A Pinch of Culinary Science

by Anu Inkeri Hopia

  Although common in the ancient Roman kitchen, the important Garum, a strong and smelly fermented fish sauce, disappeared from European kitchens for over a thousand years, but has now again emerged in modern kitchens similar to the likewise fermented Vietnamese fish sauce. From the late mediaeval times, fermentation was a good strategy to preserve fish in times and areas when salt was scarce or expensive. In the north this was definitely the case compared with the warmer Mediterranean, where they could simply make dams along the coast to let the sea water evaporate in the sun, leaving tons of salt for preserving their food. Coastal areas, such as those of Western Norway, had better access to salt via Atlantic trade routes and could use the cartloads of salt required to salt their cod, haddock, or herring. To make fermented fish, however, you need only very little salt to achieve a preserved foodstuff. It makes sense, then, that some of the most famous fermented fish in Norway are a typical inland product. Transporting large loads of salt in from the coast would have been very demanding in a time with few roads and no motorized vehicles.

  A Nordic, mildly fermented fish product is gravlax. The meaning of the word has changed over the centuries, but the original Swedish words “begravad” (buried) and “lax” (salmon) still remind us about the original recipe. According to literature on food history, this is a Swedish delicacy that originates from the Viking Age, introduced to the rest of the world after World War II. The oldest version of the preparation technique included burying freshly caught salmon or trout in tightly covered stone set earth holes. Storing the fish for a short period of time would give a slightly fermented, piquantly sour-tasting fish. Longer storage gave well-preserved but obviously quite stinky pieces of nutritious and energy-rich fatty fish that would keep for a year or even more. Putting that into one’s mouth might require the courage of a viking, indeed! Today, the most common Nordic, aromatic, fermented varieties include the Swedish surströmming (Baltic herring), Norwegian rakfisk (usually inland trout) and the Icelandic hákarl (fermented and dried Greenland shark). In many cases, fermentation was the preferred preservation method for fatty fish because salting and subsequent air-drying fatty fish would easily result in a rancid product. The anaerobic conditions necessary for fermentation prevent the easily perishable fish fat from turning rancid since this is a reaction between fatty acids and oxygen from the air.

  Past is past and vikings were vikings. The modern versions of gravlax are quite mild compared with the original ones, very mildly fermented. Today we prepare gravlax by selecting the freshest fillets of salmon, salting them lightly (often with high quality granular sea salt), adding a pinch of sugar, maybe dash of good brandy, gin, or aquavit, some sprigs of dill, and some ground rose or white pepper. Then we wrap them tightly and put them in the fridge under a light weight. After a few days of waiting, we can sharpen our best knife and cut thin slices of the most elegant delicacy that the Northern seas (or rivers) can offer us. A true harmony in flavor!

  But here the harmony apparently ends. Gravlax is not a Swedish delicacy, it is a Finnish delicacy! No, the Danes claim that it is a Danish delicacy! Norwegians are no worse, and they declare gravlax to be important part of Norwegian food heritage, a milder and younger descendant of the mentioned rakfisk still living strong side by side with its Swedish sibling surströmming. And what do we hear from Iceland? It is an Icelandic delicacy, too! Indeed, gravlax is part of culinary heritage in all these countries, and probably in many more as well. There is a great number of “original” gravlax recipes in the food literature and cookbooks, and the variations seem to have some regional character. Finns often marinate their salmon with only sea salt and flavor it with dill. If there is sugar in a Finnish gravlax recipe, the amount is moderate. Swedes and Norwegians always add sugar, from half to equal amounts compared with salt. “Why can’t they do it the right way, as the Finns do?” Anu the Finn asks. Maybe we should simply let the Danes, Finns, Icelanders, Norwegians, and Swedes continue their everlasting friendly debate over whose gravlax is the best. Preferences are personal and the familiar one is often considered the best. However, what we all can share is our interest on the phenomena taking place during, and the possible explanations behind, the “graving” process. One procedure that seems to be common to most recipes regardless of origin is the procedure that follows immediately after seasoning: the boneless fillets, skin on, are sprinkled on the meat side with the other ingredients, whereby they are placed against each other meat sides facing together and skin sides out. They are tightly wrapped and left to mature in the fridge. Obviously, one fillet must lie underneath, the other on top. So, the question arises: should the fillets be flipped to switch places during the process, or not? Some recipes tell you to flip the fish around halfway during the process while others do not mention this. Tatu the chef had reflected upon this, and one day he asked the very opportune question: “Does salt climb?” What an excellent question! In effect: does it make any difference if the fillets are flipped over? Do we get two different types of gravlax in one if we don’t flip?

