How to preserve the nutrients in vegetables

While vegetables provide starches, sugars and proteins, their main contributions are vitamins, minerals, fibre and protective phytochemicals. Their nutrient content, colour and texture are affected by the method of preparation, the length of cooking time and the volume of water used. Here are some tips for the proper methods of preserving nutrients in vegetables.

• The yellow carotene pigments are not water-soluble and are well preserved in cooking, but vitamin C and the B vitamins leach into the cooking liquid.
• Vitamin C is also destroyed on exposure to oxygen. In addition, up to 20 percent of the vitamin C in a vegetable may be lost during each minute that it takes the water to heat from cold to boiling. This is because an enzyme that destroys vitamin C becomes more active as temperature rises; however, it stops its destructive action at the boiling point.
• Vegetables should be added to water that is already boiling.
• Steaming or cooking in a small amount of water retains more than twice as much vitamin C as boiling does.
• Some cooks blanch vegetables such as beans and broccoli in boiling water for a minute or two and then plunge them into cold water to hold the colour. This is satisfactory for vegetables that are served cold, but if they are served hot, they require rapid reheating, with further loss of nutrients.
• To preserve the betacyanin in beets, avoid boiling them in water — it's best to roast, bake or microwave whole beets in their skins. Peeled or cut-up beets leach the vegetable's pigments (and thus the betacyanin is lost). In addition, boiling in water depletes the folate in beets, which is water-soluble.

Because harvested vegetables lose flavour, sweetness and texture as they use up their own food stores, the less time in storage, the better.

• Corn and peas can lose up to 40 percent of their sugar if kept at room temperature for just six hours after picking.
• Beans and stem vegetables, such as broccoli and asparagus, are known to become tough.
• Vegetables that originated in warm climates (including beans, eggplants, peppers, okra, squash and tomatoes) keep best at 10°C (50°F).
• Potatoes convert their starch to sugar below 4°C (40°F); keep them cool and out of the light to prevent the formation of poisonous alkaloids.
• Most other vegetables keep best at 0°C (32°F). The salts and sugars in their sap prevent them from freezing until several degrees colder.
• Tomatoes should not be refrigerated: the cold temperature ruins the flavour. They are best stored on the kitchen counter and used within a few days.
• Potatoes, squash and sweet potatoes are best stored in a cool, dark place, not in the fridge.
• Greens should be washed, drained, wrapped in paper or cloth towels and stored in a tightly sealed container in the fridge. If bought in airtight packaging, store as is.
• Peppers should be stored in the refrigerator, away from the fruits.

Most vegetables are safe to eat either raw or cooked.

• Lima and kidney beans and other legumes contain toxic substances that are inactivated through cooking.
• Broccoli, kale and other cruciferous vegetables harbour goitrogenic compounds that can interfere with iodine metabolism. Cooking inactivates these compounds, but eating large amounts of these vegetables raw may worsen a pre-existing thyroid condition.
• Most vegetables do not provoke allergies, but some people react to members of the nightshade family, which includes eggplants and tomatoes. Corn is another common allergen.