🌾 Post-Harvest Losses and Physiology
Types and causes of post-harvest losses and the main physiological processes responsible for deterioration after harvest.
A fruit or vegetable does not become inert after harvest. It remains a living biological system that continues to respire, lose water, ripen, and deteriorate. This is why post-harvest loss is not just a transport problem or storage problem. It is also a physiology problem.
What Post-Harvest Loss Means
Post-harvest loss means reduction in quantity or quality of produce between harvest and final consumption.
It may be:
- quantitative, such as weight or volume loss
- qualitative, such as loss of appearance, flavor, nutrition, safety, or market acceptability
This distinction matters because a commodity may still be physically present but economically downgraded or nutritionally damaged.
Post-harvest loss is not only visible wastage. Loss of quality, shelf life, or consumer acceptance is also a true post-harvest loss.Why Post-Harvest Losses Matter
Post-harvest losses are significant because they reduce:
- food availability
- farmer income
- market efficiency
- return on all pre-harvest investments
In practical terms, reducing post-harvest loss can function almost like increasing production, but without expanding land area.
This is why post-harvest management is a major part of food security and agribusiness planning.
Main Causes of Post-Harvest Loss
Losses occur due to multiple interacting causes.
Biological and physiological causes
These include:
- respiration
- ripening and senescence
- transpiration and moisture loss
- pathogen attack
Mechanical causes
These include:
- bruising
- compression injury
- cuts and punctures
- vibration damage during transport
Physical environmental causes
These include:
- high temperature
- low or high humidity
- poor ventilation
- unsuitable storage conditions
Socio-economic and system causes
These include:
- weak infrastructure
- absence of cold chain
- poor transport
- inadequate market linkage
- low awareness of handling standards
This shows that post-harvest loss is both a biological and a supply-chain issue.
Respiration: The Central Post-Harvest Process
Respiration is the process by which stored food reserves are broken down to release energy.
After harvest, respiration continues and causes:
- consumption of stored substrates
- release of carbon dioxide and water
- generation of heat
- faster ageing of the commodity
Why this matters:
- higher respiration means faster deterioration
- temperature strongly affects respiration rate
- cold storage works largely by slowing respiration
So respiration is one of the most important physiological concepts in post-harvest science.
Transpiration and Water Loss
Transpiration is the loss of water from harvested produce.
Its effects include:
- wilting
- shrivelling
- weight loss
- reduced freshness
- loss of crispness and firmness
This is why post-harvest systems often try to maintain:
- high relative humidity
- protective packaging
- reduced exposure to drying conditions
In many fresh commodities, even modest water loss can sharply reduce marketability.
Ethylene and Ripening Behavior
Ethylene is a plant hormone that strongly influences ripening and senescence.
Its post-harvest significance is especially important in fruits.
Climacteric fruits
These can continue ripening after harvest and often show a rise in respiration and ethylene production.
Non-climacteric fruits
These do not show the same ripening pattern and generally need more careful harvest at proper market maturity.
This difference is practically important because harvesting, storage, and marketing strategy depend on whether the commodity is climacteric or non-climacteric.
Chilling Injury and Temperature Sensitivity
Not all low temperatures are safe for all produce.
Some tropical and subtropical commodities suffer chilling injury when stored below their safe threshold, even if the temperature is above freezing.
Symptoms may include:
- pitting
- discoloration
- uneven ripening
- internal breakdown
This is important because “colder” is not always “better.” Storage temperature must match commodity physiology.
Why Physiology Guides Technology
Post-harvest technology works only when it respects the biology of the commodity.
Examples:
- cooling slows respiration
- humidity management reduces transpiration
- ethylene control extends shelf life
- correct temperature prevents chilling injury
This is why technology should be seen as an application of physiological understanding, not as a separate topic.
Why This Lesson Matters for the Rest of the Course
This opening lesson is the foundation for the whole elective because later topics such as:
- cold storage
- atmosphere modification
- packaging
- processing
- value addition
all depend on understanding why harvested produce deteriorates in the first place.
Summary Cheat Sheet
- Post-harvest loss includes both quantitative loss and qualitative loss.
- Losses matter because they reduce food supply, farmer income, and the value of production.
- Causes include physiological, mechanical, environmental, pathological, and system-level factors.
- Respiration continues after harvest and is a major driver of deterioration.
- Transpiration causes water loss, wilting, and shrivelling.
- Ethylene strongly influences ripening and senescence, especially in climacteric fruits.
- Chilling injury shows that temperature control must match commodity type.
- Post-harvest technology works best when built on correct physiological understanding.
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