It seems as though you have misunderstood how the IN clause works.
The IN clause will look for what is on the left in the list of values on the right. For example: WHERE id IN (2,3,5) - if id is in that list it will return true. In your case it is the opposite.
Something like this should work for what your after but there are likely to be better alternatives.
$sql = '';
foreach ($keywords AS $keyword)
{
if ($sql != '')
$sql .= ' OR ';
$sql .= "object_title LIKE '%$keyword%'";
}
$query = 'SELECT object_title FROM table WHERE '.$sql;
% is a wildcard.
Just please remember to escape the values first.