Foams are fun. Marshmallows, meringues, cakes, whipped cream, cookies, ice cream, all of these are foams.
Foams are formed by several different processes. In many foams, such as whipped cream and beaten egg whites, an interesting thing happens at the interface between water and air. In both of these foams, proteins in the foam are first denatured, which, as the name implies, means that they are changed from their natural state.
Proteins are made up of building blocks called amino acids. Some of these building blocks are attracted to water and avoid oils and fats. Others are attracted to oils and fats and are repelled by water. In the natural state of the protein, the water loving parts are on the outside of the protein, next to the water, and the water avoiding parts are tucked inside, away from the water.
Proteins are big molecules, formed of strands and sheets of amino acids, all tangled up into a shape that is important for their natural function. When we beat the cream or the egg whites, the protein unfolds, like a carefully folded origami animal would if you beat it hard with a whisk.
As the protein unfolds, it encounters oils and fats in the cream, and air. The water loving parts of the protein still stay in the water. The water avoiding parts unfold so they can stick into the fats or into the air, to avoid the water. Eventually, the air bubbles become smaller and smaller as they are beaten, and they become surrounded by a film made of protein, to which some water is still attached. The proteins can now link together to form a tough film that holds the bubble in shape and prevents the bubbles from merging together again.
In whipped egg whites, you get bubbles with a protein film. In whipped cream, you get a bubble surrounded by a film of protein, surrounded by a film of fat stuck to the fat loving parts of the protein, surrounded by another film of protein that forms the inner wall of the next bubble.
We can make our egg white foam more stable by increasing the number of places where the proteins bond together. Beating the egg whites in a copper bowl causes the amino acids that have sulfur in them to bond together where the sulfur atoms are. Linking two sulfur atoms in this way forms a disulfide bridge, a very strong bond that helps keep the protein stuck in the new position.
Adding an acid like lemon juice or cream of tartar can also help form more bonds between the proteins and stabilize the foam.
Whipped cream forms a foam, but whipped milk does not (unless it is heated with steam). The reason lies in the nature of the proteins in milk and cream, and the nature of butterfat. But mostly it lies in the amount of solid fat compared to the amount of water.
Butterfat is a liquid at body temperature (anything above about 90° Fahrenheit), but it solidifies when chilled. This is why butter melts in your mouth. To whip cream, we need chilled, solid, butter fat. As we beat the cream, we form bubbles, the proteins denature, with some parts staying in the water and some parts staying in the fat, until we end up with a film of solid fat and protein that traps the air inside, with the water in between the bubbles.
If we beat the cream too much, we can turn the whole thing inside-out, with the water trapped inside films of fat and protein, and the air gets out. We have made butter. Where cream was tiny bits of fat in liquid water, butter is tiny drops of water in solid fat. One is a liquid, and the other is a solid, but both are made of the same stuff.
To keep whipped cream stable, we need to keep the temperature low enough that the fat stays quite solid. We can also stabilize it by adding more protein, such as gelatin, or some vegetable gums. Both help to link the proteins together and hold the fat in place.
Heating milk with steam, as in a cappuccino machine, denatures the proteins and links them together, and at the same time incorporates air into the foam. When the steam cools, it becomes water again. The foam is full of air, not steam.
If cream does not contain at least 30% fat, it will be difficult to whip. Most whipping cream is about 36% fat. Reduced fat whipping creams need the help of stabilizers. Most common are cellulose based ingredients called hydrocolloids, or food gums.
As you whip the cream, it gradually becomes stiffer. Maximum stiffness happens when the cream just starts to become butter. It will be slightly yellow in appearance, and the volume will have dropped a bit. The stiffness comes from the firm butterfat that has formed larger and larger particles on its way to becoming butter.
If your recipe uses whipped cream as a structural element, such as in cake icing or rosettes on a cream pie, you will want a nice stiff cream. For toppings on strawberry shortcake or other desserts, stopping the whipping when the foam is at peak volume will make it stretch farther.
Some cakes get their foam exclusively from beaten egg whites that are folded into the batter. But most cakes and breads get their foam in a different way.
