Extraction of co-products from biomass: example of thermal degradation of silymarin compounds in subcritical water.

In an effort to increase revenues from a given feedstock, valuable co-products could be extracted prior to biochemical or thermochemical conversion with subcritical water. Although subcritical water shows significant promise in replacing organic solvents as an extraction solvent, compound degradation has been observed at elevated extraction temperatures. First order thermal degradation kinetics from a model system, silymarin extracted from Silybum marianum, in water at pH 5.1 and 100, 120, 140, and 160 degrees C were investigated. Water pressure was maintained slightly above its vapor pressure. Silymarin is a mixture of taxifolin, silichristin, silidianin, silibinin, and isosilibinin. The degradation rate constants ranged from 0.0104 min(-1) at 100 degrees C for silichristin to a maximum of 0.0840 min(-1) at 160 degrees C for silybin B. Half-lives, calculated from the rate constants, ranged from a low of 6.2 min at 160 degrees C to a high of 58.3 min at 100 degrees C, both for silichristin. The respective activation energies for the compounds ranged from 37.2 kJ/gmole for silidianin to 45.2 kJ/gmole for silichristin. In extracting the silymarin with pure ethanol at 140 degrees C, no degradation was observed. However, when extracting with ethanol/water mixtures at and 140 degrees C, degradation increased exponentially as the concentration of water increased.