Reading references and experiments illustrating the principles upon which fireless cookery is based.

1. A test of the insulating powers of different materials.

Apparatus:

One or more boxes and fittings, described on pages 9 to 11.

One or more pails of the same size, shape and material, preferably of from two to four quarts’ capacity, with close fitting covers.

• Cooking thermometer
• Wool
• Mineral wool
• Cotton batting or waste
• Excelsior
• Hay
• Sawdust
• Newspapers
• Ground cork
• Southern moss
• Pencil
• Notebook

Pack the box successively with as many of the different packing materials given above as are to be tested, following the directions given on page 15; or have several exactly similar boxes packed at the same time. For all tests fill the cooker-pail with water, bring it to the boiling point, let it boil one minute, to permit all parts of the utensil and its contents to reach the same temperature; then put it at once into the cooker-box and leave it for an equal length of time, not less than one hour. Record the temperature of the contents of the pail at the expiration of this period. In order to get a full record and a fair comparison it would be well to repeat this experiment with varying periods of time, taking the temperature, for instance, at the end of one, three, six, nine, and twelve hours. In taking temperatures do not wholly remove the cushion and cover of the pail, but slip them to one side, enough to insert the thermometer. This is, of course, a crude method of taking temperatures, but answers for purposes of comparison. If it is desired to make more accurate records this can be done by boring the cover of the box, the cushion and the pail cover, and inserting a thermometer through corks which are used to close the bored holes. The temperature can then be read while the apparatus is closed. However, the first method, if carefully done, will give probably within one degree of the correct temperature. Record the results in tabular form.

Which material do you find gives the best insulation?

Winkelmann,[4] Duff,[5] and other writers on physics give tables of the conductivity of felt, asbestos paper, paper, cotton, flannel, and other materials; but as different figures are shown, from different sources, for the same material, it is likely that the insulating power of any material used for packing a cooker will depend as much or more upon the way it is packed as upon the material used.

Experiment: Conductivity of different materials.

Take a piece of copper wire about six inches long in one hand, and a piece of steel wire of the same length and thickness in the other. Put one end of each piece in a flame, holding the wire by the extreme end. Notice which first becomes too hot to hold at the end farthest from the flame. This illustrates the different conductivity of the two materials, steel and copper. There is not a great deal of difference in the conductivity of different materials, but metals are relatively good conductors, and air is a very poor conductor.

2. Heat is carried from the pail partly by convection

, except where solid insulating material, such as wood or indurated fibre, is used; and that manner of packing which best entangles the air and prevents air currents will, therefore, most increase the effectiveness of the insulation.

Experiment: Convection.

Into a glass flask of cold water drop a few crystals of potassium permanganate, being careful not to agitate the flask. Apply a flame to the bottom of the flask. As the water becomes heated its density is reduced and it rises, forming convection currents which are coloured by the permanganate and may be distinctly seen.

Convection currents may be formed in any liquid or gas; for instance, air. By means of them heat will be carried from one part of the liquid or gas to another. Thus air heated by contact with a kettle of food will, if allowed to flow freely, carry the heat away from the food.

3. Heat is also lost by radiation.

This takes place less rapidly from a bright, highly polished surface, and for this reason “Thermos” and similar bottles are encased in polished nickle. A cooker-pail with polished outside surface retains heat better than one with a dull finish. In those cookers made with a metal outside retainer, the surface should not be painted or roughened or dulled by any means.

Experiment: Radiation.

Take two empty tin cans of the same size and shape. Wash off the paper labels. Keep one of them bright and shining, but move the other through a candle flame until the entire outer surface is smoked. Into each pour exactly the same quantity of water at the same temperature. Note carefully the temperature and the time. At the end of any given period, say one hour, again take the temperature of each. Which has lost the most heat, that in the bright can or that in the dull can?

4. The effect of different degrees or thicknesses of insulation.

Materials:

The same as those used in the experiment, section 1, with the addition of boxes of various sizes, some smaller, some larger, than the one used in the first experiment.

Pack the boxes with one or more of the various insulating materials used in the first experiment, so as to allow varying thicknesses of insulation around the cooker-pail. This should be the same or an exactly similar pail in each case. Fill the pail for all tests with an equal quantity of water, boil it for one minute, and leave it in the boxes for an equal length of time. Record the temperature maintained in each test. Keep the record in tabular form.

What thickness of insulation do you find gives the best result with the materials used in your experiment? Is it necessary to assume that the same thickness will be required with all insulating materials?

5. The effect of the density of foods upon the temperature maintained.

Materials:

• One cooker or hay-box
• Starch
• Water
• Salt
• Cooking thermometer
• Scales
• Litre or quart measure
• Notebook and pencil

Bring one or more litres or quarts of water to a boil, boil it for one minute, and put it into the cooker for one hour or more. Repeat the test, using, successively, five grams of salt to each litre, or one teaspoonful to each quart, and 5, 10, and 20 per cent. mixtures of starch with water. Record the temperatures in tabular form, and compare the results. What would you gather to be the effect of density upon the temperatures maintained?

