A Pinch of Culinary Science

by Anu Inkeri Hopia

  Whenever an archeologist makes a finding, s/he will have to interpret the data just as when we do our tasting experiments. Almost always there will be more than one possible explanation to explain the origins of the finding. The combination of rocks, coal, and fat indicate quite clearly that they were used for animal biological material. Secondly, they haven’t found human bones in the pits, which of course is no direct evidence that humans were not burnt there because our ancestors could have removed the bones after the burning ritual. So, the interpretation of the pits as some sort of “boneless tombs” is not a hypothesis that can ever be falsified, and thus the hypothesis is considered by archeologists to be somewhat speculative. After all, as the science philosopher Karl Popper wrote (in our words): every suggestion for an explanation should be testable, and if there is no way to test if your hypothesis is false, then your hypothesis has a serious problem. So, we cannot rule out that it actually happened, but there is no way to test if that interpretation is wrong either. We can choose to leave this as a possible use for cooking pits, but we will never be able to confirm or falsify it. Frustrating, but this is what researchers must learn to live with from time to time. Were the pits used for “dry cooking,” or were they lined and filled with water to be boiled by hot rocks? Plunging hot rocks into water results in different cracking patterns than burning them dry. This is a fortunate fact because the cracking pattern of the rocks found in the pits can then be used as evidence to show that water was not used in these pits. So, wouldn’t the finding of dry-cracked rocks also serve to disprove the sauna hypothesis, as it also disproves wet-cooking? As we envisage possible uses for these structures in the ground, new hypotheses appear that must be evaluated, disproved or left as possible explanations. But let us, for now, with our eyes open for other possible explanations, choose to see these pits as actual cooking pits used for cooking food for some occasion. After all, we won’t be criticized too heavily by archeologists for this choice as this seems to be the prevailing hypothesis among many of them.

  Historical timeline in the Nordic countries

  ^ The age of the cooking pits: Most of the cooking pit fields found in the Nordic countries are dated to the Bronze Age. The timelines in different parts of the world vary depending on the cultural and technological development of the region.

  The next question is: did the prehistoric Norsemen and -women use these subterranean ovens for everyday cooking for the whole family or village, or was this reserved for special occasions only? When cooking pits went out of use during the late Iron Age, was this because we invented new materials and new technologies or was it for some other reason? Archeologists putting their interpretive minds to the task, piecing all the various bits of evidence together, tell us that cooking in a pit was probably not an everyday activity; the village did not come together to cook a whole venison every other day. Excavations reveal that the pits were not reused many times, and the number of pits in an area compared with the other remains of human activity tells us that the number of pits is far too low to be a regular method for feeding the people living there. Even if the people had meat or fish only once a week. There are reasons to believe that cooking pits were used for religious purposes such as offerings in pre-Christian religious practices: blót, in old Norse language. There are, for example, remains of horse meat found in and around cooking pits near large houses, which were centers of power and homes of important chieftains. The horse had a strong symbolic meaning. Finding horse meat in a cooking pit might be related to offerings of symbolic character to demonstrate power, hierarchy, and wealth, but also a wish for good fortune in battle and agriculture. So, apparently, there are other possible explanations for why people used cooking pits than plain nutrition in a low-tech society compared with ours.

  In recent years, pit cooking has regained some popularity as both a spectacular, fascinating but also effective way to cook food for everything between five and 150 people. Indeed, it is not much more labor to cook for 50 than for five. Those of us taking the effort to dig a hole in the ground, collect rocks, kindle a fire, and wait for the hours it takes are surely seeking an experience out of the ordinary. The uniquely complex smell of cooked meat, aromatic herbs, and boiled soil reaching you the moment you lift the turf to uncover the pit is an experience that may leave its olfactory imprint on you for years. Cooking in pits are also, today, inevitably something for the special occasions rather than an everyday activity.

