Food preservation chemistry — At a given pH, which weak organic acid typically exerts the stronger bacteriostatic effect due to a higher fraction of undissociated molecules penetrating cells?

Introduction / Context:
Weak organic acids inhibit microbes primarily in their undissociated form. These neutral molecules diffuse across cell membranes; once inside the higher-pH cytoplasm, they dissociate, acidifying the cytosol and collapsing proton gradients. Food preservative effectiveness therefore depends on pH, pKa, and lipophilicity of the acid.

Given Data / Assumptions:

• All options are organic acids used or encountered in foods.
• “At a given pH” means comparing relative undissociated fractions and inhibitory power under the same acidity.
• We focus on generally observed preservative performance in foods.

Concept / Approach:
For a fixed pH, the Henderson–Hasselbalch relation shows that acids with pKa closer to the pH have higher undissociated fractions. Acetic acid (pKa ~4.76) often yields a substantial undissociated fraction in the mildly acidic range common to many foods (pH ~3.5–5.5) and also exhibits favorable membrane partitioning, giving strong antimicrobial action. Polycarboxylic acids like citric and tartaric, while acidulants and chelators, usually show lower bacteriostatic potency per mole at the same pH. Maleic acid behavior is more complex; in typical food systems acetic acid remains the benchmark household preservative (vinegar) for bacteriostasis.

Step-by-Step Solution:

Verification / Alternative check:
Food preservation literature consistently documents robust inhibition of many bacteria by vinegar (acetic acid) at culinary pH, outperforming citric/tartaric in comparable conditions for many spoilage organisms.

Why Other Options Are Wrong:

• Tartaric acid: effective acidulant/chelator but generally less potent bacteriostat than acetic at equal pH.
• Citric acid: good flavoring/chelator; antimicrobial mainly via pH reduction and metal binding, not as bacteriostatic as acetic at the same pH.
• Maleic acid: not a standard food preservative; practical use is limited.

Common Pitfalls:
Ignoring the role of pH and assuming “stronger acid = better preservative”; what matters is undissociated fraction and membrane permeability.

Final Answer:
Acetic acid