Why does oil transfer heat to food more gradually than water?

Question

While working on the homework for week 2 of Harvardx's Edx course, SPU27x Science & Cooking: From Haute Cuisine to Soft Matter Science, I came across this question:

The difference in the specific heats for water and oil is important for cooking. For example, oil heats faster and you are less likely to overcook food if you cook it in oil than water. Briefly explain these observations, using the scientific ideas from this week.

We are asked to write a short response to it, and after submitting it, we are given a grading rubric and asked to do a self assessment. Here is my response:

While it is true that oil heats faster than water due to its lower specific heat capacity, it is not true that food is less likely to be overcooked than water. The boiling point of oil is generally much higher than that of water (about 250-350 degrees Celsius compared to 100 degrees Celsius). This means that when cooking food in oil, it is very likely that your food will reach temperatures much higher than if they were cooked in water. Knowing that a liquid's temperature stays constant at the boiling point, food cooked in water will never go beyond 100 degrees Celsius, so you are less likely to burn food in water than if you were to cook in oil, as the reactions that result in burns do not occur at that temperature, but do occur in the boiling point of oils.

and the rubric we are given states:

A full answer contains at least two of the following ideas :

• Oil has a lower specific heat than water.
• Oil can be heated to high temperatures with relatively smaller input of energy than water.
• At a fixed temperature, oil transfers heat more gradually to food than water at the same temperature.
• In liquid form, oil can be heated to a higher temperature than water.

You may use different words to express these concepts, so judge your response based on the meaning of your answer, rather than expecting your wording to match this exactly.

So it looks like my response is quite different from the suggested answer. In particular, I am confused that the provided answer does not explicitly address whether it is true that food is less likely to overcook in oil. I am also unsure why oil transfers heat more gradually to food than does water.

I would really appreciate it if anyone could clarify my questions, and also give me some feedback on my response.

Answer 1

First, I don't think the question you are asking about is written well (your question about this question is better). We don't have the benefit of the course's context, but alone this question seems poor. The phrase "you are less likely to overcook food" is problematic because the techniques for cooking with oil and water are quite different, as would be expected based on the specific heats and boiling points. Although oil may transfer less heat over a fixed time period at the same temperature as water, it is unlikely you would have the oil and water at the same temperature. You simply don't cook with oil and water in the same way.

In the original question and answer, the term overcook seems to refer to the core temperature of the food. Your answer considers overcooking as burning, which is not the same thing. Perhaps they are thinking of the case where your steak will not get well done (overcooked) before you notice the outside is unpleasantly charred when pan frying in oil, but boiling the steak may result in a past-well done product without visual indication.

Answer 2

Overcook is kind of silly. It depends on time, temperature, and heat transfer rate.

The statement you will get higher hear transfer in water compared to oil at the same temperature is a little weak to me. Yes the oil will cool (lose temperature) faster in contact with the cooler surface of the food compared to water due to a lower specific heat. But it is a fluid and the temperature will be fairly constant. That oil molecule will be quickly replaced by another molecule. Viscosity would effect mixing rate. Even while in contact the oil molecule will receive heat from the molecules behind it. I get specific heat is a factor here but I am not convinced it is enough of a factor to be measured. I would tell them what they want to hear.

It would be easy enough to measure. Get a pot of water and a pot of oil each at 180 F. Cool two thermometers in the same glass of water. Place in pots and see which gets to 180 F first. Then reverse the two thermometers.

Answer 3

Going to pile on about the question being poorly/missleadingly worded. EDIT: OK, rewording, embarrassed for a physics degree to misrepresent specific heat.

On the specific heat part of the question, the specific heat of vegetable oils is about half of that of water. That means that it takes about twice as much energy to raise water in temperature the same number of degrees as oil, or it releases twice as much to the food to lower it the same amount. Effectively, it means that water, at the same temperature, has twice the cooking energy that oil does, so at the same temperature will cook the food faster and could be more prone to over-cooking it without further energy being added. But rarely is cooking done by using oil or water as a heat reservoir, it is done by continually adding energy to the media to try to maintain a somewhat constant temperature.

