Blemishes on fruits and vegetables result from a variety of causes; some imperfections are harmful but many are not. This fact sheet identifies causal factors that result in blemishes, describes the injury they cause and determines if the damage is too severe for consumption of the fruit or vegetable.

Throughout this fact sheet, methods are discussed that can help reduce damage. By learning to identify cosmetic blemishes consumers can greatly reduce waste and frequently obtain good produce at a lower cost.

Preharvest factors: genetics, natural damage, nutrition and plant health in the field

Selection of new fruit and vegetable varieties is strongly affected by the market's desire for large, brightly colored fruit with symmetrical appearance. Discoloration or non-uniformity sometimes results in the downgrading of these commodities.

For example, the presence of russet, a brownish roughened area on the skin of apples does not negatively influence eating quality. Russet on apples can be genetic, as well as induced by environmental factors such as mild frost damage. In the past, many popular apple varieties were completely covered with russet. But today, fruit with noticeable russeting often does not pass US grading standards.

Many crops with preharvest damage from field conditions are commercially graded out before being marketed. However, some damage only affects products visually and this damage is easily cut out before eating. Examples include a cut on a potato, scarring from rubbing of fruit on branches, and tissue healing processes after surface damage such as that caused by hail or insects. If the product is soft or overripe, cosmetic damage can be a genuine safety concern, especially where infection by pathogens can cause rotting.

Inappropriate fertilization of products, resulting from excessive nitrogen or inadequate calcium applications in the field, can result in formation of necrotic lesions in the tissue. One example of this, known as bitter pit or stippen, occurs in some apple varieties as brown pitting on the skin or in the flesh. This disorder can cause major losses, and as its common name suggests, leads to off-flavor in the fruit. Moreover, bitter pit can continue to develop during storage, and presence of this type of disorder is an indication of poor storability of the crop. Disorders related to low calcium can be found in many fruits and vegetables (see Table 1).

Harvest and handling factors: Bruising and vibration damage

Bruising is the most noted concern of consumers in the marketplace. Bruising can happen anywhere in the handling chain, from harvest to purchase, but occurs most frequently during harvest. Bruising can occur as a result of impact or compression damage.

Impact damage occurs when the product is dropped some height to a hard surface. This bruising is frequently caused by commercial packing lines or by harvesters who are not appropriately trained and treat the product roughly.

Compression damage occurs when a product is crushed or compressed by the weight of others on top of it. This happens if the product is soft and inappropriate containers are used. For example, the containers may be too deep, allowing too much product weight on the bottom fruit or vegetable.

Both impact and compression bruising can cause significant damage, discolor tissues, and detract from the visual and eating quality of the commodity. Even when browning does not occur, such as in the case of bruised strawberry fruit, the damaged areas look unattractively dark.

Vibration injury occurs after harvest when the products rub against each other or the carton trays during transport. A good example of this is the dark scuff marks on the skin of pear fruit when it ripens.

Postharvest storage: Senescence, chilling injury, and ethylene gas

Blemishes can develop during storage of products for at least three major reasons. The first of these is senescence problems. Senescence is defined as the phase of plant growth after full maturity that results in tissue death. This condition typically shows up as tissue browning. More rapid than normal senescence and development of brown flesh can indicate that the nutrient composition of the product is poor. A good example is senescent breakdown of apples, a condition that is associated with low calcium in the fruit.

The second set of postharvest problems is chilling injury. Whereas most apples, stone fruit, and vegetables such as asparagus, cabbage, lettuce and onions should be stored close to 32°F to maximize storage life, many products, especially those of sub-tropical or tropical origin develop chilling injury if stored at too low of a temperature.

Good examples of chilling sensitive products are cucumbers, eggplants, sweet peppers and tomatoes. These products should be stored at 45°F or above. However, they are occasionally stored with chilling insensitive products in retail markets or in the home.

Damage doesn't always occur to these products because exposure to low temperatures has to be lengthy. When damage does occur, it can be manifested as pitting of the skin, uneven ripening, loss of flavor or development of off-flavors, and higher susceptibility to decay.

A common type of chilling injury seen in supermarkets is pitting on green peppers, but usually the flavor is unaffected unless damage is severe. Additional types of injury due to chilling include skin graying on ripening bananas and failure of peaches to ripen with a juicy texture. In general, visual symptoms of chilling injury are not pronounced and are often overlooked by the consumer (see Table 2).

The third type of blemish, which is perhaps the least understood in the marketplace, is associated with mixed storage of products that produce ethylene gas with products that are injured by its presence.

Ethylene is a natural plant growth regulator, produced by "climacteric fruit", such as apples, tomatoes and bananas (see Table 3). It is a colorless gas that is thought to coordinate the ripening of these fruits. Ethylene gas is also produced by tractors and other equipment with internal combustion engines and can accelerate ripening of climacteric fruit.

Ethylene can have a number of deleterious effects on other products. These include faster yellowing and senescence of leafy vegetables, browning reactions such as russet spotting on lettuce leaves, development of bitterness in carrots, and sprouting of potatoes.

To avoid blemishes associated with ethylene gas, products that produce it should not be stored with low ethylene producers, which are sensitive to the gas. If this is unavoidable, one method of minimizing damage is to ensure that storage areas are well ventilated. Products such as potassium permanganate that absorb ethylene are available both for use in commercial storage and in home refrigerators.

Summary

The bottom line is that almost any blemish found on products at harvest is considered to be a grade defect in most markets. Often, these criteria have been developed solely for marketing purposes, but they also protect the consumer from products that may decay or deteriorate more rapidly than normal.

The consumer who is willing to sacrifice visual appearance may learn to differentiate blemishes that are purely cosmetic from those that are unsafe to eat. Whatever the growing method, food with blemishes that result from decay, causing odor, mold, or a fermented taste, should be avoided for safety reasons.

References

Postharvest Technology of Horticultural Crops. Adel A. Kader, ed., 1992. University of California Division of Agriculture and Natural Resources. Oakland, CA. 296 pp.

Postharvest An Introduction to the Physiology and Handling of Fruit and Vegetables. R.H.H. Wills, T. H. Lee, D. Graham, W. B. McGlason, and E.G. Hall. 1981. AVI Publishing Company Inc., Westport, CT. 161 pp.

The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Robert E. Hardenburg, Alley E. Watada, Chien Yi Wang, U.S. Department of Agriculture, Agriculture Handbook No. 66 (revised), 136 pp.

Prepared by:

Christopher B. Watkins, Associate Professor, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853

Margaret Wahlig Cole, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853