日本語版

Bitter crossing — 2026-09-06

Sour-bitter

すっぱ苦い (suppanigai) — our coined term

Sour-bitter refers to a state in which sourness and bitterness both arise at once, with neither clearly coming first. It is the lab's coined term. It is common in citrus juices, but it is not a simple relationship in which strengthening the acid makes bitterness disappear or fade. Depending on what the acid is combined with, and at what stage it is added, bitterness can intensify, stay unmoved, or intensify separately alongside the acid.

What this page establishes

• The term "sour-bitter" is not an existing word but one the lab has coined, and whether a headword in the same form exists in dictionaries has not been confirmed.
• It has been reported that the recognition threshold of limonin, a bitter compound in citrus, reaches its maximum at pH 3.8, at 6.5 mg/kg in orange juice.
• In natural orange juice, adding citric acid alone did not change the bitterness from limonin or nomilin, but in coffee, raising the extraction temperature has been reported to intensify bitterness and sourness together.
• In a mouse experiment, it has been reported that even when one of the pathways for sensing acid (OTOP1) was stopped, the response of avoiding strong acid remained.

How it arises

The relationship between sourness and bitterness shows a different face depending on where it is measured. We look separately at three layers: a solution in which only the compounds are dissolved in distilled water, the cells that themselves handle the reception of sourness, and actual fruit juices and drinks.

In solution

At the most basic layer, the recognition threshold itself of limonin, a bitter compound in citrus, shifts with the presence of sugar or acid. A study comparing the threshold in distilled water with the threshold in a model system containing sucrose and citric acid, and in orange juice, found that pH moved together with the concentration of the acidic component, and the conditions at which the threshold reached its maximum concentrated around a particular acidity.

Cell-level evidence supporting sour-bitter has not been found on either the sourness side or the bitterness side. What can be placed here is a finding about sourness alone: even in mice in which OTOP1, an ion channel involved in the reception of sourness, was made non-functional, the behavior of avoiding concentrated acid remained. This study does not address interaction with bitterness. It shows only that the reception of sourness itself does not close within a single receptor, and the "in cells" layer remains empty for this crossing.

In practice

In an experiment using natural orange juice, adding sucrose lowered the perceived bitterness from limonin and nomilin, but adding citric acid did not. This does not contradict the earlier claim about limonin's threshold — measured in both a model solution and actual orange juice — in which the threshold shifted with acidity. It is a difference of conditions: in the solution layer, conditions in which acid shifts the threshold have been found, whereas in the practical layer of natural juice, adding the same acid alone could not shift the perceived bitterness.

In a different experiment using the same natural orange juice, mixing limonin and nomilin 1:1 lowered each one's recognition threshold below its value alone. This is an example in which it is not the acid itself but how the bitter compounds are combined with each other that shifts the threshold.

Along the timeline of processing too, sourness and bitterness can move in the same direction rather than separately. In Italian table olives, extending the brine-fermentation period raised the sensory scores for bitterness and sourness together. This is an example in which the two rise together rather than suppressing one another.

The same directional movement has been recorded in coffee extraction as well. Brewing dark-roast beans at a high extraction temperature intensifies not only bitterness but sourness at the same time, with both attributes rising together with an increase in total dissolved solids. This is one example showing that bitterness and sourness are not in a relationship where each cancels the other out.

In language

One more thing to address here is a mix-up on the language side. This is not a finding about culinary sensory perception but about a discrimination task using test strips. Sourness and bitterness are easily confused in everyday speech, but in experimental settings they are measured as separate sensations. A study that had assessors identify sweetness, sourness, saltiness, and bitterness using test strips and answer columns separate from oral sensations such as astringency and irritation offers supporting evidence that sourness and bitterness are dimensions that can be evaluated separately. The impression of "tasting sour yet feeling bitter" is likely not a crossing of sensations but two independent sensations standing at once.

Exceptions and reservations

The name sour-bitter does not assert a rule that acid reliably strengthens, or reliably weakens, bitterness. Even for the same citrus bitter compound, whether adding acid alone moves perceived bitterness or not splits by experiment and by condition. Adding sugar often works to weaken bitterness, but adding acid works in the opposite direction depending on the system. Differences from individual variation or panel composition are also not generalized beyond the scope reported by each study.

The route of chemically removing bitterness itself is also a different thing from the route of shifting perception with acid. Debittering, which breaks down and reduces limonin through enzyme treatment, works by a different mechanism than an operation that adds acid to try to change how bitterness is felt. The two are not lumped together as the same "approach to suppressing bitterness."

