bibtex-entry-format

bibtex-entry-format is a variable defined in `bibtex.el'.
Its value is

(opts-or-alts required-fields numerical-fields)

• This variable is safe as a file local variable if its value
  satisfies the predicate which is a byte-compiled expression.
  

Documentation:
Type of formatting performed by `bibtex-clean-entry'.
It may be t, nil, or a list of symbols out of the following:
opts-or-alts Delete empty optional and alternative fields and
remove OPT and ALT prefixes from used fields.
required-fields Signal an error if a required field is missing.
numerical-fields Delete delimiters around numeral fields.
page-dashes Change double dashes in page field to single dash
(for scribe compatibility).
whitespace Delete whitespace at the beginning and end of fields.
inherit-booktitle If entry contains a crossref field and the booktitle
field is empty, set the booktitle field to the content
of the title field of the crossreferenced entry.
realign Realign entries, so that field texts and perhaps equal
signs (depending on the value of
`bibtex-align-at-equal-sign') begin in the same column.
Also fill fields.
last-comma Add or delete comma on end of last field in entry,
according to value of `bibtex-comma-after-last-field'.
delimiters Change delimiters according to variables
`bibtex-field-delimiters' and `bibtex-entry-delimiters'.
unify-case Change case of entry types and field names.
braces Enclose parts of field entries by braces according to
`bibtex-field-braces-alist'.
strings Replace parts of field entries by string constants
according to `bibtex-field-strings-alist'.
sort-fields Sort fields to match the field order in
`bibtex-BibTeX-entry-alist'.

The value t means do all of the above formatting actions.
The value nil means do no formatting at all.

You can customize this variable.