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Ministry of Agriculture, Food and Rural Affairs

Postharvest Handling and Storage

For many specialty crops, minimal information is available on the specific conditions that maximize storage-life. In some cases, the storage recommendations of a closely related traditional crop provide a good starting point. For other crops, growers will have to follow general principles of postharvest handling and storage, and experiment with different storage conditions over time.

It is impossible to generalize storage principles for all specialty crops because of the diversity of specialty crops available to be grown in Ontario. However, for most crops there are two main types: 1) those marketed dry or with low moisture levels and stored at low humidity to reduce rots, or 2) those marketed fresh with high moisture levels and that require high humidity in storage to reduce moisture loss. Crops that are often stored under dry conditions include specialty grains, oilseeds, nuts and many medicinal herbs. While storing these crops at low temperatures will maintain quality longer, this is not always required. Most specialty vegetables, culinary herbs, and fruits are marketed fresh and require high humidity to prevent wilting and prolong shelf-life. Because high humidity promotes fungal growth, these crops are often stored at very low temperatures, often just above freezing, to prevent rots. However, storing certain crops at low temperatures can result in chilling injury. This is especially true of crops of tropical or sub-tropical origin. Examples of this include sweet potatoes, basil, tomatoes, and many cucurbits. The information below provides general principles of postharvest handling and storage for the two main types of specialty crops. Consult the individual crop profiles for more specific information on each crop.

Postharvest Handling and Storage of Fresh Produce

Cooling
Rapid and efficient cooling is the most effective management practice for maintaining the quality and shelf-life of fresh produce. It is essential that only good quality produce is placed in cold storage. There are several cooling methods available for different sizes of operations and types of crops being grown. Table 1,Suggested Cooling Methods for Traditional Crops, lists the suggested cooling methods for many traditional vegetable crops. These will provide a reasonable guideline for closely related specialty crops. Cooling costs can be minimized by harvesting produce during cooler parts of the day.

Room Cooling
Containers of produce are placed into a refrigerated room and cold air from the evaporator coils slowly cools the product. It takes considerable time to cool produce to acceptable storage temperatures. The cold air does not always penetrate deep within the container. Highly perishable produce may suffer significant quality loss and hence shelf-life.

Forced-Air Cooling
Produce is placed in a refrigerated room, and cold air is pulled through the containers using high-capacity fans. An effectively designed system allows good contact between the cold air and the produce. Forced-air cooling is not as rapid as hydrocooling. However, it is adaptable to more types of produce and more flexible for smaller-scale operations. Forced-air cooling should be done quickly so produce does not lose too much moisture.

Hydrocooling
Cold water is an effective method for quickly cooling a wide range of produce in containers or bulk bins. Hydrocoolers use either an immersion or a shower system to bring products in contact with the chilled water. Hydrocooling avoids product water loss and may even add water to a slightly wilted commodity.

It is critical that water quality be maintained to reduce the spread of microorganisms, especially when hydrocooling water is recycled through the system multiple times. Use a water sanitation system such as chlorine, ozone, ultraviolet light or hydrogen peroxide to maintain appropriate water quality.

Package Icing
Some vegetables and herbs are cooled by filling the packed containers with crushed or flaked ice. Initially, the direct contact between the ice and product causes fast cooling. However, as the ice in contact with the product melts, the cooling rate slows considerably. Package icing is less efficient than forced air or hydrocooling and is not suited to many types of produce. Using liquid-ice, a slurry of ice and water, distributes ice throughout the container, achieving better contact with the product.

Table 1. Suggested Cooling Methods for Traditional Crops

Vegetables RC FA HC PI VC
Asparagus    
Beans      
Broccoli    
Brussels sprouts    
Cabbage      
Carrots (with tops)    
Carrots (without tops)    
Cauliflower      
Celery    
Cucumber    
Eggplant      
Garlic        
Herbs        
Leafy greens    
Muskmelon    
Onion (dry)      
Onion (green)    
Peas   `  
Pepper    
Potatoes        
Squash (summer)      
Squash (winter)        
Sweet corn  
Sweet potato      
Tomato      
Watermelon      
Fruit RC FA HC PI VC
Apples    
Berries      
RC - Room cooling
FA - Forced-air cooling
HC - Hydrocooling
PI - Package icing
VC - Vacuum cooling

Vacuum Cooling
This method involves water evaporation at very low atmospheric pressure. Vegetables and herbs that have a large surface area-to-volume ratio, such as lettuce and leafy greens, are best suited for vacuum cooling. There is approximately 1% product weight loss (mostly water) for every 6˚C of cooling, and thus some coolers are equipped with a water spray system that adds water to the product surface during cooling. Capital equipment costs are high, so vacuum cooling is suited primarily to large operations.

