Is this possible in a trigger?

I want to keep a history of changes on a field in a table. This will be
the case in multiple tables.

Can I create a trigger that loops the OLD and NEW values and compares
the values and if they are different creates a change string as follows:

e.g;

FOR EACH field IN NEW
IF field.value <> OLD.field.name THEN
changes := changes
|| field.name
|| ' was: '
|| OLD.field.value
|| ' now is: '
|| field.value
|| '\n\r';
END IF
END FOR;

Your help is really appreciated.

Thank you.