Figure 1. Amino acid sequence of /¡-casein A.
Figure 2. Amino acid sequence of a-lactalbumin B.
phates to a kind of cold meat slurry, then forming the matrix in which the animal fatty tissue and possibly connective tissue material is dispersed.
In general, the final temperature during processing does not exceed 15-18°C. Often flavorings, binders, or other additives are mixed in. Having this in mind, it will be clear that hardly any of the abovementioned finely comminuted products will show structures that are equal to true and simple emulsions as defined in physical chemistry.
The stability of finely comminuted meat products is— apart from production technology, formulation, the use of binders, etc—mainly determined by the quality of the meat cuts used (6,7).
Structure of Lean Meat
Lean meat generally contains about 20% protein, which can be divided into:
30-35% Sarcoplasmic proteins.
50-55% Myofibrillar or structural proteins (myosin, ac-
15-20% Stromal proteins (collagen, elastin).
The essential unit of all muscle is the fiber (8) (Fig. 3). Each fiber is surrounded by a membrane, the sarcolemma, and consists of a great number of parallel ordered myofibrils (8). In turn, each myofibril represents a similarly parallel-ordered structure of very thin protein threads, the actin- and the myosin filaments (Fig. 4).
Within the sarcolemma, surrounding the myofibrils, is the meat juice or sarcoplasma. This meat juice contains the water soluble meat proteins (wsp), consisting of enzymes, the red meat color myoglobin, and structured bodies. It is easily extracted on pressing, freezing/thawing, or chopping the meat. The myofibrillar proteins in warm
Fine elastin fibers Figure 3. Sketch of a muscle fiber.
Actin filaments Z-lines
slaughtered meat are present as free actin- and myosin filaments and are soluble in brine. Therefore, they are called the salt soluble proteins (ssp). In chilled and matured meat, however, the actin and myosin have gone into a reaction to form the complex actomyosin (7,8). This ac-tomyosin is only partly soluble in brine, depending on the condition of the meat, age of the animal, (pre)slaughter conditions, anatomic location, mechanical treatment, etc.
The nonsoluble part of the myofibrillar proteins is able to swell (take up water). In this postrigor condition the water- and fat-holding capacity of the meat are considerably lower than in warm slaughtered meat.
As indicated in the introduction, hardly any of the above-mentioned finely comminuted meat products is a true emulsion. In meat emulsion there are at least four different phase systems at the same time:
1. A true aqueous solution of salt, phosphates, nitrite, sugars, etc and a colloidal solution (sol) of sarcoplasmic (water-soluble) proteins and salt-solubilized myofibrillar proteins. This solution is the continuous phase of three different dispersed systems at a time.
2. A suspension of undissolved proteins and fatty tissue.
3. An emulsion containing emulsified fat droplets.
4. A foam; during the comminution process some air is corporated.
This system has to be stabilized against heat treatments such as pasteurization or sterilization, against smoking, cooling, frying, reheating, vacuum treatments, etc—whatever physical stress is put upon the products during conserving, storing, and commercializing.
These stabilization properties are important, not only from the standpoint of production practices, but also from
Actin filaments Z-lines
the standpoint of cost and quality characteristics, such as texture, consistency, bite, tenderness, juiciness, appearance, and palatability of the finished sausages.
The mechanism for the binding and stabilization phenomenon still is not completely understood, and there are several conflicting viewpoints expressed in the literature (8-15). It is, however, generally agreed on that the myofibrillar proteins are mainly responsible for this stability (810,16,17).
The stability of the matrix is, apart from the effect of binders, determined by the water holding and gelling capacity of the meat proteins, and, more in particular, of the myofibrillar part. In relation to the stability of fresh sausage batters, the water-binding capacity of the nonsolubilized myofibrillar proteins is decisive. Changes in the net-charge of the proteins, resulting in either attraction or repulsion of the filaments, are responsible for shrinkage and swelling of the protein network and run parallel with the decrease or increase, respectively, in water holding (18).
This mechanism of water binding may be compared to that of a sponge. The more salt-soluble proteins are solu-bilized, the better the water binding will be.
Animal fatty tissue consists of a cellular network in which the fat is enclosed. The network is built up of connective tissue and water. As long as these cell walls are intact, only minor fat separation will take place. However, on chopping the fatty tissue an increasing number of fat cells is damaged and more and more fat is set free. This free fat should be stabilized to prevent fat separation from the product. The origin of the fat (pork, beef, mutton, poultry, etc) and the anatomic location determine the amount of free fat during comminution and as a consequence the application of the fatty tissue.
Most beef, mutton, and chicken fat, as well as pork flare fat are hard to stabilize in finely comminuted meat products, unless they are pre-emulsified with a nonmeat protein such as sodium caseinate (13,19,20).
Pork fats other than flare fat (back fat, shoulder fat) are softer and consequently can be used directly in the production of the meat emulsion, because there is more fat in a liquid state.
Particularly in finely comminuted meat products, the interaction between the free fat and the meat proteins plays an important role in the stabilization of both fat and water (14).
The salt-soluble myofibrillar meat proteins have excellent emulsifying properties and are quickly preferentially absorbed in the fat/water interface (21). The sarcoplasmic proteins are relatively unimportant in this respect. In addition to their gelling capacity and their role in water binding, this emulsification is another reason to aim at a sufficient extraction of these myofibrillar proteins during first stage of chopping.
However, when the myofibrillar meat protein enters the fat/water interface, the protein structure is altered and, as a result, the myofibrillar protein is no longer capable of gel formation and water binding (8). This means that the consumption of ssp for the emulsification of free fat goes at the expense of the water binding.
Milk protein, type sodium caseinate, is a perfect emulsifying protein, which is very strongly attracted by the fat/ water interface (12). If this type of milk protein gets the opportunity to surround the free-fat particles during sausage manufacture and before the myofibrillar (ssp) proteins do this, the latter are saved for denaturation in the interface.
Research has proved that milk protein, type sodium caseinate, indeed is better and more quickly absorbed in the oil/water interface when emulsification takes place when both milk proteins and meat proteins are present in the continuous phase (Fig. 5).
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It is a well known fact that homemade food is always a healthier option for pets when compared to the market packed food. The increasing hazards to the health of the pets have made pet owners stick to containment of commercial pet food. The basic fundamentals of health for human beings are applicable for pets also.