bibtex-field-braces-alist

bibtex-field-braces-alist is a variable defined in `bibtex.el'.
Its value is

nil

Documentation:
Alist of field regexps that M-x bibtex-clean-entry encloses by braces.
Each element has the form (FIELDS REGEXP), where FIELDS is a list
of BibTeX field names and REGEXP is a regexp.
Space characters in REGEXP will be replaced by "[ \t\n]+".

You can customize this variable.