  So, is the increasing popularity of cooking in pits also of a more symbolic and experience-oriented nature, some sort of mild “food cult?” After all, those of us who try out pit cooking might seek some extraordinary experience or a unique flavor. Or do we do it to distinguish ourselves from the everyday cooking or ready-made food? One thing is certain: the cooking pit experience is a unique and recommendable one, regardless of our motivation to engage in it. Indeed, the Norwegian archeologist Lars Erik Narmo has written that such “dry-cooked” meat from a pit turns out very juicy. Upon such a statement, we, as food-oriented chemists, feel that there are some gaps to be filled: why does the food turn out juicy? Which kinds of cooking processes take place in a cooking pit? What is, if any, the difference between pit cooking and more modern methods for cooking? Do we really need to dig a hole in the ground to make this delicious food, or can we accomplish the same without engaging in a prehistoric role-play (unless we find this meaningful or fun in itself)?

  Making a cooking pit. The Northern European Bronze Age pit chef would most likely look for a flat space with fairly dry soil, not sand, and good turf that could be cut in whole pieces. S/he would then dig a round, oval, or quadrangle hole of 20–40 cm depth and from 1–3 m diameter in width, being careful that the turf was left in whole pieces and put aside. While lighting a fire in the pit, the party would collect round rocks the size of a hand, or somewhat larger, and place them on top of the burning fire. While the fire was burning, the meat would be wrapped in some material, perhaps a hide, animal stomach, birch bark, large leaves, or clay. What they used for wrapping is difficult to trace because most of these materials have long since decayed leaving no trace for the archeologists (we have not found literature stating the finding of clay with traces of food in relation to cooking pits). As soon as the fire had burnt down, the food was placed on top of the very hot rocks and coals, and the turf rolled back to cover it all. You now have a pit containing a layer of hot rocks, food on top and then turf as lid, a literal earth oven. No live fire, only hot rocks to heat the food. One skill of the expert prehistoric chef would perhaps be to know exactly how much rock was needed to heat a certain amount of a certain type of meat to perfection. Surely, if you were cooking a salmon, it would require shorter time and less rock than a whole leg of a reindeer.

  Procedure for preparation of food in a cooking pit

  It is not difficult to make your own cooking pit as long as you have the available land area, are allowed by the authorities to light a fire and have access to rocks. Archeologist Lars Erik Narmo writes that the method described above works fine, and there is no need to put rocks on top of the food. This is very similar to what in Finland and Sweden is called “robber’s roast.” Maybe the Norwegians were law-abiding people in old times, because we haven’t found any similar name for this dish in Norwegian. The food is packed in aluminum foil, wet newspapers and chicken wire. After the fire has burnt down, the coals are removed, the food placed on top of the stones, coals shoveled back on top of the food and finally covered by turf or sand. In our own cooking pit practice, which we have done with our students in Norway every year for more than a decade, we dig the hole deeper, use more rocks compared with what described in the literature, and we place the hot rocks all around the food to ensure even heating from all sides. Does this make a difference compared with the prehistoric way? We really don’t know, although Narmo, without giving further details, writes that “experiments show that it is not necessary to place rocks on top of the meat.” If you want to have rocks both below and on top o
f the food, your need to extract some of the hot rocks from the fire, place the food on the bottom rocks and put the smaller rocks back on top. This requires thick, heat-resistant gloves or some tool to handle the hot rocks. It is not easy to estimate how much rocks you need to collect, but our years of experience in pit cooking lambs legs have taught us that you probably need to collect about double the amount than you initially would think is enough. When cooking whole fish such as salmon, much less rock is needed as cooking time is shorter and the required temperature is much lower. Usually we would cook both a leg of lamb and one or two whole salmons, and some vegetables. Can you manage with one pit or do you need two? As we have the manpower, we usually dig one for the lamb and one for the fish and start the fish later. The question then remains where to place the vegetables, which will always result in a compromise in terms of cooking time. Surely there is always place for further experimenting.

