Avocado Growers Manual Postharvesting Handling If the fruit are held at 3 to 4 C once sprung, shelf life should not be affected. Care must be taken not to remove sprung fruit to a high temperature as this causes an increase in disease levels and abnormal ripening. If the fruit are removed and cooled to temperatures below 12 C when ripening, but before they are sprung, the fruit are susceptible to chilling injury. Ripening fruit should be kept at the temperature at which they are ripening to avoid development of disorders. 120 Firmometer value 100 Packing operation Following harvest fruit is transported to a Packhouse where it typically goes through the process described below. There are usually slight differences between how each individual pack shed grades and packs avocados. Bins full of fruit are stacked in a cool area prior to tipping fruit onto the packline. Once the fruit is gently tipped out, a series of conveyor belts and rollers, transport the fruit through a washer unit in some cases, a series of soft brushes in others, or a combination of both, before the fruit passes over the grading table. A few sheds also apply wax to enhance the shine of the peel, but this is not a statutory requirement. 80 No ethylene Ethylene 60 40 20 0 0 2 4 6 8 10 12 14 16 18 Days after treatment Softening of avocados treated with ethylene (red line) compared to softening of untreated avocados (black line) at 15 C (adapted from Woolf, McLeod, Roughan, Burdon and White, 1997). The important points to note are: Bins of fruit kept in a cool area before packing Two days of exposure to ethylene is sufficient to trigger ripening in avocados After 2 days of gassing, firmness will vary from hard to just sprung Avocados will not be eating ripe on removal from the ripening room Fruit will be eating ripe after another 2 to 6 days following the sprung stage if kept at 15 to 16 C or above Sometimes the fruit are eating ripe 3 days after gassing but the ripening time could be as long as 8 days if fruit maturity is not advanced After storage for 2 weeks or more, time from gassing to eating ripe is less than that for unstored fruit The later ethylene is applied after harvest to green mature fruit, the more uniform the ripening of fruit within a tray Similar techniques of controlled ripening are used for other tropical and subtropical fruits Ripening rooms used for bananas are also suitable for avocados Tipping of fruit onto grader Copyright NZ Avocado Growers Association August 2001 PG 3 of 10
Postharvest Handling Avocado Growers Manual Trained grading staff inspects individual fruit for external defects and cull out any fruit found not complying with the Avocado Industry Export Grade Standards (that is set by a committee each year). Cull fruit are often graded again for local market. Pallet of fruit ready for shipping Temperature management Grading fruit Export quality fruit then passes over a singulator that separates and aligns fruit to pass onto the mass grader. Mass graders can be between 1 and 6 lanes wide and consist of a series of chain-mounted and driven weighing cups, all moving parallel to one another with a variable speed control. As each full cup passes over an electronic weighing station early in the grader line, its weight is measured and this information is fed into the grader computer, which in turn activates a switch to tilt the cup at the correct drop point. Fruit are then either conveyed to bins or accumulate in padded areas below the grader, while packing staff places this fruit into single layer trays or double layer lugs. Once filled, trays are stacked and strapped onto pallets, which are then placed into cold rooms to await transport to the port or arrival of containers. Even under absolutely ideal handling and storage conditions, avocados will eventually age (senesce) and decay. Being such a highly perishable fruit, avocado quality is therefore heavily affected by a combination of time and temperature. The shorter the time from harvest to arrival in the market, the better the chance of good quality fruit. Combining short storage time with correct temperatures during storage (and in some cases, using controlled atmosphere storage) will maximise the chances of maintaining good fruit quality. Air temperatures can vary enormously during the harvest period and even during the course of the same day. Avocado fruit, being dark-green skinned, can easily absorb heat and this heat must be removed or reduced after picking to delay the ripening process. Very hot fruit take longer to cool to storage temperature and increases the risk of hot spots occurring in pallet lots during storage, which is why fruit should be kept cool in the shade (refer to the Harvesting chapter). Once the fruit temperature is brought down to the target level, it is important to maintain this temperature and not allow the temperature to vary widely from then on. Any significant increase in temperature during storage reduces shelf life after arrival in the market. To maintain quality and prevent internal disorders, green, unripened fruit should be stored in well-ventilated coolrooms, at a pulp temperature of 5 to 7 C. Depending on fruit maturity and their stage of ripening, avocados can tolerate slightly lower temperatures, but there is a risk of inducing cold damage to the peel. Packing fruit into trays PG 4 of 10 August 2001 Copyright NZ Avocado Growers Association
