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Disodium Pyrophosphate

Disodium pyrophosphate or sodium acid pyrophosphate (SAPP)[1] is an inorganic compound consisting of sodium cations and pyrophosphate anion. It is a white, water-soluble solid that serves as a buffering and chelating agent, with many applications in the food industry. When crystallized from water, it forms a hexahydrate, but it dehydrates above room temperature. Pyrophosphate is a polyvalent anion with a high affinity for polyvalent cations, e.g. Ca2+.

Disodium pyrophosphate is produced by heating sodium dihydrogen phosphate:
 
2 NaH2PO4 → Na2H2P2O7 + H2O
 

Disodium pyrophosphate is a popular leavening agent found in baking powders. It combines with sodium bicarbonate to release carbon dioxide:

Na2H2P2O7 + NaHCO3 → Na3HP2O7 + CO2 + H2O
It is available in a variety of grades that affect the speed of its action. Because the resulting phosphate residue has an off-taste, SAPP is usually used in very sweet cakes which mask the off-taste.[2]

Disodium pyrophosphate in baking powder, New Zealand, 1950s
Disodium pyrophosphate and other sodium and potassium polyphosphates are widely used in food processing; in the E number scheme, they are collectively designated as E450, with the disodium form designated as E450(a). In the United States, it is classified as generally recognized as safe (GRAS) for food use. In canned seafood, it is used to maintain color and reduce purge[clarification needed] during retorting. Retorting achieves microbial stability with heat.[3] It is an acid source for reaction with baking soda to leaven baked goods.[4] In baking powder, it is often labeled as food additive E450.[5] In cured meats, it speeds the conversion of sodium nitrite to nitrite (NO2−) by forming the nitrous acid (HONO) intermediate,[clarification needed] and can improve water-holding capacity. Disodium pyrophosphate is also found in frozen hash browns and other potato products, where it is used to keep the color of the potatoes from darkening.[4]

Disodium pyrophosphate can leave a slightly bitter aftertaste in some products, but "the SAPP taste can be masked by using sufficient baking soda and by adding a source of calcium ions, sugar, or flavorings.