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At home when you want to deep-fry something, if you leave the heat on for a minute or two more than you should have, the oil starts to burn and produce smoke.

Why doesn't this happen at restaurants and takeaways (UK)? Is it the specialised equipment they use? Or is there more to it? I doubt they turn off the heat on their deep fryers when not in use, cause if someone walks in and orders something that needs deep-frying, it'll take a long time for the oil to get to frying temperatures again!

Any clarification would be greatly appreciated.

  • Are you deep frying in a purpose-built fryer or in a pan/pot? – Catija May 10 '16 at 14:21
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    Restaurants, generally, don't... as I think the answers show :) – Catija May 10 '16 at 14:34
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    @Ciwan Be very careful! A few years ago I was forced out of my apartment for a day because someone in the building next to mine was doing some deep frying with poor equipment and some of the oil spilled and caught on fire. It ended up burning down the entire building and it's a small miracle that no one was killed! – Mason Wheeler May 10 '16 at 15:34
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    If in doubt, assume what you are doing is dangerous. Smoking bulk oil means you are overheating it beyond safety. At least monitor it, and at the first sign of ANY smoke, remove your vessel from the burner to instantly stop the heat input! – rackandboneman May 10 '16 at 15:36
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    And (sorry to repeat myself): If you prefer doing your deep frying stovetop (I do too), use an oil that has good safety margin (peanut oil will still be well away from disaster at 190°C) and an accurate thermometer. – rackandboneman May 10 '16 at 15:41
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Restaurants have massive fans.

Commercial deep fryers have temperature control.
Example temperature control unit:

enter image description here

And massive heating elements (notice 4 temperature controls):

enter image description here

Massive heating elements allows for even delivery of heat. When you drop frozen fish it has to kick out some heat but it is careful not to get too hot via temperature control. The temperature gradient alone causes good mixing. When you heat fresh oil you can see it stirring around.

If your oil is burning and producing smoke then your deep-fryer is too hot. Even most home fryers have temperature control. Or you are possibly using the wrong oil. A cheaper home unit will have a smaller heating unit so it will have more trouble with maintaining a constant temperature but a temperature controller near the heating element should prevent any oil from burning.

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    A domestic deep fryer would also come with temperature control (would be utterly dangerous otherwise since the heat source usually cannot be easily removed from the vessel in such an appliance). A pot/pan/wok needs temperature control in the shape of a thermometer and/or experienced user. Also, if doing small batches eg in a wok, use some more resilient oil (peanut oil for example) than generic deep fryer oil. – rackandboneman May 10 '16 at 15:33
  • Has anyone ever seen one without (appliance shaped, not stovetop)? Otherwise, we should merge. – rackandboneman May 10 '16 at 15:38
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    @JohnBargman Sounds like you should stop using that fryer! – David Richerby May 10 '16 at 19:15
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    Plus one for the first line. You don't see smoke (a visible effluent) coming out of commercial establishments because there are likely ordinances against it in your area, as there are in mine. It is however more than just a big fan; there's filters and all sorts of stuff. – Mazura May 10 '16 at 23:05
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    @JohnBargman Arrange the following in increasing order of cost: (a) stopping using your current fryer; (b) stopping using your current fryer and replacing it with a new one; (c) continuing to use your current fryer, burning your house down and having to replace everything you own. Oh, wait. I already did put them in order. – David Richerby May 12 '16 at 22:10
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Restaurants have the right tools and equipment for deep frying. I would agree that they also have massive exhaust fans to keep the smoke down. They also have the right equipment to do mass quantities of food. They have a special deep fry station, where they have gallons of the correct oil and at the right temperature. Here size or quantity of oil matters. The larger the amount of oil to fry in, less thermal shock for the oil.

At home, you would probably use 1 gallon of oil max in a home style deep fryer, and do small batches. The amount of built up heat in that 1 gallon is easily changed when you put something in it to fry. It may drop and then reheat. Or if you are using a pan or pot, same thing, the temperature is fluctuating and has a tendency to burn food if too high. In the commercial kitchen there might be 10 gallons of oil, so less drop in temperature. And temperatures matter for a good outcome.

  • I would say commercial get more shock. The afternoon is slow and dinner can be basket after basket of frozen fish. More oil and bigger basket. – paparazzo May 10 '16 at 14:25
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It is a combination of a few factors.

I doubt they turn off the heat on their deep fryers when not in use

Some do it. There are small deep fryers which have the capacity for one portion of fries, and bistros and restaurants where you sit down and wait for your order to be prepared turn them off during lull times. They take some minutes to heat up, but the customers are expecting the wait anyway. Of course, fast food joints can't do this.

Is it the specialized equipment they use

Partly, yes. Your stove is engineered to output a constant amount of energy (time modulated), no matter what you put on it. Different cooking vessels with different types and amounts of food heat up at a different rate and reach a different equilibrium temperature. While a commercial deep fryer will allow you control the temperature too to some extent, the fryer has a known shape, material and volume, allowing the engineers to work with a narrower possible range of temperatures. It will only overheat in some less usual circumstances, such as letting the oil go very low while being turned up very high, but the typical setting and volume are likely to be chosen in such a way that the final temperature is optimal for deep frying and too low for overheating.

