1) “Des” is a plural so it’s “In the cider there is always iron salts“
2/3) “en” is short for “il existe toujours dan le cidre des sels de fer qui, comme on le sait, précipitent le tanin en noir” while “celle” refer to “coloration” : “this leads to a darker coloration than the one that would be produced by the tannins’s oxydation alone“
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