It’s the “roasted mite” in the sun? Scientists reveal the truth

Cotton fabrics such as quilts or towels that have been sun dried will have a unique “Sun smell”, which is pleasant to smell. There are different opinions about the source of this sun flavor.

it was said earlier that the smell of the sun came from the body smell of the mite after it was scorched. However, there is no exact experimental evidence for this claim. The smell of roasted mites should be similar to the burnt smell of protein, far from the sun. It is also said that the smell of the sun is the smell left by ultraviolet rays in the sun after killing bacteria and other microorganisms. But the experiment proved that the ultraviolet lamp can kill the microorganism on the surface of the non fiber fabric, but it can’t smell the sun. There is also a view that the smell of the sun comes from the ozone produced by the ultraviolet radiation of oxygen. The problem with this statement is that the concentration of ozone is too low to form obvious odor, and ozone itself is fishy. There is also the sun burned cotton fiber, water evaporation said, and so on, and so on.

the team of Matthew S. Johnson of the University of Copenhagen in Denmark recently gave their answer. The team used gas chromatography to detect the components of the gas emitted from the surface of the cotton towel after drying outdoors. Compared with the air components emitted by outdoor air dried or indoor air dried towels, the outdoor drying towels emit a large number of organic compounds such as aldehydes, ketones, carboxylic acids and so on, which contain 5-9 carbon atoms. Most of them are pleasant at low concentrations, and some are also used in perfumes, such as 2- methylpropionic aldehyde. These compounds are the source of the sun flavor.

the authors put the towel washed and dried with deionized water into a plastic bag, and air was blown into the bag to blow the surface, so that the substances on the towel surface could be desorbed into the air. These substances flow out with the air and are captured and enriched by adsorbents. During the test, the organic matter is desorbed into the gas chromatography by heating the adsorbent. The corresponding component analysis of each chromatographic peak was performed by mass spectrometry combined with chromatography.

the gas chromatograms of the gas desorbed from the towel dried in the sun showed many peaks not found in the background signals. The compounds corresponding to these peaks are mostly carbonyl compounds with 5-9 carbon atoms. At low concentrations, these compounds have a pleasant aroma, very similar to the smell of the sun.

after finding the chemicals that produce the sun flavor, we need to specify how these substances are generated? Here, the authors can not make specific experimental observations, but give their conjectures. Specifically, the production of these compounds has gone through two main steps: first, ultraviolet light in the sun excites molecules such as light absorbing dyes, disinfectants and softeners on the towel, making them in the excited state and becoming active. A small amount of ozone and high reactive free radicals will also be produced in this step. Second, these highly active substances generated by ultraviolet radiation react with volatile organic compounds from the air and adsorbed on the towel surface. Volatile organic compounds form various carbonyl compounds by oxidation and protonation. These compounds are immobilized on the surface of the fabric by physical adsorption or hydrogen bonding with the carbonyl groups on the cellulose surface. This hypothesis is consistent with the following actual phenomena:

1. The washed cotton fabric has a stronger sun smell after drying – the water layer on the surface of the fabric can scatter sunlight, increase the path of sunlight on the surface of the fabric, and increase the concentration of active substances, so as to produce carbonyl compounds more easily.

2. The sun smell of fabrics often lasts for several days – carbonyl compounds are more stable on the surface of fabrics by physical adsorption or hydrogen bonding.

1) high boiling point, nonvolatile compounds cannot be detected. It is limited by the thermal desorption temperature before injection and the thermal stability of the column. New detection methods need to be developed and used.

2) substances with poor thermal stability cannot be detected. For example, in 1974, scientists pointed out that the smell of the sun might come from peroxyacyl nitrates. However, it is easy to decompose when heated, and may not exist before entering the chromatographic column, so it has not been detected.

3) the tested fabric is not limited to cotton fabric. If it is necessary to explore the effect of cellulose surface hydroxyl groups on odor retention, nylon and other chemical fibers can be used to carry out control experiments.