Why enzymes prebleach but cannot bleach

Bleaching is delignification taken to the end of the curve. After the kraft cook a few percent of lignin remains, and it is the part of the original lignin least willing to leave: more carbon–carbon linked, less cleavable by the alkali, and partly sealed inside the fibre wall. The industry measures it as the kappa number and removes it with oxidants. Enzymes enter this picture as assistants, and their role is dictated by one physical fact: an enzyme is a large molecule, and a fibre wall is a fine filter.

What the bleach actually attacks

Residual lignin is not the only chemical burden. Kraft cooking dissolves hemicelluloses along with lignin, and as the liquor conditions change through the cook, dissolved xylan re-adsorbs onto the fibres as a polymer skin over the residual lignin. Under the same alkaline conditions the glucuronic acid side groups of xylan convert to hexenuronic acids, which consume bleaching oxidant without contributing brightness and are a main source of the yellowing of finished paper. So the bleach plant spends oxidant on lignin, on a xylan mask, and on acids the lignin never asked for.

Xylanase: removing the mask

Xylanase is the workhorse because it never has to reach the lignin. It hydrolyses the accessible, redeposited xylan — the mask — at pH and temperatures close to the pulp’s own, letting the subsequent chemical stages strike the lignin with less oxidant and washing the hexenuronic-acid burden away with its substrate. Reported chlorine dioxide savings in mill practice run to roughly a fifth to a third, with the organochlorine effluent falling in proportion. The trick is that a hydrolase needs only water and access; it imposes no redox chemistry of its own.

Laccase and the mediator problem

Laccase is the honest attempt at enzymatic delignification proper: it oxidises phenolic lignin units to phenoxy radicals, with oxygen as the terminal acceptor. But the enzyme itself is far too large to enter the fibre wall, so free laccase touches only surface lignin. The workaround is the mediator: a small oxidisable molecule the enzyme converts into a diffusing radical that travels into the wall, attacks buried lignin and comes back to be recharged. Even with a mediator, the chemistry stalls against the rest of the structure — most lignin units are non-phenolic and resist the radical, and the radicals that do form can couple lignin fragments back together, undoing the depolymerisation.

Why the last stages stay chemical

Three ceilings. Access: an enzyme only ever touches polymer it can physically reach, and the deepest lignin is the whole reason bleaching is hard. Scale: a tonne of pulp carries tens of kilograms of lignin, and oxidation at that throughput through a diffusing mediator does not reach the rate chemical stages deliver. Chemistry: the final brightness steps — destroying condensed lignin and hexenuronic acids — are oxidations that bleach sequences do with oxygen, chlorine dioxide and peroxide, and no enzyme carries that capability. So enzymatic stages sit where access, not oxidising power, is the bottleneck: ahead of the heavy stages, cutting what those stages must do. The limiting quantity of the entire subject is access — something an enzyme can exploit but cannot manufacture.