What Produces Sichuan Pepper’s Unique Flavor?
The province of 四川 (Sichuan or Szechuan) directly translates to “four rivers”, and is the home of 麻辣 málà, the combination of numbing and spicy, used in Sichuan cuisine. If you’ve ever had mápó tofu or dàn dàn noodles, then you’ve been introduced to this major ingredient. The numbing portion of the málà duo is caused by Sichuan pepper, which is surprisingly neither related to chili peppers nor peppercorns. They actually do not contain capsaicin and are more closely related to citrus fruits, which explains the subtle citrus flavor profile [1,2]. There are multiple species of Sichuan pepper, all within the genus Zanthoxylum that are native to and now cultivated in different parts of Asia [2]. For the purposes of this exploration, I’ll be referring to all the different species together as the Zanthoxylum genus or simply Sichuan pepper. The part of the plant that is actually used in most cuisine is the seed husks.
Other than the tingling/numbing sensation, there are many volatile components that contribute to the Zanthoxylum’s unique flavor profile. Ji et. al. wrote a review that compiled many different studies on the different components of the Sichuan pepper [2]. The most prominent of which together produce a citrus, floral, pine, and minty flavor profile:
Paresthesia and our neurons
If you’ve ever bitten into a Sichaun pepper, the first thing you’ll notice is a tingling and numbing sensation, with a burst of herby, citrusy, and floral flavor. The abnormal sensation of this experience is called paresthesia, and is caused by the sanshool group of chemicals. Sanshools are named after the Japanese name for the Zanthoxylum plant, 山椒 sanshō, or “mountain pepper”.
The primary chemical that causes paresthesia is hydroxy-α-sanshool, and many researchers have explored how this compound interacts with our neurons. In 2013, Hagura et. al. investigated how hydroxy-α-sanshool interacts with our neurons by rubbing ground Sichuan pepper on the lip of volunteers, and then comparing the resulting sensation to a range of mechanical vibrations. The resulting data showed that hydroxy-α-sanshool interacts with the Meissner corpuscle on the end of neurons called rapidly adapting 1 afferent fibers (RA1). The Meissner corpuscle is a specialized portion of the RA1 neuron that produces a signal in response to a specific physical touch [3]. These RA1 neurons typically detect lower frequency vibrations, between 10–80 hertz (10–80 oscillations per second) [4]. But the interaction of hydroxy-α-sanshool with ths neuron tricks your brain into thinking there is a vibration, when in reality there isn’t. as a 50 hertz (50 oscillations per second) vibration [4]. This illusion is what causes you to feel the paresthesia.
Vibration is not the only sensation that is activated by hydroxy-α-sanshool. In the same year, Tsunozaki et. al. determined that hydroxy-α-sanshool also inhibits certain neurons responsible for both inflammatory and acute pain [5]. These neurons, known as Aδ mechanonociceptors, have free nerve endings and when inhibited, decrease pain sensitivity [5,6]. The hinderance of this pain receptor likely also contributes to the paresthesia caused by Zanthoxylum.
Additionally, hydroxy-α-sanshool activates a variety of other neurons that contribute to the overall paresthesia caused by Sichuan peppers [1]. When rats were exposed to ethanol (control) vs hydroxy-α-sanshool (test), the hydroxy-α-sanshool group experienced spontaneous firing of neurons sensitive to touch (tactile), cooler temperatures, and cold pain. Hydroxy-α-sanshool also activated silent (non-sensitive) neurons. In humans, it was also reported that hydroxy-α-sanshool caused an enhanced sensation of evaporative cooling. This paper also showed that hydroxy-α-sanshool impacted neurons by modulating the type or amount of stimuli that would activate the neuron:
The ways in which this chemical signal is relayed to the brain depend upon more in depth biochemical interaction and electrical impulses called action potentials. If you’d like to learn more about that aspect of sensation translation in reference to Sichuan peppers, I’ve written more on it here.
Flavor profile
Technically, flavor is what we sense when compounds are recognized by taste receptor cells that are grouped together in our taste buds. But when we discuss flavor and food on a day to day basis, we combine texture, aroma, spice, temperature, visuals, and taste into the experience. The overall flavor profile of Sichuan pepper comes from the compounds that interact with the taste receptor cells and the olfactory sensory neurons, combined with the paresthesia caused by the sanshool compounds, producing the Zanthoxylum’s unique flavor. If you were to look up why Sichaun peppers feel the way they do, you’ll mostly see articles written about the 50 hertz vibration. Covering the intricacies of how one chemical can impact multiple facets of our senses isn’t quite as catchy as “Sichuan pepper go brrrr”. So while the most well known paresthesia is the 50 hertz vibration, the combination of other pain, tactile, temperature, and acid firing neurons come together to produce the numbing sensation. I would the argue that paresthesia, flavor, and aroma should also be included in the Sichuan pepper’s flavor description.
