Que saco!

Synopsis: Tonio's father seeks to fill 90 bags with the same amount of coffee, but one of them ends up with less coffee than the rest, and leaves a warning. But Tonio and Karen did not see the warning in time and now have to figure out which is the lightest bag. What is the smallest number of weighings needed for this?
Source: Unicamp - M3, Mathematics Multimedia.
URL: http://m3.ime.unicamp.br/recursos/1165