Can you use a microwave to heat oils?

I’ve been asked about this on a number of occasions and having never really used a microwave to do this job myself I’ve always given a well thought out but personally un-tested response along the lines of:

‘well microwave ovens have been designed to take advantage of the vibration of water molecules. Oils contain very little water and therefore require a lot of energy to heat up this way.  That makes it an inefficient method at best and at worst it is dangerous as the container tends to heat up quicker than the oil and can lead to burns’.

But today something different happened.

The question came up in a forum I have joined and the question wasn’t directed at me so I had no obligation to answer. However,  I watched as half the people came back with a ‘yes of course, I always use the microwave’ and half with ‘no, microwaves are not good for that’.   Some in the not good camp did furnish their answers with a little bit of ‘microwaves kill your DNA’ but that’s another story.  What I had become fascinated with was the idea that everything I thought I knew about microwaves might indeed be wrong! With that on my mind I set about to find out some facts.

My original answer that microwaves heat water turns out to be correct.  Microwaves work best at heating polar molecules and water is that, oils are generally not polar (they are non-ionic) and so don’t convert the electromagnetic radiation to heat as effectively as water does.  This does indeed lead to them not heating up so efficiently.

But oils do heat up.  I tried it and as much as I didn’t want them too (as if they didn’t heat it would prove me, the genius right) they got warm. 

And that confused me.

So the oils MUST be able to convert the microwave radiation to heat somehow.  What is going on?

I remembered a concept called ‘specific heat capacity’  from my uni days and decided to check that out.

Specific Heat Capacity is the amount of energy needed to raise the temperature of a substance.

It turns out that water has a pretty high Heat Capacity at 4.185  (Kj/Kg.K)

Glass has a heat capacity of 0.67 (Pyrex 0.753)

In practical terms this means that water takes over 6 times as much energy to heat it 1C than glass does!

What this means in our microwave scenario is that the glass container holding your microwave cup of water will get very, very hot much quicker than the water. This may not be great in our microwave example as because the waves are coming at the glass from all angles it will get hot all over and not just where the water is, this might make it very hard to handle.  When we heat water in a glass on a hot plate the heat is coming from the bottom up so the top stays cooler for longer.

So what about oils?

I found a paper that said Olive oil has a Heat Capacity of 1.970 Kj/ Kg.K,  Linseed Oil has a capacity of 1.84, g mixed vegetable oil 1.67, Sesame 1.63, Soybean 1.97.

So these oils take less energy to heat than water (they don’t store as much energy as heat) so it can’t be that there just isn’t enough energy produced in a microwave to heat them.  Clearly if that was the case water wouldn’t heat.

AND it is true that microwaves vibrate water molecules which is how water eventually heats.

And we do know that while oils aren’t made of water they are not anhydrous – some vegetable oils and butters contain up to 15% moisture (fresh pressed Shea) although it is more usual for it them to contain less than 1%. Could THAT be heating the oil?

But then I had another idea…..

Is the heat transferring from the container to the oil?

Just like I said above the glass water container gets hot when you heat water in the microwave.  It also gets hot when you try to heat oils but this time it gets VERY hot.  I developed a theory that the microwave energy is heating the container and then the container is heating the non-polar oil.  Indeed this radiant heat is a pretty normal cooking method so why couldn’t that work….

I tried putting oil on paper and it took ages to get warm – much longer than when I had it in a glass – I’ll do a full experiment in our lab once I get a new microwave as this would be good for my chemistry classes.

I then put oil on a piece of balsa wood – why?  Because balsa wood has a specific heat capacity of 2.9 which means it takes nearly 4 1/2 times more energy to heat that.  The oil took ages to heat – again I need to do this properly but you can see that it looks to be worth the time investment……

But does exploring this theory help us to answer the original question?

Well I think it does in part.  Basically you can heat oils in the microwave but keep in mind that it is likely the heat is being transferred from your container to the oil and the oil is heating that way.  With that in mind it is an inefficient and potentially dangerous way to heat an oil when compared to putting it in a double boiler (Baine Marie).

And is that all we should be concerned about?

Actually no, during my research I found a paper published in 2003 called ‘Changes occurring in vegetable oils composition due to microwave heating’ by the National Research Centre, Fats and Oils Department, Dokki, Cairo Egypt.

This paper found that oils heated in the microwave changed chemically.  The tocopheryl content of the oils reduced, colour changed and peroxide values increased (indicating a degradation of the oils).  The oils were microwaved for 18 minutes and sampled at 2 minute intervals.  During this test temperature rose dramatically – at 2 minutes the temperature was  at 120C and by 18 minutes the temperature was up to 227C. However, I believe that it is the temperature rather than the heating method that is to blame for this oil breakdown!

During cosmetic manufacture we are careful not to take our oil phase above 100C and more usually to 70-80C.  This is a) because economically it makes no sense to heat things past their maximum melting points – heat is energy and energy costs money,  b) most cosmetic factories heat stuff using water power and water boils at 100C unless it is under pressure – most cosmetic manufacturers don’t utilise high pressure and C) heating oils excessively changes their chemistry and facilitates their degradation.

So what does that mean for our ‘can you use a microwave to heat oils’ question?

Well, if a microwave can heat an oil  to 120C in 2 minutes then it is too powerful for the job in hand and has the potential to break the oils down, taking away the natural antioxidants and ‘goodness’ that we are paying for.  That seems a bit silly.

Further, it is likely that the container will heat up so much that it will be difficult to handle.

Finally microwaves are just another thing in the lab to keep clean.

For completions sake I am still wanting to do a few more experiments but I’m happy enough with what I have found out to conclude that I for one will not be heating my oils in the microwave anytime soon.  I just don’t trust myself not too burn me, the or both of us.

Amanda x