Black coffee does not bring on acid reflex in me. Adding sugar and cream to my coffee sometimes brings on acid reflex in me. The addition of cream and sugar sometimes brings on a strong acidic reaction in the stomach. Coffee candy and milk chocolate are acidic and they bring on strong acidic reactions in the stomach.
Of all these choices, straight black coffee to me is the best way to keep coffee alkaline.
Coffee made from ground beans keeps the coffee alkaline. Instant coffee often brings on acid reflex in me.
If you go to Starbucks and order coffee, then add nutmeg, cinnamon, or Chinese almond powder in the coffee, it tends not to turn acidic in the stomach.