Grape Postharvest Operations with emphasis in developing countries

Authors: Fabio Mencarelli, Andrea Bellincontro – LAPO, Department of Food Science and Technology, University of Viterbo, Italy.
Email: mencarel@unitus.it
Giancarlo DiRenzo – Technical Economic Department, University of Basilicata, Italy.
E-mail: direnzo@unibas.it

Edited by: Danilo Mejía, PhD - Agricultural and Food Engineering Technologies Service (AGST)

2. Postharvest Operations

2.1 Harvest

Harvest represents the moment of detachment of bunches from the vine at the proper level of maturity. It should be performed without mechanical damage and product loss, as quickly as possible, minimizing the costs. At the present, hand harvest is the only harvesting system for table-grapes. The primary advantages of hand harvest are:

-     human picker can handle with care bunches avoiding mechanical damage;
-     selection of fruits starts on the vine because the picker selects maturity and appearance;
-     multiple (generally no more than two) harvest for grading the bunches on the vine;
-     minimum of capital investment.

Harvest rate depends on the grapes vine growing system, the number and expertise of workers, and packing system adopted. Very high harvest rate could be obtained using growing system called Italian pergulate (Tendone) which permits a fast harvesting considering that bunches are concentrate in the middle of the row more o less 1.7 m height from the ground; the picker picks bunches walking down the vine without need of ladder or stool (Fig.6).

The main problems with hand harvest are due to labor management and picker skill.

Moreover quality is such an important aspect in successful marketing that hand harvest is still the only method used for table grapes. Management should be very careful for effective use of hand labor. A short training period is necessary for harvesting and packaging grape according to the market request. At the present, table grapes harvesting is carried out by the picker that selects the bunch and cuts the stem using very sharp shears. Before releasing the bunch in the box, the picker generally removes brown and moldy berries. Thus, the picker performs the following operations:

-     bunch selection using color and eventually dimension as selection criteria;
-     detachment of the bunch selected by keeping the bunch in one hand and than with sharp scissor cut the stem
-     bunch trimming and cleaning (removal or decayed and moldy berries)
-     place gently the bunch in the box, without pressing or squeezing

In the case of “field packing”, table grapes may be picked, sorted and packed directly into the shipping container by the picker (Fig.16).

Generally the pickers and the packer work together: the picker detaches, trims and cleans the bunch, the packer places the bunch in the shipping container and arranges the packing materials like the sponges tissue paper that are used to improve product appearance and to reduce bruising among the bunches during the following operations. The pickers walk down the vine. The packer move all the packing materials and the table used to support the container during the filling operation. Empty and filled container are left on ground and carried by another worker. This system minimizes rehandling of the fruit, but makes the quality control on the product more difficult than in the packinghouse.

A particular type of field packing is the so called “avenue packing”. In this system pickers and packers do not work in pair, but they are located in different places. The picker picks and trims the bunches and places them into field lugs. When the lug is filled, it is transferred to the packer that use a working table located in the avenue between vineyard blocks. Packing materials are located close to the packing place. In this case quality control is still difficult, because the supervisor has to move around the vineyard in order to control the different packers

In case of field packing picker productivity is reduced in the range 50÷80 kg/h . The working efficiency depend on bunches density in the vine; bunch size, uniformity of berry color and size.

In Developing Countries where local investment are difficult, this type of harvest for table grapes more than for other fruits should be promoted for few important reasons:

In the “shed packing” (in the packinhouse), bunches are harvested by pickers and placed in field lugs untrimmed; in this case field lugs are generally made in plastic with the following dimension 0.6 x 0.4 x 0.25 m, the weight of grape is generally into the following range 18÷20 kg (Fig.17).

Usually grapes tolerate this depth, but it could be decreased to avoid bruising due to compression of bottom layers of table grapes. The lugs are vented on sides and bottom to facilitate air circulation. To avoid fruit cutting, ventilation slots are normally rounded so that inside surfaces is easy to clean and smooth to reduce fruits abrasion injury. Rough handling and impacts can increase roughness of the internal surface and create fruit abrasion problems. If wood lugs are used, they should be coated (paint or varnish-type) to reduce this problem.

The picker productivity range 100 ÷ 150 kg /h if the bunches are placed in filed lug and subsequently cleaned, trimmed and packed in the packinghouse..

The first step in fruit protection from mechanical injury is a careful field supervision. Inaccurate picking or packing procedures like over-filling the containers or beating bunches against container hard surface dramatically reduce the product quality and postharvest life of table grape.

