Commodities / Grapes
Defects and Disease |
Shelf Life |
Grape quality is relative to the intended use. Quality of grapes for juice, jam, or wine is usually measured in terms of sugar content (% soluble solids), total acidity (grams tartaric acid per liter), pH, and development of variety flavor and color. Quality of table grapes is usually measured as a combination of appearance and flavor. Freedom from defects is a major quality parameter in both processing and fresh grapes.
Grade standards have been established for the various types and uses of grapes. Refer to these USDA standards for specific information. Processors often impose minimum standards for sugar content.Flavor top
Grape flavor is related to degree of ripeness. Grapes do not continue to ripen after harvest so harvest timing is critical. Flavor in table grapes is dependent on varietal character and the ratio of sugar to acid. A ratio of 20 to 22 (with each parameter expressed as percent) is usually considered optimum. For example, grapes with 20% soluble solids and 1% total acid would taste as sweet as grapes with 16% soluble solids and 0.8% total acidity. Flavor of juice and wine grapes is based primarily on varietal character.
Development of flavor occurs best when growing conditions are optimal for the variety. Some varieties perform best under warm conditions, while others do better under cool conditions. For example, Concord grapes grown for juice and jam are produced primarily along the Great Lakes and in Washington state where cool conditions during ripening promote development of the characteristic varietal flavor. Concord grapes grown in warmer climates often fail to ripen evenly and overall quality is diminished.
Color is an important aspect of grape quality, especially for red (blue or black) processing grapes. The anthocyanins responsible for berry color develop in the grape skins during the ripening process. These color compounds are extracted from the skins during processing and are an important component of product quality.
Color development in red (blue or black) grapes is dependent on exposure of the clusters to partial sunshine. Therefore, proper canopy management is necessary for optimum color. This may involve proactive measures such as vigor management, pruning level, training system, etc., and/or reactive measures such as shoot positioning and cluster zone leaf removal.
Berry size in grapes is only a consideration for table grapes. Minimum berry diameter and bunch weights are included in the USDA standards and are based on the type and variety of grapes.
There are a number of diseases that affect grapes. Some occur primarily on the leaves and indirectly affect fruit quality. Others occur on the fruit and directly affect fruit quality.
The most common diseases affecting grapes are black rot, powdery mildew, downy mildew, Phomopsis cane and leaf spot, and Botrytis bunch rot. Miscellaneous bunch rots also occur in some years. Control recommendations can be found in the Midwest Small Fruit Pest Management Handbook and Midwest Small Fruit and Grape Spray Guide for Commercial Growers.
Gray Mold (Botrytis cinerea) can occur in the field or in storage in table grapes. Penicillium and other molds can also occur on fruit in storage.
Grape berry moth is the most common pest that affects fruit quality. This second generation of this pest feeds on developing fruit and often leads to secondary fruit rots. Japanese beetle is another common insect pest of grapes that feeds primarily on the leaves and does not directly affect fruit quality.
Shelf life is only important in grapes intended for table use. Decreased quality during postharvest handling of table grapes is most often associated with water loss and decay. Browning of the cluster stem and shelling of berries is another problem.
It is important to remove field heat as soon as possible to prevent water loss. Most table grape shippers will use forced air cooling to achieve temperatures of 32-34˚F. Grapes are stored at 30 to 32˚F and freeze at 28˚F. A shelf life of 2 weeks to 6 months can be expected, depending on variety, if these temperatures are maintained at 90% to 95% humidity. For long-term storage, controlled atmosphere and SO2 fumigation is frequently used.