Thesis

Intensive Propagation of the lactic acid bacterium Lactococcus Lactis

Swansea University, Chemical Engineering
Thesis for: Master in Philosophy (MPhil), Advisor: Dr.R.W.Lovitt

ABSTRACT 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.

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Jun 1, 2014