Memory Leak in Tika version 1.13 when parsing millions of files

We have an application using Tika which parses roughly 7,000,000 files of different types, many of the files are MSG files with attachments. This works correctly with Tika 1.9, and has been in production for over a year, with parsing runs taking place every few weeks. The same application runs into insufficient memory problems (java heap) when using Tika 1.13.

I have used lsof and file leak detector to track down open files, however neither shows any open files when the application is running. I did find an issue with open files https://issues.apache.org/jira/browse/TIKA-2015, however there was a workaround for this and this is not the issue.

I am sorry to have to report this with a level of vagueness, but with lsof turning nothing up I am a bit stuck as to how to investigate further. We are more than willing to help by testing on the basis of any ideas provided.