A couple of weeks ago, we went camping with a pair of friends. One of the highlights of our camping trips is the food. While the guys are getting the VW vans ready for the adventure, my friend and I are busy planning the dinners. Each couple is assigned to cook one meal for the whole group. On the first night, I cooked up some ratatouille using eggplant, zucchini, peppers, basil, and tomatoes from my garden. It was a summery feast perfect for a cool Pacific coast night. The next night was my friend’s turn to cook. She prepared delectable salmon cakes served alongside an orzo salad. As she was finishing up the salad, I heard a dismayed, “Oh no!” When I went over to investigate, she held up a container of basil that had turned black and wilted. I told her that basil doesn’t like temperatures below 50 degrees Fahrenheit. I followed up my statement by saying, “I think it’s some sort of defense mechanism or something.” It bothered me that I wasn’t sure about the why behind her basil’s quick demise, so I decided to investigate.
Before we address why basil hates the cold, let’s get to know a little more about this important herb. Basil is a member of the mint family. It’s native to Africa and Asia. Basil is not only important as a culinary herb, but it has been used in traditional medicine to treat digestive orders, respiratory issues, and a host of other ailments. Some of its essential oils have demonstrated the ability to kill harmful bacteria and fungi, making it a plant of major research interest.
There are nearly 150 species of basil, but the one most commonly used in cooking is Ocimum basilicum. The genus name is derived from the Greek word okimom, which means “aromatic herb.” The species name means “royal fragrance.” Given that its scientific name is all about aroma, what accounts for the fragrance differences amongst the different types of basil? It’s all about the chemical compounds found in basil’s essential oils. For example, sweet basil has high concentrations of linalool. This compound is also found in mint, citrus fruit, and cannabis. On the other hand, cinnamon basil produces more methyl cinnamate.
Interestingly enough, sweet basil and cinnamon basil are members of the same species, but they are what we refer to as cultivars. The word cultivar comes from the term “cultivated variety.” Think of Ocimum basilicum as plant with a whole bunch of specific traits ranging from flower color to leaf size to flavor. One day someone tasted a plant and said, “Wow! This one tastes like cinnamon!” Using the power of artificial selection, that plant was cultivated so that we could have a whole bunch of Ocimum basilicum ‘Cinnamon.’ Notice that the scientific name now includes the cultivar as the 3rd word, which is capitalized and in quotes. Cultivars may not be true to seed since they can cross-pollinate with other plants. Think of the resulting offspring as “plant mutts.” If those mutts taste good or have other beneficial characteristics that make them worth growing, horticulturists will use the nicer term, hybrid, to describe them.
Regardless of which cultivar one is dealing with, these plants are susceptible to cold injury. Research has shown that temperatures below 53.6o F (12o C) can cause serious problems. The initial symptoms are brown spots. Eventually, the entire leaf darkens (black necrosis) and the plant will drop its leaves (leaf abscission). Once the plant is weakened, the plant is an easy target for bacterial and fungal infection. So what’s going on?
Plant cells are surrounded by an outer cell wall. Below that is the cell membrane. The cell membrane is responsible for what goes into and out of a cell. Cold temperatures damage the membrane causing it to lose its integrity. In addition, the plant’s hormone and antioxidant levels are affected, which can weaken the plant’s defense mechanisms. Metabolic changes also occur resulting in an increase in the production of ethylene. This chemical is linked to temperature sensitivity.
Cold injury is influenced by other variables. Both older leaves and the youngest leaves found at the tip of a stem are more prone to suffer from cold injury. One study hypothesized that this was due to a breakdown of fatty acids in the cell membrane. Basil is also more susceptible to cold injury when harvested in the morning, rather than later in the day. As a matter of fact, one study concluded that the best time to harvest basil is 6:00pm to 10:00pm. It’s thought that since plant photosynthesis is diurnal, or active during the day, plants have the most stored sugar at the end of the day, which provides them with more protection. (Think of it as being able to put up a better fight if you’re well-nourished versus famished.)
So what does all this mean for the home cook? Once you’re home from the grocery store, rinse and dry off the basil stems. Place them in a glass of water and put it on the counter at room temperature. If you’re not going to use it up quickly, be sure to change the water daily or you’ll lose your herb to harmful bacteria.
One more thing. If you’re not going to cook the basil, don’t tear the leaves or chop it until you’re ready to eat. Cold injury is just one reason why basil leaves turn brown or black. Another reason is a process called oxidation. Plant cells contain an enzyme called polyphenol oxidase. When the leaves are cut, plant cells are exposed to oxygen allowing this enzyme react with other chemical compounds causing the color of the leaves to change. While it’s pretty easy to hold off on prepping the basil if you’re simply going to mix it into a salad or sprinkle it on pasta, what if you’re making pesto? How can you prevent it from turning brown? The key is to blanch the leaves in boiling water for 15 seconds, followed by a quick plunge in ice. The reason this works is because the intense heat denatures (changes the shape of) polyphenol oxidase rendering it unable to successfully engage in those chemical reactions that cause browning. Ahhhhh science... it’s a beautiful thing.
This weekend is another camping adventure. Can’t wait to tell my friend why her basil really went south.
What are you wondering about? Maybe I can answer your question in my next blog. Leave a comment!