Effect of Retting Enzymes on the Structure and Composition of Flax Cell Walls

Abstract

Commercial enzyme mixtures are tested for their possibly selective degradation of flax ( Linum usitatissimum L.) stem components in relation to the retting process in producing linen. Structural and chemical compositional results from treatments are obtained using scanning electron microscopy, histochemistry, gas-liquid chromatography, 13C cp mas nmr spectrometry, and mid-infrared spectroscopy. Flaxzyme and Ultrazym and an enriched pectinase mixture (epm), which was not developed for flax retting but is included for comparison, are tested for their activity toward cell wall components and used in various concentrations for “enzyme-retting” of flax. Ariane flax stem sections are incubated with enzymes in a rotary incubator and the fibers are manually separated from the residual core. All of the commercial enzyme mixtures have cellulase, pectinase, and hemicellulase activities, but individual enzyme activities vary. Activities against the soluble test substrates do not predict the activity against natural fibers. At about equal protein concentrations, Flaxzyme treatment appears to facilitate bast fiber removal better than the other enzymes, with Ultrazym nearly as effective and epm the least effective. The ranking of effectiveness is generally supported by the amounts of uronic acid, arabinose, and xylose removed from the stems analyzed chemically. Increased enzyme levels generally facilitate removal of matrix carbohydrates from the flax. All enzymes separate bast fibers from the lignified core and partially from the cuticle near the cut surface of the stem sections, but the enzymes do not work far from the exposed ends. Retting quality is defined more by the degree of cell wall degradation and fiber separation than by any differences in kinds of cell walls degraded by the various enzymes. The cuticle remains attached to the fiber at times, apparently reducing access of the enzymes to the matrix polysacchrides and suggesting some recalcitrance of epidermal cells (and therefore loss of cuticle) to biodegradation. Lignin remains in the middle lamellae after enzyme retting and would likely prevent separation of the fiber bundles. Some solubilzation of the inner secondary wall of the flax fiber appears to occur with Flaxzyme. The structural and chemical analyses characterize alterations in flax bast after enzyme retting and would be useful in ranking the specificity and effectiveness of cell wall degradation.