Sodium tripolyphosphate (STPP), the sodium salt of the polyphosphate penta-anion, also known as pentasodium tripolyphosphate or sodium triphosphate (different with trisodium phosphate), is a synthetic ingredient mainly used for retaining moisture in fish. The European food additive number for it is E451(i).
http://www.chinafooding.com/Sodium_tripolyphosphate-STPP
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