We measure the temperature inside the brewing chamber of every coffee maker we test.
Brian Bennett/CNETAfter brewing coffee, we take sample readings of the produced coffee liquid with an optical refractometer. Given that we factor in the amount of water and freshly ground coffee used, that data lets us calculate the Total Dissolved Solids percentage of each brew. From there we arrive at the extraction percentage. The ideal range is commonly thought to be between 18% and 20%.
We also back up measured data with a good old-fashioned taste test. Over-extracted coffee tastes bitter and sharp, while under-extracted coffee is usually weak or sour. To be certain, we brew identical test runs a minimum of three times to get a sense of the average results.
Source: cnet.com