Some characteristic phenomena of oxygen transfer in hydrocarbon fermentation

The oxygen transfer coefficient has been investigated in a hydrocarbon fermentation with a yeast, Candida petrophilum. The dynamic measurement of the volumetric oxygen transfer coefficient, K/sub L/a, has been performed during the fermentation. In hydrocarbon fermentation, K/sub L/a values and the trapping of gas in culture liquid decreased markedly as the fermentation proceeded. On the contrary, the K/sub L/a value did not change in the fermentation with glucose. Some factors possibly having an influence on the change of K/sub L/a have been studied, and it was found that the concentration of cells had a strong effect on the K/sub L/a value. Hydrocarbon-assimilating yeasts have a strong affinity to n-paraffin oil, and they prefer to adsorb onto the surface of air bubbles. The cells become a physical obstacle to oxygen transfer from air bubbles into the liquid by blocking its passage, and thus reducing the rate of oxygen transfer. The adsorption of the yeast cells onto air bubbles can occur only when n-paraffin oil exists in the culture liquid. A small amount of an antifoaming agent makes air bubbles unstable and prevents their being coated by cells, resulting in high K/sub L/a values and high quantities of air entrapped in themore » culture liquid. It has been observed that the cells adsorbed onto the surface of air bubbles can take oxygen from the liquid film around them by direct absorption. During the second half of the logarithmic growth phase, about 90 percent of the needed oxygen can be taken up by direct absorption.« less