I use MusicBrainz Picard to manage the ID3 tags on my MP3 collection and iTunes for playing them and managing my iPod. I have Picard set to remove the Composer tag from any files that don't also contain the word "Classical" in the Genre tag.
I've run Picard and it has removed the Composer tag from many of my files. However, iTunes keeps displaying the old value of the Composer field.
I have tried right-clicking on the file to update, choosing "Get Info" and clicking OK. This has no effect on the Composer field when it has been removed entirely; it only refreshes the displayed value when it has been changed from one non-empty value to another.
How can I get iTunes to stop showing the old values for tags that have been removed? Please note I don't want to lose other iTunes metadata such as ratings and play counts, so removing the files from the iTunes library and re-adding them is not an option.