Sichuan pepper

Sichuan pepper, Chinese spice made from the dried papery fruit husks of any of several species of prickly ash (Zanthoxylum) of the rue and citrus family (Rutaceae). The major Sichuan pepper species are Chinese pepper (Zanthoxylum simulans) and red Sichuan pepper (Z. bungeanum), with the green Sichuan pepper (Z. armatum) cultivated to a lesser extent. Despite the name, Sichuan pepper plants are not true peppers and are unrelated to both black pepper (Piper nigrum, of the family Piperaceae) and chili peppers (various Capsicum species, of the family Solanaceae). Their flavour is mild and somewhat reminiscent of citrus, and the spice produces a unique numbing and vibrating sensation on the tongue. Sichuan pepper is available as an infused oil, as peppercorns, or as a ground spice. Together with star anise, cassia (Chinese cinnamon), fennel, and cloves, it is a key component of Chinese five-spice powder. The fresh leaves are used as a spice in some areas of northern China and are commonly used in a fermented sauce made of soybeans and wheat flour.

Physical description

Sichuan pepper plants are fairly similar in appearance. They are deciduous shrubs or small trees with multiple trunks. Both the red Sichuan pepper and the Chinese pepper plants can reach up to 7 metres (23 feet) in height in cool climates, while the green Sichuan pepper plant is smaller. Cultivated plants are commonly kept pruned short to facilitate harvesting. The leathery leaves are pinnately compound, usually with 5–15 oval serrated leaflets, and are armed with prickles. The plants are mostly dioecious, meaning that male and female flowers are generally borne on separate individuals; however, male plants may produce a few peppercorns, and some female plants form bisexual flowers and can self-pollinate. Some varieties and cultivars do not require a male plant to produce a decent harvest. The small flowers are borne in upright clusters that usually become pendulous with the fruits. The papery fruit husks of the red Sichuan pepper and Chinese pepper are red in colour, while those of the green Sichuan pepper are green.

The species are fairly resistant to pests and plant diseases. However, in 1968 Sichuan peppercorns were banned from importation into the United States in an attempt to control the spread of citrus canker, a bacterial disease, to commercially valuable citrus trees. The ban was lifted in 2004, with the caveat that the fruits must be heated to a certain temperature before shipment to kill the bacteria, and this requirement was quietly dropped in 2007.

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Mechanism of action

While the floral, citrus flavour of Sichuan pepper is attractive in its own right, the spice induces a numbing, tingling sensation that is key to its culinary importance in Sichuan cuisine. Upon their introduction to China centuries ago, peppers (Capsicum) from the Americas quickly became a staple ingredient in many regions. It was believed that the tingling Sichuan pepper notes offset the piquant heat of chili peppers, allowing the other flavours of a dish to shine through.

The numbing quality of Sichuan peppercorns comes from a chemical compound called hydroxy-alpha sanshool, which causes a sensation of micro-vibrations across the tongue and lips. Hydroxy-alpha sanshool works by activating somatosensory neurons on the tongue, which are responsible for detecting innocuous stimuli such as a gentle touch. The activation of these neurons is similar to the mechanism of local anesthetics used in minor surgeries. Notably, this is almost the opposite mechanism of capsaicin, the heat-producing molecule of chili peppers, which triggers the tongue neurons responsible for the detection of painful stimuli to produce a burning sensation.

Related species

There are a number of related species that are used similarly in other Asian cuisines. The Japanese prickly ash, or Japanese pepper (Z. piperitum) is known as sansho in Japan. It and Z. schinifolium are cultivated in Korea and are commonly known as chopi. Indian plants include the green Sichuan pepper and Z. rhetsa.