Chapter 9 FOODS AND THE THEORY OF DIGESTION

The body is constantly in need of new material. Oxidation, as shown in the preceding chapter, rapidly destroys substances at the cells, and these have to be replaced. Upon this renewal depends the supply of energy. Moreover, there is found to be an actual breaking down of the living material, or protoplasm, in the body. While this does not destroy the cells, as is sometimes erroneously stated, it reduces the quantity of the protoplasm and makes necessary a process of repair, or rebuilding, of the tissues. This also requires new material.

Finally, substances, such as water and common salt, are required for the aid which they render in the general work of the body. Since these are constantly being lost in one way or another, they also must be replaced. These different needs of the body for new materials are supplied through

The Foods.-Foods are substances that, on being taken into the healthy body, are of assistance in carrying on its work. This definition properly includes oxygen, but the term is usually limited to substances introduced through the digestive organs. As suggested above, foods serve at least three purposes:

1. They, with oxygen, supply the body with energy.

2. They provide materials for rebuilding the tissues.

3. They supply materials that aid directly or indirectly in the general work of the body.

[pg 118]The Simple Foods, or Nutrients.-From the great variety of things that are eaten, it might appear that many different kinds of substances are suitable for food. When our various animal and vegetable foods are analyzed, however, they are found to be similar in composition and to contain only some five or six kinds of materials that are essentially different. While certain foods may contain only a single one of these, most of the foods are mixtures of two or more. These few common materials which, in different proportions, form the different things that are eaten, are variously referred to as simple foods, food-stuffs, and nutrients, the last name being the one generally preferred. The different classes of nutrients are as follows:

Nutrients:

Proteids

(Albuminoids)

Carbohydrates

Fats

Mineral salts

Water

It is now necessary to become somewhat familiar with the different nutrients and the purposes which they serve in the body.

Proteids.-The proteids are obtained in part from the animal and in part from the plant kingdom, there being several varieties. A well-known variety, called albumin, is found in the white of eggs and in the plasma of the blood, while the muscles contain an abundance of another variety, known as myosin. Cheese consists largely of a kind of proteid, called casein, which is also present in milk, but in a more diluted form. If a mouthful of wheat is chewed for some time, most of it is dissolved and swallowed, but there remains in the mouth a sticky, gum-like substance. This is gluten, a form of proteid which occurs[pg 119] in different grains. Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind of proteid which is called legumen.

Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small per cent of sulphur. Certain ones (the nucleo-proteids from grains) also contain phosphorus. All of the proteids are highly complex compounds and form a most important class of nutrients.

Purposes of Proteids.-The chief purpose of proteids in the body is to rebuild the tissues. Not only do they supply all of the main elements in the tissues, but they are of such a nature chemically that they are readily built into the protoplasm. They are absolutely essential to life, no other nutrients being able to take their place. An animal deprived of them exhausts the proteids in its body and then dies. In addition to rebuilding the tissues, proteids may also be oxidized to supply the body with energy.

Albuminoids form a small class of foods, of minor importance, which are similar to proteids in composition, but differ from them in being unable to rebuild the tissues. Gelatin, a constituent of soup and obtained from bones and connective tissue by boiling, is the best known of the albuminoid foods. On account of the nitrogen which they contain, proteids and albuminoids are often classed together as nitrogenous foods.

Carbohydrates.-While the carbohydrates are not so essential to life as are the proteids, they are of very great value in the body. They are composed of carbon, hydrogen, and oxygen, and are obtained mainly from plants. There are several varieties of carbohydrates, but they are similar in composition. All of those used as food to any great extent are starch and certain kinds of sugar.

[pg 120]Starch is the carbohydrate of greatest importance as a food, and it is also the one found in the greatest abundance. All green plants form more or less starch, and many of them store it in their leaves, seeds, or roots (Fig. 60). From these sources it is obtained as food. Glycogen, a substance closely resembling starch, is found in the body of the oyster. It is also formed in the liver and muscles of the higher animals, being prepared from the sugar of the blood, and is stored by them as reserve food (Chapter XI). Glycogen is, on this account, called animal starch. Starch on being eaten is first changed to sugar, after which it may be converted into glycogen in the liver and in the muscles.

Fig. 60-Starch grains in cells of potato as they appear under the microscope. (See practical work.)

