Abstract: A method of controlling additive delivery during a bioreaction in a bioreactor comprises running a bioreaction in a bioreactor including adding additive into the bioreactor during spaced-apart feed events, where contents of the bioreactor equilibrate during a stabilisation period after a feed event; making in situ measurements of a bulk physical property of the bioreactor contents during the bioreaction to obtain process trend data; calculating a derivative of process trend data obtained over a measurement period beginning after a stabilisation period, the derivative being a metabolic rate index (MRI); and using the MRI to determine a time for starting a next feed event. A controller for a bioreactor and a bioreactor system are configured to operate according to the method.