José Manuel Domínguez

Biotechnological production of xylitol. Part 3: Operation in culture media made from lignocellulose hydrolysates

The acid hydrolysis of lignocellulosic materials for obtaining xylose solutions and the utilization of hydrolysates for making culture media for xylitol production are reviewed. Strategies for enhancing bioconversion of hydrolysates, including microorganism adaptation and physico-chemical processing of hydrolysates, are discussed. The effects caused by other influential factors (such as type and concentration of inhibitors, degree of hydrolysate concentration, cell concentration, pH, available oxygen and media supplementation) are also considered.

Biotechnological production of xylitol. Part 1: Interest of xylitol and fundamentals of its biosynthesis

Xylitol, a polyol with growing market as a sweetener, can be produced by either chemical or biotechnological methods. The chemical properties of xylitol relevant to its food applications are summarized. The applicable production technologies (extraction from vegetables and fruits, chemical synthesis and biotechnological processes based on the utilization of bacteria, fungi or yeast) are described. Special attention is paid to the fundamentals of xylose metabolism by yeast, since this is a key factor affecting the feasibility of the most promising biotechnological methods for xylitol production. Other related technologies, involving the utilization of enzymes, combinations of enzymes and microorganisms or mixed cultures of microorganisms, are also described.

Improved xylitol production with Debaryomyces hansenii Y-7426 from raw or detoxified wood hydrolysates

Abstract Xylose solutions obtained by acid prehydrolysis of raw or NaOH-treated Eucalyptus globulus wood samples were used for making fermentation media useful for xylitol production with Debaryomyces hansenii NRRL Y-7426. Several physicochemical treatments for detoxification of hydrolysates were tested including overliming, sulfite addition, adsorption in charcoal, and concentration by evaporation. Despite the relatively low xylose concentration of hydrolysates, high xylitol yields (up to 0.84 g xylitol g −1 consumed xylose) were obtained at good productivities (up to 0.53 g xylitol l −1 h −1 ). The improvements obtained with the several strategies assayed are discussed in terms of overall productivities and product yield.

Solvent extraction of hemicellulosic wood hydrolysates: a procedure useful for obtaining both detoxified fermentation media and polyphenols with antioxidant activity

Eucalyptus wood hydrolysates were extracted with organic solvents (ethyl acetate and diethyl ether) to remove part of the phenolics derived from lignin. In order to obtain increased phenolic removal, the effects of the major experimental variables affecting extraction (type of solvent, hydrolysate to solvent volume ratio, temperature, pH and coupling of extraction stages) were explored. Under the best operational conditions, up to 84% of the initial lignin-derived compounds were extracted. The phenolic compounds extracted by solvents showed antioxidant activity. Under the best conditions, the antioxidant activity coefficient was 64% of the value found with a synthetic antioxidant (BHT). Extracted hydrolysates led to xylose solutions, allowing an enhanced fermentative production of xylitol with the yeast Debaryomyces hansenii.

Pretreatment of sugar cane bagasse hemicellulose hydrolysate for xylitol production by yeast.

Sugar cane bagasse hemicellulose hydrolyzate was prepared by dilute sulfuric acid (3% w/v) hydrolysis with a high-solid, low-liquid ratio followed by leaching. The hydrolyzate contains 11% (w/v) of fermentable sugars with xylose as the major component, which comprises up to 75% of the total reducing sugars. The neutralized hydrolyzate exhibited strong inhibition toward cell growth and ethanol production by yeasts. The inhibitory effect of hydrolyzate can be alleviated by treating hydrolyzate either with ion-exchange resins or with acidified activated charcoal.

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.

Charcoal adsorption of wood hydrolysates for improving their fermentability: Influence of the operational conditions

Xylose solutions, obtained by acid hydrolysis of Eucalyptus wood, were subjected to treatments with charcoal to remove lignin-derived compounds that can exert an inhibitory effect on subsequent fermentation stages. The effects of three operational variables (hydrolysate concentration, hydrolysate:charcoal ratio and adsorption time) on the concentrations of xylose, acetic acid and phenolics were considered. Additional experiments were performed with hydrolysates concentrated to a third of the initial volume to evaluate the effects of temperature, pH and hydrolysate:charcoal ratio (using a broader range of ratios) on the adsorption process. Raw and charcoal-treated hydrolysates were used as fermentation media for xylitol production by the yeast Debaryomyces hansenii. Treatments with a hydrolysate:charcoal ratio of 205 g/g were sufficient for fermentation to occur; a higher charge of adsorbent did not result in significant improvements.

Biotechnological production of citric acid.

This work provides a review about the biotechnological production of citric acid starting from the physicochemical properties and industrial applications, mainly in the food and pharmaceutical sectors. Several factors affecting citric acid fermentation are discussed, including carbon source, nitrogen and phosphate limitations, pH of culture medium, aeration, trace elements and morphology of the fungus. Special attention is paid to the fundamentals of biochemistry and accumulation of citric acid. Technologies employed at industrial scale such as surface or submerged cultures, mainly employing Aspergillus niger, and processes carried out with Yarrowia lipolytica, as well as the technology for recovering the product are also described. Finally, this review summarizes the use of orange peels and other by-products as feedstocks for the bioproduction of citric acid.

Biotechnological production of xylitol. Part 2 : Operation in culture media made with commercial sugars

The production of xylitol by microorganisms cultured in media made from commercial sugars (xylose with or without other sugars as co-substrates) is reviewed. The microbial strains used for this purpose and the influence of the most important operational variables (including age of inoculum, initial concentrations of cells and substrate, pH, temperature, composition of culture media, dissolved oxygen and product concentration) are considered. The existing information on xylitol production by recombinant microorganisms is summarized.

Dilute acid hemicellulose hydrolysates from corn cobs for xylitol production by yeast

Abstract Dilute acid hemicellulose hydrolysate, comprised mainly of xylose, was obtained from ground corn cobs after dilute hydrochloric acid (2%, wt) hydrolysis at 100°C for 2 h. Similar acid hydrolysate was also obtained after the corn cobs were treated with 10% ammonium hydroxide at 26°C for 24 h. Neutralized hydrolysates containing ca. 130 g/l xylose were used as the substrate for xylitol production by a xylitol-producing yeast, Candida sp. 11-2. Ammonia-treated hydrolysate was a better substrate for xylitol production. Xylose was consumed within 36 h with a specific xylitol productivity of 1.94 g/l·h and a xylitol weight yield of 0.57 g/g xylose utilized. In contrast, neutralized hemicellulose hydrolysate obtained without prior ammonia treatment required anion exchange resin treatment to be fermented by yeasts. The specific productivity and weight yield of xylitol from anion exchange resin-treated hydrolysate were comparable to that from an ammonia-treated sample.