As the cow ages, the muscles become more developed and tougher. Also there is a change in the arrangement of fat within the animal. The fat is between the muscle fibers in a younger cow. As the cow ages, there is less fat within the muscles and all of it is stored on the outside of the muscle. The degree of fat within the muscle is called marbling. When you shop for a steak, look for speckled, spotty streaks of fat in the meat. Avoid huge chunks of fat around the perimeter. The fat melts during cooking and make the meat juicy and flavorful from the inside.
There are 8 categories of beef grade (ordered from the best)
- Prime (young meat, very well marbled)
- Choice (young meat, well marbled)
- Select (young meat, lean)
- Standard (older meat, somewhat marbled)
- Commerical (older meat, less marbled)
- Utility (older meat, lean)
- Cutter (processed meat snack)
- Canner (other uses...ie dog food)
When you shop at Safeway, if it is not explicitly labeled then it is usually "commercial" or "utility" grade. Don't be tricked by the "rancher's reserve: tender beef" sticker. It doesn't mean anything.
Personal recommendations: Costco only sells USDA Choice beef as well as Nob Hill. When the prices are competitive or on sale, I would recommend getting meat from these 2 locations; it's usually about 3 grades higher than Safeway meat. "Select" is still good and usually about 10%-20% cheaper when you can find it.
Chicken and Pork have different grading systems, but they aren't as extensive or as critical to meat quality.