Catholic Recipe: Almond Milk
In the Middle Ages, animal milk was, of course, not refrigerated, and fresh milk did not stay fresh for long. Most cooks simply did not use much milk as the short shelf-life of the product made it a difficult ingredient to depend upon. Many recipe collections of the time advise that cooks should only rely on milk that comes directly from a cow, something not possible at all times, and purchasing milk was a dubious practice, for streetsellers of milk often sold wares that were either spoiled or diluted with water. Milk's use had to be immediate, in cooking or by turning into cheese and butter. It was these difficulties that forced Medieval cooks to look upon milk with great reluctance, and so having milk in the kitchen was usually unheard of.
Rather than animal milk, Medieval cooks turned to something they could depend upon, and that was the milky liquid produced by grinding almonds or walnuts. This liquid, high in natural fats, could be prepared fresh whenever needed in whatever quantities. It also could be made well ahead of time and stored with no danger of degeneration. Because of its high fat content, it, like animal milk, could be churned into butter, and because it was not animal milk, it could be used and consumed during Church designated meatless days.
Almond milk was used extensively in period; all existing cookbooks call for it, and it must have been found in literally every Medieval kitchen. It's the prime ingredient in many, many recipes, and the modern cook recreating Medieval food will have to learn its production in order to prepare the most common of dishes. Fortunately, it's easily made. I prefer the recipe of Terence Scully, as printed in Le Viandier de Taillevent, p. 315:
Combine almonds and water. Steep for 5 minutes, stirring occasionally. Sieve the mixture to remove coarse grains OR (preferably) blend mixture in electric blender until grains are absorbed. Yield - 2 cups almond milk.Recipe Source: Boke of Gode of Cookery Recipes, A by James L. Matterer, 2000