Stomach acid is neutralized in the food once it passes in the largely alkaline environment of your duodenum. Passing further in your intestine, the whole mixture is largely pH neutral, and as your cells and your intestinal flora work to extract nutrients and water out of it, it slowly compacts the residue into finally in your colon turns into feces. Whether sphincter is infallible or not has no bearing to your question, the only thing in your colon is your feces.