Are there other studies to show that microwave cooking does not kill nutrients in vegetables?

This is the second part to clarify whether microwave kills nutrients or not. If you missed part 1, please read it here

Sure, there are many studies to show that microwave oven does not kill nutrients. A more recent study carried out in Italy and published last year, 2010, shows that microwaved vegetables retain more phenolic compounds than those that are boiled.

Natella and others from National Research Institute for Food and Nutrition (INRAN) compared effect of microwaving, pressure cooking and boiling on the antioxidant and phenolic compounds in seven vegetables which are: carrot, cauliflower, pea, potato, spinach, Swiss chard and tomato.

Three cooking procedures were used: (1) boiling- 2 glasses of tap water was used to boil 250 g (equal in weight to a quarter kilogram of meat) vegetables. (2) pressure cooking- a 37.5 millilitres of tap water was used to cook the same amount of vegetables by pressure cooker and (3) no water was added to vegetables during microwave cooking (This is the correct way to microwave vegetables). They observed that all the three methods cause some loss to phenolic compounds, however, microwaving and pressure cooking is less detrimental to nutrients than boiling. To link this study with that reported by Spanish researchers, the Italians used double the weight of vegetables and yet did not add any water to micowaved vegetables.

The decrease of phenolics during cooking is caused by leaching of phenols into the cooking water. The degree of leaching depends on the cooking temperature, cooking period, and volume of cooking water. As a consequence, cooking procedures that use less water and/or a shorter time (such as microwaving and pressure cooking) can better preserve the nutrient content of vegetables.

Away from Italy, researchers in Saudi Arabia studied effect of boiling, autoclaving and microwaving to the loss of nutrients in chickenpea. Their Study established that microwave cooking is commendable for chickpea preparation, not only for improving nutritional quality but also for reducing cooking time.

Researchers from different corners of the world keep publishing papers showing that microwaving does have a destructive effect on nutrient compared to conventional cooking methods. A study carried out in Turkey in 2004 showed that boiling, steaming and microwaving had no difference on the content of phenolics and antioxidants in pepper, squash, green beans, peas, leek, broccoli and spinach.

Hamauzu, A Japanese researcher with his colleagues also showed that microwaving and boiling of Broccoli had similar effect on phenolic compounds and antioxidants. Both methods caused some losses but the losses were comparable.

Journalist point of view

It is not only scientists who have come out defending microwave oven, O’Connor; a journalist with New York Times wrote an article in October 2006 declaring that microwave does not kill nutrients. She quoted a research from Cornell University, USA, one the world’s respected University for Food Science and Nutrition. The study looked at the effects of cooking on water-soluble vitamins in vegetables and found that spinach retained nearly all its vitamin B-complex when cooked in a microwave oven, but lost about 77%  when cooked on a stove.

What cause loss of nutrients during cooking?

There are three key factors that contribute to the loss of nutrients during cooking of vegetables whether over open fire, by hot plate, gas cooker or microwave oven. If these critical factors are well controlled, losses are minimized and the cooked vegetables deliver high levels of nutrients.

Long cooking periods leads to high losses while short cooking periods are less detrimental to nutrients. Very fast cooking methods such as microwaving and pressure cooking offer an advantage of minimal loss of nutrients. On the other hand, slow cooking methods such as open fire, hot plate and gas cooker that require long periods of cooking lead to high losses of nutrients.

During the cooking, water soluble nutrients (mostly vitamins, minerals and other health promoting compounds) migrate from the food into water and eventually get lost through evaporation. High amounts of water provide a sea into which nutrients can get lost. That is why boiling and similar cooking methods that use a lot of water lead to high losses of nutrients.

High cooking temperatures destroy nutrients. Most beneficial minerals and Vitamins such as Vitamin C are sensitive to heat. They easily get lost at very high heat (temperatures). Combination high heat and long periods of cooking cause very high losses of nutrients and therefore are never recommended for cooking vegetables.

Reasons why microwaving causes minimal loss of nutrients in vegetables.

Microwave heats the food by rotating water molecules at very high speed causing them to collide and rub against each other. This leads to friction which generates heat.  Due to the heating mechanism, microwave cooks food very fast.  It is advisable not to add any water to vegetables during microwave cooking. If water has to be added, it should be very little. Combination of fast heating and water-free cooking is the reason for minimal loss of nutrients in a microwave.

Tips on cooking vegetables in a microwave oven to reduce loss of vitamins and minerals.

Here is a suggestion of how to cook vegetables with little or no loss of vitamins and minerals. After cleaning, rinse the vegetables in cold water, this leaves them with sufficient water. To make it completely water-free cooking, dry the vegetables with a kitchen towel. Place the vegetables in a covered, microwave-safe container. Do not add water (some times it is recommended to add 1 to 2 tablespoonfuls of water-this depends with the type of vegetables). Heat at the recommended power as indicated on the packaging instruction. For the vegetables bought in the open market, cook for three to four minutes or until vegetables are tender. This requires trial and error. Do not overcook.

Bottom line: Microwave cooking causes some loss of nutrients, but when compared with conventional cooking such as boiling and steaming, microwave cooked vegetables retain higher level of nutrients due to rapid heating and use of little or no water.

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