Abstract
We expect food to change over time; recipes suggest cooking times, packaging states a shelf life, and we will pay more for a 10-year-old than a 5-year-old whiskey. Some changes occur over a fraction of a second and others over several years; some improve the quality of the food, while others harm it. Whatever the mechanisms involved, controlling changes in foods to optimize quality and ensure safety is the primary task of the food technologist. We must answer two important and distinct questions about change—what can happen, and will it happen fast enough to be relevant to the food we eat? The first question is concerned with the thermodynamics of the system and the second with the kinetics. If we observe a change, then we know that it is both thermodynamically possible and kinetically viable. If we see nothing, it could be either thermodynamically impossible or thermodynamically possible but kinetically too slow to be important. For example, during baking, bread will brown rapidly, i.e., we can conclude that the browning reaction is both thermodynamically possible and kinetically viable. However, if the same dough is held at room temperature, it remains the same pale color over several weeks; either the reaction is thermodynamically impossible under these conditions or thermodynamically possible but too slow to be seen.
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Notes
- 1.
For completeness, the zeroth law of thermodynamics (oddly numbered because its position in the logical scheme was not accepted until quite late in the development of the science) states that if object A is in thermal equilibrium (i.e., in contact with and at the same temperature) with object B and with object C, then objects B and C are also in thermal equilibrium with one another. This seems a somewhat tortured presentation of an obvious statement, but it is necessary to introduce the ideas of temperature and thermometry. The third law is that the entropy of a perfect crystal at 0 K is zero and gives us a scale for entropy.
- 2.
Sometimes Eq. 1.4is seen written with the gas constantR(= 8.314 JK−1 mol−1) in place of the Boltzmann constant. The gas constant is the product of Avagadro’s number (= 6.02 × 1023) and the Boltzmann constant and is useful when the energy difference is expressed on a per-mole basis.
- 3.
Technically, nonexpansion work. A system at constant pressure will expand and contract to do work against the gas surrounding it.
- 4.
The Gibbs free energy is used for constant pressure systems; an analogous Helmholtz free energy is used for constant volume systems.
- 5.
Free energy, like mass and volume, areintensiveparameters and thus depend on the amount of material present and can be divided up on a “per molecule” basis. Other parameters, e.g., density and temperature, are independent of the amount of material present (i.e.,extensiveparameters), and so it does not make sense to try and assign a fraction to each molecule.
- 6.
This is a sorption isotherm, as they started from a dry material and added water. It would have been equally valid to start with a moist product and allow it to dry down to the different water activities, in which case they would measure a moisture desorption isotherm. There is often a hysteresis between sorption and desorption isotherms.
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Coupland, J. (2014). Basic Thermodynamics. In: An Introduction to the Physical Chemistry of Food. Food Science Text Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-0761-8_1
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DOI: https://doi.org/10.1007/978-1-4939-0761-8_1
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