Alfred B. Wagner, Frank J. Dainello, and Jerry M. Parsons
Quality in produce can be defined as the composite of characteristics that differentiates individual items within a commodity and have a significant influence in determining the level of acceptance by the consumer. High quality vegetables are one of today’s wonders with regard to the U.S. food supply. Locally produced vegetables, readily available seasonally have resulted in consumers demanding the same freshness and quality on a year round basis. Unfortunately, many Texas vegetable growers believe that once a high quality product is produced, their problems are over. In reality, their troubles could be just beginning. It has been estimated that more than 40% of perishable commodities are lost after production.
Fresh vegetables are extremely perishable and have relatively short shelf lives. They are living, respiring tissues that start senescing immediately at harvest. Freshly harvested vegetables are mostly comprised of water with most having 90 to 95% moisture content. Water loss after harvest is one of the most serious postharvest conditions. Consequently, special effort is required to reduce the effects of these naturally occurring processes if quality harvested in the field will be the same at the consumer level.
Special skills are required for proper harvesting, handling, grading and packaging of vegetables in order to insure optimum produce quality at the marketplace. It makes little difference what the quality is at harvest if it is reduced by poor handling, packaging or storage conditions. Price received for produce is determined by quality at the marketplace. Variables consumers perceive as a reflection of produce quality are ranked in order of preference as follows: crispness and freshness, taste, appearance and condition, nutritive value, and price. Studies have shown that two factors normally enter into consumers purchase decisions: competition between like items on the display shelf, and, the acceptability of the item in reference to his or her standard for that item in reference to the above variables. Consequently, producer who are able to produce and package their produce in such a way to enhance these variables are the most successful in the market place.
Because of the perishable nature of vegetables, harvesting and handling speed is of utmost importance as soon as harvest maturity has occurred. Every producer should have products reach the end consumer as quickly as possible. Unfortunately, they have no control once the produce leaves their farm or packing sheds. However, maximum speed and efficiency in handling produce on the farm or at the shed will help maintain quality. Consequently, a grower must be prepared to operate in advance of the actual harvest operation. Preharvest preparation should include lining up sufficient labor, supplies (containers and packaging items), cleaning the grading/ packing shed, and determining if all equipment is operable. Once the produce reaches harvest maturity, delays for any reason can result in major quality and crop losses. In addition, the nutritive content of produce is not static either. Biosynthetic and degradation reactions will even continue to occur during handling and storage.
Techniques to extend postharvest shelf life:
- reducing respiration by lowering temperature
- slow respiration by maintaining optimal gaseous environment
- slow water loss by maintaining optimal relative humidity
- variety selection
Postharvest handling includes all steps involved in moving a commodity from the producer to the consumer including harvesting, handling, cooling, curing, ripening, packing, packaging, storing, shipping, wholesaling, retailing, and any other procedure that the product is subjected to. Because vegetables can change hands so many times in the Postharvest sector, a high level of management is necessary to ensure that quality is maintained. Each time someone fails to be conscientious in carrying out his or her assigned responsibility, quality is irreversibly sacrificed.
Maintaining produce quality from the farm to the buyer is a major prerequisite of successful marketing. The initial step required to insure successful marketing is to harvest the crop at the optimum stage of maturity. Full red, vine-ripened tomatoes may be ideal to meet the needs of a roadside stand, but totally wrong if the fruit is destined for long distance shipment. Factors such as size, color, content of sugar, starch, acid, juice or oil, firmness, tenderness, heat unit accumulation, days from bloom, and specific gravity can be used to schedule harvest. Vegetable producers should gather as much information as possible on maturity indices for their particular commodities. The result of harvesting at an inappropriate stage of development can be a reduction in quality and yield. Table 25 of the Appendix lists the optimum harvest maturity stages for most vegetables produced in Texas.
Because of stressful conditions during the harvest season in most parts of Texas, proper timing of the harvest period is essential. For vegetables that lose quality rapidly due to high temperatures, such as sweet corn and green beans, early morning harvest prior to the buildup of field heat may be in order. However, with some vegetables which are prone to breakage, such as asparagus, late afternoon harvest when wilting generally occurs may be more beneficial.
Most postharvest problems are management problems, and therefore, are people problems. The objective of this chapter is to identify the most common problems in postharvest management and to emphasize what those involved in handling systems can do to ensure that consumers receive products of the highest quality.
