Post Harvest Handling of Blueberries
- Field Handling
- Packing Shed
- Temperature Management
- Modified and Controlled Atmosphere
- Postharvest Diseases
In order to ensure the longest postharvest shelf life for both fresh and processed fruit, a detailed postharvest handling strategy plan should be developed. Post a copy of the plan so that all pickers, field supervisors, and packing shed personnel are aware of the steps that must be followed. Be sure to develop copies, in the appropriate language, for non-English-speaking workers. Adequate training for pickers and supervisors is essential.
There are many opportunities for field bruising after the berries have been picked. If blueberries are picked into buckets in the field for later transfer to a processing flat, be sure to instruct field staff not to over-fill flats. Compression damage will result and poor cooling will occur if flats contain too many berries. Instruct truck and trailer loaders to use care when handling flats so as not to jar or drop them. Drivers should proceed slowly over rough roadways; be sure truck suspension systems are fully functional.
Protecting berries from heat deterioration begins in the field. Filled flats should never be left exposed to the sun because the fruit temperature can rise above the air temperature in less than an hour. Flats left in the shade will have fruit temperatures as much as 10 F lower than the ambient air temperature. If there is a long delay between harvest and transfer to packing shed or processor, place a light-colored tarpaulin over the flats and periodically wet it down to provide evaporative cooling. These recommendations pertain to processed as well as fresh market fruit. For fresh market fruit, however, flats are normally brought into the packing shed soon after they are filled.
For fresh operations where fruit is picked into buckets, a sorting belt will have to be installed so that under-ripe, over-ripe, and rotted fruit, as well as twigs and leaves, can be removed prior to packing. Provide for worker comfort when standing at the line by providing stools or platforms to elevate workers. Ensure good lighting over sorting belts. Be sure to instruct workers to clip fingernails, remove rings, and wear hairnets when working on the line.
Fruit should be only one layer deep when flowing along the belt. At the end of the sorting line, the berries drop into the standard pulp-paper pint containers. Frequently, extra fruit is placed in the container to overfill it by 20 percent. From there, the pints are commonly capped with a cellophane square held in place with a rubber band. After capping, 12 pints are placed in a standard corrugated shipping flat. A standard pallet load consists of 96 flats. Fresh blueberries are not washed during the sorting process. However, in order to maintain a quality pack, the sorting line will have to be washed down periodically with soap and water to remove spoiled fruit and accumulated dirt.
For processed fruit, flats are emptied onto a belt that passes beneath a blower system that removes stems and twigs. Following this, the fruit travels through a water dump tank where under-ripe fruit floats off to be used later in a juice operation. The conveyor belt next passes through another blower which dries the fruit prior to hand sorting. If the fruit is destined for bulk freezing prior to further processing, it goes directly into a shipping container.
Standard container weights are 6-1/2 and 28 to 30 lb plastic pails, or, for larger users, 400 lb metal drums lined with a 2-4 mil plastic bag. Bulk frozen product packed without sugar is referred to as “straight pack” in the industry. Various “sugar packs” are used, but the standard is known as a “4+1,” where 24 lbs of berries are packed with 6 lbs of sugar in a 30 lb pail. For individually quick frozen (IQF) product, the sorting belt travels through a freezing tunnel where the berries freeze separately. IQF fruit is commonly packed into corrugated boxes lined with a plastic bag. The standard in the industry is 30 lbs net weight of IQF fruit.
For fresh market fruit, the best management plan for ensuring optimum postharvest quality will have to include some type of temperature control. Fresh fruit is still respiring heavily when it is picked. In the process of respiration, as oxygen is consumed and carbon dioxide and heat are liberated, fruit sugars are depleted and shrinkage occurs. The rate of respiration is governed by temperature.
Figure 1 shows the rise in respiration rates for blueberries and compares this with those of strawberries. In blueberries, respiration rates are three times higher for fruit held at 50 F compared to that held at less than 40 F, and seven times higher when held at a room temperature of 70 F. The lower respiration rates of blueberries compared to those of strawberries partially accounts for the longer shelf life of blueberries.
Field heat should be removed from freshly harvested fruit as soon as possible. Research has shown that blueberries cooled to 35 F within 2 hours with forced air cooling had significantly less decay (37 to 46 percent) after 10 days’ storage at 35 F than fruit that had been cooled to 35 F within 48 hours. The rapid removal of field heat with cold forced air is referred to as precooling or pressure cooling. By pulling cold air (via convection) across the berries, field heat can be removed in approximately 1 hour (figure 2).
Conversely, in a conventional refrigerated cool room, removal of field heat is exceedingly slow and inefficient and tightly packed flats of berries may receive inadequate cooling. Moisture released by warm interior berries can lead to “sweating,” or moisture condensation on colder fruit on the outside.
Efficient operators combine a precooling setup inside of a refrigerated cold room. A thermometer should be used to monitor fruit temperature. Experiment with the width of stacks and the width of the plenum to find the most efficient arrangement in terms of temperature reduction and floor space. If fruit is left too long in the forced air tunnel, excessive desiccation can occur. There are other precooling arrangements. For a complete guide, refer to Special Publication 3311, Postharvest Technology of Horticultural Crops (see Bibliography).
After flats have been cooled, they can be left in the cold room until they are packed into market containers. As with other small fruits, blueberries should be kept under conditions of high humidity (95 percent at 32 F). Even though the flats will show condensation after removal from the cooler, this “sweating” has not been found to alter the white “bloom” of fresh berries or contribute to postharvest disease development. With careful picking, prompt precooling, and storage at 32 F, blueberries have a minimum shelf life of approximately 14 days.
