Post-harvest physiology of fruits and vegetables
Why harvested produce deteriorates, respiration, water loss, ethylene, chilling injury, maturity indices, precooling, curing, and a Q10 storage-life sum.
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A harvested tomato, lettuce or mango is still alive. Cut off from the roots and leaves that supplied it, it keeps respiring, losing water and changing until it ripens, wilts, rots or is eaten. Every step between field and consumer either slows this deterioration or speeds it up. FAO estimates that around 14 per cent of the world's food is lost after harvest and before retail, much more for fruits and vegetables in hot climates without cold chains.
Post-harvest physiology explains why produce deteriorates and how handling, temperature and atmosphere can extend its life. It is a core topic in horticulture and food technology courses.
Why harvested produce deteriorates
Respiration
Produce burns its stored sugars, acids and starch, losing weight, sweetness and flavour, and releasing heat. Respiration rates vary enormously:
| Respiration class | Examples |
|---|---|
| Very low | nuts, dates, dried fruits |
| Low | apple, citrus, grape, onion, potato (mature) |
| Moderate | banana, cherry, peach, cabbage, tomato |
| High | avocado, strawberry, cauliflower, lettuce |
| Very high | asparagus, broccoli, mushroom, sweet corn, peas |
High-respiration products, usually immature, actively growing tissues, are the most perishable.
Water loss
Most fresh produce is 80 to 95 per cent water. Losing just 5 per cent of its weight makes many fruits and vegetables visibly shrivelled or wilted and unsaleable. Water loss is driven by the vapour pressure deficit between the produce and the air, so it rises with temperature and dry air. It is reduced by cooling, high relative humidity (often 90 to 95 per cent for leafy vegetables), waxes and packaging.
Ethylene
Ethylene speeds ripening of climacteric fruits and causes damage in sensitive products: yellowing of broccoli and cucumber, russet spotting of lettuce, sprouting of potatoes, bitterness in carrots, and abscission of flowers and leaves. Ethylene producers such as ripening bananas, apples and tomatoes should not be stored with sensitive produce.
Pathological decay
Moulds and bacteria, such as Botrytis, Penicillium, Rhizopus, anthracnose and soft-rot bacteria, cause much of the loss, entering through wounds, bruises and natural openings.
Physiological disorders
- Chilling injury: tropical and subtropical produce such as banana, mango, papaya, pineapple, avocado, cucumber, tomato and sweet potato is damaged at low but non-freezing temperatures, usually below 10 to 13 °C, showing pitting, discoloration, failure to ripen and decay.
- Freezing injury below the freezing point of the tissue.
- Disorders linked to nutrition or atmosphere, such as bitter pit in apples (calcium deficiency) or injury from too much CO2.
Temperature: the most important factor
The Q10 of a process is the factor by which its rate increases for each 10 °C rise. For respiration and deterioration of produce, Q10 is commonly 2 to 3 between 0 and 30 °C, and often higher between 0 and 10 °C. Each hour at field heat costs storage life. Precooling, removing field heat quickly after harvest, is therefore essential:
- Room cooling: simple but slow.
- Forced-air cooling: cold air pulled through vented packages; widely used for fruits.
- Hydrocooling: chilled water; for products that tolerate wetting, such as sweet corn, carrots and some stone fruits.
- Vacuum cooling: rapid evaporative cooling under vacuum for leafy vegetables like lettuce.
- Package icing: for broccoli and some others.
Harvest maturity
Produce must be harvested at the right stage. Physiological maturity is when a fruit can complete ripening; horticultural (commercial) maturity is when it suits the intended use. Maturity indices include:
- days from flowering or fruit set, and heat units;
- size, shape and the fullness of shoulders (mango);
- colour of skin or flesh, and seed colour;
- firmness (penetrometer), total soluble solids (refractometer, °Brix), acidity and the sugar–acid ratio;
- starch–iodine tests (apple), oil or dry matter content (avocado), juice content (citrus);
- specific gravity (mango, potato).
Harvesting too early gives poor flavour and more shrivelling; too late shortens storage life.
Handling practices
- Harvest in the cool of the day, keep produce shaded, and avoid bruising: impact damage often appears days later.
- Curing: potatoes and sweet potatoes are held warm and humid for a few days to heal wounds with new skin layers; onions and garlic are dried to form protective outer scales.
- Sorting and grading, removing damaged and diseased produce.
- Cleaning and sanitising wash water, for example with chlorine at controlled levels.
- Packaging that protects, ventilates and fits the cold chain; modified-atmosphere packaging for some products.
- Sprout control in potatoes and onions by cool storage and approved sprout inhibitors.
How temperature changes storage life
A product keeps for 28 days at 0 °C, and its rate of deterioration has a Q10 of 3.
- At 10 °C, deterioration is 3 times faster: 28 ÷ 3 ≈ 9 days.
- At 20 °C, it is 32 = 9 times faster: 28 ÷ 9 ≈ 3 days.
- At 30 °C, it is 33 = 27 times faster: about 1 day.
So one day at 30 °C uses about as much storage life as 27 days at 0 °C. For a temperature rise of ΔT degrees in general, the rate increases by Q10(ΔT/10); for a rise from 0 to 5 °C with Q10 = 3, the factor is 30.5 ≈ 1.7.
Key points
- Harvested produce is alive: respiration, water loss, ethylene, decay and disorders limit its life.
- Respiration rates range from very low (nuts, dried fruit) to very high (asparagus, broccoli, sweet corn).
- 5 per cent weight loss makes much produce unsaleable; keep humidity high.
- Temperature is the key control; Q10 is often 2–3, so precool quickly by forced air, hydro, vacuum or ice.
- Chill-sensitive tropical produce must stay above about 10–13 °C.
- Harvest at the right maturity using colour, firmness, soluble solids, starch and other indices; cure potatoes, sweet potatoes and onions.
What the exams ask
- School-leaving and entrance papers: causes of post-harvest losses, maturity indices, and basic handling practices.
- Degree and research-fellowship entrances: respiration classes, Q10, ethylene effects, chilling injury, precooling methods, curing, and maturity standards.
- Professional examinations: designing post-harvest handling chains, loss assessment, and calculations of storage life and cooling.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do bananas go black in the refrigerator?
Bananas are chill-sensitive. Below about 12 °C their skin cells are damaged and turn brown or black, even though the flesh may still be edible.
Why should vegetables be harvested in the morning?
They are cooler and fully turgid, so they carry less field heat and lose less water before cooling.
Does washing produce make it last longer?
Only if done carefully. Clean, sanitised water removes dirt and some microbes, but free water left on produce, or contaminated wash water, can increase decay.
Sources
- Kader, A. A. (ed.) Postharvest Technology of Horticultural Crops, 3rd edition. University of California ANR Publication 3311, 2002.
- Wills, R. B. H. and Golding, J. B. Postharvest: An Introduction to the Physiology and Handling of Fruit and Vegetables, 6th edition. UNSW Press and CABI, 2016.
- FAO. The State of Food and Agriculture 2019: Moving Forward on Food Loss and Waste Reduction. FAO, Rome, 2019.
- Thompson, J. F. et al. Commercial Cooling of Fruits, Vegetables, and Flowers, revised edition. University of California ANR Publication 21567, 2008.
- Gross, K. C., Wang, C. Y. and Saltveit, M. (eds) The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. USDA Agriculture Handbook 66, 2016.
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