With all vegetables, care should be taken to prevent injury due to harvesting and handling errors. A crucial time to be aware of this is during the harvesting operation. The hands of the harvest worker are the most important hands that ever touch the product. The truly skilled worker does not inflict injury to the product. Broken skin and bruises reduce eye appeal and provide a ready access to decay organisms and enhance physiological breakdown. Although speed is an important consideration, excessive, unsupervised speed may result in a greater incidence of injury and quality losses. Therefore, time should be spent to properly train and monitor the performance of all personnel to insure maximum efficiency without sacrificing quality. Equally important is the need for periodic inspection and repair of all harvest containers, bulk bins and grading equipment to insure that these items are not causing injury to the produce.
Bruise damage will cause respiration rates and ethylene production to increase dramatically. This shortens the shelf life. There are several management practices that can reduce or eliminate harvest injury.
- Remove protruding nails or staples and smooth the rough edges on field containers.
- Harvest workers should not have long, sharp fingernails.
- Use care in dumping products from one container to another. This is one of the most common trouble spots.
- Use padding on all impact areas when possible.
- Clean sand and all debris out of all containers.
- Don’t overfill containers! Severe damage can result when stacked.
- Consider the time of day of harvest. Many products are more turgid in the early morning and bruise more easily.
Transport from field to packinghouse can be a source of injury. Roads should be maintained in good condition. Drivers should exercise care and remember that they are transporting living material. The springs and shock absorbers on trucks and trailers must be properly maintained.
Dumping or unloading at the packing shed is also a trouble spot. Dry dumping is an option for certain produce items. If a wet dump is necessary, appropriate flow control out of the dumping area is needed to minimize bruising. The packing line itself should have as few drops and shears as possible. Shears that are essential should be designed properly.
Rapid cooling as soon as possible after harvest is essential to the maintenance of optimum quality. The first consideration at harvest is removal of the produce from direct sunlight, and secondly, to precool as quickly as possible. Table X-1 lists cooling methods and guidelines for use with specific types of vegetables.
Exposure of produce to cold air in an enclosed space is the simplest and most common cooling method. Cold air normally is discharged horizontally near the ceiling so as to enable it to return through produce stacked on the floor. The precooking room may also be the storage room. This results in less handling of the product. Heat is removed slowly so the peak demand for refrigeration is less than with more rapid cooling systems. Since cooling is slow, shipments may be delayed, or in some cases the product may be shipped without adequate precooking. Certain commodities, such as snap beans, may deteriorate before cooling is accomplished. These problems may be minimized by ensuring that containers are stacked to facilitate good air circulation. Fans must be powerful enough to move the air at a velocity of 2 to 4 miles per hour among the containers, which should be vented adequately.
Table X-l. Cooling Methods for Vegetables
|Room Cooling||All vegetables||Too slow for many perishable commodities. Cooling rates vary extensively within loads, pallets, and containers|
|Forced Air Cooling (Pressure Cooling)||Strawberry, Fruit-type Vegetables, Tubers, Cauliflower||Much faster than room cooling:, cooling rates very uniform. Container venting and stacking requirements are critical to effective cooling|
|Hydrocooling||Stems, Leafy Vegetables, Some Fruit-type Vegetables||Very fast cooling; uniform cooling in bulk if properly used, but may vary extensively in packed shipping containers; daily cleaning and sanitation measure essential; product must tolerate wetting; water tolerant shipping containers may be needed|
|Package Icing||Roots, Stems, Some Flower-type Vegetables, Green Onion, Brussels Sprouts||Fast cooling; limited to commodities that can tolerate water-ice contact; water tolerant shipping containers are essential|
|Transit Cooling Mechanical Refrigeration||All Vegetables|
|Top-Icing and Channel-Icing||Some Roots, Stems, Leafy Vegetables, Muskmelon||Cooling in most available equipment is too slow and variable; generally not effective. Slow and irregular, top-ice weight reduces net pay load; water-tolerant shipping containers needed|
Adapted from Knott’s Handbook for Vegetable Growers. 3rd Ed.
1 For these methods to be effective, cold storage rooms are needed to hold the vegetables after cooling
Forced-air cooling or pressure cooling is accomplished through the use of fans and strategically placed barriers so that cold air is forced to pass through the containers of produce. This method usually takes from 1/4th to 1/10th the time required to cool produce by passive room cooling, but takes two or three times longer than hydro or vacuum cooling. Room coolers are relatively easy to adapt to forced-air. However, the refrigeration capacity of the room may need to be increased to compensate for the rapid removal of heat from the produce. Commodities should not be left on the cooler longer than the time required to reach about 1/8th of their initial temperature because water loss from the product is increased. The importance of humidity control is discussed in a later section. Cooling schedules should be calculated to avoid this problem.
