Choosing and running a desizing enzyme on cotton

What the size does, and why it has to come off

Warp yarn takes constant abrasion and tension on the loom. Size lays a thin film over the yarn, holds down the hairiness and keeps ends from breaking. Once the cloth is off the loom that film has no further job.

Left on, it spoils everything downstream. The film is water-repellent: the fabric will not wet out, part of the peroxide in the bleach bath is spent breaking down size instead of whitening, and dye cannot reach the fibre evenly. Whatever you improve in pre-treatment is capped by the size layer underneath it.

Size add-on on greige cotton is usually quoted as a few per cent up to about fifteen per cent on yarn weight. That is a class range; the quality, the loom type and the weaver's own recipe decide where in it you land. Asking the weaver for the size recipe cuts most of the trial work in the dyehouse.

Can every size be removed with an enzyme?

No. Alpha-amylase cleaves the α-1,4 glycosidic bond in starch at random points. Amylose and amylopectin chains drop to water-soluble dextrins and wash out in a hot rinse. The enzyme does not touch cellulose, which is why desizing carries no strength-loss risk the way a cellulase step does.

Everything that is not starch sits outside this mechanism.

A large share of cotton woven in Turkey carries a mixed size. The enzyme takes the starch fraction; washing has to take the rest. "We dosed the enzyme and the TEGEWA reading did not move" often starts here, and the usual response is to raise the dosage. More enzyme gets added for a polymer the enzyme cannot see.

If the weaver's recipe is not available, run three lab samples off the same cloth: one with hot alkaline wash only, one with enzyme only, one with both in sequence. The TEGEWA readings and drop test tell you which fraction leaves by which route.

Standard or high-temperature amylase?

Below a certain temperature the starch film does not swell. The enzyme cannot get into an unswollen film, works on it from the outside and stays slow. So the temperature decision is about substrate accessibility as much as about enzyme stability.

The band quoted in the literature for standard Bacillus amylases is pH 6-7.5 at 55-70 °C; high-temperature types are selected to run in the 90-105 °C band. Both are class ranges, not product specifications. Derin Kimya's own figures sit on each product page: pending from the client.

In practice the machine picks the enzyme, not the other way round. Holding 60-70 °C on a jigger or in exhaust is easy and a standard type covers it. On a continuous line where the cloth is padded and goes straight into a steamer, dwell is measured in seconds, and only a high-temperature type finishes the job in that window. A heated pad-roll belongs on the same side.

There is a second, commercial argument. An HT type lets desizing and the pre-wash share a single pass. The steam and water account usually turns on that, not on the price of the enzyme.

What changes between the three routes?

Dwell is not the only difference. On pad-batch the enzyme waits inside a closed roll together with the cloth: there is no bath and no circulation, so once the reaction is over there is no way to correct the medium. In exhaust, pH and temperature can be measured and corrected mid-cycle. On a continuous line, noticing the fault and running metres of cloth through the range happen at the same moment.

On pad-batch, the enzyme the cloth actually carries is set by pick-up. An operator who holds the bath concentration and changes the padder pressure has changed the dosage without knowing it. Continuous leaves no margin at all: slow the line and you over-treat, speed it up and you under-treat, and both can show up in the same batch on the same shift.

The set-up sequence is the same on all three:

1. Check that the greige wets out. If the wax and oil load is high, the wetting agent decision comes before the enzyme decision.
2. Measure water hardness and pH. Do not settle on a sequestering agent quantity without these two numbers.
3. Build the bath: wetting agent, buffer and electrolyte if required. Bring it to temperature.
4. Dose the enzyme diluted, with circulation running. Never pour concentrate into a hot bath.
5. Drop the bath hot at the end of dwell and make the first rinse a hot one.
6. Read TEGEWA and record it against the batch. Nothing moves to the next step before that reading exists.

Where does the removed size actually go?

The enzyme makes the size soluble. It does not take it off the cloth. Washing does that, and the distinction gets missed more often than it should: dwell right, temperature right, and the TEGEWA reading still low because the wash-off capacity was not there.

The dissolved size goes to effluent, where it carries a large share of the whole organic load of cotton pre-treatment on its own. Collecting the desizing liquor separately is usually cheaper on the treatment side than putting the combined effluent through the plant. It is a cost that never appears in the dyehouse chemical budget and always appears in the effluent bill.

Three things decide the wash-off: water temperature, how many wash boxes or rinse steps there are, and whether the flow runs counter-current. A single cold rinse can erase the result of a perfectly run enzyme bath.

Why calcium matters and a strong sequestering agent can hurt

Alpha-amylase carries a structural calcium ion that holds the folded protein in shape. Pull the calcium out and the enzyme becomes markedly more fragile to heat.

The dyehouse reflex is to sequester hardness. A strong sequestering agent takes the enzyme's calcium along with the hardness in the water. What you see is desizing performance falling while nothing in the recipe appears to have changed, and the amylase usually gets the blame.

Three ways out. Pick an amylase that depends less on calcium; choose the sequestering agent type and quantity to suit the enzyme instead of dosing a house standard blind; or add a controlled amount of a calcium salt to the bath. Which one applies depends on the product: pending from the client.

The same care applies to free chlorine in mains water. An oxidising residue kills the enzyme in the bath, and that loss cannot be bought back with dosage.

How is desizing verified?

The TEGEWA violet scale reads the blue-violet colour that an iodine / potassium iodide solution gives with starch against a nine-step card. A 1 means heavy starch residue; a 9 means no starch shows.

Measurement sequence:

1. Rinse and dry the sample. On wet cloth the colour spreads and the reading is not trustworthy.
2. Take samples from the head, middle and tail of the piece. One point is not a reading.
3. Apply the solution and respect the waiting time printed on the card.
4. Read in daylight with the card held against the fabric.
5. Take the lowest of the three points as the batch score.

Cloth going to dyeing is usually asked for 7 and above; printing and mercerising expect more. That is a class expectation. Where a customer specification exists, the specification wins.

The scale has one limit worth remembering: it only sees starch. A PVA-sized fabric reads 9 and is still full of size. That is why the drop test sits next to TEGEWA. Together the two mean something; separately each one misleads.

What goes wrong?

Incomplete desizing shows up as unlevelness after dyeing. Size is never distributed evenly across the cloth. The areas that keep it take less dye, and the difference only becomes visible after drying. By then the dyehouse is arguing about levelling agent and dye recipe, while the fault was made two steps earlier.

Size redeposits as the bath cools. Partly degraded dextrins recombine as the temperature falls and fix themselves back onto the surface. The only defence is dropping the bath hot and rinsing hot. A batch left to cool in the machine over a weekend is the textbook case.

The enzyme goes into the bath already dead. Concentrate poured straight into a hot bath, dosing next to a steam line, alkali carried in on the greige, an oxidising residue in the water: all give the same result. There is only one symptom: raise the dosage and nothing changes.

The next step is started without reading TEGEWA. The measurement takes two minutes; stripping a dyed batch takes a shift. Where the reading is not written onto the batch card, the same quality arriving a second time means building the recipe from scratch again.

Recipes travel without the liquor ratio. A dosage proven at 1:8 in exhaust does not repeat at 1:5. When a recipe moves, what has to match is the enzyme concentration the cloth sees, not the figure written against fabric weight.