Heat Chemistry

Have you ever wondered why cooked food tastes better than raw ingredients? It all comes down to chemical reactions that happen when you apply some heat.

People didn't really know about this until Louis Camille Maillard came along. Maillard was a French chemist, who studied the interaction between heat and food at the start of the 20th century. More specifically, he looked at the effect of heat on amino acids and sugars.

Amino acids are found in proteins. They're an important source of nutrients. Sugars, meanwhile, are a source of energy. Maillard discovered that heat brings both of them to life.

Maillard discovered a chemical process which became known as the Maillard Reaction.

The process starts at about 140°C (284°F). At this temperature, the sugars and amino acids start to clash and fuse with one another. This creates hundreds of new molecules, with different flavors and aromas to the original sugars and amino acids.

These reactions get more and more intense as the temperature keeps going up. At about 160°C (320°F), so many molecules are fusing together that it creates a kind of flavor crescendo – foods start tasting meatier, or nuttier, or maltier, or sweeter, than they ever would have tasted raw.

While all this is going on, food is also browning and crisping up. As we've already learned, these changes to a food's appearance and texture also contribute to better taste.

The Maillard Reaction can go too far. When we hit 180°C (356°F), another reaction kicks in.

We call this reaction pyrolysis, which is just a fancier word for 'burning'. This reaction breaks down other molecules in food, like fats and carbohydrates, which produce bitter flavors and acrid aromas which drown out everything else.

In some cases, a little bit of burning is fine. For example, some people love a bit of char on a burger or steak, which has a nice, smoky flavor.

But too much char will always ruin a dish.

So how can you harness the Maillard Reaction to improve your cooking skills?

This is where a digital thermometer can help. You know that food cooks best between 140°C (284°F) and 160°C (320°F). You also know that it starts to burn at 180°C (356°F). Now, you can use your thermometer to keep track, and stay in the ideal range.

Another thing to be aware of: if we want a more intense reaction, we can add more amino acids and sugars. For example, you might add honey (sugar) to a piece of frying meat (amino acids). Or you might brush egg (amino acids) onto a baking pastry (sugar).

We'll see plenty more examples of this later in the pathway.

Last time, we learned about the Maillard Reaction – a chemical process which generally starts at 140°C (284°F).

And here's a bit of extra information that you may or may not have noticed. That Maillard temperature is actually higher than the boiling point of water: 100°C (212°F).

In the context of cooking, this fact is extremely important. Why? Because you can't get a Maillard Reaction if you cook a piece of food in water. The water just can't get hot enough – it will turn into steam, and evaporate away, before it even gets close.

That's why vegetables don't go brown and tasty when you boil them in a saucepan.

So, if water can't get hot enough for the Maillard Reaction, what can we use instead? There are a couple of options, but the most obvious one is oil.

Oil heats up much faster than water. It can also reach higher temperatures. Some olive oils, for example, can reach temperatures of as high as 210°C (410°F). That's more than hot enough for the Maillard Reaction to occur.

Above that temperature (we call it the smoke point) an oil will burn and break down. It will start to release trails of acrid smoke, and even harmful chemicals.

But you shouldn't be letting it get that hot. Remember: at 180°C (356°F), food will start to burn. If you find yourself reaching an oil's smoke point, your food will already be black.

There are a few ways to cook food in oil, but the most common one is pan frying. Just take a frying pan, heat a layer of oil, then throw in a piece of food.

If your oil is hot enough, it'll make the moisture evaporate from the surface of food. That's the sizzling sound you hear when food is frying. Once the water is gone, the Maillard Reaction will make the surface turn tasty and crisp.

Meanwhile, heat will travel from the surface, up into the center of the food. This is a slower process, which won't dehydrate the food, or activate the Maillard Reaction. This is why the center of a fried piece of food will often be tender and moist.

Another way to cook a piece of food using oil is deep frying. This method involves immersing food in a saucepan of oil.

There is one advantage to deep frying: the transfer of heat occurs evenly to food, from all directions at once. This is different to pan frying, which heats food from below, and can potentially lead to uneven cooking if you're not regularly flipping the food.

But a piece of deep fried food will absorb much more oil than pan fried. This makes it a lot less healthy, and can also lead to a greasy, oily texture. It's also dangerous: if you spill all that boiling oil, you could suffer some severe burns.

Because of all this, as a general rule, pan frying is a better option.

Last time, we discussed how water will never get hot enough for a Maillard Reaction. But that doesn't mean it's not a useful medium for cooking.

See, while the Maillard Reaction only happens at temperatures over 140°C (284°F), lower heats can still lead to different chemical processes.

