Hmm.... must be some unusual contaminant being removed from the
glassware. In analytical chemistry, three rinses is generally
considered to be sufficient to remove all but 0.1% (or less) of any
residue. For extreme cleanliness, instead of more rinsing, nitric acid
soaks are generally used. I don't recommend that for lemons,
though.... :-)

Our home-grown lemons (miniature tree, indoors) have lots of juice and
lots of flavor but are a little low on the zest. I wish we had a
variety that was thicker-skinned.

Regards -- Terry