We take it for granted that cooking is using heat to transform raw materials into good dishes. Yet, high temperatures overwhelm ingredients to change them in a variety of ways: affecting their texture, flavor, nutrients, and digestibility. They are able to shape molecules that would not exist without their intervention.
In this article we will explore the effects and influence of heat on food and perform a scientific experiment on the dissolution of solids into liquids, a phenomenon that is as simple as it is essential for many culinary preparations.
Understanding these processes allows us to improve our skills in the kitchen, creating better dishes with less effort!
A physical matter: changes of state
Heat is a powerful agent capable of causing so-called state changes in matter, which are fundamental to many culinary techniques. A classic example is the transformation of a solid into a liquid. Ice becomes water when it is heated and, in doing so, absorbs energy in a process known as melting. Heat can then go further, turning water into vapor through evaporation. It happens when water reaches boiling point and molecules move fast enough to escape into the gas phase. Water is of course one example out of all, ça va sans dire.
These state changes are fascinating from a scientific point of view, but they are also the basis for many culinary preparations. The melting of butter is often one of the first steps in the preparation of sauces and desserts, while the evaporation of water is essential for the concentration of flavors in reductions and sauces. Another practical example involving a change of state is cheese melting. When we make a fondue or cheese sauce, heated solid cheese changes to a liquid state and can be blended with other ingredients to create flavor mixes. If we wish, we can then pass it under the hot grill of the oven or let it cool. The physical state of the cheese will change once more and we will get a dish with different textures again.
During these changes, depending on the ingredients we start with, the chemical reactions that take place resemble the endless blackboards in movie classrooms, on which professors start writing to peel off the chalk long–and soporific–hours later. On fire those same reactions can occur in minutes or, even, seconds.
Let’s review some of the most important chemical transformations that occur in the kitchen due to heat!
The chemical effects of heat on food
PROTEIN DENATURATION
Proteins are more or less complex molecules that we can find in a great many foods. Recently, fashion has it that they are associated with the term fit by implying that they are good for you, better indeed, than other very important nutrients such as carbohydrates. And so pasta, bread, and flour derivatives are looked down upon in preference to high-protein foods. This concept has no reason to exist from a scientific point of view. Moreover, the same farinaceous foods contain a certain percentage of protein, and not just a few: in wheat flour on average we find 14g per 100g of product! Beware when we reduce a food, complex in its interest, to a simple molecule: we will almost always be wrong.
There are many different proteins contained in different foods, but they are all built from a mix of building blocks called amino acids. I say mix because there are 20 different amino acids in nature, and they can combine in different ways and at different percentages depending on the protein they are to make up. Depending on the food we consider, we will find a different protein mix!
And what happens when we warm them up?
When subjected to heat, proteins tend to denature, that is, they lose what was their spatial conformation. In fact, their component atoms bind together to form even very large-in the scale of molecules, we mean-and arty structures. The precise way in which proteins are arranged in space, their shape, is essential for them to perform their function, that is, the task for which they are synthesized from a grain or legume, animal or vegetable. The bonds of the atoms that make up these molecules are strongly affected by heat, which is able to break them up by making the amino acids unwind and intertwine differently. The result in the kitchen can be different depending on the ingredient we are handling and its interaction with the other molecules in the pot.
An example of protein denaturation visible to the naked eye?
The cooking of eggs! Albumen, when it turns white from transparent, is visual evidence of denaturation of the proteins it contains. This also suggests to us that the shape that proteins take influences how light is trapped and reflected, as well as how it smells and tastes. Those who eat raw eggs know what I am talking about. I don’t, I’m not able to consume raw eggs but in short, it’s common knowledge!
Other visual examples of protein denaturation during cooking are the colors that fish or meat take on. The fish, from translucent and smooth, soft, becomes more intensely colored, opaque and crumbly to the touch. Red meat turns to brown when well cooked, and has a firmer texture. And then there are the cheeses, which can spin, dissolve in the solvent or form lumps depending on the amount of protein they contain and the amount of water they contain.
Curd, which we talked about in the article where I explained how cheeses are made, is precisely the product of the denaturation of milk proteins by an agent that has this power: to make them lose their original structure and turn them into a subspecies of net!
CARAMELISATION OR DEGRADATION OF SUGARS
We have seen that there are so many different proteins composed of unique amino acid mixes that can give very different results. In contrast, caramelization starts with very few molecules: sucrose, i.e., cooking sugar, or glucose or fructose. It only takes one of these molecules to make caramelization possible, and the result is the formation of thousands of different compounds whose precise composition and structure is still partly unknown to chemists. We know that diacetyl is responsible for the buttery notes of caramel, and that there are something like a hundred different molecules responsible for its brown color alone!
The sugar begins to liquefy at 160°C. Its molecules begin to break apart, and the fragments react with each other to form a whole series of aromatic brown compounds. As the temperature rises and the sugar molecules break down, there is an intermediate time to pay attention because the consistency of the whole thing will look like a big lump of crystals. It will take some time and patience to see the mixture liquefy. After that, depending on when we stop the cooking you will get a mixture of a color ranging from yellow to dark red, brown or, if it has been heated too much and has begun to degrade, ebony. Here, if we get to this point, perhaps it is best not to consume caramel, which will contain potentially dangerous molecules.
