Can a lemon ever taste sweet?

Can a lemon ever taste sweet?

Yes!!! It is possible.

Synsepalum dulcificum (Miracle fruit) ,when eaten, causes sour foods (such as lemons and limes) subsequently consumed to taste sweet. This effect is due to miraculin.  

The berry itself has a low sugar content and a mildly sweet tang. It contains a glycoprotein molecule, with some trailing carbohydrate chains, called miraculin. When the fleshy part of the fruit is eaten, this molecule binds to the tongue's taste buds, causing sour foods to taste sweet. At neutral pH, miraculin binds and blocks the receptors, but at low pH (resulting from ingestion of sour foods) miraculin binds proteins and becomes able to activate the sweet receptors, resulting in the perception of sweet taste. This effect lasts until the protein is washed away by saliva (up to about 30 minutes). 

Why is miracle fruit illegal?

Since 2011, The FDA has imposed a ban on importing Synsepalum dulcificum (specifying 'miraculin') from its origin in Taiwan, declaring it as an "illegal undeclared sweetener". Although this ban does not apply to fresh and freeze-dried miracle fruit, the fresh or normally-frozen berry deteriorates rapidly.

Why it is banned?

Although miracle fruit is generally considered safe when consumed occasionally as a food, the safety of long-term use of miracle fruit supplements is unknown. It's important to keep in mind that supplements haven't been tested for safety and dietary supplements are largely unregulated.