2.2 Packinghouse (shed) operations

The lug should remain in the shade to wait for the transport to the shed. At the shed, grapes are precooled as soon as they arrive from the field (generally late in the morning or in the first hours of the afternoon). Grape bunches are packed the day after or later, in relation with the market request. Often the packinghouse operations are carried on when the weather forecast does not permit the field packing for more than one day.

In the packing line, table grape is not dumped on a packing belt but it is packed directly into the shipping container (box, plastic bag, basket) from field lugs. Generally the worker stands in front of a work-table where the hole for trimmed materials, the shipping container positioned on the scale pan, and the field lug are located (Fig.18).

The worker picks the bunch from the filed lug, trims and cleans it by using small and sharp scissor, and places it in the shipping container using the packing materials. When the shipping container is ready it is moved on a belt-conveyer which carries the container to the end of the line where it is stacked on a pallet for the shipment.

The packing line is generally completed with idle roller conveyer for empty shipping container, that is located 0.40÷0.50 m over the belt conveyer. The trimming material can be collected in individual container located down the working table or, even better, on a belt-conveyer that moves the materials in the opposite direction of the packed grapes (Fig.19).


Figure 18. View of typical packing line for table grape	Figure 19.View of a modern packing line for table grape: workers use gloves, overall and cap.

Wrong design of sorting space and lack worker expertise is probably the most limiting factor of the packing facilities, considering that the main operation for product preparation are done by hand. In the packing line worker productivity range from 50 ÷ 120 kg /h: the smallest shipping plastic basket is 250÷500 gr, the maximum is 7.5 kg shipping container.

It should be considered that packinghouse operation is indispensable if wrapped, labeled and weighted container (bag, basket, tray) should be produced as required by the supermarkets. Moreover the hygienic conditions that could be guaranteed in the packinghouse make this packing system much more open to future development.

Figure 20. Scheme of typical packing line for table grape with the particular of working table. Cross section and side view (dimension in meters). A Empty field lugs; B Field lugs with table grape untrimmed; C Empty shipping container or package and package materials; D Packed grape; E Scale; F Field lugs with table grape untrimmed ready to be worked; G Lug for trimmed materials; H Light.

2.2.1 Packing and packaging materials

Packing can be viewed as simply a convenience to create the minimum commercial unit for each step of the product commercial life. For instance basket, box and pallet respectively represent the commercial unit for retail, gross and shipment.

Considering that shipping container represent a convenient unit to transfer of grape from the point of production to the point of final sale or consumption, it must be designed and used in order to protect individual bunches. This is very important in Developing Countries allowing to reduce the cost of investment if the harvest and sorting operations are performed in the field.

Some grape intended for export and some high-quality grapes for domestic market are “wrap packed”. Here, individual bunches are wrapped in tissue paper or sponge before being placed in their shipping container (Fig.21a).

Wooden containers are generally used because easy to handle in filed and resistance to high level of relative umidity in the field and during the storage (Fig.21b). Anyway the use of cardbox corrugated containers is increasing especially for grape that will be sold without prior storage (Fig.21c).

Figure 21. (a) Individual bunches are wrapped in sponge (left); (b) Wooden package with grape packed in plastic bag (middle); (c) Carton package with paper stripe separating bunches (right).

Plastic containers are even used especially in Developing Countries due to higher resistance (at relative humidity, mechanical) compare to wood and cardboard, easy to wash, light compare to wood, well perforated. For these reasons, and above all for food safety reason, the plastic container is recommendable but for the problem related to the plastic recycle, today most of the Developed Countries prefer wood or cardboard, the latter the best because is even not returnable.

The package's size changes among Countries. Generally dimension is in relation to the quantities that can be exposed, in a day or two, in the retailer. In addition, exporters should use package sizes that meet the requirements of metric shipping containers. Standardized pallet are the 1200 by 1000 mm (47.24 by 39.37 in) pallet base or 1200 by 800 mm (Europallet), which is very closely with the 48 by 40 in (1219 by 1016 mm) pallet base used by most food chains in the United States. Size of shipping packages that should be considered for export are: (outside base dimensions) 400 by 300 mm (15.75 by 11.81 in), 500 by 300 mm (19.69 by 11.81 in), 600 by 300 mm (23.62 by 11.81 in). Height range from 100 to 130 mm (3.94 by 5.12 in).

