Nitrates in Fresh, Frozen and Cooked Spinach

Whether fresh, frozen or cooked, spinach contains high levels of nitrates compared to most other vegetables. Nitrates have long been seen as villains because they can be converted into nitrites and further into cancer-causing nitrosamines, but recent studies suggest that nitrate-rich vegetables may also provide protection against cardiovascular and metabolic diseases (1).

Now, because of the effects of nitrates on health, you may be interested in learning how different forms of spinach (fresh, frozen, cooked) fare against each other in terms of nitrate and nitrite content. So, here's how storage conditions and cooking affect the nitrate content of spinach:

Fresh, Uncooked Spinach

Fresh spinach is known to be high in nitrates and is right there on top of the list of nitrate-rich foods, along with foods like arugula, celery, lettuce and red beets. A study that analyzed over 6000 samples of spinach found that on average, spinach contains 17 mmol of nitrate per kg (2). However, it is important to be aware that the nitrate content of fresh spinach can vary greatly depending on where it was grown, when it was grown, what kind of fertilizers were used, and other factors (3).

While undamaged, freshly harvested spinach can contain extremely high levels of nitrates, it contains hardly any nitrites. In fact, plant-based foods in general contain little to no nitrites right after harvesting. However, once the plants have been harvested, the nitrates can be converted into nitrites, either by microorganisms in the plants themselves or by enzymes in your mouth (4).

Storage temperature has been found to play a major role in how much of the nitrate in raw spinach is converted into nitrites during storage. When fresh spinach is stored at room temperature, nitrates are rapidly converted into nitrites. Cool temperatures, by contrast, have been found to slow down this conversion dramatically (4). In fact, a study published in the journal Food Additives and Contaminants found no significant changes in the nitrate and nitrite contents of spinach when stored at 5 °C (41 °F) for seven days (5).

Frozen Spinach

Conversion of nitrates into nitrites is inhibited under frozen storage (6). Indeed, a study published in the Journal of Food Science found no significant changes in the nitrate/nitrite content of spinach during frozen storage for up to 12 weeks (7). However, if frozen spinach is allowed to thaw at room temperature for an excessively long time, it can start to accumulate nitrates (6).

Now, although the nitrate/nitrite content of spinach does not appear to change much in the freezer, frozen spinach can contain significantly less nitrates than its fresh counterpart. That's because spinach is typically blanched before it is frozen, and nitrates leach out of vegetables when they are blanched in boiling water. In one study, the nitrate content of spinach decreased by 36 percent during water-blanching. Note, though, that spinach can also be blanched in a steamer, but steaming does not seem to reduce the nitrate content of spinach. (8)

Cooked Spinach

Boiling is an effective way to reduce the nitrate content of spinach. Research suggests that boiling spinach can reduce its nitrate content by more than 50 percent – as long as you discard the water in which it was cooked. If you wash your spinach before you boil it, the loss of nitrates will be even greater (8).

As mentioned earlier, steaming is not an effective way to reduce the nitrate levels in spinach. Also sautéing does not get rid of those nitrates—in fact, one study found that when spinach was sautéed, its nitrate levels increased by 25 percent. However, that increase can probably be attributed to the weight the spinach lost when it was sautéed. (8)

Now, before you rush to make any conclusions, keep in mind that many vitamins are heat-sensitive and/or water-soluble, which means that cooking will also destroy some of the health-protecting vitamins in spinach. For example, vitamin C, which can help inhibit the conversion of nitrites into harmful nitrosamines in your stomach (9), is easily destroyed by cooking.

The Bottom Line

The nitrates in green leafy vegetables like spinach and arugula are not thought to be harmful per se. However, as they can be converted into nitrites and further into carcinogenic nitrosamines, it is understandable that some people may be worried about them. If you are looking to maximize the health benefits of spinach while minimizing any potential risks related to the nitrate/nitrite content of spinach, here's what you can do:

• When buying raw spinach, select spinach that looks fresh and undamaged. Avoid wilted or yellowing leaves.
• Store fresh spinach in a refrigerator and never on the countertop at room temperature.
• As vitamin C helps inhibit the formation of harmful nitrosamines, you can try incorporating some vitamin C rich superfoods such as camu camu or acerola powder into some of your favorite spinach recipes.
• Vitamin E has been shown to work synergistically with vitamin C to inhibit nitrosamine formation, so make sure your diet is also rich in vitamin E (9).

Finally, variety is key to a healthy diet, which is why nutritionists frequently recommend that you rotate your greens and other vegetables. The practice of rotating vegetables helps ensure your diet stays balanced, and it helps reduce your risk of excessive exposure to any single nutrient, anti-nutrient or compound.

Sources

1. J. Lundberg et al (2018). Metabolic Effects of Dietary Nitrate in Health and Disease. Cell Metabolism, 28(1):9-22.
2. D. Hobbs et al (2013). The effects of dietary nitrate on blood pressure and endothelial function: a review of human intervention studies. Nutrition Research Reviews, 26: 210-222.
3. HealWithFood.org (2018). Nitrates in Spinach: Risks and Benefits. Last accessed: October 2018.
4. C. Colla et al (2018). Nitrate in fruits and vegetables. Scientia Horticulturae, 237, 221-238.
5. Chung et al (2004). Changes in nitrate and nitrite content of four vegetables during storage at refrigerated and ambient temperatures. Food Additives and Contaminants, 21(4):317-22.
6. W. Phillips (1968). Changes in the nitrate and nitrite contents of fresh and processed spinach during storage. Journal of Agricultural and Food Chemistry, 16 (1), 88-91.
7. B. Schuster and K. Lee (1987). Nitrate and nitrite methods of analysis and levels in raw carrots, processed carrots and in selected vegetables and grain products. Journal of Food Science, 52, 1632-1636.
8. K. Ekart et al (2013). Study on the influence of food processing on nitrate levels in vegetables. European Food Safety Authority.
9. D. Lathia and A. Blum (1989). Role of vitamin E as nitrite scavenger and N-nitrosamine inhibitor: a review. International Journal for Vitamin and Nutrition Research, 59(4), 430-8.