Internal browning (IB) is a chilling injury which can occur in unripe peaches stored at low temperatures. Affected fruit have a normal external appearance but the flesh tissue has a brown discoloured appearance.
When the flesh tissue or cell structure of fruit, is in a healthy condition, the enzymes and phenolic compounds are separated and cannot react with each another. When fruit is cut or bruised, oxygen is introduced into the injured plant tissue. In the presence of oxygen, the browning enzyme polyphenol oxidase, located in the chloroplasts, oxidises the phenolic compounds naturally present in the fruit and which leak out of cell vacuoles when cells are damaged. Similarly, if plant tissue is damaged due to low temperature, oxidative browning can manifest as IB. This disorder occurs in susceptible peach cultivars that have been subjected to low temperature storage (-0.5 °C) for extended periods, and mainly in white flesh peaches.
Causes and remedies
Similar to woolliness, IB increases exponentially the longer the cold storage period and susceptible cultivars can develop the disorder within two weeks of storage. Unlike woolliness it can develop during cold storage before shelf life. Because IB is a progressive disorder, cold storage should be kept as short as possible to eliminate the problem in susceptible cultivars (see cultivar specific information for detail). After packing peaches must be forced-air cooled to -0.5 °C within 24 hours. More IB will develop in fruit stored in the temperature range from 2 °C to 5 °C compared to -0.5 °C, and hence, the former temperatures are to be avoided.
Peaches intended for storage should be harvested mature but not ripe, typically in the flesh firmness range of 9 to 10 kg for sea export. Internal browning can be prevented by harvesting at a slightly more advanced maturity on some susceptible cultivars, for example at 8 kg flesh firmness or even a bit softer. However, this remedy is often not practical because fruit harvested more mature are difficult to pack and handle because of bruising and overripeness. In most cases this leads to low pack outs and larger variation in maturity and fruit ripeness in the carton.
To enable good pack houses handling, most peach cultivars should be harvested at a flesh firmness of 9 to 10 kg with the background skin colour changing from green to yellow. Some of the late maturing cultivars may even be harvested firmer than 10 kg (see cultivar specific information for harvest flesh firmness recommendations).
After packing, fruit can be pre-conditioned to prevent IB. Pre-conditioning can be carried out by holding fruit harvested at flesh firmness higher than
9 kg, as measured on the cheeks of fruit using a penetrometer with an 11 mm plunger, at a temperature of 20 °C and a relative humidity of 85% before commencement of cold storage. The pre-conditioning time required depends on flesh firmness of the fruit at harvest, as well as the desired pre-conditioning target flesh firmness. For good quality management it is imperative that harvest maturity is uniform. Pre-conditioning can be done by determining flesh firmness on a representative sample of 10 fruit at harvest, after 24 hours and thereafter every 12 hours. As soon as 50% or more sample fruit have reached the desired flesh firmness, the fruit consignment must be forced-air cooled to -0.5 °C immediately (see cultivar specific information for pre-conditioning target flesh firmness recommendation). The optimum flesh firmness for pre-conditioning varies between cultivars, but generally, the target firmness for yellow flesh nectarines and peaches is 6 to 7 kg and for white flesh between 8 and 9 kg (see cultivar specific information for pre-conditioning target and harvest maturity flesh firmness recommendations).