Tamping pressure shouldn't be too hard as it can lead to over-extracting and an extremely bitter taste of espresso. Polish the puck with a twisting motion. Avoid twisting as you push down though. Otherwise, you can ruin the packed coffee.
Some say, you need to tamp very hard, others say pressure really is not important. According to experts, tamp pressure is not that important, what is important is ensuring that the tamp pressure is consistent each time and that your tamping is flat and as level as possible. This ensures you have the best results.
One of the most important is the pressure applied to the grounds, which should be somewhere between 8 – 10 kg. Too little pressure and the espresso will be under-extracted and weak, while too much pressure can result in a bitter and overextracted espresso.
There is no such thing as overtamping. There is only undertamping and tamping with a sufficient amount of pressure.
The water sprays through the grounds, making something weak and unpleasant-tasting. On the flipside, tamping too hard leads to the opposite happening. Water struggles to get through the puck, and because it spends more time seeping through, your espresso becomes over-extracted.
With a slightly curled finger, spread the grounds out so that they are level with the top of the handle's basket. Tamp twice. For the first tamp, apply a light amount of pressure to form a puck shape and for the second use give it a bit more force and press heavily down to remove any obvious spaces between the grinds.
Pressurized portafilters (generally) do not need tamping down. Brush the edges of the filter basket off so there are no stray coffee grounds to get into the threads of the brew head. I like to smooth the basket down with a light tamp to make sure the grounds are even within the basket.
Common reasons for a wet espresso puck include; too little espresso in the portafilter, wrong grind size, uneven distribution of grounds, and air pockets due to improper tamping.
If your coffee puck is too soft or muddy, then you are dosing too low (Picture #3). A low dose means that you may have had channelling through your coffee, causing an uneven extraction (see the holes in Picture #3). Also, your espresso will lack body and sweetness, making it taste a little thin and over extracted.
Generally, darker-roasted coffees perform well with a smaller ratio, while lighter-roasted coffees perform better with a larger one. Borrowing language from Italian espresso nomenclature, a 1:1 to a 1:2 is a Ristretto espresso, 1:2 to a 1:3 is a Normale espresso, and a 1:3 to a 1:4 is a Lungo espresso.
2. Overfilling Filter Basket. Filling the portafilter with too much espresso during an espresso tamp creates uneven extraction from the grounds, and filling it with not enough espresso creates uneven extraction as well. Too much grounds results in a stronger and over-brewed coffee.
Too strong? If the espresso was strong, pull another shot 2 grams longer in yield (you may have to grind coarser to keep the brewing time down). Keep iterating in 2-gram increments up or down in yield until you have reached your desired strength. This will become your new target yield.
While the tamped espresso was sweet with punchy acidity and a persistent crema, the non-tamped espresso was woody, with a bitter finish and crema that dissipated almost instantly.
Weight Of The Tamper
To apply the right amount of pressure your tamper needs to weight around 1-2 pounds. Avoid tampers made of plastic and opt for one made with heavy material such as stainless steel. According to professional baristas, tampers around 1 pound seem to be the best.
This technique involves spraying or adding a 'droplet' of water to your coffee beans to reduce static-loaded coffee grinds and grind retention when single-dose grinding. For anyone who's noticed their grounds clinging to metal surfaces or flying away from the portafilter while grinding, this is a must-have.
In regular espresso machines, using the wrong coffee grind size is the most common reason for no crema. Espresso requires a much finer grind size than drip coffee or normal ground coffee for general coffee machines. Espresso grounds should ideally be finer than table salt.
Damage can occur if you overtighten the portafilter when locking it in place to brew, and wear occurs just by using the machine normally.
Keeping the portafilter warm (and receiving / tamping a dose within a warm portafilter) is supposed to result in additional oils being released in the extraction, which in theory leads to a fuller pull and more crema. It's also supposed to add to the benefits of pre-infusion.
Espresso extraction should produce a double espresso of 50-60ml in 25-30 seconds or a single espresso of 25-30ml in the same time.