Sugars.-There are several varieties of sugar, but the important ones used as foods fall into one or the other of two classes, known as double sugars (disaccharides) and single sugars (monosaccharides). To the first class belong cane sugar, found in sugar cane and beets, milk sugar, found in sweet milk, and maltose, a kind of sugar which is made from starch by the action of malt. The important members of the second class are grape sugar, or dextrose, and fruit sugar, or levulose, both of which are found in fruits and in honey.

The most important of all sugars, so far as its use in the body is concerned, is dextrose. To this form all the other sugars, and starch also, are converted before they are finally used in the body. The close chemical relation[pg 121] between the different carbohydrates makes such a conversion easily possible.

Fats.-The fats used as foods belong to one or the other of two classes, known as solid fats and oils. The solid fats are derived chiefly from animals, and the oils are obtained mostly from plants. Butter, the fat of meats, olive oil, and the oil of nuts are the fats of greatest importance as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen, and oxygen. They are rather complex chemical compounds, though not so complex as proteids. Since neither fats nor carbohydrates contain nitrogen, they are frequently classed together as non-nitrogenous foods.

Purpose Served by Carbohydrates, Fats, and Albuminoids.-These classes of nutrients all serve the common purpose of supplying energy. By uniting with oxygen at the cells, they supply heat and the other forms of bodily force. This is perhaps their only purpose.50 Proteids also serve this purpose, but they are not so well adapted to supplying energy as are the carbohydrates and the fats. In the first place they do not completely oxidize and therefore do not supply so much energy; and, in the second place, they form waste products that are removed with difficulty from the body.

Mineral Salts and their Uses.-Mineral salts are found in small quantities in all of the more common food materials, and, as a rule, find their way into the body unnoticed. They supply the elements which are found in the body in small quantities and serve a variety of [pg 122]purposes.51 Calcium phosphate and calcium carbonate are important constituents of the bones and teeth; and the salts containing iron renew the hemoglobin of the blood. Others perform important functions in the vital processes. The mineral compound of greatest importance perhaps is sodium chloride, or common salt.52 This is a natural constituent of most of our foods, and is also added to food in its preparation for the table. When it is withheld from animals for a considerable length of time, they suffer intensely and finally die. It is necessary in the blood and lymph to keep their constituents in solution, and is thought to play an important r?le in the chemical changes of the cells. It is constantly leaving the body as a waste product and must be constantly supplied in small quantities in the foods.

[pg 123]Importance of Water.-Water finds its way into the body as a pure liquid, as a part of such mixtures as coffee, chocolate, and milk, and as a constituent of all our solid foods. (See table of foods, page 126.) It is also formed in the body by the oxidation of hydrogen. It passes through the body unchanged, and is constantly being removed by all the organs of excretion. Though water does not liberate energy in the body nor build up the tissues in the sense that other foods do, it is as necessary to the maintenance of life as oxygen or proteids. It occurs in all the tissues, and forms about 70 per cent of the entire weight of the body. Its presence is necessary for the interchange of materials at the cells and for keeping the tissues soft and pliable. As it enters the body, it carries digested food substances with it, and as it leaves it is loaded with wastes. Its chief physiological work, which is that of a transporter of material, depends upon its ability to dissolve substances and to flow readily from place to place.

Relative Quantity of Nutrients Needed.-Proteids, carbohydrates, and fats are the nutrients that supply most of the body's nourishment. The most hygienic diet is the one which supplies the proteids in sufficient quantity to rebuild the tissues and the carbohydrates and fats in the right amounts to supply the body with energy. Much experimenting has been done with a view to determining these proportions, but the results so far are not entirely satisfactory. According to some of the older estimates, a person of average size requires for his daily use five ounces of proteid, two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent investigations of this problem seem to show that the body is as well, if not better, nourished by a much smaller amount of proteid-not more than two and one half ounces (60 grams) daily.53

[pg 124] While there is probably no necessity for the healthy individual's taking his proteid, fat, and carbohydrate in exact proportions (if the proportions best suited to his body were known), the fact needs to be emphasized that proteids, although absolutely necessary, should form but a small part (not over one fifth) of the daily bill of fare. In recognition of this fact is involved a principle of health and also one of economy. The proteids, especially those in meats, are the most expensive of the nutrients, whereas the carbohydrates, which should form the greater bulk of one's food, are the least expensive.