Wheat flour contains a protein called gluten, which is formed when enzymes in the flour react with precursor proteins as water is added. Gluten is gluey, and as you mix the batter or knead the dough, the little bits of gluten that form stick together and form rubber-like sheets. Stirring and folding incorporate air to form little bubbles in the sheets of protein. Yeast or other leavening agents add gas inside the bubbles and make them expand. Heating the dough further changes the protein, denaturing it into a solid.
A Bread Recipe
Basic bread is fairly simple. You want some flour, some water, some yeast, and optionally some sugar or honey, salt, and/or oil, butter, or some other fat.
What do those ingredients do, and how much of each do we need?
The flour provides the gluten, starch, flavor, and bulk of the bread.
Water is necessary to make the gluten, and allow the yeast to multiply and produce carbon dioxide gas. The yeast is there to make the carbon dioxide gas so we have a foam instead of a brick.
All other ingredients are optional. The salt is not there as a seasoning. There really isn't a lot of it in most breads. It is there to slow down the yeast. If the yeast provides too much gas too fast, it will form faster than the gluten does, and the gas will simply escape as the bubbles pop. But many recipes omit the salt. Some of the gas will escape, but the recipe usually calls for the size of the bread to double, and that will eventually happen with or without the salt.
Sugar or honey is often added to feed the yeast. But the yeast will find enough food in the flour without it. It will just grow a little more slowly, which (as we saw with adding salt) can be a good thing. But if you are making a lot of bread, and start with a small amount of yeast, you can grow the yeast you need in a little sugar water. The amount of sugar or honey is generally so small that it makes little difference to the taste of the bread.
Adding fat (oil, butter, margarine, shortening, lard, etc.) will prevent the gluten from forming large sheets. The fat gets in the way of the small sheets joining up, and "shortens" the strands and sheets of gluten. Hence the word "shortening". Some recipes have you oil the outside of the dough to keep it from sticking to pans, fingers, and breadboards. Others have you paint melted butter on top of a baked loaf to keep the crust from getting dry and hard. Neither of these uses has much or any effect on the interior of the loaf.
Bread flour is flour grown and processed to have a lot of gluten. Cake flour is designed to have less gluten. All-purpose flour is a mix, and has an intermediate amount of gluten. If you are using all-purpose flour, you probably won't have much use for shortening in bread dough, but it will make your cakes more tender and cake-like, and less bread-like. The chart above shows ten simple bread recipes collected from different sources, with the ingredient amounts converted into percentages to make it easy to see how variable a basic bread recipe can be and still end up producing very similar results. The last column in the chart is the average recipe.
Converting the average recipe into a one-loaf batch by assuming 2 teaspoons is 1% and rounding the numbers we have:
1. 144 Teaspoons of flour (3 cups)
2. 56 Teaspoons of water (1 1/8 cups)
3. 1 Teaspoon of sugar
4. 1/2 Teaspoon of salt
5. 2/3 Teaspoon of yeast
6. 1 Teaspoon of melted butter
Saving the butter for painting on the top of the loaf when the bread has baked, we mix all the other ingredients in a bowl until they are well blended.
The next step is to help to process the gluten. There are some recipes that do not call for kneading the bread. These recipes let time do the work for you. You simply let the dough work all by itself overnight and most of the next day, all alone in its bowl, covered by a damp towel.
Most recipes, however, assume you want your bread today. To speed up the formation of the sheets of rubbery gluten, the dough is set onto a floured board (so it doesn't stick to the board), and folded over and pressed flat repeatedly, for something like 8 to 10 minutes, adding flour as needed to prevent it from sticking.
This process has the side effect of removing most of all of the bubbles that may have been in the dough. To get the bubbles we need, we next let the bread sit in a place where it won't dry out (usually a greased bowl covered with a damp towel).
Let the dough rise for 15 minutes, or until it has doubled in size. The actual length of time here is not very important. Recipes vary quite a bit. Some have you let the dough double, then punch it down and let it double again. How much you play with the dough will depend on how much gluten the flour has, and how much gluten you want to develop. But the result, whether it is a soft, light cake-like loaf or a rugged firm and hearty loaf, will still be recognized as bread.
The loaf is next baked. It can be placed in a loaf pan or just on a greased cookie sheet. You can decorate the top by scoring it with a knife. You can form the dough into ropes and braid them together. All of these are just decorative variations.
