Mechanistic model of the growth of Streptococcus cremoris HP at super optimal temperatures

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Abstract
The kinetic response of Streptococcus cremoris HP to growth at super optimal temperatures is reported. The response to a step increase in temperature was shown to be transient and to result from an increased metabolic rate caused by the raised temperature combined with thermal deactivation of the cell mass present. The catabolic and anabolic activities of the cell were shown to decay at different rates resulting in an accumulation of cells capable of catabolism (energy production) but unable to reproduce. The proposed mechanism was confirmed by independent estimates of the catabolic and anabolic activities of the culture. A mathematical model based on the proposed mechanism and incorporating simultaneous exponential growth, thermal death, and catabolic uncoupling of anabolically inactive cells was developed. Experimental evaluation of the model indicated the presence of a delay in deactivation of metabolic activity in response to a temperature transient. After the inclusion of this delay in death, it was confirmed that the model was capable of prediction of the balanced growth and transient response of this organism to changes in growth temperature. The delay in death was shown to be of major significance to the control of a simulated cheddar cheese fermentation.