The Science of The Bacon, Egg, and Cheese Sandwich

A New York style bacon, egg, and cheese sandwich on a bagel (with salt, pepper, and ketchup, of course) 

    First, let’s understand what a bacon, egg, and cheese is. A bacon, egg, and cheese is a breakfast sandwich that contains- you guessed it- bacon, egg, and cheese. Bacon, egg, and cheeses take many forms, but often the ingredients are sandwiched in between two pieces of bread (usually a roll or bagel) and topped with salt, pepper, and tomato ketchup. The bacon, egg, and cheese has become a staple breakfast item, the best known to be sold from New York bodegas. You may be asking yourself, "what does a breakfast sandwich have to do with science?" Like many dishes, the seemingly simple ingredients the bacon, egg, and cheese is composed of come from not so simple origins. In this post, we will explore the science that goes into the creation of each of the bacon, egg, and cheese’s main ingredients.

Bacon

    Early forms of bacon can be traced back to China thousands of years ago in the form of salted pork belly. Pork curing methods later made their way to the Roman Empire and Europe where the term “bacoun” was used to refer to pork in general. In America, bacon refers to the belly meat of the pig, differing from the back-derived version common in Europe. Before refrigeration, curing and smoking food items was a common preservation method used to make food last longer. Over decades, bacon has become a quintessential food item for Americans. Bacon can be enjoyed on everything from burgers to donuts and very few complete American breakfasts lack the salty treat. One of the things that draws Americans to bacon is its variance in textures. Some enjoy it crispy - some enjoy it floppy - and some (including me) enjoy it somewhere in between. The variance in texture is possible due to the unique mixture of fat and connective tissue found in belly bacon.

Bacon comes from the belly section as shown above

Fat Content and Texture

    The pork (pig) belly is known for its characteristic high fat content and toughness. Uncooked, the belly of a pig has a very tough texture which serves as the only barrier between the animal's organs and the outside environment. Pork belly maintains its tough structure by being made of a high amount of connective tissue and collagen. The collagen fibers form parallel bundles that run along the meat, sheathing the muscle and making contractions more efficient. The other layer of protection pork belly has is the high amount of intermuscular fat and adipose tissue. Adipose tissue is a specialized connective tissue consisting of lipid-rich cells called adipocytes that cover the outer surface of muscles. The main function of adipose tissue is to store energy in the form of lipids (fat). Besides energy storage, fat tissue has several other significant functions which include thermal isolation, cushioning the organs and muscle, an endocrine role, and the production of numerous bioactive factors. Fat molecules are made up of hydroxyl groups on a glycerol backbone that reacts with carboxyl groups of fatty acids in a dehydration synthesis reaction. This yields a fat molecule with three fatty acid tails bound to the glycerol backbone via ester linkages, called triglycerides. The linkages binding the fatty acids to the glycerol backbone contain an oxygen atom next to a carbonyl, or C=O, group. Triglycerides can contain three identical fatty acid tails or three different fatty acid tails. The high amounts of fat and connective tissue serve the pig well while it is alive, but after it is harvested, these attributes make cooking the meat more difficult. The curing, smoking, and frying process the pork goes through turns this difficult cut of meat into something delicious.

Comparison between triglyceride (top) and monoglyceride (bottom). In the example shown above, the three fatty acids attached to the glycerol backbone are all of different lengths.

Curing and Smoking

    One of the most important steps in the bacon-making process is curing and smoking. When the pork belly is removed from the whole hog, the meat must undergo a series of steps in order to ensure its stability. The first step is curing. The point of curing is to draw excess moisture out of a food item in order to limit the possibility of mold and bacterial growth. This is done by creating a solute gradient between the inside and outside of whatever is being cured. 

