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One person's pain is another's balm in the contradictory world of nature's 'hottest' vegetable. 
The painful side was viewed 1.5 million times around the world on YouTube  recently, when police broke up a peaceful Occupy Wall Street protest at the University of California. 
The video shows one large policeman stepping forward in full  riot gear. He takes out a big aluminium can of pepper spray and shoots the substance - in a deliberate and prolonged burst - directly into the eyes of the shocked, sitting students. 
The fat cop's spray contained a 'burning' agent called capsaicin,  derived from red peppers.  Capsaicin is the ingredient that makes peppers spicy-hot. 
Red pepper belongs to the nightshade family, along with  tomatoes, potatoes and tobacco. Capsaicin is an alkaloid, which, in mammals like us, produces a feeling of heat in the mouth and throat. 
Some people enjoy the pain: there has long been a demand for capsaicin-spiced food and beverages.  Many diners go out of their way to seek  that burning sensation in  hot chilli  sauce  or salsa.    
The degree of  heat  found within a food is  measured on the Scoville Scale.  Chilli has the highest rating on the  scale, which is measured in Scoville Units  (SCU). The  formidable Cuban habanero  chilli is at the extreme outer range of the scale, measuring  300,000 SCU. 
Ironically, the heat in capsaicin is also a balm, used to relieve pain. In high concentrations, it creates a burning effect on sensitive areas of skin, and is used in creams for the temporary relief of minor aches and pains in muscles and joints. 
This soothing agent is usually applied by a therapist wearing rubber gloves and a face mask. The cream remains on the skin until the patient starts to feel the 'heat', when  it is promptly removed. 
How does capsaicin work as a treatment for pain? It binds with proteins in the body called TRPV1 receptors, which are connected to membranes that sense both pain and heat. Capsaicin activates the protein to release pain-killing endorphins and the sensation of heat. 
When 'riot control'  pepper spray comes in contact with skin, and especially with eyes or mucous membranes, the pain is sharp.  Breathing small particles of it can cause difficulty breathing, and large overdoses  can even cause death.
Hong Kong had its own capsaicin-related headline incident in 2008, at the equestrian competitions during the Summer Olympics. Four horses in the jumping events  tested positive for the substance, which resulted in disqualifications and fines. 
Putting capsaicin on a horse's legs makes them hypersensitive to pain, creating a powerful incentive to avoid hitting the barriers - a clear case of doping. Another jumper tested positive for nonivamide,  a synthetic form of capsaicin called a capsaicinoid.  
Why did mother nature develop capsaicin in the first place, then give the  hot  vegetables a bright red colour, like a public warning? 
Well, it seems to be a smart strategy to ensure wide geographical distribution of red pepper seeds. 
That strategy involves discouraging mammals, while encouraging birds, to eat the peppers, since mammals don't disperse seeds as widely as birds. 
 Birds do not feel the heat because they lack the proteins needed to sense it on their tongues.
Birds can see colours, so bright red peppers are easily spotted. Unlike animals, birds do not damage seeds by chewing them - so pepper seeds are excreted not only intact, but also surrounded by 'bird fertiliser'.
In addition, red peppers love the growing conditions under big trees - where birds commonly rest. What a smart pepper  plant.
A practical tip for the reader: capsaicin does not dissolve well in water, but it does   in fat. So, to stop the pain of a too-hot mouthful, eat something fatty or oily - do not drink hot water or  tea. It also dissolves well in alcohol.  Cheers.
Reinhard Renneberg  has been a professor of bioanalytical chemistry at Hong Kong University of Science and Technology since 1994



Scientifically speaking, spicy foods are for the birds