Effects of a One-sided Diet.-The plan of the body is such as to require a mixed diet, and all of the great classes of nutrients are necessary. If one could subsist on any single class, it would be proteids, for proteids are able both to rebuild tissue and to supply energy. But if proteids are eaten much in excess of the body's need for rebuilding the tissues, and this excess is oxidized for supplying energy, a strain is thrown upon the organs of excretion, because of the increase in the wastes. Not only is there danger of overworking certain of these organs (the liver and kidneys), but the wastes may linger too long in the body, causing disorder and laying the foundation for disease. On the other hand, if an insufficient amount of proteid is taken, the tissues are improperly nourished, and one is unable to exert his usual strength. What is true of the proteids is true, though in a different way, of the other great classes of foods. A diet which is lacking in proteid, carbohydrate, or fat, or which has any one of them in excess, is not adapted to the requirements of the body.

Composition of the Food Materials.-One who intelligently provides the daily bill of fare must have some knowledge of the nature and quantity of the nutrients[pg 125] present in the different materials used as food. This information is supplied by the chemist, who has made extensive analyses for this purpose. Results of such analyses are shown in Table 1 (page 126), which gives the percentage of proteids, fats, carbohydrates, water, and mineral salts in the edible portions of the more common of our foods.

Fig. 61-Relative proportions of different nutrients in well-known foods.

Food Supply to the Table.-The main problem in supplying the daily bill of fare is that of securing through the different food materials the requisite amounts of proteids, carbohydrates, and fats. In this matter a table showing the composition of foods can be used to great advantage. Consulting the table on page 126, it is seen that large per cents of proteids are supplied by lean meat, eggs, cheese, beans, peas, peanuts, and oatmeal, while fat is in excess in fat meat, butter, and nuts (Fig. 61). Carbohydrates are supplied in abundance by potatoes, rice, corn, sugar, and molasses. The different cereals also contain a large percentage of carbohydrates in the form of starch.

[pg 126] TABLE I. The Composition of Food Materials54

Food Materials Water Solids Proteid Fat Carbohydrates Mineral Matter Heat Value of One Pound

Animal foods, edible portion Per cent Per cent Per cent Per cent Per cent Per cent Calories55

