L.lactis is an important lactic acid bacterium, widely used in the dairy industry
nowadays. The coccus is used as a natural acidifier for the inoculation of bulk
quantities of milk in order to produce a variety of fermented products. As such, large
quantities of its biomass are necessary. The possibility of producing the cellular
biomass of the coccus in mass quantities was investigated through several techniques.
Firstly, the bacterium was grown into simple batch cultures without pH control where
the physicochemical needs of the coccus were determined. Through the determination
of the optimum nutritional conditions for the propagation of the coccus, an optimised
medium for growth occurred.
The growth efficiency on the medium was tested on a 2L STR reactor operated
batchwise with continuous pH control. The optimum pH conditions for the growth of
the coccus were determined as well as parameters such as cellular yield coefficient,
substrate and starter inoculum concentration.
The metabolism o the coccus was determined as homofermentative, mainly producing
lactic acid through measurements of the organic acids produced at the end of the
fermentation process in the STR. In addition carbohydrate consumption rate in the
optimum pH values selected was measure and the constant coefficients for substrate
consumption end product inhibition effect and a maintenance coefficient term was
determined. A simple mathematical model was constructed to describe the growth of
the coccus batchwise and a correlation was made between the experimental data
obtained from the STR fermentations and the theoretical predictions. An inhibition
term was incorporated in the model in order to describe the inhibitory effect of lactic
acid over the growth. With the use of the inhibition term a good fit between the
experimental data and the model was obtained.
The growth kinetics of L.lactis were further investigated by modelling its growth in a
continuous system and in a Membrane Bioreactor system. The feasibility and the
efficiency in all 3 systems was evaluated in terms of the volumetric cell productivity.
(g/l/h). The models were also tested against different substrate concentrations,
different starter inoculums, different dilution and flux rates. The MBR system has
found to be highly productive especially when operated in the continuous mode of
substrate feed , the volumetric cell productivity (g/l/h) (45.94 g/l/h) was over 10 times
higher when compared with the volumetric cell productivity (g/l/h) given by the
continuous system (1.4 g/l/h)and over 30 times when compared with the batch
system(0.45 g/l/h). MBR was proven to be a possibly useful system for the
development of high concentrations of cellular biomass but its practical application
has to be further investigated.