Plum fruit with gel breakdown (GB) have a normal external appearance, but exhibit gelatinous breakdown of the inner flesh tissue surrounding the stone, while the outer flesh tissue has a healthy appearance. With increasing severity the disorder spreads outwards, changing from a translucent to a brown discolouration and is associated with loss of juiciness.
GB in plums occurs due to abnormal physiological circumstances. It is not a true chilling injury since it may develop in fruit while still on the tree. This is often due to adverse climatic conditions and/or events causing maturation beyond the optimum maturity window before harvest. In plums with GB potential, the disorder develops as fruit ripens after harvest. Therefore, factors that stimulate ripening can also cause higher levels of GB. Whether GB manifests pre-or post-harvest, it is a progressive disorder that intensifies over cold storage time. Plum cultivars have varying susceptibility for GB. Generally, GB cannot be seen from the outside of the fruit unless it is extremely severe. Development of the disorder in the flesh tissue surrounding the stone, as opposed to below the skin, is ascribed to the ripening pattern in the fruit. For example, with Songold plums, more rapid ripening in the inner than the outer flesh tissue leads to more highly permeable cell membranes in inner tissue which allows cell fluids leak out of these cells. If this happens under conditions where pectins in the cell walls have metabolized to the point that they have a high fluid binding potential, gel complexes form as the cell fluids leak out the cells and this then manifests as GB around the stone. Plums with advanced GB are unpalatable. For quality management purposes, to check if plums have developed GB at any point in the handling chain, it is best to cut the fruit around the equatorial axis, twist the halves in opposite directions and visually check for signs of gelatinous breakdown which is usually more intense around the stone.
Causes and remedies
Cultivar type has a profound impact on the susceptibility to develop GB. Hence, good cultivar evaluation is important to guard against planting of unsuitable cultivars.
The maturity of plums at harvest can significantly influence development of GB in susceptible fruit during subsequent storage. Plums harvested at post-optimum maturity often become overripe, have a high susceptibility for GB and tend to develop high levels of decay. By contrast, pre-optimum maturity plums are prone to internal browning and are unpleasant to eat. Given the quality risks associated with harvesting plums outside the optimum maturity specifications, the importance of harvesting within the optimum maturity window cannot be over emphasized. This means that multiple picks through orchards are often required. Mixed maturities, where pre- and post-optimum maturity fruit is packed in boxes with optimum maturity fruit is highly problematic for buyers. 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 for pickers to identify plums suitable for harvest. During packing, colour sorting on the pack line, if feasible, can assist greatly in eliminating mixed maturities of plums in the box.
Field heat removal and forced-air cooling:
Both field heat removal and forced-air cooling have a profound effect on fruit quality maintenance and this is especially true for plums with inherent weakness. Since early season plum cultivars have a high respiration rate, the commencement of forced-air cooling as soon as possible after harvest assists quality maintenance. If harvested, packed and cooled on the same day, field heat removal is not necessary. If packed 24 hours or so after harvest, field heat removal, by holding the bins at 15.0 °C or in the case of fast ripening cultivars like Sapphire, at –0.5 °C, will help maintain quality. Generally, it is best to only use field heat removal at –0.5 °C in cases where plums cannot be packed within 48 hours of harvest.
To minimize the risk of internal disorders, the optimal forced-air cooling time to the storage temperature of –0.5 °C is between 12 and 24 hours for Pioneer, Sapphire, Purple Majesty and Souvenir. However, a cooling time of between 24 and 36 hours is recommended for most other main stream plum cultivars. In the case of possible heat wave damage, research has shown that slower forced-air cooling reduced internal disorder levels compared to faster cooling.
Cold storage regimes:
Different storage regimes are required for different plum cultivars. The cold storage regime applied has a significant impact on post-storage fruit quality. Hence, it is of cardinal importance to know what regime to use for what cultivar and why! Two regular atmosphere cold storage methods are commonly used for South African plums, namely: the single- and the dual-temperature regimes. The single-temperature regime entails holding fruit at
-0,5 °C for the entire storage period which could be anything up to 10 weeks, depending on the cultivar. The dual-temperature regime involves storage at -0,5 °C for a maximum of 10 days after harvest, followed by a warming phase at 7.5 °C for 5 to 12 days depending on the cultivar and the perceived inherent fruit quality, with the temperature again lowered to -0.5 °C for the remainder of the cold storage period. The single-temperature regime is preferable because it is much easier to use commercially. It also eliminates the time and temperature variations which occur when conducting the warming phase of the dual-temperature regime during transit in shipping containers. Because a number of plum cultivars cannot withstand initial durations longer than 10 days at low temperatures without the risk of developing internal browning, dual-temperature storage is often the only solution to enable storage for longer periods. The risk associated with dual-temperature storage is that because it stimulates ripening, it may also lead to GB in susceptible fruit if not applied correctly.
