日本語版

Research note 055 — 2026-09-07

Where Roast Aroma and Bitterness Divide — From Sesame's Roast Level

The words "roast aroma" (香ばしい, koubashii) and "bitter" are often used side by side in the kitchen. Roasting sesame, roasting tea, charring the bottom of a pot of rice — foods that have been heated strongly seem to come with roast aroma and bitterness as a set. But looking at material from the side that actually measures sensation, the two do not pass through the same gate. Roast aroma is a sensation smelled through the nose, and bitterness is one of the basic tastes received on the tongue. This note first checks where that dividing line falls, using dictionaries and sensory-evaluation vocabulary systems, then traces how aroma and bitterness move within the single operation of roasting, and finally looks at how far direct measurement of sesame actually reaches.

What this page establishes

• Sake's sensory-evaluation vocabulary system has been confirmed to place bitterness and terms related to charring and roast aroma into separate classes — taste and aroma.
• Multiple studies confirm that, in Tieguanyin oolong tea and hojicha, changing the roasting temperature or time moves aroma and bitterness separately rather than in the same direction.
• Experimental results are reported in which pinching the nose to block aroma either strengthens or weakens how bitterness is perceived.
• Direct measurement that scored the bitterness of roasted sesame itself as an independent sensory score has not yet been found.

Aroma and bitterness sit on separate shelves

The vocabulary system for evaluating sake's aroma and flavor divides 86 terms into 16 classes. Class 4, which groups burnt and caramel-like aromas, places the term 「焦臭」 (in translation: "burnt smell") at code 430, while bitterness is set up separately as Class 13 (code 1310), using tyrosol as the standard reference sample. 「焦臭」 belongs to the aroma category, and bitterness belongs to the taste category. Already at the stage where specialists built this vocabulary, the two were placed on separate shelves.

The definition Japanese dictionaries give to 「香ばしい」 (in translation: "koubashii") likewise faces only toward aroma. The Digital Daijisen defines the word as「よい香りがする。多く、食物を煎ったり焼いたりしたときの、好ましい香りにいう」 (in translation: "to have a good smell; most often said of the pleasant aroma of food that has been roasted or grilled"). There is no mention of bitterness, and no mention of the blackness of charring. The Japanese word koubashii itself was set up, from the start, as a word of smell.

From the same heat, aroma and bitterness move separately

In an experiment that roasted Tieguanyin oolong tea at 105°C for 1, 3, or 5 hours, with an additional roast of 130°C for 1 hour applied to the 5-hour batch, extending the roasting time from 1 hour to 5 hours changed the color and aroma compounds step by step, but the trained panel's bitterness score alone did not move. While aroma and color moved, bitterness alone stayed in place.

A similar split appears in an experiment in which a trained panel of 9 rated, on a 10 cm scale, the process by which the socarrat layer forms as paella cooking progresses. The more heating advanced, the more the intensity of roasted aroma rose, yet burnt smell was kept lower than in the white-rice control. Bitterness itself was not included in this evaluation scale — roasted aroma rising and bitterness rising were, from the outset, measured as separate axes.

Roast aroma has a boundary

A study that roasted 14 kinds of aracha (unrefined green tea) on a Morita-type continuous hojicha roasting machine, separating conditions from a strong roast at 100°C for 15 minutes to a deep roast at 150°C for 12 minutes, used GC-MS analysis to newly identify aroma compounds absent from green tea — 86 in hojicha and 44 in kamairi-cha (pan-fired tea). The more the roasting conditions change, the more the number of aroma compounds generated changes too.

A lightly roasted sample of the same tea leaves, roasted at 130°C for 10 minutes, carries the description 「好ましい焙煎香」 (in translation: "a preferable roasted aroma") in the paper. But a medium-roast sample, roasted further to 150°C for 12 minutes, carries the description「苦い香り、焦げた焙煎香」 (in translation: "a bitter aroma, a burnt roasted aroma"). This "bitter" is an adjective attached to an impression smelled through the nose, not a bitterness score measured on the tongue. Even so, a boundary still shows up within the aroma description itself.

A study (within the scope of its abstract) that evaluated 11 commercial barley-tea products using a 28-term flavor wheel combining aroma and taste sorted the samples into 5 clusters. At its two extremes sat a type with a sweet aroma and strong sweetness, and a type with a bitter aroma and strong bitterness. Aroma-evaluation terms and bitterness scores were moving in correspondence on the same samples.

The effect of roasting temperature shows up in a similar shape not only for coffee and tea, but for nuts too. In an experiment that varied Spanish almonds from 150°C to 190°C, the 190°C samples showed a deep color and a burnt flavor, while the 150°C samples were under-roasted. Roasting at 170°C for 10 minutes was the condition showing the highest liking (as reported within the scope of the abstract); consumers' overall liking had a positive relationship with the intensity of roasted aroma and nuttiness, while it had a negative relationship with the intensity of bitterness and burnt flavor. The range where roast aroma is preferred and the range where bitterness increases do not overlap, even along the same temperature axis.

What pinching your nose reveals

Up to this point the discussion has been about the process of roasting, but the relationship between aroma and bitterness has also been checked directly in nose-pinching experiments. When cocoa flavoring was added to a cocoa beverage without pinching the nose (that is, with the sense of smell intact), bitterness increased. In a different condition of the same experiment, adding vanilla flavoring to a milk beverage made bitter with caffeine did not significantly increase sweetness, but bitterness, unexpectedly, increased instead. The same operation of adding aroma moved bitterness rather than the sweetness it was aimed at.

