My wife and I got curious what Europeans used for sour flavors before lemons.
\nBefore lemons, Europeans had a much wider range of sour ingredients than we use today. Lemons didn't introduce acidity so much as gradually replace a collection of local sources, some of which arguably suited particular dishes better.
\nMedieval cooks loved sweet-sour flavors, often pairing vinegar or verjuice with honey, sugar, or dried fruit. Citrus was a luxury, especially in the north. In English cookbooks, citrus juice shows up around 1584 and becomes more common after that.
\nSour fruits supplied tartness in pies, and sour milk or buttermilk (later cream of tartar) helped with leavening. Nothing really replaced lemon zest, though. Spices, floral waters, and herbs filled that role with different flavors.
\nJam is the most telling case. Modern recipes add lemon juice because pectin needs acid to set. Historical cooks instead mixed low-pectin fruit with high-pectin fruit like apples or gooseberries, or simply cooked preserves down further. Crabapples, currants, gooseberries, cranberries, sour apples, and quinces can make jelly with sugar alone, and Mrs. Beeton's 1861 gooseberry jelly is just gooseberries and sugar. One caveat: for modern shelf-stable canning, don't swap these in for bottled lemon juice without a tested recipe.
\nOther cuisines had their own long-standing souring traditions rather than lemon substitutes. Examples include tamarind, amchur (dried green mango), kokum, sumac, pomegranate, and yogurt.
\nLemons probably narrowed everyday sour flavors in Northern European and American cooking. A medieval cook might use verjuice with meat, sorrel with fish, crabapple sauce with game, and gooseberries in a pudding. Today we'd squeeze a lemon over all four. That's convenient, but we've lost some distinctive flavors. Verjuice, redcurrants, crabapples, and gooseberries seem especially worth reviving for home preserving.
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