The term “mixed maturity” refers to obvious variation in ripeness between individual plum fruit in the same packaging unit upon arrival in the market. This is undesirable because it is difficult for retailers to handle and market such produce easily and effectively because of the concomitant variable shelf life and quality between individual fruits.
Mixed maturities, caused by pre- and post-optimum maturity fruit being harvested and packed in packaging units together with optimum maturity fruit is highly problematic for buyers. This problem has persisted for many years. For example, according to Davies, Boyes and Beyers (1935), bladderiness was associated with mixed maturities in export plums, where fruit which became overripe also developed bladderiness. Plums harvested at post-optimum maturity not only have a propensity to become overripe, they also have a high susceptibility to develop gel breakdown and decay. By contrast, pre-optimum maturity plums are prone to internal browning and are often unacceptably hard. Such fruit is characterised by low sugars and high acids which make them unpleasant to eat.
To avoid fruit with widely variable quality in the same packaging unit, it is important to ensure that only fruit of similar harvest maturity are packed in the same packaging units, be these punnets, thrift bags, boxes and/or pallets. Harvest maturity, as the primary cause of mixed maturity after cold storage, as well as certain secondary factors which impact on this serious quality disorder are discussed below.
Causes and remedies
For a multitude of reasons, it is important to harvest plums within the prescribed cultivar specific optimum maturity windows. These harvest maturity standards must be set based on research conducted using simulated packing, packaging, cooling, handling, cold storage and transportation conditions. It is necessary to monitor fruit maturation in orchards on a regular basis in the two week period before expected harvest. While the historic ripening patterns in specific orchards are important, fruit may ripen quicker than expected. Generally, to ensure good quality plums, suitable for the long storage durations required for sea export, it may be necessary to pick through trees more than once to ensure that only optimum maturity fruit is packed. It is important to guard against packing significantly riper fruit together with optimum maturity fruit. This is because ethylene production from the individual riper fruits can trigger other fruit in the box to ripen faster, and this is more likely to occur when shrivel sheets or grape bags are used in boxes to limit moisture loss.
Research has shown that for most plum cultivars, flesh firmness measured using a penetrometer fitted with an 11 mm plunger should be used as primary maturity indicator. Thereafter, if suitable total soluble solids levels are measured, the skin colour and fruit size best representing fruit with the correct firmness can be used by pickers to identify plums suitable for harvest.
Controlled and modified atmosphere packaging is often used to retard the ripening of fresh produce. However, these technologies have not been applied on large scale for South African plums. Therefore, it is important to test technologies like these, and even seemingly innocuous packaging types such as outer box bags, wrappers and thrift bags, etc., before commercial use, in particular for possible mixed maturity effects. This is because box liners will inhibit ripening of immature fruit through atmosphere modification, while the barrier will entrap ethylene released from advanced maturity fruit and this will accelerate ripening.
Packing and Packaging:
During packing, colour sorting of fruit on the pack line, if feasible, can greatly assist in eliminating mixed maturities of plums after storage.
The application of liners such as perforated or non-perforated bags or wrappers, can impact on mixed maturities, in particular if the dual-temperature regime is applied. This is because the air-flow barrier created by liners delays warming of fruit pulp temperatures during the warming phase and similarly, hinders cooling thereafter. The resulting variable cooling profiles for cartons of fruit is further affected by the position in the pallet and the position of the pallet in the shipping container. This can lead to variable outturn fruit quality.
Perforated shrivel sheets or perforated grape bags are commonly used to control shrivel in cartons of stored plums. These liners are effective because they increase the relative humidity in the environment surrounding the fruit by encapsulating water vapour migrating from the fruit, and this reduces moisture loss and shrivel. On the downside, box liners hinder effective temperature management because they form a partial air flow barrier which can lead to problems with overripeness.
Perforated grape bags are less ventilated than perforated shrivel sheets. Hence, while shrivel control is better in cartons of plums packed in grape bags, there is a greater probability for accumulation of the ripening hormone, ethylene. This poses a risk for development of overripeness, especially under dual-temperature storage conditions. It is important to note that ethylene accumulation which leads to faster fruit ripening can be a trigger for decay proliferation if inoculum is present, in particular, under conditions of high relative humidity such as within bag liners.
Irrespective of whether perforated shrivel sheets or perforated bags are used, it is important that the liners are properly perforated, especially in the case of bags. Variations in perforations lead to variations in cooling time during forced-air cooling and during the warming cycle of the dual-temperature regime in shipping containers. Along with other fruit quality problems, this can result in variable ripeness of fruit delivered to the market. As for all technologies used for storage of fresh fruit, only consider use of modified and controlled atmospheres if comprehensive laboratory and semi-commercial testing has proved it safe to do so.
Rough handling during harvest and packing damages fruit that may slip by undetected on the pack-line and end up in the carton. If not correctly packed and palletized, damage to the fruit skin and underlying flesh tissue can occur in the form of vibration abrasions, rub marks or bruising. Injuries to the fruit may cause rapid production of ethylene. This stimulates rapid ripening of affected fruit which can lead to overripeness and secondary development of decay at the wound site. If this ethylene wounding response is restricted to individual fruits, it can result in mixed maturities in a carton which is often a cause for rejection by the buyers. In the worst case, ethylene production from individual fruits can also stimulate other healthy healthy fruit to ripen faster than intended, and this is more likely to occur when box liners are employed.