Clothes can still smell after washing and drying. Alongside microbial activity, another process worth considering is oxidation of sebum left on the fabric.
A 2020 study using artificially soiled fabrics showed that copper added to wash water increased the formation of sebum-derived odor compounds. Combining a copper-binding ingredient with an antioxidant in detergent reduced the measured odor compounds. This was a study of how odor molecules form, not a sensory assessment of how bad the fabrics smelled.
Odor molecules can form from residual sebum
The work was conducted by Gregory S. Miracle and colleagues at P&G. Even after visible dirt is removed, sebum left in fibers can change over time and generate odors. Sebum contains oxidation-prone components such as squalene and fatty acids. Squalene is one of its oily constituents.
Autoxidation is a process in which oily components react with oxygen through chain reactions. As compounds break down, smaller, more volatile molecules form. Alongside light and heat, metal ions such as copper can catalyze these reactions.
The team therefore tried two approaches: using a chelant to bind copper and make it less available to participate in reactions, and using an antioxidant to intercept reactive molecules that propagate the chain reaction. They compared these approaches individually and in combination.
Washing, drying, and measuring artificially soiled fabric
The main tests used white fabric made of 50% cotton and 50% polyester. A fixed amount of artificial sebum soil containing squalene, fatty acids, and vegetable oil, among other ingredients, was applied and aged at 35°C for 4–7 days before washing. Each wash contained 16 soiled swatches alongside other fabrics, using commercial top-loading or front-loading machines.
The researchers used Cincinnati tap water or softened water derived from it, with controlled hardness conditions. After tumble drying for 45 minutes at medium heat, volatile compounds released from the fabric were analyzed by GC-MS, which separates mixed compounds and measures their amounts.
Three molecules formed by sebum oxidation served as odor markers, with their combined concentration compared across treatments. One main marker was 3-methyl-2-butenal. For measurement, fabric was placed in a container and held at 80°C for 90 minutes before gas sampling. The values therefore represent concentrations of marker compounds released under heated analytical conditions, not the concentrations surrounding clothes at room temperature.
Replicates combined multiple washes and multiple samples within washes. Treatments were allocated to reduce bias from machine differences. The study did not follow worn garments in everyday use or conduct olfactory-panel assessments.
Adding copper increased marker compounds
In an experiment varying copper addition, the mean marker concentration was 62 nmol/L without added copper. Adding 400 ng of copper per gram of water increased it to 120 nmol/L, and 800 ng increased it to 162 nmol/L. At 1200 ng it was 163 nmol/L, with little further increase. Each condition involved 12 measurements.
Thus, marker compounds increased with added copper before reaching a plateau in this experiment. The no-added-copper condition could still contain copper already present in tap water; it was not a comparison with completely copper-free water.
A separate experiment with smaller copper additions also increased marker compounds. Incorporating the chelant diethylenetriamine (DETA) into the detergent reduced them. DETA was investigated here as an ingredient intended to limit copper’s contribution.
With the same amount of DETA, however, the reduction was smaller at higher copper levels. The authors suggest that its limited copper-binding capacity may be one reason. Increasing another chelant did not produce the same result, showing that not every metal-binding ingredient works in the same way.
Combining antioxidants and chelants
Antioxidants also differed markedly. A comparison of seven antioxidants at the same formulation level found marker reductions ranging from 5–95%. The authors discuss whether properties such as water affinity and access to oily soil may affect their performance.
One antioxidant examined in detail was abbreviated MtBHPP in the paper. It is a phenolic compound that interrupts oxidative chain reactions, intended here to act within oily soil on fabric.
An experiment comparing combinations of the two ingredients produced the following results, with 18 measurements per condition.
- Control without additional MtBHPP or DETA: 143 nmol/L
- Formulation containing 0.037% MtBHPP: 80 nmol/L, a 44% reduction from the control
- Formulation containing 0.037% MtBHPP and 0.05% DETA: 14 nmol/L, a 90% reduction from the control
All formulation percentages are by weight in the detergent. In this experiment, addressing copper as well as oxidation yielded fewer marker compounds than using the antioxidant alone. Another comparison also found reductions with DETA alone and with the antioxidant–chelant combination.
The 90% reduction means that the sum of three specified compounds was lower than in that experiment’s control. It does not mean that perceived malodor fell by 90% or that any garment can be deodorized by that amount. The tabulated values varied; the paper reports means and standard deviations but no significance-test results for this comparison.
Visualizing compounds and interpreting comparisons carefully
The team also developed a way to turn analytical data into colors. Cotton towels soiled with artificial sebum were washed with each formulation, and colors represented marker abundance. The formulation containing both chelant and antioxidant showed lower values. This visualizes chemical measurements, not people’s olfactory impressions.
An important consideration is the copper already present in tap water. Its concentration varies from day to day, but the study did not measure it for every run and machine. Instead, control and treatment conditions within an experiment were compared on the same day using the same water source.
Consequently, concentrations from tables describing experiments performed on different days cannot simply be compared to conclude that one formulation is always better. Comparisons need to use the control within the same experiment.
This was an artificial-soil study intended to inform detergent formulation. Effects in households with different copper levels, fabrics, sebum conditions, and washing practices require separate testing. It did not test household addition of chelants or antioxidants to detergent.
Summary
On fabric treated with artificial sebum, copper added to wash water increased odor-marker compounds measured after washing and drying. In comparison, a formulation combining a copper-binding chelant with a chain-breaking antioxidant produced fewer of these compounds than the antioxidant alone.
Understanding clothing odors involves not only microbial activity but also chemical changes in residual sebum. This study suggests a detergent-design approach that goes beyond treating odors after they appear: inhibiting the reactions that generate odor molecules.
All ten authors were affiliated with P&G. The paper declares no conflict of interest. No separate research-funding statement was identified.
References
- Miracle, G. S., et al. (2020). Copper Chelants and Antioxidants in Laundry Detergent Formulations Reduce Formation of Malodor Molecules on Fabrics. Journal of Surfactants and Detergents, 23, 1125–1134. DOI; Publicly available original full text.
