Wednesday, 5 February 2014

What puts the pepper in Sichuan?

Sichuan peppercorns with a few seeds
I've just had a couple of days in Bristol doing a bunch of meetings and general network schmoozing, and while there I treated myself to a spot of Chinese cuisine. The Dynasty restaurant was near the hotel and I figured it was going to be good for two reasons. First it was above a Chinese supermarket, always a good sign, and secondly it was full of Chinese people. Since I was on my own, I had just the one dish and scoffed the lot. Ma-po tofu is a favourite of mine, it's a Sichuan dish of minced pork or beef, braised with silken tofu and lots of spices. After just a mouthful or two I could immediately tell it was packed with a very particular flavour, that of Sichuan peppercorns. This peculiar spice gives not only a hot flavour, but also a kind of numbing tingly effect to your mouth.

As I sat in the Dynasty, munching on my dinner, I had time to contemplate what on Earth was going on in my mouth. 

The prickly ash
The Sichuan peppercorn is neither a chilli, nor is it a relative of the black pepper plant (Piper nigrum, a perennial climbing vine). The Sichuan peppercorns you buy are instead the teeny, tiny dried skins of the fruit of a citrus relative called the prickly ash (Zanthoxylum simulans or Z. bungeanum). It's a shrubby bush with ash like leaves that produces clusters of tiny flowers and, in time, tiny fruit. As the knobbly, red-brown fruit dries, it splits and sheds a single black seed. The bit we use in cooking is the skin of this seed, the seed itself doesn't add anything, except a gritty texture. 

The active ingredient in the Sichuan pepper is called hydroxy alpha sanshool, or just sanshool to its friends. It's a long chain hydrocarbon with an amide group (contains nitrogen and oxygen) and hydroxy group (that's OH) at one end. It's a fairly straight forward chemical, but it has some extraordinary effects on your mouth.

Firstly, it shares a property with capsaicin, the chemical that makes chillis hot. It binds to a protein that is snappily named transient receptor potential cation channel subfamily V member 1, or TRPV1. This protein, in groups of four, forms channels in the membranes at the tips of nerve cells on your tongue. When sanshool, or capsaicin, sticks to TRPV1, the channel opens, calcium ions pour into the nerve cell and a nerve impulse starts. This impulse rushes to your brain and is perceived as burning heat because normally the TRPV1 channel only opens when the temperature goes over about 40C. 

Hydroxy alpha sanshool
So, sanshool gives you a feeling of heat, but it has another trick up its sleeve. It binds to, and opens a different membrane channel found in nerve endings in our tongues, skin and wrapped around the bases of downy hairs. Once again the biologists demonstrate a stunning ability to think up cool names for ion channels, are you ready: potassium channel subfamily K (KCNK). Normally these channels open when they feel a light touch, or the slightest movement of a hair, but when the sanshool opens them, we get a continual prickling and tingling sensation. 

It's this that gives Sichuan pepper it's unique, prickly taste. If you have never experienced this pharmacological culinary treat, seek it out. Sichuan pepper is a component of Five Spice powder, but for the full effect you need a dish like Ma-po tofu with all it's delicious mouth tingling kick. 

References:
Structure of TRPV1: Nature 504, 107–112 (05 December 2013)
Action of KCNK: Nature Neuroscience 11, 772 - 779 (2008)
Physiology of tingling sensation: J Neurosci. 2010 March 24; 30(12): 4353–4361 

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.