GELATINIZATION OF STARCHES
We find them in pasta, bread, flatbreads, rice, potatoes, and generally all grains: when we eat them, we are stocking up on starches!
What is starch?
Starch is a water-insoluble complex carbohydrate that is used as an energy reserve in plant cells. It is the most important source of carbohydrates that can be absorbed and utilized by human cellular metabolism. Starch is found in large quantities in vegetables such as tubers, cereals, and legumes and is itself made up of two sugars called amylose and amylopectin. Amylose and amylopectin within the starch granule are insoluble at room temperature and cannot be digested by our enzymes. For them to become digestible, the crystalline structure of starch must change by “breaking down” and taking on the characteristics of a gel: hence, the term gelatinization.
Gelatinization of starch is possible by heating in water: the starch hydrates, swells, and loses its crystal structure. Because amylose and amylopectin bind to water, a decrease in free water is observed during the baking of bread, pasta, etc.
Gelatinized starch forms a mass that is able to incorporate other substances, giving us the ability to create many different dishes or thicken foods that seem too liquid to us.
Fun fact: You can do part of the pasta preparation in cold water by letting it soak. I know, it turns the nose up, but it can be interesting to experiment with flavored waters whose flavors will soak into the pasta. Or you can try microwave cooking, several versions of which can be found on search engines. Although distant from our tradition, anything can be an excuse to turn the kitchen into a laboratory and test different cooking methods!
This is what great chefs do.
In these three paragraphs we have seen some examples of how heat acts on the basic molecules of food, while skipping several. Think, for example, of fats, which we have not touched on here, and how by cooking vegetables or meat in a well-heated pot the oil, or butter, is able to extract and intensify flavors and aromas. Or to Maillard reactions, a very complicated series of transformations in cooking involving foods within which sugars and proteins are found: meat, bread and fried foods are just a few examples. Think of the brown, fragrant crust, the aromas that invade the kitchen during these preparations. It’s Maillard’s reactions, just the ones that make your mouth water at the very thought!
Knowing how to control heat is also important in food safety issues, where, for example:
- Through the use of high temperatures under certain conditions, our species has been able to understand how to sterilize certain types of foods, extending their shelf life
- Incorrect use of temperature can result in the formation of health-damaging compounds, such as black burnt crust. Meat, bread, chips, vegetables-any food that is left on the stove too long can burn, and a number of dangerous and potentially carcinogenic molecules, such as acrylamide, will foment.
- In some cases, heat can accelerate the decomposition of food, especially if it is not stored properly or if it is exposed to high temperatures for long periods of time.
- More generally, cooking various foods ensures their microbiological safety and increases the digestibility of many dishes.
As we might have expected, there are far too many effects of heat on food to cover in this article, but suffice it to know that there are and to have, at the very least, a general background on them.
But now let’s get down to practice!
Experiments in the kitchen: pecorino cream cheese
Cheeses are formed by a protein lattice consisting mainly of caseins. This lattice traps water and fat within it: the proportion with which these molecules are found helps us to know whether that cheese will melt creamily and evenly, retain some of its structure, or begin to string.
We will use pecorino, preferring a medium-aged one that does not contain high percentages of water, but not minimal either. This will allow us, when heated gently, to turn it into a cream. Remember that the more aged a cheese is, the easier it is for lumps to form, so choose carefully. Let’s see how to make the cream!
need higher temperatures to break protein bonds. If we heat too much, the proteins coagulate with each other, forming lumps and expelling the water.
Ingredients:
- 50 grams of semi-aged pecorino romano cheese
- 65 grams of boiling water
- Small bowl (suitable for bain-marie)
- Saucepan
Procedure:
- We bring water to a boil in a small saucepan, meanwhile grate the cheese and put it in a small bowl. When the water boils, pour it into the small bowl with the Parmesan cheese and stir vigorously, almost to an emulsion. We leave some boiling water in the saucepan; we will need it later. We probably will not get a completely homogeneous mixture, but the result will not be bad.
Scientific comment: by pouring the hot water, the cheese fats began to melt and the protein mesh, already shattered by the grater, loosened its grip further allowing fats and water to flow out of the mesh. To break the net completely, we need to increase the temperature a little more, but without overdoing it: if we took the mixture to high temperatures, lumps would form!
- We then place the small bowl inside the saucepan in which we had boiled the water to heat the pecorino in a bain-marie. We keep the heat off, however, so that we can better control the rise in temperature. If you have a kitchen thermometer, you can measure the temperature: when we reach 55 °C we will reach the ideal creamy consistency!If you don’t have a thermometer, no harm done: your eyes will be your best tool to guide you in this experiment. Also try it several times until you get the result you are most satisfied with. My directions are partial because everything will also depend on the cheese you have purchased!
What’s next?
You will only have the choice of spreading your cream on a crostone of bread or tossing it into your freshly drained al dente pasta.
Scientific tip?
Instead of water brought to a boil, you can use water from cooking pasta or, alternatively, that contains starch. Starch has the effect of stabilizing the cheese protein network making it more difficult for them to coagulate! Starch powder is also used to thicken sauces and can be used to make cheese fondue!
