Water fills a very important place in human nutrition. It is everywhere present, generally in some useful way. All plant substance, all animal tissue, foods, and nearly all the material things with which man comes in contact in his daily life are made up of more or less water, or are associated with it. Sometimes this is very evident, as with green plants or juicy fruits. It is not so evident with wheat, flour, and corn meal. If, however, we submit almost any substance, no matter how dry it may appear, except, perhaps, glass and metals, to the heat of an oven at 100° C, we find that a material loss of weight occurs; and if we so arrange that whatever is driven off is first drawn through some substance that entirely absorbs the water which has been vaporized, we learn that the decrease in weight is nearly all accounted for by the water thus collected.

23. Determination Of Water

This fact suggests to us the chemist's way of determining the proportion of water which any particular material contains. He weighs out a certain amount of the substance and then keeps it in an oven at 212° F. for five hours, perhaps, after which it is reweighed. The difference in the two weights, or the loss, is assumed to be all water, and the percentage in the original substance is easily calculated. That portion of the material which is left behind after the water is evaporated we call the dry substance, or water-free substance.

24. Hygroscopic Water

Water is associated with plant and animal tissues in two ways, hygroscopically and physiologically. It is easy to illustrate the former way by an object lesson. If an ounce of corn meal were to be dried in an oven as described, it would, as stated, lose in weight. If it were subsequently allowed to remain exposed in the open air, it would return quite or nearly to its original weight. The loss would be due to water driven out, and the gain to water absorbed from the atmosphere, which we call hygroscopic moisture.

All solids attract moisture up to a certain proportion, which varies with the substance and with the atmospheric conditions that prevail. The surfaces of the particles of matter are ordinarily covered with a thin film of water which is thicker on a cold, wet day than on a warm, dry day; and so certain foods, when exposed to the air, weigh less at one time than at another, because the percentage of hygroscopic water varies. An equilibrium will always be established between the attraction of a substance for moisture and the tension of the vapor of water in the surrounding air, which accounts for the effect of temperature and of the degree to which the air is saturated with water vapor. As all substances do not have the same attraction for moisture, therefore, under similar atmospheric conditions, one food may retain more water than another.

25. Physiological Water

Water that is held physiologically is that which is a constant and essential part of living organisms, in which relation it is necessary to life and performs certain important functions. These functions are of three kinds: (1) The presence of water in the tissues of plants and animals gives them more or less firmness or rigidity combined with elasticity; (2) water acts as a food solvent; (3) water is the great carrier of food materials and of waste products from one part to another of the vegetable or animal organism.