  Procedures on the web indicate different cooking times, but when we cook lamb leg, it usually requires 2–2.5 hours, with 3–3.5 hours to give the best result. When using longer cooking times, the bone can simply be pulled out to leave the now very tender meat in one piece. Increasing the cooking time will not necessarily result in a very dry meat as when using an electric oven because the pit will gradually become cooler the longer you wait. When you are finished eating, make sure to put the rocks back where you found them and fill the pit with the soil and turf so that there are as few traces as possible from your activity. Archeologists recommend that you drop a small coin into the pit before filling it back up to make sure that later excavations don’t confuse your pit with a prehistoric one, spending time and resources on carbon dating material from a family party from the 2000s.

  Pit cooking—gradually turning down the heat. Cooking in a pit is quite different from what most of us do at home or in restaurants in more than one sense. Obviously, we don’t go digging holes in the ground every time we want to cook a roast, leg, or fish. But pit cooking is different from most cooking in the respect that when the food is placed in the cooking vessel or oven, the heat source has already been turned off. Ordinary oven cooking is usually conducted at a constant oven temperature, cooking in water or gravy/sauce is done at constant 100°C or lower and frying an egg in a skillet is done while the hotplate constantly feeds energy to the skillet. In the cooking pit, however, the fire has gone out before the food is added. While the firewood is burning, some of the thermal energy released in the process is transferred as heat to the rocks, which in turn become hot. The rocks have a certain heat capacity, ability to store thermal energy, and when the pit is closed we have a starting situation for the cooking process where the cold uncooked food is surrounded by hot rocks. Hot and cold objects in vicinity or contact with each other will engage in energy exchange. The hot becomes gradually colder by giving off excess energy, the cold surroundings, which include the food, become hotter by receiving this energy as various forms of heat. Through this exchange of energy, if we wait long enough, we ultimately end up with an equilibrium where everything has the same temperature. The ability to store thermal energy is a property of the material that can be measured, and for a specific material this is called the material’s specific heat capacity: the amount of energy it takes to heat 1 kg of the material one degree Celsius (or Kelvin). Materials with high heat capacity take more energy to heat and can therefore store more energy.

  When we pile food and hot rocks together in a cooking pit and cover it with the turf, we ensure that more of the thermal energy in the rocks is transferred to the food rather than to the surrounding air. In a cooking pit, all four kinds of heat are in action:

  – conduction through direct contact between rocks and food.

  – convection of hot air and steam inside the pit and inside the packages of food.

  – radiation from the hot rocks reaching nearby food.

  – condensation of water vapor, both on the surface of the food packets (humidity from the soil) and inside the packets (water from the food).

  Effectively, pit cooking combines almost all forms of heat treatment: boiling, steaming, barbecuing, oven roasting, and skillet frying in one single cooking method.

  As shown in the diagram, water has a very high heat capacity; large amounts of energy are needed to heat it up. Since most food contains a large proportion of water, this means that we need relatively large amounts of rock compared to food. This is, however, compensated by the fact that the rocks are very hot to begin with. Heat transfer is, after all, a product of heat capacity, amount of material and temperature difference. Additionally, we must take into account that much thermal energy is lost to the surroundings, that the food has to maintain a certain temperature for a certain time to be cooked and so forth. So, as result of both the energy loss to the surroundings and the difference in heat capacity between rocks and food, it is a good idea to be generous when collecting rocks for a cooking pit.

  Approximate specific heat capacity for various materials

  ^Foods have varying proportions of water, which contribute considerably to the heat capacity. The unit kJ/kg K is kilojoules per kilogram and Kelvin. That is, the energy it takes to heat one kilogram of the material one degree Celsius (or one Kelvin).

  Tender meat from the pit. When the archeologist states that food that has been “dry cooked” comes out very juicy, he particularly refers to a statement from a huntsman pit cooking a leg of moose. Having very large legs and very lean meat, moose legs would easily result in a dry chewing experience. Cooking a whole leg of moose might indeed be the ultimate test to show if pit cooking can compete with other methods. Two aspects are often emphasized for good taste and tender meat:

  1. High starting temperature. The surface of the meat is seared at a high temperature, which promotes Maillard browning reactions that produce tasteful compounds and color. This occurs in the early phase of cooking in a pit when the rocks are very hot (the rocks may be as hot as 500°C or higher).

