Lignin Isolation by Enzymatic Hydrolysis of Pulp
Yamasaki et al. first developed a technique for isolating residual lignin from kraft pulp
by enzymatic hydrolysis.1
This procedure is based on selective hydrolysis and
dissolution of carbohydrates in pulp by commercial cellulolytic enzymes, leaving lignin
behind as an insoluble residue. Pulps are typically subjected to several successive
enzyme treatments to ensure complete dissolution of the carbohydrates and to increase
the amount of recovered insoluble lignin residue. The residual lignin from this isolation
method is typically obtained in good yield, especially from unbleached kraft pulps.
Hortling et al. used cellulase and β-glucosidase to isolate residual lignin from a series of
kraft pulps.2
These laboratory-produced kraft pulps varied from 93 to 14 kappa number.
The enzymes employed in this enzymatic hydrolysis technique are effective enough to
hydrolyze practically all pulp polysaccharides in one enzyme treatment. The insoluble
lignin residue remaining after enzymatic hydrolysis was subsequently extracted with 0.5
M sodium hydroxide and the soluble fraction was acid-precipitated, yielding the residual
lignin sample. Little or no lignin was water-soluble after the enzymatic treatment of the
pulps in the 93-58 kappa number range, while 20-60% of the lignin was dissolved during
enzymatic hydrolysis for pulps with lower kappa numbers. The results indicated that the
residual lignin samples contained 65-80% lignin, 7-8% carbohydrates, and the remaining
impurities from proteins acquired during the enzymatic treatment. The content of protein
impurities was higher in the portion of the sample that was soluble in sodium hydroxide
than in the insoluble fraction, suggesting that the proteins have an affinity for the residual
lignin. Although the molecular weights of the residual lignin were found to decrease
with increasing degree of delignification, the authors suggested that the difficulty of
removing residual lignin from pulps is due partly to insoluble fractions of the residual