Salt has been used for centuries to preserve meats and fish. It works to inhibit the growth of bacteria cells, which lose water and become dehydrated in salty environments. Over the years, food scientists and manufacturers have discovered that other chemicals also can serve as preservatives.
Citric acid, an organic acid found in many fruits, especially limes, lemons and grapefruits, is one of those chemicals. It increases the acidity of a microbe's environment, making it harder for bacteria and mold to survive and reproduce. It can also be used to bind to and neutralize fat-degrading metal ions that get into food via processing machinery.
What's great about citric acid is that it does all of this without harming the organisms that ingest it. It occurs naturally in all living things and is an important intermediate chemical in a metabolic pathway known as the citric acid cycle, or Krebs cycle. As a result, citric acid doesn't cause side effects in 99.9 percent of the population and is approved by the U.S. Food and Drug Administration for use in foods and beverages [source: Driver]. Maybe that's why the chemical appeared 288 times on the fast-food menus we surveyed.
The next item on our list -- high-fructose corn syrup -- doesn't fare as well in the court of public opinion.