Lemon juice is acidic. It contains citric acid which a weak organic acid (pH = 2.3). When it hits the stomach which contains hydrochloric acid (pH ~ 1) it will have little effect on the acidity of the stomach unless the pH of the stomach acid is greater than 2.3 then it will decrease the stomach acid pH.
GERD stems from acid rising to the throat, unusually low stomach-acid levels (i.e too high a pH) may worsen the condition as the undigested food sits in the stomach. The citric acid in lemon juices seems to provide relief in such cases.
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