Cold Storage.--Man has also come to use cold to keep bacteria from growing in foods. The ice box at home and cold storage on a larger scale enables one to keep foods for a more or less lengthy period. If food is frozen, as in cold storage, it might keep without growth of bacteria for years. But fruits and vegetables cannot be frozen without spoiling their flavor. And all foods after freezing seem particularly susceptible to the bacteria of decay. For that reason products taken from cold storage must be used at once. Ptomaines.--Many foods get their flavor from the growth of molds or bacteria in them. Cheese, butter, the gamey taste of certain meats, the flavor of sauerkraut, are all due to the work of bacteria. But if bacteria are allowed to grow so as to become very numerous, the ptomaines which result from their growth in foods may poison the person eating such foods. Frequently ptomaine poisoning occurs in the summer time because of the rapid growth of bacteria. Much of the indigestion and diarrhoea which attack people during the summer is doubtless due to this kind of poisoning. Preservatives.[21]--This leads us to ask if we may not preserve food in ways other than those mentioned so as to protect ourselves from danger of ptomaine poisoning. Many substances check the development of bacteria and in this way they _preserve_ the food. Preservatives are of two kinds, those harmless to man and those that are poisonous. Of the former, salt and sugar are examples; of the latter, formaldehyde and possibly benzoic acid. Footnote 21: Perform experiment here to determine the value of different preservatives. Use sugar, salt, vinegar, boracic acid, benzoic acid, formaldehyde, and alcohol. Sugar.--We have noted the use of sugar in canning. Small amounts of sugar will be readily attacked by yeasts, molds, and bacteria, but a 40 to 50 per cent solution will effectually keep out bacteria. Preserves are fruits boiled in about their own weight of sugar. Condensed milk is preserved by the sugar added to it; so are candied and, in part, dried fruits. Salt.--Salt has been used for centuries to keep foods. Meats are smoked, dried, and salted; some are put down in strong salt solutions. Fish, especially cod and herring, are dried and salted. The keeping of butter is also due to the salt mixed with it. Vinegar is another preservative. It, like salt, changes the flavor of materials kept in it and so cannot come into wide use. Spices are also used as preservatives.