Sodium phosphate is a generic term that may refer to any sodium salt of phosphoric acid. They're commonly added to food and may serve a variety of purposes. Sodium phosphates have been well studied and are generally considered safe when used as a food additive.
Specific Compounds
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Sodium phosphate may refer to any of three specific compounds, although it most often refers to trisodium phosphate (Na3PO4) unless otherwise specified. Sodium dihydrogen phosphate (NaH2PO4) may also be called monosodium phosphate. Disodium hydrogen phosphate (Na2HPO4) may also be called disodium phosphate.
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Texturizer
Disodium hydrogen phosphate can serve as a texturizer and texture-modifying agent. This form of sodium phosphate may be added for the purpose of changing the appearance or feel of the food. A texturizing agent is frequently added to increase the shelf life of the food.
Emulsifier
All three forms of sodium phosphate can serve as an emulsifier. An emulsifier is added to allow for the uniform dispersion of two or more ingredients that would otherwise be immiscible. The most common specific purpose of an emulsifier is to prevent oil from separating from the rest of the mixture. Sodium phosphate is commonly added as an emulsifying agent to processed cheeses, processed meats and canned soups.
Leavening Agent
Sodium phosphate may be added to a baked product to help the dough rise. The most common uses of sodium phosphate as a leavening agent are in batter for breaded chicken or fish and commercially sold cakes.
Surface-Active Agents
Sodium phosphates can also be added to food to change the surface tension of the liquid components of the food. This is typically done to serve as a foaming or whipping agent.
Neutralizing Agent
Sodium phosphate can be added to food to keep it from becoming too acidic or alkaline.
Nutrient
Sodium phosphate can be added to food as a dietary supplement. These compounds provide phosphates which are an essential nutrient.