why does oil temperature rise when I put food in the pan

I tried deep frying in oil using a thermometer for the first time today, my usual method is to test temp with a small amount of food.

I was using a wok filled 1/2 full with vegetable oil. I heated it until the thermometer read 350 deg F. I then put in some Somosa that were frozen. A lot of steam and bubbles were created. I lowered the gas flame and noticed that the temperature had risen to 390 degF. My instinct tells me that the oil temp was likely to be really at 390 because the Samosa cooked faster than I expected.

I tried this a few times and consistently got the same results even with non-frozen food. The temperature did not change if I stirred the oil.

Could there be something wrong with the themometer?. I calibrated it with boiling water before I started.

Have I discovered a new form of energy? :)

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I doubt you have discovered new physics in your wok :-) You haven't described in enough detail (read, excessive, obsessively compulsive, completely over the top detail) for an assessment of what exactly is happening. The only explanation that comes to mind is that the the heat of the flame, even as adjusted, is such that it continues to add energy to the oil faster than the food and air cool it, thus leading to a net increase in temperature during the cooking interval. Another good hypothesis is your thermometer is wacky :-) – SAJ14SAJ Feb 28 '13 at 17:57
What is the reaction time of your thermometer? Most kitchen thermometers seem to have a reaction time of 10 sec. It is possible that the temperature actually started falling when you threw in the food, but the display continued climbing for some time because it has a lag. – rumtscho Feb 28 '13 at 18:19
I don't understand the downvote. – Carey Gregory Feb 28 '13 at 21:32
I think I have solved the mystery. It turns out that even though the tip of thermometer was 1 inch below the surface of the oil it was not measuring the right temp. I repeated the experiment, heated the oil to indicated 350 deg, when I spooned some hot oil further up the shaft of the thermometer and the indicated temp shot up to 400 deg. Introducing cold food to the oil causes the oil to contact the shaft of the thermometer higher up resulting in a higher temp reading. – MalcL Mar 1 '13 at 3:22

It depends on the type of the thermometer and the position of the sensor inside of it. In all likely hood, you're not measuring the oil temperature but the air/steam above the surface.

When you put the food in, it produces more flow due to the water evaporating from the food and leaving the oil and registering on the sensor.

This is sometimes tricky, because some sensors are right at the tip and if you let it sink to the bottom you have yet another problem of reading the burner contact point and not the fluid.

Your best bet is to try and keep your sensor probe somewhat horizontal and almost fully submerged..

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@mandomando has it right, I was measuring the air temperature and not the oil. I have done some more experiments and observed interesting results.

I tested two analog probes - a small Taylor and a candy/fryer CDN from Sur le Table. Both showed the same results - unless the probe is at least 2 inches into the food it can read as much as 50 degF too low. Looking at the CDN web site http://www.cdnw.com/sites/default/files/English_IRXL400en_0.pdf, I found a note saying that the probe will be inaccurate unless the probe is inserted up to a small dimple in the side of the probe. On both of my probes the dimple is 2 inches from the tip.

Conclusion - be very careful when using analog probes. In my case, using a fat frying thermometer submerged just 1/2 inch below the dimple resulted in a reading 50 degF lower than indicated.

I did the same experiment on a small Taylor digital probe and found that it read correctly at all depths.

Also, I found all of the probes indicated different temperatures. I would advise anyone using them to test them in a pan of boiling water before use.

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