Storage
Rapid, uniform cooling to remove field heat, followed by storing under proper conditions will ensure that a quality product reaches the consumer or processor.

See the individual crop profiles for recommended storage conditions for individual crops.

Storage Tips
Keep several thermometers in the storage area to help ensure the area is being uniformly cooled. Calibrate thermometers regularly to ensure that fresh produce is stored properly. Thermometers set into ice water should read 0˚C. Change the wash water regularly when washing fresh produce. This prevents the build-up of bacteria and other pathogens. Use chlorine at 150-200 ppm to eliminate rot-organisms from the surface of the product. Good storage conditions will maintain, not improve, the quality of produce in storage. Store only good quality produce.

Ethylene
Ethylene is naturally produced by most fruit and vegetables. It acts as a growth regulator and a ripening agent. Ethephon is a commercially available, synthetically produced ethylene product. It is widely used in the processing tomato industry.

Ethylene gas increases the rate of ripening of mature fruits and vegetables, and shortens shelf-life. The effects of ethylene gas depend upon:

  • the stage of produce maturity
  • the temperature and relative humidity of the storage room
  • the concentration of ethylene
  • the exposure time

Ethylene is an invisible and odourless gas, however, its detrimental effects on produce can be easily misdiagnosed. Some of the undesirable effects of ethylene are:

  • loss of green colour in some immature fruits (e.g. cucumbers and summer squash) and leafy vegetables
  • accelerated ripening of fruits (e.g. tomatoes) during handling and storage
  • russet spotting on lettuce
  • bitterness (isocoumarin) in carrots
  • sprouting of potatoes
  • abscission of leaves (e.g. cauliflower, cabbage)
  • toughening of asparagus

The specific effects of ethylene on specialty crops are poorly understood. However, unless ripening is desired, minimizing the effects of ethylene will always be beneficial.

There are many sources of ethylene in the environment. The more important ones include internal combustion engines and ripening produce. Other sources include fluorescent ballasts, decomposing produce, cigarette smoke and rubber materials exposed to heat or UV light.

Use electric rather than gas-powered fork lifts where ethylene-sensitive commodities are present. Isolate internal combustion engines from handling and storage areas. Do not let engines idle in an enclosed loading space.

Simple ventilation of handling areas can prevent ethylene build-up, as long as the incoming air is not polluted. One complete air exchange per hour is usually sufficient.

Ethylene can be used to your advantage when controlled and careful ripening of fruits and vegetables is desired. In this case, commercially available ethylene generators are used.

For more information, see the OMAFRA Factsheets:

Table 2. Chilling Injury-Sensitive Vegetable Crops

Commodity

Min. Storage
Temperature
˚C

Chilling Injury Symptoms

Asparagus 0-2 darkened, water-soaked tips leading to bacterial soft rot
Basil 7-10 wilting, water-soaked appearance
Beans (snap) 7-10 russetting, pitting, corky centres
Cantaloupe 2-5 pitting and surface decay
Cucumber 7-10 pitting and surface decay
Eggplant 7-13 flesh browning, pitting, decay of cap, fruit rot
Okra 7-10 pitting, discolouration, decay
Pepper 7-13 water-soaked appearance, darkening, fruit rot
Potato 2-10 browning, sweetening
Pumpkin 10-15 pitting, rot
Sweet potato 10 flesh discolouration, off-flavours, hard core, increased decay
Tomato (ripe) 7-13 rubbery texture, watery flesh, seed browning, decay
Tomato
(mature green)
13 irregular ripening, poor texture
Watermelon 10-15 surface pitting, loss of flavour, fading of red colour
Winter squash 10-15 pitting, poor flavour, increased rot
Zucchini
(summer squash)
5-10 surface pitting, rapid decay

Chilling Injury
Chilling injury occurs when a commodity is exposed to temperatures below its ideal storage range. The minimum storage temperature for each crop varies, as do the symptoms of chilling injury.

Chilling injury is not always easily recognized at the farm gate. Chilled produce often looks fine as it is removed from storage. However, as the temperature increases (in the grocery store or after purchase), symptoms begin to develop, and quality is quickly compromised.

Chilling injury may occur at any stage of the production and distribution line. The longer the chilling, the more significant the level of injury. The effects of multiple, short periods of chilling may be cumulative in some commodities. Table 2 provides a summary of the effects of chilling injury on several vegetable and specialty crops. These effects will likely be similar for closely related specialty crops. Some fruit crops can also be affected by chilling injury. Consult the crop profiles for specific storage requirements.

