The issue is not so much “leaving unused shots hanging around by the machine”. When making several drinks at once, sometimes you brew shots and then for whatever reason it takes you more than ten seconds to use them (ran out of milk and had to steam more, had to get something from the back room, customer had a question or a last-minute change to their drink, etc.). It’s not like you left the shots there just to hang out, but by the time you get to them they’re cold.
If you brew a shot of espresso into a glass shotglass, you will see three layers – the light foamy crema on top, the rich brown body, and the really dark bit at the bottom (the heart – don’t look at me, I didn’t make up the names). If you let the shot sit there, not only will it get cold, but the look of the shot changes to an overall dull dark brown. I can’t speak from experience as to how this changes the taste, as I don’t drink straight shots.
The ten-shot (not a morning person, Johnny?
) is a bit of an exception, as it’s not physically possible to brew ten shots and get them into a cup in ten seconds. Most Starbucks espresso machines have four brewing groups, capable of brewing eight shots at once. A good shot by Starbucks standards takes between 18 and 23 seconds to brew (trust me, there is a difference if the shot’s short or long). Even if you can get all the shots to come out within ten seconds of each other, which isn’t so hard on the newer push-button Verissimo machines but is highly unlikely on the older manual La Marzoccos, you still have to brew another pair of shots.
The ten second rule, as I recall, isn’t so much ten seconds from brewing to the customer’s hand, but ten seconds from brewing to going into the drink. If I’m making a venti (large) cappuccino, it takes me more than ten seconds to make it well, and the shot’s the first thing to go in. But the steamed milk is keeping the shot warm, and the mixing destroys the whole layer thing.
I don’t mind getting short or long shots, as long as they’re not six-second bathwater or 90-second retreads, but I wouldn’t want a drink made from a shot that’s been sitting there for much longer than ten seconds.
– Dragonblink, (ex)Barista extraordinaire