A Pinch of Culinary Science

by Anu Inkeri Hopia

  18

  Physics Takes the Cake

  You have invited friends and family to a celebration, maybe a birthday or Independence Day. Then you ask yourself: “Should I buy a ready-made sponge cake, or should I bake it myself?” The first option is maybe the safest bet, but the second will surely communicate more sincerely your love and appreciation for the guests. But, maybe, your experience from prior baking projects has been that your cakes don’t always turn out light and airy. Last time you baked such a cake it came out flat and dense, even though you followed the recipe and treated it with the utmost care, making sure you didn’t disturb or shake the light and tender sponge. You had to rush to the supermarket to get a ready-made sponge. What to do this time? The British physicist Peter Barham has a suggestion for you, claiming that his advice will ensure that your sponge cake does not collapse. Brace yourself: as soon as the cake is ready and baked in its metal spring form, you take it out of the oven and hold it 20–30 cm above the kitchen bench or a table. Then, let it fall flat down. Boom! This, he claims, will ensure that the cake will not collapse. Is he joking!? Does he expect us to trust this advice a few hours before the party? Has he tried this himself? If I try this, should I buy a ready-made sponge cake just in case? The questions and dilemmas pile up, but this claim truly begs for an experiment.

  Jokes often have some core of truth, and science jokes are no exception: “If it is green or wiggles, it’s biology. If it stinks, it’s chemistry. If it doesn’t work, it’s physics.” We are chemists and we’re used to getting our hands dirty, but can we trust the physics professor to have actually tested his claim? How many of us haven’t experienced the school science experiment that didn’t work out as expected although the book said it should? With an important cake in the oven, this is not an option. So, we decided to dive head-first and test this claim in a cake-dropping experiment. Perhaps the outcome would result in some jaws dropping as well? The experiment was conducted in a food workshop with 20 participants to witness and taste the result.

  The cake drop. Tatu the chef chose a very basic recipe for sponge cake: sugar, eggs, and flour. Sponge cake recipes may also contain milk, and even baking powder to help with the leavening, but we chose to keep it as simple as possible to avoid complicating factors.

  The two parallel cakes were baked simultaneously in the same oven. When taken out, one cake was treated with the utmost care and placed carefully to cool in the tin. The other was held horizontally 20–30 cm above the table and dropped flat down, after which it was left to cool. When cold, the cakes were cut in half and the heights measured.

  Recipe

  Ingredients

  250 ml sugar

  250 ml eggs (6 medium sized eggs)

  250 ml sifted wheat flour + some flour for the cake tin

  Procedure

  The sugar and eggs were beaten with an electric mixer until a light foam. The flour was added and gently folded in. The mixture was poured into a 32 cm springform cake tin and placed in a convection (hot air fan) oven for 1 minute at 200°C, and then 25 minutes at 180°C.

  The recipe was repeated to give two sponge cakes.

  The result. The cake that was dropped was clearly higher in the center area compared with the one that was handled with care. The latter had the typical high circumference and pit in the middle, while the one that was dropped had a more even height throughout. In the middle area, the one that was dropped was approximately 20% higher than the one handled with care—quite a spectacular result! Well, not that surprising as we had secretly pretested this in advance. We had safeguarded ourselves and did a corresponding experiment at home, one month earlier. In this experiment, the result was much the same, but with a somewhat less marked difference. In that case, we used some baking powder in the recipe, so the cake handled with care came out quite tall as well, but the one that was dropped came out 15% higher and even had a center area that was higher than around the rim.

  We are quite convinced that the British physicist’s claim is indeed correct, but what can be the explanation for this apparently counterintuitive result? This requires a closer look at cake structure and going back to our structural friends, the dispersions (as described in the sausage chapter).

  What makes a cake rise? Structurally, a sponge cake is a solid foam—a solid substance with gas bubbles distributed throughout the structure. Just like the inside of the pillow you might be sitting on right now while reading this book. If we simply stir together egg, sugar, and flour and bake it in the oven, we would get a pancake; flat and dense. The foamy cake structure is achieved if we beat eggs together with sugar to produce a liquid foam, full of air bubbles, and then gently fold in the flour to avoid too many bubbles bursting. We still have a fragile liquid foam, but somewhat more thick and viscous. When the cake enters the oven, it rises—even though no baking powder or other leavening agent has been added. The reason for this is that the water from the eggs in the batter evaporates. Liquid water turns into steam. Steam—water in the form of gas—requires more than 1000 times the space compared with liquid water. One teaspoon of water turned into steam would, in the ideal case, expand to more than 5 liters. So, when the water evaporates in the liquid batter, it ends up in the bubbles, who in turn expand radically. The whole cake increases in volume and therefore rises, the internal vapor pressure in the cake keeping it up, competing with gravity, which constantly pulls the whole cake down. The air that we beat into the batter would also expand somewhat when heated in the oven, but this expansion is not much more than 25%, and expansion of air, therefore, has a much smaller effect than expansion of liquid water into steam.

  ^ Liquid foam: Chemically speaking, a cake batter is a liquid foam. Water (blue background) is the continuous phase, and air bubbles (white) are the dispersed phase. Proteins (in particular from eggs, orange and purple) and sugar (blue) are dissolved in the water. Starch granules from the flour (yellow) float around as solid, dispersed particles.

