Limonene is a phytonutrient in a class of chemicals called terpenes. Terpenes are hydrocarbons, meaning they are made up of hydrogen and carbon, that are produced by a wide variety of plants. A big terpene producer is conifers such as pine, which produce it in resin. The word terpene is derived from the word turpentine. Terpenes contain isoprene units, which are a combination of 5 carbon atoms and 8 hydrogen atoms, and the different types of terpenes are classified according to how many of these isoprene units they contain. Limonene is a monoterpene, meaning it has two isoprene units in its chemical makeup. It is a clear liquid whose molecules can come in two different types which are mirror images of each other. L-limonene has a piney smell, and d-limonene smells like oranges. This is starting to sound like a room freshener, and in fact it is used in some solvents and cleaners. However it is also a phytonutrient that can give you important health benefits.
Limonene is found in the oil of citrus peels, and is a by-product of the orange juice industry. Industrial uses for limonene include cleaning products, and additives meant to add aroma or flavoring to a product.
Since limonene is collected and used for other products during processing of oranges, there probably isn't much of it left in packaged orange juice. You can get limonene if you squeeze your own orange juice or make lemonade with fresh lemons. Even better is using grated peels, or zest in your recipes, or using whole preserved lemons. These delicious lemons preserved in olive oil and salt can be chopped up and added to mediterranean-style dishes. Limonene is present in a variety of citrus fruits such as oranges, grapefruits, lemons, limes and tangerines.
Most of the medicinal testing has been done on d-limonene. In laboratory studies, it has been shown to induce apoptosis, which is the "death signal" kills a cancer cell, or prevents it from dividing. It also increases the levels of enzymes in the liver that can detoxify carcinogens. In animals limonene has been shown to slow the growth of cancers of the pancreas, stomach, colon, skin and liver. Unfortunately these results haven't been backed up by human trials, so there isn't any proof that limonene can fight cancer in humans. There have been studies showing that limonene slowed the growth of breast and colon cancer in humans, but the studies were too small to warrant using this chemical as an anti-cancer treatment.
Some other medical uses for limonene are to treat gallstones, as it can act as a solvent to break up cholesterol in the stones, and to neutralize gastric acid in cases of heartburn.
Is it safe to eat limonene? In human studies people given doses of limonene had nausea, vomiting and diarrhea, but didn't have any permanent ill effects. A more serious effect is skin and respiratory allergies, but this is a danger for people who are using cleaning products or pesticides with limonene. If you become sensitized to this chemical you can start to have an allergic reaction every time you come in contact with citrus. It's important to avoid contact with cleaners, solvents and pesticides. Limonene is used in a pesticide that is applied in people's homes to kill termites. Also, citrus fruits themselves are often treated with pesticides and you may get the residue in peels when you squeeze or grate the fruit. You should wash your fruit well if you are using the peel, or buy organic fruits.
Considering the nasty side effects and potential allergic reactions, it's probably not a good idea to run out and buy a supplement of limonene. But eating some fresh squeezed orange juice or some preserved lemons chopped into a sauce won't hurt you, and may have the beneficial effects of fighting cancer and heartburn.