For example, if you throw a vegetable into a saucepan full of boiling water, the temperature of the water (100°C) is enough to break down the walls of that vegetable's cells. This won't brown the vegetable, or crisp it up – but it will make it soft and tender.

Sometimes, that's just what we need. A dish would be boring if the ingredients were all crispy. It's good to include some softer components too.

As a general rule, cooking in water is also healthier than cooking in oil. But actually, it isn't always quite so simple.

When you boil vegetables in a saucepan of water, some of the vegetables' healthy nutrients will actually leach into the water. For example, in one study, a boiled carrot lost 55% of its total vitamin C.

One easy way to deal with this is to re-use that water in your dish. For example, if you were boiling some veg for a curry, you could use that water in the sauce. After all, it's now full of those minerals the nutrients that escaped the vegetable in the first place.

There's another little trick you can try with this: when you're boiling something, throw in a pinch of salt. If the water is saturated by salt molecules, the nutrients in the vegetable are less likely to leach out.

Another way to preserve more nutrients, when cooking with water, is to forget about boiling entirely. Studies have found that steaming is much healthier. A carrot, for example, will only lose about 15% of its vitamin C.

There are a few different ways to steam something. You can buy a specialist steamer, you can do it on the stove, or you can even do it in a microwave. To do it on the stove:

• - Put a couple of inches of water into a saucepan
• - Take three small balls of aluminium foil, and make a triangle at the bottom of the pot
• - Put your ingredients on a plate, then balance the plate on the balls of foil
• - Cover the saucepan, and bring the water to a boil

As the water boils, the saucepan will soon fill up with steam, and the ingredients on the plate will cook. It's a little bit fiddlier than boiling, but once you get the hang of it, it can be worth the extra work.

Two more water-based cooking options are poaching and simmering. These are basically the same as boiling, but you keep the water at a lower temperature, rather than the full 100°C (212°F).

Poaching uses water between 70°C (160°F) and 80°C (180°F). Simmering is slightly hotter, from 85°C (185°C) to 95°C (205°F). Just like steaming, these are better for nutrient retention. At lower temperatures, nutrients leach out a lot less.

These methods are also good for delicate foods, like eggs or fish. When water boils, the bubbles can bump against a piece of food, and potentially tear it apart. But that doesn't happen at these lower temperatures.

In fact, if you're poaching or simmering something, the appearance of large, violent bubbles means your water is getting too hot.

So, we've been learning about different mediums that can transfer heat into food. So far, we've looked at oil and water. Today, we'll be looking at air.

Whenever you bake or roast something in the oven, you're essentially creating a pocket of hot air, then allowing that air to transfer heat into food. The air itself is heated by some kind of heating element; a fan might also circulate the air, to keep the temperature in the oven even.

When you open the door, the hot air will quickly flow out. It's almost like boiling some food in hot water, then opening a hatch on the side of the saucepan. The water would flow out, exactly the same as the air.

Remember: hot air rises, so the top shelf of your oven will usually get hotter than the bottom shelf.

As a general rule, baking involves some non-solid foods, which you heat until they turn into solids. Bread is a good example. It starts as a non-solid ball of dough, then turns into a solid loaf.

Roasting, on the other hand, involves cooking a food which is already solid. For example, some vegetables, or a big piece of meat.

Roasting meat and vegetables is slower than frying or boiling. But this slower process will often result in softer, richer tasting food. You'll even get a bit of Maillard Reaction, which helps to crisp up the surface.

The heat is also able to penetrate deeper, which is important with a large piece of meat. If you tried to fry a roast chicken, the surface would burn before the heat ever reached the center.

Along with baking and roasting, another way to use air as a medium is to throw some food on a barbecue.

On a barbecue, the air is heated by coal or gas. This hot air rises up past the food, cooking the food and infusing it with a distinctive smokey taste. As the food drips oils and fats onto the coals, these actually evaporate then rise as well, adding more flavor to the food.

A variation on this is broiling, or grilling, which involves placing a powerful heating element above your food instead. You won't get the smokey flavor of a barbecue, but apart from that, the results will be mostly the same.

We couldn't really talk about cooking with air, without talking about air fryers. They've been around for a while, but in the last few years, they've started to become more popular.

They work by blowing hot air around a piece of food, effectively mimicking hot oil: the surface of the food dehydrates, and crisps up, while the middle of the food becomes tender.

Some people swear by air fryers. They cook food quickly, and with less oil involved, they're theoretically more healthy than pan frying.

They're certainly not a kitchen essential, but if you're interested, you should try one out.