  While today’s gravlax is a mildly preserved product, it is indeed considered a refrigerated product, the Norwegian rakfisk and Swedish surström-ming are preserved through fermentation for long keeping. The mild salting ensures that naturally present enzymes and bacteria are not inhibited and can produce acids, most notably lactic acid, that lower the pH so that other microorganisms do not thrive. Along with the protective acids, aroma compounds are also produced to give a strong and characteristic smell. This has much in common with that which happens in well-stored cheeses, also preserved by lactic acid bacteria fermentation lowering the pH in the product. An important factor in this process is anaerobic conditions, the absence of air/oxygen. Since some pathogenic bacteria also thrive under such conditions, notably botulinum bacteria (abundant in soil), which produce very harmful toxins, the manufacture of these long-stored fish products is quite strictly regulated. Home -producers of rak-fisk, for example, are warned to adhere to high hygiene standards, ensuring that the fish is at no point in contact with soil. When making gravlax, this is a negligible problem because the anaerobic storage is in fridge-cold conditions and for only a short time before the product is consumed. ×

  We decided to test the flipping issue. Tatu bought a good fresh salmon from the local market hall and cut out two identical fillets of each 620 g. Both fillets were sprinkled with 30 g sea salt and 30 g sugar on the meat side and were put on top of each other, meat side in and salt/sugar in between. The following 50 hours were used as each found most suitable, before we met for a workshop to evaluate the result.

  A pinch of salt chemistry. Table salt, sodium chloride, is not a molecule. It is a salt, among a countless number of different possible salts. Sodium chloride is composed of two different types of ions with opposite charges: sodium ions with a positive charge, chloride ions with a negative charge. In dry environment, these two attract each other, a bit like the opposite poles of two magnets. These forces give a well-organized three-dimensional structure with every other ion being sodium and chloride. The result is a beautiful octahedral structure, which also is reflected in the macroscopic structure of salt crystals visible to the naked eye.

  The electrostatic attraction between the two different ions is very strong. However, water molecules have the ability to penetrate between the sodium and chloride ions and jiggle them apart, one by one. This way, salt dissolves in water, each ion surrounded by one or more layers of water molecules, to start their individual journey in the watery world of food.

  Salt dissolves in water

  ^ Salt dissolves: Each sodium and chloride ion requires six water molecules to tear itself away from the salt crystal and flow freely around in the water. The salt is completely dissolved when all ions float around individually.

  Salmon fillets with salt

  ^ Diffusion of salt: When salt dissolves at the surface of the fillet, the salt concentration is much higher at the surface compared with the inner parts of the meat. In time, dissol
ved salt ions will spread—diffuse—into the meat through tiny channels.

  Salt dissolved in water moves around randomly, entirely based on collisions in the submicroscopic world of ions and molecules. Over time this will result in salt from areas of higher concentration to distribute more evenly throughout the water—diffusion.

  When salt is dissolved into the thin water layer that covers the food, the saline concentration on the surface is much higher than inside the food and salt ions diffuse slowly into the regions with initially lower salt concentrations wherever there are channels and gaps.

