Creme de cassis and grenadine, both part of a well-stocked bar, look similar because they're dark red, but that doesn't mean you can use one in place of the other in all situations. When possibly switching out grenadine with creme de cassis, you need to think about what you are making and for whom you are making it.
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Creme de Cassis
Creme de cassis, a sweet and fruity French cordial, is made from blackcurrants, and used in fancier drinks like kir royales or specialty Manhattans. Creme de cassis has a relatively low alcohol content, around 30 proof, and, in a pinch, mixologists can substitute it with raspberry-flavored liqueur. Because creme de cassis is a specialty liqueur, it is usually moderately priced, although you can find less expensive American-made blackcurrant liqueurs. Creme de cassis also has a special place in pop culture as it is fictional detective Hercule Poirot's favorite drink.
Grenadine, a key ingredient in both Shirley Temples and Roy Rogers, is a nonalcoholic bar syrup added to drinks mainly as an accent or splash. Grenadine shares creme de cassis's essential taste as both have blackcurrant flavors, although grenadine carries other fruit flavors as well, with pomegranate being the strongest. Grenadine is relatively inexpensive and the least expensive brands usually contain mostly artificial fruit flavorings, sugar and/or corn syrup. Grenadine's sweetness and its red color make it a cocktail ingredient staple.
With creme de cassis and grenadine having similar flavors, the question isn't so much can you substitute the other as much as should they be substitutes. The biggest concern is who will be drinking the drink. If the drinker is of legal drinking age, then creme de cassis is a fine, although more expensive, substitute. If the drinker is underage, then creme de cassis is never a suitable substitute.
Instead of grabbing for the creme de cassis, try other substitutes for grenadine. Other nonalcoholic substitutes, which are not nearly as expensive as creme de cassis, include pomegranate juice and raspberry syrup. And if you have cocktail cherries, you also have another suitable nonalcoholic substitute -- just pour the juice from the jar or tray of cherries into the drink. Each of these alternatives will sweeten the drink and give it a reddish hue.