The quality and taste with brewed coffee depends upon numerous factors such as the price or quality of the beans, the control exercised in roasting the beans, the proportions of the particles of different size obtained in grinding the roasted beans for brewing, freshness of use after grinding, and the extraction of the correct amount of soluble solids from the coffee during brewing.
Many different automatic coffee brewers have been designed for brewing and dispensing coffee in wide variety of quantities varying from single cup, ten cup (50 oz.), to one gallon and as large as ten gallon sizes. These are intended for use in homes, restaurants, factories and the like. Although these operate in a generally satisfactory manner, they are subject to the defects and disadvantages of passing all of the measured amounts of hot water through the coffee grounds and continuing to heat the coffee after brewing is completed, resulting in coffee which tastes astringent and bitter initially and worsens in taste with time of heating after brewing.
In "Facts About Coffee" published by the Pan-American Coffee Bureau it is stated, referring to brewing 1 pound of coffee:
If the coffee and water remains in contact long enough, about 51/2 ozs. of soluble Materials will be removed. However, complete extraction is extremely undesirable; it releases materials that make the beverage taste astringent and bitter. Such beverage is totally unacceptable. Extraction should be completed when only the first 2.9 to 3.5 ozs. of soluble materials have been removed from the grounds, as these will contain all of the desired flavor, aroma and color that the coffee has to give. When extraction is extended beyond 3.5 ozs. only bitter materials are withdrawn, and the coffee becomes astringent and unpalatable.
Publication No. 61, 1970 entitled "The Direct Reading Coffee Hydrometer" contains a chart labeled FIG. 4 which shows that after considerable and lengthy testing, ideal coffee may be brewed using anywhere from 1.85 to 2.60 gallons of water per pound of coffee. However, with any ratio within this range the brew may be strong, bitter, underdeveloped or weak, unless the time of contact with the water with the coffee is controlled to yield from 1.15 to 1.35% solids in solution extracted from the coffee which is equivalent to approximately 2.9 to 3.5 ozs.
In a pound of coffee, 2.9 ozs. represents 53% of the 5.5 ozs. capable of complete extraction. 3.5 ozs. represents 64% of the 5.5 ozs. (complete extraction). Therefore to obtain ideal coffee brew, between 53 and 64% only of the hot water should be allowed to contact the coffee grounds, and between 36 and 47% of the hot water should bypass the coffee grounds. Confirming this recommendation, the first cited publication for manually poured, large urns states, "If all of the water cannot be passed through the grounds within the prescribed brewing time" (which is between 4 and 6 minutes), "it may be necessary to place some of the water (up to 40%) in the bottom of the urn before brewing. This prevents over-extraction and bitterness."