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ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Bioproducts Research » Research » Publications at this Location » Publication #309497

Title: Countercurrent extraction of soluble sugars from almond hulls and assessment of the bioenergy potential

item Holtman, Kevin
item Offeman, Richard
item Franquivillanueva, Diana
item Bayati, Andre
item Orts, William

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2015
Publication Date: 3/11/2015
Citation: Holtman, K.M., Offeman, R.D., Franquivillanueva, D.M., Bayati, A.K., Orts, W.J. 2015. Countercurrent extraction of soluble sugars from almond hulls and assessment of the bioenergy potential. Journal of Agricultural and Food Chemistry. 63(9):2490-2498. doi: 10.1021/jf5048332.

Interpretive Summary: This manuscript provides quantitative analysis of nonpareil almond hulls, including free soluble sugars levels higher than reported previously in the literature. Solid/liquid ratio, steep time, and pectinase addition were optimized for highest extraction of the free soluble sugars. Pectinase application was utilized for the first time and reduced the hydrophilicity of the hulls, thereby allowing for a larger free water recovery. Sugar recovery is tied to the free water recovery as dissolution is a recovery process and as a result higher free water recovery results in higher sugar recovery. A countercurrent process was developed to efficiently extract repeated rounds of almond hulls to create a higher sugar concentration in the syrup product. After 5 rounds the fermentable sugar content was 131 grams per liter (gpL) and the total extractable sugars was 169 gpL. Modelling suggests that the equilibrium product concentration is 184 gpL. The concentrated sugar product was fermented to ethanol at high efficiency to produce a beer concentration of 7.6 % (v/v). After ethanol distillation, the thin stillage was anaerobically digested to produce 297 mL CH4 per g SCOD (soluble chemical oxygen demand) destroyed. The data suggests that 1 T dry basis hulls is capable of producing 58 gal ethanol and 93 m3 of CH4 for local use.

Technical Abstract: Almond hulls contain considerable proportions (37 % by dry weight) of water soluble, fermentable sugars (sucrose, glucose, fructose) which can be extracted for industrial purposes. Optimization found that 20 % solids content was the maximum practical solids/liquor ratio for sugar extraction and that the addition of 0.5 % (w/v) pectinase aided in maintaining a sufficient free water volume for sugar recovery. A laboratory scale experiment mimicking countercurrent extraction was utilized for solubilizing and accumulating sugars in solution. It was found that a 1 hr steep followed by three extraction (wash) stages could produce a high concentration (131 gpL fermentable sugar) syrup. Overall, sugar recovery efficiency was 88 % and 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 beer concentrations were 7.6 % (v/v). After distillation the thin stillage contained 231 gpL SCOD which was converted to biomethane at a destruction efficiency of 90 % and a biomethane potential of 297 mL/g SCOD destroyed. Overall, it was estimated that almond hulls could produce 58 gal ethanol and 93 m3 per T fresh almond hulls (db).