  In the early stages in the oven, when the cake is still practically a liquid foam—bubbles dispersed in a liquid—it is very fragile and sensitive to mechanic disturbance. If this wobbly mass gets a nudge, the bubbles of hot steam maintaining the expanded foam might find their way up and out, even leaving behind channels for more steam and gas to escape. This is the moment where the cake requires peace and quiet. However, after a while, the batter solidifies due to chemical reactions facilitated by the high temperature: gelling of starch from the flour and coagulation of proteins from eggs and flour. The liquid foam turns into a solid, but soft and brittle, foam. The cake is still in the oven, but, in addition to the internal pressure of the steam, the solid structures of gelatinized starch and coagulated proteins now hold it up. So now you may safely dance and jump around the cake as much as you wish without the fear of it collapsing.

  Then the moment arrives when the cake is ready to be taken out from the oven. How come your beautiful fluffy creation shrinks in front of your very eyes, the high peak in the middle turns into a valley, and the smooth surface of the cake is all wrinkled? The sponge still contains a considerable amount of water. When the cake is taken out, the temperature soon sinks to below 100°C, and the steam still inside the recently expanded bubbles condenses back to liquid water. The recently expanded bubbles experience a vacuum effect; they are sucked together from the inside. What looks like a cake “falling together” is actually a mild implosion. However, the cake is now a solid foam that is more rigid than it was as a liquid foam prior to baking. If we were to bake a bread, the solid foam would be rather strong and elastic, and the crust so sturdy that the bread would not contract that easily, while in a sponge cake the structure is still rather soft and brittle and prone to collapse. Channels that we so much feared would let steam escape if the cake was disturbed in the early stages of baking would now have been much welcome to let air in from the outside as steam condenses and bubbles contract.

  The oven baking process

  ^When water in the cake batter boils a
s a result of heat transfer from the oven, the air bubbles are filled with steam and expand (left-hand page, from top and downwards). When the cake is taken from the oven, the steam condenses into liquid water. The bubbles—and the cake—contract (right-hand page, top). However, if the cake is subjected to a shock just after baking, cracks will form between bubbles and on the surface of the cake. Air can then pass through these new channels to replace steam that has condensed into water (right-hand page, bottom).

  What happens when the cake is dropped? We do not know whether Barham’s advice came after an accident in the kitchen making him reflect upon this, or if the hypothesis came before the experiment. Our tests anyhow support his claim and indicate that he might have found a good way to ensure a high sponge. His explanation for why this works goes as follows: when the cake is taken out from the oven, it is a solid foam: lots of bubbles surrounded by a fragile solid dough (crumb). If this structure receives a physical shock, cracks will appear between bubbles, and on the surfaces of the cake functioning as ventilation channels. Small and rather firm bubbles become interconnected, united into a more open network. Additionally, the steam inside the cake condenses and contracts, but instead of the bubbles contracting with the steam, air from the outside can be sucked in through the cracks to fill the bubbles. Even though the crumb and crust are soft and fragile, they are strong enough to maintain the structure. And you will not have to make double the amount of cream to fill a pit in the middle of the cake.

  As to yet, we have not found any scientific studies describing such shock treatments of sponge cakes, so Peter Barham’s explanation should perhaps still be considered as a hypothesis. But it would be interesting if research could document the effect of the cake drop. Pastry chefs and bakers might have a safe route to high cakes, they could make their cakes 10–20% higher without increasing the amount of ingredients, and the customers would have cakes with fewer calories of energy.

  This time we must admit that even though it is physics, it still works in real life and not only under idealized and controlled conditions. At least, the times we have tested it. The question remains: would you put your trust in the physics if there was an important occasion at stake…?

  19

  The Great Norwegian Porridge Feud

  In 1864, a rather unusual cookbook was published in Norway bearing the name Sensible Cookery—A timely cookery—and household book. The author was a mysterious person hiding behind what was obviously a pseudonym: Clemens Bonifacius, translated from Latin meaning “The gentle helper.” The book consists of two main parts. The first is a general text on nutrition and home economics, while the second part is a collection of recipes and cooking procedures like most other cookbooks. If you were a farm wife or housewife reading this book, you would probably not experience the first part as very gentle, but rather harsh and scolding. Perhaps you would think that a more proper pseudonym for the author should be, “the strict educator,” your thoughts wandering to the time when you went to school and the teacher used the rattan stick on your hand if you hadn’t done with your homework. When opening the book, you would read that the way you cooked your porridge was entirely wrong; it was against the knowledge of modern science. And cooking porridge was not a small issue in those times, as two-thirds of the people in the relatively poor Norway (100 years before the faucets of North Sea oil started flooding) ate porridge three times a day, 300 days of the year. By today’s standards the porridge of 19th century Norway was not a very luxurious dish: coarse whole meal flour, water and a pinch of salt in a pot cooking over an open hearth.

