7. Chemistry of the action of food materials (salt, soda, acids, water, etc.) upon cooking utensils made of tin, or aluminum, when used in a cooker or hay-box.
The amount of tin dissolved by foods is indicated by the corrosion of the utensil, which can often be seen by the naked eye to be altered in appearance. The exact quantity of tin salts or other tin compounds which may be formed can only be determined by careful chemical analysis. It has been found that many canned goods supposed to be inert, such as squash and pumpkin, have a marked effect upon tin. Crude tests with a number of different foods can be made with tin, iron, aluminum, and copper utensils, as in many cases there is evidence to the eye of action upon the metals. It must be borne in mind, however, that such tests are crude and not decisive of the fact of there being no action in case no action is plainly visible. Only chemical analysis can prove this.
The action of foods upon tin cans bears a close relation to their action upon the utensils when used in fireless cookery, since there is time with the long cooking involved for similar reactions to take place in the cooker.[6]
[6] See “Food Inspection and Analysis,” by Leach, published by John Wiley Sons, New York, 1904, page 694.
Tin utensils rust badly after short use in a cooker, and thus affect the flavour of food cooked in them. This is due to the action of acids and water on the iron which forms the basis of sheet tin. When the thin plating of tin is worn off, the iron is left exposed to the action of water, etc.
Soda dissolves aluminum, and leaves a black surface on aluminum utensils. This black substance is iron, which is present with the aluminum in the utensils. To remove the black appearance, clean the utensil with acid. Do not try to remove it by scouring, as this will not do the work well, and is laborious and injurious to the pail.
Detection of poisonous metals that may be dissolved from the cooker utensils.
Experiment A. Tin.
In a tin cooker-pail boil such foods as apple sauce, tomatoes, squash, or others that act on tin, and put them into a cooker for twelve hours. Transfer them to an agate ware or porcelain utensil, evaporate them over steam until they may be burned in a porcelain dish until charred and brittle. Pulverize this charred mass, and extract it with hydrochloric acid. Filter and wash it. Saturate the filtrate with hydrogen sulphide gas; add a saturated solution of potassium acetate to neutralize the hydrochloric acid present and assist in the coagulation of sulphide of tin. Warm it slightly, filter and wash out the stannic sulphide, dry it and weight it as stannic oxide, from which the tin dissolved may be calculated.