The label on the container will contain the producer brand name, the type of product (i.e. grape), the variety, weight, and all the information required by the local regulation (producer identity, production area, etc.). Traceability coding is today very frequent. The used languages on the label should be the native language of the country receiving the product. On the container illustrations of the packed product are recommended in order to facilitate product identification and handling. Post harvest treatments should be clearly marked on container if it is required. It should be considered that fungicide treatment should have the approval of the Country importing the product.

2.2.2 Cooling system

In grape postharvest, one of the most important factor affecting product quality is water loss from the stem due to its large surface to mass ratio. The condition of the stem in terms of color and turgor is an important quality factor and an excellent indicator of the postharvest treatment. The stem green color and its freshness are necessary conditions to maintain high quality level for the market.

The rate of water loss could be especially high before and during the cooling phase, if grapes are harvested during the hot dry season. The following actions can help to protect grapes:

The use of adequate precooling techniques, rapidly removing the field and the respiration heats, can reduce the water loss because minimizes the exposure of grapes to low vapor pressure conditions. In each condition flow and temperature level of cooling air through the containers and exposure of the fruit to this air dramatically affect the grape cooling rate. Moreover the location and amount of venting of the containers (if they are present), alignment of the containers (air channels), and packing materials such as curtains, bunch wraps, and pads represent a barrier to the cooling process.

Table grape requires air precooling since most of the varieties is picked in hot season It could be performed using the following different system: room cooling or conventional system, tunnel cooling and pressure cooling. The use of eachone of these systems should be considered in term of cooling time, plant investment, and energy costs. Packaging type, design and materials dramatically affect the cooling time. Grape closed in plastic bag with the bunches wrapped in paper tissue o separated by a sponge layer and covers by a SO₂ generator pad require a cooling time 20 times longer than those packed in a open tray.

The use of existing cold storage rooms is one of the most common system. The cold air flow across the packed containers and refrigeration capacity have to be increased in order to guarantee the adequate air circulation with respect the normal flow in a cold storage rooms.

In the room precooling the distribution of pallet and container respect to the air flow is determinant. Air should flow in channels created between the rows of pallets. Pallets and container should be aligned in order to avoid obstruction of the air flow between and across the container. Adequate precooling rate are obtained if is possible to maintain on and around the product the cold air speed close to the value of 0.5 m/s and the refrigeration power inside of the cold storage room should be more than 0.28 kW/t considering a product average density of 0.20 t/m³.

It is necessary sometime to improve the air movement inside the cold room with mobile fans or/and to put the warm grapes quantity in proportion to the installed cooling power (generally this amount range between the 10÷20% of the total storage capacity of a cold storage room).

A great advantage of this technique is that the pallets may be cooled and subsequently stored in the same place without other moving. The design and subsequent use of the plant is very simple and, in relation to the precooling time which longer than other methods, the power of refrigeration machinery results lower respect to faster methods.

In contrast, the disadvantages are that longer precooling time do not permit a dynamic organization of production. Long precooling time imposes a long pause in handling and rapid shipping is not possible. In the case of table grapes for long term storage, deterioration due to the long time exposure to an incorrect temperature, makes room cooling to be too slow.

In the tunnel cold air is forced to flow around fruit trough the vented side of the container. Generally the containers are placed in the cooler so that the air must pass through the containers before returning to the evaporator surface (Fig.22). Precooling time is reduced increasing the air speed but an air speed over 3.5 m/s could injury the berries and break the paper use for packing, dramatically improving the energy consumption and plant cost (Fig.23 and 24). Using an air speed over 1 m/s is possibile to cool much more grape per day than in room cooling. Due to the short cooling time, despite the high air speed, the water loss rate from the fruit is negligible.

In tunnel cooling heat is carried away primarily by flow of air trough the bunches inside the containers rather than by flow past the outside of the containers as in room cooling. Using high air speed value and container adequately vented, more than a single row of pallet can be cooled at the same time. In this case air movement is always from colder fruit upstream to warmer fruit downstream, which avoids the sweating, but this flow cause a gradient of temperature among the parallel rows of pallets.

The use of precooling tunnel reduces cooling time, removing very fast the field heat. Potential for bunches deterioration is reduced and soon after the treatment, grapes can be stored or shipped in the best condition.

Compare to room cooling, the plant and energy cost are higher but, considering that they are specialized plant, the space required for a specific volume of product and the refrigeration losses are much more lower than in room cooling. Anyway, the use of this method requires higher cost for product movement if it is assigned to storage. Adeguate package design is required in relation to the fixed air speed and cooling time.