Also, buffering agents such as fat, milk, or polysaccharides — which weaken only bitterness without leaving their own taste behind — are not a partner for sour-bitter. Sourness is placed in this crossing precisely because it stands beside bitterness while leaving its own taste in place.

Examples from around the world

Examples in which sourness and bitterness both stand out at once are common in citrus juices and in drinks that bring out strong sourness.

• grapefruit juice — a citrus juice in which a mild bitterness overlaps with sweetness and sourness
• orange juice — a juice where claims about limonin gather at both the solution and the practical layer
• coffee brew — a drink in which extraction temperature moves bitterness and sourness together
• Americano (cocktail) — a drink that layers sourness and carbonation onto a strongly bitter spirit
• Tom Khom — a group of northern Thai and highland dishes sharing a name connected to bitterness

Try it at home

Narrow the conditions and check whether adding acid shifts how bitterness is felt.

1. Add citric acid to orange juice
   Materials: store-bought 100% orange juice, food-grade citric acid, a measuring spoon.
   Variable: the amount of citric acid, across 3 conditions — "none," "a little," and "more." Keep sugar, temperature, and vessel the same.
   Procedure: Divide the same juice into three cups and dissolve in a different amount of citric acid in each, changing only that variable. Let it sit for a few minutes, then taste in the same order and note the intensity of bitterness for each.
   Status: candidate (to be published once a tasting-comparison record is attached).
2. Vary the coffee's extraction temperature
   Materials: the same dark-roast beans, the same grind, one brewing device.
   Variable: extraction temperature, across 3 conditions — "cold water," "room temperature," and "boiling water." Keep the amount of beans, extraction time, and amount of water the same.
   Procedure: Brew all three conditions with the same beans and the same amounts, let them cool to room temperature, then taste and note the intensity of bitterness and sourness for each.
   Status: candidate (to be published once a tasting-comparison record is attached).

What is not known

• No source found: no source has been found, outside dictionaries, that confirms actual usage of the word form "sour-bitter" itself.
• Not measured: the mouse experiment looked at the response to acid, and how that response changes when bitterness is given at the same time has not been measured.
• Not measured: the effect of sugar on the recognition thresholds of limonin and nomilin is limited to findings in orange juice, and whether the same magnitude occurs in other citrus fruits or drinks has not been measured.
• No source found: no source has been found that examined, for drinks other than coffee, whether extraction or processing temperature moves bitterness and sourness in the same direction.

Between the dictionaries

Whether the term すっぱ苦い, in this same form, appears as a headword in Japanese dictionaries is unconfirmed. What the lab has been able to confirm is an example in which a grapefruit juice's product description writes 「甘酸っぱさ」 (in translation: "a sweet-and-sour taste") and 「ほのかな苦み」 (in translation: "a faint bitterness") side by side — a way of writing that keeps sourness and bitterness as separate words rather than fusing them into one. The coined term sour-bitter stands on the side of combining the two into a single word.

This differs in axis from the existing word horonigai (mildly bitter). Horonigai is a word for a state in which the other taste does not remain in the end result and only bitterness is weakened; on the lab's map it is placed on the axis of bitterness intensity. Sour-bitter is a word for a state in which sourness remains in the end result while standing beside bitterness; it is placed on the axis of the other taste. Even in a scene that looks the same — "bitterness softening" — which word it falls under is decided by whether the acid's presence remains as a taste.

Frequently asked

Does adding citric acid to orange juice make the bitterness disappear?

In an experiment using natural orange juice, adding citric acid alone did not lower the perceived bitterness from limonin and nomilin. In the same experiment, adding sucrose did lower bitterness. If the goal is to weaken bitterness, moving sugar rather than acid is what works, within the scope of this experiment.

When coffee is brewed at a high temperature, which wins out, bitterness or sourness?

In an experiment using dark-roast beans, raising the extraction temperature intensified bitterness and sourness together. This is recorded not as a relationship in which one wins and cancels out the other, but as an example in which both move in the same direction.

Are sour and bitter the same sensation?

They are measured as separate sensations. The fact that an experiment having assessors identify sourness and bitterness using separate test strips and separate answer columns succeeds is supporting evidence that the two are distinguishable dimensions. As for the reception of acid itself, it is also known that a single receptor alone cannot explain the entire response, so part of how sourness arises still remains open.

Related: Bitter crossings /Does any seasoning remove bitterness? — Sugar and salt /Citrus Bitterness Arrives Late /The Words for Bitterness /horonigai (gently bitter) (mildly bitter)