Foam stages

At the glossy "soft peak" stage, foam retains some shape but droops. The foam does not cling to the bowl. Liquid is still likely to drain to the bottom of the bowl.

At the glossy "stiff peak" stage", foam retains a well-defined edge and clings to the bowl. The foam approaches 90% air and the egg liquid is spread thin. There is just enough lubrication to make the foam creamy and mix easily with other ingredients. This is the optimum stage for mousses, soufflés, sponge cakes, and similar dishes.

If the foam is beaten past the glossy stiff peak stage, it enters into the slip-and-streak stage. The appearance is dull and dry. The foam has a crumbly consistency and begins to leak liquid. The proteins in this stage are bonding so closely that they squeeze out the liquid between them. It no longer clings to the bowl. Pastry chefs use this stage for meringues and cookies, and then stop the overcoagulation by adding sugar which separates the proteins again and absorbs the water. Past this stage, the foam loses volume and gets grainy.

Egg, bowl, and whisk

Just about any egg, bowl, and whisk can provide a good foam. Here are some tips to consider:

• Older eggs at room temperature have thinner whites and foam more quickly, but by the same token a thin egg white is more likely to drain and older yolks rupture more quickly, often leaving traces of yolk in the white.
• Fresh eggs take longer to beat but make a more stable foam.
• Cold yolks are easier to separate from whites. The whipping process warms the whites anyway.
• Egg foams can also be whipped from dried egg whites.
• Use a bowl large enough to accomodate an eightfold expansion of volume.
• If beating by hand, a large "balloon whisk" aerates a greater volume of whites at a time.
• When using a mixer, choose a beater that both spins and traces a curlicue path for a more even foam. Other mixers and beaters work, but less efficient beaters produce a denser texture.

Effects of other ingredients

• Salt weakens structure as its ions Na+ Cl- compete for bonding sites on the unfolded proteins. It's best to add salt to the other components of a dish rather than the foam itself.
• Sugar both hinders and helps foam making. When added early, it delays foaming and reduces the ultimate volume and lightness. At this point, the sugar interferes with the unfolding and bonding of the proteins. The syrup-egg mixture is also heavier and harder to spread into bubbles. It takes twice as long to foam. However, sugar improves foam stability by slowing down drainage from bubble walls. In the oven, dissolved sugar delays moisture loss until after ovalbumin has had time to coagulate and reinforce the foam. Sugar also adds its own reinforcement to the structure in the form of cotton-candy-like strands of dry sugar. Most times, sugar is added to the egg whites after the foam has already begun to form, though sometimes its added at the beginning for a more firm, dense foam.
• Water in small amounts can increase the volume and lightness of foam, but since it thins whites, it is more likely that some liquid drains from the foam.

Protecting the foam

Though proteins are responsible for giving foams structure, they can also destabilize them. If whipped too long, the proteins form tight bonds that squeeze out the water. This causes the foam to lose volume, become grainy, and separate into a dry froth and runny liquid. There are simple ways to prevent this catastrophic collapse:

The presence of copper or silver in foaming egg whites binds to reactive sulfur groups. This renders the proteins unable to form other strong bonds, which prevents proteins from getting too close to each other and collapsing the foam. For this reason, the use of a copper bowl for whipping egg whites has long been recommended. If a copper or silver plated bowl is not available, the cook may add a pinch of powdered copper supplement to obtain the same result.

Bonds with sulfur groups can also be prevented by adding an acid such as 1/8 tsp cream of tartar or 1/2 tsp of lemon juice. The acid boosts the number of available hydrogen (H) ions in the mix, making it harder for the S-H groups to shed their hydrogen and form strong sulfur bonds with anything else.

Other things to keep in check include egg yolks, fats/oils, and detergent. Traces of these three things interfere with protein bonding. Their presence does not prevent a foam from forming, but it takes longer and the quality suffers. Fats and yolks can be added once the foam is formed.

Egg foams

Usually I expand more on the basic topic, but as eggs are one of the most versatile food items, I am going to skip and focus the conversation on information that is relevant to the meringue recipe.

The foaming power of egg whites came around the 17th century. At the time, the fork was a still a novelty. Cooks used twigs, bits of dried fruit, or bundle straw to whip the whites into meringues and soufflés.

