Sugarcane straw, a low-cost biomass used in the production of second-generation ethanol, can give rise to a sugar with even greater added value and economic interest: xylitol.
Through the action of a modified version of the microorganism Saccharomyces cerevisiae, the xylose present in the material can be metabolized, resulting in the healthy and increasingly popular sweetener. This is what studies carried out at the State University of Campinas (Unicamp) show, whose results were published in the Journal of Genetic Engineering and Biotechnology.
“Currently, this potential is neglected by the ethanol industry, which fails to add value and supply the growing demand of the market and the food industry for the sweetener xylitol”, says Fellipe da Silveira Bezerra de Mello, one of the authors of the article and a researcher at the Department of Genetics, Evolution, Microbiology and Immunology at the Institute of Biology (IB-Unicamp).
“The sweet taste of xylitol is perceived by our brain as if it were sugar, but the chemical carries the benefit of not being metabolized by the intestine and not being fermented by the microorganisms that cause cavities, which gives it a gigantic potential in the world market”, completes Gonçalo Amarante Guimarães Pereira, a professor at IB-Unicamp who also participated in the research.
genetic editing
The work is a combination of two research efforts. The first involved the creation of a system for genetic modification of Brazilian industrial strains of Saccharomyces cerevisiae. This yeast does not naturally metabolize xylose, a type of sugar available in the biomass of sugarcane (and also in the trunks and leaves of other vegetables), as it does when transforming glucose into ethanol. Hence the need to create a mutant strain.
The microorganism received the gene that converts xylose into xylitol through a gene editing technique known as CRISPR-Cas9 (acronym for Set of Regularly Spaced Palindromic Repeats, which works with an associated protein, Cas), which allows for precise editing of a specific region of DNA. This is the first study of its kind using the main strains of yeast used by the Brazilian bioethanol industry and which, from now on, may serve as a basis for the work of other researchers.
After genetic editing of the S. cerevisiae, the second work front began: tests to confirm that it would be possible to use the same source of material used in the production of second-generation ethanol — hydrolyzed sugarcane straw, that is, degraded to release xylose — to obtain xylitol. In addition, at this stage, the production made by two yeasts edited in the same way was compared: one industrial and the other laboratory. Both strains were successful, but the industrial strain consistently outperformed the laboratory strain.
“The results showed that Brazilian industrial yeast can produce more xylitol in the optimal medium, which contains only xylose, without all the stress [impurezas] of sugarcane hydrolyzate. The industrial strain was also more successful in the production of xylitol using the straw hydrolyzate. [com as impurezas]indicating that this strain also has resistance to the stresses present in this environment”, says Mello.
“Conclusively, what we realized was that Brazilian industrial yeast, in addition to being very good for ethanol fermentation, is also excellent for producing other molecules, in this case xylitol, which has greater added value.”
Future
With the proof of concept completed, the researchers are now working to increase productivity through improvements in the sugarcane hydrolyzate fermentation process. One of the strategies is to supplement the culture medium to promote greater yeast growth. Another is to apply an electric current to the medium, which should contribute to the regeneration of cofactors (molecules that help in the chemical reactions necessary for the transformation of xylose) — the more cofactors, the greater the production of xylitol.
“We are carrying out a scan of possible strategies to obtain a commercial product with industrial competitiveness. In this way, the engineering area could work with the purification of the product for use by the consumer.”
Carla Maneira, Frank Uriel Lizarazo Suarez, Sheila Nagamatsu, Beatriz Vargas, Carla Vieira, Thais Secches, Alessando LV Coradini, Maria Augusta de Carvalho Silvello, Rosana Goldbeck and Gleidson Silva Teixeira also participated in the studies. The group received support from Fapesp through three projects (15/06677-8, 18/03403-2 and 16/02506-7).
The article “Rational engineering of industrial S. cerevisiae: towards xylitol production from sugarcane straw” (“Rational engineering of S. cerevisiae industrial: towards the production of xylitol from sugarcane straw”) can be read here.
