Applied Science for Housecraft

PREFACE

This textbook is meant for schools and colleges since schools have been paying more attention to skill than to science.

Attention must be drawn to the fact that certain complicated chemical symbols and equations in cookery and laundry have been omitted. These cannot be fully understood without knowledge of organic chemistry, which is not taught at school.

When a cookery lesson is presented on measuring and weighing of ingredients, the chapter on balances, weighing masses and measures should first be treated. Similarly, the scholar should first become acquainted with air pressure and the influence of pressure on the boiling point of a fluid, before the steam pressure cooker is discussed. Justice will be done to science only by presenting the subject in this manner.

I hope that once this textbook has been studied, scholars will perceive to what extent the cooking and washing processes are based upon physical and chemical principles and laws, and will realise that knowledge of these is essential for every student. By combining science with dexterity, drudgery can be eliminated from housekeeping, equipment and furniture properly cared for and housekeeping run on a more economical basis.

I should not have been able to complete this textbook satisfactorily, had I not received the assistance of a number of experts and scientists. I, therefore, mention with gratitude the names of Miss A. Wrogemann, technical adviser for domestic science, Mrs Ina Strasheim (Teachers' College, Pretoria), Mrs Judith Steenekamp (Teachers' College, Potchefstroom), Prof. Dr D. J. du Plessis (Department of Chemistry), Prof. F. K. Peters and Dr B. Spoelstra (Department of Physics), all of the University of Zululand. A special word of thanks to Dr A. Strasheim from the C.S.I.R, Mrs B. Pienaar for her diagrams, and my husband, Prof. D. J. P. Haasbroek, who assisted me in finalising the book.

E. Haasbroek

Kwa-Dlangezwa via Empangeni, South-Africa

CHAPTER I: THE BALANCE, MEASURES AND WEIGHING MASSES

When a cook wishes to bake a cake or a loaf of bread or prepares a meal, she/he always uses measures and weighing masses. Often the quantities of flour, sugar or butter are estimated without weighing or measuring them, but then attempts sometimes fail. An experienced person estimates reasonably well, but when a teacher faces a class of thirty or more inexperienced pupils, who must for the first time be taught the preparation of foodstuffs, she/he must make use of certain concepts regarding the balance, measures and weighing masses. When these are mentioned, the pupils must be capable of understanding their teacher. The terms or concepts must be explained before their use in cooking or laundry since the kitchen and laundry are the laboratories.

Pupils must, therefore, first be acquainted with the concepts, mass, weight and volume.

A. MASS

Take any object in the classroom as an example, and mass may then be defined as the quantity of matter occurring in that particular object. The mass of any item is always constant. The unit of mass in the metric system is the gram.

B. WEIGHT

The concept of weight is closely related to mass. The weight of any object is the force by which it is attracted to the centre of the earth. When you pick up a bag of flour, it appears to be heavy. This is due to the force by which it is attracted to the centre of the earth. This force, the power of gravity, varies from the equator to the poles.

Consequently, the bag of flour will not weigh the same everywhere on the earth. It will weigh more at the poles than at the equator, although the quantity of the matter (or mass) remains the same. It may appear, therefore, as though one might purchase more flour for the same amount of money at the equator than at the poles! But alas, to measure the quantity of matter accurately, use is made of the principle of balancing one mass against another, using a lever, as illustrated in the diagram.

Figure 1: Principle upon which the balance is based A. Effort; B. Resistance; C. Fulcrum

Weight is, therefore, a force and the weight of any object will depend upon its position on earth. The unit of force is the newton and a mass of 1 kilogram is attracted to the earth by a force of 9,8 newton. To determine the mass or weight of an object, we may use a variety of balances. A few examples follow.

1. The chemical balance

This balance is not used in the kitchen, but we must consider it, since the principle according to which it functions is applied to other types of scales or balances. As the name suggests, it is an extremely sensitive balance which registers even a little dust on its pans. Its accuracy is incomparable. Consequently, this balance is most suitable for laboratories where absolutely correct quantities are imperative for specific chemical reactions to take place. (Lavoisier's name may be mentioned here owing to his contribution to science by means of the chemical balance. It is well worth the while to read up more about his work.)

