Interaction of medium detoxification/supplementation and cell recycling in fermentative xylitol production

Abstract

To study the importance and interactions of pH control and delignification of a medium derived from oat hull hemicellulose, as well as the influence of supplemental nitrogen source and cell recycling, Candida guilliermondii was used as the biocatalyst in repeated batch bioconversion processes in three successive batches each lasting 144 h. The research study was conducted based on a factorial design with three factors consisting of: a) pH control (pH_s: pH constant during the process; pH_i: pH adjusted to 6.0 at the beginning of bioconversion with no control thereafter; and pH_t: pH set to 6.0 before sterilization of the medium with no control thereafter); b) medium detoxification/supplementation (Conc.: medium used directly with no treatment; AC1.25: medium treated with 1.25% activated carbon; AC2.5: medium treated with 2.5% activated carbon; AC5: medium treated with 5% activated carbon; and AC2.5-N: medium detoxified with 2.5% activated carbon and then supplemented with ammonium sulphate); and c) cell recycling [three cycles (C₁, C₂, and C₃)]. Results from the bioconversion process indicated that detoxification of the medium was the least important factor affecting product yield and productivity, while cell recycling and medium supplementation were much more important parameters which needed to be considered to achieve a successful process. Results also indicated that medium supplementation by inorganic nitrogen (ammonium sulphate) could be a requirement to achieve a consistent process performance in consecutive cycles of bioconversion using recycled biocatalyst. However, there is no need to use a supplemental nitrogen source in a single-cycle process.     

A new method for the preparation of crystalline L-arabinose from arabinoxylan by enzymatic hydrolysis and selective fermentation with yeast

Arabinoxylan (`Cellace') corn fiber, containing 28.1% (w/w) as l-arabinose and 32.8% (w/w) as d-xylose, was hydrolyzed by a crude enzyme containing -xylanase, -xylosidase and -l-arabinofuranosidase originating from the extracellular culture broth of Penicillium funiculosum. The resultant hydrolysate contained l-arabinose, d-xylose and small amounts of other mono- and oligosaccharides. The l-arabinose and d-xylose were 21.3% (w/w) and 18.7% (w/w), respectively, based on the initial arabinoxylan. Williopsis saturnus var. saturnus, which metabolizes d-xylose without using l-arabinose, was aerobically cultured in the hydrolysate at 30 °C for 96 h. The sugar solution after removal of yeast cells contained l-arabinose and d-xylose which were 20.3% (w/w) and 0.002% (w/w), respectively, of the initial arabinoxylan. The solution was decolorized with activated carbon, and deionized with cation- and anion-exchange resins. The clear sugar solution thus obtained was composed of l-arabinose and d-xylose which were 19.3% (w/w) and 0.001% (w/w), respectively, of the initial arabinoxylan. After concentration of the sugar solution under reduced pressure, followed by crystallization of l-arabinose, 16% (w/w) l-arabinose (based on the initial arabinoxylan) was obtained as crude crystals. No d-xylose was detected in the final preparation.     

Structure of Ovarian Asterosaponin-1 in the Starfish Asterias amurensis

Cultured crown gall cells of Catharanthus roseus Don (Vinca rosea L.) was found to contain brassinosteroids. These were identified as brassinolide and castasterone by GC/MS. This is the first conclusive identification of endogenous brassinosteroids in cultured cells.     

木糖发酵生产乙醇的研究

选育出一株优良的木糖发酵菌株树干毕赤酵母菌7124,并利用纯木糖优化了木糖发酵条件,利用海藻酸钠固定化树干毕赤酵母菌增殖细胞,不仅能较好满足限氧发酵条件,而且能耐较高糖浓度,使乙醇发酵浓度提高到20g/L。利用半纤维素水解液进行了乙醇发酵的初步研究,基本达到了纯木糖发酵的效果。     

纤维素生物质新型水解技术的研究进展

纤维素生物质水解技术是生物质资源转化的关键技术之一,在传统的酸水解和酶水解技术基础上,近年来出现了一些新型的水解技术,它们一般都具有绿色高效、对环境友好等特点;回顾并综述了纤维素生物质水解技术的最新进展,并对纤维素生物质水解技术的发展研究方向提出了设想.     

