Kinetics of sugars consumption and ethanol inhibition in carob pulp fermentation by Saccharomyces cerevisiae in batch and fed-batch cultures

Abstract

The waste materials from the carob processing industry are a potential resource for second-generation bioethanol production. These by-products are small carob kibbles with a high content of soluble sugars (45-50%). Batch and fed-batch Saccharomyces cerevisiae fermentations of high density sugar from carob pods were analyzed in terms of the kinetics of sugars consumption and ethanol inhibition. In all the batch runs, 90-95% of the total sugar was consumed and transformed into ethanol with a yield close to the theoretical maximum (0.47-0.50 g/g), and a final ethanol concentration of 100-110 g/l. In fed-batch runs, fresh carob extract was added when glucose had been consumed. This addition and the subsequent decrease of ethanol concentrations by dilution increased the final ethanol production up to 130 g/l. It seems that invertase activity and yeast tolerance to ethanol are the main factors to be controlled in carob fermentations. The efficiency of highly concentrated carob fermentation makes it a very promising process for use in a second-generation ethanol biorefinery.

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

“…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.…”

“…A solid waste named carob kibbles that has high content of soluble sugars (40-60 %w/v), is produced from the fruit after the carob seeds are removed for the high-added value compounds extraction [6]. We have shown previously that the sugars from the carob pod can be easily extracted with water using very low energy input and are efficiently transformed into ethanol by Saccharomyces cerevisiae [2].…”

“…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.…”

“…The first one is that S. cerevisiae, that can ferment efficiently and completely the carob sugars (sucrose, fructose and glucose) if optimal conditions are provided [2], cannot ferment lactose, the main sugar of whey. To solve this problem, a mixed population approach should be taken and a lactosefermenting yeast species, such as Kluyveromyces lactis or Kluyveromyces marxianus, widely used in whey fermentation [5,7,8] should be included in the process.…”

“…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].…”