Figure 23. Refrigeration power and cooling time in relation to the air speed in table grape precooling packed in open trays. Starting from the cooling time it is possible to calculate air speed and refrigeration power required.

Figure 24. Refrigeration power and cooling time in relation to the air speed in table grape precooling packed in plastic bag. Starting from the cooling time it is possible to calculate air speed and refrigeration power required.

2.2.3 Storage

Recommended storage temperatures for table grapes are -1 ÷ 2 °C. The relative humidity should be maintained around 95%. Although temperatures as low as -2°C have not been injurious to well-matured fruit of some varieties, other varieties of low sugar content have been reported damaged by exposure to - 1.7 °C. Generally 0°C can be considered the optimum value for storage temperature. In the cold storage rooms uniform air circulation should be provided, but air velocity should be lower than 0.1 m/s in the channels between the pallets in order to minimize the loss of moisture from the stems. Large diameter fan, possibly with double speed, should be installed in order to adapt air movement to temperature level and quantity of product stored.

High and continuous ventilation rate are required to exhaust sulfur dioxide from room air following fumigation.

The greatest change that takes place in grapes in storage is loss of water. The first noticeable effect is drying and browning of stems and pedicels. This effect becomes evident with a loss of only 1 to 2% of the weight of the whole bunch. When the loss reaches 3 to 5%, the fruit loses its turgidity and softens.

Maintaining relative humidity of 92 to 95% in grape storage is often a problem especially at the beginning of the storage season when the rooms are being filled with dry lugs. Using large cooler (aeroevaporator) surface that means a minimum specific surface of 30 cm²/m³ of cold storage room, with a specific power of 0.12 kW/m², it is possible to have an optimal DT of 3°C. The use of water for defrosting the cold surface increases the relative humidity inside the storage room despite humidification increase the risk of liquid water presence on the berries surface. For this reason this defrosting technique is not advisable in cold storage rooms for grape. The most usual system to keep high grapes turgor during the long term storage is to pack the bunches in a plastic bag; in this way respiration heat removal results more difficult but relative humidity inside the bag will be close to saturation point. In order to avoid product reheating, inside the plastic bag, air velocity in the storage room have to be increased to 0,2÷0,3 m/s.

In Developing Countries where the potential for investment is low, cold room storage is sometime impossible to propose. Since the storage requirements of table grapes are low temperature and high relative humidity, the technique called “evaporative cooling” can represent a simple, low cost technique for removing heat from grapes. The reduction of temperature is only partial, down to 15-17°C depending on the kind of construction and the outside temperature and relative humidity. The principle is to close the grapes, or other fruits, in an environment where the walls are done by water absorbing materials such as straw or bricks. The walls are wetted with water which, depending on the outside temperature and relative humidity, evaporates cooling down the storage environment which keeps even a high level of relative humidity (Fig.25). The major requirement of this system is the availability of water.

Another simple technique, depending on outside atmosphere conditions, is the “ventilated wet air” consisting in a room in which a couple of fans are located on opposite walls, one pushing the air from outside inside and the other, on the opposite wall, removing air from inside to outside (exhaust). The box of table grapes can be placed over the pallets on the floor of the room, which is successively wetted completely with a thin layer of water. During the night, when the temperature is down and relative humidity is high, the fans work at low speed creating a continuous flow of air. During the day, when the temperature starts to rise, the floor is wetted and the fan work at high speed to accelerate the water evaporation from the floor, cooling down the storage atmosphere (Fig.26).

Figure 25. Evaporative cooling can be used for table grapes storage in certain areas.

Figure 26. Similar wet air cooled can be used for table grapes storage in certain areas.

Modified or controlled atmospheres (CA) are not used for industrial application in table grape packinghouse. The use of CA condition matched with sulphur dioxide fumigation has shown that CA alone does not control decay but fruit treated with S0₂ have been stored in good condition.

2.3 Shipping

The selection of transport system and right temperature level is not enough to ensure the success of grape transport that depends on many factors like the initial temperature, the refrigeration power, the air circulation system efficiency, quantity of grape and package adopted. For best results, temperatures should be maintained as close as possible to that level of the storage temperature. However to reduce the risk of freezing injury of grape, especially of packages located close to the cold air outlet, the thermostat setting (temperature control) should be set above not lower than 0°C. .

