how does the amount of caffiene vary in different roasts of coffee?!

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I managed a Starbuck's owned Seattle's Best Cafe' and can tell you because I had to go to classes myself.

The lighter the roast, the more caffeine. Roasting leaches oils and caffeine from the bean. Breakfast blends, espresso blends, light blends are higher in caffeine, while darker roastslike French roasts have less because they are roasted longer.

The lighter and medium roasts of coffees have a higher levels of caffeine than the darker roasts which lose some of the drug during the longer roasting time.

Also check the following info about caffeine amount in some products, it may help:

Beverage (8-oz or as noted)
Starbucks Grande Coffee (16 oz) 330 milligrams
Coffee, Drip 115-175 milligrams
Starbucks Grande Latte (16 oz) 150 milligrams
Dunkin' Donuts Coffee (16 oz) 143 milligrams
Coffee, Brewed 80-135 milligrams
McDonald's Coffee (small) 100 milligrams
Coffee, Espresso (2 ounces) 100 milligrams
Coffee, Instant 65-100 milligrams
Tea, iced 47 milligrams
Tea, brewed, imported brands (avg.) 60 milligrams
Tea, brewed, U.S. brands (avg.) 40 milligrams
Tea, instant 30 milligrams
Tea, green 15
Hot cocoa 14
Starbucks Grande Decaf Latte (16 oz) 13 milligrams
Chocolate Milk (8 oz) 5 milligrams
Coffee, Decaf, brewed 3-4 milligrams
Coffee, Decaf, instant 2-3 milligrams

http://wilstar.com/caffeine.htm
http://www.volcanicacoffee.com/coffee-ar…