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ch19 - CHAPTER 19 SOLUTIONS 19.1 a Espresso coffee is...

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– 19.1 – CHAPTER 19 SOLUTIONS 19.1 a) Espresso coffee is prepared in a machine that pumps cold water at high pressure (10 – 20 bar is the range of commonly used values) into a hot water boiler, which displaces near boiling hot water. This water is then forced through a cake of ground coffee, as illustrated below. In a conventional machine, the user manually loads ground coffee into a metal filter housing, referred to as a portafilter , ensuring that the ground coffee is adequately packed, locks the portafilter under the hot water exit head, and the activates the heater. A coffee cup, placed under the portafilter, is filled with the freshly extracted espresso coffee, which is produced by the action of the high pressure hot water leaching through the ground coffee. A typical espresso machine: (A) pressure vessel, (B) portafilter holding ground coffee, (C) on/off switch, with built in pressure indicator, (D) solenoid valve for espresso coffee, (E) cup holding leached espresso coffee. A SIPOC for the preparation of coffee using an espresso machine is shown in the diagram below. Customers Coffee beans Ground Coffee Customers Our company Espresso machine Espresso coffee Customers Making Espresso Coffee # PROCESS STEPS Figure – SIPOC for the preparation of coffee using an espresso machine. b) The following are the some of the possible sources of variance in the quality of the coffee produced using the above machine: 1. Is the ground coffee fresh enough? If the coffee is too stale, this will affect the taste of the coffee.
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– 19.2 – 2. Is the ground coffee ground to the correct grade? If this is too coarse, the leaching will be insufficient, affecting the taste of the coffee. If the coffee is ground excessively, the pressure drop across the packed ground coffee will be too high, affecting the flow and leaching detrimentally. If the coffee is ground too fine, it will produce harsh bitter flavors. Serious espresso drinkers go as far as preparing their own roasted coffee beans and grinding them fresh, themselves. However, it is not reasonable to count on this extreme behavior in the majority of users. 3. Is the ground coffee packed evenly in the portafilter, and to the correct degree? Since the brew water is under high pressure, it will find the path of least resistance through the coffee. Uneven packing leads to channeling, in which the coffee surrounding the channels is over-extracted, and under-extracted elsewhere. The resulting beverage is bitter and astringent, with much of the potential good flavors remaining behind in the portafilter basket. In contrast, if the ground coffee is evenly and tightly packed, the water flows through all of the coffee uniformly. 4. Is the correct amount of coffee loaded? Insufficient coffee will lead to over- extraction and to flat and watery espresso.
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