Production and properties of <i>Aspergillus niger</i> inulinase

Abstract

Abstract An inulinase producing Aspergillus niger strain was isolated from Compositae rhizosphere soil samples. High inulinase levels were produced on a corn steep liquor (CSL)‐maltose medium in the absence of inulin at 28°C within 110 h of fermentation. Media based on CSL‐sucrose yielded high cell‐bound inulinase activity; on inulin‐based media the enzyme was mainly extracellularly produced. Both crude extra‐and intracellular inulinase preparations displayed identical pH and temperature optima with maximal activity at pH 4.3–4.4 and at 55–56°C. These properties are favourable in view of large scale inulinase application for pure fructose production. High operation temperatures would avoid microbial contamination of reactors and would allow the use of high inulin‐substrate concentrations, a limiting factor in obtaining high conversion ratios. The remarkably low pH optimum prevents colour formation and undesirable chemical side reactions. An advantageous low ratio of invertase to inulinase activity (S/I value) of 0.85 was found for the crude extracellular enzyme preparation. Crude inulin (chicory) extracts are hydrolysed faster than pure inulin. Apart from inulin (100% hydrolysis), sucrose (45%) and raffinose (20%) are also hydrolysed, and no liberation of oligomers or of sucrose from inulin was observed. These facts indicate that the A. niger enzyme is an exo‐acting inulinase. The above characteristics make this A. niger inulinase an industrially attractive enzyme for the preparation of pure fructose from inulin‐containing agricultural crops.