Use of immobilised lactase

Lactase is one of relatively few enzymes that have been used both free and immobilised in large-scale processes. The reasons for its utility has been given earlier (see Chapter 4), but the relatively high cost of the enzyme is an added incentive for its use in an immobilised state.

Immobilised lactases are important mainly in the treatment of whey, as the fats and proteins in the milk emulsion tend to coat the biocatalysts. This both reduces their apparent activity and increases the probability of microbial colonisation.

Yeast lactase has been immobilised by incorporation into cellulose triacetate fibres during wet spinning, a process developed by Snamprogetti S.p.A. in Italy. The fibres are cut up and used in a batchwise STR process at 5°C (Kluyveromyces lactis, pH optimum 6.4 -6.8, 90 U g⁻¹). Fungal lactases have been immobilised on 0.5 mm diameter porous silica (35 nm mean pore diameter) using glutaraldehyde and  g-aminopropyltriethoxysilane (Asperigillus niger, pH optimum 3.0 -3.5, 500 U g⁻¹; A. oryzae, pH optimum 4.0 -1.5, 400 U g⁻¹). They are used in PBRs. Due to the different pH optima of fungal and yeast lactases, the yeast enzymes are useful at the neutral pH of both milk and sweet whey, whereas fungal enzymes are more useful with acid whey.

Immobilised lactases are particularly affected by two inherent short-comings. Product inhibition by galactose and unwanted oligosaccharide formation are both noticeable under the diffusion-controlled conditions usually prevalent. Both problems may be reduced by an increase in the effectiveness factor and a reduction in the degree of hydrolysis or initial lactose concentration, but such conditions also lead to a reduction in the economic return. The control of microbial contamination within the bioreactors is the most critical practical problem in these processes. To some extent, this may be overcome by the use of regular sanitation with basic detergent and a dilute protease solution.

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This page was established in 2004 and last updated by Martin Chaplin
on 6 August, 2014