What exactly makes a lemon smell like a lemon and an orange smell like an orange?

When citrus fruits are pressed, in addition to the fruit juice itself, an oil present in the peel is also obtained. This is purified and made into suitable concentrations. Peel oils are the source of the strong smell we sense when we peel an orange and to a lesser extent a lemon.

Infact the peel of lemons, oranges, limes etc contain about 80% of the real 'flavour' of the fruit (but I don't suggest you go and eat the peel). This is why they include the peel in cocktails such as the Caprioska and Caprianha. Without the peel you would hardly taste the fruits.

Here's an old bartender trick: cut a firm slice of FRESH lime, orange etc peel about the size of a watch-face, 5mm thick. With a cigarette lighter heat the outside for about 10 seconds until the citric oils begin to spit out. Then quickly squeeze the peel so all the oils spray into the flame, making a small fireball.

Is this what you mean DB?

When you smell an orange or a lemon it's the essential oil that we smell. Essential oils are fragrant substances that are obtained from plants - the oils are known as volatile, that is when they evaporate into the air they leave their lemony or orangey aroma behind.

Sorry Siskin but no.

Why do they NOT smell the same ?

Orange and lemon oil contain basically the same fragrant ingredients: d-limonene, citral, decyl aldehyde, methyl anthranilate, linalool and terpineol. The differences in scent between lemon and orange oil are caused by different percentages of those components. Lemon oil contains a much larger percentage of terpineol.

Lothruin is on the right track but the interpretation/conclusion is wrong.
There IS an EXACT difference !

Appearantly the limonene in orange is a mirror molecule from that in lemon. Same components, but put together opposite from each other, which causes the difference in scent.

The same can be seen between Mint and Caraway, where the molecular structure of carvone is a mirror between the two.

Well it is "less" or "more" but it is of the "same thing"

Lothruin has a very big YO!

LIMONENE is what makes a lemon and orange smell.
You can make limonene "in a test tube" and it smells of lemons.
You can make more limonene and it smells of oranges.
The difference is because one test tube contains left-handed limonene and the other contains right-handed limonene.
They both contain exactly the same atoms but the arrangement of atoms in the limonene molecule that has a lemon smell is a mirror image of the orange smelling one.

So the next obvious question is does a racemic mixture of both optical isomers of limonene smell of lemons or does it smell of oranges?

Gmack, the language of your last enquiry suggests you might know something about this subject ?

Re an answer, I would suggest it depends whether or not the 2 versions of limonene actually chemically interact with each other.
If not then I would suggest it would be the same as having a test tube of one variant under your left nostril, and the other version under your right nostril as you have a good sniff.

Try it with an orange and a lemon , see which is "stronger" or maybe your nose/brain will get confused and smell limes or grapefruit or even cabbage soup!

Yes, DB, I suppose so unless we have dominant nostrils! The language of my question just suggests that I vaguely remember some A-level chemistry from 10 years ago.

It surely must have something to do with either or both being cis and trans. Cis meaning the same side and Trans on the other side. Both, I'm guessing, are isomers of each other to some degree.

or, as has been mentioned a Right (dextro) or Left (Levulo) enantiomer. The L-form is the one that occurs naturally. The D-forms are biologically inert -- they cannot be manufactured into proteins by biological processes.

So, since this is a natural ( I guess we could have synthetic lemon and orange oils if we prefer ), then both forms of limonene, if natural, would be levulo enantiomers.