Frozen vs Fresh Kale: Which is Better?

Supercharged with a wide range of nutrients and health-protecting phytochemicals, fresh kale has been lauded as a superfood. But is frozen kale just as good as its fresh counterpart at providing nutrients?

Turns out, there are huge differences in how easily different nutrients get destroyed in the freezer and during the blanching process that typically precedes freezing kale. Fiber, minerals and fat-soluble nutrients such as beta-carotene and vitamin K are not easily destroyed by blanching or freezing, so just like fresh kale, frozen kale is loaded with these nutrients. Water-soluble vitamins like vitamin C, by contrast, are not as stable, and fresh kale loses a significant amount of vitamin C when it is blanched and frozen. Finally, both fresh and frozen kale are rich in glucosinolates, but only fresh kale is loaded with the enzyme that transforms this biologically inactive phytonutrient into cancer-fighting isothiocyanates.

For details on how frozen kale fares against fresh kale in terms of nutritional value, read on.

Dietary Fiber

Fresh kale is a good source of fiber, with a 100-gram serving of fresh kale providing 13 percent of the Daily Value for this important macronutrient. Blanching and freezing do not destroy dietary fiber, so also frozen kale will provide your body with plenty of fiber. (1) Eating plenty of foods that are rich in fiber may help lower risk of everything from cardiovascular diseases and diabetes to obesity and certain gastrointestinal diseases (2).

Vitamins

One thing that sets kale apart from spinach and many other green leafy vegetables is that garden-fresh kale contains many times more vitamin C than most other greens. In fact, ounce for ounce, fresh kale contains even more vitamin C than oranges! Unfortunately, however, vitamin C is easily destroyed during cooking and storage, and frozen kale has been reported to contain only little more than half of the vitamin C of fresh kale. (1) That said, vegetables also rapidly lose vitamin C in the refrigerator (3), so if you are trying to up your vitamin C intake with kale, make sure the kale you are using is as fresh as possible.

Kale is also an excellent source of carotenoids which are precursors to vitamin A. In fact, when it comes to supplying your body with carotenoids such as beta-carotene, kale even beats out moringa which is known for its high carotenoid content. (1) And the best thing is, the beta-carotene in kale is not easily destroyed by blanching and subsequent freezing. In fact, blanching foods rich in beta-carotene can help improve the bioavailability of beta-carotene, making it easier for your body to absorb this important nutrient (4).

Yet another vitamin that stands out when you look at the nutrient profile of kale is vitamin K. Just one cup of fresh, uncooked kale provides over 100% of your daily need for vitamin K (1). Like beta-carotene, vitamin K is not easily destroyed by blanching and freezing (5), so also frozen kale is an excellent source of vitamin K. Even water-blanching, which can greatly reduce the amount of water-soluble nutrients such as vitamin C in foods, will not result in a huge loss of vitamin K because this fat-soluble nutrient leaches into oils, not water.

Minerals

Fresh kale is loaded with copper and manganese, and it contains appreciable amounts of calcium, magnesium and potassium. These minerals are destroyed by neither hot nor cold temperatures, but they can escape from kale leaves into the boiling water during the blanching process.

Potassium, in particular, is prone to leaching out of green leafy vegetables during blanching; however, the amounts of potassium lost during blanching are generally quite small compared to water-soluble vitamins. Plus, if you blanch and freeze kale at home, you can reduce the loss of potassium by steam-blanching the leaves instead of dropping them into boiling water. (6)

Glucosinolates

Sulforaphane is a naturally occurring isothiocyanate that may help prevent cancer through multiple mechanisms, including acting as an anti-inflammatory agent, triggering self-destruction of cancerous cells, and modulating proteins that are involved in cell division (7).

This powerful compound is formed when a glucosinolate called glucoraphanin, abundant in both curly kale and black kale (lacinato), is transformed by myrosinase, an enzyme found in Brassicas. In a whole, undamaged vegetable, myrosinase is physically separated from glucoraphanin by the plant's cell walls, but when a Brassica vegetable such as kale or broccoli is cut or chopped, myrosinase comes in contact with glucoraphanin, catalyzing its conversion into sulforaphane.

Research suggests that glucosinolates like sulforaphane are not easily destroyed by blanching and freezing. Myrosinase activity, on the other hand, is greatly reduced by blanching. (8) The good news is that myrosinase can also be produced by gut bacteria in the colon, so you may also be able to get some sulforaphane from blanched and frozen kale (9). However, studies suggest that conversion of glucosinolates into isothiocyanates and absorption of isothiocyanates are much greater following ingestion of raw Brassica vegetables than after ingestion of cooked Brassicas (10).

So, to sum up, if your goal is to get more isothiocyanates from your food, fresh, raw kale appears to be better than blanched, frozen kale.

References

1. USDA National Nutrient Database for Standard Reference, Release 28
2. J. Anderson et al (2009). Health benefits of dietary fiber. Nutrition Reviews, 67(4):188-205.
3. D. J. Favell (1998). A comparison of the vitamin C content of fresh and frozen vegetables. Food Chemistry, 62(1): 59-64.
4. K. Gul et al (2015). Chemistry, encapsulation, and health benefits of beta-carotene - A review. Cogent Food & Agriculture, 1(1).
5. S. Booth (2015). Is vitamin K lost when a food is cooked or frozen? Health and Nutrition Letter of Tuft University, July 2015 issue.
6. U.S. Department of Agriculture (2007). USDA Table of Nutrient Retention Factors, Release 6.
7. B. Mokhtari et al (2018). The role of Sulforaphane in cancer chemoprevention and health benefits: a mini-review. Journal of Cell Communication and Signaling, 12(1):91-101.
8. V. Rungapamestry et al (2008). Influence of blanching and freezing broccoli (Brassica oleracea var. italica) prior to storage and cooking on glucosinolate concentrations and myrosinase activity. European Food Research and Technology, 227(1):37-44.
9. Isothiocyanates. Micronutrient Information Center, Linus Pauling Institute, Oregon State University.
10. V. Rungapamestry et al (2007). Effect of cooking Brassica vegetables on the subsequent hydrolysis and metabolic fate of glucosinolates. Proceedings of The Nutrition Society, 66(1):69-81.