E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Crusted Port.
When port is first bottled its fermentation is not complete; in time it precipitates argol on the sides of the bottle, where it forms a crust. Crusted port, therefore, is port which has completed its fermentation.
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The crust is composed of argol, tartrate of lime, and colouring matter, thus making the wine more ethereal in quality and lighter in colour.