Fruit juices have been noted as refreshing in effect. Fruit acids serve also some cooking purposes, as tartaric acid frees carbon dioxid gas in some baking-powders.

The common acids in human foods are :

Tartaric acid in grapes (1-5%), and in currants (5.8%).

Malic " apples (.9%), blackberries (.7%), strawberries (1.4%).

Citric " " oranges (1%), lemons (7%).

Vinegar is a manufactured food-acid. It is made from apples by fermentation that converts sugar into alcohol, then acetic acid. Though vinegar is also made from wine, molasses, glucose, it is in all forms fermented. When pure any of these vinegars is satisfactory, though cider and wine are preferable. Spirit vinegar made from corn or barley malt, though cheaper to produce, is less palatable.

Adulteration of vinegar, even with water, is easily accomplished and often practiced. Law now requires that vinegar have acetic acid, 4%; solids (of apple), 1 3/5%; ash, 1/4%. Spirit vinegar may be colored and other additions made to give it the appearance of cider vinegar. No adulteration is ever advisable, and most adulteration is somewhat injurious, even when not obviously dangerous. Its object is always increase in profit. It is improved production that human health requires.

Clear vinegar is the result of completed fermentation and protection from air. During the process of acetic fermentation vinegar is cloudy and forms deposits. "Mother" of vinegar is a fungus growth associated with the acetic-acid ferment. The acidity resulting from completed fermentation inhibits growth of more ferments.

Glass, stone, or wood stopped receptacles must be used for vinegar, as it dissolves the household metals, iron, copper, tin, aluminium.