POSTHARVEST COOLING AND HANDLING OF

FIELD- AND GREENHOUSE-GROWN TOMATOES


Although considered a vegetable by most people, tomatoes are actually the fruit of several plants of the genus Lycopersicon. Tomatoes are one of the leading produce items in the United States in terms of value and volume consumes. Preserving the quality of the fruit from vine to consumer is essential to successful marketing. This fact sheet describes harvesting, handling, storage, and packaging procedures that will help maintain fruit quality. It also discusses the economic aspects of marketing the crop.
Freshness Facts

Preferred cooling method:............Room cooling
Optimum storage temperature:
Mature green....................................58 to 60 degrees F
Pink................................................48 to 50 degrees F

Tomatoes are very sensitive to chilling. Recommended storage temperatures differ with the maturity of the fruit. Precise temperature control is critical to maintaining acceptable quality.

Storage life:
Mature green....................................21 to 28 days
Pink...................................................7 to 14 days
Red....................................................2 to 4 days

Optimum relative humidity:
...........................................................85 to 95%


Tomatoes are grown in commercial quantities both in greenhouses and in the field. Fresh market tomatoes are usually marketed by fruit type. These types include full-size globe (red or yellow), plum (roma), and cherry. Consumers buy tomatoes primarily for their appearance but are attracted to repeat purchases by flavor and quality. Tomatoes are very sensitive to mishandling and improper storage conditions. Because they can be injured by either low or high temperatures, proper postharvest handling and storage methods are essential for maintaining acceptable quality and promoting long shelf life.

Harvesting

Tomatoes destined for fresh market are hand harvested, either with or without a harvesting aid. Machine harvesting is usually reserved for processing tomatoes that are especially developed for those markets.

Immature green tomatoes will ripen poorly and be of low quality. However, tomatoes harvested at the mature green stage will ripen into a product indiscernible from vine-ripened fruit. In the field, it is often difficult for inexperienced pickers to judge between immature and mature green tomatoes. A simple way to determine maturity is to slice the tomato with a sharp knife. If seeds are cut, the fruit is too immature for harvest and will not ripen properly.

Vine-ripened tomatoes should be harvested at the breaker stage to ensure the best quality. Fruit at the breaker stage, which have some interlocular gel and a pinkish red color on the inside, are sure to be mature. Such fruit can be handled and shipped better than that which has more color, and it will often bring a higher price than less mature tomatoes.

If tomatoes are to be vine-ripened, fields should be harvested often and thoroughly to narrow the range of ripeness. Harvesting every day may be desirable during the peak of the season. Remove all diseased, misshapen, and otherwise cull tomatoes from the vines as soon as they are discovered. Remove discarded tomatoes from the field to avoid the spread and buildup of diseases and insect pests.

Keep in mind that harvesting is basically a material handling process. At the peak of the harvest season, the rate of production of marketable vine-ripened fruit may reach 100 to 150 field boxes (containing 30 to 35 pounds of fruit each) or 5 to 7 pallet bins per acre per day. The harvest of mature green fruit may yeild as much as 25 pallet bins per acre. Enough containers should be available to take care of maximum harvest volume.

A considerable amount of labor is required to harvest tomatoes. It has been estimated that the distance traveled through the field to harvest 1 acre of tomatoes may be greater than 100 miles. Providing a cross break every 100 feet along the row for the loading of containers will reduce this figure considerably. This arrangement is particularly useful if a harvesting aid is used. The following guidelines can also significantly increase harvesting efficiency and reduce wasted effort:

Field Supervision

The quality and market value of tomatoes depends on the timeliness of harvest and the level of care in handling. Postharvest quality control begins in the field. Clean hands and sanitary personal habits are required of workers at all times when handling produce items. Growers are legally required to provide sanitary facilities and instructions for all workers handling produce. Careful supervision and proper instruction of the harvesting crew are essential to the success of any hand-harvesting operation. Frequent checks of harvesting pails for trash and poor-quality tomatoes is a good idea. Packing house problems and buyer complaints often are the result of a harvesting crew that has been poorly instructed and supervised.

