Cooking Science

Most people treat it as an art or craft (or mundane task), but cooking is actually a very specialized branch of chemistry. Knowing the broader principles of that science can make it much easier to understand why some techniques work and why some don't.

Rather than blindly learning hundreds of traditional ad hoc rules about what to do and what not to do, one can simply think about how various substances react to heat and understand what will happen and why.

The science presented here is in very general, non-mathematical terms and should be easy to understand by almost anyone. For a much more complete analysis (e.g. integrals and differential equations), see publications such as Douglas Baldwin's Practical Guide to Sous Vide Cooking.

Note that all temperatures given here are the temperature of the food itself, not the temperature of the cooking device. Meat in a 350°F oven could have a temperature of 130°F. It is the 130°F that is significant to this discussion, not the oven temperature.

Fast Reactions

When food is heated, some reactions happen very rapidly.

Eggs are a very visible example of this. Eggs contain several types of protein (not only white and yolk), each of which coagulates at a different temperature. As soon as each temperature is reached the transformation is quite obvious. If the temperature is kept constant it doesn't matter how long you wait, the egg won't cook any more; its temperature must be raised in order to coagulate other proteins.

The protein in meat is similarly denatured by heat. At temperatures over 95°F, muscle fibres start to contract, and continue to do so up to 175°F.

This reaction happens immediately and is entirely dependent on the maximum temperature that the meat experiences. At 100°F, the protein will contract slightly, at 135°F it will contract much more, and at 170°F it will contract a lot. It doesn't matter how long the meat is held at a fixed temperature; protein left at 135°F for one minute will contract just as much as if it were left at that same temperature for an hour.

Protein shrinkage causes the moisture trapped in the meat to be squeezed out, which means loss of moisture and drier, tougher meat.

This moisture loss is entirely predictable, the higher the temperature the meat experiences, the greater the loss, regardless of how long the meat is cooked for. It is temperature, not time that determines moisture loss.

When cooking a steak, many people think of the doneness (e.g. rare, medium) as being a factor of how long the meat is cooked for. It isn't. It is entirely determined by the temperature of the meat.

Slow Reactions

Other reactions depend upon both time and temperature. Given enough time, fat will eventually render into cracklings and oil. Connective tissue proteins (e.g. collagen) experience a conversion to gelatin. At high temperatures, these tough, chewy, almost inedible bits become soft and moist after several hours. At lower temperatures, the same process happens, but it can take up to several days for full conversion.

But note that when the conversion happens, it happens rapidly. Meat does not gradually get more tender the longer it is cooked. It remains tough for a long time, becomes tender over a short period of time, and remains tender for a long time.

This slow reaction doesn't occur only in meat. Rice and vegetables can quickly reach the same temperature as the water in which they are being boiled, but it still takes considerable time for that heat to break down starches and other tissues, thereby cooking the food.

Cooking the muscle part of meat depends almost entirely on the temperature of the meat. Cooking connective tissue and vegetables depends on both time and temperature.

Time and Temperature

For any specific food, one can produce a chart like the following showing for any temperature how long it takes for the food to become cooked, and how long it takes to become overcooked. Each type of food will have a different chart; even different cuts of the same meat (e.g. dark and white chicken) will be different.

At very hot temperatures, food cooks very quickly, and the time between raw and overcooked is quite small. For instance, once french fries are properly cooked, it can be only a minute or so before they become overcooked. Restaurants have precise temperatures for their cooking oil, and a timer that ensures that the fries are removed from the oil at just the right moment. Any sooner and they will taste somewhat hard and raw inside, and any later and they will be burnt outside and mushy inside.

At less hot temperatures, the period during which the food remains properly cooked is significantly longer. For a roast in a hot oven, there could easily be ten or twenty minutes of leeway in the cooking time.

At warm temperatures, the just-right period is much much longer, possibly many hours, or even days.

And looking at the bottom line, one can see that at cool temperatures, not surprisingly, food doesn't cook at all.