6. The effect on temperature of filling the cooker-pails one-fourth, one-half, three-quarters, and entirely full.

Materials:

• Cooker or hay-box pail of eight quarts’ capacity
• Pail of two quarts’ capacity
• “Space adjuster”
• Water
• Thermometer
• Notebook and pencil

Fill the large cooker-pail one-fourth full of water. Bring it to a boil and put it into the cooker for a definite period of time, not less than one hour. Record the resulting temperature. If desired to make the test more comprehensive, leave the water in the cooker for six, nine, or twelve hours, being careful to allow the cooker to become cold between each test. Perform the same experiment with the same pail one-half full, again when it is three-fourths full, and again when entirely full. Record the results in tabular form and compare them. Repeat these tests with a pail of two quarts’ capacity. What is the influence on temperature of having pails partially, or completely, filled?

The explanation is that evaporation takes place in partially filled pails.

7. Chemistry of the action of food materials (salt, soda, acids, water, etc.) upon cooking utensils made of tin, or aluminum, when used in a cooker or hay-box.

The amount of tin dissolved by foods is indicated by the corrosion of the utensil, which can often be seen by the naked eye to be altered in appearance. The exact quantity of tin salts or other tin compounds which may be formed can only be determined by careful chemical analysis. It has been found that many canned goods supposed to be inert, such as squash and pumpkin, have a marked effect upon tin. Crude tests with a number of different foods can be made with tin, iron, aluminum, and copper utensils, as in many cases there is evidence to the eye of action upon the metals. It must be borne in mind, however, that such tests are crude and not decisive of the fact of there being no action in case no action is plainly visible. Only chemical analysis can prove this.

The action of foods upon tin cans bears a close relation to their action upon the utensils when used in fireless cookery, since there is time with the long cooking involved for similar reactions to take place in the cooker.[6]

Tin utensils rust badly after short use in a cooker, and thus affect the flavour of food cooked in them. This is due to the action of acids and water on the iron which forms the basis of sheet tin. When the thin plating of tin is worn off, the iron is left exposed to the action of water, etc.

Soda dissolves aluminum, and leaves a black surface on aluminum utensils. This black substance is iron, which is present with the aluminum in the utensils. To remove the black appearance, clean the utensil with acid. Do not try to remove it by scouring, as this will not do the work well, and is laborious and injurious to the pail.

Detection of poisonous metals that may be dissolved from the cooker utensils.

Experiment A. Tin.

In a tin cooker-pail boil such foods as apple sauce, tomatoes, squash, or others that act on tin, and put them into a cooker for twelve hours. Transfer them to an agate ware or porcelain utensil, evaporate them over steam until they may be burned in a porcelain dish until charred and brittle. Pulverize this charred mass, and extract it with hydrochloric acid. Filter and wash it. Saturate the filtrate with hydrogen sulphide gas; add a saturated solution of potassium acetate to neutralize the hydrochloric acid present and assist in the coagulation of sulphide of tin. Warm it slightly, filter and wash out the stannic sulphide, dry it and weight it as stannic oxide, from which the tin dissolved may be calculated.

Experiment B. Aluminum.

To simplify the experiment a weak solution of malic acid may be used (seven grams per litre being about the average amount found in apples). Bring this to a boil in an aluminum cooker-pail and put it into a cooker for twelve hours. Transfer it to a porcelain vessel and add ammonia to precipitate the alumina. Filter and wash this, dry and weigh the aluminum oxide. It is probable that a smaller quantity of aluminum would be dissolved by foods of a mushy consistency than would be found in this clear solution.

8. The efficiency of home-made refrigerating boxes compared with other means of keeping foods cold.

Materials:

• One box fitted as for fireless cooking, with two or three covered crocks of at least one-half gallon capacity, packed as directed on page 37, with either sawdust, hay, straw, excelsior or paper. Sawdust is specially recommended.
• Thermometer
• Ice
• Notebook and pencil

Fill the central crock with a weighed quantity of ice. Fill one or both of the other crocks with water at room temperature. Cover the crocks and close the box. Record the temperature of the water at the end of six, twelve, twenty-four, and forty-eight hours.

Make repeated observations of the temperatures found in ordinary household refrigerators, cellars, cold storage rooms, and any other places used for keeping foods cold. Compare these with the temperatures obtained with a home-made refrigerating box. Is there any economy in using these boxes?

