Experimental cookery

Lathrop states that both iron and tin are "injurious to fruit pigments. The compounds of the pigments with the metals become quickly oxidized on exposure to air, with a marked increase in intensity of discoloration. Tin turns grapes, cherries, raspberries, and blackberries a deep purple and straw- berries a pale red. Iron produces dull brownish discolorations. Copper and aluminum are far less injurious and are therefore used wherever fruit must come in contact with metal. Copper being somewhat injurious to the color of grape, aluminum is usually used with grape." The application of this knowledge comes in cooking fruits, preserves. REACTIONS OF THE TANNINS 121 and jellies. Since tin cooking utensils are seldom used in household cooking, this metal would be injurious to color of fruits when used in taking seeds from berries with tin colanders, or when such fruits are canned in un- lacquered tin. Iron might affect the color of fruits, if such fruits are cooked in utensils from which the enamel has been chipped. Tannins The term tannin is sometimes used to denote a whole group of substances having certain characteristics in common. Sometimes it is used to denote a particular substance, i.e., gallotanic or digallic acid. The former is the meaning used in the following paragraphs. The tannins are widely distributed in the higher plants. Some plants are very rich in them; others contain very little. The amount in the plant will vary with different years and growing conditions. The tannins are found throughout the plant, but the woody part, the stems, and rootstocks are likely to contain larger amounts. They are found in many fruits, espe- cially during the immature or green stages, and in the seeds of several plants. Lathrop states that stemming of grapes prior to heating for juice extraction is advisable to get rid of astringent tannin of the stems which would be detrimental to flavor. To some extent the tannin content of fruit is not only dependent upon environmental influences, but upon inherited characteristics. The New York Experiment Station has reported a variety of peach called Sunbeam, which does not turn brown when pared. Kertesz reports its tannin content as very low, 0.0076 per cent. Reactions of the tannins. Thatcher states that chemically the "Tan- nins are either free phenol-acids, or, more commonly, glucosides of these acids." The structure of the tannins is very similar to that of the anthocyan pigments. They are divided into two general classes, known as the pyro- gallol tannins and the catechol tannins. Thatcher has reported the following characteristic reactions. Pyrogallol Variety Catechol Variety Ferric salts Dark blue Greenish black Bromine water No precipitate Yellow or brown precipitate Leather Produces bloom No bloom Concentrated sulfuric acid Yellow or brown Red or Pink Lime water Gray or blue Pink or brown precipitate precipitate Haas and Hill summarize the properties of the tannins under eight headings. Only five will be given here. 1. Tannins are mostly uncrystallizable colloidal substances with as- tringent properties. 122 FRUITS AND VEGETABLES 2. They give blackish-blue or blackish-green colors with ferric salts, a fact made use of in the manufacture of ink. 3. In alkaline solution the tannins and many of their derivatives readily absorb oxygen becoming dark in color.