This section is from the book "The Steward's Handbook And Guide To Party Catering", by Jessup Whitehead. Also available from Amazon: Larousse Gastronomique.
The harmless fruit acids used in cookery-are those named above (see acetic acid), obtained from fruits of the citrus family, lemons, limes, etc., the lees of wine and from vinegar. Oxalic acid is poison though obtained from the weed oxalis or sorrel, which we cook and is harmless. Prussic acid is a poison although it is present in minute quantities and gives the pleasant bitter flavor to the leaves and fruit of trees of the almond tribe, which are freely used. An acid stirred into a solution of cochineal changes it from purple to scarlet, hence cake icing and other substances colored with cochineal have a brighter tint if they are slightly acidulated. Lemon juice or other acids stirred into such mixtures as boiling pudding sauce, tapioca or starch jell)' and some soups, generally will change their bluish appearance to clear transparency. Lemon juice or other acid is often required to make a similar change in gelatine jelly and in strong consommes, which sometimes become too rich and viscid to pass through the strainer until cut with a dash of acid. Acids act upon copper or brass so as to brighten the surfaces. Acid and salt will clean a copper or brass vessel, the brightened surface soon tarnishes, however, unless dry polished afterwards.
Acids act upon copper and brass vessels in such a way as to produce a poison called verdigris, which forms at the edge where the air, acid and copper are in contact. Stewed fruit, cranberries, pickles, slaw, salads, etc., acquire a bitter taste and become poisonous if left standing a few hours in copper or brass, and brass spoons from which the plating is worn off become coated with a bitter tasting poison if left in dishes of fruit jelly, sauce, pickles, salads, or anything that contains an acid. Acids act on iron in such a way that sour bread dough set to rise in iron pans becomes stained as with ink. They act on tin and zinc to a less degree, but salads containing vinegar acquire an unpleasant metallic taste in tin pans, and should be made and kept in glass or earthenware bowls. Acids, like lemon slices or juice or vinegar, will whiten boiling chickens, fish, turkeys, calf's head, sweetbreads, etc., provided the vessel used to boil in be bright and new, but if an iron vessel or a tin one much worn, the action of the acid will often spoil the appearance of the fish or meat entirely by turning them blue and of a dirty color, and when such vessels with the tinning mostly worn off must be used, the vinegar or lemon juice should be omitted. (See boracic acid.) Acids dissolve sea shells and egg shells immersed in them; their action upon the lime of the shells produces a slow effervescense until the lime is all driven off.
In like manner lemon juice injures the finger nails, making them brittle and jagged. A very slight acidity or sourness is generally productive of whiteness in bread and cake, while the opposite alkalim-ity produces a yellow or dusk color, thus bread just on the point of turning sour is the whiter, and bread made with milk turns out whiter through the formation of lactic acid or the souring of the milk in it while rising, but the addition of soda to counteract the slight sourness would make it a darker shade. In cake-making the addition of lemon juice or cream tartar alone produces both lightness and whiteness, while soda or baking powder added has the effect to make the hue either dull yellow or grey.