Tomatoes should be removed from the plants by gently twisting them without tearing or causing undue damage to the fruit or plants. Rough handling will result in both visible and latent damage. Fruit should never be packed tightly into harvesting containers or allowed to remain in the sun for extended periods.

The harvesting crew should be made familiar with the maturity color chart. They should also be shown examples of the various defects they may encounter. Since mature green fruit are often harvested "to size," depending on maturity conditions, the crew should be familiar with the acceptable size range.

Harvesting Aids

Mechanical harvesting aids reduce the amount of labor for harvesting and make the work easier and more pleasant. Hand-operated carts or wheelbarrows are useful for carrying the tomatoes from the field, but motorized one-, two-, or three-row machines that transport the workers as well as the harvested tomatoes are more practical. Small self-propelled, high clearance units that travel in the row middles can be used. These machines can also be used to pull an over-the-row trailer on which additional workers and harvesting containers are carried. One or two pickers per row in the middle, harvesting rows both to the left and right, can be used, depending on the amount of fruit ready for harvest and the ground speed of the machine. A cover over the machine may be provided to protect the workers from the sun and from rain showers.

Some tomato harvesting aids are available commercially. However, many growers have found it necessary to custom build equipment or to modify existing equipment to meet their needs. High-clearance, self-propelled sprayers that have been equipped with removable seats and container racks work well for large-scale growers. With these machines, the grower can apply pesticides and harvest the fruit using the same machine without leaving out a row, as is necessary with a tractor-mounted machine. Some growers have modified self-propelled tobacco harvesters for use in harvesting tomatoes.


Rules For Manual Harvesting

  1. Keep your hands clean. Remember that you are handling a food product. The law requires you to wash your hands after each visit to the rest station.
  2. Be thorough. Harvest only those fruit that are ready. Immature tomatoes should be left for the next harvest. Pick all suitable fruit on the plant before moving on to the next one.
  3. Do not squeeze or bruise the tomatoes or allow your fingernails to penetrate the skin.
  4. Overfilling your hands will cause you to drop the fruit. This damages the fruit and lowers your picking efficiency.
  5. Do not put trash or culled fruit into the picking container with good tomatoes.
  6. Never drop or throw tomatoes into the picking container.
  7. Slowly pour the tomatoes from the picking pail into the pins from the lowest possible height.

Cleaning and Grading

Careful supervision of labor is one key to ensuring uniform cleaning, sizing, and packing of hand-harvested tomatoes. On most tomato grading lines, the fruit is dumped or floated out of bulk bins or hand field crates, which reduces bruising. Passing the fruit along a slow roller conveyer that slowly turns each tomato is the best way to inspect thoroughly for defects. Tomatoes that are misshapen, damaged, decayed, or cracked should be discarded. Firm flesh, shiny skin, and uniform color are indicators of good quality.

Tomatoes should be washed sufficiently to remove dust and foreign material by spraying them with a small amount of chlorinated water as they move over a set of soft brush rolls. Often, so little water is required that most packers make no attempt to recycle the water. Not reusing the wash water eliminates the problems with trash and disease build-up that normally occurs with wash tanks. The small amount of retained water may be removed by sponge-rubber doughnut rolls alone or in combination with an air-blast drier.

The wash water should be several degrees warmer than the pulp temperature of the tomatoes to avoid drawing water and disease organisms in the fruit. The water should be chlorinated at the rate of 125 parts per million (1 quart of 5.25 percent chlorine bleach to 100 gallons of water). The chlorine level and pH of the wash water should be checked at least hourly during the day with test papers or a meter. (A neutral pH of 7.0 is desirable.) For more detailed information on chlorination, obtain a copy of publication AG-414-6, Chlorination and Postharvest Disease Control, from your county Cooperative Extension Center.

Tomatoes are subject to a large number of postharvest diseases. Some of the more common are alternaria rot (Alternaria alternata), gray mold or botrytis (Botrytis cinerea), rhizopus rot (Rhizopus stolonifer), and sour rot (geotrichum candidum). Although the skin of tomatoes offers some protection against infection, it is easily damaged by rough handling. Pathogens can enter tomatoes through a variety of openings. Wounds such as punctures, cuts, abrasions, and cracks as well as stems and stem scars provide potential points of entry. The entry of pathogens into a surface injury is nearly a certainty. Therefore, tomatoes with surface injury should be separated promptly from sound fruit and discarded before decay can spread.

