This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Fat may be deposited around and between each muscle, i.e., inter-muscularly; between the fasciculi, interfascicularly; between the fibers, intrafascicularly; or within the fiber, intracellularly. The nodular white spaces between the fibers, Fig. 21, were occupied by fat cells, i.e., intra-fascicular fat. The fat was removed in preparation of the section, but the membrane between the fat cells still shows. Although fat deposited around and within the muscle is not important in the contraction of the muscle, that around the muscle, and particularly in roasts, bastes the roast during cooking and thus affects the palatability.
Fat content. The fat content of the body, and hence of the muscles and finally the "cuts" of meat, varies with the nutritive condition and age of the animal. Well-nourished animals have more fat distributed over and throughout the muscles. In general older animals tend to deposit more fat within and around the muscle than young ones.
It is generally conceded that the presence of fat around and particularly within the muscle increases the juiciness of the meat. Some water is deposited with the fat and this, combined with melted fat is probably responsible for the apparent increased juiciness.
Some cuts contain more fat. Cuts coming from certain locations contain relatively more fat than cuts from other parts of the body. Fat tends to be deposited first subcutaneously and around the internal organs. In general deposition of fat intermuscularly and intramuscularly comes later, and last of all its deposit within the individual fibers. The deposition of fat within the lean tissues is known as marbling, which undoubtedly increases the palatability of the meat. But to secure marbled muscular tissues a rather thick fat covering is essential (i.e., the portion of the cut lying beneath the skin) and, because meat is eaten primarily for its lean content, many housewives think this fat covering of the muscles is unnecessary. The fat covering differs in thickness in various cuts so that no definite statement can be made as to the most desirable thickness. But for a prime rib roast most persons prefer a covering of about 3/4 inch, some liking more and others less. The fat covering can become so thick and heavy that it does not materially increase the palatability and is wasteful.
Adipose tissue. Adipose tissue is largely made up of fat cells within connective tissue. For human fatty tissue Burns gives the composition: water 15 per cent, fat 82.5 per cent, and protein 2.5 per cent. Gortner states that adipose tissue often contains as much as or more water than it does fat.
Fig. 21. - Longitudinal section of muscle fibers of beef. The white nodular spaces are fat cells. The pulling apart of the fibers in preparation is shown by the long, narrow, white spaces. The burled looking part of the fibers is due to pulling of the knife edge in cutting. The cross striations show plainly in the fibers to the left of the fat cells. Magnification approximately x 200.
Size of fat cells. The size of the fat cells varies with the nutritional state of the animal. Hammond states that they may be 10 to 20u, in a starved animal, 50/x in one in ordinary condition, and up to 175u in a very fat one. The cells are smaller within the connective tissue and in the marbling fat within the muscles and larger around the internal organs and sub-cutaneously.