As mentioned before, the above temperatures are the actual temperature of the food, not of the cooking utensil or container. The transfer of heat to the food complicates the situation.

For example, if a roast is put into a 350°F oven, the outer surface will heat up to nearly 350°F. That heat will slowly move toward the centre of the roast, and the meat will be considered cooked when the centre temperature reaches 120°F. But the meat between the centre and the surface has an ever increasing temperature, so while the centre is cooked perfectly (or will be if left to stand so that the centre heats up to 130°F), the outer part will be far hotter than it should be and will be overcooked. Typically, a slice of this roast will be red or pink in the middle, with a darker, somewhat dry ring around it, and a still darker drier and tougher ring around that. When eating, people tend to cut their meat so that they get a mix of cooked and overcooked meat and it tastes okay.

But if the meat is cooked at a very low temperature, the temperature gradient will be much less, and the dry, tough layer will be much thinner, less noticeable, and ideally will correspond to the searing crust. Prime rib for instance can be cooked in a 150°F oven for many (e.g. 6) hours and be almost uniformly cooked all the way through. (And yes, that really is 150°F, not 150°C.) Prime rib is naturally tender, so of course doesn't need to be cooked for so long; the seemingly excessive time is required for the heat to penetrate to the centre without overcooking the outside. The main disadvantages to this method are:

• It takes much longer. (It's worth the wait.)
• There will be very little juice drippings produced. (Which means the juices remain in the meat.)
• The outside won't have the traditional flavourful crust. (Easily remedied by reverse searing (see below).)

This same principle applies equally to steaks, burgers, and other meats.

Techniques

Searing

Briefly subjecting the surface of the meat to a very high temperature causes the outer layer to brown and develop a much more intense flavour This transformation of the proteins is known as the Maillard Reaction (sometimes incorrectly referred to as caramelizing, which is for sugar, not protein).

In the mistaken belief that this also cauterizes the meat and seals in juices, traditionally one is supposed to sear the outside of meat before cooking it. It is much better though to do the searing after cooking it (reverse searing). Side by side comparisons show that reverse searing preserves more of the internal moisture in the meat.

In the example of the roast above, one should let the meat sit awhile to allow the internal temperatures to become uniform, and then place it in a 550°F+ oven for five minutes or so. The outer fat will render and the surface will develop a tasty crust without the drying heat penetrating far into the meat.

Steaks, burgers, etc. can similarly be browned after cooking, briefly using a very hot cast-iron searing pan or BBQ grill.

Salting

Meat is often soaked in brine (or salt and baking powder) before cooking to give it a crispier crust (e.g. roasts, turkeys, chicken wings). This traditional method works, but not nearly as well as dry brining.

The salt causes moisture to be drawn from the meat or skin through osmosis. With traditional brining, this moisture is simply lost.

Instead the salt should be applied directly to the dry meat or skin, and the meat left uncovered in the refrigerator overnight or for several days. Osmosis draws moisture from the skin and dissolves the salt, but after a while this solution is reabsorbed, remoisturizing and allowing the salt to penetrate into the meat. The salt helps break down the surface proteins and produce a much crispier tastier crust when cooked.

Sous Vide

While it can be time consuming and requires special equipment, sous vide is an almost perfect technique for controlling the maximum temperature that food experiences.

The food to be cooked is vacuum sealed in plastic wrap and placed in a bath of water. A circulator keeps the water constantly stirred and at an exact temperature.

A good quality steak can be left at 131°F for an hour and a half (or several hours, it won't make any difference), and be uniformly cooked all the way through. A poorer quality steak can be left to cook at 131°F for a day or two, be uniformly cooked all the way through, and have all its tough chewy connective tissue completely softened.

Then dry, season, and sear, and you'll have a perfect steak without any trace of the usual layer of overcooked meat. Or refrigerate, still in the bag, and a few days later use the sous vide bath to reheat to serving temperature (without fear of the overcooking that reheating normally causes).

The same technique works for a variety of foods, including chicken, legs of lamb, burgers, vegetables, and even quinoa.

