Tamping is a crucial and important process while preparing espresso. Let's explain a bit about why.
Purpose of tamping is to evenly distribute ground coffee in the filter basket for extraction in the espresso machine. During the extraction, pressured hot will flow through the ground and extract the oil of coffee, water will seek out the least resistance path, therefore ensuring that ground are tamped well and evenly distributed to achieve proper extraction.
3 common scenario usually will happen during tamping.
1) Uneven tamping
Uneven tamping will cause the water flow faster on 1 side and slower on the other side, resulting a uneven extraction that will directly affect the taste of the espresso as water face more resistance on the heavier side and take longer time and higher pressure to flow out wide the other side face less resistance there is under extract.
2) Under / over tamped
Under pressure tamping will create a lesser resistant in the filter during extraction which result an under extract taste.
Over tamped means that more than 30 pounds of pressure are applied or multiple tamp happened. Ground are compressed tightly and there is not much space for water to flow through, resulting much higher resistance during extraction and water take longer time to get out from the basket. This will increase your extraction time, even with the same dose of ground filled in.
Example : Using our Ethiopia Sidamo which suppose to produce a sweet, berries taste with proper tamping and extraction, under tamp might result a flat, sweet cup only while over tamp will result a bitter, roasty taste as all the sweetness and fruity flavor will be over powered by the longer extraction time.
3) Tamp, Tap, Tamp
Seen barista tamped and "KNOCK" the basket to get the extra ground fall into the basket, then tamp again. Please stop doing this! That extra knock just loosen the whole thing and create crack inside of the ground. This will not be fix even if you tap again. Evenly distribute the ground before your first tamp is important to avoid this happen. Only tamp once !
Get it right
Practice make perfect and consistent extraction! Pay attention on every extraction timing with the same dosage of ground in the basket. Fine tune until to get it right and consistent.
1) Level : Even and flat surface. Use your tumb and index finger to feel it tamper is leveled with the basket to know if your tamping is correct as shown at below image. It should be LEVELED if your angle get it right.
2) Pressure : Straight down with even force of 30 pound. You ground shouldn't be fall down even if you turn your basket upside down after tamping.