Tomatoes are typically separated by size on one or more sizing belts similar to the one shown in Figure 1. Size classifications are given in Table 1. A rotating bar sizer may be suitable alternative for the small producer. It is important on such a line that the belt and roller speed and the drop height be minimized and that all impact surfaces be well padded. A layer of 3/8- to 1/2-inch closed-cell foam with a smooth, washable outer surface should be adequate. Open-cell foam or scraps of carpeting are better than than nothing but are very difficult to keep clean and generally do not provide the proper level of protection. Lines should be inspected for sharp projections that might injure the fruit and should be kept free of dirt and trash. A daily cleaning with a strong chlorine solution (1/2 pint of 5.25 percent chlorine bleach per gallon of water) followed by a clean water rinse will help prevent the buildup of decay organisms on packing equipment.


Figure 1. Typical tomato washing and sizing line.

For marketing purposes, tomatoes are segregated by grade, color, and variety. Color classifications are given in Table 2 and illustrated in Figure 2. Size is not an indication of grade. Grades of field-grown tomatoes include U.S. No. 1, U.S. Combination (a mix of least 60 percent No. 1 with the remainder No. 2), U.S. No. 2, and U.S. No. 3. Greenhouse tomatoes are classed into U.S. No. 1 and U.S. No. 2 only. Many buyers will accept only tomatoes of U.S. No. 1 quality or better. Single copies of the U.S. grade standards for fresh greenhouse tomatoes (separate publications) may be obtained from the following address:

Standardization Section
AMS, F&VD, Fresh Products Branch
U.S. Department of Agriculture
P.O. Box 96456, Room 2056-South
Washington, D.C. 20090-6456

Growers should check with buyers to determine their preference for grade, color, variety, and packaging.


Figure 2. Tomato color chart showing four of the six official classifications: (a)breaker; (b) turning; (c) near the upper end of the turning range; (d) pink; (e) light red. Not shown: green, which means the surface "is completely green in color. The shade of green color may vary from light to dark," Also not shown: red, which means "more than 90 percent of the surface, in the aggregate, shows red in color."


Table 1. Size Classification of Tomatoes
Size                        Diameter (inches)
Designation           Minimum*         Maximum**
  Extra small         1-28/32          2-4/32
  Small               2-4/32           2-9/32
  Medium              2-9/32           2-17/32
  Large               2-17/32          2-28/32
  Extra large         2-28/32          3-15/32
  Maximum large       3-15/32            ---
_________________________________________________
* Will not pass through a round opening of the designated diameter when the tomato is place with the greatest transverse diameter across the opening.
** Will pass through a round opening of the designated diameter at any position.
Table 2. Color Classification of Tomatoes
The following terms may be used to describe the color of tomatoes as an indication of the stage of ripeness: Green - The surface of the tomato is completely green. The shade of green may vary from light to dark.
Breakers - There is a definite break in the color from green to tannish yellow with pink or red skin covering not more than 10 percent of the surface.
Turning - More than 10 percent but not more than 30 percent of the surface, in the green aggregate, shows a definite change in color from green to tannish yellow, pink, red, or a combination of those colors.
Pink - More than 30 percent but not more than 60 percent of the surface, in the aggregate, shows pinkish red or red color.
Light Red - More than 60 percent but not more than 90 percent, in the aggregate, shows pinkish red or red color.
Red - More than 90 percent of the surface, in the aggregate, shows red color.

Source: Standards for Grade of Fresh Tomatoes (7 CFR 51). U.S. Department of Agriculture.

Temperature and Humidity Management

Immediate and thorough postharvest cooling to remove excessive field heat aids greatly in maintaining quality and substantially lengthens the shelf life of the tomatoes. In addition, prompt and thorough cooling and washing can reduce the effects of dehydration and minimize decay. Postharvest cooling is essential for maintaining quality but will not improve the quality of a poor product.

