Coffee Appreciation

MMMMMMMMM...mouthwatering Crema there.
I never get a single stream of coffee as pictured, my machine produces two distinct streams from the seperate spouts on the...

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Why baristas like to watch

A smart barista will intently watch the stream of espresso pour out during an extraction. While appearance alone isn't definitive proof of a good cup, it does go a long way in verifying the barista's technique. Describing what the barista is looking for requires we first review a few terms, some of which are excerpted with permission from the Espresso Glossary by Mark Prince:
Blonding denotes the color transition of a pour from dark brown and tiger-striping to a light, uniform pale blond. This normally occurs in the last third of the pull and is a signal to end the pour. This overly-blond portion of an espresso is thin, nearly flavorless, and if allowed to continue too long, will dilute the body and taste characteristics of an otherwise enjoyable shot.
Channeling is the rapid passage of water through fractures in the coffee puck, which produces a thinner, under-extracted espresso. When it occurs, you'll often see sudden appearances of blond streaks in the stream of espresso; sometimes the puck will even have pencil lead-sized holes where channeling occurred.
Crema is one of the sure signs of a properly brewed shot of espresso (in non crema-enhancing espresso machines) and is created by the dispersion of gases— air and carbon dioxide —in liquid at a high pressure. The liquid contains emulsified oils, and forms a dark golden brown layer resembling foam on top of an espresso shot. [excerpted M.P.]
Extraction is the act of forcing hot water from the boiler though ground coffee, which in turn "extracts" flavors, oils, colloids, lipids and other elements that turn water into brewed coffee or espresso. [excerpted M.P.]
Golden rule is a common phrase that describes the ideal extraction time and volume for an espresso. The Instituto Nationale Espresso Italiano (PDF) provides a working definition of the characteristics of an ideal espresso, although I consider these parameters more "golden guidelines" than hard and fast rules. My own brief definition of an espresso is an extraction using approximately fourteen grams of coffee to produce a sixty milliliter double in 22 to 32 seconds (timing from the moment the pump starts).
Over-extraction occurs when too many coffee solids are extracted, resulting in a strong, harsh flavor. The visual signs are a low-volume extraction having a dark, thin crema. A dark "halo" at the edge of the cup is another classic indictor of an over-extraction, or of brew water that is too hot.
Pre-infusion: the act of pre-wetting the bed of ground coffee inside an espresso machine before actually commencing the brew. Some espresso machines do this by using the pump; water is pumped to the coffee for a second or two, and then halted for another second or two. After this pause, the pump activates again, and continues brewing the shot. Super automatics and some automatic espresso machines use this pre-infusion.
Another type of pre-infusion is called "natural" or progressive pre-infusion, and occurs in espresso machines equipped with an E61 grouphead. When the pump is activated, a secondary chamber must fill prior to full pressure being applied to the bed of coffee. This gives a 3 to 7 second saturation time for the grounds before the pressure builds up. This type of pre-infusion is preferable to pump and pause active pre-infusion.
There is a school of thought that progressive pre-infusion improves overall extraction from the coffee. [excerpted M.P.]
Tiger striping and mottling are leading visual indicators of a good extraction. Tiger striping is formed by the contrast of darker and lighter crema in the espresso stream; ideally it begins early in the pour and is sustained through the end. Mottling is the in-cup confirmation of a good extraction; it is the darker brown speckling and reddish-brown splotches formed on the surface of the crema.
Under-extraction occurs when too few coffee solids are extracted, resulting in a weak, dull flavor. The visual signs are a rapid, high-volume extraction having a uniformly light blond crema.