pH auxostat
The pH-auxostat couples the addition of fresh medium to pH control.
As the pH drifts from a given setpoint, fresh medium is added to bring the pH
back to the setpoint. The rate of medium addition is determined by the
buffering capacity and the feed concentration of the limiting nutrient and
not directly by the setpoint (pH) as in a traditional auxostat. The
pH-auxostat is robust but controls nutrient concentration indirectly. The pH
change is often an excellent indication of growth and meets the requirements
as a growth-dependent parameter as defined by Fredrickson et al.
(1970). However, the exact cause of pH change varies among organisms. It
represents the summation of the production of different ionic species and ion
release during substrate uptake. Therefore the pH can move either up or down
as a function of growth. The most common situation is pH depression because
of organic acid production and ammonium uptake. However for microorganisms
growing on protein or amino acid-rich media, the pH will rise with growth
because of the release of excess ammonia. The pH-auxostat was simultaneously
reported by different research groups and was probably first operated in the
1960's (e.g., Bungay, 1972; Watson 1969) and a detailed development was
presented by Martin and Hempfling (1976).