There is a poor man's version of sous vide, in which one uses a large insulated picnic cooler full of warm water, and meat hacuum sealed in zip-lock bags. Occasionally open the lid, check the temperature, remove some water, and replace it with warmer water.

Pasteurization

One concern that some people have is that low temperature cooking doesn't meet government guidelines for safe temperatures.

As a senior citizen, I recently received a flyer from the government warning me about this, even suggesting that whole chickens should be cooked to an internal temperature of 180°F. Imagine how incredibly dry and stringy the white meat would be.

The reason the government (and other organizations) suggest such high temperatures is because those temperatures kill the bacteria almost instantly. Such a simple rule requires no timing or judgement or intelligence on your part. It also ruins your food.

The 7 log10 pasteurization standard

Pasteurization kills most bacteria, but not all. Health professionals recommend that food be cooked enough to kill all but one in every ten million germs in order to consider it safely pasteurized. Such food can then be safely eaten immediately or stored in a refrigerator for several days.

The base-10 log of 10,000,000 is 7 (the number of zeros), hence the name of this standard.

Temperature versus bacterial survival

Doubling about every 20 minutes, most bacteria multiply rapidly at temperatures between 40 and 140 degrees Fahrenheit.

The birth rate isn't the only thing affected by temperature though. At higher temperatures, bacteria start dying, but not all at once. The higher the temperature, the faster they die. At above about 130 degrees Fahrenheit, bacteria die faster than they reproduce. So even at relatively low temperatures, the death rate can be higher than the birth rate, and given sufficient time the bacteria will eventually die off.

Consider salmonella. At 165 degrees, almost all bacteria are killed almost immediately, while at 135 degrees it takes well over an hour for enough to die to achieve the 7 log10 standard. Similarly, 145 degrees takes less than 10 minutes, while 155 degrees less than 1 minute.

Killing bacteria isn't simply a matter of temperature; just like cooking, it requires both temperature and time.

Generally any food heated to over 130°F and held long enough to cook it will end up pasteurized, and be safe to eat immediately or to store in the refrigerator.

Food slow cooked at below that temperature will not be pasteurized. It must not be cooked for more than a few hours, and must be eaten immediately.

Fish is often cooked below this critical temperature, so should be frozen awhile before cooking to kill any parasites, especially if caught wild.

Measuring

Time or Temperature?

Many recipes, especially for meat, suggest cooking for a certain amount of time, or so many minutes per pound, or similar guideline.

Ignore that advice. The only reliable way to get consistent and predictable results is by using an instant read thermometer to measure the temperature at the centre.

Metric, Imperial, or American?

Because of its simple and rational units, metric would obviously be the best choice, but so many published cookbooks use Imperial or American units that it's not always convenient. Even many recipes that include multiple measurement systems simply provide conversions to metric resulting in awkward measurements (e.g. 28 g, 355 mL), so unless you are going to rewrite and customize it, stick with the most natural units for the given recipe.

The non-metric systems can also be confused with each other, so make sure you know which one the recipe means. Americans measure volumes using fluid ounces (29.56 mL) and dry ounces (34.44 mL), while the British use ounces (28.35 mL). The British have 40 ounces in a quart, while the Americans have only 32, albeit larger, fluid ounces. There are also ounces that measure weight rather than volume, common ounces (28.35 g) and troy ounces (31.1 g). (An ounce of gold really does weigh more than an ounce of feathers.)

Volume or Weight?

Most non-metric recipes don't say exactly what they mean, so you have to take a chance and guess. Ounces of butter is probably by weight, while ounces of oil is more likely volume.

Even worse, many non-metric recipes ask for volumes of compressible substances. I gave up expecting anything useful from volume recipes long ago when I saw a recipe that called for one cup of raw spinach: a single leaf can overflow the measuring cup, yet one can pack a whole package of spinach into that same cup.

The non-English-speaking world uses metric, and gives measurements in weights, not volumes. Their recipes are so much more consistent and reliable.

