Comparison between Different Hydrolysis Processes of Vine-Trimming Waste to Obtain Hemicellulosic Sugars for Further Lactic Acid Conversion

Abstract

Trimming vine shoot samples were treated with water under selected operational conditions (autohydrolysis reaction) to obtain a liquid phase containing hemicellulose-decomposition products. In a further acid-catalyzed step (posthydrolysis reaction), xylooligosaccharides were converted into single sugars for the biotechnological production of lactic acid using Lactobacillus pentosus. A wide range of temperatures, reaction times, and acid concentrations were tested during the autohydrolysis-posthydrolysis process to investigate their influence on hemicellulose solubilization and reaction products. The maximum concentration of hemicellulosic sugars was achieved using autohydrolysis at 210 degrees C followed by posthydrolysis with 1% H2SO4 during 2 h. Data from autohydrolysis-posthydrolysis were compared with the results obtained at the optima conditions assayed for prehydrolysis (3% H2SO4 at 130 degrees C during 15 min) based on previous works. Prehydrolysis extracted more hemicellulosic sugars from trimming vine shoots; however, the protein content in the hydrolysates from autohydrolysis-posthydrolysis was higher. The harsher conditions assayed during the autohydrolysis process and the higher content of protein after this treatment could induce Maillard reactions decreasing consequently the concentration of hemicellulosic sugars in the hydrolysates. Therefore, despite the several advantages of autohydrolysis (less equipment caused by the absence of mineral acid, less generation of neutralized sludges, and low cost of reagents) the poor results obtained in this work with no detoxified hydrolysates (Q P = 0.36 g/L h, Q S = 0.79 g/L h, Y P/S = 0.45 g/g, Y P/Sth = 61.5 %) or charcoal-treated hydrolysates (Q P = 0.76 g/L h, Q S = 1.47 g/L h, Y P/S = 0.52 g/g, Y P/Sth = 71.5 %) suggest that prehydrolysis of trimming vine shoots with diluted H2SO4 is more attractive than autohydrolysis-posthydrolysis for obtaining lactic acid through fermentation of hemicellulosic sugars with L. pentosus. Besides the higher hemicellulosic sugars concentration achieved when using the prehydrolysis technology, no detoxification steps are required to produce efficiently lactic acid (Q P = 1.14 g/L h; Q S = 1.64 g/L h; Y P/S = 0.70 g/g; Y P/Sth = 92.6 %), even when vinification lees are used as nutrients (Q P = 0.89 g/L h; Q S = 1.54 g/L h; Y P/S = 0.58 g/g; Y P/Sth = 76.1 %).