Kinetic and Energetic Parameters of Carob Wastes Fermentation by Saccharomyces cerevisiae: Crabtree Effect, Ethanol Toxicity, and Invertase Repression

Abstract

Carob waste is a useful raw material for the second-generation ethanol because 50% of its dry weight is sucrose, glucose, and fructose. To optimize the process, we have studied the influence of the initial concentration of sugars on the fermentation performance of Saccharomyces cerevisiae. With initial sugar concentrations (S0) of 20 g/l, the yeasts were derepressed and the ethanol produced during the exponential phase was consumed in a diauxic phase. The rate of ethanol consumption decreased with increasing S0 and disappeared at 250 g/l when the Crabtree effect was complete and almost all the sugar consumed was transformed into ethanol with a yield factor of 0.42 g/g. Sucrose hydrolysis was delayed at high S0 because of glucose repression of invertase synthesis, which was triggered at concentrations above 40 g/l. At S0 higher than 250 g/l, even when glucose had been exhausted, sucrose was hydrolyzed very slowly, probably due to an inhibition at this low water activity. Although with lower metabolic rates and longer times of fermentation, 250 g/l is considered the optimal initial concentration because it avoids the diauxic consumption of ethanol and maintains enough invertase activity to consume all the sucrose, and also avoids the inhibitions due to lower water activities at higher S0.

Growth kinetics and physiological behavior of co-cultures of Saccharomyces cerevisiae and Kluyveromyces lactis , fermenting carob sugars extracted with whey

“…Our objective with this work was to develop a technological process to transform carob wastes into 2 nd generation ethanol, a fermentation whose technical feasibility had been previously demonstrated [2,3], but in this case using whey, from a cheese dairy effluent, as the water source to extract the sugars from the carob kibbles. To our knowledge the use of whey as aqueous solvent, to save water, had not be considered previously.…”

“…Two yeast strains were selected: Saccharomyces cerevisiae F13A [2,3] and Kluyveromyces lactis CBS 2360. Stock cultures were maintained in 20 % glycerol at -80 °C.…”

“…From previous works [2,3] it has been demonstrated that carob waste sugars, extracted with water, can be fermented efficiently by S. cerevisiae. A multi-wastes approach would enable the simultaneous management of several agro-industrial wastes, providing the high amounts of sugar and water needed.…”

“…Moreover the problems and bottlenecks of the process, when using high carob sugar concentrations, have been identified and solved by the design of a fed-batch process [2,3].…”

Using agro-industrial wastes for mixotrophic growth and lipids production by the green microalga Chlorella sorokiniana

“…Different agro-industrial wastes have been used successfully as carbon source for yeast and microalgae [22], including carob pod extract [23], corn powder hydrolysate [24], cheese whey effluent [25], urban wastewater [26] or apple vinegar [27]. In addition, the low…”