  However, salt is not the only thing that moves around in food. Water inside and in between muscle cells also diffuses and moves as result of these random movements. Only when the food is completely frozen, is the movement halted or severely slowed down. In sum, salt moves inwards toward areas of low salt concentration while water moves outwards toward areas of lower water concentration (due to the high concentration of salt on the surface). In a typical gravlax, the fish loses water and approximately 4% of its weight. Salt also affects the fish structure. The salt ions change the electric charge of the muscle fibers, forcing them apart from each other, which again results in a fish that feels moister and more tender in our mouth. A more tender muscle that has dried out sounds like a paradox, but it is indeed the case.

  Back with Tatu’s gravlax, our theoretical pondering indicated to us that it was unlikely that the upper fillet would mature/marinate differently than its lower counterpart as long as these are purely chemical phenomena. Gravity is no match against the forces of chemistry; the diffusion rate is the same whether something diffuses upwards or downwards in a solution. However, the saline water released from the fish tissue will flow downwards, and thus it is plausible that the fillet at the bottom experiences a more intensive brine bath than the one on top. But would it be possible to taste any difference? Tatu rinsed, dried and weighed the two salmon fillets, sliced them and coded them for blind tasting. Each workshop participant received a slice from each fillet, cut from the same place in the two fillets.

  The taste panel members were asked to evaluate which of the two was saltier, which was juicier, and which was more tender. They were also asked to choose which of the two they would prefer and why. There was practically no difference in weight loss between the two fillets, both had lost approximately 4% of their original weight. In the blind tasting, they were also evaluated as equally salty, that is, the panel was divided as close to the middle as possible as to which was most salty. So, does it make any difference if the fillet lies on top or underneath? Indeed, it does! The majority of the votes, nine out of 13, described the lowermost fillet as both juicier and more tender than the topmost. In their descriptive accounts, the panelists described the uppermost fillet as firmer, with a denser structure and more evenly distributed salt. The lowermost was described as sweeter, having stronger fish flavor, which also released more liquid. When asked which they preferred, the uppermost was the winner with nine votes. Interestingly, some preferred the uppermost because it was less salty while others preferred the lowermost because it was more salty. For some, the preference comes down to their personal liking in terms of saltiness.

  If we are to trust these results, there is a difference between the two treatments, and the upper fillet seems to mature slightly slower than the lower one. That is, a more rapid “graving” process in the lowermost fillet, which also experienced a steady slow stream of salty juices from above. We would guess, or hypothesize, that turning the salmon fillets halfway through the process may give a more even results in the two fillets. And if you do not flip them, you will make two types of gravlax with one single procedure. And, at the same time, you have ensured something to talk about with your guests when you ask them if they would prefer having their slices from the upper fillet or lower fillet.

  9

  The Art of Heating

  Cooking very often implies heating, that is, increasing the temperature of the food. Temperature as phenomenon basically comes down to one thing: movement. In gases and liquids, such as the hot air in an oven or water in a pan on the stovetop, the temperature directly reflects how fast the molecules are moving. In solids, where the molecules are kept firmly in their places not moving around (in fact, this is what makes an object or substance a solid), the temperature reflects how intensely the atoms or molecules vibrate. So, whenever we measure temperature in a material, we usually measure movement at the molecular level. Cook with heat therefore implies transferring energy to the food. The source of this energy can be the surface of a skillet, the water or gravy in a pan, the air in an oven, the radiating elements of a microwave oven, the coals of a barbecue grill or the rocks in a cooking pit.

  The choice of energy source will contribute to how the food ends up: a pork chop can be simmered, shallow fried, oven baked, or grilled. If you ask a physicist, they would probably say that it is this transfer of energy that is called “heat.” Such thermal energy spontaneously spreads from a place with high temperature to a place with lower temperature. According to physics, this energy transport can occur in two different manners: conduction and radiation. Additionally, energy transfer in cooking can benefit from a couple of other concepts: convection and condensation. These four concepts can be quite useful when we talk and write about food and cooking, so let’s try to put some meaning into them.

