one way degassing valve tells you that Espresso is made by the method that high-pressure hot water pushing through the ground coffee in coffee handle. So how do we identify a cup of espresso?
The flow velocity and color of the coffee flows from the coffee handle can give us much information of coffee extraction, let us know which level should we grind the coffee, the proportion of coffee and water and the time of coffee extraction.
A cup of good espresso is extracted with dark color, bold flavor, containing the richest aroma and the sticky brown coffee crema, shape of threadiness. Then the flow velocity rises, the color of coffee turns to dark caramel color. Eventually the color of coffee becomes lighter. It is important to stop extraction before the coffee is diluted like water. The color of coffee changes a lot in the process of extraction, but it is easy to find the time of final extraction and the beginning of over extraction. It will be hard to observe the how the extraction is going if you use nude coffee handle without liquid outlet. However, viewing the process of espresso extracting from this angle is quite wonderful.
When the extraction is over, it indicates that the quintessence of coffee is completely extracted, we need to move the coffee cup filled with coffee away. If the extraction continues, the aroma, the flavor and mellowness would decline.