Micro-organisms of the genus Streptomyces are cultured in stages under submerted aerobic conditions in an aqueous nutrient medium, whereby in at least the last growth-stage a carbohydrate source is present, obtained from a hydrolysate of a xylan-contg. material. Pref. hydrolysate is an acid hydrolysate and micro-organisms used are pref. able to assimilate xylan. Pref. microorganisms are Streptomyces ATCC 21175 and ATCC 21176. The acid hydrolysate used may be a filtered, non-purified hydrolysate obtained by acid hydrolysis of cotton seed husks, rough fibres from wet-milled maize, etc. In the initial growth stages ca 0.05-0.25% of a non-nutrient material such as carboxymethylcellulose is pref. added to encourage the growth of a fibrous mycelium. The glucose-isomerase produced is used for the isomerisation of glucose in glucose syrup for the foodstuff industry. The process is economical and gives high yields.
