Where is data stored when Synaptic scans DVDs?

[Richard Owlett <rowlett@cloud85.net>]

As I have limited bandwidth, I install from DVDs.
Also I frequently experiment with various options by doing a fresh 
install to a different partition.
It is annoying to keep re-scanning a set of 13 DVDs.
Where is the information stored?
/etc/apt/sources.list is obvious, but where is the data about contents 
of each DVD stored?
TIA