Beef: Shoulder 63.9 36.1 19.5 15.6 ... 1 1020

Rib 48.1 51.9 15.4 35.6 ... .9 1790

Sirloin 60 40 18.5 20.5 ... 1 1210

Round 68.2 31.8 20.5 10.1 ... 1.2 805

Veal: Shoulder 68.8 31.2 20.2 9.8 ... ... 790

Mutton: Leg 61.8 38.2 18.3 19 ... .9 1140

Loin 49.3 50.7 15 35 ... .7 1755

Pork: Shoulder 50.3 49.7 16 32.8 ... .9 1680

Ham, salted, smoked 41.5 58.5 16.7 39.1 ... 2.7 1960

Fat, salted 12.1 87.9 .9 82.8 ... 4.2 3510

Sausage: Pork 41.5 58.8 13.8 42.8 ... 2.2 2065

Bologna 62.4 37.6 18.8 42.8 ... 3 1015

Chicken 72.2 27.8 24.4 1 ... 1.4 540

Eggs 73.8 26.2 14.9 10.5 ... .8 721

Milk 87 13 3.6 4 4.7 .7 325

Butter 10.5 89 .6 85 .5 .3 3515

Cheese: Full cream 30.2 69.8 28.3 35.5 1.8 4.2 2070

Skim milk 41.3 58.7 38.4 6.8 6.9 4.6 1165

Fish: Codfish 82.6 17.4 15.8 .5 ... 1.2 310

Salmon 63.6 36.4 21.6 13.4 ... 1.4 965

Oysters 87.1 12.9 6 1.2 3.7 2 230

Vegetable foods

Wheat flour 12.5 87.5 11 1.1 74.9 .5 1645

Graham flour (wheat) 13.1 86.9 11.7 1.7 71.7 1.8 1635

Rye flour 13.1 86.9 6.7 .8 78.7 .7 1625

Buckwheat flour 14.6 85.4 6.9 1.4 76.1 1 1605

Oatmeal 7.6 92.4 15.1 7.1 68.2 2 1850

Cornmeal 15 85 9.2 3.8 70.6 1.4 1645

Rice 12.4 87.6 7.4 .4 79.4 .4 1630

Peas 12.3 87.7 26.7 1.7 56.4 2.9 1565

Beans 12.6 87.4 23.1 2 59.2 3.1 1615

Potatoes 78.9 21.1 2.1 .1 17.9 1 375

Tomatoes 95.3 4.7 .8 .4 3.2 .3 80

Apples 83.2 16.8 .2 .4 15.9 .3 315

Sugar, granulated 2 98 ... ... 97.8 .3 1820

White bread (wheat) 32.3 67.7 8.2 1.7 56.3 .0 1280

Peanuts 9.2 90.8 25.8 24.4 38.6 2 2560

Almonds 4.8 95.2 21 17.3 54.9 2 3030

Walnuts (English) 2.5 97.5 16.6 16.1 63.4 1.4 3285

[pg 128]Variety in the selection of foods for the table is an essential feature, but this should not increase either the work or the expense of supplying the meals. Each single meal can, and should, be simple in itself and, at the same time, differ sufficiently from the meal preceding and the one following to give the necessary variety in the course of the day. The bill of fare should, of course, include fruits (for their tonic effects) and very small amounts perhaps of substances which stimulate the appetite, such as pepper, mustard, etc., known as condiments.

Purity of Food.-The fact that many of the food substances are perishable makes it possible for them to be eaten in a slightly decayed condition. Such substances are decidedly unwholesome (some containing poisons) and should be promptly rejected. Not only do fresh meats, fruits, and vegetables need careful inspection, but canned and preserved goods as well. If canned foods are imperfectly sealed or if not thoroughly cooked in the canning process, they decay and the acids which they generate act on the metals lining the cans, forming poisonous compounds. The contents of "tin" cans should for this reason be transferred to other vessels as soon as opened.

Foods are also rendered impure or weakened through adulteration, the watering of milk being a familiar example. The manufacture of jellies, preserves, sirups, and various kinds of pickles and condiments has perhaps afforded the largest field for adulterations, although it is possible to adulterate nearly all of the leading articles of food. A long step in the prevention of food and drug adulteration was taken in this country by the passage of the Pure Food Law. By forcing manufacturers of foods and medicines to state on printed labels the composition of their products, this law has made it possible for the consumer to know what he is purchasing and putting into his body.

Alcohol not a Food.-Many people in this and other countries drink in different beverages, such as whisky, beer, wine, etc., a varying amount of alcohol. This substance has a temporary stimulating or exciting effect, and the claim has been made that it serves as a food. Recently[pg 129] it has been shown that alcohol when introduced into the body in small quantities and in a greatly diluted form, is nearly all oxidized, yielding energy as does fat or sugar. If no harmful effects attended the use of alcohol, it might on this account be classed as a food. But alcohol is known to be harmful to the body. When used in large quantities, it injures nearly all of the tissues, and when taken habitually, even in small doses, it leads to the formation of the alcohol habit which is now recognized and treated as a disease. This and other facts show that alcohol is not adapted to the body plan of taking on and using new material (Chapter XI), and no substance lacking in this respect can properly be classed as a food.56 Instead of classing alcohol as a food, it should be placed in that long list of substances which are introduced into the body for special purposes and which are known by the general name of

Drugs.-Drugs act strongly upon the body and tend to bring about unusual and unnatural results. Their use should in no way be confused with that of foods. If taken in health, they tend to disturb the physiological balance of the body by unduly increasing or diminishing the action of the different organs. In disease where this balance is already disturbed, they may be administered for their counteractive effects, but always under the advice and direction of a physician. Knowing the nature of the disturbance which the drug produces, the physician can administer it to advantage, should the body be out of physiological[pg 130] balance, or diseased. Not only are drugs of no value in health, but their use is liable to do much harm.

NATURE OF DIGESTION

Before the nutrients can be oxidized at the cells, or built into the protoplasm, they undergo a number of changes. These are necessary for their entrance into the body, for their distribution by the blood and the lymph, and for the purposes which they finally serve. The first of these changes is preparatory to the entrance of the nutrients and is known as digestion. The organs which bring about this change, called digestive organs, have a special construction which adapts them to their work. It will assist materially in understanding these organs if we first learn something of the nature of the work which they have to perform.

