Q.Home made jams and gellies without any added chemical preservative additives spoil in a few days whereas commercial jams and jellies have a long shelf life. Explain. What role does added sodium benzoate play?

Step 1. Spoilage of jams/jellies is mainly caused by microorganisms (moulds, yeasts, bacteria) growing in the product over time; anything that suppresses that growth extends shelf life.

Step 2. Sodium benzoate is the sodium salt of benzoic acid, and is added to commercial jams as a chemical preservative. In solution it exists in equilibrium between its ionized (benzoate ion) and un-ionized (benzoic acid) forms, with the balance set by the solution's pH -- exactly the kind of weak-acid ionization equilibrium this chapter covers (section 3.4).

Step 3. The un-ionized benzoic acid form is the one that is able to cross microbial cell membranes and interfere with the microorganism's internal metabolism, inhibiting its growth -- so its antimicrobial action depends on the medium's pH being kept low enough (acidic) that a useful fraction of the benzoate stays in this un-ionized form; jams, being naturally somewhat acidic (fruit acids), provide this environment. …

Connect microbial spoilage to the weak-acid ionization equilibrium of benzoic acid/benzoate, and note that its preservative action depend …

• Describing sodium benzoate as simply 'a chemical that kills germs' without connecting its action to the pH-dependent ionization equilibrium that determines how much of it is in the active (un-ionized) form. …