In our posts, we have talked about economies of scale in time and money that come from making extra portions. You will want to reheat that food to eat it at some point. For us, as for many, we didn’t see leftovers as quality food. The reason for that is they never were the same when reheated. After some research and experimenting, we found ways to maintain the quality of leftovers. To maintain quality we need to know the original cooking method and the changes to it that occurred to that food as part of that process. We will dive into that in this post.
Limiting Scope
This post will kick off a series of other more specific posts. Unfortunately, we feel you could write an article per type of food on how to reheat it. That is problematic time-wise and doesn’t run the steel thread of understanding the processes so others can self-help. It also doesn’t take into account a person’s individual calorie budget and serving sizes. We will get to how those change the art of re-heating.
The Basics
For brevity let’s limit our discussion to some simple ideas that we can expand on later. We would assert that most of our cooking falls into 2 main ways of transferring heat:
- Convection is the transfer of heat from the air to the food such as happens in an oven.
- Conduction is transferring heat to the food by direct contact between the heat source and the food.
We would also add there are only two ways of applying heat to cook food.
- Dry methods where the cooking process does not rely on a large amount of liquid to transfer heat. These, in our definition, are methods such as sauteing or roasting.
- Wet methods where the primary transfer is based on liquid such as water or oil. This type of cooking is how boiling, frying, and microwaving work.
Mix and Match
Almost all the food you cook will be a mix of these elements. Most cooking is about water management and reaching a safe temperature for the food. Taking that ride from raw to done imparts flavors and textures that please us such as caramelization via the Maillard Reaction. We may concentrate flavors via reductions of water as well such as in, well, a reduction. This method also applies to items like chicken stock or jams.
Where the confusion arises is that most things we cook are hybrids of the four bullet points above. What we want to do is understand what the outcome from our methods produces to reheat it. Did our cooking create something that needs to be reheated vs a dry method or a wet method? This begs the question do we reheat it through convection or conduction?
Hybrid Examples
Let’s examine two hybrids, lasagna, and fried fish, to understand what we are discussing in terms of methods vs outcomes. We found it easy to ruin both lasagna and fried fish if reheated incorrectly. When we think of lasagna we see it as a dry convection method. For fried fish, it is usually a wet conduction method. The output is a bit different though.
We would classify lasagna as a wet method like a stew. Why such a weird idea? Lasagna cooks by method provided by its water content even though the heat is powered by dry convection. It is the same process that happens when making a casserole or braising meat. Dry heat in an oven raises the temperature of the fluid in the lasagna to cook it through boiling and/or steaming. A microwave is a wet conduction method as it uses the water in the food to heat it. Practically speaking a microwave steams or boils food from the inside out. That makes lasagna a perfect candidate to reheat in a microwave.
Have you ever tried to microwave fried fish or fries? They turn out pretty nasty in our opinion but why? They are both technically cooked as wet methods of conducting cooking. The outcome of frying food is to drive most of the moisture out of the exterior of the food and create a crust locking moisture in. If that moisture is then released as steam, you get soggy fries and fish. What does a microwave do, well, it produces steam. That is why those foods need dry convection heat to reheat them
Two Other Variables
There are two other variables in our reheating discussion. They are the concepts of thermal capacity and safety. Most of us want to reheat food to preserve its original quality. In concept our matrix of methods above accounts for most items. Thermal capacity and food safety get in the way of that at times so we will set up how to deal with them here. First, let’s identify them
- The USDA states that all leftovers should be reheated to 165℉ to be considered safe.
- The surface-to-mass ratio of the food dictates how much thermal capacity it has and how long heating it thoroughly will take.
The first thing you will notice is that 165℉ is above what some foods are initially cooked to. Let’s talk about that one a little more in-depth
Is It Done?
The USDA has to deal with a broad population which means groups like the very old very young and everyone in between. That means they must put out safety information that others may not suit all. They also must account for all different parts of the food chain from the field where the cow is raised to re-heating leftover roast. That is a lot of places for bad things to happen and bugs to grow.
To us, if a steak is over 135℉ it is overcooked. That is us, not others. We sometimes go as low as 125F for a roast. That is what you will find many people consider rare and is a common way of cooking it. Personally, fish over 145℉-150℉, unless deep fried where we don’t notice, is overdone. Won’t reheating leftovers push it past the point we like it? That being our pre-packaged hamburger and chicken is getting pushed to 165℉ because we can’t trust it.
We take a ‘let’s be reasonable’ approach and use the USDA as guidelines. We buy food from known sources. When we prep and cook, we have a clean kitchen that gets cleaned regularly. We store food in the fridge or freezer properly. Nothing gets left in the zone for long unless it is intentional. Simply put, we don’t take a lot of chances so we can cook to whatever temperature we are comfortable with. You need to do you, but when you push the temps up, you aren’t re-heating, you are re-cooking.
Mass to Surface Area
This is one of the most misunderstood variables and one of the most important in heating. Here are the two axioms to know
- The larger something is, the more time it takes to cook.
- How fast it absorbs heat is directly related to its surface area where it contacts the heat.
One caveat to that is the microwave but let’s stick to the general for now. Based on these two axioms, we want to reheat food up to temp as quickly as possible without re-cooking it. What isn’t obvious is that it creates conditions where we want to use the lowest heat possible so we don’t overcook the outer part of the food before the interior reaches temp.
Important Temps to Know
One last piece of the reheating puzzle is two important temperatures. At sea level water boils and turns to steam at 212℉. The Maillard Reaction, or browning of food begins at anywhere from 280℉ to 330℉. Depending on what sugars are present, caramelization follows the Maillard reaction. Eventually, you end up in the burnt stage of Pyrolysis which, in short form, means burnt.
Why are these temps important? They are important to know so we get what we want from reheating. Do we care about a crust? Then we don’t want to create steam via a wet method. What if we want to raise the temp of something without further browning? In that case, we want to stay under 280℉
Back to the Examples
Our serving size of lasagna has a high mass-to-surface ratio. To cook it without further caramelizing, we need to reheat it at a low temp. Because we don’t want to overcook the outside before the inside is warm, we want to take our time but we also want to eat. That is a perfect case for a slow-wet method of cooking so we boil it correct? No, but close.
A microwave, when used correctly, provides a wet method of cooking that steams the lasagna from the inside out. Oddly enough, as water is removed from tomato sauce it is capable of creating a temperature higher than the boiling point of water. Those temps can cause caramelization and overcooking to occur. Yes, we have burned lasagna and melted plastic in a microwave when not careful. For that reason, we want to slowly heat our food at 212℉. For our portion size of 4-6 oz, that usually means a microwave on half power for 10 minutes in a lightly sealed container to hold in the steam.
Our fish is the exact opposite as they are usually high surface-to-mass pieces. We add it to a preheated toaster oven that is running at 275℉ degrees for approximately 10 minutes. This low dry heat helps preserve the crust without causing more browning. The fish is small enough it heats quickly without being over cooked.
Wrapping up
In this post, we have discussed some of the basic physics and chemistry involved in cooking. By recognizing how the food was created, and what it has become, you can decide how to reheat it. Truly, no one wants to eat leftovers that don’t feel close to the original food they were. If we can get them close, we can save ourselves time and energy in making one-off meals. We can also control how much we waste when we overcook.