2.1 Effect of Temperature (Refrigeration vs. Freezing vs. Room Temperature)

Experimental approach: Use identical food samples (such as tomatoes) stored under different temperature conditions.

Key observations to record: colour changes, texture (firmness/softness), odours, visible mould or decay, liquid release.

2.2 Effect of Chemical Preservatives (Salt)

Experimental approach: Compare salted vs. unsalted fish or meat samples.

Procedure:

1. Divide fish into two equal portions
2. Label one plate A (salted) and one plate B (control)
3. Add approximately 5g salt to plate A and rub gently on the fish surface
4. Leave both samples at room temperature for three hours
5. Observe and record changes

Expected observations:

• Salted sample (A): Reduced moisture on surface, less bacterial slime, slower onset of foul odours
• Unsalted sample (B): Faster development of unpleasant odours, visible bacterial growth, texture breakdown

Why salt works: Salt creates high osmotic pressure, drawing water out of microbial cells through osmosis. Without sufficient water, microorganisms cannot grow or reproduce effectively.

2.3 Effect of Heat on Food Components

Starch Gelatinization

Heat causes starch granules to absorb water and swell, breaking hydrogen bonds and releasing amylose. This transforms starch into a viscous paste.

Demonstration: Boil potato slices in water for 5–8 minutes. The slice becomes soft as starch granules swell and rupture. A control slice in cold water remains unchanged.

Protein Denaturation

Heat causes proteins to unfold and lose their natural structure. This changes texture and can lead to coagulation.

Demonstration: Heat egg white (albumin) in hot water at 70–90°C. The clear liquid transforms into an opaque white solid. The Biuret test confirms protein structure changes.

Caramelization

When sugars heat above 160°C, they break down and form new compounds producing brown colour and rich flavour.

Demonstration: Heat sugar in a dry saucepan. The white crystals melt, turn golden-brown, and finally darken to produce caramel with a characteristic nutty aroma.

2.4 Effect of Air (Enzymatic Browning)

When cut fruits and vegetables are exposed to oxygen, the enzyme polyphenol oxidase catalyzes browning reactions.

Experimental approach: Treat apple slices with different methods:

Why lemon juice works: The citric acid lowers pH, creating an environment where polyphenol oxidase becomes less active. Boiling inactivates the enzyme entirely.

2.5 Effect of Acids on Protein

Acids denature proteins and can cause coagulation, as demonstrated when lemon juice is added to egg white. The acid changes the protein structure, causing the clear liquid to become opaque and form clumps.

Step-by-Step Framework

1. Select food samples — Choose foods that demonstrate preservation effects clearly (tomatoes, fish, apples, eggs, potatoes)
2. Apply preservation method — Use temperature control, chemical preservatives, heat treatment, or limiting oxygen exposure
3. Establish control — Keep one sample untreated for comparison
4. Observe and record — Document changes in colour, texture, smell, and appearance at regular intervals
5. Compare results — Analyze which preservation method most effectively maintained food quality
6. Draw conclusions — Relate observations to the scientific principles of preservation

Recording Observations

Maintain a structured observation table:

When analyzing preservation effects, consider:

• Rate of deterioration — How quickly did each sample spoil?
• Type of changes — Were they physical (texture, colour), chemical (oxidation), or biological (microbial growth)?
• Preservation effectiveness — Which method best maintained food quality?
• Scientific explanation — Connect observations to principles such as microbial inhibition, enzyme inactivation, or osmotic pressure

• pH testing — Using indicators to measure acidity changes
• Temperature monitoring — Using thermometers to verify storage conditions
• Visual comparison — Photographing samples over time for documentation
• Texture assessment — Touch testing for firmness changes
• Smell evaluation — Detecting off-odours indicating spoilage

In Tanzania, small-scale fish vendors along Lake Victoria often use salt preservation to extend the shelf life of tilapia and dagaa (small fish) before selling them at local markets such as Buguruni or Mwananyamala. By applying the laboratory analysis skills learned in this topic, a student could advise vendors on optimal salt application rates and compare the effectiveness of sun-drying versus refrigeration in preserving fish quality during the hot season, directly reducing food loss and improving household income.