The oven for bread is generally hot, about 400° Fahrenheit (200° Celsius). The time it takes to bake is 40 to 50 minutes, or until you like the color of the crust.
When the baking is done, brush the top with the melted butter. If you like a soft crust, you can let the loaf cool in a plastic bag. For a dry, hard crust, just let it cool on the countertop.
Some recipes call for placing a pan of water in the oven along with the loaf, to keep the crust softer and speed the baking (moist air conducts heat faster than dry air). This is optional.
Now that you know why the ingredients are there, and why the processing steps are needed, you can throw away your measuring equipment and do the whole thing by eye.
Dump some flour into a bowl. Add some yeast. Add some water gradually, stirring until the dough is about the consistency you remember from the times you made bread from a recipe. Knead the dough for a while on the floured cutting board. Let it double in a greased bowl with a damp cloth cover. Form a loaf on a baking sheet, and put it into the oven. Turn the oven to 400° and bake until you like the color.
That's how bread was made for centuries before the invention of measuring cups and ovens that kept a steady temperature. Usually the yeast was just a bit of bread dough saved from the last batch. Add a little water and sugar, and your 'starter' will grow plenty of yeast for your next bread baking session.
Yeast is a convenient leavening agent (something that makes bubbles of gas in a dough). Yeast spores float in the air, and form white films on grapes and plums and other fruits with thin skins that allow the sugars to get to the surface. You can make yeast starter for your own wine, beer, or bread by culturing the white film from grapes in a little sugar water.
But some breads just use steam and hot air for leavening. Popovers are an example of steam-leavened bread. But the prize goes to popcorn. We go to all the trouble to grind wheat, add yeast, knead, and bake, all to get a foam made from seeds, when popcorn does it all with just some heat. Puffed wheat and puffed rice are made by heating the seeds under steam pressure (in a big pressure cooker called a 'gun') and then suddenly releasing the pressure (called 'firing the gun'). The whole process takes less than a minute.
So-called 'quick' breads (because you don't have to wait for the yeast to grow, or the gluten to develop) use baking soda and an acid, such as buttermilk, to form bubbles of carbon dioxide gas. Since these breads are not kneaded or left to themselves overnight, they have little gluten, and are more like cakes than a sturdy loaf.
Baking soda is sodium bicarbonate: There is a carbon atom in the middle (carbon atoms are so common that chemists don't bother putting a big 'C' where they are), with three oxygen atoms, a hydrogen atom, and a sodium atom. When sodium bicarbonate is added to water, it breaks apart into three ions (charged fragments of a molecule). These are a positively charged sodium ion Na+, a negatively charged hydroxide ion (OH-), and carbonic acid, which is what we call soda water (carbon dioxide dissolved in water).
If we add an acid, like vinegar (acetic acid), a reaction occurs, and we get sodium acetate and water as products when the acid reacts with the sodium and hydroxide ions. What is left is carbonic acid. Carbonic acid (carbonated water) fizzes, releasing bubbles, just like in your soda.
Any common acid will react with baking soda this way, so the lactic acid in buttermilk, the citric acid in lemon juice, or the acetic acid in vinegar can be used.
But bubble formation is only half of what happens in the bread. As we bake the bread, the heat causes the gas to expand. The heat also denatures the proteins and starches, allowing them to link up into solid webs, holding the shape of the bubble even after the gas has cooled. Air from the room slowly fills the spaces where the steam and hot expanded gas used to be, as the bread slowly cools.
If the baking has not completed, and the proteins are not yet firm, then allowing the temperature to drop can cause the bubbles to all shrink back down, and we say that your soufflé has fallen. This can happen with breads and cakes as well as fluffy egg dishes.
But firm protein networks are not the whole story. During baking, what started out as a foam (a collection of closed bubbles) becomes a sponge (where all the bubbles have broken to form an open network that air and water can flow through). The heat has not just firmed the proteins and made them bond together, but it has expanded the gas in the bubbles to the point that the bubble walls have broken, letting the gas escape.
This is important because otherwise the gas would cool and contract, and the resulting vacuum would crush the bubbles back into a dense mass of dough. Try putting a little bit of water into an empty aluminum soda can and heating it on the stove until it is full of steam.