    Curing can be done in two different ways depending on the desired texture. The first way that bacon can be cured is by placing pork belly in a dry mixture of salt and sugar. The meat is left in the mixture for anywhere from hours to a couple of days depending on the thickness. Moisture is drawn out of the meat, giving it a more firm texture. The second way is by submerging or spraying the meat with a liquid salt and sugar brine and allowing the meat to dry in a cool, dry environment. This method removes less moisture than the dry brining process resulting in a softer texture. This is often the method that is used in commercial factories. It is important to note that in addition to sodium chloride (NaCl), sodium nitrate (NaNO3), also known as pink curing salt, is used to preserve wet-cured cooked meats like ham and bacon. This is to prevent Clostridium botulinum, a bacterium that is present in soil and water, from growing on the meat. The spores of the bacteria can become airborne and land on food where, under the right conditions, they can reproduce. When it reproduces, it creates the toxins that cause botulism, a foodborne illness that is rare but extremely severe. Pink curing salt helps stop the bacteria, preventing reproduction and the growth of the toxin.

    In both the dry and wet brining process, spices and herbs are mixed in to impart certain flavors onto the pork; these can include pepper, garlic, onions, bay leaves, and clove. The principle behind curing is one that governs much of natural life - osmosis. Osmosis is a process by which molecules of a solvent (usually water) tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one until an equal concentration on each side of the membrane is reached. In this case, The salt and sugar on the surface of the meat create a situation where there is more solute present on the outside of the pork belly. The water from inside the cells of the meat flows along its concentration gradient out of the meat, resulting in a dried product. From there, the bacon is cold smoked (meaning the smoke is not hot enough to cook the meat) in order to impart a smoky flavor and further dry out the meat. Similar to the curing process, flavor can be imparted onto the meat by using various types of wood such as oak, maple, hickory, and apple. After the smoking and drying process is completed, the processed bacon is sliced, sealed, and shipped off to grocery stores across the country.

 Slabs of smoked bacon 

This is an example of a wet cure. the bacon is sprayed with a brining solution and hung on hooks to dry

Browning and Maillard reaction

    Once the bacon makes it to your house, it can be prepared in many different ways, but for the bacon, egg, and cheese sandwich, it is usually pan-fried. Pan-frying the bacon not only fully cooks the bacon, but also allows the desired texture to be reached through browning. The actual term for this is called the Maillard reaction. While the reaction itself is extremely complex, the basics are that many small, simultaneous chemical reactions occur when proteins and sugars in and on your food are transformed by heat; these reactions produce new flavors, aromas, textures, and colors. The Maillard reaction is responsible for the tastes present in caramel, steak, coffee, and foods cooked in butter. These preservation and cooking processes result in the creation of a beautiful thing - BACON!

Fully cooked bacon

Egg

The next part of the infamous bacon, egg, and cheese sandwich is…EGGS! As off-putting as it may be, laid eggs are the embryonic stages of oviparous animals. They are composed of multiple sections that, if fertilized and incubated, will grow into juvenile versions of their species. Because they serve as mini-homes for embryos they are stocked full of protein, vitamins, and minerals that are passed along to whatever may eat them. For most people, chicken eggs are the only type of egg they eat, but it should be known that various poultry and reptile eggs are eaten within various global cultures. The bacon, egg, and cheese sandwich are most commonly made with chicken eggs. Chicken eggs have a somewhat mild flavor when compared to other poultry which has made them a favorite across the world. While eggs are most synonymous with breakfast, they are prepared in many different ways, from soups, baked goods, stir-fries, and even raw. Though, before the egg can be fried, scrambled, or poached for one of those preparations, a lot must happen in the chicken for the egg to get to this step. But, before we can understand how an egg is made, we must understand the parts of one.

Parts of an egg diagram

Yolk. This is the yellow part near the center, where almost all the egg's nutrients and fat are found

Albumen. This is the clear part we call the egg white. It’s called this because it turns white when cooked. There are two layers of albumen: thick (near the yolk) and thin.

Chalaza. Located in the thick albumen, chalaza is simply albumen that is twisted tightly. It keeps the yolk in the middle of the egg and prevents it from sticking to the inside of the shell.

Shell membranes. The egg contents are surrounded by two thin membranes called the inner and outer shell membranes.

Shell. This is the outer covering of the egg holding everything together. The shell consists of 94% calcium carbonate.

