Postharvest physiology
Introduction
Postharvest physiology is the scientific study of the physiological processes in harvested crops, particularly fruits, vegetables, and flowers. This field encompasses the biochemical and physiological changes that occur from the time of harvest until the point of consumption. Understanding these processes is crucial for maintaining the quality, extending the shelf life, and reducing the postharvest losses of horticultural produce.
Physiological Changes After Harvest
Respiration
Respiration is a critical physiological process that continues after harvest. It involves the breakdown of carbohydrates into carbon dioxide, water, and energy. The rate of respiration is a key determinant of the shelf life of produce. High respiration rates are associated with rapid deterioration, while lower rates can extend shelf life. The respiratory quotient (RQ) is often used to measure the balance between the consumption of oxygen and the production of carbon dioxide.
Ethylene Production
Ethylene is a plant hormone that plays a significant role in the ripening and senescence of fruits and vegetables. It acts as a signaling molecule, triggering a cascade of physiological changes. Ethylene production can be influenced by various factors, including mechanical damage, temperature, and the presence of other ethylene-producing produce. Controlling ethylene levels is essential for managing the ripening process and extending shelf life.
Water Loss
Water loss is a major factor affecting the postharvest quality of produce. It leads to weight loss, shriveling, and textural changes. The rate of water loss depends on the transpiration rate, which is influenced by factors such as temperature, relative humidity, and air movement. Proper packaging and storage conditions are crucial for minimizing water loss.
Biochemical Changes
Enzymatic Activity
Enzymes play a pivotal role in the postharvest physiology of crops. They are involved in processes such as ripening, senescence, and the breakdown of cell walls. Key enzymes include polygalacturonase, which degrades pectin and softens the fruit, and lipoxygenase, which is involved in the formation of off-flavors.
Nutrient Degradation
Postharvest, the nutritional quality of produce can decline due to various biochemical changes. Vitamins, particularly vitamin C, are highly susceptible to degradation. The rate of nutrient loss is influenced by factors such as temperature, light, and oxygen exposure. Understanding these factors is essential for developing strategies to preserve the nutritional quality of produce.
Flavor and Aroma Compounds
The flavor and aroma of fruits and vegetables are determined by a complex mixture of volatile compounds. Postharvest changes in these compounds can significantly impact the sensory quality of produce. Enzymatic reactions, microbial activity, and chemical changes all contribute to the alteration of flavor and aroma profiles.
Postharvest Handling Techniques
Temperature Management
Temperature is one of the most critical factors in postharvest handling. Low temperatures slow down respiration, ethylene production, and enzymatic activity, thereby extending shelf life. However, it is essential to avoid temperatures that can cause chilling injury in sensitive crops. Optimal storage temperatures vary among different types of produce.
Controlled Atmosphere Storage
Controlled atmosphere (CA) storage involves modifying the composition of gases in the storage environment. By reducing oxygen levels and increasing carbon dioxide levels, CA storage can slow down respiration and ethylene production. This technique is particularly effective for extending the shelf life of apples, pears, and other climacteric fruits.
Modified Atmosphere Packaging
Modified atmosphere packaging (MAP) is a technique that alters the gas composition within a package to extend the shelf life of produce. This is achieved by using films with specific gas permeability properties. MAP can help reduce respiration rates, delay ripening, and minimize microbial growth.
Postharvest Diseases and Disorders
Fungal Infections
Fungal infections are a major cause of postharvest losses. Common pathogens include Botrytis cinerea, which causes gray mold, and Penicillium, responsible for blue mold. These infections can spread rapidly under favorable conditions, leading to significant spoilage. Effective control measures include proper sanitation, temperature management, and the use of fungicides.
Bacterial Infections
Bacterial infections, although less common than fungal infections, can also cause significant postharvest losses. Erwinia species, for example, cause soft rot in vegetables. Bacterial infections often result in watery, foul-smelling decay. Preventive measures include maintaining proper hygiene and using bactericides.
Physiological Disorders
Physiological disorders are non-pathogenic issues that affect the quality of produce. Examples include chilling injury, which occurs when sensitive crops are exposed to low temperatures, and sunscald, caused by excessive exposure to sunlight. Understanding the causes and prevention of these disorders is essential for maintaining postharvest quality.
Technological Advances in Postharvest Physiology
Genetic Engineering
Genetic engineering offers potential solutions for improving postharvest quality. By manipulating genes involved in ripening, ethylene production, and disease resistance, scientists aim to develop crops with extended shelf life and improved quality. For example, the development of Flavr Savr tomatoes demonstrated the potential of genetic engineering in reducing postharvest losses.
Nanotechnology
Nanotechnology is an emerging field with applications in postharvest physiology. Nanomaterials can be used for developing advanced packaging materials that enhance the shelf life of produce. Additionally, nanosensors can be employed to monitor the quality and safety of produce in real-time.
Postharvest Treatments
Various postharvest treatments are used to extend the shelf life and maintain the quality of produce. These include the use of 1-Methylcyclopropene (1-MCP) to inhibit ethylene action, hot water treatments to control pathogens, and the application of edible coatings to reduce water loss and delay ripening.
Environmental and Economic Implications
Environmental Impact
Postharvest losses have significant environmental implications. The resources used in producing and transporting crops are wasted when produce is lost postharvest. Additionally, the disposal of spoiled produce contributes to greenhouse gas emissions. Reducing postharvest losses is therefore crucial for enhancing the sustainability of the food supply chain.
Economic Impact
Postharvest losses also have substantial economic implications. They result in reduced income for farmers and increased costs for consumers. Effective postharvest management can enhance the profitability of the horticultural sector and contribute to food security by reducing losses.
Future Directions in Postharvest Physiology
Research and Development
Ongoing research in postharvest physiology aims to develop innovative solutions for extending the shelf life and maintaining the quality of produce. Areas of focus include the identification of new postharvest treatments, the development of advanced packaging materials, and the use of biotechnology to enhance postharvest traits.
Policy and Regulation
Policy and regulation play a crucial role in postharvest management. Governments and international organizations are working to develop standards and guidelines for postharvest handling, storage, and transportation. These efforts aim to reduce postharvest losses and ensure the safety and quality of produce.
Conclusion
Postharvest physiology is a vital field that addresses the challenges of maintaining the quality and extending the shelf life of horticultural produce. Through a comprehensive understanding of the physiological and biochemical changes that occur postharvest, and the development of effective handling and storage techniques, significant progress can be made in reducing postharvest losses and enhancing the sustainability of the food supply chain.