  “Mr. Bonifacius” had been travelling in central Europe, especially in Germany, to pick up the most recent scientific knowledge. He claimed that as much as one-sixth of the flour used by Norwegian farm wives for porridge was wasted. If true, this was a big deal, as he calculated that it would amount to a loss that would be clearly visible on the national budget. If only the farm wives would do things correctly, they would help their families to a more viable economy in an otherwise poor country of mostly peasants.

  What was the problem, and who was C. Bonifacius? Bonifacius criticized the farm wives’ cooking practice of being silly, old-fashioned, unscientific, and inefficient. The two prominent examples he used were how they cooked their porridge and how they roasted their coffee (it was common to buy the coffee as unroasted beans and roast one’s own coffee at home). A typical procedure when cooking porridge would be as follows, based on 5 dl whole meal barley flour:

  Heat 2 l water to the boil in a pot and add flour little by little while stirring until you have added around 4 dl flour. Let the porridge simmer for 20 minutes. Take the pan off the heat and add the rest of the flour in three rounds, stirring vigorously between each addition.

  Leaving some of the flour until after the cooking was called to “pound” or “pestle” the porridge or pounding/pestling flour into the porridge. According to Bonifacius, this way of cooking was contrary to scientific knowledge because the flour added in the end was not cooked properly and would therefore not be digestible. Starch must be heated in water to swell and become available for people’s digestive systems. If as much as one-sixth of all flour for porridge was to go through the families’ bowels undigested, that would be a great loss to the country! Bonifacius literally wrote that “the women’s order in this country is truly such that they have to learn everything anew” and “our farm wives can’t even cook a porridge properly.”

  The book was well written and sold out within the first year. It also aroused attention from scholars, particularly from the now famous sociologist Eilert Sundt, who had travelled across the rural areas of Norway to learn and understand the ways of living among the common Norwegian. When it became clear that the person hiding behind the pseudonym was the famous natural science scholar and folk tale collector Peter Christen Asbjørnsen, the stage was set for a feud in the public media between the natural scientist and the social scientist. Based on his own observations and documentation, Sundt rebutted Asbjørnsen’s harsh claims about Norwegian cooking practices with the counterclaim that these farm wives had kept their households running, and cooked their porridge, for a thousand years. Surely, their experience and everyday observations would have guided them to good and efficient ways of doing things! So, this was not only a debate between different sciences, but also a battle between science and craftsmanship, between tradition and economy—and between national identity and foreign influence.

  Typical 19th century porridge from Norway

  Ingredients

  270 g whole meal barley flour

  2 l water

  1 tsp salt

  Accompanied by

  Sour milk (buttermilk)

  A small knob of butter, or syrup and some Norwegian brown whey “cheese”

  Procedure

  Heat the water to the boil in a pot and add flour little by little while stirring until you have added about 5/6 of the flour. Let the porridge simmer for 20 minutes. Take the pot off the heat and add the rest of the flour in three rounds, stirring vigorously between each addition.

  To serve, you need two bowls. One larger bowl for the porridge with a small “butter eye,” or a small eye of brown whey cheese and syrup. The other smaller bowl contains the sour milk. The porridge is eaten as follows: you take a spoon of porridge, dip it in the “eye”, and then dip it into the sour milk. If the whole party shares the two bowls using only their individual spoons, you are close to the authentic way of eating this porridge. After the meal, you lick off your spoon thoroughly and hang it back on its regular place on the wall.

  Norwegian “brown cheese”

  This is a Norwegian delicacy, and an everyday food. Technically it is not cheese, but it is boiled down whe y with some added cream, which attains a brown color with a caramellike flavor and the structure of a some what brittle cheese. Hence, it is perhaps even more correctly described as a type of unsweetened caramel than a cheese. Since whey, the liquid by-product from cheesemaking, contains
both proteins and sugars and the product is made by extended boiling down until it becomes solid, it is among the ultimate examples of a product of Maillard reactions. Also, this way, all the components of milk are utilized to make food, both the curd (cheese proteins) and the whey, that which is left when the curd is taken out. ×

  Who was right? A couple of years after the publication of Sensible Cookery, a third person entered onto the stage. Frans Christian Faye, medical professor at The Royal Frederik’s University (now University of Oslo), held a lecture presenting empirical evidence apparently refuting Asbjørnsen’s hypothesis. He had carried out an experiment, today often done as a classical science experiment in schools: iodine is added to various foods in order to detect starch by staining the starch granules thus making them visible in a microscope. Faye had placed his research assistant on a porridge-based diet, cooked with “late addition of flour” to mimic the farm wives’ practice. The assistant was served porridge three times a day for six days, adding up to nearly 3.5 kg flour in total, followed by analysis of stool samples from the assistant. However, they were not able to find any detectable starch, so they concluded that cooking the porridge according to the procedure used by the farm wives did not make any difference; the starch was digested anyhow it was cooked. Thus, Asbjørnsen’s claim was seemingly disproved and Sundt could breathe more easily. On the other hand, Asbjørnsen’s import of foreign and modern science still had an important impact because this is said to have spurred the initiation of nutrition research in Norway. So, ethnologist Andreas Ropeid, who has written in the highest detail about this historical issue, concludes that Asbjørnsen and Sundt both got their way, and the feud had a happy ending.