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Molecular Gastronomy and the Color of Cooked Green Beans

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  • Atoms, Molecules, and Chemical Reactions in Foods

    Cooked green beans can be a vivid green color, or they can turn gradually less colorful, sometimes becoming greyish or brownish. The new science of "Molecular Gastronomy", which is (in part) dedicated to determining the chemistry behind "Culinary Dictums", or cooking traditions that are sometimes without basis[1]. One such tradition is adding salt to the water before boiling vegetables. The reasons given for this include:

    • It makes them greener
    • It makes them firmer
    • It raises the boiling point of water to make them cook faster
    • It improves the flavor.

    Figure \(\PageIndex{1}\): Cut Green Beans, showing some losing their color

    Two very important things that chemists (and scientists in general) do include making quantitative measurements, and communicating the results of experiments as clearly and unambiguously as possible. In the case of green beans, the first three reasons were proven false by careful measurements. It turns out that the only effect of the salt is to slightly improve the flavor. Adding salt does increase the boiling point of water, but the effect is almost immeasurable at the low concentrations used in cooking. So why do they change color or firmness?

    Answering that question depends on another important activity of chemists—the use of their imaginations to devise theories or models to interpret their observations and measurements. Such theories or models are useful in suggesting new observations or experiments that yield additional data. They also serve to summarize existing information and aid in its recall.

    Green beans change color because the water may be too acidic. Acidic water has a higher concentration of hydrogen ions (H+), and these replace the magnesium ion (Mg2+) at the center of the chlorophyll molecule, which is responsible for the green color of vegetables (See the molecular structure of chlorophyll below). When the magnesium ion is replaced by a hydrogen ion, the green color disappears.

    Figure \(\PageIndex{2}\): Chlorophyll

    Chefs now use powdered chlorophyll as coloring in pasta. When chlorophyll is removed from plants, it usually decomposes, but Frank S. & Lisa Sagliano used freeze-drying of liquid chlorophyll at the University of Florida 1997 to stabilize it as a powder.[2]

    The texture or firmness of cooked beans turns out to depend on the state of polymers called pectins, which are complex sugars or starches like the one shown below. If the beans are cooked in "hard water", which contains calcium ions (Ca2+) or magnesium ions(Ca2+), the ions link the polysaccharide chains, making the beans firmer (usually undesirably firm). If the water is "softened", the pectins dissolve during cooking quickly, and the beans get mushy quickly.[3]

    Figure \(\PageIndex{3}\): A polysaccharide

    Understanding the changes in molecular structure of chlorophyll or pectins depends on The atomic theory, first proposed in modern form by John Dalton. It is one of the most important and useful ideas in chemistry. It interprets observations of the every-day world in terms of particles called atoms and molecules. Macroscopic events—those which humans can observe or experience with their unaided senses—are interpreted by means of microscopic objects—those so small that a special instrument or apparatus must be used to detect them. (Perhaps the term submicroscopic really ought to be used, because most atoms and molecules are much too small to be seen even under a microscope.) In any event, chemists continually try to explain the macroscopic world in microscopic terms. The contrasting properties of solids, liquids, and gases, for example, may be ascribed to differences in spacing between and speed of motion of the constituent atoms or molecules. In the form originally proposed by John Dalton, the atomic theory distinguished elements from compounds and was used to explain the law of constant composition and predicted the law of multiple proportions. The theory also agreed with Lavoisier's law of conservation of mass. An important aspect of the atomic theory is the assignment of relative masses (atomic weights) to the elements. Atoms and molecules are extremely small. Therefore, when calculating how much of one substance is required to react with another, chemists use a unit called the mole. One mole contains 6.022 × 1023 of whatever kind of microscopic particles one wishes to consider. Referring to 2 mol Br2 specifies a certain number of Br2 molecules in the same way that referring to 10 gross of pencils specifies a certain number of pencils. The quantity which is measured in the units called moles is known as the amount of substance. The somewhat unusual number 6.022 × 1023, also referred to as the Avogadro Constant, which specifies how many particles are in a mole, has been chosen so that the mass of 1 mol of atoms of any element is the atomic weight of that element expressed in grams. Similarly, the mass of a mole of molecules is the molecular weight expressed in grams. The molecular weight is obtained by summing atomic weights of all atoms in the molecule. This choice for the mole makes it very convenient to obtain molar masses–simply add the units grams per mole to the atomic or molecular weight. Using molar mass and the Avogadro constant, it is possible to determine the masses of individual atoms or molecules and to find how many atoms or molecules are present in a macroscopic sample of matter. A table of atomic weights and the molar masses which can be obtained from it can also be used to obtain the empirical formula of a substance if we know the percentage by weight of each element present. The opposite calculation, determination of weight percent from a chemical formula, is also possible. Once formulas for reactants and products are known, a balanced chemical equation can be written to describe any chemical change. Balancing an equation by adjusting the coefficients applied to each formula depends on the postulate of the atomic theory which states that atoms are neither created, destroyed, nor changed into atoms of another kind during a chemical reaction.

    From ChemPRIME: 2.0: Prelude to Atoms and Reactions


    1. This, H. "Building a Meal", Columbia University Press, New York, 2009, p. 14
    3. Lister, T.; Blumenthal, H. "Kitchen Chemistry" Royal Society of Chemistry, London, 2005, p. 38.

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