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UnfortunatelyEDIT: My original version of this answer came from my incomplete recollection of a chapter in Kevin Liu's Craft Cocktails at Home on flavor balancing. Now that I have the book in front of me again, bitterI'm adding more relevant detail and revising the parts I got wrong. In all fairness, salient points are already covered in other answers, but I think the science here is neat.

Bitter flavors in general are veryfairly difficult to mask. This is possibly an evolutionary adaptation - bitterness can be an indication of toxicity in wild plants, so a sensitivity to bitter compounds in very minute quantities may have helped our ancestors avoid being poisoned. The concentration In order to produce flavors of bitter flavor compounds in a solution isroughly equal intensity (using a couple orders of magnitude lower thanperceptual measurement technique known as the Labeled Magnitude Scale), it is for other flavorstakes about 3000 times the concentration of sucrose (sweetness, acidity, etc.sweet) meaning that while it might take 300-400 parts per million to detect sournessequal one part of quinine sulfate, bitter compounds can be detected at 5 parts per million or lessthe principal bittering agent in tonic water. (It takes about 1500x relative concentration for sodium chloride and 50x for citric acid).

Relative concentrations aside, you get some interesting effects when these flavors begin to interact. (Sorry for the lackLiu reproduces a number of references so far. I'm drawing mostlyfigures from memory of a section on flavor balancing in Kevin Liu's Craft Cocktails at Home and I'll add some further details when I have access to the bookfascinating paper published by Green et al. at this point.)

You can compensate for excessive bitterness Perceptually, sweetness is also very difficult to a degree with sugarsuppress (as other answers have mentionedanother likely biological preference, since sweetness often indicates a concentrated source of energy in the form of sugar) and it will tend to dampen the perception of other flavors. This is one of the reasons why commercial tonic water typically includes a pretty hefty amount of sugar or corn syrup (somewhere around 30 grams per 12 oz). Acidity doesn't cover Adding sourness to the mix will suppress bitterness nearly as well. I note thatfurther; fortunately you're alsoalready doing this by using citrus. Finally, salt will substantially reduce the perception of bitterness, while remaining barely detectable in your mixes; one simple tweakthe final mixture. Salt also has the unique effect of "leveling out" sourness and bitterness when added to a solution that you could makecontains both (compare the rightmost two groupings from this figure) and as a result, Liu concludes that "Salt = magical fairy dust". Yes, that's a direct quote from the book.

The lesson to draw from all of this makes intuitive sense: since bitterness is using sweeter varietiesso difficult to cover up, you want a lot of other flavors in combination. Adding sugar for sweetness will help, and including acid from citrus will too; it sounds like orange or Meyer lemonyou've discovered this on your own. Avoid The counter-intuitive part is that adding salt will further suppress bitterness and mellow out the acidity as well.

I would add a couple other observations from personal experience. Because bitterness is detectable in low concentrations, you definitely don't want to add any more. Steer clear of citrus withthat has a distinctsignificant bitter component like(like lime or grapefruit) and be careful to avoid introducing pith (the bitter white part of the peel). If you're adding sugar, you can be much more liberal about the amount of salt you add; sweetness has a strongly suppressing effect on saltiness. You could probably get away with a teaspoon or you'll just addso in a gallon of tonic water if you're adding a couple tablespoons of sugar too. As others have mentioned, this will all be much easier to combine if you make solutions of the quininesugar and salt (for simple syrup and, ah, salt water respectively) and then combine those into your tonic instead of just adding the dry forms. 

Because of our biological sensitivity to bitternessFinally, you may be pretty limited beyond thatdon't give up on the tonic too soon. Although it's It's a bit of an acquired taste, but I find that slight bitterness can lend some depth to mixes, and some guests really enjoy that; there's a reason why the gin and tonic is so instead of discarding this entirely,popular. If the quinine is really too much for you mighteven after extensive modification, try saving it and diluting with additional seltzer or club soda. Don't toss it out; instead, try using it in small dashes in other drinks as you experiment. Trying new combinations is the absolute best way to learn what you like.

Unfortunately, bitter flavors in general are very difficult to mask. This is possibly an evolutionary adaptation - bitterness can be an indication of toxicity in wild plants, so a sensitivity to bitter compounds in very minute quantities may have helped our ancestors avoid being poisoned. The concentration of bitter flavor compounds in a solution is a couple orders of magnitude lower than it is for other flavors (sweetness, acidity, etc.) meaning that while it might take 300-400 parts per million to detect sourness, bitter compounds can be detected at 5 parts per million or less.

(Sorry for the lack of references so far. I'm drawing mostly from memory of a section on flavor balancing in Kevin Liu's Craft Cocktails at Home and I'll add some further details when I have access to the book.)

You can compensate for excessive bitterness to a degree with sugar (as other answers have mentioned). This is one of the reasons why commercial tonic water typically includes a pretty hefty amount of sugar or corn syrup (somewhere around 30 grams per 12 oz). Acidity doesn't cover bitterness nearly as well. I note that you're also using citrus in your mixes; one simple tweak that you could make is using sweeter varieties like orange or Meyer lemon. Avoid citrus with a distinct bitter component like lime or grapefruit, or you'll just add to the quinine.

