Question

Toxicity of which of the following food additive to molds and certain bacteria is related to the ibility of the organism to metabolize the three-carbon unit?

• A. Sorbic acid
• B. Benzoic acid
• C. Acetic acid
• D. Propionic acid

Hint:

Associate the "three-carbon unit" directly with Propionic Acid ($C_3H_6O_2$).
This preservative is highly specific for bakery products because it does not inhibit the action of baking yeast (*Saccharomyces cerevisiae*) at normal usage levels while being highly toxic to molds.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Organic acids are widely used as chemical preservatives in food products because of their antimicrobial activities, which are generally more effective in low-pH environments where they exist in their undissociated forms.
Key Principle:
The antimicrobial mechanism of certain short-chain fatty acids involves not only cellular acidification but also metabolic blockages caused by the ibility of the target microorganisms to process the specific carbon chain structure.

Step 2: Detailed Explation:
Propionic acid ($CH_3CH_2COOH$) is a three-carbon, short-chain monocarboxylic acid.
It is commonly used as a calcium or sodium salt in baked foods to control mold growth and prevent "rope" formation caused by *Bacillus* species.
Unlike even-numbered fatty acids (like acetic or butyric acid), which are easily metabolized via standard $\beta$-oxidation pathways, propionic acid presents a three-carbon unit.
Many molds and bacteria lack the specific enzyme pathways (such as propionyl-CoA carboxylase) required to convert this three-carbon unit into metabolizable intermediates of the Krebs cycle.
Consequently, the accumulation of the propiote ion inside the microbial cytoplasm leads to severe metabolic disruption, depletion of coenzyme A, and ultimate cell death.

Step 3: Fil Answer
Therefore, propionic acid is the preservative whose toxicity is directly related to the organism's ibility to metabolize its three-carbon unit.