Almond hulls as a biofuels feedstock: Variations in carbohydrates by variety and location in California

Abstract

Hulls of the almond (Prunus dulcis (Miller) D.A. Webb) have a high content of fermentable sugars (glucose, fructose, sucrose), and are a potential feedstock for biofuels and other uses. The state of California in the United States produces the majority of the world's almonds. Six almond varieties across seven counties in California were studied to assess the amount and variability of sugars and sugar alcohols in the hulls. Previous studies were done over 30 years ago, and don’t reflect the current range of varieties and locations. Fermentable sugars ranged from 25 to 33% of the dry weight of the hulls portion of the samples. The “as is” samples contain shells, twigs and other materials that can dilute the concentration of sugars; the Butte and Padre varieties have much higher amounts of shell than the other varieties studied. The Nonpareil variety has the highest fermentable sugars content at an average of 271kg per tonne of “as is” hulls. Average theoretical ethanol yields ranged from 176 to 130L/tonne, depending on variety. Finding an economic outlet for the wet, spent hulls is a concern.