How the Nutrients get into the Body.-The nature of digestion is determined by the conditions affecting the entrance of nutrients into the body. Food in the stomach and air in the lungs, although surrounded by the body, are still outside of what is called the body proper. To gain entrance into the body proper, a substance must pass through the body wall. This consists of the skin on the outside and of the mucous linings of the air passages and other tubes and cavities which are connected with the external surface.

To get from the digestive organs into the blood, the nutrients must pass through the mucous membrane lining these organs and also the walls of blood or lymph vessels. Only liquid materials can make this passage. It is necessary, therefore, to reduce to the liquid state all nutrients not already in that condition. This reduction to the liquid state constitutes the digestive process.

[pg 131]How Substances are Liquefied.-While the reduction of solids to the liquid state is accomplished in some instances by heating them until they melt, they are more frequently reduced to this state by subjecting them to the action of certain liquids, called solvents. Through the action of the solvent the minute particles of the solid separate from each other and disappear from view. (Shown in dropping salt in water.) At the same time they mix with the solvent, forming a solution, from which they separate only with great difficulty. For this reason solids in solution can diffuse through porous partitions along with the solvents in which they are dissolved (page 73).

By digestion the nutrients are reduced to the form of a solution. The process is, simply speaking, one of dissolving. The liquid employed as the digestive solvent is water. The different nutrients dissolve in water, mixing with it to form a solution which is then passed into the body proper.

Digestion not a Simple Process.-Digestion is by no means a simple process, such, for instance, as the dissolving of salt or sugar in water. These, being soluble in water, dissolve at once on being mixed with a sufficient amount of this liquid. The majority of the nutrients, however, are insoluble in water and are unaffected by it when acting alone. Fats, starch, and most of the proteids do not dissolve in water. Before these can be dissolved they have to be changed chemically and converted into substances that are soluble in water. This complicates the process and prevents the use of water alone as the digestive solvent.

A Similar Case.-If a piece of limestone be placed in water, it does not dissolve, because it is insoluble in water. If hydrochloric acid is now added to the water, the limestone[pg 132] is soon dissolved (Fig. 62). (See Practical Work.) It seems at first thought that the acid dissolves the limestone, but this is not the case. The acid produces a chemical change in the limestone (calcium carbonate) and converts it into a compound (calcium chloride) that is soluble in water. As fast as this is formed it is dissolved by the water, which is the real solvent in the case. The acid simply plays the part of a chemical converter.

Fig. 62-The dissolving of limestone in water containing acid, suggesting the double action in the digestion of most foods.

The Digestive Fluids.-Several fluids-saliva, gastric juice, pancreatic juice, bile, and intestinal juice-are employed in the digestion of the food. The composition of these fluids is in keeping with the nature of the digestive process. While all of them have water for their most abundant constituent, there are dissolved in the water small amounts of active chemical agents. It is the work of these agents to convert the insoluble nutrients into substances that are soluble in water. The digestive fluids are thus able to act in a double manner on the nutrients-to change them chemically and to dissolve them. The chemical agents which bring about the changes in the nutrients are called enzymes, or digestive ferments.

Foods Classed with Reference to Digestive Changes.-With reference to the changes which they undergo during digestion, foods may be divided into three classes as follows:

1. Substances already in the liquid state and requiring no digestive action. Water and solutions of simple foods in water belong to this class. Milk and liquid fats, or oils, do not belong to this class.

2. Solid foods soluble in water. This class includes[pg 133] common salt and sugar. These require no digestive action other than dissolving in water.

3. Foods that are insoluble in water. These have first to be changed into soluble substances, after which they are dissolved.

Summary.-Materials called foods are introduced into the body for rebuilding the tissues, supplying energy, and aiding in its general work. Only a few classes of substances, viz., proteids, carbohydrates, fats, water, and some mineral compounds have all the qualities of foods and are suitable for introduction into the body. Substances known as drugs, which may be used as medicines in disease, should be avoided in health. Before foods can be passed into the body proper, they must be converted into the liquid form, or dissolved. In this process, known as digestion, water is the solvent; and certain chemical agents, called enzymes, convert the insoluble nutrients into substances that are soluble in water.

