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Background  Alpha-amylases hydrolyze 1,4 α-glycosidic bonds of starch and produce malto-oligosaccharides. It is an important enzyme generally applied in textile, food and brewing industries. Enhancement in thermal stability and productivity of enzymes are the two most sought after properties for industrial use. The  Aspergillus oryzae  (Koji) has Generally Recognized as Safe (GRAS) status and safe for use in food industry. Hence, Koji strain’s development for the screening of potent mutants, hyper producer of thermostable α-amylases, with desired attributes is the need of the time.    Results  A process has been developed to improve super Koji ( A. oryzae  cmc1) strain through γ-rays treatment. The doses i.e. 0.60, 0.80, 1.00, 1.20 & 1.40 KGy gave more than 3.0 log kill. Initially, 52 Koji mutants resistant to 1% ( w/v ) Triton X-100 were selected. 2 nd  screening was based on α-amylases hyper production and 23 mutants were sorted out by measuring clearing zones index (CI). Afterwards nine potent mutants, resistant to 2-deoxy D-glucose, were screened based on CI. These were further analyzed for thermal stability and productivity of α-amylase under submerged conditions. The mutants’ M-80(10), M-100(6) & M-120(5) gave about four fold increases in α-amylases productivity. The half life of M-100(6) α-amylase at 55 °C was 52 min and was highest among the mutants. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis confirmed that mutants did not produce aflatoxins. Field Emission Scanning Electron Microscopy (FESEM) of Koji mycelia depicted that exposure to gamma rays increased rigidity of the mycelium. The potent Koji mutant M-100(6) was grown on soluble starch in 10L fermenter and produced 40.0 IU ml -1  of α-amylases with specific activity of 2461 IU mg -1  protein. Growth kinetic parameters were:  μ  = Specific growth rate= 0.069 h -1 , t d  = Biomass doubling time= 10.0 h,  Y   p /x  = Product yield coefficient with respect to cell mass = 482 U g -1 ;  q  p = Specific rate of product formation= 33.29 U g -1  h -1 .    Conclusion  It was concluded that the developed five step screening process has great potential to generate potent mutants for the hyper production of thermostable enzymes through γ-rays mediated physical mutagenesis. The developed thermostable α-amylases of super Koji mutantM-100(6) has immense potential for application in saccharification process for maltose syrup production. Moreover, the developed five step strain’s development process may be used for the simultaneous improvement in productivity and thermal stability of other microbial enzymes.    Electronic supplementary material  The online version of this article (10.1186/s12866-018-1345-y) contains supplementary material, which is available to authorized users.

Random mutagenesis of super Koji (Aspergillus oryzae): improvement in production and thermal stability of α-amylases for maltose syrup production