Galacto-oligosaccharides (GOS) are a functional food ingredient consisting of
β-linked galactose moieties with galactose or glucose at the reducing end. More
than 30 different types of di-, tri-, and tetrasaccharides were identified as the product of various enzymatic transgalactosylation. The different unit of monomer in GOS is linked by β-(1 → 2), β-(1 → 3), β-(1 → 4), or β-(1 → 6) galactosyl moieties (Mahoney
1998; Gänzle 2011). In the commercial GOS preparations, apart from tri-, tetra-, and pentasaccharides, small amount of other carbohydrates, such as glucose-, lactose- and β-(1 → 3)-, or β-(1 → 6)-linked disaccharides, is also present (Sako et al. 1999;
Splechtna et al. 2006).
Method of Production
Galacto-oligosaccharide can be synthesized by transgalactosylation of lactose, glucose, or galactose with lactose as galactosyl donor.
10.3.1.1 Enzymes Involved
Enzymatic production of galacto-oligosaccharide has been carried out using following enzymes:
1. β-galactosidase (EC 18.104.22.168)
2. β-glucosidases/β-glycosidases (EC 22.214.171.124)
β-galactosidase and β-glucosidases/β-glycosidases have both hydrolytic (lactose) and transgalactosylation activity.
The β-galactosidase can be produced from various microbial sources like bacteria, yeast, and fungi. Among these sources, yeast has been considered as an important source of β-galactosidase from industrial point of view (Panesar et al. 2006). Apart from this, other thermostable enzymes, such as β-glucosidase/β-glycosidase, have also been used for galacto-oligosaccharide production (Akiyama et al. 2001; Choi et al. 2003). Galacto-oligosaccharide production from lactose has been carried out with microbial β-galactosidases, whole cells, permeabilized cells, as well as apply-
ing immobilized techniques (Albayrak and Yang 2002; Ladero et al. 2003; Tzortzis et al. 2005; Nakkharat et al. 2006; Nguyen et al. 2007; Sakai et al. 2008; Placier et al. 2009).
The production of GOS depends on the source of enzyme and production process. High temperature (60 °C) has been observed to favor transgalactosylation reaction and GOS yield (Cardelle-Cobas et al. 2008). The production of GOS has also been carried out by various enzymes from different microorganisms, such as thermostable β-glycosidases/β-glucosidase from Pyrococcus furiosus and Sulfolobus solfataricus
(Petzelbauer et al. 2000; Boon et al. 1998; Splechtna et al. 2001) and β-galactosidase
from Sirobasidium magnum and Penicillium simplicissimum (Onishi and Tanaka 1997; Cruz et al. 1999). The yield of GOS varied from 15 to 77 % by these enzymes with the lactose conversion of 45–95 % (Torres et al. 2010). GOS has been purified by continuous nanofiltration achieving a yield of 81–98 % for oligosaccharides (trisaccharide), 59–89 % for disaccharides, and only 14–18 % for the monosaccharides (Goulas et al. 2002).
Whey (by-product of dairy industry) is rich in lactose content and can be used for GOS production (López Leiva and Guzman 1995). The conversion to oligosaccharides not only depends on the reaction time but also on the initial concentration of substrate. The production of GOS has been carried out using an immobilized Aspergillus oryzae β-galactosidase on glutaraldehyde-treated chitosan beads in a plug reactor from whey.
The maximum yield of GOS was found to be 26 % of the total saccharides on a dry weight basis for an initial concentration of lactose of 300 g L−1 (Sheu et al. 1998). At pilot scale membrane reactor, oligosaccharides have also been produced by hydrolysis of whey permeate (2,000 L) from Maxilact (Kluyveromyces lactis β-D-galactosidase).
A yield of 31 % oligosaccharide was observed from whey permeate containing 20 % lactose and 0.5 % enzyme (Foda and Lopez-Leiva 2000).
Different substrates (lactose, ultrafiltration whey permeate, and recombined whey) with different enzyme concentrations (0.15–15 U mL−1) have been applied for GOS synthesis. The maximum production of GOS has been observed with ultrafiltration whey permeate using an enzyme concentration of 1.5 U mL−1 (HelleroVá and Čurda 2009). Continuous production of GOS has also been carried out from both lactose feed solution and whey, with PVA-immobilized β-galactosidase in a packed bed reactor. A maximum GOS production of 30 % of total sugars was achieved using 40 % lactose feed solution, whereas 15 % of total sugars have been obtained in case of whey (Jovanovic-Malinovska et al. 2012).