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Given citric and tartaric acid can help make a good fondue, how on earth does the non-acidic citrate work? What's the exact science behind sodium citrate? Do they all work by removing the effects of calcium, allowing casein to more freely move and associate with fat globules or membrane-free fat? Is calcium phosphate or calcium citrate somehow involved? Is it the negative charge of all these weak acids that's important?

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A key compound affecting texture in cheeses is colloidal calcium phosphate - calcium and phosphate dispersed throughout the fat, and water, and casein matrix.

Typically, heating would cause the casein to clump and expel the water, fat, and calcium phosphate from a nice even suspension without the aid of an emulsifier. pH has an effect on structure, but the effect of citrate salt on pH is negligible. Two articles explain the effect of citrate:

The Pastorino article concludes that citrate binds with calcium and frees phosphate, resulting in decreased protein-protein bonding (casein clumping) and increased protein-water bonding (emulsification).

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  • Based on this, though quite rigid in a molecular sense, it seems you could expect xanthan gum's anionic nature to trap some calcium as well and function like citrate, but calcium could just act as a cross-linker and prevent the gum from doing any good. At a pH around 6, the tiny citrate is doubly anionic, which matches the doubly cationic nature of calcium--at pH 8, mostly triply anionic.
    – Confused
    Aug 2 at 20:12

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