Egg Formation

    The egg is formed in the reproductive tract of a female chicken. The reproductive tract is divided into two major parts: the ovary and the oviduct. The ovary is where the yolk is added. When the yolk reaches the right size, it is released during ovulation. Ovulation is caused by the release of hormones. The released yolk is then picked up by the infundibulum, a funnel-shaped segment adjacent to the ovary. It is up to 9 centimeters long in the laying hen and has the function of searching for and engulfing the yolk that has just been released from the follicle into the adjacent ovarian pocket or body cavity. It is here that fertilization must take place. The yolk then passes to the magnum and isthmus, where the albumen shell membranes are added. Finally, the egg makes its way to the shell gland, where the shell and any shell pigments are added, before being pushed out. The eggs that most people consume are not fertilized, meaning that it has not fused with sperm from a male chicken. This also means that the egg has no chance of growing into a chick or chicken. Similar to humans, female chickens release eggs regardless of whether they are fertilized, the only difference being that chickens release eggs on a daily cycle rather than monthly.  

    Now that we have formed an egg, the next step is to cook it. For the bacon, egg, and cheese sandwich, the egg is usually pan-fried until the whites and yolk are fully set.

Hard fried egg (white and yolk are set) 

Cheese

    Cheese is a dairy product made from the curdled milk of land mammals, almost always grazers. Cheese-making is referred to in ancient Greek mythology and evidence of cheese and cheese-making has been found on Egyptian tomb murals dating back over 4,000 years. It is theorized that cheese may have been discovered accidentally by the practice of storing milk in containers made from the stomachs of animals. Most cheese is made from the milk of dairy cows specifically bred for dairy production. Cheese is aged and curdled in a variety of methods to produce many types of cheese.

Cow lactation

    Like all mammals, cows only produce milk after giving birth. Dairy cows must give birth to one calf a year to continue producing milk. After pregnancy, the levels of the hormones estrogen and progesterone fall initiating the hormones oxytocin and prolactin to increase and initiate lactation. Although various hormones take effect in milk production, prolactin is the key chemical. To increase milk production and ensure lactation year around, dairy cows are often given additional hormones. The milk produced contains the vitamins, calories, proteins, and hormones necessary to support a growing calf. It is due to its nutrient density that milk has become a staple beverage worldwide.

Curdling 

    In order to turn milk into cheese, it must go through many enzymatic processes. The first step is pasteurization, or rapid heating of the milk to kill any bacteria present. After the milk has been cooled, enzymes called rennet are added to the milk to begin coagulation. Rennet is the general name given to enzymes that act on the proteins in milk. It is derived from the stomach of adolescent ruminants (baby hoofed herbivores like calf and lambs) while their diets are still limited to milk; in their stomachs these enzymes aid in digestion. The milk is allowed to set until it reaches a pudding-like consistency. The curdled milk is then cut into very small pieces and mixed, causing the milk to separate into cheese curds and whey. Curds are the solid protein-bound pieces of curdled milk that will later turn into cheese and whey is the liquid that separates out. After the milk is separated, the liquid whey is drained off and the cheese curds are kneaded to form more protein within the solids. Salt is added and the cheese curds are then formed into blocks. From here, the cheese can be taken down numerous different paths.

Workers are kneading and packing cheese curds into blocks 

Aging

    What separates cheese from cheese curds is aging. Depending on the flavor and texture of the desired cheese, the curds will be aged for months up to years. During this aging process, moisture leaves, lactic acid fermentation occurs, and certain molds are allowed to grow. Longer-aged cheeses often have a harder, more grainy texture, and don't melt very well. Examples of this are cheeses like parmesan, aged cheddar, and Pecorino Romano. These cheeses often gain characteristic flavors like nutty and coffee notes. Longer-aged cheeses also tend to have a distinctly funky smell from the fermentation process. As delicious as those long-aged cheeses are, most cheese eaten on sandwiches like the bacon, egg, and cheese are not aged for as long. These cheeses include American, swiss, provolone, and young cheddar. These cheeses tend to have more moisture, a milder flavor, and a lower melting point. These characteristics make them great for sandwiches!

Cheese is allowed to age in a temperature and humidity controlled room, often on wooden shelves like those pictured above.

Assembly

Now that we have all of our ingredients the assembly of the perfect breakfast sandwich can finally begin. Our crispy bacon, fried egg, and American cheese can be stacked on a roll or bagel, topped with ketchup, and devoured. In my opinion, knowing the science behind arguably the best breakfast sandwich just adds to the joy of my first bite!

A fully assembled bacon, egg, and cheese sandwich