Exercises.-1. How does oxidation at the cells make necessary the introduction of new materials into the body?

2. What different purposes are served by the foods?

3. What is a nutrient? Name the important classes.

4. What are food materials? From what sources are they obtained?

5. Name the different kinds of proteids; the different kinds of carbohydrates. Why are proteids called nitrogenous foods and fats and carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteids; without water.

7. What per cents of proteid, fat, and carbohydrate are found in wheat flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts?

8. State the objection to a meal consisting of beef, eggs, beans, bread, and butter; to one consisting of potatoes, rice, bread, and butter. Which is the more objectionable of these meals and why?

9. State the general plan of digestion.

[pg 134]10. Show that digestion is not a simple process like that of dissolving salt in water.

PRACTICAL WORK

Elements supplied by the Foods.-The following brief study will enable the pupil to identify most of the elements present in the body and which have, therefore, to be supplied by the foods.

Carbon.-Examine pieces of charred wood, coke, or coal, and also the "lead" in lead pencils. Show that the charred wood and the coal will burn. Recall experiment (page 114) showing that carbon in burning forms carbon dioxide.

Hydrogen.-Fill a test tube one third full of strong hydrochloric acid and drop into it several small scraps of zinc. The gas which is evolved is hydrogen. When the hydrogen is coming off rapidly, bring a lighted splinter to the mouth of the tube. The gas should burn. Hold a cold piece of glass over the flame and observe the deposit of moisture. Hydrogen in burning forms water. Extinguish the flame by covering the top of the tube with a piece of cardboard. Now let the escaping gas collect in a tumbler inverted over the tube. After holding the tumbler in this position for two or three minutes, remove and, keeping inverted, thrust a lighted splinter into it. (The gas should either burn or explode.) What does this experiment show relative to the weight of hydrogen as compared with that of air?

Nitrogen.-Nitrogen forms about four fifths of the atmosphere, where, like oxygen, it exists in a free state. It may be separated from the oxygen of an inclosed portion of air by causing that gas to unite with phosphorus. Place a piece of phosphorus the size of a pea in a depression in a flat piece of cork. (Handle phosphorus with wet fingers or with forceps.) Place the cork on water and have ready a glass fruit jar holding not more than a quart. Ignite the phosphorus with a hot wire and invert the jar over it, pushing the mouth below the surface of the water. The phosphorus uniting with the oxygen fills the jar with white fumes of phosphoric oxide. These soon dissolve in the water, leaving a clear gas above. This is nitrogen. Place a cardboard under the mouth of the jar and turn it right side up, leaving in the water and keeping the top covered. Light a splinter and, slipping the cover to one side, thrust the flame into the jar of nitrogen, noting the effect. (Flame is extinguished.) Compare nitrogen with oxygen in its relation to combustion. What purpose is served by each in the atmosphere?

[pg 135]Oxygen.-Review experiments (page 114) showing the properties of oxygen.

Phosphorus.-Examine a small piece of phosphorus, noting that it has to be kept under water. Lay a small piece on the table and observe the tiny stream of white smoke rising from it, formed by slow oxidation. Dissolve a piece as large as a pea in a teaspoonful of carbon disulphide in a test tube, pour this on a piece of porous paper, and lay the paper on an iron support. When the carbon disulphide evaporates the phosphorus takes fire spontaneously. (The heat from the slow oxidation is sufficient to ignite the phosphorus in the finely divided condition.) What is the most striking property of phosphorus? What purpose does it serve in the match?

Sulphur.-Examine some sulphur, noting its color and the absence of odor or taste. (Impure sulphur may have an odor and a taste.) Burn a little sulphur in an iron spoon, noting that the compound which it forms with oxygen by burning has a decided odor.

Other Elements.-Magnesium. Examine and burn a piece of magnesium ribbon, noting the white compound of magnesium oxide which is formed. Iron. Examine pieces of the metal and also some of its compounds, as ferrous sulphate, ferric chloride, and ferric oxide or iron rust. Sodium. Drop a piece of the metal on water and observe results. Sodium decomposes water. It has to be kept under some liquid, such as kerosene, which contains no oxygen. (It should not be touched except with the fingers wet with kerosene.) Chlorine. Pour strong hydrochloric acid on a little manganese dioxide in a test tube, and warm gently over a low flame. The escaping gas is chlorine. Avoid breathing much of it.

