In raspberry, for every hour delay in cooling, shelf life can be reduced by one day

The paper by Dr. Marvin Pritts, Department of Horticulture, Cornell University, explains the main aspects regarding rasberries and related fruits growing and postharvest. It includes a section on Taxonomy and Commercial Importance, the Commercial Industry, Varieties, Morphology and Anatomy of Fruit, Chemical and Nutritional Composition, Handling and Storage of Fresh Produce, and Fruit Uses. The complete work is available on the link and the pdf (both, below), and the section regarding Handling and Storage, follows, divided in two parts, harvest and postharvest considerations. Handling and Storage of Fresh Produce Harvest ConsiderationsRaspberries have one of the highest respiration rates of any fruit. This, coupled with their thin skin and sugary interior, makes them among the most perishable of all fruits. With any given variety, fruit on a bush will ripen over a period of several weeks. Harvesting the same planting frequently (once every two days) is critical. Fruit harvested before it is fully ripe will have a much longer shelf life than fully ripe or overripe fruit, but will be lower in sugar and anthocyanins. The optimum stage of maturity for the raspberry occurs when the berry first becomes completely red, but before any darker hues develop. Fruit quality for fresh market raspberries usually declines as the season progresses. Marketing channels must be open before the first berries ripen, as these are likely to be the highest quality and have largest size for the season.? Berries should not be touched before harvest, and only undamaged berries with good appearance should be placed in the pack. The magnitude of injury caused by human pickers can be so great as to mask any other causes of deterioration. Overripe or damaged berries should be harvested and discarded because they are susceptible to moulds. Botrytis is the most common pathogen of raspberry fruit. Once the mould growing on overripe berries sporulates, large amounts of inoculum will be present to infect other ripening fruit. Overripe berries also attract ants, wasps, and other pests. Containers holding approximately 150 g of raspberries are typically used. Wide, shallow containers are preferable to deep containers; each should have no more than four layers of raspberries to prevent crushing. Many different types of containers are available, but among the most popular today are plastic clamshells. Post-harvest ConsiderationsThe objective of post-harvest handling of raspberries is to slow the respiration and transpiration rate of fruit. Respiration and transpiration result in shrinkage and reduced sweetness. Conditions that slow the respiration process are low temperatures, high carbon dioxide levels, and low oxygen in the storage chamber. Transpiration is slowed by high humidity. Temperature is the easiest environmental variable to modify for extended storage of raspberries. A 5C reduction in temperature reduces the respiration rate by approximately 50%: at 0C the respiration rate is 24 mg of carbon dioxide per kg of raspberry per hour; at 5C the rate is 55; at 10C the rate is 92; at 15C the rate is 135; and at 20C the rate is 200. Rapid movement of cold, humid air through the berries is essential during the first few hours after harvest to slow respiration and prevent deterioration. Large growers may have a precooling facility specifically designed for removing field heat, and an additional storage cooler. For every hour delay in cooling, shelf life can be reduced by one day. Growers take advantage of natural night cooling by harvesting fruit as early in the morning as possible. Mechanical harvesting is often done at night. Once the berries are cool, containers are wrapped in plastic to prevent water loss from the fruit and condensation on the berries when they are removed from the cooler. The plastic is not removed until the temperature of the berries warms to near the ambient temperature. The storage room can be maintained as low as -1C. Berries will not freeze at or above this temperature because the sugars in the fruit depress the freezing point. When temperature is lowered, the amount of moisture in the air is reduced. For raspberries, it is critical to maintain a humid atmosphere (90-95%) simultaneously with low temperature to prevent water loss from the fruit. Special cooling units designed to maintain a high humidity are required for raspberries. At 25C and 30% relative humidity, fruits lose water 35 times faster than at OC and 90% relative humidity. A high carbon dioxide (15-20%) and/or low oxygen (5-10%) atmosphere will reduce respiration and mould growth. Botrytis, Rhizopus, Alternaria, Penicillium and Cladosporium can cause post-harvest fruit rots, depending on storage temperature and carbon dioxide level. Modified atmospheres are used frequently when raspberries are transported long distances. Special semi-permeable wraps are sometimes used by shippers to create a modified atmosphere (low oxygen, high carbon dioxide) within individual containers. Low-oxygen atmospheres will extend the shelf life of raspberries, but bad-tasting aldehydes and alcohols can accumulate in the raspberry fruit when oxygen is limited for an extended period. Off-flavors and browning also develop when raspberries are held under elevated carbon dioxide levels for an extended period. Raspberries are considered to be at moderate risk of physiological injury from high carbon dioxide or low oxygen atmospheres. The loss of raspberries from harvest to the consumer?s table has been estimated at more than 40%. A 14% loss occurs from farmer to wholesaler, a 6% loss occurs from wholesaler to retailer, and 22% is lost between the retailer and consumer. Most of these losses are due to poor handling of berries after harvest. ? Source:RASPBERRIES AND RELATED FRUIT, Dr. Marvin Pritts, Department of Horticulture, Cornell University?s College of Agriculture and Life Sciences, Ithaca, NY 14853