Grape should be loaded in truck trailer or container at the optimum temperature level and this level should be strictly maintained in order to preserve the grape quality during the transport period. Adequate cooling rate could not be obtained using the truck refrigeration system that is generally designed to hold the product loading temperature. In order to maintain the temperatures uniform throughout the load with a variation of ±0.5 °C, leakage from the insulated surface or from the doors gasket, long exposure to sun radiation, inadequate air circulation should be avoided because they produce dangerous and local increasing of grape thermal level. Horizontal airflow refrigeration system is the most common for air circulation in van-truck. It could be increased with channels, ducts or local vents that improve air distribution and circulation trough the stacked product especially far from the refrigerant i.e. close to the door. In conventional van-truck it is necessary to prevent obstruction of the discharge air duct and to ensure the air suction by the air-refrigerant fans. For this reason it is advisable to use a spacer between the front of the van container and the first row of pallet of the van or container. The pallets have to be loaded tightly, front to rear in the van containers. The presence of loose space among pallets could cause falling of packages with product damage, and obstruction of the air flow (Fig. 27).

2.4 Pest control

Medium and long-term storage of table grapes is possible not only because of the improved refrigeration practices but also because of the development of effective fumigation practices to control decay. Long-keeping varieties, ie Emperor, Ribier, and Calmeria, Italia ecc. further contribute to the longterm storage of this commodity.

The main fungus causing decay of grape during the storage at 0°C is Botryris cinerea Pers. This fungus is capable to grow at this storage temperature. Spores of this mould are killed by the fumigation if they are present on the surface of the berries. but if the infection started in the vineyard before harvest SO₂ (sulfur dioxide) fumigation is unable to disinfect the grapes.. In fact SO₂ inactivates spores and reduces their growing rate, so in infected berries the mould continues the development (in relation to the storage temperature) and it could became visible in few hours after removal of SO₂ from storage environment (room or single package).

SO₂ fumigation reduces the mould distribution inside the package due to its action on the spores and it prevents the formation of 'nests' of mouldy berries.

Stem color is greatly affected by SO₂ fumigation. SO₂ presence in the storage room avoids stem browning and give it a light green or amber color. After long term storage stem and pedicel result much more elastic than without fumigation, so it reduces berries detachment during the transport.

Table grapes may be fumigated in storage room, in the transport vehicle, or within individual packages. Various combinations of these methods also may be used. Fumigation in transport vehicle is more complex to remove SO₂from the internal trailer or container atmosphere before door opening or during the transit.

The minimum toxicant concentration and the time required to perform this action is generally considered 100 ppm for 60 min. This means that the same action could be performed using double concentration for half hour or the opposite. Maturity in term of sugar percentage in the juice, grape phisiological condition at harvesting time represent the main factors that affects this choice.

In South Italy storage room, table grapes generally are fumigated using a concentration of 0.2÷0.3 % S0₂ of room volume for 15÷20 min, followed by periodically applications (7-10 days) of 0.1÷ 0.2 % S0₂ for 15 to 20 minutes. Concentration and time adopted for fumigations depends on the particular weather conditions during the harvesting time (September-December). Rapid alternance of high and low temperature and wet weather conditions (high relative humidity or rainfall) increase the risk of Botrytis attack, so it is necessary to dry the berries surface and as soon as possible to fumigate the bunches in order to kill the spores eventually present on berries surface.

Longer time are required for lower concentration, some experience reports good results using fumigation with 0.01÷0.02 % S0₂ for 60 to 120 min. Automatic control equipment has been developed to maintain storage atmospheres that contain 20 ppm S0₂ over periods of three or more months. Grapes stored in such an atmosphere absorb 10-15 ppm S0₂. More recent work has shown that S0₂ concentrations of 7-10 ppm in the room atmosphere provide satisfactory decay control without excessive S0₂ injury.

Gas distribution should be done quickly and uniformly in the storage room volume. Nozzles uniformly distributed on the ceiling could guarantee uniform distribution. Most common is the installation of one nozzle in front of each continuous running fan present in the cold storage room. Proper pallet alignment and proportionate fan capacity that guarantee uniform air distribution are required. Gas penetration in the package should be improved reducing to the minimum the free volume around or over the pallet in order to avoid that the gas could by-pass the channels between the pallets and though the package.

In large cold storage room gaseous S0₂ is generated by liquid sulfur dioxide that is vaporized using hot water and it flows to the storage room through the delivery line. Precaution must be adopted to provide enough air flow to mix the pure gas before it reaches the fruit. Large amount of S0₂ can be released in few second using this system.

