STUDIES ON THE GROWTH OF ASPERGILLUS NIGER STRAINS ON TAPIOCA STARCH

Abstract

A wild type A. niger strain was found to grow on the substrate containing tapioca starch in suspension by secreting amylolytic ezymes extracellularly in order to produce simple sugars from starch for growth. This strain, An was subjected to mutation by uv and g-ray irradiation. The various aspects of growth of An and mutants derived from it, on tapioca starch medium, were examined:- a) production of amylolytic enzymes and conditions affecting this production. b) formation of simple sugars. c) protein enrichment of the tapioca starch substrate. d) production of citric acid. e) use of genetic analysis in improving the production of amylolytic enzymes and protein enrichment of the starch. Mutagenic treatment of the wild type strain gave variants, some of which exhibited an improved production of amylolytic enzymes, reducing sugars, protein and citric acid compared to the wild type strain, when grown on tapioca starch medium. Diploid and haploid variants with improved amylolytic activity and protein enrichment over their parental haploid strains were obtained via the parasexual cycle. The superior amylolytic enzyme producing strains were found to have a high activity of enzymes in the medium, with a maximum activity of about 670,000 international units (i.u.) in 100 ml of medium, after the growth of mutant strain, 551. The basic conditions allowing maximum amylolytic activity were a temperature of incubation of 37°C, a pH of tapioca medium of 5.4, with NaN03 as the nitrogen source. A good amount of reducing sugars was produced by the high amylolytic strains, with a maximum amount of about 1.5g in 100 ml of medium, afte the growth of mutant strain, 551. This medium was used for the successful cultivation of yeast strains which did not grow in the original tapioca medium. The growth of A. niger strains in tapioca medium gave protein enrichment in 2 ways - firstly, in the form of fungal mycelia and secondly, by enrichment of the medium itself. The protein content of the mycelia was 23 fold that of the tapioca medium; the prote in content of the medium following the cultivation of the fungal strain reached a maximum of 3 times more than the control. The growth of A. niger strains in tapioca starch medium also resulted in citrate production. An equivalent amount of citric acid was found to be produced using 2% tapioca starch as the substrate as when 20% dextrose was used.