the hydrolysis of inulin (Gill et al. 2006). Inulobiose and other higher oligofructosides were produced using soluble (inulobiose and DP3 oligosaccharides as product; 72 % yield) and immobilized endoinulinases (higher content of inulobiose; 83 % yield) using batch fermentation (Yun et al. 1997a). Immobilized system has also been applied for the production of inulo-oligosaccharides using immobilized enzymes or enzyme-producing whole cells. Continuous production of inulo-oligosaccharides (83 % yield) has been achieved using immobilized endoinulinases produced by Pseudomonas sp. and inulin as substrate (Yun et al. 1997b). Continuous production of inulo-oligosaccharides (82 % yield) has also been reported using immobilized polystyrene-bound endoinulinase and chicory juice as substrate for 28 days at 55 °C (Yun et al. 2000). The inulo-oligosaccharide with DP2 to DP4 and DP2 to DP8 was produced at 45 °C after 72 h with partially purified (pH 6.0 and 50 % yield) and purified (pH 5.0, 70 % yield) A. ficuum endoinulinase, respectively, using 50 g L−1 inulin and enzyme concentration of 10 U g−1 substrate (Zhengyu et al. 2005). Pseudomonas mucidolens endoinulinase gene expressed on Saccharomyces cerevisiae cell surface resulted in hydrolysis of inulin (Jerusalem artichoke) with 2.31 U mL−1 of activity at temperature of 50 °C and pH of 7.0 and formation of inulotetraose (F4) as major product along with inulobiose (F2), inulotriose (F3), and inulopentaose (F5) formation (Kim et al. 2008). Similarly, K. marxianus CBS 6556 endoinulinase gene INU1 which has been expressed on citric acid producing Yarrowia lipolytica hydrolyzed 77.9 % inulin and resulted in formation of inulin-oligosaccharides (mono-, di-, and minor tri-) at 50 °C, pH 4.5 with initial 12 % inulin concentration and inulinase activity of 181.6 U g−1 within 10 h of incubation (Liu et al. 2010).