Thanks to a test called PRAL, meaning Potential Renal Acid Load, we know which foods have an acidifying or alkalizing effect because of its mineral composition, which may in turn will be reflected in the urine.
It is important to note that this test is not entirely reliable because with its methodology burn the foods and it is from the read of the ashes where the results are obtained. It is not 100% accurate because the sugar in the food is burnt first and this is one of the most acidifying substances to the body, and it is not reflected in the resulting mineral.
For example, we could add the banana, which is classified in most of the lists as a very alkaline fruit for its high potassium content, but the truth is that it is not; in fact, is an acidifying fruit because it contains a lot of sugar and it is first thing burnt in the PRAL test. This leads us to talk about which foods alkalize and which don't and in which proportion.
Therefore, among foods that acidify we can find: sugar in any of its forms, dairy (milk, cheese, yogurt and butter), the refined carbohydrates and flours (white rice, white bread, pizzas, pastries, etc.), animal protein (all kinds of mammal meat, poultry, seafood, fish, although the latter to a lesser extent), processed foods with preservatives, dyes and chemical additives. Also very cooked food as well as coffee, alcohol, snuff, drugs and medicines.
On the other hand, among alkaline food we can find: all vegetables, especially vegetables and leafy, green herbs and fruits with a low glycemic index such as tomatoes, grapefruit, lime, lemon and avocado. Seeds and raw nuts. Cooked or sprouted legumes. All the germinated. Quinoa, millet, buckwheat and all raw cold pressed oils such as olive, flax, avocado, grapeseed, sesame, coconut, etc. especially pure spices and ginger and curcuma, herbal teas, unprocessed salt like Himalayan, Celtic, and other salts of natural salt and extracted by natural processes and sun-dried.
In this chart you will find the Free Introductory Guide to Food and Alkaline lifestyle. You will find a summary of the main foods based on their categorisation from more alkaline to more acidic.