Why are winter vegetables the sweetest? I've always wondered if fresh vegetables in the winter are sweeter because they are a little more scarce. There aren't many fresh fruits in the garden to compare them to and there’s also a certain satisfaction to pulling a buried, hidden carrot root from a barren field, wiping off the dirt, and eating it. Similarly, all the brassicas (broccoli, cabbage, kale, etc) seem to get sweeter in the winter. This phenomenon got me interested, and I set out to learn more.
Here's what I found out:
When temperatures begin to dip, many vegetable plants start to produce chemicals that protect them from freezing. Starches in carrots convert into sugars and thus the carrots become very sweet. Certain chemicals are more resistant to cold. For example, sugar has a lower freezing temperature than water. Many plants that have adapted to survive cold periods bring their sugar content up as they are exposed to increasingly cooler temperatures as a defense mechanism.
This is similar to why you add antifreeze to your radiator. In your car’s radiator propylene and/or ethylene glycol act like the sugar in carrots, and these active ingredients in typical automotive antifreeze protect against freezing. With the glycol ingredients suspended in water, the liquid in your car’s radiator will not freeze and rupture your radiator. The glycol prevents ice crystals forming in the water in your radiator, while the antifreeze chemicals (especially sugar) in the plant's intracellular and intercellular regions prevent the plant cells from getting damaged by expanding ice crystals.
Ethylene glycol and propylene glycol is added to automotive antifreeze to prevent freezing, but it also raises the boiling point of the water. It's a more efficient heat conducting liquid, which is the main job of a car’s radiator: to move the heat from the engine to the radiator in the front of the car where the motorized fan and wind carries the heat away. You'll also find propylene glycol in foods like ice cream as well, keeping that smooth texture without it getting ice crystals.
How cold and how sweet can plants get?
Now with every type of plant there is a threshold for cold exposure that they can endure before they go from sweet and tasty to mush and funk. While temperature dips that are only slightly sub-freezing may cause leafy green plants some minor cellular damage, they generally repair themselves after several days of warmer temps and the texture and flavor improves. Additionally, the ice crystals that form during a cold snap can cut and damage the cell walls if the plants are handled while frozen. For this reason it's best to avoid any harvest or handling of vegetables while they are frozen.
One more thing to keep in mind? Wind protection.
A great example of this is with a winter vegetable plant like kale. In an unheated hoop house it will grow well, even if the temps drop nearly as low as outside. The reasons for this are the thin plastic walls that help stop the wind and keep plants at a standstill through the freezing. Less motion means less of a chance for those intracellular ice crystals to start smashing up the cell walls like a needle in a balloon store. (It should be mentioned that this also helps with the windchill factor).
Carrots, kale and other winter-hardy crops like cabbage really do get sweeter with the colder weather. They continue to sweeten right up to the point of freezing, and then start to show signs of cold damage. This phenomenon is a natural defense against a freeze, but does not make them totally impervious to it. When it gets too cold the plant cell walls are damaged and the texture we hope for in our hardy greens begins to decline in quality.
Growing vegetables in the winter can be challenging, and it's no wonder why much of the US vegetable production moves south in the cooler months. The sugar-producing cold response in vegetables is a defense mechanism, though in many cases the cold comes fast and kills the crop altogether. On the other hand, summer carrots may be safe from this danger but will probably never come close to the winter sweetness. And those carrots that are shipped in from California? Most likely they never saw a frost and can't compare to the deliciousness of locally grown carrots in winter.
Thanks for keeping up with us at the Seed Undergound. As always we welcome any questions, comments, or winter vegetable recipes! Hope you're keeping warm and enjoying the holiday season in whatever best fills your soul!
Further reading and articles referenced for this article.
Winter Harvest Handbook by Elliot Coleman.