Some smoothies do give me heartburn most of the time. When I had my Esophageal pHmetry doctors told me to eat/drink anything that caused me heartburn and I did. I drank 3 berry smoothies, which usually makes me feel like I could spit fire, and my stomach handled it well. The pHmetry didn't show any spikes in acidity, everything was normal.
Can you explain why you say you don't have heartburn? It sounds like you're describing canonical heartburn symptoms.