Grazing animals that eat their native diet of grass have more polyunsaturated fat in their meat than animals fed grain and other types of foodstuff. This is one of the reasons that grass-fed meat is better for your health. But polyunsaturated fats are prone to oxidation and oxidized meat can have a rancid or “off” flavor, and the meat spoils more quickly. It was long thought that grass-fed meat would suffer this fate.
Studies show that grass-fed meat is less likely to oxidize than ordinary feedlot meat. Why? The answer is that there are more antioxidants in grass than grain, and these protective substances keep the polyunsaturated fat from oxidizing. When you eat meat from a grass-fed cow, you are consuming more polyunsaturated fat, more antioxidants, and the meat is less likely to spoil.
Mercier, Y., P. Gatellier, M. Renerre (2004). "Lipid and protein oxidation in vitro, and antioxidant potential in meat from Charolais cows finished on pasture or mixed diet." Meat Science 66: 467-473.