Advanced postharvest training for horticultural crops


This set of online training modules is designed to support smallholder and subsistence farmers with postharvest operations intended for export sales, as well as university students, by supplying advanced postharvest concepts that will enable them to transition into larger markets and meet higher quality standards for export.

Part of a three-level training series, this advanced level is aimed at farmers with a greater background or understanding of postharvest handling practices required for selling horticultural crops in large or export markets. This set of lessons provides rigorous concepts of topics including postharvest basics, sanitation, food safety, packaging and transportation.

These lessons were originally created for farmers and audiences in Latin America, but are widely applicable to small-scale farmers elsewhere in the world. 

Users can control the progression of each narrated training topic by starting and stopping lessons as well as switching between topics. Notes are also made available for each topic that can be accessed at the top control bar of the video player. 

Postharvest basics

This 17-minute postharvest basics lesson looks at more detailed compositional and physiological changes that occur during postharvest handling of fruits and vegetables.

Ethylene plays an important role in the ripening of fruits and vegetables and can be controlled to promote more uniform ripening. This lesson provides an example of the natural ethylene production of bananas and their ripening process being less uniform over time compared to bananas that are treated with external ethylene providing uniform ripening over less time. 

The composition of horticultural crops can be impacted by three main factors including the cultivars used, pre-harvest environmental factors, and postharvest treatments. Pre-harvest environmental factors impacting compositional changes of fruits and vegetables include climate, culture, and harvesting stage. Postharvest treatment factors are environmental, handling methods, and the time between harvest and consumption.

Transpiration and water loss are common in fresh fruits and vegetables as some contain up to 90% water. During postharvest, fruits and vegetables begin to lose water content which affects quality, appearance, texture, and nutritional value. Examples of percent water loss and their potential affects are provided to show how changes develop as more water is lost. A 0.5% water loss may show some increased activity of cell wall enzymes while a 6% water loss will result in a reduction of textural quality such as softening, limpness, or a loss of crispness or juiciness. Percent water losses that are unmarketable are also provided for various fruits and vegetables. 

Luis Cisneros, of Texas A&M University, is the presenter of this lesson.

Grading and sorting

This 6-minute grading and sorting lesson addresses the process and conditions needed for appropriate classification and selection of fruits and vegetables destined for export markets. Classification and selection should be conducted under a covered or shaded structure that is clean. Pack houses are small sheds or larger warehouses that can be used to provide ample space and clean conditions for sorting products. Workers should maintain sanitary handling practices such as the use of hairnets when sorting through products. Large tables or structures raised off of the ground will allow high visibility of large quantities of products that can make it easier to identify damaged products or those that may not meet export market standards. 

Ivanna Vejarano, of the Panamerican Agricultural School, Zamorano, is the presenter of this lesson.

Quality measurement procedures

This 11-minute quality measurement procedures lesson provides an overview of more complex methods for measuring visual appearance such as color, shape, texture as well as sensory factors such as smell and taste. Several types of measurements or evaluations can be conducted to determine the quality of products. For visual appearance, color scales exists to help compare a farmer's products to market standards. These scales focus on uniformity of color and intensity. Other measurements of color use colorimeters which measure the reflection of light on the surface of the product. 

Other methods for measuring color can include liquid chromatography which evaluate the pigments within fruits and vegetables. Extraction of products are mixed with solvents to be injected into the equipment for analysis. The resulting data gives a more specific value of the pigments as well as antioxidant capacities. Other measurements for taste include analyzing the pH and titratable acidity helping to understand the sourness of fruits and vegetables which can better inform stages of maturity for maximum quality. For aromas, gas chromatography can be used to measure the characteristic smell of horticultural crops. 

Ana Silvia Colmenares, of Universidad del Valle Guatemala, is the presenter of this lesson.  

Curing underground storage organs

This 14-minute curing bulbs, roots, and tubers lesson take a deeper look at principles and practices of curing. In roots and tubers, one process that takes place as a result of wounding crops due to harvesting is suberization. Corky tissue develops at the site of the wound during curing, preventing water loss and sights for bacterial growth. This process is similar to a scab forming on human tissue that will help to maintain the quality of crops during curing.

More advanced structures that can be used for curing are controlled environments. These are more sophisticated indoor rooms or warehouses for curing that provide greater ability to control temperature and airflow. Fans, heaters, and vents are used to establish uniform and highly efficient curing in less time. Bins or plastic crates can be used for roots and tubers to increase the amount of air flow and warm temperature over the crops. Bins can be stacked allowing more surface area to be exposed for contact with air flow. 

Lisa Kitinoja, of the Postharvest Education Foundation, is the presenter of this lesson.

