Hi,
Thanks for posting your query at DoctorSpring.com. I am Dr.R.K and I am pleased to assist you.

Coffee whether it is Barista or Instant is known to increase the acidity in stomach. The symptoms you get is due to increased acidity as a result of drinking coffee. You can confirm it with an upper GI endoscopy. Medications like prilosec would provide relief.

You need not be concerned. As your doctor has said, it is due to gastritis only.

I hope that answers your question. I would be glad to answer any follow-up queries.
Regards,
Dr.R.K

That could possibly because of the method of preparation. The caffeine is not directly responsible for the acidity and gastritis symptoms . It is the various other stains and chemical in the coffee.

Instant coffee in my understanding is made wet following the roasting. This is then freeze dried . So during these process it is possible that the gastritis causing stains are removed. In direct brew coffee the process is more direct and hence more risk.

I would recommend Prilosec. This will act as diagnostic tool also. If there is response then the acidity is the root cause for the symptoms.

Another possible, less likely cause is the type of coffee. Particular brands and species can differ in their capacity to produce gastric irritation. One particular varient (i think its arabica) is know to cause more gastritis.

Hope this helps
Feel free to ask followups and clarifications.
Thank you