  Conduction—when hot and cold surfaces touch. One of the most striking experiences of conduction that probably every human being at some point experiences is when you burn your hand as you accidentally touch the ovenproof dish in your oven, or place your hand on the hot skillet. Conduction occurs when two objects are in direct contact with each other and one has higher temperature than the other. The molecules in the skillet, which is of solid metal, are vibrating intensely. When these come in contact with the surface of some food, or your hand, they collide with high intensity into the molecules on the surface of the food (hand), thus transferring some of their energy by setting the molecules of the food into motion. If the food remains in the hot skillet, a domino effect occurs from the outer parts inwards. This domino effect, a gradual conduction between outer and inner parts of the food, is one reason for the food eventually becoming hot all the way through. Substances that are efficient heat conductors are consequently efficient for heating food. A metal, YES. Styrofoam, NO.

  Some metals are better conductors than others. If you have had the pleasure of stirring a cup of hot tea or coffee with a silver spoon, you might have noticed that the spoon feels hotter than your everyday steel spoon. This is because silver conducts heat much more efficiently, by this domino effect conduction, through the length of the spoon to your fingers. On the other hand, when one of our prehistoric forefathers made a primitive skillet by placing a flat rock on top of a burning fire, it was rather inefficient compared with the metal pans used in the later metal ages. S/he might be able to heat the rock to fry one side of the meat, but the rock is not very efficient in continuously transferring the heat from the fire to the food, so s/he would have to move the meat around to new spots every time the meat was turned.

  Liquids and gases are comparably poor conductors of heat, one reason being that the distance between the molecules is larger than in solids. So, the collisions between the cooking medium (gas or liquid) and the food are necessarily less frequent compared with cooking on a solid surface such as a metal. It takes longer to heat food in an oven, where the heat transfer medium is air, compared with having the food in direct contact with the hot metal surface of a skillet. Since water has much higher density than air, water is a much more efficient heat transfer medium; the molecules are more densely packed and collide more often with your food. So, cooking potatoes in water is usually quicker than baking them in the oven, even though the temperature in the oven might be much higher than in the water, boiling at 100°C.

  Relative conductivity compared with water for selected materials

  ^From air to silver: The table shows rel
ative conductivity for various materials relative to water. Metals are good conductors, but even among these the differences are large.

  A barbecue radiates. Feel the sun warming an otherwise chilly, spring day, or feel the warmth sitting beside your wood-fired oven. In both cases, you experience thermal radiation warm your cold toes. Heat through radiation is different from conduction in that there needn’t be any physical contact between what is hotter and colder. In radiation, energy moves via electromagnetic waves, a form of light. If we leave out microwaving for a while, it is basically infrared radiation we use when we use radiation for cooking. The radiation is absorbed at the surface of the food, where molecules are set into motion, and the food is heated. The energy source (the coals in a barbecue, the hot walls or broiling elements of an oven) throws packages of energy around in all directions. When a molecule at the surface of the food is hit by one of these, it is energized and starts vibrating or moving around faster. An example of a food that is cooked purely by infrared radiation is the Finnish way of cooking salmon called “loimulohi,” flame-roasted salmon. You fasten a salmon fillet to a plank by driving wooden plugs through it and then place the plank upright by the side of the fire. If you don’t place it too close to the fire, the only heat reaching the fish is the radiation. The hot air from the fire move upwards and will not reach the food (see convection below).

  Microwave cooking also relies on radiation but in a somewhat different fashion than infrared radiation from hot bodies. While infrared radiation affects basically all types of molecules and materials, microwaves are more selective. They are particularly efficient in setting water molecules, and, to a certain degree, fat molecules, into rotating movement. Consequently, almost all of the infrared radiation is absorbed at the surface layers of the food, while microwaves may travel several centimeters into the food before being absorbed. Thus, food that is microwaved is heated just as much from the inside out as from the outside in. When a molecule absorbs an energy package in the microwave frequency range, it is set into intense motion. This results in small areas of very high temperature. Therefore, you would want to wait for a while for the energy to spread from the hot spots to the cooler regions in your food via conduction.