Science Buddies: "Ask an Expert"

Hi, I have a science fair project and my topic is "The effect of lemon juice, orange juice, and salt water on the freshness of apples. I read online that lemon juice can help preserve apples, but I don't get that. So my question is "What is in lemon juice that helps apples stay fresh"? I would look it up online, but I am required to ask an expert as one of my resources. If you can include a little background on your expertise, that would be great! Thank You!

Hi,

Here is a list of answers given to other people who have asked this question on science buddies. I am sure you will find a good answer here:

https://www.sciencebuddies.org/science- ... clmby#1057

My backgound: PhD Chemist

Best regards,

Barrett L Tomlinson

Thank you! This helps alot!

Lemon juice is very acidic, have very low pH. When an apple becomes brown (or an avocado) it is because when you cut the fruit, some of the cells are broken and they release some chemicals including phenolic compounds and enzymes. Phenolic compounds are used by plants in order to protect themselves against microorganisms that attack them when the plant is injured. When you cut the apple, the phenols and the enzymes react with each other. The oxygen in the environment aids in this reaction. This results in the browning of the apple.
One way to inhibit the formation of the brown color (due to oxidation) is to inhibit the function of the enzymes that are being released by the injured cells. If there are no enzymes, the reaction of the phenols cannot occur. Well, enzymes have specific temperature, pH and concentration of salts needed for them to be able to function. If you add lemon juice, you are decreasing the pH of the environment where the enzyme is released, the enzyme changes the conformation (it becomes denatured) and now the enzyme cannot function anymore. Another way to avoid the formation of the brown color is to inhibit the action of the oxygen by “coating” the apple with something for example mayonnaise that won’t allow the air to react with the enzymes and the phenols.
Hope this helps.
Olga R. Kopp