The amount of protein in a specific cut of meat will vary
according to the proportions of lean tissue, bone, and
fat. While the protein content of fish and meat are each
approximately 15% to 20%, there’s less difference
among the other factors in fish, though fat content does
vary according to the species.
Fat in most fish is invisible, intermingled with the tissues
as oil. High-fat fish are more distinctive in flavor than
lean ones because their flavors are dissolved in their
fish oils. In contrast, fat in meat generally surrounds
the muscles and is often removed, though it is visible
in some cuts of meat as marbling (white lines that look
a little like a road map or a river delta).
The structures of fish and meat are similar in that they
both have muscle fibers and connective tissues. In meat,
connective tissue binds together bundles of fibers within
the muscle, surrounds the individual muscles, and attaches
muscle to bone. Fish has shorter muscle fibers and less
connective tissue than meat, and the connective tissue
is more delicate and positioned differently. In fish,
connective tissues lie mainly in thin sheets that separate
orderly layers of muscle fibers.
This combination of factors—less and more tender
connective tissue and shorter muscle fibers—is why
fish cooks more quickly. Connective tissues in fish are
also transformed to gelatin at a much lower temperature
than the connective tissues in meat. As fish’s connective
tissues dissolve, muscle fibers separate easily and the
fish becomes flaky, a sign that it’s ready to eat.