Myosin is the meat protein principally responsible for creating a cohesive gel that binds a sausage mix or meat batter together. Why is "bind" important? Because, when you think about it, a sausage is something of a paradox: you start by cutting, chopping, or grinding meat and fat into separate pieces, and then you want it all to hold together as something new, a sausage or a pâté. Without myosin (or some other gelling agent), you would just have loose meat in a casing or slurry in a pan, which does not hold together.
In addition to binding the sausage mix together, myosin ensures a flavorful, juicy meat mix. When you extract enough myosin it will bind with free water in the mix, helping to retain it, while the water also disperses the myosin, allowing it to form a continuous matrix or web that traps fat particles, preventing them from coalescing and leaking from the meat mix when it's cooked. As Gauri S. Mittal poetically describes it, "The fat encased by protein is the basic unit in the ideal sausage emulsion. On subsequent heating, this protein coagulated and held the fat globules in a rigid web. Each fat particle was entrapped within this matrix, like honey in a honeycomb."
Probably one of the most important parts of the sausagemaker's art – and one of the least understood and hardest to master – is understanding what contributes to a proper bind as well as what can interfere with it, and selecting materials and working out methods accordingly. Not enough bind, and fat and liquid will be lost, leaving your sausage dry and crumbly. Too much bind (or not enough fat in the mix to create soft spots in the bind), and the sausage will be tough and rubbery.
Where Myosin Comes From
The proteins myosin and actin make up the smaller bundles or "myobrils" within muscle fiber and are responsible for the contraction and relaxation of our muscles. Hence they are generally referred to as contractile myofibrillar proteins. Together, they make up about 55-60% of the total muscle protein of vertebrate skeletal muscle, with the thicker myosin myofilaments yielding about twice as much protein as the thinner actin myofilaments. Actin alone does not have binding properties, but in the presence of myosin, acto-myosin is formed, which enhances the binding effect of myosin.
When it comes to binding ability, not all meat is created equal. Bull meat will yield almost twice as much protein as beef tongue and three times more than beef jowls. More mature animals, and animals that have access to the outdoors and a chance to exercise, will typically yield more myosin. For this reason, game meat yields significantly more myosin than meat from domesticated animals, and meat from shoulders and hams will have more binding ability than lesser exercised muscles. And some seafoods, such as shrimp, lobster, and crawfish have high binding power. Matters are further complicated by the fact that not all myosin is created equal. For example, myosin from the white meat of chickens has significantly more binding ability than myosin from the red meat. Finally, treatment of the animal prior to slaughter and of the carcass after slaughter can affect the binding ability of the meat. Undue stress just prior to slaughter results in pale, soft, exudative (PSE) meat, and this condition also leads to protein denaturation and reduces its extractability. Finally, "hot-boning" or removing and grinding the meat before the carcass has entered rigor mortis, although the practice is now rare in the US, will increase its binding ability due to phosphates naturally present in the meat.
Extracting Myosin and Processing
Extracting myosin from muscle meat requires salt. The myofibrillar proteins (of which myosin is the principal one) are only soluble in water in a concentrated salt solution (ideally at least 6%). This is a much higher percentage than you want in your final meat mix (typically 1-2%), so the best way to extract myosin is to cube or coarsely grind the lean meat, add the full amount of salt that your recipe calls for, and allow it to sit for a few hours or a few days before final grinding or chopping. The salt (sodium chloride) will extract water from the meat and form a concentrated brine ideal for extracting myosin.
Other salts can be even more effective. Phosphate salts, mainly sodium tripolyphosphate and tetrasodium pyrophosphate (usually sold in a blend as "special meat binder" by sources such as Butcher-Packer) are highly effective at extracting myosin, even at very low levels (typically 0.3%, around a couple grams per kilo, or less). So for low-sodium recipes, these can be very helpful in achieving a proper bind. Although phosphates have been approved for food use, some people have raised health concerns about them as an "unnatural additive." The fact is that phosphates are naturally present in meat before rigor mortis sets in, which is why "hot-boning," as mentioned above, increases the binding ability of meat.
