Grape storage in a controlled atmosphere
Almost all these fruits are pre-dried and many of them are stored for different periods of time before consumption. On the other hand, for fresh consumption, the fruits of Vitis species are largely limited to distribution on the local market.
The grapes grow hard and must be at full maturity at the time of picking because the entire ripening period must be on the stump. “Mature” here has psychological significance, that is, it is the state in which the fruit looks pleasant to the eye and can be eaten with satisfaction. However, the grapes should not be passed from the ripening, as this predisposes them to serious problems after harvesting: the weakening of the stem / tail of the bunch for some varieties , such as the seedless Thompson variety, which causes the detachment of the berries and the increased probability of being attacked by decomposition microorganisms. The danger of fruit decomposition increases in case of exposure to rain or excessive wet weather before harvesting (favorable conditions for infecting in the field with Botrytis cinerea.
Cooling and storage:
Grapes are vulnerable to the drying effect of air due to the relatively high ratio between surface and volume, especially those with tails. The condition of the coditis is an important quality factor and an excellent indicator of the treatment that has been applied to the fruit. The tail should be kept fresh , green, not only in appearance , but also because it becomes brittle by drying and can break. The tail of a bunch of grapes, unlike other fruits, is the part that is held when handling; if this breaks the fruit is lost for any practical purpose, even if the berries are in excellent condition. Therefore, special attention should be paid to operations that minimize moisture loss.
The rate of water loss is high especially before and during pre-ripening, because the grapes are normally harvested in conditions of high temperature and dry air. The heat in the field must be quickly removed after harvesting to minimize the exposure of the grapes to low vapor pressure conditions. The volume and temperature of the pre-drying air, the speed with which it passes over / among the containers and the accessibility of the fruit to this air are significant factors in the heat elimination rate. These factors are drastically influenced by the location and ventilation of containers, their alignment (air channels) and packaging materials.
Table grapes are initially cooled with a forced air cooling system. A pressure gradient is so given that a positive flow of cold air between the fruits is created from one ventilated end of the container to the other end . The containers are arranged so that air must pass through the container before returning to the cooling surface. The pre-hardening time is usually 3-6 h, depending on the packaging system and air flow.
The recommended storage temperature for Vitis vinifera grapes (European or Californian type) is -1°C. relative humidity should be 90-95 %. Although temperatures of – 1.7 °C did not affect the well – ripened fruits of some varieties, other varieties of grapes with a lower sugar content were damaged by exposure to-0.5 °C. grape deposits must ensure uniform air circulation in containers. The fruits must be forcibly cooled by Air jet to less than 4°C before storage. During the initial storage, a well-destroyed air flow rate of 52 l/s per ton of grapes is required for the completion of cooling. After the fruits have been pre-dried, the air velocity must be reduced to a value that allows uniform temperature maintenance throughout the room (no more than 0.05 to 0.1 m/s in the channels between the legs of the stack).
The biggest change in stored grapes is the loss of moisture. The first notable effect is the drying and browning of the tail and the stem of the bunch. This becomes evident even at a loss of only 1 to 2% of the fruit mass. When the loss is 3% to 5 % the fruits lose turgor and velvety.
Maintaining a humidity of 90 to 95% in grape warehouses is often a problem , especially at the beginning of the storage season when the packages are dry. Each package absorbs 0.15 to 0.3 kg of water in a month and the less moisture is introduced into the room the more is extracted from the fruit. Spray humidification is effective for minimizing shrinkage. With an adequate balance of water and air pressure, the right nozzle type, a fine spray can be obtained even at -0.5 °C.
Smoking / fumigation:
Vinifera grapes should be fumigated with sulfur dioxide after packaging to prevent or delay the spread of decomposition microorganisms. The surface treatment sterilizes the fruit, and in particular the lesions produced during handling.
Fumigation with SO2 in the warehouse prevents new infections but does not control those that have been contacted in the field. Most of the time they are not developed long enough to be identified at harvest and as a result are the primary cause of decomposition in the warehouse. Harvey describes a method of measuring infection in the field to predict decomposition during storage ; the forecast indicates batches that are safe and can be safely stored and those that have a higher probability of decomposition and should be marketed earlier.
The common practice at initial cooling is to fumigate the fruits in the evening. In this way the preraciation is not delayed and fumigation can be done after the departure of the staff. This initial treatment is the responsibility of the frigotechnical staff.