Bacteriology of Insulating Boxes

9. Temperatures which kill disease and putrefactive germs, or check their growth.

It is taken for granted that the student of this subject will be more or less familiar with the nature of bacteria and the elements of bacteriology. It will be recalled that bacteria are a vegetable form of life; that, like all plants, they have, under certain conditions, the power of growth which is shown, largely, by their reproduction; and that under other conditions they are killed. When their growth is merely checked, they are in a dormant state, or perhaps form spores, in either of which cases they are ready to develop as soon as their environment permits. Temperature has much to do with the state of bacteria. If the temperature and other conditions are such that they are in an active or growing state, they will multiply with enormous rapidity. When in food stuffs they effect certain changes by reason of the products which they form as a result of their life processes, or of the alteration in the food materials, owing to their abstraction of some chemical elements or compounds used for their nutrition. When bacteria form unpleasant smelling or tasting substances we speak of them as “putrefactive bacteria.” Those which, if introduced into the bodies of humans or animals, will cause diseases, are called “disease bacteria.” Foods are liable to contain both kinds; and, therefore, it is, obviously, wise to do all that is possible to kill them or prevent their growth.

Most forms occurring in foods grow best at from 80 degrees to 98 degrees Fahrenheit. Few bacteria grow at above 100 degrees, and, if kept at 125 degrees, the weaker ones soon die. After subjection to a temperature of 150 degrees to 160 degrees Fahrenheit, for ten minutes, if water is present, almost all kinds are killed unless they are in the spore state. Prolonged boiling will often be resisted by spores. Dry heat is not as effective in killing bacteria as moist, and a higher temperature must, therefore, be reached to effect this end. Below 70 degrees Fahrenheit the growth of bacteria is more and more retarded, but not entirely checked until freezing point is reached. The popular idea that freezing may be relied upon to destroy bacteria is not true.

The bearing of these facts upon the subject of bacteria in foods cooked in insulating boxes is evident. Whether foods are cooked or kept cold, care must be taken that such a temperature is reached that bacteria may not grow.

In application of these principles we see that foods must be heated sufficiently to kill bacteria before it will be safe to subject them to the comparatively low temperature of the cooker for the long period necessary. This is one reason why foods in large pieces, such as roasts of meat, whole vegetables, and moulds containing a mass of food, must be boiled for a considerable time before being put into the cooker. Heat will not penetrate at once to the centre of such foods, and they would be likely to ferment or putrefy unless boiled long enough to heat the centre beyond the point where bacteria thrive. The fact that meats, cereals, and other foods have been known to sour or ferment, even after such boiling, if left in the cooker for a very long time, may be explained by the fact that, though all growing bacteria were killed, spores, which resisted the boiling, might have been present in the food, and when it cooled to a point conducive to the germination of these spores, and remained at this temperature for long, they might have developed, become active, and produced the objectionable changes characteristic of their kind.

In the case of foods to be kept in refrigerating boxes, a temperature considerably below 70 degrees Fahrenheit must be maintained. 50 degrees Fahrenheit, or lower, will be found an excellent preventive of germ growth.

Mr. L. A. Rogers has written a clear and concise description of the nature, growth, and conditions necessary to combat bacteria such as are found in food, in his paper entitled “Bacteria in Milk,” published in the Yearbook of the Department of Agriculture, 1907, pages 180 to 196.

Other books which give information on this subject are “Bacteria, Yeasts, and Molds in the Home,” by Conn, and “Household Bacteriology,” by S. Maria Elliott.

Yeasts and moulds also may take part in the changes which spoil foods; but the temperature conditions which control bacteria would be practically the same for them.

10. Cooking temperatures of different starches.

Experiment: Cooking starch.

Pare and grate one or more potatoes. Wash the gratings by placing them in a cheesecloth bag and immersing them in cold water. Squeeze and press the contents of the bag until no more starch seems to pass through the cloth. Let it settle, pour off the water; add clear water and let the starch settle again. Pour off the second water. Take one tablespoonful of the starch, mix it with one cupful of cold water. Heat it slowly over a moderate fire, stirring it constantly, and recording the temperature at which the mixture becomes noticeably clearer and thickens.

Repeat this experiment with corn-starch; wheat starch, washed from wheat flour, as is done with the grated potato; with starch washed from rye flour; and, if desired, with rice, bean, pea, oat and tapioca starches, also.

“Food and the Principles of Dietetics,” by Hutchison, gives, on page 378, a list of different starches and the temperatures at which they gelatinize.

In a bulletin entitled “Digestibility of Starch of Different Sorts as Affected by Cooking,” by Edna D. Day, Ph.D. (U. S. Dept, of Agriculture, Office of Experiment Stations, Bulletin No. 202, page 40), we read that starch takes up water at 60 degrees to 80 degrees Centigrade (140 degrees to 176 degrees Fahrenheit) and forms a sticky, colloidal substance known as starch paste, in which form it is very easily digested. Long boiling, at least to the extent of three hours, does not make it more quickly digestible.

