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Physical and Chemical Changes in Food Background Information
All elements have a nucleus made of uncharged neutrons and positively charged protons.
Negatively charged electrons move around the nucleus. Different elements have different numbers of protons,
neutrons, and electrons. The protons of one atom can be attracted to the electrons of another atom. Chemical
elements combine by connecting together through the bonding of the negatively charged protons in the
element's nucleus, with positively charged electrons that are moving around another element's nucleus. An
atom can form as many bonds as it has electrons to share.
Two or more atoms can combine to form molecules. Molecules come in a wide variety of shapes
and sizes. Molecules move all the time. They move faster when energy in the form of heat is applied. If
enough heat is applied, the vibration of the molecules breaks the chemical bonds of protons and the electrons
break apart. Higher temperature causes more vibration. As bonds formed by protons and electrons break, the
molecular bonds also break, and matter changes its physical state from solid to liquid to gas.
Changes like cutting and mashing change only the shape of a substance. These are physical changes.
Pouring a liquid from one container to another or boiling, melting, or freezing a substance is a physical change.
When custard freezes into ice cream, it is a physical change.
Freezing often turns a liquid to a solid by forming crystals. If water freezes slowly, larger ice crystals
form. We can see the results of this in freezing fruits. Bread freezes well because it has low moisture content.
Foods that don't freeze well include things that have been thickened and the moisture has formed a gel, like
pudding and mayonnaise. Also, things thickened with cornstarch-puddings and fillings-break down during the
freezing process as do whipping cream, yogurt, and salad dressings.
Freezing most vegetables isn't a problem since one of the reasons for cooking vegetables is to
soften them up, and freezing starts the process. Once thawed, the vegetable is softer than before the freezing.
At a temperature above its melting point, a substance cannot exist as a solid - changing to a liquid;
at its boiling point, a substance cannot exist as a liquid - changing to a gas. Such physical changes are called
changes of state, and they can be used to classify and separate mixtures. Food scientists use knowledge of
changes of state to separate food mixtures because physical changes often make new mixtures or break down
existing mixtures.
For example: salt can be separated from water by heating the solution to boiling, turning the water to
steam, letting the steam escape to a separate container, and distilling the steam into pure water. The salt's
boiling point is so much higher than the water's that salt will remain behind in the original container. In some
parts of the world, sea water is changed to drinking water in this way.
Changes in physical and chemical state of matter have to do with atoms and molecules bonding and
moving apart. Three kinds of bonding occur: covalent (shared electrons), ionic (electrons of one element
captured by another form new element), and hydrogen (temporary link between molecules). Hydrogen bonds
are the weakest. Covalent bonds are ten times as strong. Strong bonds contain less energy than weak ones.
The more energy a compound has, the more it will be likely to change its physical and chemical state.
Both physical and chemical properties are always accompanied by energy changes. Physical properties of a
substance are concerned with mass, weight, volume, color, crystalline slope, melting point, boiling point, and
refractive index (ability to bend light).
The physical state of substances in scientific equations is shown by (g) for gas, (l) for liquid, (cr) for
solid, and (ag) for a water solution. (Cr means crystal and indicates the change of state from a liquid to a crystal
or solid state.)
All foods can be analyzed in terms of their chemical makeup. Any substance that is either produced
by or used in a chemical process is a chemical. A food scientist considers water, carbohydrates, protein, fat,
sugar, and flour (among others) to be chemicals. Each has a specific chemical composition. For example,
sugar is produced by plants from carbon dioxide and water. Water is produced by combining hydrogen and
oxygen.
Products with claims like "no chemical added" or "all natural" can be misleading. No food is free of
chemicals. Some chemicals do have dangerous properties while others contribute to our comfort. As you
study food science you will be able to prepare, preserve, and evaluate food in terms of its chemical
composition.
The way you live your life would be drastically different without food science, the practical
application of chemistry to food. Instant rice and iodized salt seem so familiar to us that we do not stop to think
about the extensive research that went into their production. Another example is adding vitamins A and D to
milk. It is another thing to add these vitamins. It is another to have the milk taste good.