Xylitol is a natural pentasaccharide. In nature, xylitol is found in many fruits and vegetables such as apricot, raspberry, straw, lettuce, and broccoli. As a sweetener, it has attracted more and more attention in recent years because it does not cause dental caries and has a high sweetness. The metabolism of xylitol does not require the regulation of insulin and is clinically used as a substitute for sucrose in the food of diabetic patients. Its energy is similar to sucrose, so it can be used by diabetics for infusion. At the same time, its metabolic process does not require the action of glucose-6-phosphate dehydrogenase, and it is also an ideal sweetener for glucose-6-phosphate dehydrogenase deficiency patients. Recently, researchers from Iowa State University in the United States have found that xylitol has some effect to prevent lung infection. Xylitol, which is the main chemical production method, is present in fruits and vegetables, but its content is very low. Direct extraction is not only difficult but also economically poor. Currently, the industry mainly produces xylose by catalytic hydrogenation. Xylitol is widely used as a raw material for producing xylitol, and agricultural and forestry by-products rich in xylan such as corn cob, bagasse, straw, bark, sawdust, etc. can be used. The crude material is treated with dilute acid, and the hemicellulose is partially hydrolyzed to obtain xylose-based hydrolyzate. After proper treatment, the sugar is reduced to alcohol by hydrogenation under Raney nickel catalysis. The hydrogenation solution is evaporated and crystallized. Get xylitol. China is a big producer of xylitol with an annual output of 20,000 tons, which accounts for half of the international market. Although the source of raw materials for industrial production of xylitol is abundant and inexpensive, the production process is rather complicated. The raw materials must be refined to a certain degree to avoid catalyst poisoning; the catalytic hydrogenation reaction needs to be performed under high temperature and high pressure; and the reaction will reduce all the sugars in the raw materials to the corresponding sugar alcohols, and the product contains not only xylitol but also sorbose Other sugar alcohols such as alcohols and arabitol, these sugar alcohols are similar in nature and difficult to separate. Therefore, the production cost of xylitol is relatively high and the price is about 10 times that of sucrose. Biotechnology has achieved major breakthroughs in biotechnical production of xylitol by reducing xylose to xylitol under the catalysis of xylose reductase in microbial cells. Hydrogen is derived from hydrogen atoms in the reduced coenzyme and hydrogen ions in water. No separate hydrogen production is required. This simplifies the production process and makes it safer and more energy-efficient, but it requires a portion of the sugar to regenerate the reduced coenzyme. Xylitol Production Using Natural Microorganisms Microorganisms used to produce xylitol are mainly yeasts. There are many yeasts in nature that can use xylose as the only carbon source. Xylitol is the intermediate of xylose metabolism of these microorganisms. Different microorganisms have different ability to metabolize xylose, and the speed and quantity of xylitol accumulation are also different. Xylitol can be produced rapidly and in large quantities by xylitol-rich strains. The yeast producing xylitol has the most Candida species. Meyrial, Eleonora, Kim, Yahashi, Lee, et al. reported on the production of xylitol by the fermentation of xylose by Candida, which belong to different species. In addition, Dominguez, Sampaio, Nahlik et al. reported on the production of xylitol by fermentation of xylose from Debaryomycetes. Leathers et al. reported on the production of xylitol from the fermentation of xylose with Pichia pastoris. The production of xylitol in China is also useful. Xylitol research. Using natural microbes to produce xylitol, a portion of xylose must be used for the regeneration of coenzymes, so the conversion rate of xylitol by xylose is not so high. The use of genetic engineering methods to construct xylitol-producing engineering bacteria is an effective way to increase the conversion rate. Xylitol Production by Genetically Engineered Bacteria Currently, most genetically engineered bacteria producing xylitol have cloned the xylose reductase gene from Pichia pastoris or Candida so that they can be expressed in Saccharomyces cerevisiae. Saccharomyces cerevisiae can't ferment xylose by itself. After expressing the xylose reductase gene, xylose can be reduced to xylitol. The produced xylitol can't be further metabolized to regenerate coenzyme. Therefore, high conversion rate close to the stoichiometric value can be obtained. . However, coenzyme regeneration is necessary to complete the biotransformation, and it is necessary to supplement other substances to achieve coenzyme regeneration, usually supplemented with glucose, and the consumption of glucose is 0.5 to 1 times of xylose. Engineering bacteria reduce xylose consumption but increase glucose consumption. The price of glucose is lower than xylose, which is the advantage of engineering bacteria. However, from the perspective of xylitol production, the natural bacteria and the engineering bacteria themselves have their own advantages. Only by analyzing the specific xylitol-producing ability of the specific strains can the advantages and disadvantages be compared. The main indicators for judging the ability of microorganisms to produce xylitol are conversion rate and yield. Application Prospects and Prospects The use of biocatalysis instead of chemical catalysis can reduce the production cost of xylitol. The resilience of the cells can be adapted to cruder raw materials; biochemical reactions can be performed at room temperature and pressure; the substrate specificity of the biocatalyst ensures that only xylitol is present in the product, which makes biotechnical production of xylitol very broad Prospects. Although European countries, the United States, Japan, Korea, and China have all conducted biotechnological production of xylitol, there are only a few companies that have truly achieved industrialization. The main reason is that most strains do not produce enough xylitol. According to Leathers, most of the natural strains producing xylitol currently have a conversion rate of 0.56 to 0.74 g/g and a yield of 0.2 to 0.5 g/lh. The engineering bacteria have a conversion rate of approximately 1 g/g (but glucose is required). The yield is from 0.6 to 1.0 g/lh. There is a certain gap between this level and industrial production. Although some literatures reported species with high conversion and yield, these reports are experimental results under specific conditions and are not suitable for practical applications. From the analysis of xylitol production process, if the conversion rate of the natural bacteria in the whole process of suitable production reaches 0.7g/g or more, the yield reaches more than 2g/lh, or the engineered bacteria can be cultured with inexpensive medium and the yield Above 2g/lh, biotechnological production of xylitol can be industrialized. At present, some research work, including our country, has been able to reach or close to this level of some strains. Therefore, large-scale production of xylitol by biotechnology has basically been without technical obstacles. The green production process has broad prospects In order to realize the green production of xylitol, the existing acid hydrolysis process can be considered to be replaced by enzymatic hydrolysis. After the raw material is pretreated, the hemicellulose is partially degraded and dissolved in water, and then xylan is converted after being completely hydrolyzed with xylanase to xylose. The entire process does not produce pollution, but the realization of this process still needs improvement of the raw material pretreatment process and the improvement of xylanase activity. Another green production line for xylitol is starch or glucose. As early as 1969, Onishi et al. reported that starting from glucose, xylitol can be obtained by three-step fermentation of three microorganisms. Due to the long process, low yield, and no application value, this route has long been shelved. Recently, China has selected yeast strains that can efficiently convert glucose to arabitol (an isomer of xylitol). Suzuki et al. reported on breeding of bacteria that efficiently converted arabitol to xylitol. The production of xylitol with glucose shows the possibility again. With modern biotechnology, the above yeast and bacterial genes involved in the production of xylitol are concentrated in one microorganism, making it possible to produce xylitol with starch or glucose on an industrial scale. In this way, not only can green production be achieved, but also the production process can be greatly simplified, possibly bringing about a true revolution in xylitol production.
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