Buffering capacity as an indicator of physiological state in a continuous culture ofCandida utilis

Proton production and consumption pathways in yeast metabolism. A chemostat culture analysis

In this investigation, a method for the accurate quantitative determination of net proton production or consumption in biological cultures has been devised. Cells are cultured under constant pH conditions. The specific rate of proton production or consumption by the culture (qH+, mmol h-1 per g biomass) is proportional to the mmol of base or acid required to maintain constant pH per unit time, and this equivalence is independent of the buffering capacity of the culture medium. The above method has been applied to chemostat cultures of Candida utilis growing on glucose or glycerol as carbon source, and different nitrogen sources. The results indicate that the nitrogen assimilation pathway alone determines the value of qH+, and a fixed stoichiometric relationship between nitrogen uptake rate qN (meq h-1 per g biomass) and qH+ has been found for each nitrogen source employed. Thus, qH+/qN values of +1, 0 and -1 were found for ammonium ions, urea and nitrate respectively. Under oxidative metabolism, the contribution of carbon catabolism to the value of qH+ was undetectable. Sine qN may be related to growth and production of type 1 compounds in fermentation processes, the parameter qH+ was incorporated into a model of growth and energy metabolism in chemostat culture (Castrillo and Ugalde, Yeast 10, 185 - 197, 1994), resulting in adequate simulations of experimentally observed culture performance. Thus, it is suggested that qH+ may be employed as a simple and effective control parameter for biotechnological processes involving biomass-related products.

Phenomenological theory of substrate-induced acidification with application to Candida utilis dissimilating ethanol

The mathematical theory of substrate-induced acidification by microorganisms was based on special formulation of mass and charge conservation laws for distinct species. The strength of the theory was tested on the ethanol-induced acidification by Candida utilis in pure water and in phthalate buffer. We showed that the theory may be used to approximate, extrapolate and predict the course of apparent acidification including the effects of buffering capacity of the broth. It makes it possible to formulate mathematical models that are suitable for process simulation and computer control of bioreactors when pH is used as a state variable.

Effect of external pH on ethanol oxidation by Candida utilis

The external pH affects both ethanol and oxygen uptake rates by nongrowing cells of Candida utilis suspended either in distilled water or in phthalate buffer. The buffering properties of organic acids control the maximum rates of exogenous respiration and ethanol uptake. The substrate limitation of ethanol uptake rate and endogenous respiration rate increase proportionally with increasing hydrogen ion concentration in the medium.

Effect of external pH on acidification and excretion of ethanol intermediates by Candida utilis

External pH affects the acidification induced by ethanol. The apparent specific rate of acidification depends on the dissociation properties and production of carbon dioxide, acetic and lactic acid which are intermediates of ethanol oxidation. The organic acids are transported by Nernst-Einstein diffusion. We designed and identified a new and simple mathematical model that allows us to describe the effect of external pH on nonstationary transport of dissociated intermediates of ethanol oxidation.