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I was wondering whether it is possible to pickle food using hydrochloric acid (HCl), rather than acetic acid. I am not very familiar with the biology of the fermentation process involved in pickling and I do not know if HCl would give rise to a suitable environment. Is HCl-based pickling possible? What percentage of HCl should be used?

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Short answer
Weak organic acids are more effective for food preservation then strong mineral acids, mainly because undissociated weak acids can cross the cell membrane and disrupt cell physiology from within.

Background
Natural fermented, or pickled materials are traditionally cured in a brine solution (salt, water, sometimes spices and sugar). Desirable anaerobic bacteria convert carbohydrates to acetic acid that “pickle” and preserve the food. The brine protects the vegetables from aerobic organisms. However, most contemporary pickles are not fermented at all; instead they rely upon the direct addition of acetic acid (vinegar, a product of fermentation), and usually heating for additional preservation. This way the acetic acid production through the fermentation process is simply bypassed.

So the basis of the traditional and contemporary pickling is the addition of organic acids like lactic and acetic acids. These small organic acids are known to be more effective against bacteria than inorganic mineral acids like HCl (Rode et al., 2010)

Acetic acid is a weak acid. In undissociated form, it is lipid soluble and able to rapidly diffuse through the plasma membrane (source: World Nutrition Science blog). It is thought that diffusion of acetic acid and other organic acids into the bacterial cell is the main mechanism of action behind its preservative effects. Preservation by acidification is especially effective in certain species of pH-sensitive bacteria such as E. coli and Salmonella. In contrast, strong mineral acids are nearly completely ionized in solution and ions do not pass the cell membrane's hydrophobic lipid bilayer.

The mechanism of action of small organic acids in food preservation are thought to be at least threefold:

  • Once diffused in the cell, organic acids will dissociate because the internal pH is near or above neutrality and thereby lower the bacteria's internal pH and disrupt cell physiology;
  • The anionic part of the organic acids, which cannot escape the bacteria in its dissociated form, will accumulate within the bacteria and disrupt many metabolic functions. This is one advantage of using organic weak acids like acetic acid, because they are normal constituents of the cells. Acetic acid and lactic acid for example are both intermediates in glucose metabolism. Accumulation of these metabolites will disrupt the glucose metabolism essential to the cell's survival.
  • Eventually, upon progressing accumulation, osmotic pressure increases become mechanically incompatible with bacterial life.

Another advantage of acetic acid is that it is a natural metabolite in human metabolism, and because it has been used in food preservation for thousands of years it is not classified as a food preservative, at least in the EU, and can be freely added to foods. Last but not least, acetic acid has a certain taste to it that may be preferable over inorganic acids.

Reference
- Rode et al., Can J Microbiol (2010); 56(9): 777-92

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