The addition of _____ produces sulphur dioxide gas which destroys and prevents the entry of microorganisms in food.

Question

Accepted Answer

The correct option is 1:
Potassium metabisulphite ($\text{K}\_2\text{S}\_2\text{O}\_5$) is a potent chemical food preservative specifically known for its ability to release sulfur dioxide ($\text{SO}\_2$) gas upon dissolving in water, especially in the acidic conditions typical of many preserved foods like fruit pulps, squashes, and jams. This release of $\text{SO}\_2$ is the core of its powerful antimicrobial action. Sulfur dioxide is a highly effective sterilizing agent because it actively interferes with the essential metabolic, respiratory, and reproductive processes of microorganisms, particularly yeasts and molds, which are the primary causes of spoilage in high-sugar and acidic products. The gas destroys and prevents the growth and entry of these microbes, offering robust protection. Because it also possesses bleaching properties, Potassium metabisulphite is the preservative of choice for light-colored products where maintaining the original color is important, such as in white grape juice or applesauce.

The incorrect option is 2:
Sodium benzoate is one of the most widely used chemical food preservatives, but its mechanism of action is completely different from what is described in the question. It does not produce sulfur dioxide gas. The preservative power of Sodium benzoate is derived from its conversion into benzoic acid in an acidic medium ($\text{pH}$ $2.5 \text{ to } 4.0$). The un-dissociated benzoic acid then enters the microbial cells and disrupts their internal function, inhibiting the growth of molds, yeasts, and some bacteria. It is typically used in clear, acidic foods like carbonated drinks, pickles, and some fruit juices.

The incorrect option is 3:
Citric acid is a naturally occurring organic acid, widely present in citrus fruits, and is a common food additive. Its primary uses are as a flavouring agent to impart a sour taste, an acidulant to control $\text{pH}$, and a metal chelator to prevent oxidation (antioxidant synergy). While lowering the $\text{pH}$ does create an unfavorable environment for many spoilage bacteria, its preservative effect is generally weak and secondary to its main functions. Crucially, Citric acid is purely an acid and does not release any sulfur dioxide gas, which is the key characteristic mentioned in the question.

The incorrect option is 4:
Potassium benzoate belongs to the same family of preservatives as Sodium benzoate, and thus shares an identical mechanism of action. When added to an acidic food product, Potassium benzoate dissociates to release the antimicrobial agent, benzoic acid. This acid is effective against microorganisms by interfering with their cellular processes, not by releasing a gas. The use of potassium benzoate is often preferred in the industry simply to help producers meet demands for low-sodium food items, making it a functional substitute for Sodium benzoate, but its chemical process does not involve the production of sulfur dioxide.