Many sausage recipes do not add salt until all the ingredients are being mixed together, after grinding and just prior to stuffing. This can be problematic, as the salt will have very little time to extract myosin. In addition, the fat and any liquid in the recipe will further dilute the salt and limit its effectiveness. To overcome these problems, recipes often call for extended or vigorous mixing in order to try and bring sodium ions into contact with the meat. This can break down the cellular structure of the fat, making it more prone to leak rather than hold its contents (a result referred to as "smearing" the fat). This results in a dry sausage, as fat is lost during cooking. If salt is not added until the mixing stage, better practice is not to add any liquid and to let the sausage mix sit for a day or more before mixing again with liquid, in order to disperse the myosin that has then been extracted.
When liquid is added at the same time as the salt, a lot of that liquid will simply leak out of the mix, passing right through the casing, because there is insufficient myosin to bind it. In an experiment done by J. Kenji López-Alt, he compared batches of sausage made from meat that sat with salt for varying lengths of time. He found that sausage that had been salted only 1 hour prior to grinding lost half as much liquid as unsalted sausage when cooked and only one quarter as much if it was salted eight hours prior to grinding.
Monitoring temperatures during processing can also be critical to maintain the binding of fat and water in the mix, especially in the case of meat mixes that are minutely chopped – a process that generates significant heat. As the melting point of the fat is reached, the protein bond around the individual globules begins to rupture, allowing the fat to coalesce, and resulting in significant loss of fat and water during cooking. So processing temperatures need to be kept below the melting point of the fat being used: 8ºC (46ºF) for poultry, 12ºC (53ºF) for pork, and 18ºC (64ºF) for beef.
And, finally, the amount of myosin needed in order to achieve a proper bind is related to the size of the meat and fat particles. The more finely minced everything is, the more myosin required to bind to all the elements and encase them in a stable matrix.
Consequently, very fine meat mixes as well as mixes that do not add salt until late in the process usually require a supplementary source of protein in order to develop sufficient bind.
Protein Supplements and Replacements
To supplement myosin, there are other sources of protein that can be added to achieve a sufficient bind, such as eggs, concentrated soy protein, whey isolate protein, or nonfat dry milk (NFDM).
Transglutaminase enzymes (sold under the brand name Activa) are powerful agents for crosslinking proteins that otherwise would not naturally bind together. Only small amounts (around 0.25% of Activa) are necessary to achieve a good bind. These can be indispensable for making sausages or meat mixes with ingredients from which it's not possible to extract myosin, such as cooked and cured meats, cheese, etc.
These are the binding agents of interest to the sausage maker, as they gel and form stable bonds at cooking temperatures. Meat mixes that will be served at room temperature or chilled, such as pâtés, terrines, or meat pies, often rely on other additives that gel at low temperatures.
Gelatin is a meat protein derived from collagen with a great affinity for water. In terrines made of cooked meats (from which myosin can no longer be extracted), such as brawn or headcheese, a reduced broth containing plenty of gelatin is often the only binding agent. In addition, there are starches made from grain, such as wheat flour and cornstarch, which require cooking to release the starch molecules and then gelatinize as they cool. Root starches, such as potato, arrowroot, and tapioca, do not need precooking and gelatinize at lower temperatures.
When cooked, the myosin proteins set up or gel into a rigid framework that gives a sausage its "snap" or "bite." As Gauri writes, "On heating, the protein coagulates and holds the fat globules in a rigid web." This is something we've all seen when cooking an egg: the protein in the egg white sets up, becomes opaque, and is no longer runny. Once cooked, there's no reversing the process. This process is pretty much complete at 55ºC or 131ºF.
Above that temperature, myosin will shrink and coagulate further. As Harold McGee writes, muscle fibers "begin to shrink in width as soon as cooking begins, and start to shorten when the tissue reaches 130ºF (54ºC). By about 170ºF (77ºC), the cells have shrunk as much as they can, and are beginning to develop cracks and breaks." As they shrink, "the coagulating proteins are squeezing out the water that used to separate them from each other and that used to be trapped in their coiled structure. It's the same thing that happens when you twist the strands of a wet rope more tightly together, or squeeze a very wet towel." This is why a medium-rare steak is juicy, while one cooked well-done has dried out.
