Part 6

PREPARATION OF FOOD

=Digestibility.= There are comparatively few kinds of food that can be eaten uncooked. Various fruits, milk, oysters, eggs, and some other things may be eaten raw, but the great mass of food materials must be prepared by some method of cooking. All the common vegetables, such as potatoes, turnips, carrots, beets, and the different grains, such as rice, wheat, corn, oats, etc., neither taste good nor are easily digestible until their starch, cellulose, and other constituents have been changed from their compact indigestible form by the action of heat. Some one has spoken of cooking as a sort of artificial digestion, by which nature is relieved of a certain amount of work which it would be very difficult, if not impossible, for her to perform.

=Flavors.= The necessity of cooking to develop, or to create, a palatable taste is important. The flesh of fowl is soft enough to masticate, but only a person on the verge of starvation could eat it until heat has changed its taste and made it one of the most savory and acceptable of meats. Coffee also well illustrates this point. When coffee is green--that is, unbrowned--it is acrid in taste, very tough, even horny in consistency, and a decoction made from it is altogether unpleasant. But when it is subjected to a certain degree of heat, for a certain time, it loses its toughness, becomes brittle, changes color, and there is developed in it a most agreeable flavor. This flavoring property is an actual product of the heat, which causes chemical changes in an essential oil contained in the bean. Heat not only develops but creates flavors, changing the odor and taste as well as the digestibility of food.

=Effects of Cold.= Some foods are better for being cold; for example, butter, honey, salads, and ice-cream. Sweet dishes as a rule are improved by a low temperature. The flavor of butter is very different and very much finer when cold than when warm. It is absolutely necessary to keep it cool in order to preserve the flavor.

CHEMICAL AND PHYSICAL CHANGES

=Chemical Changes.= Since many of the changes which cooking produces in the different food materials are of a chemical nature, it is well to consider what constitutes a chemical process. This idea may perhaps be best conveyed by a few experiments and illustrations, the materials for which may be easily obtained.

=Exp. with Cream of Tartar and Bicarbonate of Soda.= Mix two teaspoons of cream of tartar with one of bicarbonate of soda, in a little warm water. A union of the two substances follows and they neutralize each other; that is, the cream of tartar is no longer acid, and the soda is no longer alkaline. Owing to the power of chemical affinities a separation or breaking up of these compounds takes place, and new substances, carbonic acid and rochelle salts, are formed out of their constituents. The effervescence which is seen is caused by the escape of the carbonic acid.

=Exp. with Hydrochloric Acid and Soda.= Put a few drops of chemically pure hydrochloric acid into a little water; then add soda. A violent effervescence will follow. Continue putting in soda until this ceases, when the reaction should be neutral. Test it with litmus-paper. If it turns blue litmus-paper red, it is acid; if red litmus-paper blue, it is alkaline. Add acid or soda, whichever is required, until there is no change produced in either kind of litmus-paper. The results of this experiment are similar to those in the first one, namely, carbonic acid and a salt. In this case the salt is sodium chlorid or common salt, which is in solution in the liquid. Evaporate the water, when salt crystals will be found.

=Oxid of Iron.= A piece of iron when exposed to the weather becomes covered with a brownish-yellow coating, which does not look at all like the original metal. If left long enough it will wholly disappear, being completely changed into the yellowish substance, which is oxid of iron, a compound of oxygen and iron, commonly called iron rust.

=Burning of Coal.= A piece of coal burns in the grate and is apparently destroyed, leaving no residue except a little ashes. The carbon and hydrogen of the coal have united with the oxygen of the air, the result of which is largely the invisible gas, carbonic acid, which escapes through the chimney.

=Formation of Water.= Water is formed by the union of two invisible gases, hydrogen and oxygen. It bears no resemblance whatever to either of them. Its symbol is H_{2}O.

All these are examples of chemical changes.

=Definition of Chemical Change.= Chemical changes or processes may be defined as those close and intimate actions amongst the particles of matter by which they are dissociated or decomposed, or by which new compounds are formed, and involving a complete loss of identity of the original substance.

=Physical Changes.= Mix a teaspoon of sugar with an equal amount of salt; the sugar is still sugar, and the salt remains salt; and they may each be separated from the mixture as such.

Water when frozen is changed from a liquid to a solid; its chemical composition, however, remains unchanged.

Water converted into steam by heat is changed from a liquid to a gas, but chemically there is no difference between the one and the other. Steam, water, and ice are forms of the same substance, the difference being physical, not chemical, and caused by a difference in temperature.

Lead melted so that it will run, and the solid lead of a bullet, are the same thing.

These illustrate physical changes.

=Definition.= When substances are brought together in such a way that their characteristic qualities remain the same, the change is called physical. It is less close and intimate than a chemical change. The transition from one state into another is also frequently only a physical change, as is seen in the transformation of water into steam, water into ice, etc.

ELEMENTS

One feature of the work of the chemist is to separate compound bodies into their simple constituents. These constituents he also endeavors to dissociate; and if this cannot be done by any means known to him, then the thing must be regarded as a simple substance. Such simple bodies are called elements.

=Definition.= An element then may be defined as a simple substance, which cannot by any known process be transformed into anything else; that is, no matter how it is treated, it still remains chemically what it was before. Gold, silver, copper, iron, platinum, carbon, phosphorus, calcium, oxygen, hydrogen, nitrogen, and chlorin are examples of elements. Once it was believed that there were but four elements in the world--earth, air, fire, and water. Then it was learned that these were not elements at all, but compounds, and the number of elements increased, until now sixty-eight are admitted to be simple primary substances. Some of these may in the future be proven to be compounds. Sulphur is at present in the doubtful list.

=Oxygen.= Oxygen is an element. It is an invisible gas, without taste or smell. It is the most abundant substance in the world, and an exceedingly active agent, entering into nearly all chemical changes and forming compounds with all known elements except one--fluorin. It is a necessity of life and of combustion. It constitutes about two thirds of the weight of our bodies and one fifth of the weight of the air.

=Hydrogen.= Hydrogen is a gas. It is the lightest substance known. It unites with oxygen to form water, and, as will be seen later, enters into the composition of the human body.

=Nitrogen.= Nitrogen is also a gas, but, unlike oxygen, is an inactive element. It supports neither fire nor life. It is not poisonous, however, for we breathe it constantly in the atmosphere, where its office is to dilute the too active oxygen. A person breathing it in a pure state dies simply from lack of oxygen.

=Carbon.= Carbon is a solid and an important and abundant element. It is known under three forms: diamond, graphite, and charcoal. The diamond is nearly pure carbon. Graphite (the "black-lead" of lead-pencils), coal, coke, and charcoal are impure forms of it. Carbon is combustible; that is, it burns or combines with oxygen. In this union carbonic acid is formed, and there is an evolution of heat, and usually, if the union be rapid and intense enough, of light. It is the valuable element in fuels, and in the body of man it unites with the oxygen of the air, yielding heat, to keep the body warm, and energy or muscular strength for work (Prof. Atwater). The carbonic acid formed in the body is given out by the lungs and skin.

=Other Elements.= There are many other elements about which it would be interesting to note something, such as calcium and phosphorus (found abundantly in the bones), magnesium, sulphur, sodium, iron, etc. Samples of these may be obtained to show to pupils, and descriptions given and experiments made, at the discretion of the teacher. Of the four most abundant elements of the body and of food,--oxygen, carbon, hydrogen, and nitrogen,--it is extremely important that some study be made, and if the apparatus can be procured, that it be of an experimental nature rather than simply descriptive.

AIR