Other proteins can also make foams. One simple protein is gelatin, which is used to make marshmallows.
Marshmallows are made by cooking a sugar syrup to the firm ball stage (240° Fahrenheit, 116° Celsius), and beating that into gelatin that has softened in cold water.
As with other protein foams, the gelatin will denature as the hot syrup and whisking cause it to form links with itself, forming a sturdy net, with the syrup attracting the water loving parts of the protein, leaving the oil loving parts facing the air in the bubbles.
We can divide a marshmallow recipe into parts, and look at why each part is there. The first part is sprinkling gelatin powder into cold water. This is to soften the gelatin without changing the shape of the protein molecules. We only want them changing shape when we beat in the syrup.
The next part of the recipe is making the syrup. Most candy recipes are primarily about controlling the crystallization of sugar. This is done by controlling the concentration of sugar in the water, making sure there are no seed crystals falling into the syrup to prematurely begin crystallization, and providing simple sugars in addition to the sucrose (a complex sugar made up of the two simple sugars glucose and fructose).
Simple sugars bind more tightly to water than sucrose does, and so they don't crystallize as easily. There are two basic ways to get simple sugars into a syrup. The first is to simply add them. A candy recipe that calls for corn syrup in addition to sugar is doing just that. Corn syrup is mostly simple sugars.
The second way is to break up the sucrose into its two simple sugars. You do this by heating it in the presence of an acid. A candy recipe that calls for cream of tartar (tartaric acid) is doing that. Other recipes might call for vinegar, lemon juice, or other acids to cook with the sugar.
The third part of a marshmallow recipe is where we actually make the foam. Generally the syrup is slowly added to the gelatin mixture while the beaters are running at medium speed (to prevent splashing), and then the speed is turned up to high for a good 15 minutes or so, to denature the proteins and form a stable foam. The foam is then turned out into a greased pan to sit for half a day (8 to 12 hours) to let the protein net complete bonding.
Often the bulk of the recipe deals with controlling the stickiness of the whole mess. Pans are greased, lined, or lined and greased. A mixture of corn starch and powdered sugar is spread out over a cutting board and the cooled foam is dumped out of the greased pan onto it, and the powder is generously applied to cut surfaces as the candy is divided into bite sized pieces. The knife used to do the cutting is oiled with vegetable oil for the first cut, and then kept coated with the powdered sugar and corn starch for subsequent cuts.
Marshmallows were originally a way of making the cough suppressant in the root of the marsh mallow plant palatable. The starch in the root was mixed with egg white as the protein, and the syrup was beaten in. Some recipes still mix in beaten egg whites with the gelatin foam.
You can make foam without protein. The suds in the dishwater are made of just water and detergent. Detergent, like protein, has a part of the molecule that loves water, and a part that avoids water in favor of air or fats. It is used to clean up greasy dishes because the fat loving ends attach to the fat, and the water loving ends prevent fat globules from coalescing together by making a protective coat around them.
But soap suds are not a stable foam. Eventually, all the bubbles pop, and you are left with plain dishwater again.
But if we make the foam out of something that hardens when it cools, then we don't need a protein to make the foam stable.
A kind of candy called honeycomb is just that sort of foam. You start by cooking a sugar syrup (sucrose and simple sugars) to 300° Fahrenheit (150° Celsius), the so-called hard-crack stage. This is the stage just before the sugar burns (caramelizes) and turns brown. When this syrup cools, the result is hard candy, like a lollipop.
As in any candy cooking, we are careful not to let any sugar crystals fall into the syrup from the side of the pan. This would cause premature crystallization, and result in a grainy texture instead of a glassy one.
When the syrup has reached 300°, we add baking soda and quickly beat the syrup. The high heat causes the baking soda to decompose into sodium carbonate and carbon dioxide bubbles. Note that this happens with heat alone, and unlike earlier, when we talked about quick bread, no acid is needed.
The syrup will foam up and triple in volume as we whisk it.
The last step is to pour the foam out into a pan prepared (as with the marshmallows) with grease, lining, or both, to prevent sticking.
This kind of candy will absorb moisture from the air easily, and become a soggy mess unless it is coated in chocolate, or kept in a sealed container. Of course you could just eat it all right away.