Introduction

Onions (Allium cepa L.), a staple in global cuisine, are cultivated extensively and hold significant economic importance. However, the post-harvest phase poses challenges, with substantial losses due to physiological degradation, sprouting, and rotting. Understanding the interplay of storage methods, environmental factors, and physiological changes is key to reducing these losses. This blog synthesizes insights from multiple studies to provide a comprehensive overview of onion storage dynamics.

Physiological Changes During Storage

Onions undergo a variety of biochemical and physical changes during storage. These include:

• Moisture Loss:Moisture loss is a significant factor leading to weight reduction in stored onions. It is primarily caused by high temperatures and low humidity, which accelerate dehydration and transpiration. Traditional storage methods often exhibit the highest rates of moisture loss due to inadequate environmental controls. Moisture loss can also result from rotting and sprouting, further reducing marketable yield.
• Sprouting and Dormancy: Onions naturally enter a dormancy phase post-harvest, which varies in length depending on the cultivar, storage conditions, and pre-harvest practices. High temperatures initially suppress sprouting but contribute to other forms of deterioration. As dormancy ends, sprouting emerges as a critical challenge, reducing both the weight and quality of onions. Sprouting is particularly pronounced in poorly ventilated storage systems.
• Respiration: Increased metabolic activity during storage, especially when dormancy is broken, leads to carbohydrate catabolism. This heightened respiration generates heat, further accelerating deterioration. Proper ventilation can mitigate the effects of respiratory heat, but inadequate systems often exacerbate the problem.
• Chemical Composition: Chemical changes during storage include variations in dry matter content, fructans concentration, and pyruvic acid levels. High fructans levels correlate with improved storability, while pyruvic acid content influences onion pungency and flavor. These changes significantly impact consumer preferences and market value.

Optimal Storage Conditions

• Temperature: Onions store best at temperatures between 25°C and 30°C, with relative humidity levels maintained between 65% and 70%. These conditions minimize physiological losses, including moisture loss and rotting. In regions with tropical climates, achieving these parameters can be challenging without advanced infrastructure.
• Humidity: While higher humidity reduces moisture loss, it also creates conditions favorable for fungal growth. Striking a balance is critical to minimizing losses. Controlled humidity systems help maintain this equilibrium effectively.
• Ventilation: Proper air circulation is essential to regulating temperature and humidity within storage facilities. Ventilation systems, whether forced or natural, ensure uniform conditions across the storage area. Forced ventilation, in particular, has been shown to significantly reduce sprouting and rotting, preserving overall quality.
• Cold Storage: Although cold storage is effective for long-term preservation, its adoption is limited in many regions due to high costs and infrastructure requirements. Nonetheless, it is widely used in developed countries to maintain a steady supply of onions year-round.

Comparative Analysis of Storage Systems

1. Forced Ventilation:
   • Provides the most consistent environmental conditions.
   • Maintains the lowest sprouting percentage and highest marketability rates after extended storage periods.
   • Ideal for commercial operations where quality and volume are critical.
2. Natural Ventilation:
   • A cost-effective solution that combines simplicity with reasonable efficiency.
   • Suitable for small-scale farmers and local markets.
   • Requires strategic design to ensure uniform air circulation throughout the storage area.
3. Traditional Methods:
   • Commonly used in rural and resource-limited settings.
   • Characterized by high losses due to poor control over temperature and humidity.
   • Marketability rates drop significantly over time, highlighting the need for improvement.

Strategies for Enhancing Storability

Several practical measures can be employed to extend onion shelf life and reduce post-harvest losses:

• Curing:Proper curing involves drying harvested onions under controlled conditions to remove excess moisture from the outer layers. This process strengthens the protective skin, reducing susceptibility to pathogens. Curing is particularly effective in improving storage outcomes when conducted immediately after harvest.
• Growth Regulators:The application of chemicals such as Maleic Hydrazide (MH) can delay sprouting by inhibiting metabolic activity. These regulators are especially useful in regions where high ambient temperatures shorten storage life.
• Cultivar Selection: Selecting onion varieties with inherent resistance to sprouting and rotting can significantly enhance storability. Varieties with high dry matter content and fructans levels are particularly well-suited for long-term storage.
• Controlled Atmosphere Storage:Advanced storage systems that regulate oxygen and carbon dioxide levels help suppress metabolic activity, extending the shelf life of onions. While effective, these systems are often cost-prohibitive for small-scale farmers.

Advanced Research Insights

Recent research has uncovered valuable insights into the storage dynamics of onions:

• Temperature-Humidity Interplay:Studies highlight the importance of maintaining stable temperature and humidity levels to minimize physiological losses. Fluctuations can accelerate deterioration and reduce overall yield.
• Biochemical Changes:The concentration of fructans and other carbohydrates within the onion bulb influences its storability and flavor profile. High-fructan varieties demonstrate better resilience under extended storage conditions.
• Antioxidant Properties: Red onions, rich in phenolics and flavonoids, exhibit enhanced antioxidant activity. These compounds contribute not only to health benefits but also to improved storage performance.

Challenges and Opportunities

While significant progress has been made in understanding onion storage, challenges persist:

• Economic Constraints:The high costs associated with advanced storage systems limit their accessibility, particularly in developing regions. Finding cost-effective solutions is crucial to addressing this barrier.
• Infrastructure Gaps: Many rural areas lack the necessary facilities to implement advanced storage techniques. Investments in infrastructure development can bridge this gap, enabling broader adoption of improved methods.
• Farmer Awareness:Educating farmers about best practices in onion storage and providing access to affordable technologies can have a transformative impact on reducing post-harvest losses.

Conclusion

Onions are a vital agricultural commodity, and optimizing their storage conditions can significantly enhance marketability and reduce waste. While forced and natural ventilation systems offer promising solutions, integrating advanced technologies with traditional practices can create scalable, cost-effective storage solutions. Collaborative efforts among researchers, policymakers, and farmers are essential to ensuring year-round onion availability and price stability.

By leveraging insights from research and practical innovations, the agricultural community can transform onion storage into a sustainable and economically viable endeavor.

References

1. Hatem, M. H., Shehata, S. A., AbdEl-hay, Y. B., Karima, F., & AbdEl-Gwad, B. A. (2014). Effect of Storage Conditions on the Quality of Onion Bulbs. Misr J. Ag. Eng.
2. Shivakumar & Chandrashekar, S. Y. (2014). Physio-Chemical Changes During Post Harvest Handling of Onion. Agricultural Review.
3. Ogata, K. (2009). Physiological Studies on the Storage of Onion Bulbs.