Determination of the MDTM protein concentration.
The protein concentration of MDTM extract, calculated from the percentage of free nitrogen content (N %) x 6.25, was 30 mg /g (n = 3).
The lean pork trimmings were identical within treatment experiments. The amount of MDTM protein added to the trimmings during blending was approximately 5% of original protein amount. The pH of MDTM was 6.4 (n = 3).
SDS Polyacrylamide Gel Electrophoresis (SDS-PAGE)
The SDS-PAGE is a sensitive method commonly used to detect molecular changes in postmortem muscle (Koohmaraie et al., 1984). The presence of actomyosin in the MDTM extract was determined by comparing the molecular mobility of the predominant bands in the lane containing the MDTM extract (Figure 1, lane 2) with the mobility of the actomyosin standard (Figure1, lane 3). Under denaturing conditions in 10% gels purified actomyosin migrates as two separate bands consisting of myosin (lane 3, upper arrow) and actin (lane 3, lower arrow) of approximately 200 kDa and 45 kDa, respectively. The molecular mass of the bands in the MDTM extract were determined by comparing their mobilities with those of the bands in the molecular weight standards (Figure 1, lane 1). The major protein constituents observed in the MDTM extract are most likely myosin heavy chain (200 kDa), actin (45 kDa), based on their molecular retention (Mr) on 10% discontinuous SDS-PAGE and their differential salt solubility properties (Figure 1, lanes 2 and 3). Thus, the proteins migrating at approximately 200 kDa and 45 kDa (Figure 1, lane 2) are most likely extracted actomyosin. In addition the protein with a molecular weight of 18 kDa is likely myoglobin (Figure 1, lane 2). Precipitation of the MDTM extract with 7 vols cold double deionized water resulted in a precipitate, highly enriched in myosin and actomyosin (Figure 1, lane 3). Differential salt solubility is one of the functional properties of myosin crucial to the formation of thermally set meat gels crucial to the production of high quality processed meat products (Wick et al., 1996; Hamm, 1975 & 1986; Wilding et al., 1986).
Effect of MDTM Extract in Sausage Formulation
The protein concentration of STB was 0. The protein concentration of the MDTM extract was determined to be 30 mg/ml-1. The protein concentration of whole muscle is generally between 160-220 mg/g-1 tissue (Lawrie, 1985). A 3.3 lb (1.5 kg) batch, at a meat:brine ratio of 100:35 (w/w), consists of 1110 g pork trimmings and 390 g of STB or MDTM extract. Each preparation would consist of 178 to 244 g protein from the muscle source. The total protein in the sausages manufactured from normal or PSE pork trimmings processed with STB would be between 178 to 244 g per 1.5 kg batch. The amount of protein contributed by blending normal or PSE pork trimmings with MDTM would be 12 g per 1.5 kg batch Thus, the total protein in sausages manufactured from either normal or PSE pork trimmings and MDTM extract was 190 to 256 g per 1.5 kg batch. This indicates that the total protein in sausages manufactured employing MDTM extract increased approximately 0.8%. These results suggest that the improvement in the physicochemical properties of sausages manufactured by employing MDTM extract is due to the inclusion of actomyosin and not merely the increase in total amount of protein.
Cooking Loss
Cooking loss analyses demonstrated that when a 1:3 (MDTM: STB, w/w) blend was added as an ingredient during the manufacture of cooked pork sausages, the cooking loss of sausages prepared from PSE meat as well as sausages prepared from normal pork trimmings was reduced (P < 0.05). There were no differences in the cooking loss among the three groups of sausages made from: 1) normal pork trimmings and MDTM, 2) normal pork trimmings and STB or, 3) sausages made from PSE pork and MDTM extract (P > 0.05). The mechanisms controlling cooking loss are not fully understood. However, the most likely explanation for the loss of water during cooking in an atmosphere of 100% relative humidity is that the proteins begin to lose their ability to bind water upon heat denaturation.
The reduction in the cooking loss exhibited in the sausages made from PSE and the MDTM extract is most likely due to the presence of the functional myofibrillar proteins, myosin and actomyosin, present in the MDTM extract. The absence of a difference in the cooking loss (P > 0.05) between sausages made from PSE with STB and sausages made from PSE and MDTM extract (Table 1), is likely due to insufficient concentrations of the myofibrillar proteins (0.8%) present in the MDTM extract to bind water in the freshly prepared meat batter. Since actomyosin is the predominant functional protein responsible for the water binding properties of muscle and processed meats it is likely that increasing the amount of actomyosin added to sausages prepared from PSE will likely result in significant decreases in cooking losses. Experiments are currently underway to study the effect of adding higher concentrations of myofibrillar proteins derived from MDTM on processed meat products made from PSE meat.
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Table 1. The effects of added STB and MDTM extract on rigidity, cooking loss, and water holding capacity (WHC) of comminuted pork sausages1. |
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| Pork | Brine | Rupture force (g) | Cooking Loss (%) | WHC (%) |
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| PSE | STB | 1584 ± 107a | 14.5 ± 0.5a | 60.8 ± 3.4a |
| PSE | MDTM | 2041 ± 264b | 13.9 ± 0.5ab | 63.0 ± 2.6a |
| Normal | STB | 2995 ± 283c | 12.8 ± 0.4b | 61.6 ± 2.0a |
| Normal | MDTM | 3081 ± 116c | 12.4 ± 0.8b | 63.5 ± 1.4a |
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a,b,c Means followed by different letters within the same column are significantly different (P < 0.05); n = 6. 1PSE = pale soft exudative pork, STB = standard brine, and MDTM = mechanically deboned turkey meat. |
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Texture Analyses
The rigidity of sausages mPSEade from PSE was lower than sausages made from normal pork (P < 0.05). In contrast, sausages manufactured from PSE including an MDTM extract exhibited approximately a 30% increase in rigidity compared to sausages made employing STB alone (P < 0.05). According to previous result in 3.1, the amount of MDTM protein added to the trimmings during blending was approximately 5% of original protein amount. This small amount of added protein seem have significant intrinsic effect on the firmness of PSE sausage (P < 0.05). However, the difference in the rigidity between sausages made from normal pork trimmings with either MDTM or STB was not significant (P > 0.05). This was probably due to high concentrations of functional myofibrillar proteins already present in the normal pork muscle. Therefore the contribution of the MDTM extract to the functionality of the meat system was minimal.
Water Holding Capacity
Results of WHC analyses demonstrate little difference in WHC among all four groups of uncooked sausage batters regardless of type of brine employed (P > 0.05). The difference in the ability of myofibrillar proteins derived from normal or PSE meat to bind water was only observed as differences in cooking loss in the sausages made from the two sources, but not WHC. Prior to thermal gelation, myosin and actomyosin from PSE or normal muscle appear to have the same capacity for binding water. However, the capacity for binding water changes upon thermal denaturation through an unknown mechanism. Therefore, these observations may be a reflection of different mechanisms of water binding in native and thermally denatured myofibrillar proteins at the different pH environments.
The WHC of sausages formulated with MDTM extract had no difference from those formulates with STB (P > 0.05), even though MDTM extract has a lower pH (6.4) than standard brine (7.6). It is generally believed that the water holding capacity of muscle and meat is lowest around pH 5.0, and increases with increasing alkalinity (Wismer-Pedersen, 1971). These observations indicate that the myosin/actomyosin present in MDTM extract was the more dominant factor to reduce cooking loss and retain WHC of the sausages, and capable of overriding the pH effects. Increasing the concentration of actomyosin in the MDTM will likely result in a significant decrease in the cooking loss and an increase in WHC of sausage batters formulated from PSE pork meat.