  2. Moderate or low temperature over a longer period of time. For the meat to stay juicy it should be given sufficient time at moderate temperature so that tough connective tissue (collagen) is transformed to soluble and tender gelatin. Also, other chemical processes occur to produce flavor compounds. Although rather poorly controlled, the temperature in a cooking pit is constantly decreasing, resulting in an increasingly milder cooking.

  During cooking, the meat has been tightly wrapped only lying in its own juices. This should prevent severe overheating as well as stop moisture from escaping. If the guests should be somewhat delayed, we can be less worried about the food staying too long since the temperature in the pit is constantly decreasing. So, the cooking pit method has at least two safety mechanisms against severe overcooking built into it.

  Still, although not intentionally, we have been able to overcook food in a pit. In that case, we were a bit too industrious and used more rock than necessary. Cooking delicate products such as fish has an inherent greater risk for overcooking, and potatoes may occasionally turn out burnt and dry. Opening the pit to check on the food is not a strategy we would recommend because it is quite a lot of work, and when it has been opened, it is not easy to put the lid on, and much heat will have escaped. So, if time allows, we usually cook the food a bit too long rather than too short. Occasionally we use a digital cooking thermometer just to have some clue about how things are going (be prepared that the thermometer probe might be broken in the process). Cooking in a pit is not an accurate science, it is rather an experience-based practice that usually turns out very well. After all, the Bronze Age shaman didn’t have a digital thermometer lying around. Today, however, we can do such experiments to test whether our theoretical assumptions can be confirmed by measurement.

  What goes on underground? When cooking with students, we usually monitor the temperature inside the food. But we also track the temperature in various places in the pit, including in the ground just outside the pit. This way we can get some hints to what goes on, how a co
oking pit works and if much energy is lost to the surroundings.

  Lamb leg in cooking pit

  ^ Temperature zones in a cooking pit: The graphs show temperature development measured during cooking of a leg of lamb. The temperature was measured in the meat, at the surface of the meat, two places inside the pit and in the ground just outside the pit.

  Our assumptions about the temperature development in the pit and the food seem to be well supported by our measurements. As seen from the temperature in the ground just a few centimeters outside the pit, it seems well insulated and not much of the heat is lost to the immediate surrounding ground. This cooking experiment lasted just above two hours, which gave a well-done leg of lamb based on the appearance and texture with a brown seared surface. Although not particularly dry, it was not very juicy either. It was less dry, however, than expected from meat cooked to 86–90°C, as the graph shows. Interestingly, the outer and inner parts of the meat are getting closer in temperature as time passed even though the pit was still 30 degrees warmer. A notable result was that the outer part of the meat was rapidly heated in the beginning, but after a while the temperature evened out. Apparently, the meat was able to transfer energy inwards quite efficiently, rather than being much overcooked in the outer parts before being ready along the bone. This is in good agreement with a contemporary experiment described in the chapter about tempering the meat.

  It would have been interesting to leave it for another half or full hour, but this time the time constraint of the teaching session did not allow for this. Previous experiments with longer cooking times have shown that the meat indeed does not become drier, but it becomes more tender, falling apart and resulting a pulled-meat structure. Even if the cooking method is sometimes called “dry cooking,” the food is so tightly wrapped that a large amount of water from the meat itself cannot escape the food. Dry-cooked meat will end up rather moist. So, our experiences and experiments, although somewhat crude, do indeed support the claims by the archeologist that cooking food in a pit produces a delicious result. However, the question remains: does the food come out better if we spend the better of a day digging, collecting rocks, burning a sack of firewood, and waiting for another few hours? Most likely, it would be possible to achieve a very similar leg of lamb using techniques and tools from your own kitchen. But you would miss out on a great day out and the unique sensory feast that is opening a steaming pit. Perhaps the ultimate distinction from ready-made food produced in a modern factory would be to take on the role of the prehistoric chef, or shaman, from 3,500 years back in time, while at the same time being able to withdraw to the warmth of our living room whenever we want? Isn’t that indeed participating in some sort of joyful ritual, going back in time to sit at the table with our ancestors?