Introduction
Have you ever wondered where the heat in a chilli comes from? The answer lies in a fascinating interplay of chemistry, genetics, plant physiology and environmental conditions.
Most chilli growers know or at least have heard that the heat of a chilli is caused by chemical compounds called capsaicinoids, of which capsaicin is the most famous. But there are others including dihydrocapsaicin, nordihydrocapsaicin etc. There is a receptor in humans called the TRPV1 receptor which is responsible for detecting capsaicinoids which then leads to the feeling of that burning sensation when you eat a hot chilli. Interestingly, birds can eat chillies without feeling any heat sensation because their TRPV1 receptor is insensitive to capsaicin. This is believed to be an evolutionary adaption since birds are one of the main agents of seed dispersal. Despite being odourless and tasteless, capsaicinoids are very potent and studies have shown that humans can perceive them at concentrations as low as 1 ppm.
Two of the most frequently asked questions we get are “Why are my chillies not hot?” Or “I have grown these chillies before and this year they are not as hot as the ones I have previously grown.” And similarly, we get the opposite questions such as “why are my Pimento de Padron or Trinidad Pimento so hot?”
I have trawled through as many of the peer-reviewed published studies as I can find which looks into chilli heat. And my aim is to summarise the key points in this blog. Spoiler alert… the heat of a chilli is determined by both its genetics and environmental factors. But much more about this below.
The myth of poor-quality seeds
Firstly, there is a myth which is sometimes purported that the variability in the heat of certain chillies which are supposed to be hot, or a specific heat level is due to poor quality seeds. There is nothing inherent to the quality of the seeds which can affect the heat of a chilli.
It is sometimes suggested that when a chilli is not the desired or expected heat that it should be that it is due to seeds from cross-pollinated plants, i.e. seeds collected from plants that have not been isolated to reduce the likelihood of cross pollination.
As I will describe later on, the reasons for heat production in chillies is complicated, involving a mix of genetics and environmental factors.
On the point about poor isolation of plants, there is a possibility that a plant grown from seeds collected from open pollinated crops is a random hybrid. However, whilst this is a possibility, the probability of this occurring is low. I would argue that it is extremely unlikely that a random hybrid was created and then these seeds collected and the first generation (a.k.a F1 – which stands for first filial generation) grown from these seeds were good enough to result in a fruit that bears all the resemblance to its expected lineage except that it’s heat was different. Indeed, all F1 hybrids will be a 50/50 mix of both parents; hence, it is very unlikely that an F1 will be identical to one particular parent in all aspects apart from heat level. Chillies are easily self-pollinated – the anthers (the male reproductive organ where the pollen is located) is less than 1mm away from the stigma (the uppermost sticky part of the female reproductive organ of a flower which is receptive of the pollen).
I should caveat the part about poor isolation of plants by saying that there are studies which have shown that apart from the generic controls on capsaicinoid production, there are quantitative genetic inheritance effects which control the level of heat over generations. What this means is that, unlike simple Mendelian inheritance (where traits are controlled by a single gene with clear dominant/recessive effects, e.g. eye colour in humans), quantitative traits are influenced by:
- Many genes (polygenic inheritance) - each gene contributes a small, additive effect to the phenotype. For example, in human height, dozens (even hundreds) of genes contribute to the overall variation.
- Environmental influences - the environment (nutrition, climate, lifestyle, soil fertility, etc.) also plays a large role in shaping the phenotype.
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Additive effects + interactions:
- Additive gene action - each allele adds to the trait in a cumulative way.
- Dominance and epistasis - genes can interact with each other, modifying the effect.
What this basically means is that with intentional crosses over successive generations, there is the tendency for hotter varieties to get hotter and vice versa, and mild varieties can become milder or even heatless. This, of course, relies on intentional breeding efforts over multiple generations to lead to a specific resulting heat level that one is seeking. So, again, it seems to me that the likelihood that this happens via non-isolated plants unintentionally over many generations is very low, especially when grown at the of scale 100’s of plants. At this scale the natural averaging process that comes with harvesting fruits from so many plants could easily offset any undesirable and unintentional hybrid. I will discuss these genetic influences in more detail in the sections below.
Where do the capsaicinoids occur in chillies?
