C. Lynn Knipe
from Phosphates as Emulsifiers. Encyclopaedia of Food Science, Food Technology and Nutrition. Academic Press Limited. London. 1992.
The emulsifying capacity of phosphates in meat emulsions may take several approaches depending upon one's definition of emulsification. A finely chopped meat mixture is conventionally referred to as a "meat emulsion." However, this is now thought to be a misnomer. The so-called meat emulsion consists of solid fat particles dispersed in a mixture of water and many fibrous particles, including connective tissue and muscle fibers. A true emulsion is a stable suspension of two liquids (oil and water, which are not normally soluble in each other. It might be more appropriate to refer to a meat emulsion as a matrix, whose stability is dependent upon the water-holding capacity or binding capacity of the meat proteins in the matrix. However, a finely chopped meat mixture will often be referred to as an emulsion in the remainder of this chapter.
If the proper combination of meat ingredients is combined with the proper processing procedures (e.g., grinding, chopping, eumlsifying), a stable emulsion will be prepared which will hold up well during the cooking process in a smokehouse. Examples of meat emulsions are bologna or wieners, which have such fine meat particles that they are not distinguishable on the smooth product surface. However, if either the quantity or quality of meat ingredients or the processing methods are inadequate, the meat mixture will be unstable and result in a poor quality product upon cooking.
If it is assumed that a meat emulsion is not a true emulsion, then phosphates are probably not true emulsifiers but stabilizers of meat emulsions. There are many factors involved in the stability of meat emulsions which can be affected by the addition of inorganic phosphates. The main effects of phosphates in finely chopped meat systems are on the pH, ionic strength, protein extraction, divalent cation binding and viscosity reduction.
Phosphates and pH
Phosphates affect the pH of both water and meat. The pH values of one percent solutions of the phosphates approved for use in meat products, by USDA-FSIS, appear in Table 1. While phosphates also affect the pH of meat, this effect, as determined by standard pH measurements, is somewhat less than their effect in water, due to the buffering capacity of meat. Alkaline phosphates increase meat pH in the range of 0.1 to 0.6 units, depending upon the phosphate chosen. The following list indicates the decreasing order of impact on increasing meat pH: pyrophosphates, tripolyphosphates, and hexametaphosphates. Hexametaphosphates are considered to be rather neutral and often have not effect in raising meat pH. Acid pyrophosphates often, in fact, decrease the pH of meat systems.
Table 1. pH values for 1% water solutions of USDA-approved phosphates
Inorganic phosphate pH
tetrasodium or tetrapotassium pyrophosphate 10.2
sodium or potassium tripolyphosphate 9.8
disodium or potassium orthophosphate 8.8
sodium polyphosphate, glassy 7.0
sodium metaphosphate, insoluble 6.5
monosodium or potassium orthophosphate 4.4
sodium acid pyrophosphate 4.2
Source: Ellinger, 1972
Water-holding capacity of meat emulsions
Water-holding capacity of meat is greatly affected by pH. Water-holding capacity 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. Water-holding capacity 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 protein 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 water-holding capacity 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 mortis.
Increasing or decreasing the pH away from the pI will result in increased water-holding capacity by creating a charge imbalance. A charge imbalance is a predominance of either positive or negative charges which will result in a repulsion of charged protein groups of the same charge. This repulsion results in increased capacity for water retention and could be compared to the effect of like-charges of two magnets.
Phosphates and ionic strength
Phosphates also affect the ionic strength of meat mixtures. Inorganic phosphates ionize in water to form polyelectrolytes. This ionization causes a masking of positive sites on the meat proteins which further causes a electrostatic repulsion of the proteins. An electrostatic repulsion between meat proteins allows for more space between proteins for binding water, which increases water-holding capacity. This is especially true for thethe long chain phosphates, such as tripolyphosphates and hexametaphosphates. 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.
Protein extraction and solubilization
During the formation of a meat emulsion, meat proteins are extracted from the fibrous muscle structure and are solubilized into a solution, much like dissolving instant tea. This protein extraction is enhanced by high ionic strength and pH of the solution that the protein is in. Once solubilized, with additional chopping or mixing, the liquid proteins are dispersed around bundles of muscle cells and fat particles. When this mixture is then cooked, this protein solution denatures and coagulates (like egg whites) to form a gel around the muscle bundles and fat particles. This gel serves to stabilize the matrix of the meat mixture or emulsion. Phosphates serve a very valuable role in extracting proteins from muscles. In fact, tetrasodium pyrophosphate is known to have a "specific effect" on meat proteins. Specific effect means that tetrasodium pyrophosphates are able to extract muscle proteins to a degree much greater than normally possible considering tetrasodium pyrophosphates' effects on ionic strength, pH, etc.
Phosphates and divalent cations
Phosphates also bind divalent cations, which are native to meat proteins and which are detrimental to finished processed meat quality. Divalent cations, such as calcium, magnesium, and iron are often found in untreated or unsoftened water supplies, as well as meat, and are known to decrease the water-holding capacity of meat. This is believed to be due to the bridging and binding between several groups of proteins, similar to what occurs during rigor mortis. Another theory is that divalent cations screen the negatively charged protein groups. Since divalent cations exhibit greater attraction for meat proteins than monovalent cations, such as sodium, it is concluded that they also exert a greater influence on the water-holding capacity of meat than monovalent cations.
Viscosity of meat emulsions
Phosphates are also important in the physicochemical aspects of meat processing. For example, phosphates reduce the viscosity of meat mixtures. This is important because in the chopping or mixing steps, meat temperatures will rise due to frictional energy produced as the chopper or emulsifier knives are cutting and moving through the meat mixture. Chopping emulsions containing a soft fat, such as pork fat, for example, to excessively high temperatures will result in an unstable product.
Also, when meat is finely comminuted to make a stable sausage product, the fat particles need to be reduced to a small enough size so that the extracted meat proteins can coat or entrap the fat. If the fat particles are too large we will not get a smooth, stable emulsion but if the fat is chopped too much the fat surface area may be too large or too many fat cells may be broken to yield a stable product. By reducing the viscosity, a mixture can also be chopped or mixed longer, to either reduce fat particle size or extract more protein to further stabilize the mixture. Without phosphates, long chopping or mixing times would result in unstable products.
Phosphates and sodium chloride
The functionality of phosphates is greatly affected by the addition of common table salt or sodium chloride to meat emulsions. The effects of combining salt and phosphates in meat emulsions is considered to be synergistic, which means that the measurable effects of the two ingredients combined is greater than the combined effects of the two ingredients added individually. It seems that phosphates exert more effect on pH and protein solubility and salt exerts more effect on ionic strength and water-holding capacity.
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