Proteins, carbohydrates, and fats are the principal structural components of living cells. Lipids are a class of food chemicals that include fats and oils. They are the most concentrated source of food energy providing nine calories of energy per gram. In our bodies, lipids carry the fat-soluble vitamins, contribute to food flavor, and help us feel full after eating. Fats and oils are available in many forms; fats are solid at room temperature while oils are liquid at room temperature.
According to the Institute of Food Technologists, fats, like all food components, undergo deteriorative changes which result in undesirable flavors and odors with time. These changes in fats are given the term "rancidity." Rancidity can be of two types, hydrolytic and oxidative. Hydrolytic rancidity is caused by a breakdown of the fat into glycerol and fatty acids. This is the type of rancidity that gives "rancid" butter its bad flavor.
Oxidative rancidity results from oxidation of unsaturated and polyunsaturated fatty acids. The products of these chemical reactions produce undesirable flavors and odors. These flavors sometimes develop in foods such as peanut butter, potato chips, and crackers. Manufacturers are permitted to add antioxidants to some foods to slow down this oxidative deterioration. The antioxidants normally used are butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiary butyl hydroquinone (TBHQ), and propyl gallate. You may see these terms on the labels of some foods. In some cases, the antioxidant is incorporated in th the packaging material. The anitoxidant slowly diffuses into the packaged food product during storage, thus protecting the snack. Another means of slowing down oxidation is to package the food so that it is protected from light, moisture, and oxygen--three things that accelerate oxidation.
Try this experiment to demonstrate typical off-flavors in fat caused by oxidative rancidity.
Does Light Cause Potato Chips To Become "Stale?"
Materials You Will Need:
Fresh potato chips
Pint or quart canning jars with lids
1. Wrap a pint or quart canning jar with aluminum foil. Tape the foil in place so that no light can enter the container.
2. Place fresh potato chips in the foil-wrapped jar and in a similar clear jar without foil around it.
3. Taste the potato chips are rate their flavor on a 5 point scale, where 1 = extremely dislike the flavor and 5 = extremely like the flavor. Enter the data on the Day 0 line in the table below.
4. Place the two jars on a window sill where they will be exposed to sunlight. Turn each jar one-quarter turn each day (or every 24 hours).
5. Taste the potato chips from each jar at intervals of 1-2 days for 1-2 weeks. The length of time for this experiment is dependent on the amount of sunlight that the jars are exposed to. Enter the data in the table below.
6. Make a graph of your data, noting the flavor of the potato chips stored these two ways versus storage time. The y-axis should be the flavor score and the x-axis the time in days.
The potato chips in the jar wrapped in auminum foil retained their desirable flavors. The potato chips in the clear jar developed off-flavors" over time.
Potato chips protected from UV light did not undergo lipid oxidation and retained their desirable flavors, whereas potato chips not protected developed "off-flavors."
So, can you see now why potato chips that you purchase in the store are packaged to exclude light? What's more, the packaging also excludes water vapor and oxygen. Let your students design an experiment to test these lipid oxidation factors!