How do they cool ready meals despite the rule of "do not reheat"?
March 24, 2014 1:38 PM   Subscribe

Conventional wisdom teaches us not to re-heat food. (Many do anyway, but that's another story). Many prepared meals are cooked (i.e. heated up) and then chilled and put into packets ready to be reheated (by oven or microwave). These ready meals often also say "do not reheat" on them. Some of them go into more detail with a curiosity-inducing sentence: "This ready meal has been cooled using a specialised process and should not be re-heated after home heating". My question is... what is this specialised process - why is it OK for them to cook and cool food ready to be re-heated, but not me! Is it just a sterile environment, or something more fascinating?
posted by cdenman to Food & Drink (16 answers total) 1 user marked this as a favorite
What conventional wisdom says never to reheat?

You're not supposed to keep heating and cooling and reheating and re-chilling and reheating forever (because every time you do that, you invite bacteria to move in and get breeding), but it's perfectly OK to reheat food one time and consume all of it.

I think the warnings are because the meals are designed to be consumed as one serving, and the company doesn't want the liability of someone getting sick because they ate the turkey on Monday, the corn on Tuesday, the potatoes on Wednesday, and the peas on Thursday, reheating and thawing repeatedly over the course of a long time.

My guess is that the meals are flash-frozen at the point of manufacture. This is a safer way to chill hot foods.
posted by Sara C. at 1:42 PM on March 24, 2014 [6 favorites]

Where are you seeing these messages? I have never seen them, and I also disagree with your premise that conventional wisdom tells us not to re-heat food.

Conventional wisdom tells us that some things taste/look okay after reheating, and some others don't do as well. Conventional wisdom tells us that the way in which you reheat food, the type of heat you use (say, conventional oven vs. stovetop vs. microwave) will make a big difference. And conventional wisdom tells us that the way you package the food to be reheated -- in a covered pot, for example, or wrapped in foil, or not, will have an impact on the quality of the final product.

Otherwise, I can only surmise that the packaging you refer to is stating not to reheat that food in that container more than once -- perhaps, a boil-in-bag type of situation where the bag is not meant to be functional more than once, or after it has been opened?
posted by BlahLaLa at 1:46 PM on March 24, 2014 [4 favorites]

I'm wondering where this wisdom is conventional? I mean, leftovers?

A lot of packaged foods have "do not reheat" or "Best by Date" on them so that you'll eat whatever is in the bag at it's optimal taste. It doesn't mean you can't reheat it, it just means it'll probably be gross tasting if you do.
posted by Ruthless Bunny at 1:46 PM on March 24, 2014 [2 favorites]

I suspect the specialized process is flash freezing and you don't have the equipment to do that in your home, which is why it's "not ok" for you to do it.
posted by disaster77 at 1:59 PM on March 24, 2014

I've never been taught to not reheat food that has been cooked. I have worked in professional kitchens. And I've eaten reheated leftovers for decades.

We were taught (in those kitchens) to cool cooked food as quickly as possible; if it was something that was supposed to be eaten hot, it could then be reheated to that temperature safely assuming there were no issues with storing it between cooking and reheating.
posted by rtha at 2:02 PM on March 24, 2014 [4 favorites]

I have actually toured plants where they make ready meals. I think this is a product of two things.

(1) Most food production plants (or at least the ones I've toured) use flash-freezing methods. TV dinner plants have huge super-cool freezers and the production lines pass the ready meals through these freezers to flash-freeze. Your conventional freezer can't freeze the leftovers as quickly, which means larger ice crystals form during the freezing process.

(2) Ready meals are designed to be at their highest and best possible use after one reheating. Two reheatings mean that the meal is sub-par in taste and texture.

At least some food warnings (e.g. some expiration dates) are as much about ensuring that the consumer's experience of the food is good as they are about food safety. BigFood wants to make sure that you enjoy your ready meal. If reheating it makes it bland and weirdly textured, well, they don't want you to do that.

I'm pretty sure you're fine on the food safety front as long as you following the USDA guidelines or similar. But your leftovers may taste funky.
posted by pie ninja at 2:02 PM on March 24, 2014 [1 favorite]

The usual concern about food contamination is microorganisms (mostly bacteria) and their waste products. Extreme temperatures (hot and cold) are inimical to life which means they usually kill contaminating organisms, but won't dispose of their corpses or waste products, which may themselves contribute towards unhealthiness or off flavors.

