Acid cellulase bio-polishing: pH, temperature and the strength trade-off

What acid cellulase does, and what it does not

Acid cellulase hydrolyses the β-1,4 glycosidic bond in cellulose. Fibre ends standing out of the yarn body are cut at that bond and removed from the surface by mechanical action. The result: a smoother face, cleaner-reading colour, softer handle.

What it does not do is end pilling. The enzyme only removes the fibre ends present on the surface at the time of treatment. In wear, new ends work their way out of the yarn. Bio-polishing delays the onset and reduces the severity; it does not eliminate pilling. Any commitment given to the customer has to be framed that way.

The enzyme also does not distinguish its target. The same enzyme that cuts the surface fibre cuts cellulose wherever it makes contact. So the measure of the process is not "how clean is it" but "how much did we lose".

Why is the working window so narrow?

An enzyme is a protein, and its activity depends on its three-dimensional shape. Change the pH and the charged groups on its surface change, distorting the geometry of the active site. Raise the temperature and the structure unfolds without refolding.

The window quoted in the literature for the acid cellulase class is pH 4.5-5.5 at 50-55 °C. That is a class range; Derin Kimya's own figures are on each product page: pending from the client.

Leaving the window has two different consequences, and they should not be confused:

This distinction matters on the floor. The reflex "nothing happened, raise the dosage" spends money and fixes nothing when the real problem is temperature.

How is pH held?

The most common mistake is setting bath pH once with acetic acid and walking away. Alkali carried in on the cloth, bicarbonate in the water, and the machine itself all push pH up during the run. The pH measured at the end can sit half a unit above the start, and that takes a noticeable share of the activity with it.

The answer is a buffer: an acetic acid / sodium acetate system does the job in this pH region. Buffer capacity has to be enough to absorb the alkali the cloth brings in.

Sequence:

1. Rinse properly before the enzyme step. Caustic left over from scouring or bleaching consumes the buffer before the process starts.
2. Fill the bath, bring it to temperature, set pH with the buffer.
3. Verify by measurement. Do not calculate dosage before pH is confirmed.
4. Add the enzyme. Never pour concentrate straight in. Dilute it and dose with circulation running.
5. Measure pH again mid-cycle. If it has drifted, revise the recipe upward on buffer, not on enzyme.

How is strength loss controlled?

Process intensity is the product of three variables: enzyme quantity, time and mechanical action. Only one of them behaves linearly.

Dosage. Once the enzyme-substrate reaction is saturated, adding more enzyme does not add speed. Past that point dosage is pure cost.

Time. The most linear control. Hold the dosage, step the time, and build a curve for that specific fabric. In the lab, pull four samples at 20, 30, 40 and 50 minutes and compare weight loss and handle.

Mechanical action. The same enzyme at the same dosage works far harder in a high-speed jet than in an overflow. This is the main reason recipes do not travel between machines.

Weight loss is the working measure because it is fast to determine and moves in the same direction as tensile strength loss. On critical orders, run a strength measurement alongside the weight-loss curve; the relationship between the two depends on fabric construction, and once established it holds for that quality.

Target weight loss depends on the fabric, the yarn and the handle being asked for: pending from the client.

Why deactivation cannot be skipped

When the clock runs out the enzyme is still active. Even after the bath is dropped and rinsing starts, enzyme left on the cloth keeps working. A quiet strength loss accumulates in the time before drying, and it cannot be traced afterwards because every process record looks normal.

Cellulase is stopped two ways, and in practice both are used together:

• pH shift. Soda ash or caustic takes the pH clearly outside the enzyme's working range.
• Temperature. The bath goes above the denaturation threshold and is held there.

Which pH and which temperature are enough depends on the enzyme: pending from the client.

Deactivation is doubly important when dyeing follows. Residual activity keeps cutting cellulose in the dyebath, and the resulting strength loss gets charged to the dyeing step instead of the enzyme step.

Before or after dyeing?

Both are used.

Before dyeing. Dye lays onto a clean surface, there is no fastness risk, and weight loss is calculated against greige weight. The downside: the hair removed is undyed, so the gain in colour brightness is smaller.

After dyeing. The surface hair removed is dyed, so colour brightens noticeably. The risk: the acid bath can shift shade on some reactive dyes, and on dark shades the dyed fibre released into the bath can re-deposit on the surface. Dark shades always go through a lab trial first.

The usual rule: after dyeing on pale and medium shades, before dyeing on darks and on anything with a fastness risk.

Four recurring mistakes

1. Unbuffered pH. A one-shot acid addition does not hold through the cycle. Symptom: the same recipe works on some batches and not others.
2. Carrying a recipe between machines. A recipe validated on an overflow runs too hard in a high-speed jet. Symptom: unexpected strength loss with an unchanged recipe.
3. Incomplete deactivation. Symptom: a systematic gap between lab sample and production batch, with production always losing more.
4. Poor wetting. On fabric with low absorbency the enzyme does not reach the whole surface at once and the result comes out uneven. The drop-test figure coming out of pre-treatment is a precondition for the enzyme step.