Raw, freshly-picked vegetables have the highest nutrient density however a plate of raw vegetables holds little appeal on a cold Irish day. Therefore, when we enjoy cooked vegetables, we must accept a loss of some nutrients as certain nutrients are sensitive to heat, light, acid and alkaline solutions or they are soluble and are therefore lost in cooking water that we throw away after cooking.
What is the best way to cook and enjoy vegetables and retain a greater amount of nutrients?
1. Steaming vegetables or microwaving at low temperature. Note: the vegetable is not immersed in water. Boiling is best avoided.
2. Cooking Times depend on the vegetable and your taste . For me, spinach takes 1 minute in my steamer, asparagus 3 minutes, broccoli 5 minutes but carrots take 10 minutes.
3. Enjoy slightly crunchy vegetables over soft, well-cooked vegetables – the longer the cooking time, the greater the nutrient loss
4. Use filtered water to cook the vegetables and retain the water to use as stock for a soup or casserole dish but only if the vegetables are organically grown as pesticides may leach into the water.
5. Add a little butter or oil to cooked green and orange vegetables to help the absorption of the fat-soluble nutrient, Vitamin A
Microwaving is better than boiling vegetables for nutrient retention
One study1 of swiss chard and green beans showed that microwaving is better than boiling or lightly frying for B1. Another study2 of fresh broccoli, cauliflower, potatoes and frozen corn and peas by boiling, steaming, microwave boiling and microwave steaming showed nutrient retention was highest in foods cooked by microwave steaming, followed by microwave boiling, followed by steaming, and then by boiling. The second study analysed vitamin C, vitamin B6, calcium, and magnesium. All other study conclusions that I read, reported no bad affect from microwaving over traditional cooking of vegetables.
What nutrients are lost by cooking?
Group 1. Vitamin C, B1, B5, B3, Folate, iodine
Group 2. D, B12, Zinc, Selenium (lost in high processing temperatures – ready meals etc best avoided)
Group 3. Some soluble salts leach out in water – potassium , calcium, magnesium. Do not add salt to cooking water if you boil your vegetables.
Why microwaves are considered by some people to be unhealthy?
My main concern with microwaving (and any form of heating food) is the effect of heat on plastics surrounding the foods. Plastics are dangerous to humans when ingested. I avoid all forms of plastics when heating foods. Remove foods from their containers and use glass, pyrex, and all microwave-safe ceramics. Avoid metal and aluminum pans and utensils when using a microwave.
The second concern is that not all areas of foods contain the same amount of water therefore microwave heating is uneven and this may lead to cold sections but more importantly, some bacteria in or on the food not being destroyed. This is more of a concern with meat products.
A third concern that needs additional research is the effects of microwaving on protein foods such as meats and eggs. A recent chemistry paper reported that microwaves cause a higher degree of “protein unfolding” than conventional heating3 This involves a structural change to food rather than a nutrient loss from food.
How do microwaves cook food?
Microwaves generate electromagnetic waves. The energy in the waves causes water in food to vibrate quickly. As the water molecules vibrate, the friction between them creates heat. The heat cooks the food.
I don’t like microwaves. I have one and use it only to re-heat soup. I find they create an unnatural heat in food. I can detect this when I eat out and get food heated in a microwave. However I can see the benefit of them for people with very busy lives.
1. Orzaez Villanueva M. T. et al. (2000) Modification of vitamins B1 and B2 by culinary processes: traditional systems and microwaves. Food Chemistry. 71:4, December 2000, Pages 417–421
2. Schnepf M.& Driskell J. (1994) Journal of Food Quality. Vol 17:2; 87-89
3. Hettiarachchi A et al. (2012) Formation of β-Lactoglobulin Nanofibrils by Microwave Heating Gives a Peptide Composition Different from Conventional Heating. Biomacromolecules.13:9; 2868–2880