Producing emulsified products without fat pockets

By Oscar Esquivel, Ph.D on 3/1/2008
MeatingPlace.com

The most abundant proteins in muscle fibers, actin and myosin, are insoluble in water, but can be solubilized through physical action in solutions containing 4 percent to 5 percent salt. A protein exudate is then obtained, which is capable of coating other components of the meat system, such as fat particles, muscle, connective tissue and other inclusions.

Typically, an emulsion consists of a discontinuous phase composed of fat and other, larger particles dispersed in a continuous aqueous phase of solubilized protein material. These coated components form a matrix that coagulates upon heating, forming the typical solid and elastic structure of sausages and meat emulsions. A meat emulsion is the result of the efficient formation of an emulsifying phase, which coats suspended fat particles and physically restrains them after the protein matrix gels due to heating. Fat particles are then prevented from coalescing and forming fat pockets.


Factors affecting emulsion stability
Emulsion stability is very important to the quality of sausages and emulsified products and to prevent defects such as fat pockets and mushy texture.

"Fatting out" in a meat emulsion can be associated with considerable moisture losses due to formation of channels during the cooking process. Moisture and liquefied fat can leak and migrate to the product surface through these channels. In stable products, moisture loss and/or fat coalescence will be minimal.

Fat stability is mainly affected by three factors: the biophysical properties of the emulsifying film coating the fat particle, the gelation properties of the protein matrix and the physical characteristics and cellular integrity of the fatty tissue. Salt concentration and type of salt also affect the amount of extracted protein, which has a direct impact on emulsion stability. There is evidence that decreasing sodium chloride levels from 2.5 percent to 1.5 percent might reduce emulsion stability and produce undesirable texture. By reducing the amount of salt, there's also a decrease in the amount of solubilized protein available to coat and physically restrain the fat particles.

The amount of added fat and its relative surface also are important factors to consider for emulsion stability, given that it is necessary to fully coat the total surface of fat particles with a film of solubilized protein. Assuming there's enough soluble protein in the aqueous phase of the emulsion, the meat batter is more stable when fat particles are smaller (greater relative surface) leading to more efficient distribution of the fat particles throughout the continuous protein phase. However, if the fat relative surface area is greater (from excessive chopping) than the mount of available soluble protein, then the film coating the particles becomes too thin and more susceptible to physical disruption when fat expands during the thermal process. The result is an unstable emulsion, fat pockets and a loose and less flexible texture.

Increased thickness of the protein film may also influence emulsion stability. High temperatures at the end of the chopping process may result in a protein film that's too thick and inflexible, which will tend to fracture during fat expansion upon cooking. Thinner protein films will form a series of pores that could act as "escape valves" to hot expanding fat.

Several factors related to fat's characteristics can influence emulsion stability. Some reports mention that it is easier to emulsify short-chain saturated fatty acids than their longer-chain counterparts. The degree of saturation is also important; in fatty acids with similar chain length, it is easier to emulsify those that are less saturated. Similarly, there seems to be a relationship between the melting point of the fat and its degree of dispersion and absorption by the inter-phase protein film.

Meat batter viscosity decreases at temperatures closer to that of the fat melting point, and since fat particles are less dense than the aqueous phase, they tend to float to the surface. These floating particles are less likely to be coated by the protein film and will be prone to coalesce during the thermal process. Research that has investigated various types of fat shows that regardless of the final chopping temperature, at lower levels of fat addition, all kinds of fat yield stable emulsions. However, at higher levels of fat addition, those with higher melting points are more stable. As a rule of thumb, aim for final chopping temperatures below 63 degrees F, 53 degrees F and 43 degrees F for fats of beef, pork and poultry origin, respectively.

During cooking, the protein matrix undergoes various changes. From 104 degrees F to 122 degrees F, myosin starts to denature, and the gelation process starts. Close to 140 degrees F, collagen solubilization initiates. Finally, the structure is consolidated between 158 degrees F and 176 degrees F, when actin denatures.

Fat within this system starts to liquefy before thermal denaturation of the proteins and is completely molten before the protein matrix gelation is completed. Therefore, it is essential to have a balanced interaction between the fat and the protein film at the emulsion inter-phase that could stabilize liquid fat before gelation of the protein matrix. Both the inter-phase protein film and the protein matrix must be stable and cohesive enough that the exudation of melted fat through the structure and the subsequent formation of coalescing spots in void spaces are prevented.


Estimating emulsion stability
Estimating emulsion stability involves measuring the amount of fat and/or moisture released after the cooking process. Measuring electrical conductivity of the raw batter also is a good index of stability, given that an aqueous phase in the presence of salt would conduct electricity better than a system where a fat separation process has started.

Making a graphic of electrical conductivity as a function of time during the chopping process allows you to visualize the point where structural changes in the emulsion start. Some people even report that devices with fiber optics technology could predict the moment when an emulsion becomes unstable.

However, even though these points could be estimated, it would be too late to correct the problem. These techniques are useful research tools to establish historical records that eventually could be related to specific conditions. Unfortunately, no visible, reliable signs are readily available that could tell a processor how stable a meat batter is. Establishing good records and being consistent with manufacturing practices are still the best alternative.