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Whenever I use the oven to prepare frozen French fries, the instructions tell me to pre-heat the oven to X degrees, and then put the fully frozen French fries inside when it's ready.

But it's often a waste of time to stand around and wait for it to slowly get ready, so I tend to put them in before it's shown with the light indicator that it's at the right temperature. Also, I take out the French fries from the freezer at the same time as I start the oven, and spread them out on a thing, so they are ready to go into the oven. Maybe this make them thaw a little bit.

Since the times specified on the package never seem to be accurate at all, it doesn't seem like it matters. But maybe it does?

Why should I wait until the very last minute to take out the frozen French fries from the freezer, and put them into the oven only when it's fully reached the temperature specified on the package?

Is there something about doing it in my "impatient" way which fundamentally ruins the food? Or have they simply timed it "directly out of the freezer, immediately at this degree" at the French fries research lab?

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    For the general case, we already have this question, cooking.stackexchange.com/questions/10243. Since the OP mentioned French fries, I thought that we may make the question about them specifically, instead of closing outright as a duplicate.
    – rumtscho
    Feb 6, 2023 at 10:01
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    Have you had negative experiences from doing it this way?
    – RedSonja
    Feb 6, 2023 at 15:17
  • @RedSonja Hard to tell. Not really, I suppose...
    – A. E.
    Feb 6, 2023 at 15:18
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    If your oven has a preheat setting and you use it, don't put food in until that cycle has completed. Don't ask me how I know. OK, spoiler: the oven uses a higher temperature (or uses more elements or the broiler element) to shorten the preheat time. Feb 6, 2023 at 17:57
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    If you have already tested it works, the question "am I ruining the food" makes no sense. Assuming you know what fries are supposed to be like.
    – Mr. Boy
    Feb 7, 2023 at 16:54

6 Answers 6

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For fairly forgiving things like french fries, I've never found it makes a difference. Typically I turn the oven on, get the chips (which is what we call them here) out of the freezer, and the baking sheet out of the cupboard, put the chips on the sheet, put the rest away, then put them in the oven. Maybe 2 minutes of preheating. They take maybe 2 minutes longer. This is with a modern electric oven, that reaches the set temperature in about 10 minutes. It may use the grill element to start preheating, but it's not a fan oven.

It saves a little energy, but not much, as for short cooking the majority of the energy is in preheating.

Most cook-from-frozen things are probably OK this way, as they have to defrost and cook, and defrosting will start nicely at lower temperatures. I wouldn't do this for baking, or for anything thick, so not pizza (even pre-made frozen), and even chunky potato wedges are marginal (I'd be cooking those from raw room temperature potatoes).

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    Something I quite often do is cook oven chios for dinner while preheating the oven thoroughly to bake bread - that's what I did tonight
    – Chris H
    Feb 7, 2023 at 20:35
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    I don't preheat for frozen pizza. I just put in the maximum time from the box. It comes out right. My oven heats fast.
    – Joshua
    Feb 8, 2023 at 17:56
  • @Joshua mine is pretty quick, but I generally make my pizza from scratch (occasionally buying a chilled ready-made one). I think I've cooked less than 1 frozen pizza per year for the last 5 years! So I haven't tested enough to be comfortable doing it.
    – Chris H
    Feb 8, 2023 at 20:30
  • I use a frozen pizza as a base for adding a whole bunch of other toppings, but even then my gas oven doesn't always finish preheating before I've popped in the pizza. I normally add a couple minutes to offset how it's not at full temperature, and I've never had a problem with it coming out "badly" cooked. If one of my pizzas isn't fully cooked, it's because of the amount of extra toppings I've added, rather than not letting it fully preheat. It doesn't seem to matter if it's thin, thick, or rising crust, either. Feb 8, 2023 at 22:48
  • @ChrisH: I happen to know how to tell if a pizza is done by looking so if I undershoot no biggy. I just set two more minutes on timer.
    – Joshua
    Feb 9, 2023 at 1:36
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The primary reason that instructions on the packaging almost always tell you to pre-heat is that different ovens heat up differently, so both to the people who write the instructions, and those following it, counting from the time the oven has reached a certain heat is much easier - it's always the same time. If you know your oven, you can estimate how fast it'll heat up, and depending on that add a couple minutes to the baking time. (there's also some more complications like how it heats up, with upper and lower heating elements, etc. that depend on your oven type, I'll gloss over those)

There are some foods where putting it into a hot oven is crucial because you want the outer crust to harden quickly, e.g. when baking bread. Most frozen convenience food is forgiving and I've been putting it in while the oven is heating for many years and never noticed a difference.

