Cooks often marinate meat, unaware that most marinades toughen meat. To tenderize meat such as pork sirloin, a marinade must contain an acid, which breaks down proteins and allows meat to absorb additional moisture. But most marinades contain highly acidic ingredients that ultimately draw moisture out of meat. The key to tenderizing pork is to use milk, which has low acidity and is a potent marinade. Hunters use it to soften tough game meat, and dairy-based marinades are renowned tenderizers in American, European and Asian cuisines.
Proteins in meat are rigid, folded bundles. If the proteins break down, or “denature,” they unfold and form a mesh-like structure. The mesh traps moisture from the marinade, which makes the meat more tender. Acids break down proteins. But if meat marinates in acid too long, the fibers will contract into new, tighter bundles and squeeze moisture out of the flesh, making the meat tough and dry. The acids common to most marinades -- vinegar, citrus juice and wine -- are so strong that they cause proteins on the surface of the meat to start contracting before the flesh below the surface is sufficiently tenderized.
Milk is an ideal marinade because it’s only mildly acidic. It also contains calcium, which catalyzes enzymes in the meat to break protein bonds. Neither the calcium nor the meat’s natural enzymes cause the proteins to contract and toughen. Since it tenderizes the flesh gently, milk has sufficient time to penetrate deep into the meat before the acid can start to adversely affect the surface proteins. Only milk-based marinades tenderize meat thoroughly without simultaneously damaging some of it.
Whole milk is effective, but buttermilk and yogurt are optimal marinade bases due to their acidity, which is slightly higher than the acidity of milk. Many Southern cooks use buttermilk marinades when they fry chicken or pork chops. Indian cooks often marinate meat in yogurt, particularly for curry dishes. But whole milk is a perfectly suitable choice -- Italian cooks traditionally marinate or braise pork in milk. Regional cuisines may favor certain dairy products, but you can use them interchangeably.
Milk can’t buffer the pork from stronger acids. If you really want to include another acid -- such as orange juice or balsamic vinegar -- for flavor, do not marinate the meat for more than two hours. Instead, baste the meat with the juice or vinegar while the pork cooks. Herbs and spices are safe to include in marinades and will enhance the flavor of the pork.
You do not need a lot of marinade to tenderize the pork effectively: 1/2 cup of marinade per pound of pork is sufficient. Ideally, the meat should soak for four hours, but any time from one hour to overnight is acceptable. Only marinate raw, unfrozen pork; frozen meat does not absorb fluid.
Cover and refrigerate the pork while it marinates to ensure food safety, according to the United States Department of Agriculture. Pat pork dry to remove excess marinade before cooking, unless you are breading the meat or cooking the marinade and pork together to make sauce. The marinade is not safe to consume unless you cook it with the pork for the same amount of time. Do not reuse the milk, even on food that you intend to cook thoroughly.
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