Water sanitation and food safety

This 25-minute water sanitation and food safety lesson presents various types of water uses during postharvest and their impacts on contamination and food safety. Most water for agriculture use comes from rain, underground, or surface sources. Underground water sources that are deep will have less contact with pollutants and need to be adequately designed. Surface water that is moving may be less susceptible to contaminants, yet still water will have a higher likelihood to accumulate more contaminants and pollutants. The main types of irrigation are drip, furrow, and spray and each of these experience different water contact with plants. Drip irrigation has little to no direct contact with plants. Furrow irrigation is based on the width between planting rows where irrigation occurs and also has little contact. Spray irrigation has the greatest contact with plants which cannot be avoided. Overhead irrigation, if not done properly, can also impact nearby fields due to runoff. While many irrigation systems are filtered, they do not catch microbial pathogens.

Many fruits and vegetables undergo some form of washing during postharvest. Typically this will include spraying or tank washing. This lesson provides a comparison of the coliforms present when using each method, with spray washing having a significantly lower count. Water chlorination is used to help disinfect the water during washing. The most commonly used form is sodium hypochlorite, commonly referred to as bleach. While low cost, sodium hypochlorite is highly reactive and its efficacy depends on the pH levels. Constant monitoring is also needed when considering the use of chlorination to treat water for washing.

Alejandro Castillo, of Texas A&M University, is the presenter of this lesson.

Packaging materials for fresh produce

This 21-minute packaging for fresh produce lesson surveys the numerous types of packaging materials used for fruits and vegetables and their respective strengths and weaknesses. Common materials used for horticultural crops include wood, plastic, cloth, and other natural and synthetic materials. Wood and cloth, while low cost and readily available, can damage produce and reduce the overall quality during storage and transportation. Natural materials also lack uniformity and strength to maintain the quality of fruits and vegetables. Plastic is commonly used for its strength, uniformity, smooth surface, and the ability to wash for reuse. 

Another technique that is being used to package fresh produce is Modified Atmoshphere Packaging (MAP). MAP changes the air composition around the product with the goal of promoting its shelf life. The interaction between the packaging material and the produce is important as fruits and vegetables continue to respire after harvest. If packaging for fresh produce has a certain level of permeability in which it balances the respiration of the produce with the surrounding atmoshphere, an equilibrium is established that can help maintain the product's shelf life. Carbon dioxide and oxygen are the key components that are balanced using MAP using film permeability. Two kinds of MAP are passive and active. Passive MAP refers to the equal exchange of gases entering and exiting the package. Active MAP refers to removing atmospheric gases from the packaging using a vacuum and replacing it with a desired mixture.  This lesson provides a series of models and pictures to illustrate the process of MAP and its benefits for fruits and vegetables.

Eleni Pliakoni, of Kansas State University, is the presenter of this lesson. 

Cooling and temperature management

This 34-minute cooling and temperature management lesson provides advanced principles of maintaining temperature of horticultural crops through different technologies and methods. Temperature can have a significant affect on the quality of fruits and vegetables by altering its visual appearance, texture, and taste. Simple measures can be implemented from the point of harvesting to maintain temperature control by harvesting early in the day and using shaded structures on the farm during harvesting to prevent sun and heat exposure.

In addition to evaporative cooling and hydrocooling, rapid cooling or precooling, is a method that can be employed from the point of harvest to extend quality. In order to exercise precooling, it is important to understand the minimum temperature a crop can be cooled to as well as the appropriate method to do so. Rapid cooling brings the pulp temperature of a fruit or vegetable down to its recommended lowest temperature in a short period of time. This has proven to extend shelf life compared to longer lag times between cooling methods during postharvest. The 7/8 cooling principle is used in which 7/8 of the field heat of a product is removed during rapid cooling leaving the remainder to cool during storage. Cooling efficiency can be calculated based on the time (length of exposure to a cooling medium), temperature of cooling medium, and turbulence (contact and mixing over crop).

Another cooling medium that can be used is ice. Ice, made into a very fine state, can be mixed with water and packed with products that can withstand significant contact with water. Water and ice must remain sanitary if used to keep fruits and vegetables cold. Slurry or slush ice, made from mixing fine ice and water, can be pumped into individual containers. Containers with good drainage are used for excess water to drain out. One drawback of using ice is that certain vegetables like leafy greens may damage the stems and other sensitive leaves as ice is forcefully injected. Shipping companies also experience safety hazards and issues with water continuing to drain out of packages. 