There are also cases where aroma weakens bitterness instead. In an experiment that added aroma to a grapefruit-flavored model beverage, increasing the aroma concentration from 2.5 mg/kg to 10 mg/kg lowered the bitterness score, measured on a 10-point scale, from 4.77 to 4.27. The stronger the aroma, the weaker the bitterness became — the opposite direction from the cocoa and vanilla examples.

Aroma can either strengthen or weaken bitterness, and which way it moves is not fixed in advance. That roast aroma and bitterness move separately within the operation of roasting is not only because the amounts of compounds happen to fall out that way — it is also because the way aroma and taste are perceived, as senses, is not a relationship that pushes each other in only one direction.

The case of sesame

Sesame is an ingredient for which this boundary has not yet been fully confirmed by direct measurement. A study that roasted white sesame seeds in an electric oven at 170°C, 200°C, and 230°C, for 1 to 20 minutes, tracked the amounts of free sugars and free amino acids. The total amount of free amino acids fell sharply under both conditions, though by different degrees — to 23% of the unroasted level at 170°C for 20 minutes, and to 44% at 230°C for 10 minutes — and free sugars, mainly sucrose, had almost disappeared at 230°C for 10 minutes. The authors consider that these components contribute to taste mainly through sweetness, together with sourness and bitterness.

In this study, however, bitterness was not measured as an independent sensory score. No study that took both an aroma description and a bitterness score on the same sample of roasted sesame was found within the range this research lab searched. Where sesame's roast aroma and bitterness diverge remains, for now, only an inference from the amounts of compounds.

The boundary is drawn where the nose is pinched

Summing up the material reviewed so far, the boundary between roast aroma and bitterness lies not among the compounds, but between what disappears and what remains when the nose is pinched. What disappears when the nose is pinched is roast aroma; what remains even with the nose pinched is bitterness. Using the same sesame, this boundary can also be checked at home. Roast sesame from the same bag into 3 degrees — light, medium, and dark — and taste each one both with the nose pinched and without, writing down the intensity of bitterness using the same words or the same numbers each time. If a bitterness remains unchanged even with the nose pinched, that is bitterness on the side of taste, not aroma.

Frequently asked

Are roast aroma and bitterness the same sensation?

In sake's sensory-evaluation vocabulary system, the two are placed on separate shelves. Bitterness is treated as an independent class of basic taste, using tyrosol as its standard reference sample. Terms related to roast aroma, such as burnt smell, are meanwhile classified into a class on the aroma side. Japanese dictionary definitions likewise explain 「香ばしい」 (in translation: "koubashii") as a word for a pleasant aroma, with no description that makes its relationship to bitterness explicit.

Does stronger roasting strengthen both bitterness and roast aroma together?

Not necessarily. In an experiment that roasted Tieguanyin oolong tea at 105°C for 1, 3, or 5 hours, bitterness is reported to have stayed stable even as roasting time was extended. In a hojicha roasting experiment, even for the same kabuse-grown stem tea, the evaluation is recorded as changing from a "preferable roasted aroma" at a light roast of 130°C for 10 minutes to a "bitter aroma, a burnt roasted aroma" at a medium roast of 150°C for 12 minutes. A study examining the char (socarrat) in paella found that the intensity of roasted aroma rose as cooking progressed, while burnt smell was kept lower than in the white-rice control, but a bitterness scale itself was not included in that evaluation. How roasting strength and bitterness or roast aroma move depends on the ingredient and the conditions.

Does how bitterness is perceived change when aroma cannot be sensed?

There are reports both that it changes and that it changes in the opposite direction. In an experiment on a cocoa beverage with cocoa flavoring added while the nose was not pinched, bitterness is reported to have increased. In an experiment that added vanilla flavoring to a milk beverage made bitter with caffeine, on the other hand, sweetness was not significantly increased, but bitterness, unexpectedly, is reported to have increased instead. In an experiment on a model beverage with a citrus aroma added at a high concentration, there is also a report that bitterness and its aftertaste weakened instead. Whether aroma strengthens or weakens bitterness produces different results depending on the type of aroma and the beverage.

What is not known

• Direct measurement data that scored the bitterness of roasted sesame itself as an independent sensory score has not yet been found.
• Which differences in roast degree or manufacturing method correspond to barley tea's 「苦い香りで苦味が強い」 (in translation: "bitter aroma, strong bitterness") type has not yet been confirmed.
• For the study measuring the relationship between almond roasting temperature and bitterness/roast aroma, the original paper's statistics and panel size could not be confirmed.
• Data that organizes, across cases, the conditions that decide whether aroma strengthens or weakens bitterness (such as the type and concentration of aroma, or the type of beverage) has not yet been found.

Related: koubashii (Japanese roasted-aroma descriptor) /bitterness /sesame seed roasting /Tieguanyin oolong tea infusion /Roast-bitter /Research note 036: Roasting Makes Bitterness, and Deeper Roasting Changes It /Research note 045: Butter Roasting and Nanyang Kopi — a Roast That Wraps Around Bitterness