Postharvest Handling Avocado Growers Manual Condensation weakens packaging with the water being absorbed into the wood or cardboard. At low storage temperatures and high RH small changes in temperature can result in excessive condensation and faster water loss from the fruit. Post harvest quality problems The postharvest quality problems typically found on avocado fruit that has been coolstored and shipped to market or after postharvest handling includes: rots, bruising, uneven ripening, vascular browning, chilling injury and mechanical damage. Rots: are often the most common and severe problems found in fruit at retail. The incidence and severity of rot development in a line of fruit will often determine the shelf life of the fruit. Rots can appear on the fruit when they are green and unripe or once the fruit have changed colour and softened to eating ripeness. Rots in green fruit most commonly appear as spotting with or without visible fungal fruiting bodies. Most rot is caused by the anthracnose fungus (Colletotrichum gloeosporioides), the symptoms of which are, small, light brown circular spots that enlarge rapidly changing colour to dark brown or black. The centres of the spots may be slightly sunken and pink bodies can develop within the spots. The rot penetrates deeply into the flesh in a hemispherical shape. Stem-end rot is characterised by a dark brown rot that develops during ripening at the stem end and is often associated with vascular browning where the water conducting threads turn brown or black in strands to the blossom end of the fruit. Usually, stem-end rot is not visible until the fruit has been cut open. Vascular browning can be found in immature fruit but is most commonly associated with stem end rot. Small increases in the size of a stem end rot can lead to large increases in the severity of vascular browning. Stem end rot and vascular browning Fuzzy spotting Bruising: occurs in avocado fruit as a result of impact damage, caused by dropping fruit during packing or handling, or pressure where the fruit are crushed by over packing, package base sag, package collapse or shifting of loads during transport. Damage in hard and sprung fruit is visible as small, discoloured areas in the flesh between the seed and the skin. Occasionally the bruised areas have cavities. Bruising is usually more severe in soft, ripe fruit compared hard, green fruit. Anthracnose fungal bodies on a ripe fruit PG 6 of 10 August 2001 Copyright NZ Avocado Growers Association
Avocado Growers Manual Postharvesting Handling Bruising Flesh discolouration Uneven ripening: is where areas of flesh around the seed remain hard as the fruit ripen. This problem occurs frequently in export fruit and is most common in early season fruit. However, as the fruit become increasingly mature as the season progresses the incidence of uneven ripening declines to insignificant levels. Uneven ripening is also thought to be associated with water stress during the growth and development of the fruit. Avocados ripened at temperatures above 20 C can also have a high incidence of uneven ripening. Chilling injury: can occur as an injury to the skin of the fruit and as a blackening of the flesh. Damage to the skin can appear to be similar to anthracnose spotting but tends to be browner, sharply defined and superficial not penetrating into the flesh. These fruit will often ripen normally to acceptable eating quality. Internal chilling damage makes the flesh look dirty and unattractive. Uneven ripening Flesh discolouration: is where the flesh changes colour to grey or black, appears to be decayed and has a foul odour. Late season very mature fruit stored at too low temperatures or prevented from breathing properly when ripening can often exhibit blackened flesh around the bottom of the seed. Avocados stored with other ethylene producing fruit, such as apples, are also prone to flesh discolouration. Flesh discolouration is also a symptom of chilling injury especially in fruit stored for a long time. Copyright NZ Avocado Growers Association Chilling injury to the skin In Hass, chilling injured fruit will not colour normally as they ripen, remaining green or a two-toned green to black colour. Severe chilling results in irregular softening of the flesh, off-flavours and an increase in ripe rots. Avocados are most susceptible to chilling injury at the start of ripening to the sprung stage. Mechanical damage: is where the skin of the fruit has been rubbed or abraded so that the tops of bumps or nodules have been removed. The damage usually heals leaving the fruit appearing spotted. Damage to the skin makes the fruit more susceptible to fungal infection especially if the fruit get wet or are dipped. August 2001 PG 7 of 10