
Subreddit: r/AskScience
User: u/rtfbear1
Original Post:
Why are the vast majority of foods acidic?
Most foods and common cooking ingredients have a pH < 7. Tofu seems to be among the minority of basic foods. Why don't humans eat more basic foods? Is there something about how humans evolved to the diet they have or life in general which causes the living matter we eat to be generally acidic?
My Response:
Hi, food scientist here.
Many food-borne pathogens do not survive below a pH of 4.6; it’s one of the major hurdles used in food technology and processing to ensure that food is microbiologically safe, along with heat treatment, salt, etc. In any case, many non-pathogen and beneficial microorganisms produce organic acids during fermentation. There’s a theory that much of the fruit that humans and hominids consumed throughout our evolutionary history were at least partially fermented after they’d ripened.
Regardless, humans have associated a higher degree of safety with acidic foods, such as fermented food products. Between cooking and fermentation, these processes helped ensure food did not make humans sick. So many of our food ingredients followed along this track through empirical experimentation, intuition, and accident.
Basic foods are more likely to contain pathogens. This is not always the case, and certainly there have been happy accidents in which foods have been covered in wood ash or treated with potash, caustic lime, lye, and other basic ingredients. But these require a thorough understanding of how fire works and the byproducts produced. So it’s actually quite advanced technology in the grand scheme of human existence to create basic. Some are naturally present such as natron (sodium carbonate), but these are not universally present in nature. So it’s harder to find basic deposits and use them for food preparation.
Many pathogenic microorganisms can survive in slightly basic conditions. A key example is Bacillus cereus, which can grow at pH as high as 9.3. This makes basic foods less likely to survive the basic requirement of being free of danger by early humans. So through selection, basic foods became a big no-no, especially before the development of pasteurization. Tofu is a strange side product that is slightly more resistant to pathogen growth despite being basic, largely due to how it’s produced (soy bean milk needs to be boiled and coagulated with calcium sulfate, so it has already been pasteurized at least once).

Basic foods also do not always taste very good. Many tend towards soapiness because basic conditions accelerate the cleavage of fats into fatty acids and glycerin. And humans are extremely sensitive to the bitterness of free fatty acids and soaps, likely because it’s associated with rancidity. Basic conditions can also lead to very slow, but uncontrolled hydrolysis of proteins to peptides, many of which are bitter as well. Many other chemical reactions occur more readily in basic conditions versus acidic conditions which affect the flavor, texture, and consistency of a food.
However, so long as there are sugars, acids can be produced by fermentation. So a very wide variety of foods can go through this process, including carbohydrate- and starch-rich foods. It’s a relatively simple way to render food safe from microbiological contamination, and prevent foods from spoiling through mold growth. And this process occurs naturally without much stimulation - yeast, lactic acid bacteria, and acetobacter (which convert ethanol into acetic acid) species are all very prevalent in nature. Many species of lactic acid bacteria can also survive high concentrations of salt, which also has a protective factor during fermentation, so humans learned when food are salted, they’re likely to go through lactic acid fermentation and be even more resistant to pathogen growth.
The underlying intuition is that if it’s acidic, it will be less likely to cause food poisoning and illness.
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