strength
2.8.1. Meat fibre orientation
The orientation of muscle fibres at the binding surface affects the binding properties of the meat. Purslow, Donnelly, & Savage (1987) tested the binding strength of meat pieces by applying crude myosin in between meat pieces and then cooked at 80 °C. They found that the muscle fibres were perpendicular to the adhesive junction (90°/90°) was stronger than when both meat pieces were parallel (0°/0°) to the junction, or when one meat piece had fibres perpendicular and the other had fibres parallel to the adhesive junction (90°/0°). Figure 2.4 shows the orientation of the meat pieces against the layer of the adhesive for a 90°/0° junction.
Figure 2.4. Two possible orientations of meat fibres during meat binding.
The 90°/90° junctions showed that when the two pieces of meat were pulled apart the fracture occurred within the myosin gel used to bind the meat pieces. While for the 90°/0° and 0°/0° junctions the fracture was within the muscle fibres of the meat piece with fibres parallel to the junction. It can be concluded that the strength of the meat with
Meat fibre perpendicular
to the adhesive junction
(90°)
Layer of adhesive
consisting mainly of
myosin
Meat fibre parallel to the
adhesive junction (0°)
90°
26
fibres perpendicular to the adhesive layer is greater than the strength of the myosin adhesive layer, which is greater than the strength of the meat with fibres parallel to the adhesive layer (Donnelly & Purslow, 1987). However, studies by Farouk, Zhang, & Cummings (2005) showed that the fibre alignment for restructured beef steaks did not produce any significant difference in the binding strength, either when the fibres were aligned parallel, perpendicular or a mixture of both. Their result might be due to the use of the enzyme transglutaminase as the glue rather than the crude myosin used by Purslow et al. (1987). The binding junctions of the sample by Farouk et al. (2005) were numerous while only one binding junction was tested by Purslow et al. (1987). These orientation variables and the nature of the “glue” could play an important role in the binding strength of neighbouring meat pieces.
2.8.2. The effects of heat treatments on gel-network formation by myosin
The proteins in meat are important, especially myosin, because they act as glue for attaching meat pieces together in the production of reformed meat products. Heat treatment is often used to allow the myosin to coagulate and form a network, thus binding the neigbouring pieces of meat. Myosin forms a gel network in four sequential steps. These are: 1) the unfolding of the myosin heads (S1 region), 2) disulfide bonding and hydrophobic interactions between the myosin heads causing intermolecular interaction of S1 regions, 3) the unfolding of myosin tail or Light Meromyosin (LLM), and 4) interaction of the tails to form a gel network (Lanier, Yongsawatdigul, & Carvajal-Rondanelli, 2014). It appears that both the head and the tail of myosin play roles in forming the gel network with the myosin head role involving sulfhydryl groups, while the myosin tail consists of conformational changes of the helix-coil of the rod (Samejima, Ishioroshi, & Yasui, 1981).
The thermal denaturation of myosin was reported to have occurred at transition temperatures of 47, 54, 57 and 63 °C by Smyth, Smith, & O’Neill (1998). However, the S1 regions start to unfold when the temperature rose to 37°C (Burke, Zaager, & Bliss, 1987). This was consistent with the results of Smyth, Smith, Vega-Vargas, & O’Neill (1996), which showed that the unfolding of the S1 region of chicken breast muscle occurred at 36 °C and with aggregation when the temperature reached between 45 to 54 °C. Smyth et al., (1998) suggested that the forming of disulfide bonds occurred between 47 to 53 °C, which
27
contributed to gel network formation. For the myosin tail, King & Lehrer (1989) reported that the helix structure of the myosin tail unfolded in three transitions at 43, 47 and 53 °C. At 47 °C however, the unfolding process occurred at the S2 region. The formation of a myosin gel by heating has been demonstrated by Morita & Yasui (1991) who examined changes occurring to the hydrophobicity of the myosin tail during heating. At 35 to 40 °C, the hydrophobic amino acid residues of the myosin tail started to be exposed and reached a maximum exposure at 65 °C. When the meat was heated to temperatures higher than 65 °C, it resulted in a decrease in the hydrophobicity of the myosin tail. This could be due to the hydrophobic residues taking part in protein-protein interactions, where a network is formed to produce a gel for meat adhesion. The helix content of the myosin tail or the LMM began to decrease at 30 °C and attained a minimum at 70 °C. Therefore, it can be concluded that, to have maximum myosin gel formation, the reformed meat would be heated to between 65 to 70 °C. This would maximise the bonding between meat pieces. However, this may not always be the best temperature range as different meat samples from different species-containing different fibre types, salt and phosphate levels, and pH of the meat-could mean that different temperature ranges would be optimal either for myosin denaturation or aggregation (Lanier et al., 2014; Visessanguan, Ogawa, Nakai, & An, 2000).