Abstract: This paper reports a mutagenic strain MS5.1, a strain, mutagenised from an original strain Bacillus stearothermophilus BS3.232 by N-methyl- N′-nitro-N-nitrosoguanidine(NTG), which effectively secreted a new kind of enzy me(neoamylase) that can catalytically make starch into isomalto-oligosaccharide. The activity of MS　5.1 producing enzyme is increased by 53.8% compared with BS3.232 under the same conditions. The results of thin-layer chromatography of the prod ucts from starch catalyzed by the enzyme demonstrated that the enzyme can be used to produce iso maltooligosaccharide from starch, including isomaltose, panose, isomaltotriose, isomaltotetose. The optimum conditions which secreted neoamylase were confirmed by orthogonal test and the results showed that the most important par ameter is the temperature of fermentation and the optimum conditions to secrete the enzyme were: the strain MS5.1 should be cultured with superior to 60% aerati on under 55 ℃ for 24 h.

Key words: isomaltooligosaccharide, neoamylase, Bacillus stearothermophilus, s ecretion