The aroma of mat rush, or igusa, that evokes new tatami mats combines not only green, grassy notes but also components reminiscent of sweet caramel, vanilla, and fruit. A 2010 study sought the compounds behind this aroma and showed that post-harvest drying plays an important role in its formation.
Looking for compounds that reach the nose, not simply the most abundant ones
The aim was not merely to list substances released by mat rush, but to identify candidates contributing to its characteristic aroma and determine where in processing that aroma develops. The researchers compared fresh rush harvested in Kumamoto Prefecture, rush dried after mud treatment, and rush dried without mud treatment.
They first examined how to extract the aroma from crushed rush. Ether alone did not adequately recover the original aroma, but adding a small amount of water improved its intensity and quality. They therefore adopted extraction with ether containing 1% water, followed by separation of volatile compounds at low temperature under high vacuum.
For the analysis, compounds were separated by gas chromatography and smelled by a person at the outlet. The extracts were then diluted in successive fourfold steps, with two assessors checking how far they could be diluted before each odor was no longer detectable. This flavor-dilution factor, or FD factor, helps identify compounds that remain perceptually prominent in the extract. It should be distinguished from a compound’s concentration or the overall odor intensity of a tatami mat.
Caramel and vanilla notes among the green aromas
There were 51 odor-active peaks, of which 43 were identified or tentatively identified by comparing mass spectra, chromatographic retention, and the odors of reference compounds. These included substances present at such low levels that their mass-spectrometric signals were weak, and others whose molecular assignments remained tentative.
The 12 peaks that remained detectable at high dilution included green-smelling (E,Z)-2,6-nonadienal, sour-smelling isovaleric acid, sweet caramel-like furanones, grape-like methyl anthranilate, and vanilla-like vanillin. One of the 12 was an unidentified compound with green and earthy qualities.
Even an aroma described simply as “tatami-like” brings together small elements with different characteristics. The descriptions here refer to the separated compounds as they were smelled. They do not mean that whole rush smells exactly like vanilla or grapes; rather, they offer clues for understanding a complex aroma.
The difference before and after drying was greater than the difference made by mud treatment
A second central question was the processing stage at which the aroma develops. Rush intended for tatami facing is generally treated in clay-containing muddy water and then dried for a long time at a relatively low temperature. The paper describes this processing as approximately 50–70°C for more than 10 hours.
Comparing fresh and dried rush, many compounds had higher FD factors after drying. This comparison also corrected for dry-matter content to account for the greater amount of water in fresh rush. It therefore took into account the difference that would arise simply because removal of water puts more plant material into the same sample weight.
Major odor compounds were also readily detected in rush dried without mud treatment. Vanillin, for example, showed the same high FD factor in dried samples with and without mud treatment. In contrast, compounds such as green-smelling nonadienal and some caramel-like components had FD factors one dilution step higher in the mud-treated samples.
The authors therefore considered drying particularly important in forming the characteristic aroma, with mud playing a relatively minor role. The experiment did not establish that the two dried samples were identical. Rather, differences associated with mud treatment were small compared with the large differences between fresh and dried rush.
Possible formation pathways with parallels in food aromas
The researchers considered the identified compounds in light of known chemical reactions. Possible pathways included heat-related reactions involving sugars and amino acids for caramel-like furanones, lipid degradation for aldehydes with green or fatty odors, and reactions originating from ferulic acid for vanillin and related compounds.
These were hypotheses about formation pathways based on the compounds’ properties, not the results of directly tracking each reaction within rush. Even so, they support the idea that drying a plant can generate new aromas as well as preserve existing ones. Measuring precursors and reconstructing rush aroma by mixing candidate compounds remain further steps for verification.
Summary
A diverse range of compounds were identified as candidates contributing to the characteristic aroma of rush, with sour, sweet caramel-like, fruity, and vanilla-like qualities alongside green notes. Many compounds had higher FD factors after drying than in fresh rush, and major compounds were present even in rush dried without mud treatment, highlighting the importance of the drying stage.
The smell of new tatami can be considered in terms of both compounds originally present in the plant and changes that occur during processing. This study provided a foundation for connecting those compounds with how the aroma develops.
The paper lists all seven authors as affiliated with Ogawa & Co., Ltd. It acknowledges the Kumamoto Prefectural Agricultural Research Center for providing rush samples and advice. No separate funding or conflict-of-interest declaration was found.
Reference
Kumazawa K, et al. (2010). Identification and Formation of Volatile Components Responsible for the Characteristic Aroma of Mat Rush (Igusa). Bioscience, Biotechnology, and Biochemistry, 74(6), 1231–1236. Original research / Open full text