Tomatoes destined for distant markets or tomatoes in the pink or light red stage should be cooled immediately after harvest to avoid becoming overripe before reaching the consumer. Placing containers of warm tomatoes in a refrigerated space, known as room cooling, is recommended. To aid room cooling and prevent the buildup of heat of respiration, containers of tomatoes should be loosely stacked with space between the containers to allow for sufficient air circulation.

Tomatoes are very sensitive to chill injury. The recommended storage temperature varies with the maturity of the fruit (for mature green, 58 degrees F; for pink, 50 degrees F). Proper temperature control is critical to quality and shelf life. Mature green tomatoes cannot be held at temperatures that delay ripening any appreciable time. When they are stored for several weeks at 55 degrees F, they often develop decay and fail to ripen properly. The optimum temperature for ripening mature green tomatoes is from 65 to 70 degrees F. At temperatures above 80 degrees F, mature green tomatoes will appear to ripen but may not have the best eating qualities. A temperature of 58 degrees to 60 degrees F is best for slowing the ripening of mature green tomatoes and preventing existing decay.

Mature green tomatoes stored at temperatures below 50 degrees F are susceptible to decay by Alternaria that may occur during subsequent ripening. Chill injury is cumulative and is a function of both temperature and exposure time. For example, comparable decay may be expected in mature green tomatoes held at 0 degrees F for six days or 5 degrees F for nine days. Mature green tomatoes may also be exposed to nighttime temperatures below 50 degrees F for a week or more. Some studies indicate that storage of tomatoes in humidities above 90 percent can increase the incidence of decay.

Light red tomatoes can be stored for two weeks or longer at 50 degrees F. Longer storage may result in reduced retail shelf life. Ripe tomatoes may be stored at lower temperatures than mature green tomatoes. Several days at 40 degrees F may be acceptable, but longer storage at this temperature will result in loss of color, firmness, shelf life, and especially taste. Under extreme circumstances, firm yet well-ripened tomatoes may be stored for as long as three weeks at 33 degrees to 35 degrees F. Such tomatoes will have almost no shelf life and very poor flavor and color.

Pink to firm-red greenhouse grown tomatoes may be stored at temperatures of 50 degrees to 55 degrees F. Less mature tomatoes should be ripened at 70 degrees F before being stored at 50 degrees to 55 degrees F.

Ethylene Treatment

Ethylene is a naturally occurring, odorless, tasteless gas produced by many types of produce, including tomatoes. Mature green tomatoes exhibit accelerated ripening in the presence of ethylene. In commercial practice, mature green tomatoes are exposed to supplemental ethylene treatment to hasten ripening and to ensure uniform ripening throughout a lot. Tomatoes may be exposed to an ethylene concentration of 100 to 150 ppm for 24 to 48 hours at a temperature of 70 degrees to 75 degrees F and 90 percent relative humidity. Immature tomatoes may be ripened with the application of ethylene, but the resulting fruit will not be of good quality. Likewise, fruit beyond the breaker stage will not benefit from the application of ethylene since the process has been initiated already by the tomato's own ethylene. However, there is some evidence that additional ethylene may speed the ripening process.

Although there are many large, commercial tomato ripening facilities, small-scale growers and packers often find it convenient to build and operate small on-farm facilities. A tomato ripening room must be nearly airtight to prevent the escape of ethylene. Small ethylene gas generators may be purchased or rented for this purpose.

Care should be exercised in mixing tomatoes with other types of produce in storage and transit. The ethylene gas given off by many ripening fruits (such as apples, cantaloupes, and bananas) will hasten the ripening of tomatoes located nearby. Common storage of mature green tomatoes with ripe tomatoes should be avoided also for the same reason.

A mixture of 3 percent oxygen and 97 percent nitrogen will extend the life of mature green tomatoes up to six weeks at 55 degrees without noticeable decline in appearance and taste. Lower oxygen levels will produce an off-flavor.

