The Origin of ketchup from Salt: A World History by Mark Kurlansky via DelanceyPlace.com. Ketchup began in English kitchens as anchovy sauce, and borrowed the Indonesian word for fish sauce, kecap. Variations were created using mushrooms, walnuts, lemons, and eventually tomatoes (referred to widely in prior centuries as "love apples"):
"In eighteenth-century England, anchovy sauce became known as ketchup, katchup, or catsup.
To make English Katchup
Take a wide mouth'd bottle, put therin a pint of the best white wine vinegar, putting in ten or twelve cloves of eschalot peeled and just bruised; then take a quarter of a pint of the best langoon white wine, boil it a little, and put to it twelve or fourteen [salt cured] anchovies washed and shred, and dissolve them in the wine, and when cold, put them in the bottle; then take a quarter of a pint more of white wine, and put it in mace, ginger sliced, a few cloves, a spoonful of whole pepper just bruised, and let them boil all a little; when near cold, slice in almost a whole nutmeg, and some lemon peel, and likewise put in two or three spoonfuls of horse radish; then stop it close, and for a week shake it once or twice a day; then use it; it is good to put into fish sauce, or any savory dish of meat; you may add it to clear liquor that comes from mushrooms.
-- Eliza Smith, The Compleat Housewife, posthumous 16th edition, 1758
"Ketchup derives its name from the Indonesian fish and soy sauce kecap ikan. The names of several other Indonesian sauces also include the word kecap, pronounced KETCHUP, which means a base of dark, thick soy sauce. Why would English garum [or fish sauce] have an Indonesian name? Because the English, starting with the medieval spice trade, looked to Asia for seasoning. Many English condiments, even Worcestershire sauce, invented in the 1840s, are based on Asian ideas.
"Whether it is called garum, anchovy sauce, or ketchup, a large dose of salt was an essential ingredient. Margaret Dods cautioned in her 1829 London cookbook that 'catsups, to make them keep well, require a great deal (of salt).' The salt in ketchup originally came from salt-cured fish, and most early anchovy ketchup recipes, such as Eliza Smith's, do not even list salt as an ingredient because it is part of the anchovies. But the English and Americans began to move away from having fish in their ketchup. It became a mushroom sauce, a walnut sauce, or even a salted lemon sauce. These ketchups originally included salt anchovies, but as Anglo-Saxon cooking lost its boldness, cooks began to see the presence of fish as a strong flavor limiting the usefulness of the condiment. Roman cooks would have been appalled by the lack of temerity, but Margaret Dods adds at the end of her walnut ketchup recipe:
Anchovies, garlic, cayenne, etc. are sometimes put to this catsup; but we think this is a bad method, as these flavours may render it unsuitable for some dishes, and they can be added extempore when required. -- Margaret Dods, Cook and Housewife's Manual, London, 1829
Ketchup became a tomato sauce, originally called 'tomato ketchup' in America, which is appropriate since the tomato is an American plant, brought to Europe by Hernán Cortés, embraced in the Mediterranean, and regarded with great suspicion in the North. The first known recipe for 'tomato ketchup' was by a New Jersey resident. All that is certain about the date is that it had to be before 1782, the year his unfashionable loyalty to the British Crown forced him to flee to Nova Scotia.
"The first published recipe for tomato ketchup appeared in 1812, written by a prominent Philadelphia physician and horticulturist, James Mease. Already in 1804 he had observed, employing the term used for tomatoes in the United States at the time, that 'love apples' make 'a fine catsup.' Mease said that the condiment was frequently used by the French. The French have never been known for their fondness for tomato ketchup, so it is thought, given the date, that the French he was referring to, were planter refugees from the Haitian revolution. To this day, a tomato sauce is commonly used in Haiti and referred to as sauce creole.
LOVE APPLE CATSUP
Slice the apples [tomatoes] thin, and over every layer sprinkle a little salt; cover them, and let them lie twenty-four hours; then beat them well, and simmer them half an hour in a bell-metal kettle; add mace and allspice. When cold, add two cloves of raw shallots cut small, and half a gill of brandy to each bottle, which must be corked tight, and kept in a cool place. -- James Mease, Archives of Useful Knowledge, Philadelphia, 1812
"Ketchup remained a salted product. Lydia Maria Child, in her 1829 Boston cookbook, The American Frugal Housewife, advised in making tomato ketchup, 'A good deal of salt and spice is needed to keep the product well.'"