This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Burns states, "Energy is the underlying cause of all changes in matter. This does not seem a very satisfactory definition, but, so far, it is the only one possible. . . . Energy, then, is that which produces an effect on our senses." It is measured by its power to do work.
Energy in some form is often applied to the materials used for food products. This energy may be electrical, mechanical, or in the form of heat.
An electric current passed through a food may be used to cook it. Very interesting experiments are being carried out along this line of work by the Household Equipment Department at Iowa State College. One of the striking results is the very short time required to cook the food. The passage of an electric current is used by some companies to pasteurize milk and by some to sterilize fruit juices.
Mechanical energy is used to beat, stir, fold, knead, or grind food.
The frequency of application of heat to foods does not need to be mentioned, for this is what the term to cook means. Foods may be cooked by radiant heat; or by transmission of the heat by conduction, i.e., from particle to particle; or by convection, which is the diffusion of heat through a gas or liquid by movement of the gas or liquid particles. A combination of these methods may be used to transmit the heat. Oven cooking employs all three.
Kinetic energy is due to motion. It may be due to motion of the sub-stance itself or of the particles composing it. It is directly available for work. Potential energy is associated with position, i.e., composition, stress, or strain. Kinetic energy is necessary to liberate it to perform work.
Just as matter is indestructible, though its form may be changed, so is energy indestructible. This constitutes the first law of thermodynamics. The law of Hess states that the amount of heat generated by a chemical reaction is the same whether it takes place all at once or in steps.
The second law of thermodynamics concerns the degradation or dissipation of energy. In practise, some of the freed energy is converted into heat, which is diffused among the surrounding objects, and, so far as work is concerned, is lost. Burns states the law simply: "Every change takes place at the cost of a certain amount of available energy." He adds that the second law lends itself to the deduction that the cause of all change is the tendency of energy to attain the same uniform degree of intensity as that of its environment. This means that any system tends to change to the most stable state.
Bread during baking tends to attain a physico-chemical equilibrium. But the temperature attained during baking is much higher than the storage temperature. Freshly baked bread does not taste or feel like bread 24 hours old. Bread can be stored so that it loses no moisture, yet readjustments in the loaf take place, so that staling occurs. If bread is stored at temperatures of 60°C. or above, the texture remains more like that of freshly baked bread and staling is not perceptible. But at this temperature bacterial changes occur readily. If the bread has not lost considerable moisture, upon reheating it, it acquires the characteristics of freshly baked bread.
The principle of Le Chatelier gives the factors of equilibrium: "Every system in chemical equilibrium, under the influence of a change of any single one of the factors of equilibrium, undergoes a transformation in such direction that, if this transformation took place alone, it would produce a change in the opposite direction of the factor in question. The factors of equilibrium are temperature, pressure and electromotive force, corresponding to three forms of energy; heat, electricity and mechanical energy."
The symbol pH is very commonly used in present-day literature. Some explanation of the term and its relation to hydrogen-ion concentration is desirable. Perhaps it will be best to review briefly what is meant by hydrogen-ion concentration. In a solution of hydrochloric acid in water, the molecule of the acid consists of an atom of hydrogen united to an atom of chlorine. Hydrochloric acid molecules are found in the solution, but not all the acid remains in the molecular form. Part of the acid molecules are ionized into hydrogen ions and chlorine ions, the degree of ionization depending upon the concentration, the more dilute the acid solution the greater the percentage of hydrochloric acid ionized. The hydrogen ions are positively charged, and the chlorine ions are negatively charged. In symbols, the ionization of hydrochloric acid is expressed as follows:
When a substance is ionized in solution the solution conducts an electrical current.
When water is ionized it gives both hydrogen ions and hydroxyl ions. Since the ionization of water is very slight, the amount of hydrogen and hydroxyl ions in a liter of water is not great.
