Chlorine-free denim bleaching with laccase: what it changes

When does denim need bleaching at all?

Stone and enzyme washing work by abrasion. The indigo-bearing shell of the yarn is lifted in places and the white core shows through. If the order asks for contrast, the job ends there.

Not every order asks for it. When the whole garment has to come up a shade, or tone will not settle across a batch, abrasion has nothing left to give. Bleaching does not take indigo off the cloth; it destroys the colour where it sits.

In denim that has long meant sodium hypochlorite or potassium permanganate. Both deliver, and both leave a bill on the fabric and on the line. Laccase works the same direction without chlorine: the laccase enzymes page holds the range.

How does laccase fade indigo?

Laccase is an oxidoreductase. It uses oxygen to oxidise certain aromatic structures, and in denim the target is the central double bond of indigo. Break it and the molecule falls apart into colourless fragments.

The difference from cellulase is in the mechanism. Cellulase tears away the fibre carrying the dye. Laccase destroys the dye in place, and no weight leaves the garment, because the enzyme does not act on cellulose.

In most applications the enzyme does not reach the indigo directly. A mediator does the travelling: laccase oxidises it, it reaches the dye and takes the electron, then returns to be re-oxidised. Mediated systems fade appreciably harder, and denim recipes are usually built that way.

The third condition is oxygen. The reaction consumes dissolved oxygen, so aeration is as much a part of the result as the recipe. The same dosage in a closed drum with little agitation fades less, and that belongs to the machine.

Hypochlorite, permanganate, ozone, laccase

The column to read is the right-hand one. Hypochlorite is billed twice: on the fabric, where the oxidant takes cellulose with the dye and the tensile loss cannot be bought back, and on the line, where residual chlorine has to be reduced in an antichlor bath. Skip that bath and the fault surfaces weeks later in a warehouse.

With permanganate the weight is in the neutralising. Stop short of it and the cloth yellows and the fibre weakens, so a metabisulphite step belongs to the recipe. The plant then deals with the manganese, a cost landing on the water bill rather than the chemical budget.

What laccase takes out is concrete: no antichlor or metabisulphite step, no oxidative strength loss on the cloth. What it adds is a slower reaction and a deactivation step. That is the comparison to put to a supplier, rather than a claim about being green.

Buyers often arrive at laccase from the commercial side. Where a brand restricted substance list limits chlorine bleaching, the laundry has to change route, and compliance documentation for the replacement is requested from the supplier and filed.

Which effects does it cover, and which not?

Laccase acts everywhere the liquor reaches, all at once. What it delivers is shade lifting and shade cleaning: a blue running grey comes back cleaner, the clean cast a washing plant is usually after.

Two jobs stay outside it. Local effects are the first: sprayed fading on knees and seat is permanganate work, and laccase chooses no area. Near-white shades are the second. Laccase lifts the tone without taking denim to white, and an order written for that level keeps an oxidative route on the table.

The third use comes after abrasion rather than instead of it. Indigo left in the liquor and on the undyed components loses its colour, blueing on pocketing and sewing thread falls back, and the contrast built by the stone step stays visible. Laccase settles back-staining by destroying the dye, not by dosing one more chemical.

Strength of effect is set on time and on the oxygen supply, read together. Two machines on the same dosage and time give different shades if their load factors differ. On a new quality, step the time, pull a sample at each step, and take the approval off that sample.

What order do the steps go in?

1. Desizing and rinsing. Size and carried-over alkali upset the enzyme bath.
2. Stone or enzyme wash. Contrast is made here, not by the laccase; see neutral cellulase stone washing.
3. Hot rinse. Indigo and enzyme from the cellulase bath do not belong in the next one.
4. Laccase bath. Set pH and temperature to the product window, keep the liquor aerated, follow the shade on a sample.
5. Deactivation. Take pH and temperature outside the enzyme window.
6. Rinse and soften.

The request to merge the two enzyme baths comes up often. The working windows of cellulase and laccase do not always overlap, and an oxidative bath lowers cellulase activity. In practice they are two baths with a hot rinse between them.

Two details get skipped at set-up. The first is what the garment carries in: alkali from desizing or from the cellulase bath pushes the laccase bath outside its pH window, and the drift is seen after the run. The second is the water, where an oxidising residue shows up as shade differences nobody can account for.

Where is the pH and temperature window?

The working region quoted in the literature for the laccase class is mildly acid, and the window shifts with the mediator in use. That is a class description, not a product specification. For DK LCC and DK LCC S the pH, temperature, time and mediator details are given in the TDS and confirmed with a trial sample against your own conditions. The other enzyme families are compared on the enzymes category page.

Missing the window does not give one symptom. Too far to the acid side, or too hot, and the enzyme denatures: the shade does not lift, the shift raises the dosage, and the bath was carrying dead enzyme all along. Drift alkaline and the enzyme survives but slows, so the time runs out with the shade halfway there.

The shade decision is made in the lab. The same recipe leaves a different tone on fabric dyed to a different indigo depth, so it is re-proven when the order fabric changes. Read the colour on a dried sample.

Why deactivation cannot be skipped

Laccase does not stop when the clock does. Enzyme left on the garment after the bath is dropped keeps oxidising, and a load can sit wet for hours before the dryer.

The softener bath makes it worse: those baths are usually set slightly acid, which is where laccase is comfortable.

Looking for strength loss here is the wrong place. Laccase does not touch cellulose, so the risk is shade drift. The colour measured after drying reads lighter than the one taken as the load left the machine. The difference between the first and last garment of a batch starts here: the first piece goes straight to the dryer, the last waits wet.

Stopping is done with pH and temperature together. pH alone is not enough, because its effect is reversible: rinse water that brings the pH back into the enzyme's band brings the activity with it. Which values are enough is given in the product TDS. How each enzyme class is stopped is set out in enzyme deactivation.

What goes wrong?

The shade drifts after drying. Deactivation was skipped or half done. The colour signed off at the machine reads lighter after the dryer, and the spread inside the batch widens.

The liquor is not aerated. Dosage and time are right and the result is still weak. In a closed drum at a high load factor the dissolved oxygen goes quickly, so load factor and agitation change first, not dosage.

Contrast is expected from laccase. The enzyme lifts the whole garment and chooses no area. A recipe that needs an abrasion effect is not repaired by raising the dosage; the abrasion step is missing.

A recipe travels without the indigo depth. On deep raw denim the same time lifts less, so recipes move together with the fabric quality. Dropping hypochlorite also drops the antichlor bath and changes the number of rinses; the new sequence is built from the start.

The cellulase bath is carried over. Enzyme and free indigo brought across make the laccase bath unpredictable, and without a hot rinse the two baths stop adding up to anything planned.