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Use of immobilised invertase

Invertase was probably the first enzyme to be used on a large scale in an immobilised form (by Tate & Lyle). In the period 1941 -1946 the acid, previously used in the manufacture of Golden Syrup, was unavailable, so yeast invertase was used instead. Yeast cells were autolysed and the autolysate clarified by adjustment to pH 4.7, followed by filtration through a bed of calcium sulphate and adsorption into bone char. A layer of the bone char containing invertase was included in the bed of bone char already used for decolourising the syrup. The scale used was large, the bed of invertase-char being 2 ft (60 cm) deep in a bed of char 20 ft (610 cm) deep. The preparation was very stable, the limiting factors being microbial contamination or loss of decolourising power rather than loss of enzymic activity. The process was cost-effective but, not surprisingly, the product did not have the subtlety of flavour of the acid-hydrolysed material and the immobilised enzyme process was abandoned when the acid became available once again. Recently, however, it has been relaunched using Brimac^TM, where the invertase -char mix is stabilised by cross-linking and has a half-life of 90 days in use (pH 5.5, 50°C). The revival is due, in part, to the success of HFCS as a high-quality low-colour sweetener. It is impossible to produce inverted syrups of equivalent quality by acid hydrolysis. Enzymic inversion avoids the high-colour, high salt-ash, relatively low conversion and batch variability problems of acid hydrolysis. Although free invertase may be used (with residence times of about a day), the use of immobilised enzymes in a PBR (with residence time of about 15 min) makes the process competitive; the cost of 95% inversion (at 50% (w/w)) being no more than the final evaporation costs (to 75% (w/w)). A productivity of 16 tonnes of inverted syrup (dry weight) may be achieved using one litre of the granular enzyme.

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