Muscular steatosis is a condition where muscle fibers have been replaced by fatty deposits, which creates a striking appearance in cuts of meat with it. Sometimes this condition is simply called “steatosis,” “callous,” “calloused lean” (other spellings include “callus,” “woody callused”), or because it is observed sometimes in the ribbed carcass, in this case, it is called “calloused ribeye.”
Swatland (1994) stated that this condition is most often found in beef and pork and is thought to be caused by muscle damage or nerve degeneration where fatty tissue infiltrates into this area. Sometimes it appears to be a highly marbled area, but it will be contrasted by areas of slightly or much lower marbling adjacent to it. In severe cases, a large portion of the muscle will be solid fat. Below are photos of muscular steatosis.
Chances are that muscular steatosis occurs most often when there has been some sort of injury in the animal’s life, although Swatland (1994) further stated that it can be found in animals that have vascular abnormalities, have had biopsies taken, or is observed in muscles used when animals rear up on their hind legs.
There are several articles from Japan on this condition (Koji and Saburo, 2006; Nade et al., 2009; Takahaski et al., 2011), which is called “SHIKORI,” and is considered a defect in the Japanese Meat Grading Association program. One of the muscles that appears to be most affected by muscular steatosis is the M. trapezius, which is exposed during the presentation of beef carcasses in the Japanese grading system because of ribbing between the 6th-7th ribs.
Koji, I., and F. Saburo. 2006. Effects of gender, bodyweight and beef carcass traits in the development of muscular abnormality in Japanese Black cattle. Journal of the Japan Veterinary Medical Association 59: 555-557.
Nade, T., N. Takahaski, K. Kuchida, T. Ishii, and N. Kimura. 2009. Morphological characteristics of the callused M. trapezius from Holstein steer using computer image analysis. Nihon Chikusan Gakkaiho (Japanese Society of Animal Science) 80: 77-82.
Swatland, H. J. 1994. Structure and Development of Meat Animals and Poultry. CRC Press, Boca Raton, Florida.
Takahashi, N., T. Nade, and N. Kimura. 2011. Regional SHIKORI occurrence rate of fattening Holstein steer carcasses shipped from a farm with high SKIKORI occurrence. Nihon Chikusan Gakkaiho (Japanese Society of Animal Science) 82: 139-145.