Propylene glycol alginate (PGA, additive E405) is a chemically modified seaweed gum: an ester of alginic acid in which many of the acid groups are capped with propylene glycol. That small change makes it stay smooth and stable in acidic, calcium-rich liquids that would make ordinary alginate clump — so it is the go-to emulsifier and foam stabiliser for vinaigrettes, fruit drinks and, classically, the head on a glass of beer.
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