In pure water the concentration of hydrogen ions and hydroxyl ions is equal at 22°C. The reaction of water varies at various temperatures, so that the following discussion is confined to room temperature or 22°C. The concentration of the hydrogen ions and hydroxyl ions is 1/ 10,000,000 of a gram-molecular weight (mole) each per liter. A normal solution of hydrogen ions contains 1 gram-molecular weight (1 gram) of hydrogen ions per liter of solution; a normal solution of hydroxyl ions contains 1 gram-molecular weight (17 grams) of hydroxyl ions per liter of solution. A liter of pure water contains 1 / 10,000,000 of a mole of hydrogen. This requires many figures to express as a fraction, so for convenience the concentration is expressed as follows:1 / 10,000,000 = 10-7. Again for convenience Sorensen proposed to disregard the minus sign and use the numerical value of the exponent 10 to express the reaction corresponding to the concentration of hydrogen ions.
Thus 1 / 10,000,000 = 10-7 = pH 7
When the concentration of the hydroxyl ions is equal to that of the hydrogen ions, the concentration of hydroxyl ions is also 1 / 10,000,000 of a mole or 10-7. This is the neutral point. The term pH is used to denote the concentration of the hydrogen ions only. Sometimes pOH is used to denote the concentration of the hydroxyl ions. Thus when the concentration of the hydrogen ions is pH 7, that of the hydroxyl ions is pOH 7. If the concentration of the hydroxyl ions is multiplied by the concentration of the hydrogen ions a definite product is obtained. In fractions this would be written thus:
1 / 10,000,000 X 1 / 10,000,000 = 1 / 100,000,000,000,000
But in the exponential notation it is expressed as follows:
10-7 X 10-7 = 10-14
When the hydrogen-ion concentration in a solution is increased, the hy-droxyl-ion concentration is decreased, so that the product of the concentration of the hydrogen and hydroxyl ions always gives 10-14, and this is a constant for the product of these two ions. Thus in a solution that has a pH 6 the hydrogen ions exceed the hydroxyl ions but the product of their concentrations is the constant 10-14. In a solution that has a pH 6 the concentration of the hydrogen ions expressed in fractions is 1 / 1,000,000 of a mole of hydrogen. In the same solution the concentration of the hydroxyl ions is 1 / 100,000,000 of a mole. The product of these two concentrations gives the constant. Expressed in exponential notation it is 10-6 X 10-8 = 10-14. In a solution that has a pH 8 the hydroxyl ions exceed the hydrogen ions but the product of their concentration is again 10-14. When the concentration of the hydrogen ions is pH 3 or 10-3 then the concentration of the hydroxyl ions is 10-11 and their product is again 10-14.
Literally, the term pH means to a power. It is used to express the reaction of a fluid, that is, its degree of acidity or alkalinity, but it does not do this directly, as is shown in the above equations. It is an inverse logarithmic function, deprived of its minus sign. Experimentally it is determined electrometrically and is really a number obtained from determining the electromotive force (E.M.F.) of a substance in a suitable apparatus and by using this value of (E.M.F.) in a formula, computing the pH.
Hydrogen-ion concentration refers to the concentration of the ionized or active ions per liter of substance. The pH value does not represent this directly but for all practical purposes may be taken as a value representing it.
The relation of hydrogen-ion concentration and pH values in solutions of varying normalities is given below.
Grains of hydrogen ion per liter
The above arrangement shows that the pH value is not an arithmetical series or ratio but varies according to the logarithmic notation. Thus pH 2 is not one-half of pH 1 but one-tenth of it.
The number of times the hydrogen-ion and hydroxyl-ion concentrations of a solution exceed that of pure water may be shown as follows. The arrangement is by Alexander.
The pH values are determined by two methods: the principal one is by electrometric determinations, the other is by the use of indicators. The latter is a rapid and a very useful way to determine an approximate pH value and to check apparatus. For an accurate determination the electro-metric method must be used.
Number of times that the H- or OH-ion concentration exceeds that of pure water
pure water, neutral