A common misconception is that when you double brew coffee, you double the caffeine. You don't. Double-brewed coffee increases the caffeine only slightly; the biggest change is the stronger flavor. As "Perfect Coffee at Home" authors Michael Haft and Harrison Suarez wrote in The Atlantic, "strong coffee has almost ... everything to do with the ratio of coffee to water in your cup." It also has something to do with the type of coffee beans you start with.
The fresher the coffee beans -- or grinds -- you start with, the more flavor they'll have. Seek out a bold, dark roast, and then keep the beans in an airtight container away from light, heat and moisture. If you're using whole beans, wait until the last minute to grind them; grinds lose more of their flavor over time.
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Bad coffee of any kind can be just as due to the water as to the beans. Start with clean, clear, cold water when you're making double-brewed coffee, in particular. Tap water is fine, but if you have a purifier, use it. The purer the water, the purer the flavor of the beans.
Method 1: Brew Coffee Twice
This method of double brewing means doubling the steps and the time. Make coffee as you normally would in your coffee machine, with your normal ratios of coffee grinds to water -- usually 1 to 2 tablespoons per 6 ounces of water.
When it's finished brewing, let the coffee cool to at least 200 degrees Fahrenheit. Clean the filter.
Add another 1 to 2 tablespoons of fresh grinds per 6 ounces of brewed coffee to the filter, and add the brewed coffee to the reservoir. Making this coffee with the fresh brew instead of water will double the concentration of the final result.
Method 2: Use More Coffee
The second method is simpler, and simply entails using more grinds. The higher the ratio of coffee to water, the stronger the coffee, so for a truly double-brewed coffee, use twice as many grinds as you normally would. For 6 ounces of water, this usually means 2 or 3 tablespoons of coffee grinds.
Everyone's tastes are different, so experiment with different coffee-to-water ratios to find the double brew for you. Make small changes to find the sweet spot, and write that method down.
If your coffee machine allows you to control the temperature of the water in the pot, brew it to about **200 degrees Fahrenheit**; hotter waters tend to burn the coffee, giving it a sour taste.