Human Foods and Their Nutritive Value - Harry Snyder (e book reader pc TXT) 📗

id="pgepubid00160">Fig. 76.—Microscope and Accessories. 1, eye-piece or ocular; 2, objective; 3, stage; 4, cover glass; 5, slide; 6, mirror.

Use of the Microscope.—Special directions in the use of the microscope will be given by the instructor. The object or material to be examined is placed on a microscopical slide. Care should be exercised to secure a representative sample, and to properly distribute the substance on the slide. If a pulverized material is to be examined, use but little and spread it in as thin a layer as possible. If a liquid, one or two drops placed on the slide will suffice. The material on the slide is covered with a cover glass, before it is placed on the stage of the microscope. In focusing, do not allow the object glass of the microscope to come in contact with the cover glass. Focus upward, not downward. Special care should be exercised in focusing and in handling the eye-piece and objective. A camel's-hair brush, clean dry chamois skin, or clean silk only should be used in polishing the lenses. Always put the microscope back in its case after using.

Experiment No. 1 Water in Flour

Carefully weigh a porcelain or aluminum dish. (Porcelain must be used if the ash is to be determined on the same sample.) Place in it about 2 gm. of flour; record the weight; then place the dish in the water oven for at least 6 hours. After drying, weigh again, and from the loss of weight calculate the per cent of water in the flour. (Weight of flour and dish before drying minus weight of flour and dish after drying equals weight of water lost. Weight of water divided by weight of flour taken, multiplied by 100, equals the per cent of water in the flour.)

How does the amount of water you obtained compare with the amount given in the tables of analysis?

Experiment No. 2 Water in Butter

Carefully weigh a clean, dry aluminum dish, place in it about 2 gms. of butter, and weigh again. Record the weights. Place the dish containing butter in the water oven for 5 or 6 hours and then weigh. The loss in weight represents the water in the butter. Calculate the per cent of water. Care must be taken to get a representative sample of the butter to be tested; preferably small amounts should be taken with the butter trier from various parts of the package.

Experiment No. 3 Ash in Flour

Place the porcelain dish containing flour from the preceding experiment in a muffle furnace and let it remain until the organic matter is completely volatilized. Cool, weigh, and determine the per cent of ash. The flour should be burned at the lowest temperature necessary for complete combustion.

Experiment No. 4 Nitric Acid Test for Nitrogenous Organic Matter

To 3 cc. of egg albumin in a test tube add 2 cc. of HNO3 (conc.) and heat. When cool add NH4OH. The nitric acid chemically reacts upon the albumin, forming yellow xanthoprotein. What change occurs in the appearance of the egg albumin when the HNO3 is added? Is this a physical or chemical change? What is the name of the compound formed? What change occurs on adding NH4OH?

Experiment No. 5 Acidity of Lemons

With a pipette measure into a small beaker 2 cc. of lemon juice. Add 25 cc. of water and a few drops of phenolphthalein indicator. From the burette run in N/10 KOH solution until a faint pink tinge remains permanently. Note the number of cubic centimeters of KOH solution required to neutralize the citric acid in the lemon juice. Calculate the per cent of citric acid.

(1 cc. of N/10 KOH solution equals 0.00642 gm. citric acid. 1 cc. of H2O weighs 1 gm. Because of sugar and other matter in solution 1 cc. of lemon juice weighs approximately 1.03 gm.)

1. What is the characteristic acid of lemons? 2. What is the salt formed when the lemon juice is neutralized by the KOH solution? 3. Describe briefly the process for determining the acidity of lemon juice. 4. What per cent of acidity did you obtain? 5. How does this compare with the acidity of vinegar?

Experiment No. 6 Influence of Heat on Potato Starch Grains

With the point of a knife scrape slightly the surface of a raw potato and place a drop of the starchy juice upon the microscopical slide. Cover with cover glass and examine under the microscope.

In the evaporating dish cook a small piece of potato, then place a very small portion upon the slide, and examine with the microscope.

Make drawings of the starch grains in raw and in cooked potatoes.

Experiment No. 7 Influence of Yeast on Starch Grains

Moisten a small portion of the dough prepared with yeast and with the stirring rod place a drop of the starchy water upon the slide. Cover with cover glass and examine under the microscope.

Repeat, examining a drop of starchy water washed from flour.

Make drawing of wheat starch grain in flour and in dough prepared with yeast.

Experiment No. 8 Mechanical Composition of Potatoes

Wash one potato. Weigh, then peel, making the peeling as thin as possible. Weigh the peeled potato and weigh the peeling or refuse. Calculate the per cent of potato that is edible and the per cent that is refuse.

Experiment No. 9 Pectose from Apples

Reduce a small peeled apple to a pulp. Squeeze the pulp through a clean cloth into a beaker. Add 10 cc. H2O and heat on a sand bath to coagulate the albumin. Filter, adding a little hot water if necessary. To the filtrate add 5 cc. alcohol. The precipitate is the pectose material.

1. Is the pectose from the apple soluble? 2. Is it coagulated by heat? 3. Is it soluble in alcohol?

Experiment No. 10 Lemon Extract

To 5 cc. of the extract in a test tube add an equal volume of water. A cloudy appearance indicates the presence of lemon oil. If the solution remains clear after adding the water, the extract does not contain lemon oil.

