Xylitol, an approved food additive (E967), is a five-carbon polyol, with a hydroxyl group attached to every carbon atom (pentahydroxy sugar alcohol). Its sweetness is similar to sucrose’s, providing although 40% fewer calories (2.4 kcal / g) than sucrose (4.0 kcal / g). The discovery of xylitol was first made in 1891 by the German chemist Emil Fischer (1852-1919) and his group. It has also been found in many edible vegetables and fungi, such as lettuce, berries and mushrooms, while being considered as an intermediate metabolite in carbohydrate metabolism during the conversion of L-xylulose to D -xylulose.
Among its important characteristics are the high solubility, the low glycemic index, the fact that it is not toxic, but also its anti-cariogenic properties. In addition, xylitol does not participate in Maillard reactions, thus it does not diminish the nutritional value of proteins.
In the European Community, the safety of xylitol usage has been recognized since 1984 and the Agency of Food and Drugs Administration of the United States (Food and Drug Administration – FDA) characterized it as “Generally Recognized as Safe” (GRAS) in 1986 and “Safe for Teeth” in 1994. Additionally, the human body can obtain a maximum amount of 20-30 g of xylitol and the total consumption must not exceed 50 g per day. Large doses can cause diarrhea and intestinal dysfunction due to the inability of the intestinal bacteria to metabolize it. As a result, xylitol compared to other polyols, such as sorbitol (E420), mannitol (E421), maltitol (E965), lactitol (E966) etc., is widely used in food production such as ice cream, chocolate, cookies, candies, jam, gums and drinks.
Xylitol is accounting for 12% of the total polyol market share and is rapidly growing worldwide due to the rapid increase in chewing gum sales and the existence of a consumer audience that seeks to consume products that are healthy. Thus, its use has started to spread in the pharmaceutical industry, in order to produce throat pastilles, multivitamin tablets, syrups, cough drops, and especially toothpaste and mouthwash, not only due to the anti-cariogenic properties it imparts, but also thanks to its ability to reduce tooth decay and the formation of oral biofilms, mainly against Streptococcus mutans to the dental plaque and saliva. Xylitol also prevents the transmission of S. mutans from mothers to children. Some studies claim that the cariostatic characteristics of xylitol against this bacterium are due to the reduction of glycosyltransferase expression to the microorganism. Others indicate that xylitol changes the cell structure by influencing the polysaccharide that surrounds it, and creates a more dilute and diffuse cell wall. Apart from S. mutans, other studies have shown the effectiveness of xylitol usage over other microorganisms that are harmful for our mouth’s health, such as: Haemophilus influenzae, Pseudomonas aerugionosa, Staphylococcus aureus and Streptococcus pneumoniae. More importantly, xylitol is effective for the treatment of several diseases such as diabetes and diabetic hyperglycemia, anaemia, osteoporosis, and acute otitis media.
According to Islam (2011), xylitol’s consumption not only reduces the blood glucose concentration, but also improves the glucose tolerance levels and does not interfere with the metabolic pathway of insulin, compared to sucrose. Xylitol may also be used as a substitute for sugar for overweight patients, like people suffering from obesity and other metabolic diseases, suppressing appetite for sugar. Therefore, this polyol can be considered as the best non-sugar sweetener compared to common sugar, maintaining all the parameters related to diabetes at more physiological, safe and stable as possible levels.
Although xylitol was discovered in 1891, during the second world war, Finland was the first country which started the mass production of xylitol, because there was no available sugar (sucrose). The production process of this polyol initially involved the extraction of xylan, a polysaccharide derived from hardwoods, such as birch. Then, the hydrolysis of xylan to D-xylose, a monosaccharide, was followed by the hydrogenation of D-xylose to xylitol. After the end of the war, as sucrose was available again, the production of xylitol decreased.
The industrial production of xylitol has been present for nearly four decades from 1970 onwards. D-xylose solutions are subjected to a catalytic chemical hydrogenation under high-temperature (80-140 °C) and pressure (up to 50 atm), until the formation of the polyol is achieved. However, due to the need of pure xylose, the expensive and specialized equipment, for high temperature and pressure conditions, required for the production process of xylitol, this traditional chemical process becomes quite expensive. Several studies have been examining the existence of alternative chemical pathways, with emphasis on biotechnological processes, in order to reduce the cost of processing and production, and in addition, finding solutions for the recycling and utilization of industrial wastes and raw materials such as vegetable residues, corn cobs, coconut shells, sunflower stalks and bamboos. All these wastes can be hydrolyzed with dilute acids for the extraction of D- xylose, which can be transform to xylitol using microorganisms.
Therefore, biotechnological production of xylitol, which is less costly, can be an alternative solution against the conventionally used chemical process. Xylose together with the glucose are important sugars derived from hydrolysis of lignocellulosic biomass, including other sugars such as arabinose, galactose, the mannose and rhamnose (all of them have countless applications both in the food and pharmaceutical industry). Then, various kinds of microorganisms, such as bacteria, filamentous fungi, and mainly yeasts of the genus Candida, such as Candida tropicalis based on Rao et al. (2006) or according to Prakash et al. (2011) Debaryomyces hansenii, can be used for the fermentation of xylose, corn fiber and sugarcane hemicellulose hydrolysates to xylitol.
The main xylitol production locations are North America, Europe, Japan, South China, India and South Asia. Its production increases rapidly, with an expected growth rate of 4.08% of the global market from 2017 to 2024.