Fast flow and the resulting thin and sour shot of espresso can be frustrating. It’s a sign that water is not encountering enough resistance as it passes through the coffee puck, pulling out only the brighter notes. The culprit is usually obvious: the grind is too coarse, the dose is too low, or the puck is not evenly compressed. Developing the ability to determine which of these is contributing the most to a fast shot turns a failed shot into a diagnostic tool, rather than a wasted one.
Grind size will have the biggest impact on flow rate so that is where you want to start. If the shot is coming through in under 15 seconds, grind a bit finer and test it again. It is not about reaching a certain time, but more about having a shot that comes out dark and slow, then thins out. I have seen new baristas increase tamping pressure to slow the shot down when it is coming through too fast. This is not the proper fix because there is only so much pressure you can apply to the coffee in the portafilter and there is no way you can make up for improper particle size. Grinding finer will increase the surface area of the coffee, making the water move slower through it and pull out more flavor.
A non-uniform bed of grounds is just as bad as any other and can channel water through weak spots. If the grounds are heaped up in one area of the basket, or lumpy from grinding, water will find a path of least resistance. Getting the grounds flat before tamping will go a long way to forming a uniform puck. Tapping the portafilter gently, or stirring with a blunt object can also eliminate lumps. If you’re experiencing any spitting or jetting, then channeling is definitely a factor.
Even just a few quick shots will show you just how sensitive espresso can be. Start with a standard shot, then change nothing but grind a bit finer for a second shot. Pay attention to the duration, the stream’s consistency, and the color as it changes from dark brown to light blond. If you have time, make a third shot with another slight tweak to see if it tastes better or worse. This helps you trust that you are making thoughtful modifications, rather than just trying your luck.
If the flow is still too quick after adjusting the grind size, then you need to look at the dose and the freshness of your coffee. If your dose is too low, there will be too much room for the water to flow through. If your coffee is too old, it will offer less resistance, as the gasses will have escaped. A small adjustment in dose or a fresh batch of coffee will help return the body and crema to your shot. Instead of thinking of a fast flowing shot as an error, use it as a diagnostic tool to help you achieve a slower, more satisfying shot that is full and rounded in flavor.