Galactooligosaccharides find widespread use in the industry as prebiotic compounds. A number of processes have been developed for the production of galactooligosaccharides. Some processes involve the use of μ-galactosidase enzyme obtained from different microbial sources, example Aspergillus oryzae, Bullera singularis, Candida, Kluveromyces sp., Bacillus circulans, Lactobacillus bulgaricus, Streptococcus thermophilus, and Bifidobacterium sp., (Akiyama et. al., 2001, Rabiu et. al., 2001, Tzortiz et. al., 2005, Jorgensen et al., 2001, Shin et. al., 1998, U.S. Pat. No. 5,032,509, EP 0 272 095 A2).
Use β-galactosidase enzyme or whole cells in immobilized matrices in place of free whole cells or enzymes has also been reported (Akiyama et. al., 2001, Rabiu et. al., 2001, Tzortiz et. al., 2005, Jorgensen et al., 2001, Shin et. al., 1998, U.S. Pat. No. 5,032,509, EP 0 272 095 A2). A combination of β-galactosidase enzyme and Saccharomyces cerevisiae cells has been co-immobilized in calcium alginate beads to produce ethanol from whey (Axelsson et al., 1991, Lewandoska et al., 2003 and Tanaka et al., 1986). The objective was to cleave lactose to glucose and galactose and thereby yeast can produce more ethanol.
The processes involving immobilized whole cells or enzyme over the free enzyme has certain advantages (1) catalytic power is stabilized (2) the immobilized matrices can be recycled which reduces cost and (3) products can be isolated in a simple manner. However, the use of immobilized enzymes depends upon the cost benefit and technical feasibility factors. In some processes, the extraction and purification of enzyme is costly and in some cases the enzyme denatures after extraction. Under such conditions, the use of immobilized or coimmobilized whole cells has added advantages over the immobilized enzymes (Yuan et al., 1992, Kiss et al., 1999).
The most prevalent method of whole cell immobilization is cell entrapment in hydrocolloids like alginate, carrageenan, polyacrylamide, agarose, gelatin, gellan gum (U.S. Pat. No. 5,175,093, U.S. Pat. No. 5,288,632, U.S. Pat. No. 5,093,253, U.S. Pat. No. 4,572,897, U.S. Pat. No. 5,070,019, U.S. Pat. No. 5,759,578, U.S. Pat. No. 5,939,294 and U.S. Pat. No. 5,034,324, Birnbaum et al., 1981). JP2005042037, JP5815243, and JP56113289 describe the use of polyvinyl alcohol with polyethylene glycol and boric acid as a successfully alternative to other hydrocolloids.
U.S. Pat. No. 5,766,907 mixed microbial cells in calcium chloride solution containing small amount of xanthan gum and then dropped into sodium alginate. The capsule membrane formed by ionic bond between calcium and alginate prevented swelling of the membrane and resulted in a high concentration of microbes within the capsule.
U.S. Pat. No. 5,034,324 discloses that polyvinyl alcohol has a high affinity for microorganisms and provides mechanical strength and durability sufficiently high for use in any reactor, and high resistance to water and chemicals. Jianlong et al., (1995) used acryl amide as polymerizing agent in the polyvinyl alcohol matrix with boric acid as cross-linking agent to overcome the swelling of polyvinyl alcohol gels in aqueous solution.
However use of immobilized cells has following disadvantages;                a. The matrix and Cross linking agents must comply regulatory approval for the use in food grade conditions.        b. Mechanical stability of immobilized beads with cells is poor        c. Constraints in the diffusion of substrate and products and hence the efficiency of bioconversion is less than in free cells.        d. Because of low conversion efficiency, the product separation form unreacted substrate and impurities is a challenging task        
U.S. Pat. No. 716,451 describe mixing of the saccharide solution, obtained after hydrolysis, with ethanol and passing through activated carbon column to remove the mono and disaccharide components. The galactooligosaccharide component is eluted using pure ethanol. This downstream processing results in substantially pure galactooligosaccharide solution However, this process is not efficient due to the loss in the yield of galactooligosaccharides. Also, it is not economical due to the use of ethanol.
To overcome the limitation of activated carbon column treatment, EP 0 272 095 A2 and U.S. Pat. No. 5,032,509 describe loading of the saccharide solution on a strong cation exchange resin followed by elution of galactooligosaccharides with water at 60 to 80° C. This process can improve the yield of galalctooligosaccharide. However, it is not cost effective because of the use of costly strong cation exchange resin.
Therefore, the need for a cost effective process that results in the production galactooligosaccharides of high purity and yield galactooligosaccharides continues to prevail.