Reader, food writer and nutrition student Louise Fulton Keats was kind enough to alert me to a document from the US Department of Agriculture regarding nutrient losses from food during cooking.
It is well-known that some vitamins are fragile and begin to diminish over time, when subjected to light, and when exposed to heat as we do with cooking. Folate is fragile, as is vitamin C, but some vitamins are more sturdy, such as biotin and vitamin D which can handle a stir fry.
The United States Department of Agriculture document on the nutrient retention of fresh and cooked foods is enlightening. If you read the figures be aware that they are only an average and will depend upon whether you cooked your vegies so they retained their crispness or until they cried for mercy. The fact that the figures are given to the nearest 5% shows they are just a guide. Some of the figures are quite old too and our ability to analyse nutrients has improved (although far from perfect).
Vitamin C, thiamin and folate are the most fragile of the vitamins. We can get adequate vitamin C from a salad and a couple of fresh fruit a day as we really only need 45 mg daily (half a capsicum has about 50 mg of C). Getting our daily needs of folate is trickier as the amount needed daily, about 400 mcg, means lots of green leafies, avocadoes, bananas, Vegemite and bread (because bread-making flour is fortified with folic acid). More is required during pregnancy, so women considering a family take a folic acid supplement.
May I add that freezing vegetables is a great way to retain their nutrition. Vegetables at room temperature lose about half their vitamin C in three days. As frozen vegetables are blanched before freezing they too will lose 20% of their vitamin C, with the remaining 80% not budging even after three months of freezing.
Minerals are generally unfazed by heat. Iron stays the same whether in an ice floe or in volcanic lava. The calcium, iron, magnesium and zinc content of food will remain steady independent of the quality or cooking time of the food. Nuts will provide almost the same amount of minerals whether fresh or roasted.
It is often said that the mineral content of fresh produce is on the decline. This was refuted in a report for Food Standards Australia New Zealand, although one study doesn’t necessarily prove a point. As the report says, much depends upon the season, variety, geography and level of ripeness of fresh produce so it becomes difficult to compare a vegetable now with one of yesteryear. Despite that, the report states that any minor differences “would be very unlikely to be of dietary significance.”
There is more to food nutrition than just the essential vitamins and minerals. There are bioactive compounds, including the broad class of antioxidants, that work in our favour. The antioxidant level in stored fruit and vegetables remains fairly constant until the produce begins to spoil, after which you will plonk it in the compost bin (you do have a compost bin, don’t you?).
Many of you have already heard that the lycopene in tomatoes is more bioavailable from cooked tomatoes and that beta-carotene is more bioavailable from cooked carrots. The heat breaks down plant cell walls so they can release more of their nutrition. Cooking makes digestion more efficient for many foods, including meats and legumes. Cooking also makes food tastier, kills nasty bacteria and is an important part of defining most cultures.
What was interesting to Louise and myself were the figures on alcohol. I, and others, often parrot the view that cooking will evaporate any alcohol that you have added to the dish, such as wine or brandy. Baking a food with alcohol for 30 minutes evaporates only 60% of the alcohol. It takes over 2 hours of cooking before nearly all the alcohol has left the dish. By then, may of those fragile vitamins have probably followed suit.
What does it all mean?
Most readers are lucky in that they can buy a range of fresh, good quality food and they can cook it quickly, so the small amount of nutrient losses doesn’t adversely affect their health. It is clear that we benefit from both raw food (salad, nuts, fruit) and cooked food.
It is also clear that the person with the greatest control over the nutrient content of food is you, the consumer. Buy as fresh as possible, store to minimise nutrient loss and then eat soon after purchase as is practical. And don’t cook it to within an inch of its life.
Kevers, C., et al 2007. Journal of Agricultural & Food Chemistry 55: 8596-8603