The use of ice for cooling, either by package icing or by bulk application to the top of a load, is one of the oldest cooling methods. The use of slurries and mechanization of the process have made this a popular method for some commodities like broccoli. Cartons must be able to withstand free water. Once iced, they should be placed in a refrigerated storage unit or transit vehicle. As the ice melts, high relative humidity is present surrounding the produce. A rule of thumb for the cooling capacity of ice is that, to cool a produce from 95F to 35F, the weight of ice needed is equivalent to 38% of the produce weight. Most packed cartons have enough free space to include this amount of ice. It is essential that only high quality ice made from potable water be used.
Hydrocooling is one of the most efficient of all methods for precooking. Produce is drenched with cold water, either on a moving conveyor or in a stationary setting. In some cases, commodities may be forced through a tank of cold water. Hydrocooling is an excellent method for bulky items such as sweet corn, peaches, or cantaloupes. Good water sanitation practices must be observed and once cooled, the produce should be kept cold. The cold water must come in direct contact with the product, so it is essential the containers be designed and filled in such a way that the water does not simply channel through without making contact. Studies have shown that some commercial hydrocoolers are not adequately insulated and as much as half of the energy for refrigeration is wasted.
Commodities may be enclosed in a sealed container from which air and water vapor are rapidly pumped out. As the air pressure is reduced, the boiling point of water is lowered, so the product is cooled by surface water evaporation. Vacuum cooling works best with products that have a high surface to volume ratio, such as lettuce or leafy greens. The method is effective on produce that is already packaged providing there is a means for water vapor to escape. Moisture loss from the commodity is generally within the range of 1.5 to 5.0%. Generally, about 1% of the weight is lost for each 10oF the product is cooled. This can be reduced by wetting the product before cooling. Vacuum chambers vary in size from very large, about rail car size, to smaller portable units that may be taken to the field.
The use of wet pads and fans to cool the air in greenhouses is a common practice. This can be adapted to the cooling of horticultural commodities, however, the benefits are few compared to other methods of cooling. With this method air temperature can be reduced to near the wet bulb temperature, but not below it. Therefore, most items cannot be cooled adequately by this method. It does offer the benefit of surrounding the produce with air of very high relative humidity, which helps to reduce shrinkage. Roadside markets, or other outlets in which long term storage is not needed, may be the most practical application for an evaporative cooler.
The cooling methods described here represent only a few of the available temperature management procedures. Once a product is cooled to the lowest safe temperature, it should be kept cold. Transit vehicles, such as trucks, rail cars, etc. should be precooled before loading. Their refrigeration units are not designed to remove field heat, only to maintain the low temperature during transit. Forklift operators and other handlers should be aware that undue exposure to warm air will result in loss of quality. For some commodities, a few hours at warm temperatures can result in some loss of the product.
The process by which harvested vegetables sustain life from its stored food reserves is termed respiration.
The following is a simplified equation for respirations:
Sugar + Oxygen = Carbon Dioxide + Water + Energy
An understanding of respiration is useful in estimating the need for refrigeration since a portion of the energy generated by respiration is given off as heat. The amount of heat given off is a function of both the respiration rate of the commodity and the temperature at which it is stored. In general, the respiration rate doubles with each 10C (18F) rise in temperature, and as the respiration rate increases, the shelf life decreases. It should be obvious that the respiration rate and the heat generated by respiration can be minimized by appropriate temperature management throughout all steps in the postharvest handling scheme. In fact, temperature is the single most important factor to control in the postharvest environment. To ignore this is to lose money.
Environmental activists and the media have raised serious concerns over the safety of our food supply. Their concerns have been focused on pesticide use on and residues in our fruits and vegetables. However, a more eminent danger lies in the potential for borne illness due to microbial contamination of fresh produce. These organisms can readily contaminate fresh produce during any number of operations associated with the production, handling, packaging, processing, distribution, and preparation of produce for consumption. As a result, significantly higher incidences of illnesses and deaths have been found to be caused by microbial contamination than from pesticide poisoning. The danger of contamination of produce by microbial human pathogens is increased with the use of untreated manures or improperly handled compost on crops grown in, on or near the soil surface. Such products area potent source of the potentially deadly bacteria Escherichia coli 0157:h7, Salmonella and Cryptosporidium. Consequently, another important facet of market quality demands is clean fruit. Most vegetables should be washed after harvest to remove dirt, pesticide residue and to freshen up wilted items. With the proper design of a produce shed, the washing and cooling processes can be combined into one continuous operation. Proper design and maintenance of packing sheds can go a long way in reducing the incidence of contaminated produce being shipped to supermarkets. The perceived seriousness of the problem has caused many of the nation’s largest supermarket chains to demand that growers or grower/shippers to have an independent third party audit of their operation and facilities to insure their products are grown, packed, and shipped using good agricultural practices that reduce the incidences of microbial contamination.
