Did you know that by peeling a tomato you could lose 25% of the calcium, 32% of the magnesium and 29% of the vitamin C? (1) In fact, many fruits and vegetables contain a large proportion of their nutrients just below the surface, so peeling is not a good idea.
The dangers to vitamins are mainly heat, light and oxygen. The main danger to minerals is water. Some vitamins are less stable than others. The most unstable are folate (B9), thiamine (B1) and vitamin C. Vitamin C is therefore used in a lot of research as the benchmark for nutrient loss. More stable are niacin (B3), K, D, biotin (B7) and pantothenic acid (B5). (2)
We cannot all grow our own food and eat it within moments of harvesting. Being aware of what different processes can do will help us minimise nutrient loss.
For example, if you keep spinach at room temperature for 7 days all the vitamin C will have gone. (3)
As mentioned above, many nutrients tend to lie close to the surface so excessive trimming is not advised.
Cooking in water causes leaching of nutrients, particularly sodium, potassium and calcium. To reduce the loss, cook with as little water as possible, reduce the surface area of the food (use big chunks) and save the cooking water for soups or stews. A quick stir fry is a better option.
Let’s look at tomatoes again…
Let’s look at some storage options…
Before food is canned it is usually blanched in steam or water. The water soluble vitamins can easily be destroyed by blanching. The food is heated once canned, to kill any unwanted organisms. However, once the food has gone through this process very little further deterioration is observed, making it a useful long term storage system with no refrigeration issues. Depending on the water used for canning, mineral content, such as calcium, can actually increase. (3)
There will be some nutrient loss in the preparation for freezing, but after that nutrients remain quite stable. There will then be losses during the thawing and cooking. Commercially, freezing can happen soon after harvesting, so frozen food can actually retain more nutrients than fresh fruit and vegetables that have spent a lot of time in transport and storage. The food is blanched before freezing. This involves boiling or steaming for a short time to stop the enzyme activity that would decrease the flavour, colour and texture. Blanching time is critical. Too short and enzyme activity can actually be stimulated; too long can lead to more nutrient loss. (4, 5)
This can reduce vitamin C content. It does, however, decrease the likelihood of food spoilage.
- Use fresh ingredients whenever possible.
- Store foods carefully, keeping food cold and sealing in airtight containers.
- Remember light, heat and oxygen can damage vitamins.
- Keep vegetables in the crisper section of the refrigerator.
- Fruits are not stable for long periods of time in the refrigerator and will deteriorate rapidly.
- Try washing or scrubbing vegetables rather than peeling them.
- Use the outer leaves of things like cabbage and lettuce wherever possible.
- Cook food quickly.
- Steam, roast or grill vegetables rather than boiling them.
- Water used for cooking is the biggest danger to minerals. (If you do boil vegetables, save the water for other uses, e.g. soups.)
Got any tips or questions of your own about preserving nutrients? Leave us a comment below, we’d love to hear from you.