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Countercurrent Extraction of Soluble Sugars from Almond Hulls and Assessment of the Bioenergy Potential

Holtman, Kevin M., Offeman, Richard D., Franqui-Villanueva, Diana, Bayati, Andre K., Orts, William J.
Journal of agricultural and food chemistry 2015 v.63 no.9 pp. 2490-2498
almond hulls, biogas, ethanol, fructose, glucose, methane, polygalacturonase, sucrose, syrups, washing
Almond hulls contain considerable proportions (37% by dry weight) of water-soluble, fermentable sugars (sucrose, glucose, and fructose), which can be extracted for industrial purposes. The maximum optimal solids loading was determined to be 20% for sugar extraction, and the addition of 0.5% (w/v) pectinase aided in maintaining a sufficient free water volume for sugar recovery. A laboratory countercurrent extraction experiment utilizing a 1 h steep followed by three extraction (wash) stages produced a high-concentration (131 g/L fermentable sugar) syrup. Overall, sugar recovery efficiency was 88%. The inner stage washing efficiencies were compatible with solution equilibrium calculations, indicating that efficiency was high. The concentrated sugar syrup was fermented to ethanol at high efficiency (86% conversion), and ethanol concentrations in the broth were 7.4% (v/v). Thin stillage contained 233 g SCOD/L, which was converted to biomethane at an efficiency of 90% with a biomethane potential of 297 mL/g SCODdestroyed. Overall, results suggested that a minima of 49 gal (185 L) ethanol and 75 m3 methane/t hulls (dry whole hull basis) are achievable.