Browsing by Author "Peinado, J. M."
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- A model of the specific growth rate inhibition by weak acids in yeasts based on energy requirementsPublication . Quintas, Célia; Leyva, J. S.; Sotoca, R.; Loureiro Dias, M. C.; Peinado, J. M.Zygosaccharomyces bailii, a spoilage yeast, capable of metabolic activity in food environments with low pH, low a(W) and in the presence of weak acid preservatives was chosen for a study on the effect of benzoic acid on growth parameters. In batch cultures, under controlled pH, this food preservative inhibited growth, decreasing the specific growth rate (mu) and the yield coefficient (Y-S) on glucose. Data obtained at pH 3.5, 4.0 and 4.5 showed that this inhibition was exclusively promoted by the undissociated form of the acid since the effect was independent of pH when the concentration of the acid was expressed in this form. Moreover, the relationship between the values for mu and Y-S, provided evidence that the specific consumption rate of glucose (q(S)) was not affected by benzoic acid, indicating that the inhibition of growth should be completely explained by a decrease of Y-S. The outcome of parallel experiments performed in continuous culture was that the decrease of Y-S was due to an increase of the maintenance coefficient (m), defined as the fraction of q(S) diverted from growth to cope with stress, represented in this case by the presence of the preservative. Based on these results a model was built, assuming that m increased hyperbolically with the concentration of benzoic acid, from zero in the absence of the acid up to q(S) when growth was completely inhibited. The concentration of the acid, for which m=q(S)/2, is a constant (K-W), and represents a measure of the tolerance for a preservative, in this case benzoic acid. The simple equation mu/mu(0)=1 +/- W/K-W predicts the value of mu for a concentration (W) of the preservative, requiring the knowledge of two parameters: the specific growth rate in the absence of the preservative (mu(0)) and K-W. The equation fitted very well the data of the effect of benzoic acid on the specific growth rate of Z bailii, having K-W=0.96 mM benzoic acid. The model was also validated with other spoilage yeasts grown in the presence of benzoic acid in microtiter plates in an automated spectrophotometer. The values obtained for K-w under these conditions confirm Z bailii as the most tolerant (K-W=2.1 mM) followed by Pichia sp. (K-W=0.78 mM), Saccharomyces cerevisiae (K-W=0.53 mM) and Debaryomyces hansenii (K-W=0.11 mM). (c) 2004 Elsevier B.V All rights reserved.
- Growth kinetics and physiological behavior of co-cultures of Saccharomyces cerevisiae and Kluyveromyces lactis, fermenting carob sugars extracted with wheyPublication . Rodrigues, Brígida; Lima-Costa, Maria Emília; Constantino, A.; Raposo, Sara; Felizardo, C.; Gonçalves, D.; Fernandes, T.; Dionísio, Lídia; Peinado, J. M.Alcoholic fermentation of carob waste sugars (sucrose, glucose and fructose) extracted with cheese whey, by co-cultures of Saccharomyces.cerevisiae and Kluyveromyces lactis has been analyzed. Growth and fermentation of S. cerevisiae in the carob-whey medium showed an inhibition of about 30% in comparison with water-extracted carob. The inhibition of K. lactis on carob-whey was greater (70%) when compared with the whey medium alone, due to osmolarity problems. Oxygen availability was a very important factor for K. lactis, influencing its fermentation performance. When K. lactis was grown alone on carob-whey medium, lactose was always consumed first, and glucose and fructose were consumed afterwards, only at high aeration conditions. In co-culture with S. cerevisiae, K. lactis was completely inhibited and, at low aeration, died after 3 days; at high aeration this culture could survive but growth and lactose fermentation were only recovered after S. cerevisiae became stationary. To overcome the osmolarity and K. lactis' oxygen problems, the medium had to be diluted and a sequential fermentative process was designed in a STR-31 reactor. K. lactis was inoculated first and, with low aeration (0.13 vvm), consumed all the lactose in 48 h. Then S. cerevisiae was inoculated, consuming the total of the carob sugars, and producing ethanol in a fed-batch regime. The established co-culture with K. lactis increased S. cerevisiae ethanol tolerance. This fermentation process produced ethanol with good efficiency (80 g/l final concentration and a conversion factor of 0.4g ethanolig sugar), eliminating all the sugars of the mixed waste. These efficient fermentative results pointed to a new joint treatment of agro-industrial wastes which may be implemented successfully, with economic and environmental sustainability for a bioethanol industrial proposal. (C) 2016 Elsevier Inc. All rights reserved.
- Kinetic and energetic parameters of carob wastes fermentation by Saccharomyces cerevisiae: crabtree effect, ethanol toxicity, and invertase repressionPublication . Rodrigues, Brígida; Peinado, J. M.; Raposo, Sara; Constantino, Ana; Quintas, Célia; Emília Lima-Costa, MariaCarob 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.