It’s the most often asked question for chilli lovers – ‘how hot is that chilli?’ The heat in the fruit comes from chemical compounds called capsaicinoids, of which the two most common are capsaicin and dihydrocapsaicin. These two typically make up around 90 % of the total capsaicinoids present in any given chilli, but there are many more capsaicinoids, all of which will have an impact on the flavour, and crucially, the heat of a chilli.
Most chilli lovers will have heard of the Scoville test and scale. This was originally introduced in the early years of the 20th century and involves extracting the active components of the chillis with alcholol, adding sugar and then serially diluting the resulting solution until no further heat can be detected on tasting. The more dilutions are required, the hotter the chilli was originally. Although this might seem somewhat unscientific, in reality at the time the test was introduced this was considered a reasonable test, as human sensitivity to capsaicin can be measured in parts per million (ppm), far better than any other analytical technique at the time.
In the 21st century testing is somewhat more refined, and analysis is typically performed using High Performance Liquid Chromatography (HPLC). This is a technique that involves separating the individual components and then seeing how much of each one is present. The resolving power of HPLC is very good, and HPLC analysis of capsaicin and dihydrocapsaicin is now the most common way of determining content. From the quantities of the two capsaicinoids found a trivial calculation leads to an estimated Scoville rating.
While HPLC is a good, solid technique, there are some problems with it. It relies on the use of standards, small amounts of known compounds, to calibrate the instrument. This is not a problem for capsaicin and dihydrocapsaicin, but the many other compounds involved in chilli flavour and heat are rarely looked for or quantified, yet may play a significant role in the flavour and heat of a fruit. It is also not clear what the link between a capsaicin measurement in weight for weight and the Scoville scale should be. Typically pure capsaicin is given a rating of 16,000,000 Scoville units, and this means that a chilli that is 5 % capsaicin by dried weight would rate at 800,000 on the scale. However the heat provided by other components is not as well determined, for instance in some instances dihydrocapsaicin is rated as 15,000,000, whereas others simply treat capsaicin and dihydrocapsaicin as the same. The calculation for the ASTM test gives it a rating of ?? of that of capsaicin itself. Clearly there is some confusion, and this is just for dihydrocapsaicin! Many other capsaicinoids are found in the chillis we eat, and will contribute to the heat we detect in the mouth.
Recently at the University of Bath a project has looked at using nuclear magnetic resonance (NMR) to quantify the levels of capsaicinoids in a wide range of chillis. NMR is closely related to magnetic resonance imaging (MRI), a way to look inside the body for medical applications . NMR has the advantage that all the components can be seen at the same time, and no standards are required for the minor components. Our technique employs the same extraction of the dried sample with a solvent as for HPLC, at which point the NMR spectrum of the resulting solution is studied, following the addition of a spike of a known compound to allow us to see how much of the capsaicins are present.
The project is still in its infancy, however recent results comparing the levels of capsaicinoids determined by NMR with those seen by HPLC has given a very reassuring correlation (see chart below).
Chillis studied in the project include some very hot beasts – not least Katie and Lucy, both Capsicum Chinense, which have around 9-10% capsaicinoids by weight, and thus would be in the region of 1.3-1.5 million units on the Scoville scale, depending on how the calculation is done.
We are currently looking at using the NMR method to determine the presence of the more minor capsaicinoids in the extracts.
In the future we aim to investigate the effects of the growing environment on the heat of the resulting chillis. This will entail growing several different varieties (ideally a mild, and medium and a hot variety) under a set of different conditions, including water stressed, nutrient starved and temperature controlled. Some research has suggested that water stress leads to higher heat contents in chilli fruit, but only for chillis that are mild to medium in heat. The hot variety showed little variation. One of the aims of the project is to help the amateur chilli grower to get the ‘best’ from their plants.
Article by Tim Woodman (Bath University)
1 The full name for MRI should really be nuclear magnetic resonance imaging; when introducing the technique it was felt that the ‘nuclear’ would be off-putting for patients and it was dropped. We have no such qualms at the University, and in reality although we look at ‘nuclei’ NMR does not involve radiation, other than radio waves
2 For assessing a particular variety of chilli, ideally at least 10 pods from a range of plants should be studied. In our work so far only single pods have been looked at.