The first thing we should know is that peppers that sting do so because they contain substances called capsaicinoids. This curious name has its origin in what was assigned to its botanical genus, Capsicum, according to some derived from the Greek κάπτω (from gobble, supposedly referring to spicy), and according to others from the Latin capso (box or capsule, for the fruit). Be that as it may, this plant has its origin in America, from where it came to Europe brought by Columbus. Today the most common cultivated species is Capsicum annuum L., but some varieties of pepper contain capsaicinoids, spicy, and others do not.
Whether a variety can sting or not at all depends fundamentally on whether it is capable of synthesizing these capsaicinoids. To do this, the plant must form two precursor substances (vanillylamine and 8-methyl-6-nonenoic acid) from the amino acids phenylalanine and valine, respectively.
An enzyme called capsaicin synthase then combines those two precursors to form capsaicin. This can then be transformed into other capsaicinoids with varied structures, but all of them are characterized because they sting.
Eye! They bite mammals, but not other animals such as birds. Mammals have a protein called TRPV1 that helps us perceive different stimuli, such as high temperature. But TRPV1 is also activated by capsaicinoids, hence the sensation of heat you feel after eating a hot pepper.
Birds have a different TRPV1 protein, which is insensitive to capsaicinodes. That is why birds can eat peppers without being stung and disperse their seeds.
In peppers, the synthesis of capsaicinoids is regulated at different levels. A first requirement is genetic: the pepper variety in question must present the functional allele of the Pun1 gene, which makes the plant have capsaicin synthase. This is the case of the Padrón variety, but also of many others such as jalapeños or habaneros. If they have Pun1, they can sting.
Another gene necessary for peppers to bite is pAMT. If it presents any alteration, vanillylamine cannot be formed, and then instead of capsaicinoids, capsinoids are formed, which look like the first ones, but do not sting.
Genetic regulation, especially by Pun1, is decisive, but then there is a finer regulation of the synthesis pathway in response to different factors.
The first factor that regulates capsaicin content is development. The age of both the entire plant and the fruit will affect the itching.
Once the fruit sets, it begins to grow, increasing in size over time. As the fruit grows, the synthesis of capsaicin gradually increases until it turns red (ripe) and reaches its maximum degree of spiciness. Thus, a larger fruit is usually spicier than a small one, since it is generally older and has more capsaicin.
However, this rule is not infallible. On the plant there is always some fruit that grows little and remains small even if it is ripe.
Likewise, the age of the plant influences the itching. As the plant ages, the capsaicin content of its fruits increases. This is the reason why Padrón peppers are hotter at the end of the season, between the end of August and October.
Finally, if the plant is stressed for any reason, the synthesis of capsaicin in its fruits increases.
This has been observed in experiments in which treatments that altered some environmental factor have been applied to plants. Thus, excess or deficit of water (irrigation), excess or deficit of nutrients (fertilization), salinity conditions, infection by pathogens (diseases), etc. They make the peppers hotter.
There are studies that relate this stress response to the possible functions of capsaicinoids for the plant. For example, its antimicrobial properties could be useful against pathogens that attack pepper.
In practice, when we are going to eat a plate of Padrón peppers, it is difficult to know with complete certainty whether a specific one is going to be spicy or not.
If none sting, they may be from a hybrid line or specifically selected not to sting, and probably do not have the functional Pun1 allele. If they are authentic, some will surely sting.
But that's the fun, enjoying an authentic "Galician roulette", and that some bite and others don't.
This article was originally published on The Conversation.
José Díaz Varela is a professor at the University of Plant Physiology at the Universidade da Coruña.