Climate impacts on fruit physiology which in turn determines the inherent quality potential of stored fruit. The nearer to harvest date, the more dangerous sub-optimal climatic conditions can be. The precise impact of climate on fruit quality is difficult to accurately predict because it varies from year to year and impacts on multiple complex factors in the plum tree and its fruit. Rain and cool conditions followed by days of ambient temperatures higher than 38 °C are known to increase levels of internal disorders, including GB. Similarly, heat waves, roughly defined as conditions where ambient temperatures reach 35 °C for 3 days, or 38 °C for two days, or 40 °C for one day just before harvest, often result in sunburn and internal disorders. Under these conditions it is thought that the tree draws water from the fruit to cool the growing tips. With the cooling system in the fruit no longer functioning effectively, the temperature in the fruit itself can reportedly reach up to 15 °C higher than ambient. This simply “cooks” the fruit, in particular, the tissue around the stone where the vascular tissue is concentrated. This then manifests as GB. During heat waves it is important to ensure that the soil moisture in the orchards is adequate, as this helps to reduce the risk of heat damage and shrivel. Fruit maturity permitting, it is best not to harvest in extreme temperatures. As a general guideline, 28 °C is regarded as the maximum “safe” temperature for harvesting plums.
During or after high risk climatic conditions, fruit which are immature on the tree have a better chance of not being damaged. Therefore, if harvested a few of days after the problem weather conditions have abated, the internal quality will hopefully be satisfactory. In the case of mature fruit that must be harvested because it has ripened beyond the middle of the optimum maturity window, it is good practice to cut a sample of fruit through the equatorial axis to conduct an internal inspection for flesh tissue damage. Flesh tissue damage detected at harvest, be it GB or otherwise, indicates a very high quality risk. As progressive disorders, the levels of internal disorders will increase with longer storage so extended storage of such fruit is high risk. After potentially adverse weather, even if no GB is detected at harvest, it is recommended to do an internal examination of the fruit again after forced-air cooling. If indications of tissue breakdown are detected, the fruit quality is suspect and storage should be kept as short as possible.
In the late 1980’s a fair amount of research was done to evaluate controlled atmosphere storage in land based stores and shipping containers to achieve longer storage for plums. Reasonable results were achieved for some cultivars in both laboratory and semi-commercial trials. A combination which seemed to work quite well was 3% O2 / 5% CO2. This technology, which was mainly applied using purpose built shipping containers systems to deliver the atmosphere modification during transit, did not take off in South Africa. This was probably primarily due to insufficient benefit versus cost. Similarly, considerable research has and is still being done on the use of modified atmosphere bags without any significant market penetration in South Africa to date.
It is important to test technologies like controlled and modified atmosphere, and even seemingly innocuous packaging types such as perforated bags, perforated wrappers and thrift bags, etc., very carefully, before using them commercially. Only use these if comprehensive research by reputable organizations in South Africa has proved them suitable for the specific cultivars and storage conditions.
A product which has contributed significantly to assist quality management of certain plum cultivars is SmartFreshSM with the active ingredient 1-Methylcyclopropene. This ethylene blocker is applied to fruit in air-tight cold rooms before or after packing, or to packed fruit in the shipping containers, just prior to export. SmartFreshSM slows down fruit ripening and consequently lowers the risk of GB which can also sometimes be used to achieve longer storage. Refer to Plum – SmartFreshSM Protocol link for specific instructions on how SmartFreshSM is to be used. For plums with good inherent quality, SmartFreshSM can assist in achieving longer storage, by a week or more. However, if the inherent quality is sub-optimal, it is best to use SmartFreshSM to try and minimize GB development without trying to extend storage.