Study of the factors affecting cook yield and moisture retention of muscle foods
This work focused on the factors that affect the cook yield and moisture retention of different muscle foods at reduced salt concentrations (≤ 150 mM). Before studying the water-holding capacity (WHC) of different muscles, which was evaluated by cook yield and moisture retention upon heating and pressing, solubilization of myofibrillar proteins of chicken and cod muscles in water were first investigated. Certain possible solubility inhibiting (PSI) polypeptides and exposure to low pH affected the water solubility of myofibrillar proteins. The PSI proteins acted like a barrier that prevented the rest of the myofibrillar proteins from disorganization, swelling and final solubilization in water. M-protein (160 kDa), α-actinin (95 kDa), amorphin (87 kDa) and desmin (56 kDa) were tentatively identified as the PSI polypeptides in the chicken breast muscle. Re-addition of these PSI polypeptides to the PSI-extracted chicken mince inhibited the full solubilization of the chicken mince. Exposure of the cod myofibrillar proteins to a low pH could cause protein denaturation and subsequently the loss of extractability in water. M-protein (160 kDa), a 103 kDa polypeptide and α-actinin (95 kDa) were tentatively identified as the PSI polypeptides in cod muscle. By extracting the acidified cod mince with a solution of physiological ionic strength at neutral pH promoted the process to reverse the low pH effect on these three PSI polypeptides, thus the muscle proteins solubility was re-established.
The cook yield and moisture retention of the washed chicken mince at low ionic strength (25-150 mM sodium chloride) was governed by the balance between the driving force and the constraints against swelling of the myofibrils. Washing the chicken mince with moderate salt at neutral pH increased electrostatic repulsive force and the osmotic potential of myofibrillar proteins, and also solubilized the constraint components associated with myofibrillar structure. Therefore, improved water-holding capacity was observed. By lowering the pH of chicken mince previously extracted with moderate salt at neutral pH to 6.3, the improved WHC was lost, suggesting that pH might be a dominant factor in controlling WHC.
Examining the aqueous phase (press juice) after centrifugation showed that pH adjustment from 5.8 to 7.0 can selectively solubilize some proteins in chicken muscle. The adjustment of pH to neutrality also improved the cook yield and moisture retention of unwashed chicken muscles. It is hypothesized that these polypeptides represent proteins which are soluble in the aqueous phase of the muscle at neutral pH but much less so at pH 6 and their solubilization in situ may be involved in the water-holding properties of the muscle. After pH adjustment, the net negative charges of muscle proteins increased; and also removed some of the polypeptides that hold structural elements like thick filaments and Z-disks together. Removal of these polypeptides from where they are normally located in the muscle cell structure at neutral pH allows the components of the structural elements, for example, myosin of the thick filaments, to be released or to expand and to pull in water by the increase of electrostatic repulsive forces and the entropy of the system. The proteins thus unfold more extensively during heating resulting in a better cook yield and moisture retention upon heating and pressing.
Mechanical force such as mincing and homogenizing, can damage the muscle cell structure, thus removing some of the constraints associated with the cell membrane or the linkage between the myofibrils. Therefore, cook yield and moisture retention after heating and pressing were improved in the ground and homogenized muscle.