The Food and Drug Administration in the interest of protecting our nation’s food supply has developed a set of guidelines for minimizing microbial food safety hazards for fresh fruits and vegetables. The guidelines are briefly stated below. A complete copy of the guidelines can be obtained from:
Food Safety Initiative Staff, HFS-32
U. S. Food and Drug Administration
Center for Food Safety and Applied Nutrition
200 C street S.W.
Washington, D.C. 20204
Food Safety Guidelines:
Agricultural Water: Water quality should be adequate for its intended use. Where water quality is unknown or cannot be controlled, growers should use good management practices which minimize the risk of contamination such as protecting surface waters, wells and pump areas from uncontrolled livestock or wildlife access to limit the extent of fecal contamination. Grass sod waterways, diversion berms, runoff control structures, and vegetative buffer areas may prevent polluted runoff water from contaminating agricultural water sources and produce crops. Where water quality is unknown, try to avoid direct contact of the water to editable crops. Test water source(s) frequently for microbial contamination.
Manure and Municipal Biosolids: Use only properly treated manure and biosolids to fertilize vegetables. Situate, if practical, vegetable production fields away from other potential sources of contamination such as nearby composting or manure storage areas, livestock or poultry operations, nearby municipal wastewater or biosolids storage, treatment or disposal areas, and high concentrations of wildlife in the growing and harvesting and packing environment.
Worker Health and Hygiene: Establish hygiene practices for worker appropriate for the agricultural environment (field, packing shed and transportation operation) and provide worker training on the hygiene practices required. Contact of fresh produce with fecal matter is the major cause of food borne illnesses. Most of which is a result of the lack of proper worker hygiene practices. Also infectious diseases, accompanied by diarrhea or open lesions, that include boils, sores, or infected wounds are a source of disease causing microorganisms. Sick or injured workers with these symptoms should be requested to remain at home or be assigned to duties in which they do not come in contact with fresh produce.
Sanitary Facilities: Producers should describe the appropriate number of toilets to the number of workers, proper hand washing facilities, maximum worker to restroom distance, how often such a facility should be cleaned, and how the waste is to be disposed to avoid contamination of worker and produce.
Field Stations: Attempts should be made to prevent cross-contamination of fresh produce during preharvest and harvest activities that may result from contact with soils, fertilizers, water, workers, and harvesting equipment. Clean harvest containers and storage facilities prior to use, discard broken or damage bins, field containers and bulk load equipment periodically, remove as much dirt as possible in the field, and maintain harvest equipment.
Packing Sheds: Maintain buildings, fixtures, and other physical facilities, and their grounds. Practice good sanitation within the packing shed. Clean pallets, rest rooms, containers, grading and packing lines daily. Develop a vigorous pest control program.
Sanitation within the Shed: Problems of rough handling and poor temperature management are compounded when poor sanitation practices exist in the postharvest environment. Packing sheds and storerooms should always be clean and neat. Products left on the floor under machinery will rot and contaminate the air with spores of decay causing organisms that may then infect other commodities. The packing line itself should be left free of produce each day and cleaned regularly. Bulk bins, storage buckets, and other containers should be cleaned and disinfected regularly.
The risk of decay always increases with the exposure of commodities to water. Dump tanks, which are used to minimize bruising during dumping of fruits and vegetables, can be a particular problem if not managed properly.
The following general management practices should be observed:
- Start with clean water each day.
- Chlorinate to approximately 100 ppm free chlorine. This may be done with chlorine gas or with either liquid or granular hypochlorite.
- Adjust the pH to the range of 6.0 to 7.0. Chlorine gas has little effect on pH but hypochlorites raise pH, so some adjustment is usually necessary. Use caution of strong acid or base is used to make this adjustment.
- For certain commodities, especially tomatoes, the water should be heated about 10F warmer than the fruit.