Quantity of SO2:
Other products should not be treated together with the grapes or even in the vicinity so that the gas touches them, as most can be very easily affected by it. Since grapes can also be affected, they must be exposed to the minimum amount of SO2 required, which depends on :
– the decomposition potential and the condition of the fruit .
– the amount of fruit to treat .
– type of containers and packaging materials .
– air velocity and air distribution uniformity .
– room size .
– losses through non-tensioning and Wall absorption .
The amount of SO2 required for effective control of Botrytis varies with the length of time the grapes are exposed to gas contact. Is the dosage of SO2 for the destruction of Botrytis spores / mycelium 100 mg / kg?h.
Traditional fumigation can be used for initial treatment when the grapes are received at the warehouse or for weekly fumigation during long-term storage. Relatively high concentrations of SO2 are added for 20 to 30 min. and then the remaining gas is vented from the room.
Initial traditional fumigation can use either circulating air or forced air fumigation. Each is used either in combination with initial cooling or as a separate operation. In circulating air fumigation, the Air jet passes over, but not among box-pallets. The penetration of SO2 as deep as possible in the boxes depends on the speed of the air passing over the pallets , the type of the box and the packaging materials. For maximum penetration a minimum speed of 0.7 m / s is required.
Air flow systems for forced air fumigation are the same as systems for forced air cooling. SO2 is introduced into the air in the chamber and is forced through the boxes, resulting in rapid penetration, regardless of the package.
The airflow for many forced air fumigation Chambers is typical for forced circulation exchangers ( around 0.5 l / s per kg of stored product); however a good penetration of SO2 was observed even with a lower air flow (0.25 l/s per kg) and slower cooling.
The maximum concentration of SO2 allowed for initial fumigation is 10000 mg / kg. Although few operators regularly use this level in small circulating air fumigation rooms, many operators use 5000 mg/kg for initial fumigation. The levels for a particular location must be determined using SO2 dosimetric tubes.
According to the new regulations it is forbidden to release any amount of SO2 into the atmosphere, so water is used to remove SO2 from the room without ventilation. The most effective systems pass chilled air through a nozzle with water. Water can absorb SO2 at a rate of 0.86 kg SO2 per 1000 L water, if the water is at 0 °c and becomes completely saturated with fumigant. At 21 °C water will absorb only half the amount of SO2. This water cannot be reused and must be disposed of. To increase the amount of SO2 absorbed in the water, sodium / potassium hydroxide can be added.
In total fumigation, the amount of SO2 introduced is balanced with that absorbed by the fruit, the boxes and the chamber itself. This method is used only associated with preraciation. For complete gas absorption , the gas will be kept in contact for at least a few hours. Without preraciation the grapes will be exposed to hot air which can lead to dehydration of the cod. By the simultaneous use of fumigation and pre-drying the fruits will be cooled quickly and fumigated effectively.
Frequency of fumigation in warehouses:
Deposits should be fumigated regularly and frequently to prevent the spread of mycelium from affected to adjacent grains. The rate of mycelium development varies with temperature. The temperature of the grain during storage must be as close as possible to – 0.5 ° C. Industrial experience and test results show that 7 days is the appropriate interval to prevent the spread of Botrytis.
Normal storage time can vary from variety to variety and is most affected by the attention given to the selection and preparation of fruits. The grapes must be harvested at the optimum time of maturity for storage. The stalk and pedicle (bunch stem) must be well developed and the fruit must be firm and ripe.
Soft and underdeveloped fruits are not stored.
Cooling to a temperature of 4 to 7 ° C is recommended for fruits in transit to the warehouse.
Materials and practice of refrigeration systems:
SO2 is corrosive so the following actions are required to ensure equipment durability and economic operation:
– SO2-resistant aluminum alloy heat exchanger battery; can be covered with a material to prevent chemical attack; aluminum sheet housing with food grade stainless steel fasteners.
– fan protection grille made of mild steel treated and covered in electrostatic field; encapsulated fan motors with cast iron or cast steel housing with very good quality paint coating; fan motor shaft made of a nickel-based material.
– pipes to vaporizers: aluminum or stainless steel pipes are used; the fixing rods will also be made of stainless steel.
Place manual and control valves outside the warehouse to minimize corrosion and easy maintenance
Place the evaporators as close as possible to the outer walls to minimize the length of the pipes between the control valves and the evaporators.