The balance has two pans attached to a beam, and the earth exerts its force of attraction on both pans and their contents (see sketch above). Different masses may therefore be compared with each other and the mass, rather than the weight of the object, may be determined. The chemical balance balances one mass against the other and registers the same reading anywhere on earth. It does not measure the force of attraction of the earth, since an equal force is exerted upon each of the two pans. The mass of an object is determined by putting it in one of the scale pans and balancing it with the standard masses in the other scale pan.

Figure 2: The Chemical Balance

A. Beam with screw (m) and three agate knife-edges, a, b, c; B. Stirrup; C. Fork; D. Pointer; E. Pillar; F. Plumb-line; G. Graduated scale; H. and I. Balance pans; J. Lever to set balance in motion; K. Platform; L. Adjustment screw with which to level pans

For domestic purposes, we are interested in the amount of a substance only and not the force of gravity. Therefore we use mass and not weight in recipes.

2. The spring balance

This balance operates on the principle that the spring, from which the object (bag of flour) is suspended, is stretched by gravity. The extent to which it stretches determines the weight. It is based on Hooke's Law which reads: The stretch of spiral spring is directly equal to the force causing the stretch, provided that the limit of elasticity is not exceeded.

Since the force of gravity is not equal throughout the world, the expansion of the spring will consequently vary accordingly. This does not create any problems for the housewife as the difference is only slight. A quantity of sugar which weighs 1 newton at the equator will weigh 1,008 newton at the poles. The higher the altitude the lower the force of gravity. Thus, sugar measuring 1 newton on Mount Everest, will weigh 1,004 newton at sea-level. The spring balance will indicate these differences since it measures weight. The chemical balance measures the mass and will consequently register the same quantity in kilograms everywhere, even on the moon.

Usually, a housewife need not be very accurate. Therefore spring balances are calibrated i.e. marked in mass units (kg) instead of weight units. This can be done because there is a definite relation between the mass of a substance and the force of gravity.

a) Advantages

This is a cheap scale and may be easily carried and handled. Readings are taken rapidly because indicator C marks the mass (weight) at once. This suits the busy housewife who wishes to do her work as quickly and as easily as possible.

(b) Disadvantages

The spring scale indicates different values for the same mass (bag of flour) at different places on the earth. Spring scales for household purposes are not usually finely graduated grams so that it is difficult to measure small amounts accurately. Furthermore, the spring is inclined to weaken if its limit of elasticity is exceeded. This is inevitable when the balance is overloaded. The balance, therefore, loses its accuracy and the housewife experiences failures when preparing food. The fact that the objects must be suspended from the balance for the purpose of weighing provides practical difficulties.

Figure 3: The Spring Balance

A. Steel spring; B. Graduation of scale; C. Indicator; D. Scale pan

3. The balance scale (platform type)

This kind is most suitable for use in kitchens and shops. It may be noticed from the diagram that it functions according to the same principle as the chemical balance: it balances one mass against another. The pans are not suspended from the beam, but are mounted on the extremes of a double beam. The weighing masses are placed on the solid platform at the one end, while the other end carries a pan which holds the ingredients of which we must determine the mass. The double beam supports these containers more satisfactorily. It is not as sensitive as the chemical balance but the beam comes to rest sooner so that readings are taken more rapidly. There is a variety of models obtainable.

Figure 4: Balance scale (platform type)

A. Scale pan; B. Platform for weighing masses; C. Base

4. The spring balance mounted on a pedestal or base

This balance has a spiral spring, but contrary to the previously described spring balance, the pressure is exerted upon the spring when an object is weighed. When the object is placed upon the balance pan, beam B presses down on the spring, which in turn forces pointer C to move clockwise across the dial D. This spring is also subject to the same defects as the one previously discussed, and is influenced by the force of gravity. But one may weigh objects with relative ease when the proper method is used. Place a sheet of paper or a light container on the balance pan and set the pointer at O. Gradually fill the pan with the ingredient concerned until the pointer indicates the required mass (weight).

Figure 5. Spring balance mounted on a pedestal

A. Balance pan; B. Beam; C. Pointer; D. Dial; E. Pedestal

5. General hints for using the balance

(a) When large quantities are weighed, keep the capacity of the balance in mind.

(b) Be careful when weighing small amounts. Slight errors in quantity may make a marked difference in the texture or flavour of the food.

(c) When not in use, the balance pan must never be used as a storage place for junk or for heavy objects.

(d) Wherever possible, put the balance out of action when it is not in use.