- Minimize exposure time in the tank. Never load the tank with produce unless the packing line is working.
- Clean out the tank thoroughly at the end of each day.
- Double check all of the above frequently.
- Transport vehicles should always be cleaned and sanitized before loading.
The same general rules for managing dump tanks also apply to hydro coolers. Even though microbial growth is slower at low temperature, refrigeration does not kill microorganisms, so hydro coolers can spread decay in the same manner as a dump tank.
The condition of rinse water, e.g. chlorination pH, etc., is not as critical as dump tank or hydrocooler water because the fruit is not immersed. However, any time commodities are in contact in water, it is wise to practice good sanitation.
Adjustment of water pH has been a major shortcoming of packinghouse managers in South Carolina, even though pH has a critical effect on the efficiency of chlorine to disinfect (Table X-2). Equipment now is available for automatic control of pH and chlorine content. Many vegetables are not treated with fungicides or other means of chemical decay control. Since we have no way to cure a decay problem, we must prevent it from occurring. Good sanitation, in addition to appropriate temperature management and handling practices, may be the packers’ only resource for decay control.
Table X-2. Effect of pH on Amount of Active Chlorine in Solution
|pH||% Active Chlorine|
Appearance plays a major role in vegetable sales success. Therefore, a grower should pay special attention to maturity, size, color, shape, and freedom from blemishes and dirt when grading and sorting produce. Each package or display bin should contain fruit having similar qualities. If vegetables are to be shipped, container size is an important consideration. A good rule of thumb is, the further a grower is from the market, the greater the marketing cost, and the greater the requirement for careful grading and packaging. Table 43 in the Appendix, lists commonly used containers and weights for fresh vegetables. A good package should protect the product during storage and transit, facilitate temperature management, protect from water loss, be compatible with any special treatments required by the product, and be compatible to existing handling systems. Standardization of containers has been a major challenge for Post harvest technologies and little progress has been made, in practice, to reduce the variety of packages in use.
One of the newest developments in packaging has been the use of materials that allow for modification of the atmosphere within the package, thus the term modified-atmosphere (MA) packaging. This involves the use of plastic films that allow depletion of oxygen and accumulation of carbon dioxide within the container, which will increase the shelf life of some commodities. However, some products develop off-odors or have increased bacterial growth under MA, and much research is ongoing to define which products respond best to MA. It should be noted that MA will not substitute for good temperature management. The high cost of MA may be wasted if other aspects of Post harvest management are ignored.
Proper storage and/or transit conditions should be used if vegetables are to be held prior to sale or if they are to be transported over a great distance. Ideal vegetable storage conditions can be found in Table 35 of the Appendix.
One of the major problems encountered during storage of certain vegetables is chilling injury. Descriptions of chilling injury symptoms are given in Table X-3. The most and least sensitive vegetables to chilling injury are given in Table X-4.
Another important consideration in order to maintain optimum storage conditions is relative humidity. Relative humidity is defined as the amount of water present in air relative to the maximum amount that the air can hold at that particular temperature. It is usually expressed as a percentage. Small fluctuations in temperature can cause wide fluctuations in relative humidity. This critical component of the Post harvest environment is often overlooked. Products stored at less than optimum relative humidity will suffer excessive water loss and begin to shrivel. Many vegetables are unacceptable for marketing if weight loss reaches 5% because of their undesirable appearance and undesirable textural changes that may accompany water loss. Leafy vegetables are among the less tolerant crops to dehydration.
The use of large refrigeration coils for heat exchange allows them to be operated with fewer differentials between coil and air temperature, thereby reducing the amount of moisture removed from the room by condensation on the coil. However, under almost all circumstances, water vapor needs to be added back to the air. There are a number of humidifying systems available for this purpose. In the absence of a humidifying system, water can be applied to the floor and walls to temporarily increase relative humidity. For long term storage, automated systems are essential. Managers of packing houses, storage facilities, etc. should have at their disposal some means of measuring relative humidity.
