Testing hotspots in your oven – using kitchen science

Every avid baker or cook knows their oven almost instinctively, learning its eccentricities over time. These foibles include uneven temperatures and, despite modern fan technologies, when an oven has aged a little it is likely to develop hot and cold zones rather than uniform temperature.

It is dificult to know exactly what these hot or cool spots are doing or where they are in your oven unless you test. I have a fun, kitchen science way of testing for hotspots. You might even want to get your children to help you (though of course be careful as it does involve a hot oven and hot sugar).

Once you know which areas in your oven might be cooler and which might be hotter, you can use this to your advantage. Or, you might find out that your oven is perfect throughout (although the chances are slim!).

Good cooks know that ovens fluctuate and informed recipes might tell you to put the tray on the top, middle or bottom shelf making use of the fact that heat rises, for example. This isn’t something that’s just confimed to non-fan ovens, although it is more profound without a fan.  As someone who uses their oven a lot, you’ll be instinctively turning bakes round mid-way through cooking time or knowing to place roasting potatoes on the top rack so they crisp up nicely as you’ve got to know how yours work a little – but testing it will give you more knowledge.

Temperature variations can seriously affect your food. This is especially important in baking, where chemical reactions caused by certain temperatures are required. It’s less important for ‘normal’ cooking as you can just leave your roast or casserole in a bit longer. For example, incorrect temperatures or hot or cold spots can seriously affect the rise of a cake.

Which? has a fantastic online advice guide on ‘Why oven temperature matters‘ which delves in to the effect of even slight variations of temperature on the finished quality of cakes. Hot and cold spots in your oven can affect your bake as dramatically as setting the temperature incorrectly or will cause lopsided or irregular finished products.

There are a number of free online resources and advice guides on the Which?  website that cooks and bakers might be interested in as well as this one, such what all the attachments do on your stand mixer. (Do search by appliance type, then look for the advice guides on that page).

effect-of-oven-temperature-on-baking-455659
Image (used with permission) from the Which? advice guide ‘Why oven temperature matters’

What about oven thermometers?

Oven thermometers may seem like a good idea – but they are only measuring the point in the oven where you have placed them. This could be a hot spot, it could be a cold spot or perfect – do you know which? A guage is not indicative of what’s going on everywhere in your oven, unless it your oven is definitely a constant temperature throughout (and if it is I wonder whether you might need a gauge?).

The most likely area for a cold spot to appear is close to the door, because some heat may leak from the seal (especially in older ovens) and it’s furthest from the heating element. And this is where guages are placed so they can be read through the glass…

Understanding the test

This whole test is based around the fact that the melting point of sucrose is 186°C  / 367°F (actually sugar does not ‘melt’ but rather decomposes – but that’s for another time!).

Also, some sugars have a subtly different chemical formulation which will affect this melting point a little, but as this is not a lab-worthy experiment so this will matter little to us. Do try to use normal white table sugar, but it doesn’t really matter too much.

As the melting point is 186°C and oven temperatures go in 5°C intervals, this is how we will approach the test.

A note about Farhenheit. Fahrenheit is less easy to get close to the melt point of sugar because it jumps on your dial in larger increments – usally in 25°F jumps. 180°C can be mapped to 350°F and 190°C as 375°F. This means it will be very difficult for a user of a Fahrenheit oven to get the equivalent of 185°C close to sugar melting point, which should be 162.5°F. If your oven goes incrementally you may not be able to do this at all. If your dial moves smoothly try to get it halfway between 250°F and 375°F 0 I cannot guarantee this will work at all as I do not have access to a Fahrenheit model to test this).

First, we heat the oven to 180°C  – if anything melts at this stage your oven is very hot. We should expect to find the sugar intact (although you may find some of the sugars are starting to turn)

Then we heat to 185°C – you may find some (or all) of the sugars melting or melted. This indicates a slightly hot area but it’s only just shy of the proper temperature, so this is fine

Next, heat to 190°C – all the sugar should be melted by now. Any sugar that isn’t a little pool of caramel indicates a cool spot in your oven. I’d expect to see it either completely melted or starting to change (cool spot). Any sugars that are still crystalline indicate a very cold spot indeed

What you will need:

  • Your oven to be cold when you start (don’t pre-heat)
  • Empty your oven of all baking trays etc, just leave its wire racks in place
  • Kitchen foil
  • Granulated white sugar
  • Pen and paper to capture your results (use the PDF print out below if you like)

How to do the test

  1. Cut up ten 10cm squares from the foil and make a little cup with each of them. The easiest way to do this is press each square into a small dish.foil_Fotor
  2. Draw yourself a little oven map such or use this PDF print out I’ve created:

DOWNLOAD and PRINT OUT Oven Temperature Map Slide1

  1. I found it easiest to jot down the temperature when the sugar melted, then mark those that melted early (ie at 180°C) with red and those that melted late (ie 190°C) with blue.
  2. Place five foil dishes on each wire rack in your oven – one in each corner and one in the middle on both racks. This will give you a good reading (ie corners and the middle of both top and bottom shelves)
  3. Close the oven door and turn the oven on to 180°C – be sure to use your fan option.
  4. When the oven temperature is achieved, give it a minute then open and check the little foil pots. Have any melted or begun to melt? Jot down on your oven map which ones have changed. If sugar has melted (or begun to melt) at any points in the oven, these are hotspots.
  5. Close the door again and turn the temperature up to 185°C
foilmelted
Foil pots – the one at the top has melted sugar, the one in the foregraound is just starting to change (note it’s not white anymore and the crystals are less apparent)
  1. Give it a minute after it reaches temperature and check the post again. Note down which ones are melted or melting. This is close to being perfect temperature
  2. Close the door again and turn it up to 190°C
  3. All the sugar should be melted by this stage, ideally. Any that are still crystalline are very cold spots, and that are on their way to melting aren’t quite as bad but you still need to be aware of them.

What now?

Now that you are armed with this basic map of your oven and hot and cold spots, you can use it to your advantage. Place cakes in the most stable regions: an area with no cold or hot spots. Place food that needs the Maillard browning reaction in a hotter area. Open the door and ‘waft’ the oven with a tea towel half way through cooking (never to be done with cakes or anything that will sag!) to distribute the heat and help the work of the fan. Is it cold near your door? Check your seals are working and whether you can replace them.

I hope this helps you. I’d be so interested to hear if you try this and discover somethign about your oven. For me, I always assumed my oven was running very hot but it’s fine throughout, except at the very front on both racks where the fan clearly isn’t strong enough to push the heat round (or I have a bad door seal that needs looking at) as it’s cold towards the door.

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