A cup of flour can weigh anywhere from 125g to 145g, depending upon how it is put into the measuring cup. If you are going to do any significant baking, give up on volume measurements. Measuring flour by volume will produce different results every time.

And if you are going to do a significant amount of baking, buy a weigh scale that provides baker's percentage, such as the MyWeigh KD8000. The same recipe can be trivially used for large or small amounts without your having to do any conversions.

Cooking

If only slow reactions are involved, deciding how to cook something can be easy. If fast reactions are involved, one should choose the lowest practical temperature in order to generate the longest possible range of cooking times, and then use that latitude to allow the slow reactions to take place.

To properly cook, one first needs to know the correct internal temperature for the type of food. Then, based on that temperature, one can easily determine the minimum cooking time to achieve pasteurization once that internal temperature has been reached.

As an added benefit, so long as that required internal temperature is not exceeded, the food can be left to cook for extra time without fear of its actually overcooking. Overcooking is caused by too much heat far more than by too much time.

After cooking meat, it's a good idea to let the meat sit for a few minutes after cooking to allow the heat to distribute more evenly and for the juices to be reabsorbed as it cools. If the meat reaches only 150 rather than 165, letting it sit and keep warm for five minutes will be more than enough time to complete the process. Even if one is going to cook by sous vide and the temperature is already evenly distributed, that five minute waiting time before serving will continue to pasteurize the meat.

The following are presented only as examples. There are many sources of much more detailed recipes available.

Vegetables

Most vegetables cook at 183°F and above, and since higher temperatures tend to destroy vitamins and create mushy textures, the ideal way to cook vegetables is to hold them at 183°F for between half an hour and a couple of hours, depending upon the type of vegetable.

This works especially well for corn on the cob bagged with salt, pepper, and butter (one half hour is enough).

Salmon

Salmon can be cooked at a very low temperature, only 115°F, for 30 to 45 minutes. The protein is fully cooked at this temperature, but it remains very moist and tender.

Note that this is below pasteurization temperature, so it should not be held for hours, and must be eaten without refrigeration.

Quinoa

The sous vide technique takes all the guesswork out of cooking quinoa. Bag the quinoa with 150% by weight water (e.g. 100g grain, 150g water) and a little butter, salt, and herbs, and immerse for one hour at 180°F. It will be perfect every time.

Burgers

Gently shape freshly ground beef (200-250g) into patty shapes, avoiding compressing or mixing it. Do not mix anything else with it, but season its surface with salt and pepper. Place into individual ziplock bags and hacuum seal, again avoiding compression. Immerse in 130°F for 40 minutes to 4 hours.

Remove from bags, dry, reseason, sear, and eat. It might look somewhat raw, but it is actually fully cooked and pasteurized, and deliciously rare and tender.

Corned Beef

Prepared uncooked corned beef usually comes with awful directions telling you to boil it for an hour or two. At that temperature the proteins contract so much that there is more than 50% moisture loss, resulting in something tough and dry that is hardly worthy of the name.

Instead, sous vide it at 160°F for 36 hours and the result will be tender and juicy.

Chicken and Turkey

Chicken legs cook best at 165°F and breasts cook best at 144°F, which presents a problem when cooking whole chicken (or turkey). Traditional methods end up overcooking the white meat, leaving it dry and stringy.

The simplest and best solution is to cook the parts separately at different temperatures, but for aesthetic reasons, people like to present an intact bird.

Spatchcocking the bird before cooking makes an excellent second choice though. Do not butterfly (remove the breast bone and keel bone), but rather remove the entire spine. The flattened bird then has the white meat pushed together and the dark meat spread out loosely, which means that when cooked in an oven the dark meat will heat up more quickly than the white meat, which means that the white meat will reach 144°F at about the same time as the dark meat reaches 165°F.

For a crispier skin, dry the bird, rub with a mixture of 2 parts baking powder and 1 part salt, and leave uncovered in the refrigerator at least overnight, or better yet for a day or two, before baking in a 450°F oven.