Capsaicinoids are produced and start to accumulate in the epidermal cells of the placenta 20 days after the flower opens. Epidermal cells are cells which are near the surface of plant tissue. The placenta is the white tissue that is on the inside of a chilli to which the seeds are attached and is more prominent closer to the stem. It is easily visible when you cut open a bell pepper since there is more placental mass in these large peppers. The capsaicinoids accumulate in blisters on the surface of the placenta and is easily visible as droplets of yellow-coloured droplets. These blisters are very thin and can easily break, which results in the yellow-coloured oily liquid running down the insides when the fruit is cut open.
The genetics and chemical process involved in capsaicinoid production
There is one key chemical compound called vanillylamine which is vital for the production of capsaicinoids. Vanillylamine is an organic compound that plays a central role in the biosynthesis of capsaicinoids and it is derived from vanillin which is the same compound responsible for vanilla flavour.
The dominant gene which controls the ability for chillies to synthesise or produce capsaicinoids is called Pun1 or C which is mapped to chromosome 2. It is this gene which is hypothesised to cause the production of capsaicinoid synthase, which is an enzyme that is needed for the final step in the production of capsaicinoids. In other words, capsaicinoid synthase is hypothesized to be the molecule that is produced as a result of the expression of the Pun1 gene.
Heatless varieties usually have mutations or deletions in the Pun1 gene. There is also a second location on the chromosomes of chillies for the control of capsaicinoid production and it is called loss of vesicle (Lov). This second location have been found in Capsicum chinense and Capsicum chacoelse. In these species, varieties that have no heat are missing the blisters (or vesicles) which fill with capsaicinoids, hence the term ‘loss of vesicle.’
Interestingly, despite the studies which have hypothesized that capsaicinoid synthase is produced at Pun1, there have been experiments which have shown that heatless chillies can produce capsaicinoids if they are stored under specific conditions such as continuous light. And curiously, if placental extracts from heatless fruits are exposed to vanillylamine and other associated chemicals, they can produce capsaicinoids. This suggests that Pun1 may not actually directly be responsible for producing capsaicinoid synthase, rather it may be more of a regulatory gene in the overall process for capsaicinoid production.
The above genetics follow simple Mendelian control patterns of dominant/recessive inheritance. However, there is also evidence of quantitative genetic inheritance (as described above) which influence the heat of chillies. Studies have shown that there are significant additive gene effects when cross breeding chillies which means that repeated back crossing and selection can lead to increases/decreases (depending on the parent involved) of capsaicinoid content. These studies have shown that there is a lack of distinctive classes of heat level in cross-bred chillies, which indicates that heat in chillies is quantitatively inherited.
This is all very complicated, so my takeaways are:
- There are certainly specific genes in chillies that control the ability of a plant to produce capsaicinoids at all. If these genes are absent the plant simply can’t produce any.
- Once the genes which are needed for capsaicinoid production and accumulation are present in parents, it is then down to quantitative genetic inheritance effects which influence the heat of an offspring.
- Finally, once the right genetics are in place, the environment then affects the actual amount of capsaicinoids which are produced.
Environmental factors
Any environmental changes which stress a plant will affect the heat level of a chilli. It has been shown that stresses such as elevated night-time temperatures and lack of adequate watering can increase the amount of capsaicinoids which are produced in a fruit.
There are studies which have shown that there is a lot of variability in the heat of chillies on the same plant. So, it is not surprising that when you grow a few chillies in a greenhouse or windowsill at home you can get chillies with lots of different heat levels. These growing environments do not maintain constant conditions, and certainly here in the UK, there is very little consistency in daylight, temperatures etc. across the growing season. These effects on chillies across a single plant or let alone several plants may not be easy to identify if the average person is comparing the heat of Carolina Reapers for example. Reapers are so extremely hot that it would be very difficult to tell the difference in heat across several fruits if you are doing a taste test.
However, if you are comparing the heat levels of Pimento de Padron peppers for example, then it is easy to tell which one is hotter than the rest – it only takes a small amount of capsaicinoids in these peppers to cause them to be hotter than expected. We often get asked why our Pimento de Padron peppers are hotter than those from the supermarkets, and the answer is simply that we aren’t growing ours in controlled environments compared to some of the large industrial growers in Europe. And it is quite likely that producers in Spain for example, if they aren’t growing in environmentally controlled spaces, benefit from more stable conditions across their growing season compared to the UK. We might easily get a couple of days of blisteringly hot weather followed by a week of cloudy cool conditions.