The lower specific heat of oil means that it is easier to heat or cool than water, making if more efficient at transferring energy from a heat source, or from its reserve, to another object, so it should simply cook faster than water. For perspective, in very rounded numbers, the specific heat of water is a bit over 4, we will ignore units, they are boring but it is the amount of energy required to heat a weight of water by 1 degree. Vegetable oil, is about 2. Copper, supper efficient, is 0.385. The smaller the number, the better it is at transferring heat.

High temperature oil frying I would not concede is any less likely to over cook food unless poor heating elements and a small reservoir is used which cannot maintain the temperature. It will tend however to overcook in a different manner, burning the outside while retaining some moister, while overcooking in water will simply dry out meats. But I will have to disagree with the basic contention of the question.

Answer 4

I think the problem is that you are comparing frying, which uses higher temperatures to boiling or simmering, which uses lower temperatures, because of the vastly lower boil point of water.

If this wants to be answered in a cooking context, one might compare time needed to cook meat using a confit method vs poaching or simmering, since both cooking liquids would be slightly under 100 C, and an apples to apples comparison can be made, thus dealing with the "heat transfer at a fixed temperature" part of it.

Yeah, I'm not sure that the "less likely to be overcooked" thing is necessarily true, because, unless you are doing a confit, the temperature one uses for water vs oil is rarely "fixed" when you look at both.

Answer 5

Specific heat capacity, on its own, tells you very little about the heat transfer properties of a particular medium as thermal conductivity adn viscosity amongst other things will all play a part.

Perhaps the most significant distinction is that boiling water will be at a pretty much fixed and stable temperature ie around 100 degrees C and unless it is in a sealed container (ie a pressure cooker) it physically can't get any hotter than that regardless of how much heat you put into it. But this is nothing to do with its specific heat capacity. On the other had oil is likely to catch fire before it reaches its boiling point but will reach a much higher temperature (depending on the oil used) and so is much better for rapid, high temperature cooking.

So if you cook things in boiling water it is indeed pretty much impossible to burn them (unless the pot boils dry) , but it is still entirely possible to overcook them.

Having said that this isn't the most important reason for choosing either oil and you are much more likely to be interested in how the cooking medium interacts with the food itself.

Answer 6

• Oil has a lower specific heat than water.
• Oil can be heated to high temperatures with relatively smaller input of energy than water.
• At a fixed temperature, oil transfers heat more gradually to food than water at the same temperature.
• In liquid form, oil can be heated to a higher temperature than water.

You're missing the primary factor: psychological. YOU are less likely to overcook the food.

"At a fixed temperature, oil transfers heat more gradually to food than water at the same temperature." is a thoroughly wrong assertion. First, with lower specific heat it will cool down faster, and then with higher temperature it will transfer its heat more rapidly. And maintaining that fixed temperature is much harder, as unlike water, it doesn't stop at 100C.

Oil heats to a higher temperature in normal use. It transfers heat from the stove heater to food much faster, higher temperature cooks the food much faster, its lower specific heat means the transfer is faster, and you use much less of it than water, so again, faster heat.

These would all be factors that would allow overcooking the food really, really fast.

So to prevent it, you watch your skillet, keep stirring, observe it at all times, remove it from heat as soon as it's cooked - pay close attention not to overcook your food throughout the whole process.

In case of boiling, you leave the pot on the stove and go do something else while the food simmers. And you're far more prone to miss the moment it's cooked, and let it overcook, simply because the process takes way too long to be controlled constantly.

While all physical factors - like temperature limit, heat penetration, heat distribution and so on, talk in favor of water when it comes to overcooking, precisely because frying is so prone to overcooking that it requires constant attention, you're less likely to overcook something with oil.

Answer 7

Im not an expert chemist so Im taking a wild guess here: Could it be about "oil and water don't mix", a.k.a solubility, and waters ability to better penetrate the surface of the food we want to cook? I would guess most food contains a substantial amount of water as compared to the amount of oil/grease/fat and that the oil cooks the food only at the surface while the water cooks the food all the way through?