Enhancement of starch conversion efficiency with free and immobilized pullulanase and α‐1,4‐glucosidase

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Abstract
Glucoamylase and pullulanase were immobilized on reconstituted bovine‐hide collagen membranes using the covalent azide linkage method. A pretanning step was incorporated into the immobilization procedure to enable the support matrix to resist proteolytic activity while accommodating an operating temperature of 50°C. The immobilized glucoamylase and pullulanase activities were 0.91 and 0.022 mg dextrose equivalent (DE) min−1 cm−2 of membrane, respectively. Immobilized glucoamylase had a half‐life of 50 days while the immobilized pullulanase had a half‐life of 7 days. This is a considerably improved stability over that reported by other researchers. The enzymes were studied in their free and immobilized forms on a variety of starch substrates including waxy maize, a material which contains 80% α‐1–6‐glucosidic linkages. Substrate concentrations ranged from 1% to a typical commercial concentration of 30%. Conversion efficiencies of 90–92% DE were obtained with free and immobilized glucoamylase preparations. Conversion enhancements of 4–5 mg of DE above this level were obtained by the use of pullulanase in its free or immobilized forms. Close examination of free pullulanase stability as a function of pH indicated improved thermal stability at higher pH values. At 50°C and pH 5.0, the free enzyme was inactivated after 24 h. At pH 7.0, the enzyme still possessed one‐half its activity after 72 h. Studies were conducted in both batch and continuous total recycle reactors. All experiments were conducted at 50°C. Experiments conducted with coimmobilized enzymes proved quite promising. Levels of conversion equivalent to those obtained with the individually immobilized enzymes were realized.