Background

Meat color, in particular intensity and stability, is significantly influenced by mitochondrial activity via two mechanisms. (1) Oxygen consumption decreases red color development and intensity when mitochondrial respiration out-competes myoglobin for oxygen and results in dark colored muscle. (2) Mitochondria influence color stability via mitochondria-mediated metmyoglobin reduction, a process that limits surface discoloration. Given the negative effects of postmortem storage on mitochondrial function, it is likely that increased aging time will (1) improve color intensity by supplying myoglobin with oxygen and (2) limit color stability by decreasing mitochondria-mediated metmyoglobin reducing activity. The objectives of this research were to assess the effects of aging time and temperature on beef color intensity and stability, as well as to better understand the role of aging time and temperature in mitochondrial function.

Methodology

Methods for Objective 1: Longissimus lumborum (n = 15) were divided into 6 roasts, each 5 cm thick, and vacuum packaged. Three of the 6 roasts within each loin were aged at either 0°C or 5°C. Of the 3 roasts within each temperature, 1 roast was aged in vacuum packaging for either 15, 30, or 45 days. Two aging temperatures x 3 aging times resulted in the following 6 treatments: (1) 15 days aging at 0°C, (2) 15 days aging at 5°C, (3) 30 days aging at 0°C, (4) 30 days aging at 5°C, (5) 45 days aging at 0°C, and (6) 45 days aging at 5°C.

After aging, each 5 cm-roast was fabricated into 2 steaks. Each steak was used to determine both initial color intensity and color stability. Metmyoglobin reducing activity and lipid oxidation also were measured. Increased metmyoglobin reducing activity and decreased lipid oxidation are associated with improved color stability.

Methods for Objective 2: Myoglobin and mitochondria isolated from beef aged at different temperatures and times were used to provide a better fundamental understanding of the mechanism by which aging influences beef color. After aging, mitochondria were isolated and used to assess the effects of storage temperature and time on oxygen consumption (determinant of color intensity) and metmyoglobin reduction (factor influencing color stability). Decreased mitochondrial oxygen consumption is associated with a more intense bloomed color. More mitochondrial reducing activity indicates improved color stability.

Findings

As aging time was increased, color intensity or bloom also increased (the development of red color was greater with longer aging). Furthermore, color intensity was greater following aging at 5°C compared with 0°C. However, increased aging time and temperature resulted in less color stability after storage and packaging. As a result, warmer temperatures and longer storage times resulted in more surface discoloration. The effects of aging on color are partially dependent on mitochondria, and postmortem changes in mitochondrial activity play a significant role in color intensity and stability.

Implications

Meat color plays a vital role in consumer purchasing decisions. Nearly 15% of retail beef is discounted in price due to surface discoloration (Smith et al. 2001). As a result, failure to optimize color life results in approximately $1 billion dollars of lost revenue every year (Smith et al. 2001). Economic benefits associated with shelf-life improvements can only be obtained when we fully understand color chemistry.

Increased storage time and temperature can benefit initial bloomed color intensity (red color development) following aging. However, this benefit is negated by a significant decrease in color stability. As a result, increasing storage temperature and time will limit color stability following aging.