Now, nothing is 100% contaminant free, but food is pretty safe as long as any microorganism contaminant is of small quantity and limited duration, so it hasn't had time to muck up your nice good food. This basically means that as long as a product has spent almost all of its existence in a microorganism-hostile state, it's safe. Ready-meal manuacturers achieve this by taking piping-hot food and freezing it very, very quickly.

The problem is that home preparation doesn't actually achieve this goal. You cook your food at a nice hot temperature, eat some of it, and then put the leftovers in the freezer, where it gradually cools to a point where it kills beasties which have established a toehold in the food. And while a few hours' worth of waste produced by a tiny bacterial collective might be a small enough risk for me or you to consume without a qualm, it's not quite enough to make a major corporations' lawyers sleep well at night, so they tell you not to do it.
posted by jackbishop at 2:03 PM on March 24, 2014 [1 favorite]

Here's a PDF on reheating and cooling food, from Food Standards Australia New Zealand, a bi-national Government agency. The take-away from this is that slow heating or cooling invites bacteria to multiply, especially in certain items (meats, dairy, and other protein-dense foods).
posted by filthy light thief at 2:03 PM on March 24, 2014 [1 favorite]

I always thought that instruction referred to the packaging in which the food is originall heated, which is only guaranteed to function as expected once.

This way, you can't sue them for burning yourself when it falls apart or dissolves into your food.
posted by Lyn Never at 2:13 PM on March 24, 2014 [8 favorites]

Agree with Lyn Never -- I read it as "don't reheat in this package," not "don't reheat this food." I reheat things all the time and I'm living to tell the tale.
posted by jabes at 2:15 PM on March 24, 2014 [3 favorites]

Your conventional freezer can't freeze the leftovers as quickly, which means larger ice crystals form during the freezing process.

Aside from the food safety concerns, this is really the big one for refreezing and reheating food. Household fridges aren't fast enough and don't go low enough to avoid big ice crystals in food.

Big ice crystals tear cell walls apart, leading to unpleasant mushiness. Big crystals/broken cell walls and exposure to air (if you don't use a home vacuum sealer) means dehydration and freezer burns on the food too.

Loss of texture and subsequent freezer burns are two of the major aesthetic reasons multiple freeze-thaw cycles are not recommended for most food. It turns into a mushy, crusty paste.
posted by bonehead at 2:51 PM on March 24, 2014

Every time you heat cold food or cool hot food, it passes through the danger zone where food-borne bacteria can grow. An answer on Seasoned Advice to the question "How many times is it safe to reheat chicken?" shows how repeated heating and cooling of food causes it to spend more time in the danger zone.
posted by grouse at 2:53 PM on March 24, 2014

The type of food matters a great deal, of course. A stock doesn't care about ice crystal size very much, but shrimp and fish are pretty much impossible to refreeze, for example.
posted by bonehead at 2:54 PM on March 24, 2014

Is it just a sterile environment

This is the crucial point. If you heat food to the point it is sterile, and then hermetically seal it, you can heat & cool it again until the cows come home without any bacterial ill-effects (colour, texture & taste issues notwithstanding).

If, however you heat & open it, allowing it to cool & reheating it is an altogether different proposition. You MAY do it in a safe manner, but the manufacturer can't rely on that, so simpler to just say "Don't".
posted by HiroProtagonist at 7:21 PM on March 24, 2014

If you cannot reheat your prepared meal leftovers, and you still want to eat, you will buy another packet. Just a guess, but I think food safety and taste concerns are a convenient excuse for selling more food.
posted by Dr Dracator at 11:43 PM on March 24, 2014

We were taught not to reheat food twice. So if you cook a large meal, its ok to reheat the leftovers but only reheat as much as you can eat, you shouldn't reheat leftovers that have already been reheated.
So ready-meals have been heated once and you reheat them at home but you shouldn't reheat them again if you don't finish it all.
posted by missmagenta at 4:00 AM on March 25, 2014

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