Can I subsitute grapefruit for either lemon or orange in baking

I'm sure you could but I don't think you'd be happy with the results.

A quick breakdown of the amount of sugar and citric acid (gives the acid tang to citrus fruits) to grapefruit, orange and lemon juices per 100 grams (roughly 100 ml).

• Grapefruit has 7-8 g. sugar and 2.5 g citric acid.
• Orange has 8-9 g. sugar and 0.9-1.7 g citric acid.
• Lemon has 2.5 g sugar and 4.5-5 g citric acid.

Please don't ask me to provide references as I soon discovered that no one site (or even 3 or 4) has the needed information together in one place. I had to search through multiple sites and calculate the differences on amounts given with different units used (ml vs oz).

Comparing these differences, you might think grapefruit could be substituted for oranges if you added more sugar. What makes grapefruit unsuitable though is its bitterness. Grapefruit contains Naringin, the compound which gives it the bitter taste. In fact, naringin is the reason why people who take certain medications, especially some cardiac meds, need to avoid grapefruit or its juice as it can interfere with the liver's metabolism of these drugs.

But getting back to the suitability of using grapefruit, if you're prepared to adjust sugar and you don't mind grapefruit's bitterness, it's worth trying. Best check, too, with anyone else who plans on eating the dessert.

What works for me is to buy lemons on sale if they're good - heavy and juicy - grate all the rind, then juice the lemons and freeze them. Grated rind freezes very well and doesn't lose any flavour either. It makes it so much more convenient for me when I want to cook with lemons. I do the same with oranges, especially the rind. I also keep citric acid in my pantry as I use it often when I cook. It can be used with oranges to make a delightful orange meringue pie as regular oranges lack enough tang.

I happen to be one who really enjoys the taste of grapefruit, and yes, I grew up enjoying them with salt to counter the bitterness. If you like the taste, it does work. If you do not like the taste, well, salt is a flavor enhancer so you are making a taste you do not like to begin with stronger, not going to work in my experience.

There are some things that grapefruit can substitute with, for instance in drinks. If you enjoy grapefruit, then a grapefruit margarita can be very refreshing without being overly sweet for instance. But, baking is a different beast. Jude correctly points at the acid and sugar differences, and just the difference in taste is enough to put many recipes off, but for an experiment, try it, you might like it. I would not have high hopes though. And even as one who enjoys grapefruit, I find the zest way to bitter to even consider it as a substitute. I would have another concern though, that being actual chemical reactions.

Many medicines carry interaction warnings with grapefruit due to levels of flavonoid and furanocoumarin compounds they contain. These chemicals inhibit some enzyme reactions causing some medicines to increase or decrease their actions. I am not a chemist, so I am only speculating, but I would also think there is a real chance that using grapefruit would effect the chemistry of your baking if more than a small amount was use. Not likely harmful at all, but it could very well cause a flop product.