What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Read Online Free PDF Page A

a substance at a higher temperature to an adjoining substance that is at a lower temperature.
    But there are two important differences between the creamed coffee and the black coffee: (1) the cup of creamed coffee contains slightly more liquid because of the added volume of the cream, and (2) the creamed coffee is cooler than the black coffee.
    Difference number 1 means that the creamed coffee with its larger volume will take more time to cool off. That is, more heat must be removed to lower its temperature by any given number of degrees. (A bathtub of water takes more time to cool than a bucket of bathwater of the same temperature.) Difference number 2 has the same result: the slightly cooler creamed coffee will cool off more slowly than the slightly hotter black coffee, because the smaller the temperature difference between a hot object and its surroundings, the slower will be its rate of cooling. So immediate creaming wins again.
    My advice is to add the cream as soon as possible. The coffee will be hotter by as much as a degree or two at drinking time, and I’m sure your life will be much the better for it.
    I’m pleased to report that this problem was the subject of a careful scientific experiment led by the college student Jonathan Afilalo and published in the spring 1999 issue of the Dawson Research Journal of Experimental Science . This is a most impressive journal that publishes papers on original, professional-quality research by undergraduate students at Dawson College in Montreal, Quebec.
    Sidebar Science: Cooling it
    THE HIGHER the temperature of an object, the faster it will lose its heat by radiation. That’s the Stefan-Boltzmann Law. Also, the bigger the temperature difference between two objects in contact with each other (such as coffee and air, for example), the faster the hot one will lose its heat to the cooler one by conduction. That’s Newton’s Law of Cooling. There are precise mathematical formulas for both of these laws, but I see no reason to burden this page with them. I’ll return to Newton’s Law in Chapter 9.

    The cooling of a cup of coffee when the cream is added two minutes after pouring (curve 1), and when the cream is added ten minutes after pouring (curve 2). Adding the cream earlier yields hotter coffee at drinking time.
    The students’ experiment came to the same conclusion as I did, as shown by their measured cooling curves plotted in the graph above. In curve 1 the cream was added two minutes after the coffee had been poured, whereas in curve 2 it wasn’t added until ten minutes after pouring. Note that thereafter the temperature in curve 1 remained about a degree and a half higher than in curve 2. Early addition of cream does keep the coffee hotter.
    When a calorie is not a calorie
    There is a difference between what a chemist calls a calorie and what a nutritionist calls a calorie. The chemist’s calorie is the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius, whereas the nutritionist’s calorie, the calorie you see in diet books and on food labels, is the amount of heat energy required to raise the temperature of one thousand grams (a kilogram) of water by one degree Celsius. Obviously, then, a nutritionist’s calorie is a thousand times bigger than a chemist’s, and the chemist would call it a kilocalorie, or kcal.
    In this book I find myself in the awkward position of being a chemist writing about food for an audience that spans both camps. For consistency in this book, and if my chemistry colleagues will forgive me, I will use the word calorie in the nutritionist’s sense unless otherwise noted. In many cases, I use the word calories simply to mean an unspecified amount of heat energy, in which case the chemist/nutritionist dichotomy doesn’t matter.
    For those chemists who are not appeased, here is a supply of kilos to insert in front of the word calorie whenever you encounter it in the book: kilo kilo kilo kilo