# Bio-based antimicrobial and odour finishes

Why protonated chitosan disrupts bacterial envelopes, the enzymes that actually generate body odour, wash durability testing, and why a plant-derived active is still a biocide under EU law.

Chitosan works because it is a polycation, which means it stops working above its pKa — and odour control is a different problem from killing bacteria.

Source: https://en.bioecon.ru/docs/forestry-biomaterials/fibers-textiles/health-functional-bio-textiles/
Updated: 2026-09-04



A functional finish has to do three things at once: act on a target, stay on the fibre through repeated laundering, and remain acceptable on skin. Bio-based actives — chitosan, plant polyphenols, organic acids — change the chemistry of the first requirement and usually make the second harder.

## Chitosan is active only while it is charged

Chitosan is chitin that has been deacetylated far enough to expose primary amines along the glucosamine backbone. Those amines have a pKa around 6.5, so in mildly acidic conditions they protonate and the polymer becomes a dense polycation. Bacterial envelopes are anionic — teichoic acids in Gram-positive cell walls, lipopolysaccharide in Gram-negative outer membranes — so the polycation binds electrostatically, disorganises the envelope and increases permeability; it also chelates divalent metals that the outer membrane needs for stability.

This mechanism carries its own ceiling. Above about pH 6.5 the amines deprotonate, the charge disappears and activity falls away. Skin surface pH is normally between roughly 4.5 and 5.5, which is favourable, but sweat, soap residue and occlusion all push it upward. The same charge requirement also governs application: chitosan is insoluble in neutral water, so it is applied from dilute acid or used in quaternised form, which keeps a permanent positive charge at the cost of the acid-switch behaviour.

## Odour is an enzyme problem, not only a bacterial one

Fresh apocrine sweat is close to odourless. Odour is produced when skin bacteria release volatiles from non-volatile conjugates: corynebacteria carry an Nα-acyl-glutamine aminoacylase that liberates 3-methyl-2-hexenoic acid from its glutamine conjugate, and a C–S lyase that releases sulfanylalkanol thiols from cysteinylglycine conjugates. A textile can therefore intervene in three different places — kill or suppress the bacteria, inhibit the enzyme, or adsorb the volatile after it forms — and these are not interchangeable. An adsorptive finish reduces perceived odour without any antibacterial claim, which is a materially different regulatory position.

## Durability is the discriminator

Almost any active works on a freshly finished swatch. What separates finishes is whether the active is physically deposited or covalently anchored, since detergent and mechanical action strip deposited material quickly. Efficacy is measured under ISO 20743 or AATCC 100 by inoculating the fabric and counting survivors, and durability by repeating that after standardised domestic laundering cycles under ISO 6330. A claim without a stated wash count is not a claim about a garment.

## The regulation applies to the active, not its origin

In the EU an antimicrobial textile is a treated article under the Biocidal Products Regulation, 528/2012. Article 58 allows it on the market only if the active substance is approved for the relevant product type, and any claim about biocidal properties must be substantiated and disclosed. Plant-derived or fermentation-derived origin gives no exemption: if the effect claimed is antimicrobial, the substance is a biocidal active and needs approval like any other. This is the practical reason many bio-based finishes are sold with odour-control rather than antibacterial claims.

A last point argues for restraint. The skin carries a resident community — staphylococci, cutibacteria, corynebacteria — that occupies niches and competes with pathogens, and broad, persistent, leaching antimicrobials in everyday clothing act on that community as much as on odour formers.