Stored food, both perishable and durable, may be considered an ecosystem, meaning a system that includes a group of organisms and their environment. The interactions between the physical, chemical, and biological factors within this ecosystem lead to changes in the quality and nutritive value of the stored product.
Knowledge of these factors is essential if the quality and quantity of stored products are to be maintained. This article discusses the environmental factors that influence both the quantity and quality of a harvest. These factors include temperature, relative humidity, moisture, atmospheric composition, and light.
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Effect of Temperature on Post-Harvest Life of Crop Produce
Temperature is the most important environmental factor influencing the postharvest life of a produce. Temperature and moisture are determining factors in accelerating or delaying the complex phenomena of biochemical transformation (especially the “breathing” of the grain) that are responsible for grain degradation.
Additionally, they directly affect the speed of development of insects and microorganisms (moulds, yeasts, and bacteria), as well as the premature and unseasonal germination of grain.
The relationship between temperature and moisture content is established to determine the area of influence of significant degradation phenomena, such as the development of insects and moulds and the germination of grain.
It is observed that the higher the temperature, the lower the moisture of the grain must be to ensure good conservation of the products.
Due to their influence on the speed of development of these degradation phenomena, temperature and moisture content condition the maximal storage duration.
Temperature affects the rate of all biochemical processes and is therefore fundamentally important in any storage system. Together with moisture content, it largely determines the storage life of grain. The temperature of the stored product and the surrounding air are both significant.
Insect, mite, fungal, and mycotoxin development, germination loss, and baking qualities are affected by temperature. At typical grain storage temperatures, the biological activity of insects, mites, fungi, and grain doubles for every 10°C increase in temperature.
At low temperatures, insect breeding ceases and less moisture is available for potential pests in cold grain. Grain should therefore be cooled immediately after drying and before storage. Cooling during storage also equalizes temperature gradients and prevents moisture translocation.
Biochemical activity due to moulds, insects, and others can also generate heat during storage, as can heat transfer to or from the store fabric by radiation. Ventilation of the store, especially air movement directly through the stored product, is one of the main ways of modifying storage temperatures.
Temperature greatly affects water loss. Lower temperatures also slow pathogen development. Temperatures outside the optimal range can cause chilling, freezing, or heat injuries. The severity of these injuries depends on storage duration and temperature.
Among them, chilling injury is the most common and threatening to storage. The critical temperature for chilling injury is generally below 5–13°C, depending on the produce and maturity stage.
Symptoms vary among commodities but often include brown discoloration, necrotic pitting, and increased susceptibility to decay. Intermittent warming and the application of substances like essential oils, salicylic acid, jasmonic acid, and calcium chloride are reported to delay or prevent chilling injury.
Effect of Moisture Content on Post-Harvest Life of Crop Produce
The moisture content of stored grain depends on the relative humidity of the air. When relative humidity is below 65–70 percent, many grain-degradation phenomena slow down or are completely blocked.
In this regard, the “safeguard” moisture content is defined as the content corresponding to equilibrium with air at 65–70 percent relative humidity.
Table 4.2 presents the recommended moisture content for long-term storage of various grains in hot regions. Many stored products are hygroscopic, meaning they can absorb and release water like a sponge.
They consist of dry matter and water. Moisture content (m.c.) expresses the weight of water in a product as a proportion of its weight. The most common method of expression is the wet basis, in which the weight includes both dry matter and water.
Formulas:
m.c. (wet basis) = (weight of water in sample ÷ wet sample weight) × 100%
m.c. (dry basis) = (weight of water in sample ÷ dry sample weight) × 100%
These formulas can convert between wet basis and dry basis moisture content.
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Effect of Relative Humidity on Post-Harvest Life of Crop Produce
Relative humidity is more useful than absolute humidity for determining how living organisms or biological materials respond to surrounding air. For instance, air’s ability to dry water from crops depends on how saturated it is—its relative humidity.
In food storage, relative humidity around the product is critical in controlling spoilage by micro-organisms. Water vapour in the surrounding air interacts with the water in the stored product. Therefore, relative humidity depends on water vapour pressure, temperature, and the moisture content of the product.
Problems arise when temperatures drop at night and relative humidity increases. If the drop is significant, the air can reach 100% relative humidity, and water begins to condense. This should be avoided in contact with stored commodities.
Storage buildings may also have issues. A warehouse roof may become cold at night while warm, moist air from stored produce, such as a grain stack, rises. The air cools on contact with the cold roof and reaches saturation.
Condensation then occurs on the underside, dripping onto the grain stack and causing mould-related spoilage. A similar issue can occur when goods are shipped from tropical to temperate regions in steel containers.
While the containers remain warm in the ship’s hold, they cool rapidly in cold weather upon unloading. Warm moist air from the produce condenses on container surfaces, causing “internal raining,” which damages the commodity.
Since fruits and vegetables contain 80–95% water by weight, favorable environmental conditions are essential to reduce transpiration. Higher relative humidity (85–95%) slows water loss. However, it can also stimulate pathogen growth and weaken packaging materials, such as cardboard boxes.
Effect of Atmospheric Composition on Post-Harvest Life of Crop Produce
Grains, microorganisms, and insects are living organisms requiring oxygen. Storing grain in low-oxygen environments causes insect death, halts microbial activity, and slows or stops biochemical grain degradation. This supports grain conservation but may reduce germination potential.
Regulating gas concentrations around stored produce helps reduce respiration and extend shelf life. Lowering O₂ and increasing CO₂ levels can slow the deterioration of fresh horticultural crops.
However, the outcome depends on the commodity type, cultivar, maturity, and temperature. Modified atmosphere packaging (MAP) is a helpful system that adjusts the atmospheric composition within the packaging headspace.
During respiration, O₂ is consumed, and CO₂, ethylene, and water vapour are produced. MAP materials allow these gases to transfer at rates that maintain favorable internal conditions, thereby preserving the produce.
MAP slows respiration and other metabolic processes, reduces ethylene sensitivity, reduces some physiological disorders (such as chilling injury), and may inhibit pathogen development.
Effect of Light on Post-Harvest Life of Crop Produce
Light can also trigger abnormal changes in product quality and influence some biological processes. For example, light exposure in potatoes leads to chlorophyll production, which results in greening and solanine formation a compound known to be toxic to humans.
Frequently Asked Questions
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