Why does the extract containing lemon oil become cloudy on adding water?

Experiment No. 11 Vanilla Extract

Pour into a test tube 5 cc. of the extract to be tested. Evaporate to one third. Then add sufficient water to restore the original volume. If a brown, flocculent precipitate is formed, the sample contains pure vanilla extract. Resin is present in vanilla beans and is extracted in the essence. The resin is readily soluble in 50 per cent alcohol. If the alcohol is removed from the extract, the excess of resin is precipitated, or if free from alkali, it may be precipitated by diluting the original solution with twice its volume of water. Test the two samples and compare.

(Adapted from Leach, "Food Inspection and Analysis.")

1. Describe the appearance of each sample after evaporating and adding water. 2. Which sample contains pure vanilla extract? 3. State the principle underlying this test.

Experiment No. 12 Testing Olive Oil for Cotton Seed Oil

Pour into a test tube 5 cc. of the oil to be tested and 5 cc. of Halphen's Reagent. Mix thoroughly. Plug the test tube loosely with cotton, and heat in a bath of boiling saturated brine for 15 minutes. If cotton seed oil is present, a deep red or orange color is produced. Test two samples and compare.

Halphen's Reagent.—Mix equal volumes of amyl alcohol and carbon disulphid containing about one per cent of sulphur in solution.

(Adapted from Leach, "Food Inspection and Analysis.")

Experiment No. 13 Testing for Coal Tar Dyes

Dilute 20 to 30 cc. of the material to 100 cc.; boil for 10 minutes with 10 cc. of a 10 per cent solution of potassium bisulphate and a piece of white woolen cloth which has previously been boiled in a 0.1 per cent solution of NaOH and thoroughly washed in water. Remove the cloth from the solution, wash in boiling water, and dry between pieces of filter paper. A bright red indicates coal tar dye. If the coloring matter is entirely from fruit, the woolen cloth will be either uncolored or will have a faint pink or brown color which is changed to green or yellow by ammonia and is not restored by washing. This is the Arata test.

(Adapted, Winston, Conn. Experiment Station Report.)

1. Describe Arata's wool test for coal tar dyes. 2. What is the appearance of the woolen cloth when the coloring matter is entirely from fruit? 3. What effect has NH4OH upon the color? 4. Why is NaOH used? 5. Why may not cotton cloth be used instead of woolen? 6. What can you say of the use of coal tar dyes in foods?

Experiment No. 14 Determining the Per Cent of Skin in Beans

Place in an evaporating dish 10 gm. of beans, 50 cc. of water, and ½ gm. of baking soda. Boil 10 minutes or until the skins are loosened, then drain off the water. Add cold water and rub the beans together till the skins slip off. Collect the skins, place on a watch glass and dry in the water oven for ½ hour. Weigh the dried skins and calculate the per cent of "skin."

1. What does the soda do? 2. What effect would hard limewater have upon the skins? 3. How does removal of skins affect food value of beans and digestibility?

Experiment No. 15 Extraction of Fat from Peanuts

Shell three or four peanuts and with the mortar and pestle break them into small pieces. Place in a test tube and pour over them about 10 cc. of ether. Cork the test tube and allow it to stand 30 minutes, shaking occasionally. Filter on to a watch glass and let stand until the ether evaporates, and then observe the fat.

1. What is the appearance of the peanut fat? 2. What is the solvent of the fat? 3. What becomes of the ether? 4. Why should the peanuts be broken into small pieces?

Experiment No. 16 Microscopic Examination of Milk

Place a drop of milk on a microscopical slide and cover with cover glass. Examine the milk to detect impurities, as dust, hair, refuse, etc. Make drawings of any foreign matter present.

Experiment No. 17 Formaldehyde in Cream or Milk

To 10 cc. of milk in a casserole add 10 cc. of the acid reagent. Heat slowly over the flame nearly to boiling, holding the casserole in the hand and giving it a slight rotary movement while heating. The presence of formaldehyde is indicated by a violet coloration varying in depth with the amount present. In the absence of formaldehyde the solution slowly turns brown.

Acid Reagent.—Commercial hydrochloric acid (sp. gr. 1.2) containing 2 cc. per liter of 10 per cent ferric chlorid.

(Adapted from Leach, "Food Inspection and Analysis.")

1. How may the presence of formaldehyde in milk be detected? 2. Why in this test is it necessary to use acid containing ferric chlorid? 3. Describe the appearance of the two samples of milk after adding the acid reagent and heating. 4. Which sample showed the presence of formaldehyde?

Experiment No. 18 Gelatine in Cream or Milk

To 20 cc. of milk or cream in a beaker add 20 cc. of acid mercuric nitrate and about 40 cc. of H2O. Let stand for a few minutes and filter. Filtrate will be cloudy if gelatine is present.

Add ½ cc. of a dilute solution of picric acid—a heavy yellow precipitate indicates gelatine.

Acid Mercuric Nitrate.—1 part by weight of Hg, 2 parts HNO3 (sp. gr. 1.42). Dilute 25 times with water.

Experiment No. 19 Testing for Oleomargarine

Apply the following tests to two samples of the material:

Boiling or Spoon Test.—Melt the sample to be tested—a piece about the size of a chestnut—in a large spoon, hastening the process by stirring with a splinter. Then, increasing the heat, bring to as brisk a boil as possible and stir thoroughly, not