Enzyme Technology
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
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