6. Some uses of the balance

(a) Weigh the meat, sugar or other foodstuffs delivered at your home. Such control is essential for institutions such as hospitals and hostels, which place large and expensive orders.

(b) When making jam, first measure the fruit accurately and then calculate the exact mass of sugar required accordingly.

(c) When making home-made scouring agents, bleaches, etc., the finest results are obtained when the ingredients are accurately measured.

These are but a few examples of the numerous purposes for which the balance may be used in the kitchen and elsewhere.

C. VOLUME

We need not weigh substances only, but we must also measure them. In the kitchen, we generally do this by means of containers. Therefore, we say that the volume of any substance is the amount of space which it occupies. To measure quantities, we require certain recognised measures. In the home, use is often made of measuring spoons and the measuring cup when measuring certain volumes of ingredients such as flour, butter, milk, etc. The litre is used when measuring large quantities for hospitals, hostels or receptions.

1. Measurement of volume

1 litre = 1 000 millilitres (ml) = 1 cubic decimetre (dm³)

Volume is further expressed in terms of cubic metres and millilitres (or cubic centimetres). The millilitre is always used in the laboratory. Although it is general practice to use measuring spoons and measuring cups in the kitchen, these measurements indicate volume only. Volume is, however, affected by the density of the ingredient, e.g. sugar is twice as heavy as flour. Thus 250 g of sugar fills 1 cup whereas 250 g of flour fills 2 cups. The volume also depends on how tightly the ingredients are pressed into the measuring utensil. A cup of flour which has been tightly filled will take 3 tablespoons more flour than one only lightly and correctly filled with flour, i.e. one lightly filled with previously sifted flour. Butter which has been taken from the refrigerator, and directly measured into a cup, will have a mass somewhat less than a cupful of butter at room temperature. This may be accounted for by the fact that it is literally impossible to fill the cup completely with the hard, frozen butter owing to the quantities of air spaces between the solid lumps. It is, therefore, more accurate to use a recipe providing the masses of ingredients so that they may be weighed accurately. It is essential always to use the correct proportions of all ingredients for success in cookery.

Since American cookery books are often used in the Republic of South Africa, we should note that their measures are based upon the American gallon (D.S.A. gallon). The American measuring cup (¼ of the American ¼gallon) contains 236,6 ml cold water. The content of our measuring cup (¼ of our litre) is 250 ml. Owing to this difference one finds variations in cookery books of the masses and weights for recipes. For measuring purposes use the following standard measuring cups, tablespoons and teaspoons:

Cup = 20 T = ¼ litre = 250 ml.

Tablespoon = 12,5 ml.

Teaspoon = 5 ml.

EXERCISE I

1. Sketch the spring balance and name the various parts.

2. Define mass and weight. Explain why the one is determined by means of a spring balance while the other is determined by means of a chemical balance.

Discuss two types of balances which are used in the home. Which do you prefer, and why?

Which precautions would you take to ensure long and good service from your kitchen balance?

One cup of vinegar is used in a recipe for three cups of salad dressing. How many litres of vinegar would you require for a reception for 240 guests, assuming that ⅛ cup of salad dressing is sufficient per person?

Use different-sized cups in your domestic science kitchen, fill each with water and then measure those quantities in a measuring cylinder graduated in ml. Determine by this method which cups are most suitable for use as standard measuring cups.

CHAPTER II: HEAT AND TEMPERATURE

A. HEAT AND TEMPERATURE

Heat plays an important role in the life of plants, animals and human beings. We cannot live without it on earth. During the cold winter months, some plants lose their leaves and people wear warm clothes to protect themselves against the cold. Should the sun cease to shine, all life would be destroyed within a very short time. But what is HEAT?

The sensation known as heat is caused by moving or vibrating atoms and molecules in any object. In cold objects, molecules vibrate or move more slowly than in heated ones. The molecules in metals vibrate faster when heated and do not move like those of gases. The molecules of some substances move more readily than others. The powerful movements of the molecules in the flame cause the molecules of the metal of the kettle to vibrate. They set the molecules of the water in motion. As the flame continues burning, the kettle and the water become hotter, i.e. the molecules of kettle and water respectively vibrate and move faster. When the water boils, some molecules change into steam and escape with such energy that they even lift the lid of the kettle.

Heat is a form of energy.

Large quantities of water require more heat to boil than smaller quantities. The latter boils when the other