C. Lynn Knipe, 1987. Meat Emulsions. Proceedings of the Eighth Annual Sausage and Processed Meats Short Course. ISU Extension Service CE 2113
A true emulsion is a colloidal suspension of two immiscible liquids (i.e. two liquids which are not soluble in each other, as in the case of oil and water). Typically, a non-polar liquid is dispersed within a polar liquid. A third component, an emulsifying agent is required for the development of a stable emulsion. The emulsifying agent acts at the interface between the fat and water to prevent coalescence of the fat (i.e. to prevent fat pockets or caps). Good examples of oil-in-water emulsion are mayonnaise and milk.
We have conventionally referred to a finely chopped meat mixture as a meat emulsion. This is somewhat a misnomer as the so-called meat emulsion consists of solid fat particles dispersed in a liquid continuous phase. It might be more appropriate to refer to this mixture as a meat batter. However, for the remainder of this presentation, we will use meat emulsion.
Meat proteins serve as the emulsifying agent in a meat emulsion. To form a stable meat emulsion, these proteins must surround the finely chopped fat particles before cooking. Meat proteins vary in their basic composition of amino acid side chains. Amino acid side chains consist of either positively or negatively charged groups and may be either polar (like water) or non-polar (like fat) in nature.
Myosin, the major structural protein of meat, is the most important of the proteins for fat emulsification and water holding capacity of processed meats. It is believed that myosin may bridge the oil-water interface as the non-polar amino acid residues of the myosin tail would be attracted to the fat cell surface while polar amino acid residues of the myosin head would be associated with the water phase.
There are a number of factors that affect the ability of myosin to emulsify fat. The first factor has to do with the basic condition of the meat at the time of use. At the time of slaughter, myosin in animal muscles is in a readily usable state, however, after rigor-mortis has set in, myosin combines together with actin to form actomyosin. Actomyosin, while not a poor water and fat binder, is not as good as myosin and not as readily solubilized at the salt levels that we use in meat emulsions.
There are other factors involved in the stability of meat emulsions which can be affected by the addition of various dry ingredients. Water holding and emulsifying capacities of meat are greatly affected by pH. Water holding capacity (WHC) of meat could be compared to the action of a sponge and is important to meat processing in that as proteins are able to hold more water they become more soluble. WHC in meat is at a minimum at what is called the iso-electric point (pI) of proteins. The pI is the pH at which all side chain groups are charged. At this point, equal positive and negative charges on the protein result in a maximum number of salt bridges between peptide chains and a net charge of zero. The pI of meat (where WHC is at a minimum) is in the pH range of 5.0 to 5.4 which is also the pH of meat after it has gone through rigor.
Increasing or decreasing the pH away from the pI will result in increased WHC by creating a charge imbalance. A predominance of either positive or negative charges will result in a repulsion of charged protein groups of the same charge and increased capacity for water retention. This repulsion could be compared to the effect of like-charges of two magnets.
Numerous researchers have observed solubility of the salt soluble muscle proteins with increased pH. Other researchers have shown that fat emulsifications increased with increasing pH.
Many factors affect the pH of emulsions. The rigor state of the meat used will be the base pH of your emulsion. At slaughter, the pH of animal tissue is approximately 7.0. Within 1 to 2 hours, pre-rigor salted meat will have a pH of 6.0 to 6.5. The addition of sodium chloride has been shown to increase the pH slightly. Cooking also increases the ph of meat. But most important, alkaline phosphates will increase pH of meat to the greatest extent.
The WHC of a meat system is greatly affected by the addition of salt. To fully understand this effect, one must consider the action of both the cation and the anion, which result from the ionization of salt. These ions must be absorbed by the protein to affect WHC.
The effect of salt can be explained by the fact that the chloride anion, being larger, would be less hydraded than the sodium cation. The less hydrated ions are drawn closer to the charged protein group due to their smaller radii. The chloride ion should therefore have more of an effect on screening of oppositely charged groups at pH above the pI than sodium resulting in a net increase in negative charges and a shift of the pI of muscle to lower pH values. An increase in negative charges results in repulsion of the protein groups and an enlargement of the space available for absorption within the muscle.