Xylitol bioproduction: state-of-the-art,industrial paradigm shift,and opportunities for integrated biorefineries

Abstract

Recent advances in biomass conversion technologies have shown a promising future toward fermentation during xylitol production. Xylitol is one of the top 12 renewable added-value chemicals that can be obtained from biomass according to US Department of Energy (USDOE). Currently, xylitol accounts for approximately US$823.6 million of annual sales in the market, and this amount is expected to reach US$1.37 billion by 2025. This high demand has been achieved owing to the chemical conversion of hemicellulosic hydrolysates from different lignocellulosic biomasses, which is a costly and non-ecofriendly process. Xylose-rich hemicellulosic hydrolysates are the major raw materials for xylitol production through either chemical or biotechnological routes. Economic production of a clean hemicellulosic hydrolysate is one of the major bottlenecks for xylitol production on the commercial scale. Advancements in biotechnology, such as the isolation of novel microorganisms, genetic manipulation of xylose metabolizing strains, and modifications in the fermentation process, can enhance the economic feasibility of xylitol production on the large scale. Furthermore, xylitol production in integrated biorefineries can be even more economic, given the readily available raw materials and the co-use of steam, electricity, and water, among others. Exploring new biotechnology techniques in integrated biorefineries would open new markets and opportunities for sustainable xylitol production to fulfill the market’s growing demands for this sugar alcohol. This article is a review of the advancements reported in the whole biotechnological process for xylitol production, and involve pretreatment technologies, hemicellulosic hydrolysate preparation, xylose conversion into xylitol, and product recovery. Special attention is devoted to current metabolic engineering strategies to improve this bioprocess, as well as to the importance of xylitol production processes in biorefineries.     

Ethanol production from eucalyptus wood hemicellulose hydrolysate by Pichia stipitis

Ethanol production was evaluated from eucalyptus wood hemicellulose acid hydrolysate using Pichia stipitis NRRL Y-7124. An initial lag phase characterized by flocculation and viability loss of the yeast inoculated was observed. Subsequently, cell regrowth occurred with sequential consumption of sugars and production of ethanol. Polyol formation was detected. Acetic acid present in the hydrolysate was an important inhibitor of the fermentation, reducing the rate and the yield. Its toxic effect was due essentially to its undissociated form. The fermentation was more effective at an oxygen transfer rate between 1.2 and 2.4 mmol/L h and an initial pH of 6.5. The hydrolysate used in the experiences had the following composition (expressed in grams per liter): xylose 30, arabinose 2.8, glucose 1.5, galactose 3.7, mannose 1.0, cellobiose 0.5, acetic acid 10, glucuronic acid 1.5, and galacturonic acid 1.0. The best values obtained were maximum ethanol concentration 12.6 g/L, fermentation time 75 h, fermentable sugar consumption 99% ethanol yield 0.35 g/g sugars consumed, and volumetric ethanol productivity 4 g/L day. (c) 1992 John Wiley & Sons, Inc.     

Fermentation of sugar cane bagasse hemicellulose hydrolysate to l(+)-lactic acid by a thermotolerant acidophilic Bacillus sp.**

Sugar cane bagasse hemicellulose, hydrolyzed by dilute H2SO4, supplemented with mineral salts and 0.5% corn steep liquor, was fermented to L(+)-lactic acid using a newly isolated strain of Bacillus sp. In batch fermentations at 50 degrees C and pH 5, over 5.5% (w/v) L(+)-lactic acid was produced (89% theoretical yield; 0.9 g lactate per g sugar) with an optical purity of 99.5%.     