Table X-3. Chilling Injury Symptoms1
|Product||Lowest Safe Temperature (°F)||Symptoms|
|Asparagus||32/36||Dull, gray green, limp tips|
|Bean, lima||34/40||Rusty brown specks, spots|
|Bean, snap||45||Pitting and russeting|
|Cucumber||45||Pitting, water soaked spots|
|Eggplant||45||Surface scald, Alternaria rot, blackening of seeds|
|Muskmelon||36/41||Pitting, surface decay|
|Honeydew||45/50||Reddish tan discoloration, pitting, surface decay, failure to ripen|
|Watermelon||40||Pitting, objectionable flavor|
|Pepper, Sweet||45||Sheet pitting, Alternaria rot on pods and calyxes darkening of seed|
|Pumpkin and Winter Squash||50||Decay, especially Alternaria rot|
|Sweetpotato||55||Decay, pitting, internal discoloration, hard core when cooked|
|Tomato, mature green||55||Poor color when ripe, Alternaria rot|
|Tomato, ripe||45/50||Water soaking and softening, decay|
1 Stored at low but nonfreezing temperatures
Table X-4. Susceptibility of Fruits and Vegetables to Freezing Injury1
|Most Susceptible||Least Susceptible|
|Beans, snap||Brussels Sprouts|
1Most = injured by one light freezing
Least = lightly frozen several times without serious damage, but shelf life likely reduced
Recommendations for Selecting Vegetables
Beans are subject to chill injury and russeting if held at temperatures below 40°F. Chill damage may begin to show within three days. If held at 32°F they will russet and become pitted and lose moisture rapidly. Optimum temperatures range between 45 and 50°F. Beans stored too long or at too high a temperature are subject to various soft rots.
Washing beans before refrigerating will help to retain moisture content. Snap beans require air circulation. Containers should be stacked to allow maximum air circulation.
The recommended temperature for storing broccoli is 32°F. If in good condition and stored with adequate air circulation, broccoli will last 10 to 14 days. Storage beyond this time can cause leaves to discolor, buds to drop off, turn yellow and tissue to deteriorate. Age can cause stem hardening and/or decay. To avoid dehydration, sprinkle crushed ice or water on the broccoli as soon after harvest as possible. Broccoli should be iced when transporting to markets.
Cabbage should be stored at 32°F. Cabbage loses moisture easily if held at room temperature. Cut heads do not hold up well in storage. If held at 32°F, cabbage will last 10 to 14 days.
These melons are highly perishable and the shortest possible storage time is recommended. Temperatures should be held between 40 and 50°F. The melon must mature on the vine before it is harvested. It does not increase in sugar content after picking, although it will soften and change types of sugars. Only a mature melon will provide the sweetness, texture, flavor and juiciness that characterize cantaloupes at their finest. A mature cantaloupe is well netted or webbed with a smooth rounded, depressed scar at the tip end. Cantaloupes for shipping are usually harvested while firm and prior to reaching full maturity in order to avoid damage during transit. A few days at room temperature will soften the fruit. Cantaloupes should be carefully handled to avoid damage. Bruising, scuffing of the netting, puncturing or cracking can cause more rapid deterioration through moisture loss or decay. Under optimum conditions a cantaloupe will deteriorate from field fresh to fair within 12 to 16 days. The operator can expect a 5 to 7 day shelf life. Because cantaloupes generate a significant amount of ethylene gas mixed load shipment with ethylene sensitive produce should be avoided.
The recommended temperature for storing carrots is 32°F. Carrots will wilt and dry if not kept sufficiently moist. When properly handled, carrots can be stored for several months. Fresh market carrots generally are harvested before reaching full maturity. Avoid storing with ethylene producing commodities such as cantaloupes, tomatoes, and most fruits.
Cauliflower should not be held for any length of time. However, if in good condition, with temperatures held at 32°F, it will keep for several weeks. Cauliflower should be stored with the head down to prevent accumulation of moisture on the curds. A good quality head of cauliflower will be white or creamy white in color. Curds should be clean, firm and compact with jacket leaves fresh and green. Small leaves extending through the curd do not influence the eating quality. Ethylene gas will accelerate decay and brown spotting. Do not store or ship with ethylene producing produce.
If held at 32°F, celery will keep well in storage. Celery should not be sprinkled if it is packaged, which encourages decay. Celery should be brittle enough to break easily with fresh, crisp and clean stalks. The inside stem should be smooth. If it feels rough or puffy, the celery is likely to be pithy. Leaflets should be fresh or only slightly wilted. Light green stalks with glossy surfaces offer the best taste.
Cucumbers will keep well for 10 to 14 days when temperatures are maintained between 45 and 50°F. Temperatures below 45F for more than two days will cause chill damage. Chill injured cucumbers will develop water-soaked spots, pitting or tissue collapse. Extensive decay will develop when removed from low temperatures. Temperatures above 50F will cause the product to ripen rapidly and take on a yellow coloring after about 10 days. Ripening will be accelerated if cucumbers are stored with ethylene producing commodities. Adequate humidity is also important as cucumbers are subject to shriveling from loss of moisture. They are waxed to help prevent dehydration.
