Hydroxy-alpha-sanshool, found in Sichuan pepper, makes lips and tongue tingle and go numb. It fools touch receptors into firing as if the skin were vibrating at around 50 times a second.
What it is¶
The sanshools are a group of alkylamides found in the husks of Zanthoxylum species — the plants that give sichuan-pepper, Japanese sansho and Nepalese timur. The most studied, and the one chiefly responsible for the famous tingle, is hydroxy-α-sanshool. Related compounds (α-sanshool, hydroxy-β-sanshool and others) vary in how strongly they tingle, burn or numb.
The sensation they produce is unlike anything else in the spice cupboard: a fizzing, buzzing vibration on the lips and tongue, followed by a mild numbness that can last for several minutes. In sichuan-cuisine it is half of málà, "numbing-hot", the combination of Sichuan pepper and chilli that defines dishes such as mapo tofu and hot pot.
Sanshool is concentrated in the reddish husk (the dried fruit wall). The shiny black seeds inside contribute grit and bitterness and are usually discarded.
How it works¶
🔬 A touch illusion
For years the tingle was assumed to be a form of heat or chemical irritation. Research published in 2008 by David Julius's laboratory showed that hydroxy-α-sanshool acts mainly on touch-sensitive nerve fibres. It blocks a set of "two-pore" potassium channels (KCNK3, KCNK9 and KCNK18) that normally keep these neurons quiet. With the brakes off, the nerves fire as though the skin were being touched or vibrated.
A 2013 study at University College London put a number on the illusion. Volunteers had sanshool applied to their lips and were then asked to match the tingle to a mechanical vibration applied to a fingertip. They matched it, on average, to a frequency of about 50 hertz — fifty buzzes a second, within the range the body perceives as "flutter". The tingle is, quite literally, felt as a vibration.
Sanshool also activates TRPV1 and TRPA1 — the receptors for capsaicin and mustard oils — to a lesser degree, which adds a faint warmth. The overall character, though, is tactile rather than hot, which is why it pairs with chilli rather than competing with it. Many eaters find that the numbing takes the edge off chilli burn, allowing more of both.
Other tingling plants¶
Sanshool is not unique. Similar alkylamides occur in several unrelated plants.
| Plant | Compound | Notes |
|---|---|---|
| Sichuan pepper (Zanthoxylum spp.) | Hydroxy-α-sanshool | Citrusy, floral, strong tingle |
| Sansho (Zanthoxylum piperitum) | Sanshools | Japanese; in shichimi-togarashi, with eel |
| Timur (Zanthoxylum armatum) | Sanshools | Nepal; grapefruit-like aroma |
| Buzz buttons (Acmella oleracea) | Spilanthol | Flower heads sold as "Szechuan buttons"; intense numbing |
| Echinacea roots | Alkylamides | Mild tongue tingle, noted by herbalists |
Zanthoxylum belongs to the citrus family, Rutaceae, which explains the lemony, floral aroma of the husks — limonene, linalool and geranyl acetate among others (see aroma-compounds).
In the kitchen¶
Sanshools are unstable. They oxidise on exposure to air and light, and the tingle fades noticeably within months of grinding. Whole husks keep better, but still lose power over a year or two.
💡 Getting the most tingle
Buy whole husks with few seeds and twigs, from a shop with good turnover. Toast them gently in a dry pan for a minute until fragrant, cool, then grind. Use the powder within a few weeks. Sichuan pepper oil (huajiao oil) captures both aroma and tingle in fat, and keeps better than the ground spice once sealed and stored in the dark.
Sanshool dissolves well in oil and poorly in water, so Sichuan pepper is usually infused into hot oil — for chilli-crisp and red chilli oil (see making-chilli-oil) — or sprinkled over a finished dish. Excessive heat damages both the aroma and the tingle, so the oil should be hot but not smoking when it meets the husks.
It also appears in chinese-five-spice, where its role is aromatic rather than numbing in most commercial blends.
History¶
Sichuan pepper was a pungent spice in China long before chillies arrived from the Americas in the 16th and 17th centuries (see how-chillies-spread-around-the-world); the málà combination is comparatively recent. In the United States, imports were banned from 1968 until 2005 because the plant can carry citrus canker, a bacterial disease of citrus trees; the ban was lifted once heat treatment of imported husks became standard.