Vaccum cooling is another mechanism that is extremely effective but of higher cost. Vacuum cooling requires a strong tank or cylinder that can withstand high pressures. Crops are placed on pallets and wet with water, and then sealed in a chamber. A vacuum pump will then remove air from the chamber, lowering the internal temperature which alters the boiling point of water. This process is best suited for leafy greens.

Regardless of the cooling method used, it is important maintain ambient pulp temperature of products from the point of harvest. Reducing the time between harvest and cooling is critical to preserving quality. Utilizing the appropriate method for cooling based on the crop and available resources will ensure minimal damage. In addition, maintaining cold chain throughout all points of postharvest handling until products get to the consumer will preserve freshness and quality resulting in better products for consumers and reduce losses. 

Steve Sargent, of the University of Florida, is the presenter of this lesson.

Storage practices for fresh horticultural crops

This 18-minute storage practices lesson explores the temperature requirements for specifics types of horticultural crops as well as the recommended storage structures that can be used for each. Different fruits and vegetables have optimal temperatures they can be cooled to as well as relative humidity. Beets, broccoli, carrots, and cabbage can be stored between zero and two degrees celsius using a cold room as the appropriate cooling mechanism. Onions and garlic are best cooled between 65-70% relative humidity using ventilated storage units. Avocados, bananas, guava, citrus, and tomatoes on the other hand, should be cooled between 13-15 degrees celsius using a zero energy cool chamber or charcoal cool room. 

The time that crops can remain in cold storage is also critical to determine the point at which deterioration and damage may occur. A scale is presented from very highly perishable to very low perishability with corresponding crops and times for storage. Very highly perishable products can be stored for up to two weeks, such as broccoli, mushrooms, peas, and ripe tomatoes. Very low perishability crops can be stored for 16 weeks or more including tree nuts and dried fruits.

Some key tips for maintaining adequate storage practices for horticultural crops include maintaining a high relative humidity. This will help reduce the rate of water loss and reduce perishability. Relative humidity will often need to be 95% or higher. Relative humidity gauges or monitoring devices can be used to frequently test storage conditions. Misting products can also help to maintain a high level of relative humidity. during storage, it is important to conduct routine checks to inspect the crop conditions, determine if structure maintenance is needed, as well as ensure no pests are present. 

Lisa Kitinoja, of the Postharvest Education Foundation, is the presenter of this lesson. 

Transportation of fresh horticultural crops

This 9-minute transportation of fresh fruits and vegetables lesson takes a deeper look at how to determine the best mode of transportation for horticultural crops and the importance of maintaining the cold chain throughout each one. Determining which method of transportation is best is dependent on the perishability of the crops and the distance they need to travel. In almost all forms of transportation, refrigeration is a critical component which allows the maintenance of the cold chain, keeping fruits and vegetables at cool temperatures through postharvest handling. 

Precooling is an important factor in maintaining the cold chain. Refrigerated trailers and marine coolers should be cooled to the desired temperatures prior to loading horticultural crops. Only fruits and vegetables that have been stored at cool temperatures should be loaded as refrigerated units for transportation are not built to load warm products. Refrigerated units circulate air differently to cool crops. Top-air trailers move air down the length of the cargo and must be stored properly as well as have vents to allow the air to pass through. Bottom-air delivery, common in marine containers, moves air from the refrigeration unit in the front, under the crops, and moves the air up above through the load to be circulated back through the unit in the front. 

Due to the various types of air delivery systems, cartons need to be designed and selected for the appropriate transportation method. Cartons for top-air delivery systems need to have vents on the sides allowing air to move along the length of the load. For bottom-air systems, vents need to be constructed on the top and bottom of the cartons allowing air that comes from the bottom of the trailer to move up through the products to the top to be recirculated. Temperature recording should be conducted during transportation. Ideally, this will include three monitors from the front, center, and back of the trailer. Once vehicles arrive at their destination, it is important that the cold chain be maintained through unloading using shaded or refrigerated locations. This will help to maintain consistent temperatures and the quality of the products all the way to the consumers. 

Jeffrey Brecht, of the University of Florida, is the presenter of this lesson.

Water loss

This 17-minute water loss lesson covers an overview of the Psychrometric Chart. This chart includes useful information such as relative humidity and water vapor pressure to better illustrate how water moves from one environment to another and how fast water loss may occur under different environments. This chart helps producers and students determine the relative humidity of environments in which their horticultural crops are being stored. Because evaporation impacts relative humidity, it is important to understand the rate of water diffusion and how vapor pressure impacts that rate. Water moves from areas of higher vapor pressure to lower vapor pressure and the cooler produce can be stored, the less water loss will occur. 

Mark Ritenour, of the University of Florida, is the presenter of this lesson.


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