Commodity Compatibility
Storage interactions occur between certain crops even if they have the same environmental requirements. Some crops give off volatile substances that interfere with the successful storage of others. For example, apples, pears, peaches, plums, apricots and tomatoes give off ethylene gas which, even in low concentrations, can cause undesirable effects in many specialty crops (see Ethylene,). Potatoes may impart an earthy flavour to fruit, particularly at high temperatures. Do not store incompatible commodities in the same building unless special ventilation arrangements have been made.

Washwater Sanitation
Water is needed in many vegetable-packing systems to move, clean or cool produce. Inadequate sanitation of this water can lead to higher levels of postharvest decay and food safety concerns. Chlorine is an effective sanitizer when used as outlined below:

  • Start with drinking water quality.
  • Maintain free chlorine concentration between 100 and 150 ppm.
  • Maintain washwater solution pH between 6.5 and 7.5.
  • Change water daily or when it becomes turbid or cloudy (organic matter deactivates chlorine).
  • Monitor free chlorine levels and pH constantly (minimum every hour) and replenish as required.

Sodium hypochlorite, calcium hypochlorite and chlorine gas can be used in washwater sanitation. Note that chlorine gas decreases the water pH while sodium hypochlorite and calcium hypochlorite increase water pH. If the water pH requires adjustment, this can be accomplished with muriatic acid (HCl) or citric acid.

Ozone, ultra-violet light and hydrogen peroxide are also effective methods of washwater sanitation. For more information on these methods, see the OMAFRA website.

Storage Sanitation
Cleaning and sanitation in horticultural storage and packing areas are/is extremely important. Crop residue left from season to season is a prime source of the pathogens that cause postharvest rots. Dried bacterial slime and fungal spores will survive on the walls and other surfaces for years. Consider every part of the storage system, including the floors, walls, ceilings, doors, refrigeration system, electrical conduits, piping, containers, pallets and machinery. A good sanitation program involves four steps: cleaning, disinfecting, rinsing and drying.

Cleaning
Remove debris, such as soil, wood, stones and metal, by brushing, vacuuming, scraping and high-pressure washing. Use a high-pressure washer with hot water and non-foaming industrial detergent. Water should be at least 38˚C. Cleaning compounds such as detergents reduce the surface tension of water so that dirt and debris may be loosened and flushed away.

Disinfecting
Disinfecting follows the cleaning step. Dirt and residue will prevent the disinfectant from coming into contact with all surfaces. Many common disinfectants are deactivated by organic matter.

There are many products and procedures for sanitizing storages and equipment. Always refer to the product label for specific rates, exposure time and other environmental restrictions. Exposure time is generally 10 minutes, however this may vary with the chosen disinfectant.

Wear protective clothing (waterproof garment, safety glasses, rubber boots and gloves) during disinfecting and check the product label for specific safety instructions.

Rinsing and Drying
Rinse from the top down with clean (preferably drinking-quality) water. This will remove chemical residues and protect equipment from corrosion. Leave the storage area open to air dry. If possible, force outside air into the area. This will help remove unpleasant odours that may be transferred to the produce next season. Ensure no stagnant water remains in the storage area. Lingering humidity may encourage the development of mould and rot organisms.

Postharvest Handling and Storage of Dry Produce

The goal for most specialty grains, oilseeds, and nuts is to have low moisture levels to prevent decay and prolong shelf-life. If these are not sufficiently dry in the field, postharvest drying may be required. The target moisture levels for specific specialty crops are not always available. If available, these are included in the crop profiles. There are many different systems for postharvest drying of grains. For more information on these systems, consult OMAFRA Publication 811 Agronomy Guide for Field Crops.

Some specialty crops, such as medicinal herbs and aromatic plants, are typically dried to less than 10% moisture after harvest. This results in a stable product that can be stored for extended periods with minimal loss of quality. For some of these crops, drying costs can be minimized by allowing the plant material to lay out in the field for a few days prior to collection. For other crops, this can reduce quality and should be avoided. Drying temperatures vary depending on the crop. To preserve quality, most medicinal crops are dried at relatively low temperatures.

All of these crops will require storage at low relative humidity to avoid re-hydration of the product and the promotion of fungal growth. However, storage temperature is not as important for these crops. While storage life can be extended through low temperature storage, many will store well at room temperature. For some dried products, freezing will further extend storage life. The storage temperature will depend on the financial benefits of extending storage life compared to the costs of cooling or heating.

Seabuckthorn milling

Postharvest processing of haskaps

Sweet potato storage