Pré-tratamento de bagaço de malte cervejeiro com ácido oxálico para solubilização de hemicelulose e obtenção de xilitol e compostos orgânicos voláteis

Abstract

The lignocellulosic biomass of various plants has been studied in recent decades as a renewable matrix for obtaining ecologically sustainable energy sources, such as second-generation ethanol (2G) and value-added products, driving the implementation of a circular economy. Brewer's spent grain (BSG) is produced in large quantities by the brewing industry, is a low-cost raw material, with availability independent of seasonality, rich in structural carbohydrates, such as glucose and xylose, which through technological processes can be industrialized and reinserted into the economy. Thus, the present work aimed to obtain ethanol, xylitol and volatile organic compounds from the fermentation of a hydrolyzate rich in pentoses from the acid pre-treatment of malt bagasse. To this end, a series of hydrolysis tests with oxalic acid were conducted, with the best result being obtained with 4.5% catalyst and 24 minutes of reaction, in an autoclave, at 125.6 °C, with solubilization of more than 99% of hemicellulose and concentration of 14.9 g/L of xylose in the liquid fraction. The hydrolyzate from the pre-treatment step was then subjected to fermentation by three strains of Saccharomyces cerevisiae, one industrial (PE-2), and two genetically modified to allow the production of ethanol from xylose and increase tolerance to fermentation-inhibiting substances (MP-P5 and JDY-01). The analyses were conducted using a rotational central composite design to evaluate the effect of fermentation temperature and pH of the hydrolyzate on carbohydrate intake, xylitol, ethanol and volatile organic compound formation by the three strains. At the end of the experiments, an inhibitory effect exerted by the accentuated amount of oxalic acid, or oxalate, its conjugate base, present in the hydrolysates (about 45 g/L) from the pre-treatment was noticed. As a result, it was not possible to verify the accumulation of xylitol in any test, and the F12 experiment (pH 3.0 and 20 °C), with the MP- P5 strain, was the only one to present ethanol production, with 1.76 g /L generated from the consumption of all the glucose and a part of the xylose present in the medium. Even so, it was possible to identify the production of some volatile organic compounds by the yeasts, with emphasis on phenethyl alcohol, superior alcohol with a rose and honey aroma that has antimicrobial and antifungal properties, and 4-Hydroxy-2-methylacetophenone, a compound phenolic acid with important antioxidant, antitumor, analgesic and antiseptic properties, which strongly appeared in the chromatograms, with a peak area of, on average, 4% and 45%, respectively, and with potential for recovery. In addition, the F26 test (pH 9.0 and 40 °C), where the JDY-01 strain was used, showed an accumulation of linoleic acid with a peak area corresponding to 44%, which would allow its recovery. BSG, therefore, proved to be suitable for use in biorefineries, and with the proper techniques, it can become a vital part of implementing a circular economy.