BACKGROUND The objective of the study was to evaluate the influence of the oxygen transfer rate (OTR) on poly(gamma-glutamic acid) (gamma-PGA) production and molecular weight and the carbon source utilization in cultures of Bacillus velezensis 83 developed in shake flasks. RESULTS The cellular metabolism of B. velezensis 83 was affected by the OTR conditions. At maximum oxygen transfer rates (OTRmax) of 12, 17 and 19 mmol L-1 h(-1), glucose was mainly used for cell growth and gamma-PGA production. In contrast, under oxygen-limiting conditions (2.5 and 5.7 mmol L-1 h(-1)), the carbon source was also directed to acetoin, 2,3-butanediol and CO2 production. Changes in OTRmax significantly impacted gamma-PGA production, its maximum mean molecular weight (MMWmax) and its depolymerization kinetics. In cultures developed at an OTRmax of 2.5 mmol L-1 h(-1), the bacteria synthesized a polymer with a MMWmax of 2400 kDa, whereas at OTR greater than 5.7 mmol L-1 h(-1) a polymer with a MMWmax of 2850 kDa was obtained and the gamma-PGA concentration increased up to 2.8-fold. In addition, gamma-PGA depolymerization, occurring even in the presence of glucose, increased at non-limiting OTR conditions. CONCLUSIONS The production, yield and molecular weight of gamma-PGA are determined by OTR conditions. Our results showed, for the first time, that there is a direct relationship between OTRmax and the gamma-PGA depolymerization rate in B. velezensis 83 cultures. (c) 2020 Society of Chemical Industry