High-quality creme de cassis should have many layers of flavor than simply sugar. Blackcurrants will add acidity to the drink, along with berry and floral notes. Your creme de cassis liqueur should include the following ingredients: alcohol, blackcurrants, and sugar. Furthermore, the alcohol content must be 15-20%. Unopened creme de cassis liqueur bottles can last for years if stored properly. Once opened, crème de cassis will only last for a few months in the fridge, like chilled champagne.