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I think there are several reasons, but it all comes down to getting a consistent result across different ovens.

For example - my oven uses the broiler element to preheat. Warm up is very quick, which is great, but anything in there during will be burnt to a crisp.

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  • Odd nobody else mentioned this. Most North American ovens use the top element to preheat which will, as you say, destroy any food in the oven. It's been years since I lived in Europe, can't remember if it was the case there.
    – miken32
    Apr 14, 2023 at 23:03
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Bacteria, the main reason why we want to keep our frozen foods frozen, can double in volume in just 20 minutes in an environment between 40°F - 140°F (5°C - 60°C). As such, the theory is that you don't want your products in an apparatus that is still heating up, as that maximizes time in this "danger zone." Also why you don't want to pull out your food and thaw it ahead of time in most cases. Most packages have these kinds of instructions probably as a defense against food poisonings from a legal perspective.

That said, many smaller products go from frozen to fully cooked in under 10 minutes, especially if you have a smaller convection oven or air fryer, which can often reach cooking temperatures in as little as a minute. In fact, in my air fryer, if I leave in French fries for the package's minimum time, they come out burnt and crispy, I have to actually subtract time and not preheat. Knowing how quickly your cooking apparatus heats up and how long it takes to fully cook food is something you'll learn the more you experiment with it.

Consider how long it takes your oven to reach full temperature. If it takes more than 10 minutes, definitely consider some preheating. If your oven reaches temperature quickly, preheating is optional. Also, observe how long it takes for your food to be done over time. Experiment with cooking times and temperatures until you find a comfortable mix. Also, when cooking with someone else's cookware, you should aim on the higher end for food safety reasons. Once you're more familiar with the oven/air fryer, you can adjust as necessary.

Having some sort of temperature probe available can also be handy. I have a portable one that I can stick inside whatever, the main compartment sits outside with a thin wire that goes inside, the needle goes in the food. It works well, and has resulted in much better cooking than I'd ever have without, especially meats. One should definitely take food safety seriously, as bad food can be literally fatal, but the instructions on the packaging are written by lawyers for the manufacturer's legal safety. With some experimentation and measurements, you can find what the minimum safety is for your own cooking, which varies with altitude, temperature, and cooking apparatus.

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    I'm assuming the the packaged french fries are industry made within a clean environment, so there should not be much bacteria to double to begin with. Feb 8, 2023 at 15:04
  • @ViktorMellgren Don't know about your part of the world, but here in the States it is not unusual to get food recalls due to Listeria, etc.
    – JS.
    Feb 8, 2023 at 19:29
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Timing is usually just an indication as it will depend on the amount of food you want to cook at the same time, the size of the oven and other oven characteristics.

For this reason a lot of frozen food that is supposed to be cooked from frozen will usually, in my experience, indicate the timing with "cook X mins or until Y", with Y being a qualitative description like "golden brown" or "crispy".

I also never pre-heat for a significant amount of time, usually turning on the oven at the same time as getting the tray ready, and I add about 5 mins to the time given on the package and then judge the result at the end, adding time if the crispiness isn't enough, it usually only happens with frozen fries.

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It mostly depends on how long it takes for your oven to reach the correct temperature. I’ve found that pre-heating isn’t usually necessary, in most cases, with my oven. However, it’s important to remember that, just like with small children, dogs, or even humans, no two ovens are exactly alike.

Some ovens tend to heat up quickly, and others take a very long time to get warm. Some can cook delicious food extremely fast, to perfection, while some can’t make anything edible, no matter how slowly their temperatures rise. Food that can be cooked without being pre-heated in my oven may very well need to be pre-heated, if, instead, it was cooked in your inferior oven.

The importance of pre-heating your oven, before cooking those French fries you have, unfortunately, is simply impossible for me to say, as, sadly, I haven’t yet inserted my ingredients into your smoking hot appliance, or felt the touch of your frozen French fries in my cold, moist hands.

The only real advice I can give you, at this time, is just that you try not to get too discouraged, and continue to show your oven the love and kindness it deserves. Though it may not be as good as other people’s ovens, like mine, try not to think of it as a competition.

No matter how pathetic and stupid of an oven you have, it’s still your oven, and, just like the old song says, “every oven’s beautiful in it’s on way.” Never forget that. God bless 🙏🏿 🍟

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