Eggplants are very sensitive to extremes in temperature. They are held best at 45 to 50°F. Lower temperature will cause chill damage. Bruising is a common problem when careful handling is not employed. Decay will show up as dark brown spots on the surface.
Recommended storage temperature is 32°F. Temperatures higher than 32F will shorten the shelf life of lettuce. Temperature must be maintained as close to 32°F as possible. A storage temperature of 38F will cut the storage life in half. However, lettuce also is damaged easily by freezing; therefore the storage room must be kept above the freezing point at 32°F. Respiration is also a problem with increased temperature. As the temperature rise, so does the respiration rate, which cuts storage life. Leaf lettuce respires at about twice the rate of head lettuce. If temperatures are kept below 35°F, russet spotting is generally kept under control. However, ethylene producing items such as apples, pears and cantaloupes can increase russet spotting. Head lettuce is more susceptible to the problem than other varieties, air circulation is important for lettuce. Lettuce should be stacked so maximum circulation is attained.
Melons should be stored at 50°F. Melons are quickly affected by extremes of either very hot or very cold temperatures. Careful consideration should be given when storing melons with other commodities. Table 46 of the Appendix lists ethylene sensitive commodities. Ethylene gas produced by melons can cause certain vegetables to age and can cause butter to develop an off taste. Some characteristics to look for among the various melons comprise:
Ripeness is indicated by a yellow rind color and slight softening at the blossom end (opposite the stem scar). The flesh should be soft, creamy white, sweet and juicy.
Melons are round at the base, coming to a point at the stem end. They will have a gold and green rind that is smooth with no netting and little ribbing. When ripe they will show a softening of rind at the large end, have a golden skin and rich aroma.
These will have a creamy yellow color and velvety surface, but color will vary according to origin. If the outer surface is white with a greenish tint, it is unripe and lacking sugar content. A hard, smooth feel to the rind also indicates the lack of maturity.
The best storage temperature is 32°F. Even under optimum conditions, mushrooms do not keep fresh more than one week after harvest. Under 32°F mushrooms will keep in prime condition for five days, for two days at a temperature of 40F and for one day if temperature is allowed to reach 50°F. Because of their high respiration rate, mushrooms require good air circulation. Mushrooms are comprised mainly of water (90%) and must be protected from dehydration. DO NOT package mushrooms in plastic bags. DO NOT wash mushrooms. Discard large (over 4″) over mature pods.
Store at 40 to 45°F. Discard large, (over 4″) over mature pods.
Onions should be stored in a dry cool location. Cured onions can be stored at 32°F to retard sprouting. The best onion will be hard and firm. They should be stored dry. They may require additional drying after harvest in order to store well. 90°F with circulating air enhances drying and curing.
Green Onions are highly perishable and should be stored at 32°F. Higher temperatures will result in yellowing and leaf decay. Crushed ice is helpful in supplying moisture.
Under optimum conditions peppers should not be held for more than 5 to 7 days. The recommended temperature is 45 to 50°F. Peppers are subject to chill damage and pitting if temperatures fall below 45°F. Higher temperatures, above 50°F will encourage ripening or development of red coloring and rapid development of decay.
Store in a cool, dark location. Potatoes should not be refrigerated below 45°F or they will convert starch to sugar resulting in a too sweet flavor. Light is also a factor as even small amounts of light will cause potatoes to turn green. Potatoes, which have been stored below 42°F, can be conditioned by storing a week to 10 days in temperatures of 50°F or slightly higher. This will convert the sugar back to natural starch and restore the balance that gives potatoes their fresh flavor.
Radishes will store best at 32°F. As with carrots, topped radishes will last longer than bunched radishes.
Store at 32°F with high humidity.
For storage periods of less than four days hold summer squash at 32 to 40°F and then use immediately when removed. If longer than four to five days, temperatures should be 45 to 50°F. Storage life under optimum conditions usually is two weeks for fair to good quality.
Sweet corn requires immediate cooling after harvest. Use of hydrocooling or top icing with immediate refrigeration at 32°F gives best results in preventing conversion of sugars to starch. The loss of sugar is 4 times faster at 70°F than at 32°F. Never bulk store sweet corn unless heavily iced because of its tendency to heat through the pile. Maximum of 4 to 8 day shelf life when stored properly.
