Finding Answers to pomegranate questions

Disease-related challenges in the early years stunted the industry’s growth, but by 2005 production volumes started to increase steadily, mirroring the global demand trajectory. Pomegranates are currently the 18th most consumed fruit in the world, and are expected to break into the top 10 within the next decade, as consumers’ appreciation increases for the anti-inflammatory and anti-hypertension properties of pomegranate seeds.

Despite the local industry’s fledgling status, SA has already built up a significant export market. In the last 11 years, annual export value has grown from R33 million to R132 million. Main export markets are the European Union and Russia, followed by the UK. In terms of supply, local growers compete with Chile, Australia, Peru and Argentina.

Although the demand for this niche fruit is increasing exponentially, worldwide supply is still low, which creates lucrative export opportunities for the South African industry. However, to leverage these, innovative, science-based tools and quality standards have to be developed, including postharvest management in the areas of packaging, storage, transport, and the detection and control of postharvest diseases and disorders.

Though one of the youngest in the world, SA’s pomegranate industry has already established itself as a world leader in terms of generating reliable and detailed statistics, and has enabled various research projects to support growth in the industry.

The latest of these was a two-year study into the impact of different storage temperatures as a means to control false codling moth (FCM), on the quality of different pomegranate cultivars.

The problem

False codling moth is a phytosanitary risk that can derail an entire fresh fruit export programme. Therefore, pomegranate producers and exporters are eager to find a way to eliminate the insect while also preserving internal and external fruit quality.

To this end, a study jointly funded by the Pomegranate Association of SA (POMASA), an industry body, and the Postharvest Innovation Programme (PHI), set out to assess:

• the fruit quality of pomegranates stored at different temperature regimes.
• the effect of different cold sterilisation regimes on FCM mortality rates.

The study included three cultivars: Acco, Herskowitz and Wonderful, and was conducted by Ambrose Trout, a researcher at ExperiCo.

Due to restrictions imposed in 2020 related to SA’s coronavirus response, some of the study’s parameters could not be assessed. Hence, work had to continue in 2021 to validate the findings of previous seasons. “We needed to determine the effects of different temperature regimes on fruit quality in an attempt to establish a cold-sterilisation protocol for controlling FCM in pomegranates,” explains Trout.

Study methodology

Waxed pomegranates were sourced from the Sonlia packhouse in the Wellington region and subsequently subjected to different single- and dual-temperature cold-storage regimes at the ExperiCo facilities in Stellenbosch.

The fruit was evaluated for external and internal disorders after 23 days and then 30 days in cold storage, followed by seven days at 20°C, simulating shelf-life conditions.

In addition, prior to cold storage, a sub-sample of fruit was inoculated with FCM larvae (instar level 5) and stored at the same temperature regimes to determine mortality after 23 and 30 days respectively.

The cold sterilisation technique used simulated the phytosanitary standards of certain import markets. It is necessary to verify the post-storage results for controlling FCM and chilling-related injury, which presents as browning on the surface, albedo, septum and/or arils, to establish if it will be possible to use cold sterilisation for the Acco, Herskowitz and Wonderful cultivars.

Results

The 2020 research showed that single- and dual-temperature regimes at -0.5°C, 1°C and 5°C successfully controlled FCM larvae in all three cultivars after 23 and 30 days in storage, both at the start and the end of shelf-life. Following inoculation, neither alive nor morbid larvae were recorded, thus highlighting the effectiveness of cold sterilisation to control FCM.

As far as browning is concerned, the results were less conclusive. While surface browning on Acco was successfully managed across all treatments for 23 days of storage, none of the regimes proved effective when the fruit had been stored for 30 days.

An early DT (T2) regime controlled browning to acceptable levels for Herskowitz after 23 and 30 days’ storage, but none of the temperature regimes were successful for Wonderful.

The study further revealed that chilling-related surface browning is a bigger issue in pomegranates than browning of the albedo, septum and aril. Generally, surface browning increased the longer the storage duration, especially for Acco, as well as over the seven-day shelf-life period, especially for Herskowitz and Wonderful.

Conclusions and the way forward

None of the temperature and storage regimes in the study could successfully control surface browning induced by chilling injury. However, a comparison of the post-storage results of the 2020 and 2021 seasons suggests that seasonal variation in orchard conditions and, consequently, inherent fruit quality, influence the effectiveness of storage regimes in controlling browning disorders.

Overall, increased browning following the seven-day shelf-life period highlighted the risks associated with fruit warming after cold storage.

Based on results to date, it is clear that further research is needed in order to establish a cold-sterilisation solution that does not compromise fruit quality. The fact is that the causes of browning in pomegranates are not simple. Therefore, finding the right solutions will require an approach that combines different aspects of harvesting maturity, cold temperature regimes, product handling during logistics, and packaging formats.

But fortunately, the pomegranate industry does not have to start from scratch. Lessons can be taken from how the stone fruit industry approached browning issues attributed to immature harvests, storage temperatures and impact forces during handling.

“The big step forward is that we can use various temperature regimes to effectively eliminate FCM infestation, and that dual-temperature regimes are potentially a better option for managing browning disorders than only using a single-temperature approach,” concludes Trout. Future work in this regard will focus on establishing a handling guide for each cultivar.

Featured image: Chilling related aril browning