Packaging

Tomatoes may be packed in a variety of containers (Table 3), depending on the intended market. Customarily, individual tomatoes packed in flats are positioned with the stem scar down.
Table 3. Common Tomato Packages

Vine-Ripened

20-pound two-layer flat
30-pound 1/2-bushel carton
50- to 55-pound bushel basket
10-pound carton
25-pound carton (seldom used)
Mature Green 25-pound loose carton Cherry 15-pound 12-pint flat

Economics and Marketing

Growers in most states produce fresh market tomatoes, but Florida and California provide the bulk of domestic supplies. Florida availability peaks in April, May, and June of each year, although substantial quantities are available in the December-through-March period. California is the major producer of fresh market tomatoes from July through November each year. Considerable numbers of fresh market tomatoes are imported from Mexico; 10 million or more pounds of tomatoes were shipped to U.S. markets every month during 1992. Mexican tomato shipments to the U.S. usually peak during the December-through-March period, with smaller shipments in summer and fall months. Tomatoes also rank as the leading greenhouse vegetable grown in the United States. Most greenhouse tomatoes are grown for sale in late spring and early summer.

Domestic per capita consumption of fresh tomatoes has remained stable since the mid-1980s at 15 pounds per person annually. For the most part, growth in tomato consumption has been limited to the canned tomato sector as increased consumption of tomato-based ethnic foods (such as salsa and pizza sauces) has spurred 13 percent increase in processed tomato consumption since 1985. Despite stagnant growth in recent years, fresh market tomatoes remained second to potatoes in 1992 cash crop receipts received by U.S. farmers and ranked fourth as measured by domestic quantities shipped (behind potatoes, lettuce, and onions).

North Carolina has three distinct selling seasons for fresh market tomatoes: (1)late spring when greenhouse tomatoes are sold; (2) the early summer period from June through mid-July when supplies are available in the eastern and piedmont areas of the state; and (3) late summer and early fall when supplies are concentrated in the western (mountain) area. The western part of the state accounts accounts for two-thirds of the state's production. For western North Carolina producers, primary supply competitors include growers in the central coast area of California, the Delmarva peninsula, Michigan,Ohio, and New Jersey. Because to the late summer harvest period, many North Carolina tomato growers must also compete against supplies available from home gardens and a number of minor (but seasonably important) producing regions in many states. Because supply availability is usually greatest in late summer, market competition is severe and average grower prices reach their seasonal low at that time (Table 4). These circumstances often provide marketing problems for many western North Carolina tomato growers.

Fresh market tomatoes are an expensive crop to grow and harvest. Production, harvesting, grading, and packing expenses for 1 acre of staked tomatoes may exceed $9,000, or about 21 cents per pound. In addition, specialized production and handling equipment must be used. High production cost combined with the extreme perishability of fresh tomatoes creates uncertain market conditions for growers. Although some producers of field-grown tomatoes market their crop directly, most use brokers. Brokers estimate that about one-third of the fresh crop is sold to terminal market buyers and to area packer-shippers. Slightly more than one-half of the crop is sold to chain stores and brokers, with the remainder sold directly to consumers via community farmers' markets.

In the past, a majority of production in the western part of the state has consisted of vine-ripened tomatoes (sold in 20-pound containers). During the last five years, however, sales of mature green tomatoes have increased. In part, this increase occurred because of higher shipping, harvesting, and handling losses for vine-ripened fruit and a narrowing in price differences between mature green and vine-ripened tomatoes. Market opportunities exist for both vine- ripened and mature green tomatoes, but high grading and packing standards are required. Stabilization of per capita consumption, the possibility of increased competition from Mexican and domestic growers, and variability in prices contribute to industry concerns about the future. However, experienced, progressive producers who adopt proper postharvest handling practices will continue to find tomato production profitable.


Table 4. North Carolina and U.S. Average (August) Grower Prices for Martketable Fresh Tomatoes (All Sizes) from 1988 through 1993

Price per Hundredweight
1988 1989 1990 1991 1992 1993 1988-1993 average
North Carolina $20.70 $15.40 $19.00 $16.90 $20.20 $22.85 $19.18
United States $38.30 24.60 26.00 21.90 23.60 24.75 26.53


The following publications in this series on the postharvest cooling and handling of North Carolina fresh produce are available from your county Cooperative Extension Center.



Prepared by:
M.D. Boyette, Extension Agricultural Engineering Specialist
D.C. Sanders, Extension Horticulture Specialist
E.A. Estes, Extension Economist