You can turn almost any kitchen recipe into a science lesson, maybe even a science project for school. Let's look at fermentation in pickle-making. Salinity, pH, oxygen and temperature are all variable factors which determine the outcome of your pickles.
To make pickles, you have to create proper conditions to encourage "good" bacterial growth, while slowing the growth of the "bad" bacteria. The good bacteria digest sugars in the vegetables to create lactic acid. Lactic acid gives pickles a distinctive sour taste and discourages growth of bacteria that will spoil the pickles.
The salinity (concentration of salt) is an important factor in the pickling process. If too little salt is used in the brine solution, bad microorganisms will grow quickly enough to cause spoilage. At the proper concentration of salt, lactic acid bacteria have an edge on the bad bacteria. The lactic acid is produced, creating flavor and lowering the pH of the solution. This more acidic environment keeps the bad organisms at bay. Too much salt keeps the lactic acid-producing bacteria from growing, and cucumbers will not pickle. Some yeasts can also grow more quickly, consume lactic acid to raise the pH and allow spoilage.
It is vital to keep oxygen away from the fermenting vegetables. Oxygen encourages the growth of bad bacteria and promotes spoilage. Also keep pickles and brine covered during fermentation.
Lactic acid-producing bacteria are happiest at 70 to 75 degrees F. Higher or lower temperatures can give spoiling bacteria an advantage and ruin your pickles.
An interesting science project might be varying the above factors to see how the pickling process is affected. Of course, do not eat any results that might be spoiled.