Matrix polysaccharides of oat coleoptile cell walls

Separation of component polysaccharides in extractable fractions of the noncellulosic matrix of Avena sativa coleoptile cell walls shows that the principal classes of polymers present are glucuronoarabinoxylans (GAX) and iodine-negative hemicellulosic β-glucans. Rhamnogalacturonan is a minor component. GAX contains about 5–10% glucuronic acid and its 4-O-methyl ether, arabinose in amount almost equal to xylose, and a small amount of galactose; some subfractions contained appreciable amounts of glucose and galacturonic acid but these may derive from separate, contaminating polysaccharides. From the sedimentation and diffusion coefficients and intrinsic viscosities of one subfraction each of the GAX and of the hemicellulosic glucan that had been purified to apparent homogeneity by criteria of sedimentation and borate electrophoresis, MWs of about 200 000 were calculated by two methods. The viscosity characteristics and gel-forming ability of the hemicellulosic glucan give evidence of appreciable molecular interactions which suggest that this polymer is an important structural component of the cell wall.     

Structure of hemicellulosic polysaccharides of Avena sativa coleoptile cell walls

The glycosidic linkage compositions of intact and, in some cases, enzyme-degraded polysaccharides extracted from the cell walls of oat coleoptiles and subsequently purified have been examined. A major component is shown to be a glucuronoarabinoxylan similar in structure to those described for a variety of other monocots. The noncellulosic glucan component is a β-linked polymer containing both 1,4- and 1,3-linked glucosyl residues in a ratio of 2 to 1. Analysis of the oligosaccharide produced by ‘lichenase’ digestion of this β-glucan suggests that the the 1,3- and 1,4-glucosyl linkages repeat in regular fashion. A small amount of xyloglucan polysaccharides like those described for cell walls of dicots was also detected.     

Hydrolase-Mediated Resolutions with Carbonic Acid Derivatives

It is shown that in the presence of crude PPL (Sigma type II) carbonic acid diesters ent-2 can be hydrolyzed enantioselectively. In contrast to hydrolysis of carboxylic esters working under pH-stat conditions is not necessary anymore. Vice versa, the enantioselective acylation of glycidol with carbonic acid derivatives, e.g. carbonic acid anhydrides, proceeds less selectively.     

Hydrolysis of newspaper polysaccharides under sulfate reducing and methane producing conditions

The initial decomposition rates of cellulose and hemicellulose were measured using toluene to specifically inhibit the microbial uptake of hydrolysis products during the degradation of newspaper under sulfate reducing and methane producing conditions. The amount of glucose and xylose accumulation in the first 2 weeks of incubation period was higher in the sulfate reducing condition compared to the methane producing condition. It was estimated that 28 and 6% of initially loaded cellulose in the sulfate reducing condition and the methane producing condition was hydrolyzed, respectively. Accordingly, the newspaper-cellulose hydrolysis rate constant was estimated to be 6.7 times higher in sulfate reducing condition than in methane producing condition. Based on the glucose accumulation patterns, when sulfate reducing bacteria (SRB) were inhibited by anthraquinone and molybdate (Na₂MoO₄), it may be suggested that SRB might have contributed to the hydrolysis of cellulose, while their effect on the hydrolysis of hemicellulose could not be elucidated.     

Aqueous extraction of sugarcane bagasse hemicellulose and production of xylose syrup

At the optimum level of severity, the aqueous extraction of sugarcane bagasse, an abundant agricultural resdue, gave, depending on the degree of comminution, 60% to 89% yield of xylose, most of it in the form of a water soluble xylan. A process for producing xylose-rich syrups was conceived and tested, consisting of aqueous extraction, acid hydrolysis of the concentrated aqueous extract centrifugal clarification of the hydrolysate, and recovery of the acid by continuous ion exclusion. The cost estimate indicates operating costs on the order of $0.12 to $0.15/kg xylose, in the form of xylose-rich molasses. (c) 1995 John Wiley & Sons, Inc.