Apricot 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.
In apricot fruit GB is often classified as a chilling injury. This is not strictly true since in extreme circumstances this internal disorder may develop in fruit on the tree. However, it is a progressive disorder that also develops during cold storage. Some apricot cultivars grown in South Africa have a high propensity to develop GB during the storage periods of approximately 4 to 5 weeks which may be required for export. Generally, GB is not externally visible unless it is extremely severe. Consequently, it will only be visible during arrival inspection or upon eating. The fruit tastes bland and musky and has a somewhat dry and gelatinous texture! For quality management purposes, to inspect apricots for GB, it is best to select a representative sample of the fruit and to cut each 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. In apricots with inherent GB potential the disorder often develops under conditions which stimulate rapid ripening after harvest.
Causes and remedies
There is a strong association between the risk of apricots developing GB and cultivar. The cultivars in probable order of decreasing susceptibility for GB development during cold storage are: Supergold, Bebeco, Charisma, Soldonne, Peeka and Imperial. While genetics influences storage potential to a large degree, optimal handling can significantly assist with managing the fruit quality.
The most common of cause of GB is harvesting all or some fruit at an advanced post-optimum maturity. For good quality, it is essential to harvest apricots intended for storage within the prescribed optimum maturity window. Flesh firmness is a valuable tool, additional to the other harvest maturity parameters such as skin colour, total soluble solids and the wring test, to determine optimum maturity. Flesh firmness should be determined using a penetrometer fitted with a 6-mm plunger. Typically, apricot populations which have more than 30% fruit with a flesh firmness less than 2.0 kg at harvest have a higher potential to develop GB than firmer fruit. Hence, it is recommended to regularly determine the flesh firmness of fruit in orchards, commencing at least one week prior to the expected harvest date. Skin colour may vary from season to season and is not always a good indication of firmness. The aim is to harvest at a flesh firmness of 2.0 kg or higher, but within the other harvest maturity parameters.
Sub-optimal temperature management at any point in the handling chain can lead to GB. A remedy to assist in limiting GB in apricots is to pack fruit as soon as possible after harvest (within 6 hours), force-air cool in about 12 hours, and cold store as near to -0,5 °C as possible, while taking care to avoid breaks in the cold chain throughout. It is also important not to cool the fruit to pulp temperatures below -0.5 °C to limit the risk of freezing damage. It is best to store apricots for the shortest duration possible. Since GB is a progressive disorder, the incidence and severity increases with longer storage, and in particular, during shelf life after cold storage. It is good practice to curtail storage of apricots to 28 to 38 days depending on cultivar and packaging.
Weather has a huge impact on fruit physiology, which in turn determines inherent quality potential. The nearer the harvest, the more risky sub-optimal climatic conditions are. The precise impact of climate on fruit quality is difficult to predict because it varies from year to year and impacts on multiple complex factors in the apricot tree and fruit. Rain and cool conditions followed by days of ambient temperatures higher than 38 °C have repeatedly proved to be high risk.
Heat wave conditions are also detrimental for fruit quality. Experience suggests that ambient temperatures reaching 35 °C for 3 days, or 38 °C for two days, or 40oC for one day just before harvest are a cause for concern. It is speculated that under these conditions 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 fruit can reach as much as 15 °C higher than ambient. This simply “cooks” the fruit flesh tissue, in particular in the area around the stone where the vascular tissue is concentrated. This then manifests as GB. Externally, fruit exposed to heat wave conditions may also develop sunburn and in extreme cases even sunken lesions.
During or after adverse 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 problematic weather has abated, there is a reasonable chance that the quality will be satisfactory. In the case of mature fruit that must be harvested because it has ripened past the middle of the optimum window, it is good practice to cut fruit through the equatorial axis and do an internal inspection for flesh tissue damage, which in this case could be GB, flesh breakdown or pit burn. Irrespective of the disorder type, flesh tissue damage detected at harvest indicates a very high quality risk. Being a progressive disorder, GB levels will increase with longer storage. After risky weather, even if no GB is detected at harvest, it is recommended to follow up with a second internal examination after forced-air cooling. If signs tissue breakdown are detected the fruit quality is risky and the probability for GB after cold storage is high.
Experience with South African apricot cultivars has shown that even small modifications of the gas composition of the atmosphere surrounding the fruit can cause extremely high levels of GB. Hence, no bags or wrappers must be used unless comprehensively tested.