Because of our biological sensitivity to bitterness, you may be pretty limited beyond that. Although it's a bit of an acquired taste, I find that slight bitterness can lend some depth to mixes, so instead of discarding this entirely, you might try saving it and diluting with additional seltzer or club soda.

EDIT: My original version of this answer came from my incomplete recollection of a chapter in Kevin Liu's Craft Cocktails at Home on flavor balancing. Now that I have the book in front of me again, I'm adding more relevant detail and revising the parts I got wrong. In all fairness, salient points are already covered in other answers, but I think the science here is neat.

Bitter flavors in general are fairly difficult to mask. This is possibly an evolutionary adaptation - bitterness can be an indication of toxicity in wild plants, so a sensitivity to bitter compounds in very minute quantities may have helped our ancestors avoid being poisoned. In order to produce flavors of roughly equal intensity (using a perceptual measurement technique known as the Labeled Magnitude Scale), it takes about 3000 times the concentration of sucrose (sweet) to equal one part of quinine sulfate, the principal bittering agent in tonic water. (It takes about 1500x relative concentration for sodium chloride and 50x for citric acid).

Relative concentrations aside, you get some interesting effects when these flavors begin to interact. (Liu reproduces a number of figures from a fascinating paper published by Green et al. at this point.) Perceptually, sweetness is also very difficult to suppress (another likely biological preference, since sweetness often indicates a concentrated source of energy in the form of sugar) and it will tend to dampen the perception of other flavors. This is one of the reasons why commercial tonic water typically includes a pretty hefty amount of sugar or corn syrup (somewhere around 30 grams per 12 oz). Adding sourness to the mix will suppress bitterness further; fortunately you're already doing this by using citrus. Finally, salt will substantially reduce the perception of bitterness, while remaining barely detectable in the final mixture. Salt also has the unique effect of "leveling out" sourness and bitterness when added to a solution that contains both (compare the rightmost two groupings from this figure) and as a result, Liu concludes that "Salt = magical fairy dust". Yes, that's a direct quote from the book.

The lesson to draw from all of this makes intuitive sense: since bitterness is so difficult to cover up, you want a lot of other flavors in combination. Adding sugar for sweetness will help, and including acid from citrus will too; it sounds like you've discovered this on your own. The counter-intuitive part is that adding salt will further suppress bitterness and mellow out the acidity as well.

I would add a couple other observations from personal experience. Because bitterness is detectable in low concentrations, you definitely don't want to add any more. Steer clear of citrus that has a significant bitter component (like lime or grapefruit) and be careful to avoid introducing pith (the bitter white part of the peel). If you're adding sugar, you can be much more liberal about the amount of salt you add; sweetness has a strongly suppressing effect on saltiness. You could probably get away with a teaspoon or so in a gallon of tonic water if you're adding a couple tablespoons of sugar too. As others have mentioned, this will all be much easier to combine if you make solutions of the sugar and salt (for simple syrup and, ah, salt water respectively) and then combine those into your tonic instead of just adding the dry forms. 

Finally, don't give up on the tonic too soon. It's a bit of an acquired taste, but I find that slight bitterness can lend some depth to mixes, and some guests really enjoy that; there's a reason why the gin and tonic is so popular. If the quinine is really too much for you even after extensive modification, try diluting with additional seltzer or club soda. Don't toss it out; instead, try using it in small dashes in other drinks as you experiment. Trying new combinations is the absolute best way to learn what you like.

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Unfortunately, bitter flavors in general are very difficult to mask. This is possibly an evolutionary adaptation - bitterness can be an indication of toxicity in wild plants, so a sensitivity to bitter compounds in very minute quantities may have helped our ancestors avoid being poisoned. The concentration of bitter flavor compounds in a solution is a couple orders of magnitude lower than it is for other flavors (sweetness, acidity, etc.) meaning that while it might take 300-400 parts per million to detect sourness, bitter compounds can be detected at 5 parts per million or less.

(Sorry for the lack of references so far. I'm drawing mostly from memory of a section on flavor balancing in Kevin Liu's Craft Cocktails at Home and I'll add some further details when I have access to the book.)

You can compensate for excessive bitterness to a degree with sugar (as other answers have mentioned). This is one of the reasons why commercial tonic water typically includes a pretty hefty amount of sugar or corn syrup (somewhere around 30 grams per 12 oz). Acidity doesn't cover bitterness nearly as well. I note that you're also using citrus in your mixes; one simple tweak that you could make is using sweeter varieties like orange or Meyer lemon. Avoid citrus with a distinct bitter component like lime or grapefruit, or you'll just add to the quinine.

Because of our biological sensitivity to bitterness, you may be pretty limited beyond that. Although it's a bit of an acquired taste, I find that slight bitterness can lend some depth to mixes, so instead of discarding this entirely, you might try saving it and diluting with additional seltzer or club soda.