The stability of levan produced by Zymomonas mobilis 113 "S" was investigated during storage of culture liquid at different pH and temperatures. Levan was not degraded if stored in a culture liquid for 120 h at 30 or 25 degrees C, but at 55 or 60 degrees C the degree of degradation was inversely proportional to pH. Cell biomass and levan sediment components did not cause the degradation of levan. The influence of organic acids (lactic-, acetic-, and gluconic) and HCl as levan hydrolyzing agents at pH 3.7, temperature 30 degrees C or 55 degrees C was investigated in a model solution. ne hydrolytic effect was similar in the case of organic acids-ensuring more fructose formation, while HCl ensured higher amounts of fructo-oligosaccharides (FOS) and/or low molecular weight levan. Consequently, acid hydrolysis acts as a main levan degrading factor during storage in a culture liquid. (c) 2004 Elsevier Ltd. All rights reserved.