In order to treat all the bunches in the package, the air in contact with the berries should be moved, so in the cold storage rooms fans should be preset for continuous functioning during the fumigation. At the end of the fumigation, the S0₂ is removed from the storage room either with an exhaust fan or by dissolution of the gas in the water depending on local regulation.

Other factors that may influence the selection of S0₂ concentration are the relative humidity in the storage room and the type of package used. High relative humidity (90 to 95%) like those required in cold storage rooms for table grape and the use of wooden lugs, due to the absorption, reduce the effective S0₂ concentration in the storage atmosphere.

Decision about the S0₂ dosage should be taken considering the grape potential decay that depends on the weather conditions in the last days before the harvest.

The best SO₂ removing system in terms of efficiency and environmental impact consists of atmosphere washing system. It is a tower connected to the cold storage room by pipes. A low power centrifugal pump, installed at the base of the tower, provides the closed cycle circulation in the tower of the atmosphere containing SO₂ (from the bottom) against water (raining from the top). SO₂is removed from the atmosphere by mean the solubilization in the water. Furthermore, the use of water in closed cycle permits enrichment of the ambient of water vapour and therefore maintains a high hygrometric level (close to saturation) in the storage room (Fig.28).

Figure 28. Typical installation for grape sulphure dioxide fumigation. 1- Cold storage room (in not sealed and adequately protected require jacket system); 2- Jacket system; 3-Addition fan for air circulation; 4- SO₂ tank; 5- SO₂ dosimeter; 6- SO₂ vaporizator; 7- fan; 8- air inlet; 9-water flow meter; 10 air washing tower.

SO₂ readily forms a corrosive acid in combination with water so all the metal surfaces should be in stainless steel or should be protected. The internal surface of cold storage rooms, built with sandwich panel should be protected with epoxy resins . Gas tight doors and fans should be in stainless steel, air cooler in stainless steel or normal cooler treated with tin plate.

Interesting for Developing Countries, in order to adapt old storage rooms to SO₂ fumigation the jacket system could be adopted. Using this system the following advantages are obtained:

-     refrigeration of table grape is indirect, thus it is possible to reach high relative humidity levels;
-     easy to manage
-     different size plastic room can be created and placed in one single cold room

Table grapes are stored within the polyester wrappers placed inside normal cold-stores and the refrigeration of the product occurs indirectly through the walls of the wrappers and not by effect of direct ventilation onto the fruit. The jacket system permits the division of large refrigerated space into several cells (wrappers) of small capacity that give the opportunity to improve efficiency of loading and unloading operations. The main parts of the plant are: sulfur dioxide dosimeter, SO₂ gasificator generally based on hot water, groups of fans to distribute SO₂ and to circulate cold air across the product, SO₂ removing system similar to the one above described based on water dissolution. The foregoing discussion of storage fumigation was based primarily on practices followed in Italy and United States. Fumigation procedures in other countries, such as South Africa, include the continuous exposure of table grapes to low concentrations of S0₂ in storage.

Several SO₂-producing compounds have been developed for in-package fumigation of table grapes. This technique is mainly useful for table grape subjected to long term shipment but when are not available plant adequate for fumigation it result the best solution for long term storage of table grape.

The SO₂ generators contain sodium bisulphite between layers of polyethylene coated paper. The generators are placed at the top of the box when the grapes are packed and provide protection of the fruit for as long as three months. The generators are generally used with bunches packed in polyethylene bag in order to prevent SO₂dispersion in the storage environment. The release of SO₂ depends on the relative humidity inside the bag and so it is hard to obtain standard concentration in all the boxes. This is the reason because it is easy to find SO₂ injury in the berries and in the stem.


Pressure cooling plant. The fan produce low pressure level in the space between the pallets row. The pressure level should be lower -30 mm H₂O respect the environmental pressure. The air speed around the product is generally maintained around 0.8÷1 m/s

Cold wall cooling plant. The fan produce low pressure level on one side of pallet. The pressure level should be lower -30÷40 mm H₂O respect the environmental pressure. The air speed around the product is generally maintained around 1.0÷1.2 m/s. The pallets cooling result more uniform respect to other methods. This results more effective for plastic bag rapped product.

Tunnel precooler. The fan produce a high pressure level on one side of the tunnel (about 20 mm H₂O) respect the environmental pressure. The air speed around the product is generally maintained around 2.5÷3.5 m/s. The air cross the pallets row. Cooling rate result lower respect to other methods.