Meat tenderness and muscle growth: is there any relationship?☆
Introduction
To meet consumer expectations, the United States meat industry has identified solving the problem of inconsistent meat tenderness as a top priority. This requires a detailed understanding of the processes that affect meat tenderness and, perhaps more importantly, the utilization of such information by the meat industry. Beginning with the decade of the 1990s, the United States meat industry has accelerated the adoption of new technologies to meet consumer expectations. For example, while beef products from several companies now carry the label of ‘guaranteed tender,’ just a few years ago such products could not be found in any retail case. Other recent developments include the use of marinades and case-ready products. These recent developments indicate the increased likelihood of adoption of new technologies by the United States meat industry to improve consistency in meat tenderness.
Eating satisfaction results from the interaction of tenderness, juiciness, and flavor. However, as outlined previously (Koohmaraie, 1995), there is little variation in juiciness and flavor of beef under production practices in the United States; therefore, reduction/elimination of tenderness variation should result in reduction/elimination of variation in eating quality. The objectives of this manuscript are to review the mechanisms of muscle growth, the biochemical basis for meat tenderness, and the relationship between these two important processes. This is not meant to be a comprehensive review of the literature.
There seems to be a trend among many to put improper importance on a mean for a given trait of a given study. For example, it is very common to see statements to the effect that a given percentage of meat tenderization occurs by day 1 postmortem, or that no change in meat tenderization occurs by day 3 postmortem, or most of the meat tenderization occurs after day 3 postmortem and since calpain has lost most of its activity, it cannot be involved in postmortem meat tenderization. The fact is that none of these changes occur uniformly among animals and to assume that they do is to underestimate the dynamic nature of postmortem changes. To illustrate this point, consider the data we generated a number of years ago to demonstrate the changes in shear force value of cooked lamb longissimus from slaughter until 14 days of postmortem storage (Fig. 1 a, b; Wheeler and Koohmaraie, 1994). The mean shear force value at 24 h postmortem was 8.66 kg. One could look at this data and conclude that at 24 h postmortem meat is very tough, that tenderization has not yet occurred, and that tenderization will begin sometime after 24 h of postmortem storage (Fig. 1a). If one plots the same curve for each individual lamb, an entirely different interpretation emerges (Fig. 1b). The range in shear force values at 24 h postmortem was from 5 to 13 kg. Similar information was presented in Fig. 1a as standard deviation (SD 2.01 kg), but often when discussing the literature the SD is entirely ignored and mean is the only data considered. It is important to avoid such generalizations and oversimplifications that can lead to erroneous conclusions. In this example, three of 11 sheep are very tender at 24 h, two acceptable and six tough, so for five of 11 animals extensive tenderization has occurred in the first 24 h.
Section snippets
Muscle growth
Hyperplasia (increase in cell number) and hypertrophy (increase in cell size) are the determinants of muscle mass. If we define hyperplasia as the actual number of cells, this trait is controlled by embryonic cell proliferation. However, if hyperplasia is defined as the DNA content, then it is determined by the prenatal cell proliferation and postnatal growth and development of satellite cells. Therefore, animals born with a greater number of muscle cells (e.g., double muscled cattle) have
Meat tenderness
Before discussing the relationship between the mechanisms of muscle protein degradation and meat tenderness, it is appropriate to provide a summary of the biological basis for meat tenderness variation (for reviews see Koohmaraie, 1992b, Koohmaraie, 1994, Koohmaraie, 1996). We demonstrated that lamb longissimus has intermediate shear force values immediately after slaughter, toughens during the first 24 h, and then becomes tender during postmortem storage at 4°C. Because sarcomere length
Meat tenderness and muscle growth
If muscle growth is the result of hyperplasia (increase in cell number during embryonic development and/or increase in DNA content due to satellite cell activity), no negative effect or maybe even a positive effect (see later) on meat tenderness is expected. The best example to support the above statement is the case of double-muscled cattle. Double muscling in cattle is the result of an inactivating mutation in the myostatin gene (Grobet et al., 1997, Kambadur et al., 1997, McPherron et al.,
Conclusions
Although indirect evidence supports the proposed mechanisms of myofibrillar protein turnover and the pivotal role that calpain plays in this process, it is important to note that there is no direct evidence in support of this theory. Hence, it is important to be able to design and execute the appropriate experiments that directly substantiate this proposal. In our assessment, we have to develop an experimental system that would allow exclusive manipulation of the calpain proteolytic system
Acknowledgements
We are grateful to Marilyn Bierman and Carol Grummert for secretarial assistance.
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