There is something to be considered besides the mere starch in cooking starchy foods, and the fact that potato starch will form paste at 149 degrees while rice starch requires 176 degrees does not mean that less cooking will be needed for potatoes than for rice. The woody fibre or other constituents of foods, as well as their density and difference in size, must be taken into account.

11. Cooking temperatures of proteids.

Egg Albumen

In the bulletin entitled “Eggs and Their Uses as Food,” by C. F. Langworthy, Ph.D., published as Farmers’ Bulletin, No. 128, by the U. S. Department of Agriculture, the statement is made that “egg white begins to coagulate at 134 degrees Fahrenheit. White fibres appear which become more numerous until at about 160 degrees Fahrenheit the whole mass is coagulated, the white almost opaque, yet it is tender and jelly-like. If the temperature is raised to 212 degrees Fahrenheit, and continued, the coagulated albumen becomes much harder and eventually more or less tough and horn-like; it also undergoes shrinkage. It has been found by experiment that the yolk of egg coagulates firmly at a lower temperature than the white.”

It also says that these changes in the albumen suggest the idea that it is not advisable to cook eggs in boiling water in order to secure the most desirable result.

Experiment A: To show the changes that take place in egg white at various temperatures.

Materials:

• Test-tube and holder
• Beaker or saucepan of water
• Thermometer
• Egg white

Put the white of egg into the test-tube. Insert the thermometer. Hold the test-tube in the pan of cold water to the depth of the egg white. Gradually heat the water and observe the temperature at which the first change in the egg albumen takes place. Notice also the temperature of the water at this point. Continue the experiment until the water in the outer vessel has boiled ten or twenty minutes, noting the temperatures at which the various changes occur.

Experiment B: To show the temperatures obtained in the proper cooking of eggs.

Materials:

• Fireless cooker
• Eggs
• Water
• Thermometer

Cook eggs as directed for soft-cooked eggs on page 190, observing the temperature of the water after the eggs are added to it, and when they are removed from the cooker; also the condition, flavour, etc., of the eggs.

Cereal Proteids

Professor Harcourt, in his bulletin, “Breakfast Foods,” published by the Ontario Department of Agriculture, pp. 20 and 29, says that long cooking of cereals renders the protein more digestible. The cooking which he describes was carried on in a double boiler, and, therefore, below boiling temperature, and in this respect is similar to fireless cookery. He says that while short cooking, which was done at boiling temperature, seemed to make cereal proteids less digestible, the long cooking at below boiling temperature, which followed, somewhat changed them and made them more digestible.

While little study appears to have been made of the digestibility of cereal proteids when cooked for a long time at a low temperature, it is probably fair, in the absence of further definite information, to assume that, like animal proteids, it is better to cook them at a low temperature such as that of the fireless cooker, than at the temperature of boiling water or higher.

Meat Proteids

In the bulletin entitled “A Precise Method of Roasting Meat,” by Elizabeth A. Sprague and H. S. Grindley, published by the University of Illinois, a study is made of the temperatures at which the changes take place from raw meat to “rare”; from “rare” to “medium rare,” and from this to “well done” meat. The authors found that if the centre of the meat is between 130 degrees and 148 degrees Fahrenheit (55 degrees and 65 degrees Centigrade), it is rare; if it is between 148 degrees and 158 degrees Fahrenheit (65 degrees and 70 degrees Centigrade), it is medium rare; and if it is between 158 degrees and 176 degrees Fahrenheit (70 degrees and 80 degrees Centigrade), it is well done. They found no advantage in cooking meat in a very hot oven (385 degrees Fahrenheit, or 195 degrees Centigrade), but rather a difficulty to keep it from burning; that in an oven which was about 350 degrees Fahrenheit (175 degrees Centigrade), the meat cooked better; and that in an Aladdin oven which kept the meat at about 212 degrees Fahrenheit (100 degrees Centigrade), it cooked best of all; that is, it was of more uniform character all through, more juicy, and more high flavoured. This seems to point to an advantage in fireless cookery for meats, and practical experience bears it out.

The initial heat of the insulated oven serves to sear and brown the meat, and when this heat is reduced by the cooling of the stones, the low temperature found to be best for completing the roasting is obtained. With regard to meats cooked in water in the cooker, experience has shown that they become well done and are more tender than when boiled, showing that the temperatures necessary to reach that degree of cooking are obtained even in the centre of a large piece of meat, without toughening or hardening the outside of the meat, as is done when more intense heat is applied.

The hardening effect of long cooking at a high temperature on meat proteids can be demonstrated by broiling a tender piece of steak until it is rare, cutting off a small piece, continuing the broiling for a few minutes, cutting off another piece and comparing these pieces with the remainder, which should be broiled until very well done.

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