This is significant because it means that if you cook sausages to the temperatures called for by most recipes and required by Public Health authorities in the US (an internal temperature of 160ºF for ground beef, pork, veal, and lamb, and 165ºF for ground turkey and chicken), you will have disrupted the protein matrix and squeezed out much of the fat and liquid that you worked so hard to trap in the sausage mix. Such high cooking temperatures are called for in order to pasteurize the sausage and ensure a sufficient reduction of pathogens in the meat mix. But, as the authors of Modernist Cuisine rightly point out, pathogen reduction is a function of both time and temperature. The USDA's own time and temperature tables show a 7-log reduction in pathogens at 158º takes 0 seconds. To achieve the same level of pathogen reduction at 135ºF takes 37 minutes. Low-temperature cooking is best done with specialized equipment, such as a controlled-temperature water bath, but it is not out of reach of the home cook.
In addition to the final cooking temperature, the rate at which a meat mix is brought up to temperature also affects the gelling of the protein and its binding properties. In general, the slower the better. A slower cooking rate results in a stronger and more cohesive gel with better fat binding properties.
In short, the way most sausages get cooked – putting them on the grill right over the hottest part of the fire or searing them in a sizzling hot pan – is in fact the least desirable if the goal is a tender, juicy, cohesive sausage.
Suspension Not Emulsion
Culinary schools, numerous books, and even scientific articles often refer to sausage mixes or meat batters as "emulsions." There is even a whole category of sausages specifically called "emulsified sausages," which includes the humble hot dog.
Strictly speaking, this is not accurate.
An emulsion is defined as a mixture of two immiscible liquids, one of which is dispersed in the form of small droplets or globules in the other liquid. Liquid that forms the small droplets is called the dispersed-phase, whereas the liquid in which the droplets are dispersed is called the continuous phase. Mayonnaise, a mixture of oil dispersed in water, is a classic example. With meat batters and sausage mixes, we are not dealing with liquids.
In distinction, as Asghar lays out succinctly, "solids dispersed in a liquid (continuous) phase are designated as suspensions. In view of these definitions, the so-called meat emulsions (of frankfurters or similar sausages, and of several luncheon meat products) are not true emulsions. They can better be regarded as meat suspensions. In fact, all known meat emulsions consist of multiphase systems in which the continuous phase (called matrix) is a complex hydrophilic colloidal aqueous solution of salts and soluble proteins. Solid compounds, such as insoluble proteins, fat particles, and other insoluble components of muscle tissue and spices are dispersed and immobilized in the matrix to give body to the resulting product." "Hence," he concludes, "the properties and behavior of [so-called] meat emulsions could not be evaluated in terms of the classical theory relating to emulsion."
Myosin dissolved in the aqueous phase acts like an emulsifying agent, surrounding and coating the dispersed fat particles. So it may help to think of it like an emulsion. But treating meat batters just like an emulsion can lead to bad practice. Recipes sometimes specify intensive kneading or mixing, in order to make a proper "emulsion," but this leads to fat smearing and is detrimental to the texture of a good sausage. If sufficient myosin has been extracted, it simply needs to be evenly dispersed, not whipped into an "emulsion."
James C. Acton , Gregory R. Ziegler , Donald L. Burge Jr. & Glenn W. Froning (1982) Functionality of muscle constituents in the processing of comminuted meat products, C R C Critical Reviews in Food Science and Nutrition, 18:2, 99-121, DOI: 10.1080/10408398209527360
Elton David Alberle et al, Principles of Meat Science
Ali Asghar , Kunihiko Samejima , Tsutomo Yasui & Robert L. Henrickson (1985) Functionality of muscle proteins in gelation mechanisms of structured meat products, C R C Critical Reviews in Food Science and Nutrition, 22:1,27-106, DOI: 10.1080/10408398509527408
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Harold McGee, On Food and Cooking: The Science and Lore of the Kitchen.
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Nathan Myhrvold et al, Modernist Cuisine
Gauri S. Mittal, "Structural Changes in Meat Emulsions During Cooking at Various Process Conditions and Formulations," Food, Agriculture & Environment vol. 2 (1), 116-121.