It is likely that you can reach this temperature with your stove too. From your description, it sounds like you choose a stove setting which is too high, and the oil doesn't reach an equilibrium temperature, but continues to heat up while you're frying. Also, it seems that you are using relatively little oil compared to the mass of the food, so that the thermal mass of the food cools it down and it doesn't burn while frying. Check the oil temperature with a thermometer and adjust, and you will probably find a stove setting which can hold 1-2 liters of oil at a relatively stable temperature after a sufficiently long period of preheating.

Or is there more to it?

The other part would be the fat itself. If you are reaching for your standard high linoleic oil every time you want to deep fry, it can be that its smoke point is lower than the optimal frying temperature. These places use fats specifically designed for deep frying, with rather high smoke points. So even if their temperature fluctuates between, say, 160 and 190 Celsius, it still doesn't smoke.

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    I don't agree that commercial fryers heaters are designed for a specific heat output. Every one I have used has temperature control. It is also a safety thing. If the oil got low a fixed heat could cause a fire. – paparazzo May 10 '16 at 14:14
  • OK, I have worked with commercial fryers, but never too closely, so wasn't aware that it also has a feedback mechanism. This means they have an even more complex (and better) design than I was aware of. – rumtscho May 10 '16 at 14:42
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    @Paparazzi Thank you for your observation. I still think that designing a fryer which fries without overheating when used with an average amount of oil is easier than designing a stove which will heat the oil in a random vessel to the optimal temperature. I changed my argument to reflect that, but removed the claim of fryers being designed for only one heat output. – rumtscho Aug 9 '16 at 22:13
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    Agree a fryer that does not maintain a temperature would be easier. It would fail on consistent cooking and safety. An oven is based on temperature. I don't get how easier is better. Just how would you design for "without overheating" absent monitoring the temperature. – paparazzo Aug 9 '16 at 22:25
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    @rumtscho I think serious fryers basically always have temperature control, not just a "power" setting (even home fryers do nowadays), so it doesn't seem crazy to think they'd just shut off if the temperature gets too high. But agreed, under normal use they'd definitely never reach that point. – Cascabel Aug 9 '16 at 22:38
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The answers attesting to commercial equipment thermostat control are correct. But, it's also about the oil. You didn't say what oil you use at home.

Restaurants typically use fryer oil with a smoke point in the mid-400's F. For cooking, they set the thermostat in the 375 F range, well below the smoke point. Maybe you have the wrong oil and/or using too much heat.

  • I use regular sunflower oil from Aldi/Lidl ..etc – J86 May 11 '16 at 9:42
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    Pure sunflower oil smoke point is about 440 F. You should be OK. 375 F is a good deep fry temp---you should use a thermometer and try and hit no higher than that. – Paulb May 11 '16 at 10:36
  • Does oil packaging mention anything about smoke point? – J86 May 11 '16 at 13:05
  • It might. But I used Google. First hit jonbarron.org/diet-and-nutrition/… – Paulb May 11 '16 at 14:39
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Commercial deep fryers are controlled by thermostats so that they are kept at a controlled temperature electronically and can be left on more or less indefinitely.

By contrast if you just put a pan full of oil on a hob you have some control over the heat input but there is no automated feedback mechanism to regulate the temperature and so it is quite hard to keep it at an accurate constant temperature.

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If you are properly deep frying, you should never allow the oil to get to the point it burns. Commercial fryers are temperature controlled to maintain a consistent temperature. If you are frying in a pan, it is the duty of the cook to monitor the oil and ensure that the burner setting is lowered, if you the temperature of the oil gets too high.

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Depends on how YOU do your frying, I guess, if you are wondering why they are different in their results.

If you crank the heat onto high setting, and add food when it gets hot enough, keeping that heating element or flame on a high setting will first get the oil back up to good frying temperature, but then will keep heating it.

Any kind of frying-specific equipment has a temperature setting, it won't heat beyond that particular frying temperature.

What causes an oil to smoke and break down? Different oil sources have different chemical compositions. They have different temperatures as their "smoke point" where they start to smoke and the heat breaks down their chemical structure. Oils like olive oil and canola oil have their smoke points below where one would want to deep fry french fries, for instance. Refined peanut oil has a very high smoke point and is a standard for deep frying or stir frying because of that.

While we might throw whatever we have handy into a pan or pot for frying, a restaurant is probably going to just use an oil that is more friendly to deep frying, as well.

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  1. Smoke point: different oils have different smoke points. Google for a chart. Butter has a very low smoke point, ad therefore burns easily. Use an oil with a high smoke point such as canola or peanut oil.
  2. The only way to measure oil temperature properly is with an immersion probe built specifically to measure oil temperature. Buy one.
  3. Even if the oil is below the smoke point, little bits of food and coating can be left in the oil when you are frying. That stuff will burn up right quick. That's when you need to toss or filter your oil.

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