Since introducing Trinidad Pimento to the UK which are heatless, we have been asked why they are are sometimes hot. This can either be from the fresh fruits we sell or the plants or seeds that customers buy and grow. Despite what I said at the start about the myth about poor quality seeds and non-isolated plants, when we grow Trinidad Pimento for seed collection for the season ahead, we ensure that we taste several fruits from each plant to rule out the possibility that we may have inadvertently grown a plant from an unintentional hybrid (however unlikely this may be). But despite these measures, anyone growing Trinidad Pimento or any other heatless varieties that are genetically capable of producing capsaicinoids, can end up with hotter-than-expected fruits due to environmental effects. In a study conducted by the famous Paul Bosland from the Chilli Pepper Institute at the New Mexico State University, they demonstrated that the environment has a very strong effect on the heat level of chillies. And astoundingly, their study revealed that the environmental component can have a stronger influence on heat level than the genotype! So the next time you are growing chillies, pay attention to the environmental conditions that you can control, such as watering. And if you do end up with hotter than expected fruits you can be sure that your environmental conditions have certainly had a role to play.
References
Blum E, Liu K, Mazourek M, Yoo EY, Jahn MM, Paran I (2002) Molecular mapping of the C locus for presence of pungency in Capsicum. Genome 45:702–705.
Blum E, Mazourek M, O'Connell M, Curry J, Thorup T, Liu K, Jahn M, Paran I. Molecular mapping of capsaicinoid biosynthesis genes and quantitative trait loci analysis for capsaicinoid content in Capsicum. Theor Appl Genet. 2003 Dec;108(1):79-86. doi: 10.1007/s00122-003-1405-y. Epub 2003 Sep 13. PMID: 13679988.
Garces-Claver A, Gil-Ortega R, Alvarez-Fernandez A, Arnedo-Andres M (2007) Inheritance of capsaicin and dihydrocapsaicin, determined by HPLC-ESI/MS, in an intraspecific cross of Capsicum annuum L. J Agric Food Chem 55:6951–6957.
Guzman, I., Bosland, P.W., O’Connell, M.A. (2011). Heat, Color, and Flavor Compounds in Capsicum Fruit. In: Gang, D. (eds) The Biological Activity of Phytochemicals. Recent Advances in Phytochemistry, vol 41. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-7299-6_8
Hall, R.D., Holden, M.A. & Yeoman, M.M. The accumulation of phenylpropanoid and capsaicinoid compounds in cell cultures and whole fruit of the chilli pepper, Capsicum frutescens Mill. Plant Cell Tiss Organ Cult 8, 163–176 (1987). https://doi.org/10.1007/BF00043153.
Harvell K, Bosland PW (1997) The environment produces a significant effect on pungency of chiles. HortScience 32:1292.
Iwai K, Lee K-R, Kobashi M, Suzuki T (1977) Formation of pungent principles in fruits of sweet pepper, Capsicum annuum L. var. grossum during post-harvest ripening under continuous light. Agric Biol Chem 41:1873–1876.
Iwai K, Suzuki T, Lee K-R, Kobashi M, Oka S (1977) In vivo and in vitro formation of dihydrocapsaicin in sweet pepper fruits, Capsicum annuum L. var. grossum. Agric Biol Chem 41:1877–1882.
Harvell, K. P. & Bosland, P. W. (1997). The Environment Produces a Significant Effect on Pungency of Chiles. HortScience, 32(7), 1292–1292. https://doi.org/10.21273/HORTSCI.32.7.1292.
N. Sukrasno, M.M. Yeoman, Phenylpropanoid metabolism during growth and development of Capsicum frutescens fruits, Phytochemistry, Volume 32, Issue 4, 1993, Pages 839-844, ISSN 0031-9422, https://doi.org/10.1016/0031-9422(93)85217-F.
Stewart C, Kang B-C, Liu K, Mazourek M, Moore SL, Yoo EY, Kim B-D, Paran I, Jahn MM (2005) The Pun1 gene for pungency in pepper encodes a putative acyltransferase. Plant J 42:675–688.
Stewart C, Mazourek M, Stellari GM, O’Connell MA, Jahn M (2007) Genetic control of pungency in C. chinense via the Pun 1 locus. J Exp Bot 58:979–991.
Zewdie Y, Bosland PW (2000) Capsaicinoid inheritance in an interspecific hybridization of Capsicum annuum x C. chinense. HortScience 125:448–453.
Zewdie Y, Bosland PW (2000) Evaluation of genotype, environment, and genotype-byenvironment interaction for capsaicinoids in Capsicum annuum L. Euphytica 111:185–190.