Sweet potatoes should be cured before storing by holding at 80 to 85°F and 90 to 95% relative humidity for 4 to 7 days. After curing, reduce temperature down to 50 to 60°F. At lower temperature, sweet potatoes are subject to chill injury, decay, hard core and pithiness.
Tomatoes that are picked mature green and properly handled will ripen into a fully colored product with good flavor. Temperatures recommended for ripening fall between 50 and 70°F. Tomatoes are subject to chill damage at temperatures below 50F and lose flavor quickly. Care in handling is crucial. Bruising hastens spoilage and shortens the shelf life. DO NOT REFRIGERATE BELOW 50 DEGREES!!! Tomato quality is dependent upon proper harvesting and handling methods.
Holding watermelons for several days at room temperature can improve flavor and coloring. At 50°F and below the watermelon loses color, develops an off flavor and becomes pitted. At higher temperatures they tend to decay. Immature melons do not ripen to a great extent, once harvested, total sugar does not increase.
When fruits or vegetables are removed from a low temperature to a higher one, moisture often condenses from the air on the cool surface of the commodity. This is known as sweating; the higher the relative humidity of the outside air, the more marked it becomes. This is because the dew point of the air is at or above the temperature of the commodity. Sweating should be prevented or minimized whenever possible, particularly with onions and the more tender fruits, because it may favor decay. This does not mean that when products sweat after removal from a refrigerated room they will decay; it does mean that conditions are more favorable for decay than if the surfaces remain dry until consumed. Sweating can be prevented to some extent by allowing fruits and vegetables to warm gradually. Air movement over the product while it is warming is helpful in drying the surface.
Storage of Mixed Commodities
At times it may be necessary to store different products together. This may or may not be safe. With some products there is a cross-transfer of odors. Also volatiles such as ethylene are emitted by some products that may be harmful to others. Combinations that should be avoided in storage rooms are apples or pears with celery, cabbage, carrots, potatoes, or onions; celery with onions or carrots; and citrus fruit with any of the strongly scented vegetables. Table 45 in the Appendix list the ethylene sensitive commodities. Pears and apples acquire an unpleasant earthy taste and odor when stored with potatoes. It is recommended that onions, nuts, citrus fruit, and potatoes each be stored separately. Lettuce, carrots and greens are damaged when stored with apples, pears, and many other fruits and some vegetables because of the ethylene that is given off from these products as a natural emanation. Very low concentrations may produce adverse effects. Ethylene also stimulates ripening of many fruits and vegetables. This ripening effect is negligible at low temperatures (e.g., 32°F), but it may have an effect at higher temperatures. For this reason, products such as cucumbers, peppers, and acorn squash, in which retention of green color is desirable and which need to be stored at 45 to 50°F, should not be stored with apples, pears, tomatoes, or other ethylene producing crops such as cantaloupes, Honeydew melons, and tomatoes.
Simple ventilation of storage rooms is the cheapest way of removing excess ethylene providing the outside air is not polluted. Overripe and rotten commodities generate substantial quantities of ethylene and should be removed from storage on a regular basis. Internal combustion engines release some ethylene in their exhaust and should never be left running in an enclosed storage area. Commodities that are particularly sensitive to ethylene should be handled with electric forklifts.
Several commercially available materials either absorb ethylene directly or convert it to inactive compounds. Certain types of activated or brominated charcoal absorb ethylene; however, some cheaper materials utilize potassium permanganate to oxidize ethylene to simple carbon dioxide and water. Purafil is an example of a commercial available absorbent.
Manipulation of the storage atmosphere, whether in large storerooms or in small packages, can reduce the detrimental effects of ethylene. In general, reducing oxygen and increasing carbon dioxide serves this purpose and is a commercially acceptable procedure for some products. Other chemicals that directly inhibit either ethylene production or ethylene activity are not cleared for Post harvest use on vegetables.
It is sometimes necessary to store or transport different types of commodities in a common area. In order for this to be done successfully, the products must have similar Postharvest requirements. Items shipped in mixed loads must be compatible in terms of the following:
- Temperature, i.e. chilling tolerance or sensitivity
- Relative humidity
- Rate of ethylene production
- Sensitivity to ethylene
- Emanation of volatiles with odors
- Absorption of odors
Compromises may be necessary. Conditions suitable for the most valuable portion of a load should be the main criterion for deciding how to set conditions for the whole lot.