Physical agitation usually breaks structure, but in egg whites it actually creates structure. Agitation of the sticky egg white transforms it into white foam, a cohesive structure that holds its own when mixed and cooked.  Whipping exerts physical stress that unfolds the compact protein molecules. Air becomes incorporated into the whites. It creates an imbalance of water and air that promotes bonding of globulin proteins and ovotransferrin. The bonded proteins become a solid network that pervades the bubble walls and hold water and air in place. Thus the egg white becomes a semi-solid mass of minute bubbles called a foam.

Left alone the foam eventually collapses. Heat transforms it into a permanent solid. Ovalbumin does not contribute much to the foam network, but heat makes it unfold and coagulate. In a cooked foam, ovalbumin more than doubles the amount of solid protein reinforcement in the bubble walls. As water evaporates, the semiliquid foam turns into a solid.

Tips on egg handling and storage

• Egg grades are classified by the USDA as an approximation of quality at the time the egg was laid. Only the top two grades, AA and A are seen in stores.
• Eggs have the ability to remain edible for weeks as long as they are kept intact and cool. Egg deterioration begins as soon as the egg is laid. The egg suffers changes in pH from acidic to alkaline. At more alkaline pH, the egg whites clear as proteins repel each other instead of forming light-blocking complexes. The whites become more runny. The yolk becomes weaker due to an influx of water that increases its volume and stretches its membranes.
• One can test egg freshness by placing eggs in a container filled with water. Fresh eggs sink rapidly to the bottom. Older eggs stay afloat longer.
• An egg deteriorates as much in a day at room temperature as in four days under refrigeration.
• It's best to buy eggs out of a cooler, not from an open shelf. Keep cold.
• An airtight container slows moisture loss and absorbion of odors.
• To freeze eggs, remove them from the shell. Freeze whites separate from yolks. It's best to mix yolks with a 1 tsp salt, 1 tbsp of sugar, or 4 tbsp lemon juice per pint to prevent proteins from aggregating and obtaining a pasty consistency once thawed. To freeze whole eggs, half the treatment for yolks.
• Salmonella infection of eggs is low, but still a problem. Salmonella is killed if eggs are cooked at 140 degrees F/60 degrees C for 5 minutes, or at 160 degrees F/70 degrees C for 1 min.
• If eggs are to be consumed or used raw, try pasteurized eggs or freeze dried eggs.
• One can tell the difference between a cooked egg vs a raw egg by giving it a spin on its side. If its smooth, then it's cooked. If it is wobbly, it is raw.

The white

The white accounts for two thirds of the shelled egg's weight. It is made of 90% water. The rest is made up of proteins, minerals, fatty materials, vitamins, and glucose.

The proteins in the whites play important roles in protecting the embryo. Some proteins block the action of digestive enzymes. Others bind tightly to vitamins and/or iron. One protein inhibits the reproduction of viruses, and another digests the cell walls of bacteria. The egg white is a chemical shielf against infection and predation.

The following proteins are important for the cook:

• Ovomucin accounts for <2% of albumen but has the most influence on the eggs culinary value. Ovomucin pulls together the proteins into an organized structure. It is what makes the thick egg white thick. It gradually desintegrates, so older eggs have a more runny texture.
• Ovalbumin is the most dominant egg protein. Its role is not entirely understood but it is thought to help inhibit protein-digestive enzymes. It is the only egg protein to have reactive sulfur groups, which strongly contributes to the flavor, texture, and color of cooked eggs. Ovalbumin becomes more heat resistant as the egg ages, so fresher eggs need less cooking than older eggs.
• Ovotransferrin holds tightly to iron. It is the first protein to coagulate when an egg is heated, thus it determines the setting temperature. Whole eggs set at higher temperature than egg whites because ovotransferrin binds to the iron in the yolk and becomes more resistant to coagulation. Ovotransferrin changes color when bound to metals, which is why whites whipped in a copper bowl turn golden and whites supplemented with ground-iron turn pink.

Egg whites are highly immunogenic. Portions of ovalbumin tend to be responsible. Sensitivity to egg whites develops early in life. Pediatricians recommend that infants and children under 1 do not consume egg whites.