Understanding heat levels in chillies

Spiciness in chillies is measured in Scoville Heat Units or SHU. The measurement is based on the concentration of four chemicals found in peppers, collectively called capsaicinoids, of which capsaicin is one component.

Scoville Heat Units are named after Wilbur Scoville, an American pharmacist, who in the early 19th century devised a taste test that measured the heat level of a chilli.  This measurement was the highest dilution of a chilli pepper extract at which no heat could be detected by a taste panel. By its very nature this test was subjective, and it has now been superseded by a laboratory test using a process called high performance liquid chromatography (HPLC).

Aji Crystal chillies
Stage of ripeness effects the heat level of chillies.

The HPLC test is fully objective and gives an accurate measurement of the SHU of any sample tested. However, any SHU test, no matter how accurate, only gives a snapshot of the heat of any chilli variety. Obviously, the genetics of a chilli variety has the over-riding influence on heat level of the fruit. However, within the genetic limits of a particular variety the fruit from different plants may differ. In addition, the growing conditions and the stage of ripeness of the fruit also have significant influences on the heat level of each fruit. Given this, the heat level of any variety will vary from fruit to fruit and from plant to plant in the same crop; let alone between seasons (e.g. summer and autumn harvested fruit), and from year to year and site to site. Generally, the hotter the variety the greater the range of heat levels found in that variety.

Another variable confusing any interpretation of heat level is how much water a chilli fruit contains. HPLC tests are done on a dry matter basis. So two chillies with the same SHU result will appear very different to a taste panel if one has a dry flesh and the other is very juicy. 

In addition, to the natural variation in the heat level of chillies, each person’s perception of heat of the same chilli is different. What one person finds unbearably hot another might consider to be pleasantly warm.

With all these variables heat levels should always be treated as a guide only, and never as a definitive result.

However, SHU levels are very useful as a guideline, and so, as a very rough guide, we at Sea Spring Seeds have categorise heat levels according to the following formula:

Sweet: 0 SHU
Very mild: up to 2,000 SHU
Mild: 2,000 – 5,000 SHU
Medium: 5,000 – 25,000 SHU
Hot: 25,000 – 50,000 SHU
Very hot: 50,000 – 200,000 SHU
Extremely hot: 200,000 – 750,000 SHU
Superhot: over 750,000 SHU


SHU levels quoted by Sea Spring Seeds

At Sea Spring Seeds we give the heat level of all, except some of the very mildest, varieties. The levels we quote are of fruit grown on our nursery in West Dorset and tested at Warwick HRI (formerly Horticulture Research International). The samples we send to HRI are always made up of fully ripe, whole fruit picked from every plant growing in that crop. In other words we aim to get a result that gives us an average heat level of the whole crop.

Because of the wide variations in heat levels between individual fruit, tests done on a single fruit are notoriously unreliable.