In order to extend the shelf life even further, studies have examined the effect of reducing postharvest spoilage due to moisture loss and fungal diseases by either over-wrapping (shrink-wrapped) filled pint containers with various types of semi-permeable film wraps (referred to as modified atmosphere packaging or MAP), or by placing filled flats in cold storage under conditions of various concentrations of carbon dioxide and oxygen (referred to as controlled atmosphere storage, or CA). MAP storage reduced moisture loss considerably, but the high humidity developing in the shrink-wrapped packs increased the incidence of fungal disorders.
In CA trials, filled flats were placed in chambers of various atmospheric concentrations at 32 F. The best treatment consisted of 1.8 percent oxygen and 12 percent carbon dioxide. After 46 days of storage under these conditions, 97 percent of the berries were rated as having very good quality. Initial work with both MAP and CA thus appears promising. In the absence of either MAP or CA, however, the proven technique of storing fruit at 32 F at a high relative humidity remains the most promising technique of gaining the longest shelf life for freshly harvested fruit.
Most fresh blueberries that are shipped throughout the western United States are sent via over-the-road, refrigerated truck trailers. A diesel motor powers a refrigeration unit that sends cold air from the front of the trailer to the rear through a canvas air delivery chute. When the cold air hits the rear doors of the trailer, it is deflected down and back toward the front of the trailer through and under the flats of berries. Flats should be loaded on wooden pallets to ensure good return flow.
In addition, a centerline loading pattern, as opposed to sidewall loading (flats stacked up against trailer sides), should be used. Use bracing materials, and stretch plastic over the flats to keep the load from touching the sidewall of the trailer. The trailer used to transport the berries must be cool (below 35 F) prior to loading, as the refrigeration system will not be able to keep up with rise in temperature from natural fruit respiration.
Fresh blueberries can also be shipped by air to market destinations that would require too much time to reach by truck. Commonly, the insulated LD3 containers are used for larger shipments into major hubs, while the smaller Series E container is used for smaller shipments. The refrigerated LD3 container has a maximum cargo weight capacity of 3,100 lbs, including 125 lbs of dry ice to help keep the temperature of the fruit from rising during shipment. Growers either rent these containers from the airlines or purchase their own.
Grower experience has shown that the insulated LD3 containers do not maintain the fruit temperature below 40 F during transit. There is simply not enough insulation to overcome the warm cargo hold that the containers are placed in during the flight to East Coast markets. The Series E container must be assembled by the grower. It consists of insulated styrofoam panels that are held together with strapping tape. The E container has a maximum cargo weight of 482 lbs. When gel ice is added to the flats of berries and the box is sealed, fruit temperature can be maintained below 40 F while the fruit is in transit. Fruit that has been sent to the Orient by Northwest shippers has arrived in satisfactory condition.
The principal postharvest disease-causing organisms include grey mold (Botrytis cinerea), ripe rot or anthracnose (Colletotrichum gloeo-sporoides), and soft rot (Rhizopus nigrans). Grey mold is the most important postharvest disease for blueberries from all major producing regions in North America. Infection produces a soft, watery decay followed by the development of grayish-white mycelium on the berry surface. Grey mold is more of a problem if harvest occurs during cool, rainy weather. Decay is not evident until after the berries are placed in cold storage. Fungicides applied during bloom to control Botrytis blight will help considerably in the control of postharvest infections.
Ripe rot, sometimes referred to as anthracnose fruit rot, only occurs in the Pacific Northwest if an extended period of rain occurs during harvest. As fruit begins to ripen and turn blue, the first indication of infection is a softening and puckering of the blossom end of the fruit. During wet weather, masses of salmon-colored spores are produced on all infected plant parts. Symptoms may not be seen until after harvest when the salmon-colored spores are seen inside the cellophane-covered baskets. Fungicides applied during bloom for the control of Botrytis blight help prevent postharvest infections of ripe rot.
Rhizopus soft rot is a potential problem if blueberries have not received prompt cooling after harvest. Rhizopus nigrans is reported to not grow at temperatures below 50 F. Infections are characterized by the presence of leaking berries, white mycelial growth, and emergence of spore-bearing heads which are white at first but later change to a dull black. Infections require the presence of free water on the surface of the fruit, as well as a break in the fruit surface.
The physical attributes of the berries themselves can influence the degree of postharvest decay development. Research has shown that fruit with a 16:1 sugar-acid ratio experienced an 8 percent breakdown when held at 40 F for 18 days, while fruit with a sugar-acid ratio of 32:1 had a 28 percent breakdown under the same conditions.
This fact sheet is adapted from Oregon State University Extension Publication PNW215, Highbush Blueberry Production. The authors of Highbush Blueberry Production are – Oregon State University: Bernadine Strik, Glenn Fisher, John Hart, Russ Ingham, Diane Kaufman, Ross Penhallegon, Jay Pscheidt and Ray William; Washington State University: Charles Brun, M. Ahmedullah, Art Antonelli, Leonard Askham, Peter Bristow, Dyvon Havens, Bill Scheer, and Carl Shanks; University of Idaho: Dan Barney. PNW215, Highbush Blueberry Production can be purchased from the Department of Extension & Experiment Station Communications, Oregon State University. How to Order