On its journey from the farm to your table, some of our fresh foods lose up to 70% of their vital nutrients.
There are some aspects of this that, sadly, we cannot control. But there are measures you can take which go some way towards preserving your food's nutritive value:
Losses during storage of fresh produce
In the US, it's been measured that most fruit and vegetables travel up to around 4 000km from the farm to the plate. Similar conditions exist in South Africa, where fruits and vegetables grown in the Cape or Lowveld are transported all over the country.
Vitamin C (ascorbic acid) is volatile, and when exposed to the oxygen in the air, it is oxidised and destroyed. Very acid foods like tomato and orange juice are more stable, as their pH slows down this process. Nevertheless:
When processed foods are stored:
- If you store fresh vegetables, they steadily lose vitamin C (for example, green beans lose more than 90% of their vitamin C content within 16 days of being harvested and refrigerated, and broccoli loses 50% of vitamin C and beta-carotene within just five days).
- Fresh orange juice stored in cartons has massive losses of vitamin C. Interestingly, this doesn't happen if the juice is stored in glass bottles.
- Tomato juice, on the other hand, retains more vitamin C when canned, compared to juice kept in glass bottles.
Plant oils (sunflower, canola, olive oils) are our best sources of vitamins E and K. If these oils are exposed to oxygen, they lose these vitamins at an alarmingly high rate. Other factors, such as high temperatures, light, alkaline pH and the presence of certain metals (e.g. copper and iron), speed up the destruction of vitamins E and K.
Other vitamins are also light-sensitive. For example, vitamin B2 or riboflavin and vitamin B6 are extremely light-sensitive and some foods can lose up to 80% of their vitamin B2 in 12 weeks when exposed to fluorescent light. That's why it's important to store oil in a dark, cool cupboard.
Losses due to chopping
Vitamins and minerals are lost when vegetables and fruit are washed, and when they are chopped up, as the surface area that is exposed to air, light and higher temperatures is greatly increased.
If food is pureed to make instant soups or smoothies, the degradation of vitamins and minerals is even greater.
Canning and freezing
The canning process destroys twice as much vitamin C, niacin (B3) and riboflavin (B2) and more than three times the amount of thiamine (B1) as freezing. This is because canned products are exposed to high temperatures during processing.
Generally speaking, frozen foods are a more nutritious bet than most other processed foods. But foods must be frozen rapidly, stored in airtight packaging (to prevent exposure to oxygen in the air) and thawed rapidly.
Although blanching (very brief exposure to boiling water for a few seconds to inactivate enzymes that could cause spoilage) may cause slight losses, the overall nutrient retention in blanched frozen foods is higher than in foods that are not blanched before freezing.
Heating and reheating
Heating and reheating of foods (something that often occurs in industrial cafeterias, hospitals and other food-service operations) will result in high losses of vitamin C, folic acid and vitamin B6.
Although vitamin A, thiamine (B1), riboflavin (B2), and niacin are relatively stable under these circumstances, the foods served from industrial kitchens usually lack most nutrients.
This is one of the reasons why people who eat cafeteria food or stay in hospitals or old-age homes for long periods of time experience deficiencies of essential nutrients.
Microwave cooking is unfairly slandered. In fact, foods prepared correctly in the microwave, at low power, retain more nutrients than foods cooked by conventional methods. Provided you use only a small amount of water, your microwaved food will be more nutritious than food prepared on the stove or in the oven.
Text copyright: Dr I.V. van Heerden, DietDoc
13 August 2007
(Jane Rambergg & Bill H McAnalley (2002). From the farm to the kitchen table: A review of the nutrient losses in foods. Glycoscience & Nutrition, Vol 3, No 5, 1-12.)
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