A 7 percent brine solution is ideal for solubilization of myosin. Using meat which contains 65 percent moisture, one would want 4.5 percent salt in contact with the meat for maximum extraction of proteins. This can be accomplished during chopping by adding the salt initially with the meat but with no additional water. This extraction is also enhanced by using ice instead of water to reduce the emulsion temperature to 28 to 30F. Once these proteins are extracted, they remain soluble with the addition of water.
Divalent cations, such as Ca (calcium), Mg (magnesium), and Fe (iron) are often found in untreated water supply and are known to decrease the WHC of mea. This is believed to be due to the bridging of actin and myosin and binding between other groups of proteins. Another theory is that divalent cations screen the negatively charged protein groups. Since divalent cations exhibit greater affinity for myosin than monovalent cations such as sodium, it is concluded that they also exert a greater influence on the WHC of meat than monovalent cations.
Increasing the total charges in the meat system will also increase WHC. This can be done by adding phosphates, especially the long chain phosphates such as tripolyphosphate (TPP) and hexametaphosphate (HMP). This effect is hard to separate from the pH effect but the long chain phosphates have numerous charges along the chains to tie up water.
Fat emulsification involves, as mentioned previously, the reduction of fat to a small enough size that the extracted protein can coat or entrap the fat. Of the fat particles are too large we will not get a smooth, stable emulsion but if the fat is chopped too much the surface area may be too large or too many fat cells may be broken to yield a stable product.
We should not ignore the physical phenomena behind the stability of meat emulsions. Prior to cooking, meat emulsions obey Stokes law:
V = rate of separation of fat
The major factor in this equation is D. In other words, as the fat particle size decrease, the emulsion stability increases providing there is sufficient protein to adequately coat the fat particles. As chopping continues, emulsion temperatures cause the surface tension of the fat particles to decrease. This decrease in surface tension further enhances the particle reduction process and rapidly increases the surface area of the fat particles. As the surface area increases, more protein is required to form the protein layer that surround the fat globules.
The viscosity of the emulsion may be extremely important in stabilizing meat emulsions by physically preventing coalescence of the dispersed phase. This explains why emulsions allowed to stand for long periods of time may not form stable products.
Fat source also affects the stability of emulsion products. Beef fat and pork leaf fat are generally not considered to be good emulsifying fats. On the other hand, ham and belly and shoulder fats are considered to be good for producing stable emulsions.
Chopping or emulsifying temperatures will also affect emulsion stability, particularly with different fat sources. If beef fat is the predominant fat used, final emulsion temperatures should be either 40F or 70F. This is apparently due to multiple crystallization temperatures of beef fat. Pork fat on the other hand should be chopped or emulsified to temperatures between 55 and 60F. Chopping emulsions containing pork fat to higher temperatures will usually result in fat caps.
Fat emulsification is also affected by the duration of chopping or emulsifying. There is a time-temperature effect on fats during emulsifying. Reaching the above mentioned temperatures alone may not guarantee stable emulsions. There needs to be sufficient time to reduce fat particle size, and if temperature is kept low, additional chopping time will further stabilize an emulsion. However, chopping time must be minimized if emulsion temperatures are high.
Considering all that has been discussed so far, the following procedure is recommended for production of stable emulsion:
Note: The above procedure can be adapted for mixer/emulsifier systems by completing all four steps in the mixer. The temperature before adding fat meats (step 3) may be lower as one needs to allow 10 to 12F temperature rise per pass through most emulsifiers. Finished emulsion temperatures should not exceed those listed in step 4 above.
Finally, perfectly good emulsions can be destroyed by improper cooking methods. In general, problems result if emulsions are cooked too fast, at too high of a relative humidity, and to too high of a finished temperature. For more details on cooking, see Irwin Waters presentation.
In conclusion, there are four major factors that are important to emulsion stability and they include: