The flavor of a tomato is determined by the amount of sugar and acid present. The more sugar compared to acid the sweeter and milder the taste. If the acid level is high compared to sugar then the fruit tastes sour. Sugar is measured by a refractometer; acid level is determined by titration or by measuring pH. Taste panels usually rate tomatoes with a balanced ratio of sugar to acid as the most pleasing. In many cases tomatoes considered mild in taste and thought to be low acid tomatoes have about the same level of acid as those judged as having high acid; the key is the sugar level. The sugars involved are fructose and glucose. The acids are citric acid and malic acid. The sugars increase with maturation but take a pronounced rise coincident with pigment formation. Maximum acidity occurs with the first pink color and then declines. There are no standards for sugar or acid content in the U.S. Standards for Grades of Fresh Tomatoes.

Cloudy weather (decreased light) and high nitrogen decrease sugar. Fruit maturing during longer days have more sugar than fruit maturing late in the season under shorter days. Water stress increases sugar concentration.

Both sugars and acids are products of photosynthesis. If foliage is damaged by disease or reduced by insect feeding or excessive pruning the capacity to produce sugars and acids is decreased.

Tomatoes harvested at the mature green stage will have less sugar and more acid than tomatoes harvested near table ripe.

The longer a tomato is in storage the greater the loss of aroma and development of off flavors. Ethylene treatment does not affect the flavor of tomatoes harvested at mature green; it shortens the time required for ripening. Exposure to chilling temperatures (below 50˚F) should be avoided to prevent off flavors.

Lycopene is the dominant pigment in red tomatoes along with smaller amounts of beta-carotene, a yellow pigment. Yellow or orange tomatoes contain beta-carotene and other carotenoids. Pink tomatoes have lycopene in the flesh and a colorless skin to give the pink color.

Tomatoes are packed and sold according to amount of color the fruit show. There are six colors specified in the grade standards from mature green to red.

Lycopene synthesis is reduced at temperatures above 86˚F resulting in yellow shoulders on many varieties. Green shouldered varieties seem to be more susceptible than those having uniform ripening.

Grey wall and blotchy ripening are disorders that can reduce marketable yields significantly. The specific cause has not been identified, but numerous environmental conditions and pathogens have been associated with the problems. Low potassium, high soil moisture, cool and cloudy conditions, tobacco mosaic virus and bacteria are all part of the complex. Some varieties are affected less than others, but none are immune. Recent research on yellow shoulder disorder in processing tomatoes indicates that soil potassium, magnesium and organic matter content, along with soil pH, can be used to predict whether the disorder is likely to show up in a particular field. Information on this research is available from The Ohio State University.

Stink bug feeding on tomato fruit can cause a condition called cloudy spot. White to yellow irregular spots occur just below the fruit skin where the insect has injected a enzyme which prevents normal color development. Recommendations for control are available in the Midwest Vegetable Production Guide for Commercial Growers or through your local Cooperative Extension Service.

Size is not related to eating quality. United States grade standards define size classes for standard tomatoes based on transverse diameter. In the marketplace, sizes are often defined by how many tomatoes fit into one layer of a standard 2-layer tomato box, e.g. the size 6X6 refers to tomatoes of a size that will fit 6 rows of 6 in a box. For beefsteak types, the market for small tomatoes (2-1/8" to 2-9/32") is almost always less than for larger fruit. Even the next size up (2-1/4" to 2-17/32" [6 x 6]) is not in great demand. Varieties and practices that produce large average fruit size will provide a greater return in this market, and so are considered necessary for the best quality. Average fruit size is generally increased when one or more branches (suckers) below the first flower cluster are removed early in the growing season. Increasing plant population tends reduce fruit size but not yield.

United States Standards for Grades of Fresh Tomatoes say that misshapen means that the tomato is "decidedly kidney-shaped, lop-sided, elongated, angular or otherwise decidedly deformed." Buyers want a pack that is uniform in size and shape; when it is not, the pack may be regarded as having low quality.

Disease

Many of the defects and diseases affecting the fruit can be graded out in the packing shed and a high quality product can be delivered. Some defects like cracking, puffiness, catfaces and scars have permissible dimensions described in the United States Standards for Grades. Most diseased fruit can be left in the field or graded out, however, some may not be evident at the time of packing, including anthracnose and in some cases early blight.

Water management is important in minimizing blossom-end rot and fruit cracking. These two defects plus shoulder checking account for a high percentage of the marketable yield loss growers experience almost every year. Shoulder checking is very fine cracking of skin of the fruit. It is closely associated with rain, especially after an extended dry period. Some work suggests that boron applications to the soil or sprayed on the foliage may reduce the incidence of shoulder checking, especially on coarse textured soils. Properly managed trickle irrigation is ideal for managing water and nutrients throughout the growing season.

Maintaining a vigorous vine by controlling insects and disease is fundamental to producing a crop of quality tomatoes. Early blight (Alternaria solani) and anthracnose (Colletotrichum coccodes) are fungal diseases that are threats every year. Early blight can reduce vine productivity and infect the fruit. Anthracnose can infect immature fruit and not be detected at the farm. When the fruit ripens the infection becomes destructive. In some years bacterial diseases are destructive to both the vine and the fruit. Normally infected fruit can be graded out but it can be a quality problem of immense proportions to the grower, especially if the infection comes early in the growing season.

Insect management is normally not difficult and fruit quality is maintained. Recommendations for control are available in the Midwest Vegetable Production Guide for Commercial Growers or from your local Cooperative Extension Service.

Leaving diseased and defective fruit in the field and careful handling to avoid bruising are important to maintain quality.

Tomatoes are subject to increased infection by soft rot bacteria (Erwinia carotovora) and several fungal pathogens when dump tanks are not managed correctly. Due to the porous nature of the stem scar, soft rot and other pathogens may enter if the tomatoes are too deep in the tank or are in the tank too long. The dump tank water should be 10˚F higher than the highest fruit pulp temperature. The tomatoes should not be submerged more than two feet and a single layer of fruit in the tank should be maintained. Tomatoes should be in the water less than two minutes if possible.

Shelf life of a tomato is determined by the stage of maturity at which it is harvested, the condition of the fruit at harvest, the temperature at which it is stored and the variety.

The optimum storage temperature for maintaining postharvest quality of mature green tomatoes is 55˚ to 60˚F; for light red (USDA Color Stage 5) is 50˚ to 55˚F and for Firm-Ripe (USDA Color Stage 6) is 44˚ to 50˚F for 3 to 5 days. Mature green tomatoes can be stored 14 days prior to ripening at 55˚F without significant reduction of sensory quality and color development. Tomatoes of all stages of maturity should be stored at 90 to 95% relative humidity.

Storage time can be doubled in controlled atmosphere. Low oxygen levels (3% to 5%) delay ripening and the development of surface and stem scar molds without severely reducing sensory quality.

Ripening green tomatoes is best at temperatures from 55˚F to 77˚F; 90 to 95% relative humidity and using 100ppm ethylene. Ethylene treatment lasts 24 to 72 hours. Once ripening is initiated, the best temperature to maintain sensory and nutritive quality is 68˚F.