POSTHARVEST COOLING AND HANDLING OF
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.
Mature green....................................21 to 28 days
Pink...................................................7 to 14 days
Red....................................................2 to 4 days
Optimum relative humidity:
...........................................................85 to 95%
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:
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.
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.
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: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."
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.
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.
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.
Vine-RipenedMature Green Cherry
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.
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.