Fragrance ingredients that suppress scalp odor can do more than change its impression with a scent: some act on reactions that produce odor compounds. A 1998 study developed experimental systems in which fatty acids formed from sebum and searched for fragrance ingredients that reduced those fatty acids. It also found that some fragrance ingredients reacted with fatty acids to form esters.
The aim was to investigate scalp-odor compounds, develop methods for assessing materials that inhibit their formation, and test them in shampoo and conditioner. The paper explored the idea of adding a pleasant scent through reactions involving sebum, microorganisms, and enzymes.
First, examining odor three days after washing
The investigation of odor causes involved eight women, two each in their 20s, 30s, 40s, and 50s. Their scalp odor was assessed immediately after washing with unperfumed shampoo and conditioner, and again three days later, during which they used no hair cosmetics. Immediately after washing there was almost no odor; after three days, oily and fatty characteristics were common despite individual differences.
Hair extracts contained fatty acids including myristic, palmitic, and oleic acids. Air was also passed into a cap covering the head, and compounds released from it were collected for 30 minutes. Both short- and long-chain fatty acids were detected, ranging from acetic acid to palmitic acid.
Using these findings, the researchers prepared a fatty-acid mixture. They reported that a model combining long-chain fatty acids with very small amounts of butyric acid, isovaleric acid, and other compounds was judged similar to scalp odor by perfumers and members of the public. From these observations, the authors considered mixed fatty-acid odors important to everyday scalp odor. The number of assessors and statistics for this reconstruction were not provided, and the proportion of overall scalp odor contributed by each compound was not established.
Recreating fatty-acid formation from sebum in the laboratory
Triglycerides in sebum are molecules in which fatty acids are linked to glycerol. When an enzyme called lipase breaks those links, free fatty acids are released. The researchers sought to use this reaction to assess fragrance ingredients.
One system used the skin-associated microorganisms Malassezia furfur and Staphylococcus epidermidis. They were cultured in lipid-containing media at 35°C for three days, and fatty-acid formation was analyzed. Another system combined commercially available reagent lipase with a sebum-like mixture of triglycerides, squalene, and cholesterol.
Fatty-acid formation was confirmed in both systems. In the lipase system, triglycerides decreased and their constituent fatty acids appeared, along with characteristics resembling scalp odor. Squalene and cholesterol, meanwhile, showed no major changes. This provided an assessment method that generated candidate odor compounds and tracked changes in their amounts.
Fragrance ingredients can reduce fatty acids in more than one way
About 70 synthetic and natural fragrance ingredients were examined in the lipase system, and about 20 representative ingredients from this group in the microbial systems. In lipase experiments with ingredients such as benzyl alcohol and geraniol, some fatty-acid signals were approximately 5–20% of those measured without a fragrance ingredient. These percentages were based on fatty-acid peak areas in an experimental system, not a corresponding percentage reduction in scalp-odor intensity.
In the microbial systems, some ingredients reduced fatty acids while visibly reducing microbial growth, whereas others reduced fatty acids without obvious growth inhibition. The authors discussed antimicrobial effects as a possible explanation for the former and inhibition of enzyme activity for the latter.
Another important finding concerned reactions that converted fatty acids into different molecules. In the benzyl-alcohol system, the researchers detected not only fewer fatty acids but also newly formed esters, including benzyl laurate, benzyl myristate, and benzyl palmitate. These result from fatty acids combining with an alcohol-type fragrance ingredient. Trace benzyl palmitate was also reported in extracts collected the day after benzyl alcohol was applied to a person.
Thus, the same finding of fewer fatty acids can have different explanations: inhibiting microbial growth, inhibiting enzyme reactions, or converting products into esters. The authors highlighted the possibility of using enzymes to alter fatty-acid-derived odors as an important outcome. However, they did not investigate how much each detected ester contributed to a pleasant scent on the head.
Fatty-acid reductions were also observed with shampoo and conditioner
Based on the experimental results, the researchers formulated a bouquet-style fragrance and added it at 0.45% to shampoo and 0.3% to conditioner. In a questionnaire after use, 75% of the 36 people who said their scalp odor usually bothered them or bothered them somewhat reported perceiving an odor-suppressing effect. These 36 people represented 46% of the overall survey group; this questionnaire tally is separate from the initial investigation involving eight women. The 75% figure reflects users’ impressions, not an improvement rate established by comparison with unperfumed products.
For chemical analysis, the two halves of the head were washed separately with fragranced and unperfumed shampoo and conditioner, left to air-dry, and compared for emitted compounds 24 hours later. The fragranced side had smaller fatty-acid peak areas and a lower total than the unperfumed side. A practical finding was that changes appeared not only in scent impressions but also in the measured compounds.
However, the paper does not report participant numbers, variability, or statistical tests for the half-head experiment. The finished fragrance was also a mixture, so the respective contributions of antimicrobial action, effects on enzymes, and esterification are unknown. The authors discussed the prospect of using these effects without substantially changing skin microorganisms, but did not measure the microbiota before and after use.
Summary
This study investigated fatty acids associated with scalp odor, developed assessment methods that generated fatty acids using microorganisms and lipase, and identified fragrance ingredients that reduced their formation or remaining amounts. With some ingredients, conversion of fatty acids into esters was also confirmed.
The prototype shampoo and conditioner produced lower analytical fatty-acid signals after 24 hours than the unperfumed side, and some users perceived odor suppression. Although reporting of details such as assessment numbers was limited, the study demonstrated a way to consider fragrance ingredients not only as materials that add scent, but also as materials that participate in odor-forming reactions.
At the time of the research, all three authors were affiliated with POLA Corporation. The acknowledgments recognize research cooperation from Ogawa & Co. No separate research-funding or conflict-of-interest statement was found in the paper.
Reference
Yamamoto, H., Shimosato, I., & Okada, M. (1998). Study of Fragrance Materials on Controlling Head Odors Formation. Journal of Society of Cosmetic Chemists of Japan, 32(1), 26–32. Original paper
