Does Microwave Cooking affect the Nutritients in Food?

Previously, we looked at some misconceptions surrounding the use of microwaves, specifically the cancer claim together with the often shared “phone test”.

Today’s article is a follow up to that article and so it may be worthwhile to read that first as we covered some of the basics regarding what a microwave oven is and how it works.


The Claims

A quick search online regarding this topic reveals a rather troubling fact – there seems to be very varying views.

One end claims that microwaving is no different from other cooking methods, meaning it can cause loss of nutrients, though in some situations it may even be better in retaining some of these nutrients.1, 2, 3

The other end claims microwaving food “zaps the nutrients right out of your food” and even stands to change in the food’s chemical structure, all the while providing some science to back up this statements.3, 4

This are no doubt very confusing but here's what we found.



How does a Microwave Cook Food?

microwave
Microwave Oven

So by now you should know that a Microwave oven cooks food on the principle that it contains water. The microwaves excites this water molecules and this is what generates the heat used to cook the food.

However, the heating occurs a little different from your typical cooking methods like cooking on an oven where the outside of the food is heated first and as you cook more, the inside gets cooked.

In a microwave oven, it’s often said that the food cooks from the “inside out” but that’s not entirely true. Microwaves can only penetrate food to the depth of 1 to 1½ inches (2.54 – 3.81 cm) so in much thicker foods that will not happen.

Such foods, will cook through the conduction of heat from the outside parts into the inside where the microwaves cannot reach.6 Of course, for thin foods that statement holds true and the food will usually cook evenly.

Still, you might have noticed that food from the microwave is sometimes cold in some parts. This is because by design the microwave oven has “cold and hot spots” – spots where the microwave energy is less concentrated and more concentrated respectively.

Therefore, the distribution of this energy is not uniform and that’s why a rotating turntable is employed to ensure the food passes through both spots.

This however doesn’t guarantee that the energy is distributed evenly, and this is further compounded by other factors such as the properties of the food itself (there's a good explanation on the second answer here).


Nutrients Zapped out or not?

Now onto the big question, does the microwave cause the loss of nutrients in the food? We believe the answer to that is both yes and no. Just like other methods of cooking, be it frying, grilling, boiling or baking, depending on the food in question, some nutrients will be lost.

For instance, if you overcook your greens by boiling, some water-soluble nutrients will leach into the water or be lost with the steam. Similarly, with a microwave, depending on how you use it and the type of food, some nutrients stand to be lost and in some cases retained more compared to some traditional methods of cooking.

The reason for this is that a microwave satisfies the conditions that make a cooking method ideal for retaining nutrients i.e.  
  • it cooks the food much quickly, 
  • uses a minimal amount of liquid, and 
  • exposes the food to heat for a much shorter time.

But what does the science say?



The Science

In one study that investigated the effect of microwave cooking Broccoli, it was found out that there was a general decrease in the levels of all the studied compounds except for mineral nutrients which were stable under all cooking conditions.

Vitamin C however showed the highest losses mainly due to degradation and leaching into water.7 The study goes on to recommend that to minimize losses of nutrients in the microwave, longer cooking times and high volumes of cooking water be avoided.

Another study investigating the loss of Vitamin B12 loss in foods cooked by microwaving, found out that the foods (raw beef, pork and milk) studied lost approximately 30-40% of vitamin B12 due to the degradation of vitamin B12 molecule by microwave heating.8

In a third study, four leafy vegetables were subjected to three different types of cooking - conventional, pressure cooking and microwaving. The study found out that there was no significant difference in the nutrient content due to the three different methods of cooking used.9

In a fourth study investigating the influence of cooking methods on antioxidant activity of vegetables, found out that microwave cooking along with griddling and baking alternately produced the lowest losses, while pressure-cooking and boiling lead to the greatest losses.10

In a similar study investigating the effect of different cooking methods on total phenolic contents and antioxidant activities of mushrooms, they found out that Microwaving was better at retaining total phenolics than other cooking methods (steaming, pressure-cooking, frying and boiling).11

This WebMD article says that cooking spinach with microwave retains almost all of its folic acid while boiling can make it loose up to 70%. We are also cautioned here that due to the uneven heating property in microwaves, degradation of nutrients is likely to be more in the hot spots.

We can continue quoting more studies here but the general idea one gets from this and other similar studies, is that microwaving food with regards to this question, boils down to how one uses it.



Retaining Nutrients when Cooking with the Microwave

To ensure you retain nutrients when cooking using the microwave, it's recommended:
  • To preserve Vitamin B12 use your oven to cook your meat and milk products instead of microwaving them. This also includes breast milk.
  • Microwave food for the shortest time required using little to no cooking water (vegetables especially). Do not overcook.
  • Cover the food while microwaving to retain moisture and ensure uniform heating.
  • Use a relatively low power setting that ensures the food is cooked quickly but not overheated.

So to conclude, most of the science available currently doesn’t seem to point to microwave cooking causing more nutrient loss than traditional cooking methods, and in some cases even points to the opposite - that it preserves more nutrients due to its shorter cooking times. However it recommends proper use to ensure retention of nutrients.

You can read some of the referenced articles below to find out more.

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