Growth and α-amylase production by Aspergillus oryzae during continuous cultivations

Abstract

Continuous cultivations of an α-amylase producing strain of Aspergillus oryzae were carried out using a chemically defined medium with glucose as the growth limiting component. For steady-state cultures the recovery of carbon was about 99%, indicating that all major carbon components i.e. biomass, carbon dioxide and α-amylase were measured. The rates of sugar consumption, oxygen consumption and carbon dioxide production are all linear function of the specific biomass growth rate: r i = 1 y ix μ + m i . Measurements of the steady-state residual glucose concentration showed that A. oryzae has a very efficient glucose uptake system with an estimated saturation constant of about 6 mg l −1. From pulse experiments applied to steady-state cultures, it was shown that the glucose uptake in A. oryzae is regulated at least at two levels. One level of regulation is connected with the size or activity of the catabolic machinery i.e., enzymes in the glucose uptake system, which seem to be built up within 30–40 min. The maximum specific glucose uptake rate was, however, found to depend on the size of the protein synthesis system (PSS) which is estimated through the stable RNA content of the cells. Regulation of this level is determined by time constants in the range of several hours. Experiments showed a significant repression of a-amylase by glucose even at 10 mg 1 −1 glucose. It was furthermore observed that the specific α-amylase production is closely coupled to the growth of the fungus with a low α-amylase production rate at low specific growth rates, suggesting that growth of the mycelium is crucial for high production of extracellular proteins.