Table of Contents:

Lactulose

Lactulose (4-O-β-D-galactopyranosyl-D-fructose) is a ketose disaccharide having wide range of applications in food and pharmaceutical sectors. It is composed of fructose and galactose linked through glycosidic bonds.The linkage between galactose and fructose is neither cleaved by human digestive enzymes nor absorbed in the small intestine. In the colon, lactulose stimulates the growth of Bifidobacteria sp. and Lactobacillus sp. leading to the production of a large number of short-chain fatty acids (Méndez and Olano 1979).

Method of Production

Lactulose can be synthesized chemically or by enzymes from various microorganisms.

10.4.1.1 Enzyme Involved

Biocatalytic production of lactulose has been carried out by using the following enzymes:

1. β-galactosidase (EC 3.2.1.23)

2. β-glycosidase (EC 3.2.1.21)

10.4.1.2 Process

Currently, for commercial utilization, lactulose is produced by alkaline isomerization of lactose. To obtain high yields, it is advantageous to use high amounts of inorganic catalysts, such as boric acid or aluminate. This approach results in expensive separation of by-products that cause difficulty in product purification and waste management (De Harr and Pluim 1991; Zokaee et al. 2002; Aider and de Halleux 2007). The problems associated with lactulose production by chemical synthesis can be solved by an enzymatic transformation process which seems to be a useful approach for clean production and easy purification of lactulose (Tang et al. 2011).

Microorganisms, such as K. lactis, S. solfataricus, Arthrobacter sp. (β-galactosidase), and P. furiosus (β-glycosidases), have been applied for both lactose hydrolysis and transglycosylation leading to lactulose synthesis (Lee et al. 2004; Kim et al. 2006; Mayer et al. 2010; Tang et al. 2011).

The optimum reaction conditions for lactulose production using permeabilized cells were 40 % (w/v) lactose and 20 % (w/v) fructose at temperature 60 °C and pH of 7.0. Under these conditions, the permeabilized cells produced approximately

20 g L−1 lactulose (Lee et al. 2004). The use of thermostable β-galactosidase can further enhance the production of lactulose (Kim et al. 2006). The continuous enzymatic production of the lactulose through transgalactosylation has also been developed using free and immobilized (Amberlite IRA-93 or Eupergit® C) thermo-stable β-glycosidase (Mayer et al. 2010). Dual-enzymatic system consisting of immobilized lactase and immobilized glucose isomerase in organic-aqueous twophase media using lactose and fructose has also been applied for the synthesis of lactulose (Xiao et al. 2010). Currently, purified β-galactosidase from Arthrobacter sp.

LAS has been used for lactulose production to reduce the nonenzymatic browning during biotransformations (Tang et al. 2011). The optimum pH and temperature for lactulose synthesis by this β-galactosidase were 6.0 and 20 °C, respectively.

Alkaline isomerization of lactose present in cheese whey ultrafiltrate permeate was carried out by the addition of boric acid (Hicks et al. 1984). However, expensive separation and purification steps were involved in this procedure. The synthesis of lactulose from dairy by-product (i.e., whey) can be carried out by using an enzyme

(β-galactosidase) in the presence of fructose. The controlled enzymatic transgalac-tosylation of lactose in whey ultrafiltration permeate can improve the efficiency of lactulose synthesis. The factors that influenced the lactulose synthesis efficiency were enzyme source, substrate concentration, and also the ratio of lactose and fructose added to the reaction mixture (Adamczak et al. 2009; Jaindl et al. 2009).

 
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