Lemon juice is a carbohydrate (sugar) and therefore doesn't need a high acid stomach to digest it. Protein needs an acid stomach because at a ph level higher than about 4.5 protein digesting enzymes would not be active. Some protein, about 15% is digested directly in the stomach. Vegetables are neutral. (I also read somewhere that unwanted acid is expelled quickly through the lungs). When oil and acid are consumed together the acid is not expelled as it assists the stomach’s digestive process and moves the semi-emulsified fat into the duodenum.
Therefore it is less taxing on the digestive system to eat; protein with vegetables and with acid fruit, or carbohydrate with vegetables. Mixing protein and carbohydrates or acid fruits with carbohydrates increases the burden on the digestive system. This is the basics of a “combination diet” for folks who suffer some digestive problems including ulcers.
Stomach (Peptic) ulcers are largely caused by bacteria (Helicobacter pylori) and can be aggravated by complex meals that sit in the stomach for ages.
Once the stomach has been acted upon the food and it is passed into the duodenum bicarbonate is released from the liver and neutralises the stomach acid increasing the PH level from 1 to 2 to 6 to 7.