It’s interesting to consider why we generally call the feeling from Sichuan pepper to be “numbing” rather than “vibrating”, given our neural response. For one, it can be easy for us to misinterpret certain sensations, as there is often a disconnect between what our brain perceives and the actual stimulus [7]. The brain is quite plastic, and things like sensory adaption (the ability to dynamically adjust to stimuli) and selective attention (the ability to focus on certain stimuli) can affect the way we interpret inputs [8, 9]. Additionally, in 2019, Fix et. al. showed that there was a mean decrease in reported pain when an injection was given with a vibrating kinetic anesthesia device, suggesting that mechanical vibration can have a numbing effect when it comes to the perception of pain [10]. Hydroxy-α-sanshool’s affect on neurons other than RA1 can also inform the way we perceive the Sichuan pepper’s effects. By making Aδ mechanonociceptors less sensitive, painful physical touch is less likely to trigger the neurons, contributing to a numbing sensation. Other cool, tactile, acid, and silent neurons firing and fluctuating in sensitivity to certain stimuli can also trick our brains into thinking we have been numbed.
While I believe that to fully experience the Sichuan pepper, you do need to feel the tingling and numbness, there is a wonderful floral and citrus aroma that comes from the volatile chemicals that can often be overpowered by the paresthesia, especially for the Sichaun pepper novices out there. If you’d like to experience the aromatic side of these little seed husks without the paresthesia, I would suggest Sichaun pepper frozen custard:
Ingredients
Frozen Custard
- 284g/1¼ cups whole milk
- 223g/½ cup + 1 tbsp granulated sugar
- 1g/¼ tsp kosher salt
- 12g/5 tbsp dried red Sichuan peppers
- 454g/2 cups heavy whipping cream
- 84g/6 large egg yolks
Sesame Swirl Toping
- 70g/⅓ cup unrefined brown sugar
- 0.5g/⅛ tsp kosher salt
- 64g/4 tbsp Chinese sesame paste (black or white sesame, well stirred)
- 57g/4 tbsp water
Method
- Infuse Sichuan peppers: Bring milk, sugar, and salt to a simmer. Remove from heat and add Sichuan peppers. Cover and steep for 1 hour before straining the mixture to remove spent seed husks.
- Prepare ice bath: In a bowl over ice and water, add heavy cream and keep in fridge until ready to use.
- Make custard: Reheat infused milk. While this is reheating, whisk egg yolks in a bowl surrounded by damp cloth to prevent the bowl from slipping. Temper eggs by slowly pouring one ladle of infused milk into egg yolks while whisking. Continue adding ladles of infused milk until approximately half of the milk has been added. At this point, add the egg mixture back to the saucepan and stir on low until the custard coats the back of a wooden spoon or silicone spatula.
- Cool custard: Strain the custard into heavy cream on the ice bath and whisk. Use plastic wrap to cover the custard, pressing it into the surface to ensure no film forms. Refrigerate overnight.
- Make sesame swirl toping: Heat brown sugar, salt, and water until sugar is completely dissolved, and let cool completely. Add sesame paste in 4 rounds, stirring to incorporate completely each time. The toping should be viscous, but pourable. I’d liken it to slightly thicker honey. Refrigerate until needed.
- Churn custard: Follow the instructions on ice cream maker and churn until frozen custard is at a soft serve consistency. At this point, transfer frozen custard to loaf tin or some freezer container.
- Mix sesame swirl: Add ⅓ of the frozen custard, ⅓ of sesame swirl, and layer until all of the custard and sesame toping has been added. Use a chopstick or knife to swirl the custard. Don’t over-mix, as this will cause the flavors to meld and it gets difficult do parse the Sichuan pepper and sesame flavors.
- Set frozen custard: Put container in freezer for at least 2 hours.
When I made this frozen custard, adapted from the MaLa Market’s blog, I found the flavor to be a subtle floral, citrus, herbal, and bright, without the numbing/tingling effect [11]. I would highly suggest setting aside some of the ice cream or at least tasting it before swirling in the sesame drizzle so you can get a taste of just the Sichuan pepper flavor.
I am no food scientist, but my guess for why the hydroxy-α-sanshool takes a back seat to the aromatic flavors in this particular recipe is that the steeping step favors the incorporation of the volatile chemicals, and less the hydroxy-α-sanshool. There are both lipophilic and hydrophilic volatile compounds found in the Sichuan pepper, and since milk is a water and fat emulsion (mostly water), these compounds become incorporated into the ice cream [2]. Hydroxy-α-sanshool is highly lipophilic, so it is possible that more fat or a greater proportion of Sichaun pepper would impart more of the tingling sensation into the ice cream[12].
Other recipes highlight the hydroxy-α-sanshool effects much more, exemplified by this article that describes Kurtwood Farm’s Sichuan pepper ice cream to have a numbing effect [13]. When I spoke to the Kurtwood Farms owner, Kurt Timmermeister, I was told that there is a drastic difference between the production of ice cream at home and in a commercial setting. While this above recipe infuses the Sichaun pepper into the milk, an ice cream shop would make a batch custard and then add the different flavors before churning and freezing. This supports my original suspicions that more fat would extract more of the lipophilic hydroxy-α-sanshool, as a custard batch would already include the egg yolk and maybe the cream. Kurt also mentioned that on Kurtwood Farms, he has access to fresh Sichuan peppers that he cultivates, which would greatly increase the flavor compared to the dried product that most people would have access to [14]. Increasing the amount of Sichuan pepper and crushing them would also increase the amount of all the compounds transferred into the ice cream. So while my adapted recipe increases the Sichuan pepper and fat amounts compared to the original, adding even more and/or infusing the flavor later could give your batch a sprinkle of paresthesia.
