“The primary purpose of packaging is to maintain fruit quality – to deliver the best quality to our markets,” says Dr Johan Fourie, Postharvest Pathologist at ExperiCo Agri-Research Solutions. “We need to re-establish confidence in South African table grapes.”
To better understand how various packaging components and environmental conditions influence table-grape quality, the ExperiCo team has been running a three-year SATI-funded project. So far, they have studied the interactions between bunch carriers, liners, and SO2-release systems.
SO2-release patterns
The researchers logged SO2 levels inside punnets to determine the SO2-release patterns of four SO2-generating systems. This article summarises the results of the three SO2 sheets that were tested.
For the trials, the researchers packed Sugrathirtyfive grapes in clamshell punnets. The punnets were enclosed with an SO2 sheet and a moisture-absorbing sheet inside 112 x 4 mm liners and placed in cartons.
The cartons were forced-air cooled to minus 0.5 °C. They were then stored for eight weeks at minus 0.5 °C, followed by three days of shelf life at 7.5 °C.
The SO2 levels spiked to 80–90 ppm within the first hour of bag closure. At this point, forced-air cooling began, and SO2 levels declined rapidly to below 10 ppm within the next hour.
Two of the sheets generated SO2 levels that fluctuated between 0.5 ppm and 1.5 ppm during forced-air cooling, rising quickly to 3–6 ppm when forced-air cooling was stopped at around 21 hours after bag closure. Their SO2 production then trended towards 2 ppm over the next four days.
The third liner remained at around 0.5 ppm during forced-air cooling, gradually rising to 1 ppm over the next four days.
For the rest of the storage period, all the sheets generated SO2 levels of 0.5–1 ppm.
No significant difference in decay control or SO2 damage occurred between the SO2 sheets. There was significantly less decay in all SO2 treatments compared with the control without SO2 sheets.
Decay control
To assess the efficacy of the SO2 sheets against common postharvest pathogens, the researchers conducted inoculation and wounding trials. In the inoculation trials, they infected wounded Sugrathirtyfive berries with either Alternaria alternata, Botrytis cinerea, or Penicillium expansum. In the wounding trials, they wounded but didn’t infect the berries.
For both trials, infected or wounded berries were placed into punnets of Sugrathirtyfive grapes, which were then packed and handled as described above.
Decay caused by A. alternata and B. cinerea was significantly less for all three SO2 sheets than the control without an SO2 sheet. There were no significant differences between the SO2 sheets and the control of P. expansum decay.
Some of the treatments with SO2 sheets exhibited berry split, but there was no consistent pattern related to sheet type across the trials. Low levels of SO2 damage were observed.
The effect of bunch carriers and liners
“With packaging, you need to achieve a balance between too much and not enough moisture, which is where liner perforations come into play,” says Fourie. “The other component is balancing the level of SO2 to suppress decay.”
In general, increased perforations of bunch carriers and liners reduce humidity and condensation risk, but may also reduce SO2 levels, potentially below the levels required for decay control.
On the other hand, too few perforations and excessive moisture are associated with high SO2 levels, which may damage the grapes.
To investigate the interactions between packaging components, the researchers assessed different combinations of three SO2 sheet types, three liners (96 x 5 mm,
112 x 4 mm, and needle-scarred micro-perforated), and two punnets (clamshell and open-top).
The experiments were done with IFG Ten and similar handling protocols to those described above. Punnets were seeded with berries infected with either B. cinerea or P. expansum. Non-inoculated punnets were also included to evaluate natural infections.
The overall results showed that all three SO2 sheets significantly reduced postharvest decay compared with the control, which had no SO2 sheet.
More decay and less SO2 bleaching occurred in the clamshells than in the open-top punnets. The increased decay in the clamshells may be due to lower SO2 exposure or higher moisture levels.
Berry split and SO2 damage were more common in the less-ventilated microperforated liners than in the 96 x 5 mm and 112 x 4 mm liners. This supports the hypothesis that higher moisture and SO2 levels are found inside less perforated liners.
Wetting before packing
Rainfall at harvest time increases the risk of berry split and fungal infections in table grapes. To simulate the effect of rain, the researchers harvested grapes and either packed them dry, sprayed them with rainwater and packed them wet, or sprayed them with rainwater and let them dry before packing.
The researchers included Crimson Seedless, Sugrathirtyfive, and IFG Ten. They used combinations of two SO2 sheet types, two liners (96 x 5 mm and needle-scarred microperforated), and two punnets (clamshell and open-top).
In these trials, IFG Ten had far less decay than the other cultivars. Crimson Seedless tended to have less berry split but more SO2 damage than the other cultivars.
Grapes packed wet developed more decay as well as more berry split and SO2 damage than grapes that weren’t wetted. The increased SO2 damage is likely because increased moisture promotes SO2 release, and SO2 dissolves in water on the berry surfaces, forming sulphuric acid.
Decay development didn’t differ between liner types, but more berry split and SO2 damage occurred in the less-ventilated, microperforated liners.
Temperature breaks during storage
To investigate the interaction between packaging systems and temperature breaks, the researchers used the same cultivars and packaging combinations as those used in the wetting trials. Three temperature breaks were tested: four hours at ambient temperature in the first week of cold storage, 48 hours at 7.5 °C in the first week of cold storage, and three breaks at 7.5 °C between the first and seventh weeks of cold storage.
As seen in the other trials, grapes packed in microperforated bags were more prone to berry split and SO2 damage. Clamshells also appeared to increase susceptibility to SO2 damage during extreme temperature abuse.
The take-home message from all these trials is that there’s a Goldilocks zone for SO2 levels – too high damages berries and too low doesn’t control decay. Liner and punnet perforations influence SO2 levels directly through ventilation and indirectly through moisture levels.
The release profile of SO2 systems matters because initial high concentrations sanitise the grapes and reduce decay development during subsequent storage.
“The key components in table-grape packaging all have different objectives,” summarised Fourie. “However, they ultimately complement each other. The packaging and management practices are all part of an integrated approach that’s required to maintain grape quality.”
