It was a routine shopping experience in Warkworth. I was considering changing the diet of my dog, Luna, from kibble to meat. The shop manager advised us about the benefits of a more natural diet of protein for dogs, reminding us of their distant evolutionary origins as wolves. She also advised that some cooked pumpkin along with the protein would be desirable for the good health of the dog. A question arose: should we steam, roast or microwave the pumpkin? The manager asserted that because microwave ovens involve some form of radiation, it would seem likely that the vitamins and other nutrients in the pumpkin would be damaged or depleted.
The question arises: Is microwave cooking safe for dog food or (by extension) for human food? This is a question about which there is much public uncertainty. A respected source of advice for a healthier life, the Harvard Medical School, has considered these common anxieties about cooking with microwaves and has concluded that this form of cooking retains more vitamins and minerals than almost any other cooking method and advises that microwave cooking can indeed be healthy. The reassurance about the use of microwaves comes from the fact that these radiation waves have energies similar to common radio waves, but have even lower energy and so are even more innocuous.
Microwaves in ovens are designed to produce energies that are extremely selective in energizing water or moisture. The microwaves heat the water molecules and then the hot water molecules transfer their heat to the food. So, what does this mean for the nutrients in, say, pumpkin? Nutrients such as vitamin C are usually quite susceptible to heat and are also usually leachable by hot water. Both heat damage and leaching are time dependent.
The longer the heating, the greater the depletion of nutrients. Boiling pumpkin in water on a conventional stove therefore can significantly deplete the levels of vitamin C and other nutrients.
Another example is broccoli, which if boiled in water loses glucosinolate, a desirable sulphur containing a valuable anti-cancer component. Microwave steaming, using a small quantity of water, is preferable to boiling broccoli in water as the former retains more glucosinolate in the vegetable. Conventional steaming would have similar advantages, but would expose the food to nutrient leaching for a longer, or at least less controlled, period.
Another cooking method, roasting, will usually involve much higher temperatures that are likely to decompose nutrients to a far greater extent. Roasting may also cause surface carbonisation of the food, which raises other health issues.
As the Harvard Medical School report notes: The healthiest cooking method is one that cooks quickly, heats food for the shortest time, and uses as little liquid as possible. In other words microwave cooking, which can retain more minerals and nutrients than almost any other cooking method, is generally healthier.
Ralph Cooney is a Fellow of the Royal Society of NZ and former Dean of Science at The University of Auckland. He is currently working on a government science research contract and lives in Warkworth.
Professor Ralph Cooney