Beatriz Rivas

Submerged Citric Acid Fermentation on Orange Peel Autohydrolysate

The citrus-processing industry generates in the Mediterranean area huge amounts of orange peel as a byproduct from the industrial extraction of citrus juices. To reduce its environmental impact as well as to provide an extra profit, this residue was investigated in this study as an alternative substrate for the fermentative production of citric acid. Orange peel contained 16.9% soluble sugars, 9.21% cellulose, 10.5% hemicellulose, and 42.5% pectin as the most important components. To get solutions rich in soluble and starchy sugars to be used as a carbon source for citric acid fermentation, this raw material was submitted to autohydrolysis, a process that does not make use of any acidic catalyst. Liquors obtained by this process under optimum conditions (temperature of 130 degrees C and a liquid/solid ratio of 8.0 g/g) contained 38.2 g/L free sugars (8.3 g/L sucrose, 13.7 g/L glucose, and 16.2 g/L fructose) and significant amounts of metals, particularly Mg, Ca, Zn, and K. Without additional nutrients, these liquors were employed for citric acid production by Aspergillus niger CECT 2090 (ATCC 9142, NRRL 599). Addition of calcium carbonate enhanced citric acid production because it prevented progressive acidification of the medium. Moreover, the influence of methanol addition on citric acid formation was investigated. Under the best conditions (40 mL of methanol/kg of medium), an effective conversion of sugars into citric acid was ensured (maximum citric acid concentration of 9.2 g/L, volumetric productivity of 0.128 g/(L.h), and yield of product on consumed sugars of 0.53 g/g), hence demonstrating the potential of orange peel wastes as an alternative raw material for citric acid fermentation.

Carbon material and bioenergetic balances of xylitol production from corncobs by Debaryomyces hansenii.

The effect of oxygenation on xylitol production by the yeast Debaryomyces hansenii has been investigated in this work using the liquors from corncob hydrolysis as the fermentation medium. The concentrations of consumed substrates (glucose, xylose, arabinose, acetate and oxygen) and formed products (xylitol, arabitol, ethanol, biomass and carbon dioxide) have been used, together with those previously obtained varying the hydrolysis technique, the level of adaptation of the microorganism, the sterilization procedure and the initial substrate and biomass concentrations, in carbon material balances to evaluate the percentages of xylose consumed by the yeast for the reduction to xylitol, alcohol fermentation, respiration and cell growth. The highest xylitol concentration (71 g/L) and volumetric productivity (1.5 g/L·h) were obtained semiaerobically using detoxified hydrolyzate produced by autohydrolysis‐posthydrolysis, at starting levels of xylose (S0) and biomass (X0) of about 100 g/L and 12 gDM/L, respectively. No less than 80% xylose was addressed to xylitol production under these conditions. The experimental data collected in this work at variable oxygen levels allowed estimating a P/O ratio of 1.16 molATP/molO. The overall ATP requirements for biomass production and maintenance demonstrated to remarkably increase with X0 and for S0 ≥ 130 g/L and to reach minimum values (1.9–2.1 molATP/C‐molDM) just under semiaerobic conditions favoring xylitol accumulation.

Fermentation kinetics and chemical characterisation of vino tostado, a traditional sweet wine from Galicia (NW Spain).

BACKGROUND Grapes after harvesting are air dried and pressed in order to concentrate sugars, acids and flavour compounds to produce vino tostado (toasted wine), a wine with intense aroma and flavour notes and high residual sugar concentration. In order to get a better knowledge of the difficulties involved, several fermentations were conducted at 12 and 28 degrees C using 0, 15 and 30 g hL(-1) ammonium sulfate and 0, 25 and 50 g hL(-1) exogenous commercial yeast (Saccharomyces cerevisiae var. bayanus) to study the kinetics of sugar consumption and ethanol, acetic acid and glycerol production. RESULTS Fermentation kinetic parameters were calculated and metal concentrations and antioxidant activity were analysed. CONCLUSION The spontaneous fermentation at 12 degrees C and all fermentations conducted with the commercial yeast gave vino tostado of adequate quality, while the spontaneous fermentation at 28 degrees C was sluggish. High-temperature fermentations led to sweeter wines with higher volumetric productivities, although low-temperature fermentations produced better wines in terms of higher glycerol and lower acetic acid levels. Fructose was the only sugar to be consumed during spontaneous fermentations, while both glucose and fructose were consumed during fermentations of the inoculated musts, with preference for each monosaccharide depending on temperature.

Maintenance and growth requirements in the metabolism of Debaryomyces hansenii performing xylose-to-xylitol bioconversion in corncob hemicellulose hydrolyzate.

In order to improve the biotechnological production of xylitol, the metabolism of Debaryomyces hansenii NRRL Y‐7426 in corncob hemicellulose hydrolyzate has been investigated under different conditions, where either maintenance or growth requirements predominated. For this purpose, the experimental results of two sets of batch bioconversions carried out alternatively varying the starting xylose concentration in the hydrolyzate (65.6 ≤ S0 ≤ 154.7 g L−1) or the initial biomass level (3.0 ≤ X0 ≤ 54.6 gDM L−1) were used to fit a metabolic model consisting of carbon material and ATP balances based on five main activities, namely fermentative assimilation of pentoses, semi‐aerobic pentose‐to‐pentitol bioconversion, biomass growth on pentoses, catabolic oxidation of pentoses, and acetic acid and NADH regeneration by the electron transport system. Such an approach allowed separately evaluating the main bioenergetic constants of this microbial system, that is, the specific rates of ATP and xylose consumption due to maintenance (mATP = 21.0 mmolATP C−molDM−1h−1; mXyl = 6.5 C‐mmolXyl C−molDM−1h−1) and the true yields of biomass on ATP (YATPmax = 0.83 C‐molDM molATP−1) and on xylose (YXylmax = 0.93 C‐molDM C‐molXyl−1). The results of this study highlighted that the system, at very high S0 and X0 values, dramatically increased its energy requirements for cell maintenance, owing to the occurrence of stressing conditions. In particular, for S0 > 130 g L−1, these activities required an ATP consumption of about 2.1 molATP L−1, that is, a value about seven‐ to eightfold that observed at low substrate concentration. Such a condition led to an increase in the fraction of ATP addressed to cell maintenance from 47% to 81%. On the other hand, the very high percentage of ATP addressed to maintenance (>96%) at very high cell concentration (X0 ≥ 25 gDM L−1) was likely due to the insufficient substrate to sustain the growth. Biotechnol. Bioeng. 2009;102: 1062–1073.