Q. When I shop for filleted fish in the supermarket, I often find them bathing in a milky solution. Is this some sort of sanitizer? What happens to fish fillets behind the scenes, anyway?

A. The milky solution might come from one of three sources: One, it might be thaw-drip, the water that oozes out of previously frozen fish. The proteins of frozen fish are not as capable of holding water as they are in the fresh state due the cellular damage caused by ice crystals. Two, it might be from melted ice; when stored after business hours, fish fillets are usually iced down (a piece of plastic film is placed over the fillets, but sometimes water leaks through). Or three, it might be from rinsing the fillets; fish handlers sometimes rinse older fillets before setting them out for sale. This provides a fresher appearance and slows down microbial growth.

The milky appearance comes from some of the smaller proteins, which are only partially soluble in water. They reflect light, making the water around each fillet opalescent.

The use of sanitizers on fish fillets is regulated by the FDA's term "Good Manufacturing Practice." That is, there are a number of compounds available that fish processors can use within certain specified limitations to reduce the bacterial populations in the outsides of the fillets. Not all processors use them.

There are four kinds of solutions to which fish fillets might be exposed. These are used more by fish fillet processors than by individual seafood stores or supermarkets. There is no law, however, to keep stores and supermarkets from using them; the fish are supposed to be labeled at the point of sale. This is rarely done. They are:

Brine: A simple salt solution has been traditionally used with the flat fish (sole and flounder) in order to add flavor, tighten the muscles and whiten the fillets.

Phosphate dip: Large fish processors selling frozen or fresh fillets sometimes dip them 30 to 60 seconds in a tripolyphosphate solution. This GRAS (generally recognized as safe) food additive reduces thaw-drip and drip-loss when frozen fillets are defrosted and when fresh or frozen fillets are cooked. When used according to industry and government standards, the sharp flavor of phosphate salts is not noticeable but the texture of the thawed and cooked fillets is much improved.

Phosphate-sorbate-citrate dip: This is another popular dip, again used mostly by processors of frozen and fresh fillets. The sorbate -- usually potassium sorbate -- has an antibiotic effect and keeps the fillets fresh-smelling. Sorbate is much-used by the dairy industry in the production of cheeses and cheese derivatives. And citrate is a buffer. That is, it regulates pH (acidity) of the dip so it is most effective.

Chlorinated water spray: Almost all processors rinse fish fillets with chlorinated water. The permitted concentration is 5 to 10 parts per million (0.0005 to 0.001 percent). That's 5 to 10 times the concentration of drinking water. Chlorinated water leaves little if any chlorine residue, but significantly reduces the surface bacterial populations. Answers By Tom Neuhaus Special to The Washington Post Q When I shop for filleted fish in the supermarket, I often find them bathing in a milky solution. Is this some sort of sanitizer? What happens to fish fillets behind the scenes, anyway? A The milky solution might come from one of three sources: One, it might be thaw-drip, the water that oozes out of previously frozen fish. The proteins of frozen fish are not as capable of holding water as they are in the fresh state due the cellular damage caused by ice crystals. Two, it might be from melted ice; when stored after business hours, fish fillets are usually iced down (a piece of plastic film is placed over the fillets, but sometimes water leaks through). Or three, it might be from rinsing the fillets; fish handlers sometimes rinse older fillets before setting them out for sale. This provides a fresher appearance and slows down microbial growth.

The milky appearance comes from some of the smaller proteins, which are only partially soluble in water. They reflect light, making the water around each fillet opalescent.

The use of sanitizers on fish fillets is regulated by the FDA's term "Good Manufacturing Practice." That is, there are a number of compounds available that fish processors can use within certain specified limitations to reduce the bacterial populations in the outsides of the fillets. Not all processors use them.

There are four kinds of solutions to which fish fillets might be exposed. These are used more by fish fillet processors than by individual seafood stores or supermarkets. There is no law, however, to keep stores and supermarkets from using them; the fish are supposed to be labeled at the point of sale. This is rarely done. They are:

Brine: A simple salt solution has been traditionally used with the flat fish (sole and flounder) in order to add flavor, tighten the muscles and whiten the fillets.

Phosphate dip: Large fish processors selling frozen or fresh fillets sometimes dip them 30 to 60 seconds in a tripolyphosphate solution. This GRAS (generally recognized as safe) food additive reduces thaw-drip and drip-loss when frozen fillets are defrosted and when fresh or frozen fillets are cooked. When used according to industry and government standards, the sharp flavor of phosphate salts is not noticeable but the texture of the thawed and cooked fillets is much improved.

Phosphate-sorbate-citrate dip: This is another popular dip, again used mostly by processors of frozen and fresh fillets. The sorbate -- usually potassium sorbate -- has an antibiotic effect and keeps the fillets fresh-smelling. Sorbate is much-used by the dairy industry in the production of cheeses and cheese derivatives. And citrate is a buffer. That is, it regulates pH (acidity) of the dip so it is most effective.

Chlorinated water spray: Almost all processors rinse fish fillets with chlorinated water. The permitted concentration is 5 to 10 parts per million (0.0005 to 0.001 percent). That's 5 to 10 times the concentration of drinking water. Chlorinated water leaves little if any chlorine residue, but significantly reduces the surface bacterial populations.