The food is produced as homogenous pellets or extruded pieces, the intention being to minimize the variation in nutritional intake between animals.
[1] The basic type of lab block is made from mainly grains, typically corn, followed by soy, fish meal, animal byproducts, and very high levels of both soluble and insoluble fibers; the ingredient list is provided, but not the proportions.
For very specialized use, there's also the "purified diet", which is assembled from individual substances (e.g. casein for protein, corn starch for carbs, soybean oil for fat, cellulose for fiber) in proportions known by the researcher.
[1] Lab animals with compromised immune systems (e.g. nude mice) may require sterilization of food and special packaging.
Food made by extrusion cooking is typically already near-aspetic, but blender-mixed purified diets tend to require irradiation or autoclaving.