Chemistry on a skewer: are scientists good at cooking barbecue

Kebab preparation, from the chemist's point of view, is a complex process, at each stage of which a large number of subtle and interrelated reactions take place. If you approach the matter wisely, the recipe for a good kebab will be comparable to individual methods of organic synthesis - or even surpass them. And, as in a full-fledged scientific experiment, in the preparation of barbecue there are many details on which the optimization of the process depends - and therefore the taste and aroma of the final product.

So, in order to cook a kebab, you need to perform two main steps: marinate the meat and fry it over charcoal. But first, let's figure out what meat is - from the point of view of chemistry.

Meat

What we call meat and buy in the store disguised as pork and beef is actually the skeletal striated musculature of animals. Unless, of course, we will not consider by-products, for example, the heart, which are not used for barbecue. In addition to the muscle tissue itself, adipose and connective tissue, which is adjacent to them, is also referred to meat.

Muscle tissue has a curious structure. We are accustomed to the fact that the cells of our body are usually very small, invisible to the eye. The structural unit of a muscle is a muscle fiber - and this is one large cell several centimeters long and hundreds of micrometers in diameter. It is formed by the fusion of thousands of other cells, due to which there can be several thousand nuclei in the muscle fiber.

The main property of muscle fibers is the ability to contract. This is how we (and other animals) move our limbs - and more. This is provided by special proteins - actin and myosin. They are elongated molecules that form long bundles inside cells. Under the influence of external factors (nerve impulse), these bundles begin to move relative to each other, pulling towards the center. The entire fiber is divided into separate links - sarcomeres, fastened together.

In addition, meat contains large amounts of the proteins elastin and collagen in the connective tissue. They are largely responsible for the mechanical characteristics of meat (toughness, etc.). Protein myoglobin is responsible for the color of meat. In general, meat is largely a protein product, but, of course, there are enough fatty layers in it.

Pickling

The meat is marinated in order to solve several problems at once: to make it softer, to give it an additional flavor and to carry out primary antimicrobial processing.

Collagen molecules, which determine the hardness of meat, normally form strong fibers, fibrils. This assembly takes place under the influence of hydrogen bonds - the attraction between partially charged (polarized) amino acid fragments. Exactly the same bonds arise between water molecules - between the hydrogen atom of one molecule and the oxygen of another.

Many marinades are acidic due to the presence of acids in them - most often acetic (for example, in wine, mayonnaise or vinegar), lemon and lactic acid. Soy sauce and teriyaki sauce also have an acidic medium - they contain a large amount of pyroglutamic acid, as well as succinic, citric, formic and acetic acid.

This means that there are many hydrogen cations in marinades that are able to bind to protein molecules and protonate them. This changes the distribution of charges in molecules and disrupts the fine structure of hydrogen bonds, which leads to a change in the geometry of protein molecules. As a result, proteins are denatured: collagen and actin fibers swell, soften, collagen gradually dissolves.

The same effect can be achieved without the use of acids. For example, some tropical fruits, such as papaya and pineapple, contain enzymes that break down elastin and collagen into single amino acids, and bacterial and fungal proteases can similarly break down muscle fiber proteins. There are physical methods of softening meat - holding at pressures of the order of several thousand atmospheres, which also leads to denaturation of proteins.

The speed at which meat is marinated also depends on the composition of the marinade. For example, the presence of alcohol in the marinade has been shown to speed up the marinating process. This is due to the fact that the lipid membrane of cells dissolves better in alcohol than in water. Various auxiliary substances, such as tannins in wine and beer, also play a role in tenderizing meat.

It is worth noting that pickling does not always lead to meat softening. In some situations, excessive marinating (in the presence of too much acid or alcohol) loses water and becomes too hard. The same effect can be achieved by overcooking the meat - then most of the water will simply "fly away" from it.

The second most important effect is antimicrobial. But not only acids are responsible for it, but also other components of the marinade, such as onions. Quite a lot of studies have been devoted to various methods of destroying harmful organisms in meat; in one of the most curious, the authors suggested adding processing in an ultrasonic bath to the standard scheme of marinating meat in beer.

