After harvest, fruits, vegetables, and other plant products remain alive and continue to metabolize. They respire, transpire, and undergo biochemical changes that affect their quality. The goal of post-harvest management is to slow these changes without causing injury, preserving quality for as long as possible .
Key insight: Post-harvest losses are not just about quantity—they're also about quality. A fruit may look fine but have lost flavor, nutrients, and texture. Understanding the biochemistry of deterioration helps us maintain all aspects of quality .
As we learned in Module II, harvested produce continues to respire, consuming sugars and producing CO₂, water, and heat. The respiration rate determines storage life:
Asparagus, broccoli, mushrooms, spinach (days)
Apples, pears, potatoes, onions (months)
Temperature is the most important factor affecting respiration. The Q₁₀ temperature coefficient for respiration is typically 2-3, meaning respiration rate doubles or triples for every 10°C increase .
The cold chain—maintaining optimal temperatures from harvest to consumer—is the single most effective post-harvest technology. For example:
Every 10°C reduction roughly doubles storage life—but only until the optimum temperature for that commodity is reached. Below that, chilling injury occurs .
Fresh produce is 80-95% water. After harvest, they continue to lose water through transpiration, leading to:
Water loss rate depends on:
Management: High humidity (90-95% RH), rapid cooling after harvest, and protective packaging reduce water loss.
Texture changes are primarily due to modifications in cell wall structure, especially pectin degradation.
| Enzyme | Target | Effect |
|---|---|---|
| Polygalacturonase (PG) | Pectin (middle lamella) | Breaks down pectin, cells separate → softening |
| Pectin methylesterase (PME) | Pectin (demethylation) | Modifies pectin, affects calcium binding |
| Cellulase | Cellulose | Weakening of cell walls |
| Expansins | Cell wall loosening | Allow other enzymes access |
The first genetically engineered food approved for sale (1994) was the Flavr Savr tomato, which used antisense RNA to suppress polygalacturonase (PG) expression. This slowed softening, allowing tomatoes to ripen longer on the vine before harvest, improving flavor while maintaining firmness for shipping .
Mealiness (a dry, grainy texture) occurs when cells separate cleanly rather than breaking. This is common in overripe or poorly stored apples and peaches. It results from abnormal pectin degradation and loss of cell adhesion .
After harvest, sugars can:
Organic acids are respired, reducing acidity over time. The sugar/acid ratio often increases during ripening but may decrease during prolonged storage as sugars are consumed .
Aroma volatile production is dynamic post-harvest:
Vitamins, especially vitamin C, decline after harvest. The rate depends on:
| Vitamin | Stability | Typical loss during storage |
|---|---|---|
| Vitamin C | Very unstable | 50-80% loss over weeks |
| Folate | Unstable | Significant losses |
| β-carotene | Moderately stable | 10-30% loss over months |
| Vitamin E | Moderately stable | Moderate losses |
Freshly squeezed orange juice has high vitamin C. But within days of refrigeration, vitamin C begins to decline. After 1 week, losses can be 10-20%. Pasteurization and storage in cardboard cartons (oxygen-permeable) accelerate losses—which is why juices are often packaged with nitrogen flushing and sold in oxygen-barrier containers .
Color changes after harvest include:
Broccoli florets are actually unopened flower buds. After harvest, they continue to develop—the buds open, revealing yellow petals. This is accompanied by chlorophyll degradation (yellowing) and loss of quality. Rapid cooling to 0°C and high humidity slow this process dramatically .
Many tropical and subtropical fruits (banana, mango, tomato, cucumber) are damaged by temperatures above freezing but below a critical threshold (typically 5-13°C). Symptoms include:
Cause: Membrane lipid phase transition at low temperatures, leading to loss of membrane integrity and cellular dysfunction .
When produce freezes, ice crystals form, rupturing cells. Upon thawing, tissues become water-soaked and rapidly decay. Most fresh produce should never be frozen .
Natural aging process leading to tissue breakdown, yellowing, and eventual death. Hormonally regulated (ethylene promotes senescence) .
| Technology | How it works | Target | Examples |
|---|---|---|---|
| Rapid cooling | Removes field heat quickly | Slows all metabolism | Forced-air cooling, hydro-cooling, vacuum cooling |
| Refrigerated storage | Maintains optimal low temperature | Slows all metabolism | Cold rooms, refrigerated transport |
| Controlled atmosphere (CA) | Reduces O₂, increases CO₂ | Slows respiration, ethylene action | Apple storage (up to 12 months) |
| Modified atmosphere packaging (MAP) | Creates modified atmosphere within package | Slows respiration, water loss | Fresh-cut salads, berries |
| Ethylene management | Remove ethylene or block its action | Slows ripening, senescence | Potassium permanganate scrubbers, 1-MCP (SmartFresh™) |
| Edible coatings | Creates barrier to gas exchange | Slows respiration, water loss | Wax on apples, citrus; chitosan on strawberries |
| Heat treatments | Brief heat exposure | Kill pathogens, induce stress resistance | Hot water dip for mangoes (fruit fly control) |
1-methylcyclopropene (1-MCP) is a gaseous compound that blocks ethylene receptors, preventing ethylene from initiating ripening and senescence. It's used commercially on apples, pears, tomatoes, and other fruits. Apples treated with 1-MCP remain firm and crisp for months, even after removal from cold storage .
Ethiopian mango exporters face long shipping times to Middle Eastern and European markets. Success requires:
Understanding post-harvest biochemistry is essential for each of these steps .
| Quality attribute | Biochemical change | Management strategy |
|---|---|---|
| Respiration | Sugar loss, heat production | Cold storage, CA, MAP |
| Water loss | Wilting, weight loss | High humidity, packaging, wax coatings |
| Texture | Pectin degradation (PG, PME) | Cold storage, calcium treatments, 1-MCP |
| Flavor | Sugar/acid changes, aroma loss | Optimal harvest timing, cold storage |
| Nutrition | Vitamin degradation | Cold storage, minimal processing |
| Color | Pigment degradation, browning | Cold storage, antioxidants, blanching |
Discuss your answers in the course forum.