How much work does it take to describe the aroma rising from a cup in the names of compounds? From a researcher’s perspective, it is a long process of separating, smelling and counting.
Reports from 1973 onwards trace ways of extracting aroma and examining it with human noses and chemical analysis. For each, we look at what was used, what was measured and how far the conclusions extended.
How has hojicha’s aroma actually been studied?
Reports on hojicha’s aroma go back at least to 1973, when a comparison of green tea with hojicha roasted at 200°C for several minutes reported 21 pyrazines in hojicha and substantial increases in them through roasting. The broad outlines of that aroma were drawn half a century ago.
The reading goes in three stages. A list of compounds changes with the extraction method, so the figures have to be read together with how they were obtained. Aroma strength does not correlate with gas-chromatogram peak size, so quantity cannot identify the strongest-smelling compounds. And AEDA is used alongside it: an aroma extract is diluted in stages and the greatest dilution at which a human nose still detects a compound becomes its FD factor.
According to a review by the Industrial Research Institute of Ishikawa, Yamanishi and colleagues compared aroma compounds in green tea with those in hojicha roasted at 200°C for several minutes. In 1973, they reported 21 pyrazines in hojicha and substantial increases in those compounds through roasting. In the same year, Hara and colleagues reported that hojicha contained many pyrazines, pyrroles and furans formed during roasting.
A 1999 paper also recounts that Yamanishi and colleagues identified 66 compounds in hojicha aroma. The broad outlines of that aroma had already been drawn half a century ago.
Change the extraction method and the compound list changes
The sources drawn on here are a review and a bōcha study from the Industrial Research Institute of Ishikawa, a 1999 paper in the journal of the Agricultural Chemical Society of Japan, a 2018 metabolomics paper on roasted stem tea, a NARO report, and a Grants-in-Aid research report. Samples and extraction methods differ from report to report, and so do the dilution steps. Each study’s FD factors therefore stand on their own.
The 1999 paper describes drawbacks in the SDE and reduced-pressure distillation methods previously used to prepare tea aroma concentrates: water-soluble volatile compounds were difficult to recover, and components in the leaves could change during sample preparation. It judged these methods unsuitable for investigating the aroma pattern encountered when drinking tea, and instead extracted the infusion with an organic solvent.
When the National Agriculture and Food Research Organization (NARO) examined roasted tea, it obtained 0.1 mL of aroma extract from 200 g of tea using reduced-pressure and high-vacuum distillation, then diluted it 10-, 100-, 1000- and 10000-fold with dichloromethane. In the Industrial Research Institute of Ishikawa’s bōcha research, a stir bar was placed in the extract to adsorb compounds, which were then released by heating. For comparisons among commercial products, compounds were collected for 30 minutes from the air in a vial containing 0.5 g of ground tea warmed to 80°C.
The question is not simply which method is right: the methods differ in what they can extract. Reading a list of compounds also means considering the extraction method that produced it.
