During storage, the respiration rate of avocado fruit is influenced by a range of factors such as pre-harvest quality, maturity, storage temperature, storage atmosphere and the application of ethylene inhibitors. To ensure the optimal storage atmosphere, controlled atmosphere (CA) methodology was developed in the early 1990s, and South African avocados were first exported under CA conditions in 1995. Since then, it has become an established industry practice.
Industry trials and laboratory experiments conducted by Dr Frans Kruger and his team at Lowveld Postharvest Services (LPS) in 2019, showed that the BA combination delivered similar post-storage ripening patterns to CA. Feedback received from back-to-back commercial trials performed by industry members supported the observations.
Building on this research, during the 2019 season LPS studied the effects that artificial CO2 applications have on the fruit respiration rate. They applied dry ice to packhouse cold rooms and refrigerated trucks. The impetus for this line of enquiry was threefold:
- A previous PHI project has indicated that the artificial administering of CO2 inhibited ripening to a greater extent than CO2 that builds up naturally.
- Previous research has also shown that the period between harvest and the application of controlled atmosphere (CA) is crucial in terms of landing hard avocados with minimal physiological and pathological disorders in the country of import.
- Shortages of containers in recent seasons have resulted in the referenced period becoming unacceptably long, and significant losses for all involved.
Dr Kruger found that temporary enrichment of the storage atmosphere with CO2 in the form of dry ice during the period that the fruit would have been traveling by refrigerated truck from the packhouse to the harbour, may suppress the respiration rate of the fruit.
In 2020 work started on commercialising this technique by carrying out three sets of laboratory-based trials with the Hass cultivar. The results were promising as a solution to the conundrum of door-to-door CA/BA. Currently, most avocados exported from SA are stored for one to two weeks under regular atmosphere (RA) conditions, followed by two to three weeks on the ship under CA before being returned to RA in the country of import. Door-to-door CA is not used often because of the cost and the logistics involved when fruit from more than one packhouse is exported in the same container.
Most door-to-door consignments occur during the late season to reduce the incidence of soft landings of Hass, Fuerte and Pinkerton avocados. However, a packhouse may not have the necessary stock to make up count-dependent, door-to-door consignments for specific clients. The insertion of dry-ice-derived CO2 into trucks will enable the exporter to assemble consignments in the harbour, using stock from different runs and/or packhouses.
Furthermore, door-to-door CA/BA is critical for the successful storage of the Maluma cultivar. This variety has a fast metabolic rate with a high prevalence of fruit with dead seeds that ripen prematurely during the export process. To prevent this, the avocados must be placed under CA/BA within four days after harvest.
Drawing on this entire body of research, Dr Kruger and his team embarked on the final leg of their research into CA/BA procedures during the 2021 season, with a study that aimed a) to quantify the ripening inhibition effect of artificially administered CO2 and b) to develop and commercialise appropriate application procedures.
Methodology
Maluma avocados from two different producers were selected for the study. Given its storage challenges, the Maluma cultivar was the ideal candidate on which to test the newly developed CO2 enrichment technology.
A total of 1 864 export pallets were followed during the study; 1 675 were from Producer A and 189 pallets from Producer B. Of these, 697 pallets were exported under door-to-door CA conditions. A further 275 were exported under regular atmosphere (RA) in refrigerated trucks (no dry-ice CO2 enrichment) to Cape Town harbour. A third batch of 892 pallets were treated with dry-ice-derived CO2 in refrigerated trucks en route from the packhouse to the harbour.
The feedback received from the European clients was used to assess the efficacy of the treatments. The number of pallets that contained fruit with quality problems (primarily soft landings and grey pulp, especially in fruit with dead seeds) was recorded for each consignment.
At the beginning of the 2021 season, a shortage of CA containers prevailed. This was unfortunate for Maluma producers, as it created quality problems due to the long periods from packing to CA containerisation. However, it was beneficial to the researchers, since the efficacy of the artificial CO2 applications could be thoroughly tested under commercial conditions.
Study results
During February 16.7% of the pallets from Producer A that were transported under RA conditions contained fruit that exhibited postharvest quality problems (primarily grey pulp in fruit with dead seeds). In contrast, only 3.7 % of the pallets that were exported under door-to-door CA conditions held fruit with postharvest defects. However, from February to March the incidence of grey pulp in the door-to-door consignments increased from 3.7% to 11.8%. This was due to Maluma fruit from orchards with a high nitrogen content being extremely sensitive to maturity increases.
In the case of non-door-to-door consignments, the incidence of postharvest problems decreased from 16.7% under RA trucking conditions in January to 11.8% under CO2 enrichment conditions during March. This lowered incidence, which was similar to that of the door-to-door consignments, occurred despite the fruit being more mature, thus confirming the efficacy of the CO2 treatments.
In the case of Producer A, the mean period from harvest to CA containerisation of the door-to-door consignments exported during February were 3.6 days for the problem-free pallets, and 3.7 days for pallets with grey pulp. For the pallets that were trucked without CO2 enrichment, the mean time between harvest and CA containerisation in Cape Town were 7.8 days for unaffected fruit and 8.1 days for affected fruit.
During March the mean periods from harvest to CA containerisation of the door-to-door consignments decreased to two days for unaffected fruit and 1.7 days for affected fruit. During this month, the periods for the consignments that received CO2 enrichment increased to 8.1 days for the unaffected fruit and 8.8 days for affected fruit.
In the case of Producer B, 46% of the pallets contained fruit showing grey pulp symptoms during March and April. Most importantly, the period from harvest to CA-containerisation was 9.8 days for the unaffected pallets and 12.2 days for the affected fruit. This is much longer than the recommended four day RA period for the Maluma cultivar. It would thus appear that an extended period from harvest to CA containerisation plays a major role in the development of grey pulp.
Recommendation
Based on the results, LPS recommends that the period between harvest and CA-containerisation should not exceed one week when using dry-ice-derived CO2 enrichment on Maluma fruit in refrigerated trucks. However, laboratory results suggest that these strict measures do not apply to Hass.
As a result, the newly developed CO2 enrichment techniques will give exporters more flexibility by significantly reducing the need for door-to-door carting of shipping containers. It will also facilitate the assembling of orders in Cape Town harbour and contribute to reducing soft landings and grey pulp in late-season Hass. There is, furthermore, a possibility that only CO2 enrichment and no CA could be applied with Hass, should empty market conditions prevail during the early- or mid-season.
During the 2022 season, the treatments were extended to also include cold rooms. A total of 4 597 pallet treatments were performed, 3 263 in cold rooms and 1 334 in trucks. Due to the logistical problems currently being experienced by the industry, some of the pallets were treated more than once. It is envisaged that, in time, the dry-ice applications will be replaced with CO2 gas injections from mass-storage tanks.
