The most crusty; having the thickest, hardest, or most prominent crust, or the most grumpy and irritable (informal usage).
From crusty (adjective) plus the superlative suffix -est meaning 'most.' It's the superlative form expressing the highest degree.
The crustiest bread comes from an old baker's trick—spray water on the dough while it bakes and increase the oven temperature just before baking ends, which forces rapid steam escapes and creates that crackling, audibly crunchy exterior.
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