It should be noted that the second stage of shashlik cooking starts the synthesis of some carcinogens - harmful substances that can potentially cause cancer. This applies in particular to the products of the charring of fat dripping onto the coals. These include benzo [a] pyrene and other polyaromatic hydrocarbons.

Another class of carcinogens arising from charring of meat are heterocyclic amines. These substances are able to form complexes with DNA and affect the vital activity of cells. One study even found Dietary Benzo [a] Pyrene Intake and Risk of Colorectal Adenoma to correlate frequent consumption of smoked or grilled meat with certain cancers. Accordingly, it is recommended to reduce the use of such substances as much as possible. But pickling can help here too.

There are several studies by Portuguese and Spanish chemists that indicate that certain types of marinades reduce the likelihood of these carcinogens forming. For example, marinating in dark beer partially inhibits the Effect of Beer Marinades on Formation of Polycyclic Aromatic Hydrocarbons in Charcoal-Grilled Pork from the formation of polyaromatic hydrocarbons, and to reduce the proportion of heterocyclic amines formed, marinades based on wine, beer, or even those containing tea should be chosen. In general, the effect of marinades on the formation of polyaromatic hydrocarbons in general is still not well understood. Other possible inhibitors include onions, garlic, spices, and citric acid pickles.

Frying

Marinating, due to the denaturation of most of the proteins, significantly speeds up the cooking process. This avoids prolonged exposure to heat and the evaporation of too much water. Along with the acceleration of protein denaturation, charcoal frying initiates many other chemical processes in meat.

The first of these is the well-known Maillard reaction. It is she who is responsible for the formation of strong-smelling organic substances, which give a special smell to fried meat. Amino acids found in meat and sugars enter into this reaction. As a result, complex heterocyclic compounds are formed, derivatives of furan, thiophene, alkylpyridines and pyrazines.

The specific flavor profile for each type of meat is different, it is determined by the ratio of the concentrations of thousands of aromatic substances formed during frying. In the case of fried chicken and pork, the condensation products of cysteine with sugars, such as 2-methyl-3-furanethiol and its dimer, as well as 2-furylmethanethiol, play an important role in the aroma.

Of course, other amino acids also react with sugars. Methionine, for example, interacts with sugars and degrades to methional, a substance that smells like fried potatoes.

It is clear that proteins and sugars are not only found in meat. Therefore, the Maillard reaction plays a role in the aroma of other dishes as well. For example, baked goods (and some types of rice) smell like 2-acetylpyrroline, a reaction product between proline and sugars. In small quantities, this substance also occurs in fried meat.

The second chemical process is fat charring. Fats are esters of glycerol and organic fatty acids such as stearic, palmitic, and so on. When heat treated, they are chemically converted to aldehydes such as hexadecanal, hexanal, and so on. Interestingly, roast beef contains more aldehydes than chicken and pork, which makes them taste different. And the characteristic lamb smell is due to 4-methyloctanoic and 4-methylnonanoic acids.

The third process is the reaction between the products of the carbonization of fats and the products of the Maillard reaction. These are all kinds of alkanethiols, alkylpyridines, alkyl derivatives of thiophenes, pyrroles, thiopyrans, thiazoles, and so on. The alkyl part in them arises from the fatty component, and the heterocyclic part from the Mayar component.

In addition, other reactions involving amino acids occur when roasting meat. Thus, cysteine and glutathione form trithiolans and dithiazines during heat treatment, which also make a significant contribution to odor.

The taste and aroma of kebabs are given not only by the decomposition products of amino acids, sugars and fats, but also by the products of coal combustion. Among them, it is worth highlighting syringol (its name, by the way, comes from the Latin name for lilac, Syringa vulgaris) and guaiacol - they are formed during the breakdown of lignin, a binder for cellulose molecules in wood. These substances give the kebab (or barbecue) its characteristic smoke smell.

Dozens of technical details of the cooking process affect the ratio of aromatic substances in the finished kebab: temperature, duration of roasting, choice of coal, meat, marinade, marinating time. And this is a great opportunity to, armed with a scientific method, find your own optimal recipe for barbecue yourself and, maybe, even write a scientific article about it - with a particularly juicy description of the experimental part.