Eli5: why are some foods not advised to be reheated, or heated more than once? Why do some have to be cooked from frozen and other defrosted
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Some foods don’t just get warm and cold, the heating process actually triggers a chemical process *or* alters the chemical structure of the food. Bread is a good example, stale bread is feels hard because it’s chemical structure has changed. Heating it returns much of that structure to it’s original form making the stale bread soft and fluffy again, but also removes a lot of water. Now, after it cools, the bread will get hard and nasty again but this time it’s not stale, it’s dried out. Now you *can’t* just reheat it to get it fluffy again because it’s actually lost water.
The other reason is the shoulder times – during defrosting (but prior to cooking) and during cooling (but prior to freezing) are windows where bacteria can thrive and reproduce. One cycle is *probably* safe enough but multiple cycles will just increase the chances of contamination to too high a degree. This is the reason why you should defrost meat in your refrigerator and not on a counter or in the sink. Left out, the surface of the meat warms quickly becoming like a bacterial Jersey Shore where they grow and contaminate and make everything disgusting. On second thought, that’s just like normal Jersey Shore. But the inside is still frozen. At least in the fridge no single part of the meat ever gets into the dangerous temperature range while it defrosts.
Finally, because “Big Food” just likes telling us little people what to do. Like, you’re not the POLICE guys, you’re not the BOSS OF US.
That depends on the food!
Rice for instance is not safe to be eaten even if it is reheated to sterilising temperatures because B. Cereus releases toxins upon death that you cannot “cook away”. This is the case for some foods and freezing the food does not prevent it either.
Frozen foods I would argue simply has to do with getting the cooking time standardised. If you don’t thaw the food its easier for the company to know the exact conditions it’ll be in when you start cooking, whereas thawed food will have various degrees of thawed. This way your food is often cooked “perfectly” or safely rather.
Lastly, thawing and refreezing causes damage to cells. Thisll make your food taste funky 🙂
When food gets warmed, the bacteria in it can multiply much much faster than when it’s refrigerated or especially when it’s frozen. That means that if it’s warmed up again and again, food that’s prone to spoilage anyway gets way more chances to spoil. This used to be more of an issue as production facilities were not as sterile in the past and these days you can pretty safely thaw and refreeze even raw meat many times – although it’s best to only thaw it enough to portion it and refreeze pre-portioned amounts.
As far as some foods being intended to be thawed then cooked while other foods are intended to be cooked from frozen that’s just a matter of the physical properties of the particular dish. For example a frozen lasagna wants to get browned on top while the underlying layers remain somewhat separate. Baking it from frozen cooks the outside while basically just defrosting and warming the inside (it’s precooked). If it were completely thawed you would overcook the inside and end up with a more homogenized mushy texture. That’s the general reason food is meant to be cooked from frozen – to crisp the outside while only thawing/heating the inside.
The advisability of reheating or heating certain foods more than once depends on factors such as the type of food, its ingredients, and the specific handling and storage conditions. Here are some general reasons why certain foods may not be advised to be reheated or heated more than once:
1. **Bacterial growth:** Reheating food allows bacteria to multiply, especially if the food was not stored or handled properly. Certain bacteria can produce toxins that are not destroyed by heat, and consuming such toxins can lead to foodborne illnesses.
2. **Nutrient degradation:** Some foods, especially those rich in vitamins and antioxidants, may experience a loss of nutritional value when reheated. The heat can cause the breakdown of certain sensitive nutrients.
3. **Texture and taste changes:** Reheating can alter the texture and taste of certain foods, making them less palatable. For example, overcooking may lead to dryness or changes in the consistency of certain dishes.
4. **Formation of harmful compounds:** Repeated heating of certain oils or fats can lead to the formation of harmful compounds, such as trans fats or oxidized fatty acids, which may have health implications.
As for the difference between cooking from frozen and defrosted, it often relates to safety and evenness of cooking:
– **Cooking from frozen:** Some foods are designed to be cooked from frozen to ensure that they reach a safe internal temperature throughout. Cooking from frozen also helps maintain the quality and texture of certain items.
– **Defrosting before cooking:** For other foods, especially larger items like roasts or whole birds, defrosting is recommended to allow more even cooking. This prevents the outer layers from cooking while the inner parts are still frozen.
It’s important to follow specific food safety guidelines, storage instructions, and reheating recommendations provided by health authorities or on food packaging to minimize the risk of foodborne illnesses and to ensure the best quality of the food.
The two big reasons:
* Some bacteria leaves stuff that makes you sick behind that can’t be heated away. So even if you kill the bacteria the food’s tainted. Think of it like if I poured poison on the food. You can’t reheat that and be safe. Not all foods attract this kind of bacteria, but “do not reheat” foods likely do.
* Some foods change their chemical composition when heated. So if you re-heat them it’s the same as overcooking them. They won’t be dangerous, but they’ll be less pleasant to eat.
The main problem is food labels don’t have to tell you the difference between “it is not safe to reheat this” and “it might taste bad reheated”. Sometimes they happen to tell you, but you might have to go look it up for each individual food.
(For example, my dog’s raw food says not to defrost it in a microwave, but explains it’s because if the food is cooked it loses a lot of the nutritional value for a dog. That means it’s not *unsafe* to defrost it, but that if I’m doing so every day carelessly my dog might end up with some deficiency.)
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