Batch size: 5 gallons (19 liters)
1.9 lb (862 g) corn sugar/dextrose
1.9 lb (862 g) table sugar/sucrose
Fermentis SafAle US-05 plus Fermaid K or other yeast-nutrient blend
Heat 5 gallons (19 liters) of water until steaming, then turn off the heat, add the sugars, and stir until dissolved. Boil for 10 minutes, then add 2 g yeast nutrient. Chill to 68°F (20°C), aerate, and pitch the yeast. After 24 hours, add 2 g more of yeast nutrient, and the same the following day. When fermentation is complete, add flavorings and adjust as desired, then package and carbonate.
This is where the fun begins. Your clean hard seltzer is a blank canvas for a variety of flavorings. Here are a few options:
Fruit: Add 2–3 lb (1–1.5 kg) of low-pectin, crushed or pureed fresh fruit (such as raspberry or pineapple) for a bright and natural fruit character. To be on the safe side, consider adding ¼ tsp (1.2 ml) of pectic enzyme to fight haze that might be caused by fruit pectins. Wouldn’t want to cloud up that crystal-clear seltzer (though it will taste great either way).
Coffee: Cold-steep 8 oz (227 g) of your favorite artisanal coffee and add to taste—if you think hard seltzer is cool, wait till you show up to a party with hard coffee.
More Flavors: Get your hands on your favorite liquid flavor extract and add to taste. This is probably the simplest option, and you get a huge range of flavors from which to choose: PB&J, vanilla and allspice, butter almond—there’s no end to the possible combinations.