Composition of the Nutrients.-The simplest way of determining what elements make up the different nutrients is by heating them and studying the products of decomposition, as follows:

To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen.-Place one half teaspoonful of powdered starch in a test tube and heat strongly. Observe that water condenses on the sides of the tube and that a black, charred mass remains behind. The black mass consists mainly of carbon. The water is composed of hydrogen and oxygen. These three elements are thus shown to be present in the starch. The experiment may be repeated, using sugar instead of starch.

To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur.-Place in a test tube some finely divided proteid[pg 136] which has been thoroughly dried (dried beef or the lean of hard cured bacon). Heat strongly in the hood of a chemical laboratory or some other place where the odors do not get into the room. First hold in the escaping gases a wet strip of red litmus paper. This will be turned blue, showing ammonia (NH3) to be escaping. Next hold in the mouth of the tube a strip of a paper wet with a solution of lead nitrate. This is turned black or brown on account of hydrogen sulphide(H2S) which is being driven off. Observe also that water condenses in the upper part of the tube and that a black, charred mass remains behind. Since the products of decomposition (H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen, sulphur, and carbon, these elements are of course present in the proteid tested.

To show the Presence of Mineral Matter.-Burn a piece of dry bread by holding it in a clear, hot flame, and observe the ash that is left behind. This is the mineral matter present in the bread.

Tests for Nutrients. Proteids.-Cover the substance to be tested with strong nitric acid and heat gradually to boiling. If proteid is present it turns yellow and partly dissolves in the acid, forming a yellow solution. Let cool and then add ammonia. The yellow solid and the solution are turned a deep orange color. Apply this test to foods containing proteid such as white of egg, cheese, lean meat, etc.

Starch.-(a) Place a small lump of starch in one fourth of a pint of water and heat gradually to boiling, stirring well. Then add enough water to form a thin liquid and fill a test tube half full. Add to this a few drops of a solution of iodine. (Prepare by dissolving a crystal of iodine in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in water and add water to this until it is a light amber color.) The starch solution is turned blue, (b) Cut with a razor a thin slice from a potato. Place this in a weak solution of iodine for a few minutes and then examine with the microscope, using first a low and then a high power. Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).

Dextrose, or Grape Sugar.-Place a solution of the substance supposed to contain grape sugar in a test tube and add a few drops of a dilute solution of copper sulphate. Then add sodium hydroxide solution until the precipitate which first forms is redissolved and a clear blue liquid obtained. Heat the upper portion of the liquid slowly to near the boiling point. A little below the boiling point the blue color disappears and a yellow-red precipitate is formed. If the upper layer of[pg 137] the liquid is now boiled, the color deepens and this may be contrasted with the blue color below. Apply this test to the sugar in raisins and in honey.

Fat.-Fat is recognized by its effect on paper, making a greasy stain which does not disappear on heating and which renders the paper translucent. Try butter, lard, or olive oil. Also show the presence of fat in peanuts by crushing them in a mortar and rubbing the powder on thin paper. If the substance to be tested contains but little fat, this may be dissolved out with ether. If a drop of ether containing the fat is placed on paper, it evaporates, leaving the fat, which then forms the stain.

To show the Effect of Alcohol upon Proteid.-Place some of the white of a raw egg in a glass vessel and cover it with a small amount of alcohol. As the albumin (proteid) hardens, or coagulates, observe that the quantity of clear liquid increases. This is due to the withdrawal of water from the albumin by the alcohol. Since the tissues are made up chiefly of proteids, a piece of muscle or of liver may be used in the experiment, instead of the egg, with similar results.

To illustrate the Digestive Process.-To a tumbler two thirds full of water add a little salt. Stir and observe that the salt is dissolved. Taste the solution to see that the salt has not been changed chemically. Now add a little powdered limestone to the water and stir as before. Observe that the limestone does not dissolve. Then add some hydrochloric acid and observe the result. State the part played by the acid and by the water in dissolving the limestone. Apply to the digestion of the different classes of foods.

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[pg 138]