Preservation of foods by adding high concentrations of salts and sugars primarily works by: (Identify the physicochemical mechanism preventing microbial growth.)

Introduction:
Food preservation with salt or sugar relies on water activity control. This question probes understanding of osmotic effects that inhibit microbial growth in preserved foods like jams or cured meats.

Given Data / Assumptions:

• High solute concentrations draw water out of microbial cells.
• Most bacteria require high water activity to multiply.
• Yeasts/molds vary in tolerance but are still limited by extreme hypertonicity.

Concept / Approach:
Adding salt or sugar creates a hypertonic environment around cells, causing water efflux and plasmolysis. Reduced water activity (a_w) means essential metabolic processes slow or stop, leading to stasis or death.

Step-by-Step Solution:
Relate solute concentration to tonicity. Explain that hypertonic surroundings pull water from microbial cytoplasm. Connect plasmolysis to growth inhibition and shelf-life extension. Select the option describing “hypertonic environment.”

Verification / Alternative check:
Water activity models show microbial growth thresholds; high sucrose/NaCl lowers a_w below these thresholds.

Why Other Options Are Wrong:

• Raising pH: Many preserves are acidic, not alkaline.
• Lowering osmotic pressure: Opposite of what occurs; osmotic pressure increases.
• Hypotonic environment: Would drive water into cells, not out.

Common Pitfalls:
Confusing pH effects with water activity; both may contribute, but tonicity is primary here.

Final Answer:
creating a hypertonic environment is the principal preservation mechanism.