By Donna Michael
Special to The Courier-Journal

Q: I am planning my vegetable garden now and trying to decide on hot peppers. I like jalapenos but have heard that there are hotter varieties. Can you suggest a few for me to grow this year? Also, what makes peppers hot and how is the heat level determined?

A: The popularity of Southwestern and Mexican food has increased over the years. More and more gardeners are looking for hot chili pepper varieties and planting them. The consensus among pepper fanatics is that the hotter they are, the better.

If you’re looking for a mouth-burning, eye-watering pepper, many longtime growers say the rule of thumb is the smaller the pepper, the hotter it will be. Ask a scientist, however, and it gets a bit technical.

To know where peppers get their heat, you need to look within the plant’s tissue and its chemical structure.

Bitter alkaloid compounds called capsaicinoids, which include capsaicin, are produced in the cells near the membranes (ribs) and some cells of the outside walls of the peppers.

The presence of the compound is dependent on a single gene. If that gene is lacking, as it is in bell peppers, there is no heat.

The seeds of chili peppers do not produce the compounds, but because of their close proximity to the ribs, where concentrations of alkaloids are the highest, the seeds absorb the capsaicin. Remove the interior membranes along with the seeds, and the heat of the peppers goes down considerably.

Slight differences in the physical structure of capsaicinoids among pepper varieties can create different heat reactions in the mouth.

The measurement of the heat in a pepper began in 1912 with a pharmacologist named Wilbur L. Scoville. The heat factor is therefore rated in Scoville units. Capsaicinoids are measured in parts per million, with one part per million equal to 15 Scoville units. Bell peppers have a value of 0, and habanero peppers can be 200,000 to 300,000. Jalapeno peppers receive a rating of only 2,500 to 5,000.

The units are based on a taste test that measures the amount of sugar water needed to dilute the heat of the pepper to the point where it is not detected to the taster. This test, once an actual taste test, is now done by liquid chromatography.

Many chili judges have gone to a more simplified rating known as the chili pepper heat scale, which has a rating system on a 1 to 10 scale, with 10 being the hottest. With this scale, jalapenos receive a 6, hot Thai peppers a 9 and habaneros a 10.

Depending on how hot you want your pepper to be this year, here are a few varieties and cultivars that you can grow:

Habanero-chocolate, 285,000 Scoville units

Red Savina habanero, 570,000

Scotch Bonnet, 150,000-300,000

Thai pepper, 100,000-350,000

Using the relative heat ratings, Santa Fe Grande gets a 6, Pequin an 8.5, Serrano Seco a 7.5 and Tepin an 8.

There are many mail-order sources for chili peppers. One is Cross Country Nurseries in Rosemont, N.J., which touts itself as having the world’s largest selection of sweet and chili peppers. You can get a free catalog by going to www.chileplants.com. You can also visit local nurseries.

Send questions to “Garden Spot,” The Courier-Journal, P.O. Box 740031, Louisville, KY 40201-7431. Donna Michael is the Jefferson County horticultural agent with the Kentucky Cooperative Extension Service. You can call her at (502) 569-2344.