How can South African postharvest practices improve?

By Anna Mouton

South Africa faces similar challenges to Chile, Peru and Australia when exporting table grapes to the northern hemisphere, especially to Asia. Dawie Moelich, technical and market access manager at SATI, explored the contrasting approaches of these countries to cold-chain practices and packaging, and discussed the benefi ts of a systems approach to postharvest quality maintenance.

Back to basics

“Postharvest quality already starts in the vineyard,” said Moelich, “with the selection of the cultivar and the site. The perception that postharvest quality starts when you cut the bunches from the vine is not the full picture.”

Managing the quality of table grapes is complicated by the physiological differences between the rachis and the berries. Moelich explained that aging is a natural process and that optimum harvest maturity is critical to maintaining quality during storage and shipping.

“The strongest tool available to slow down the inevitable loss of quality is temperature,” emphasised Moelich. The key is to reduce the time during which the product is not at optimal temperatures, and to get the product to the market as quickly as possible.

Moisture loss from grapes is increased by exposure to high temperature and low humidity directly after harvest, by delayed cooling and by larger perforations in liner bags. The physical characteristics of the rachis also infl uence water loss. “You will never be able to pack a feeble, thin rachis, and maintain it in a fresh condition to a long-distance market, no matter what packaging or postharvest methodology you use.”

Postharvest quality already starts in the vineyard, not when you cut the bunches from the vine. Tegnologie Suid-Afrikaanse Tafeldruifbedryf

The evolution of South African postharvest practices

South Africa built a reputation in the 80s and 90s as a quality supplier to Europe. Cooling cycles were very long — 60 – 72 hours — and non-perforated liner bags were standard. Dual-release sulphur-sheet packaging was the preferred option.

At the time, SA produced mostly seeded grapes with strong skins and strong rachides, recalls Moelich. Cultivars could only be planted for export after the storage potential had been evaluated in detail.

In the new millennium, there was a shift to seedless varieties, which are generally more sensitive to deterioration during storage. There was a need to investigate alternative packaging methods. “A tremendous amount of applied postharvest research was conducted, testing a wide range of packaging variables,” said Moelich.

“Recommendations were developed for specific cultivar groups according to their specific postharvest challenges.” Unfortunately, the industry resisted uptake of these measures, which were felt to complicate stock-keeping and operations in the pack house.

The tendency was to adopt liner bags with larger perforations, so as to facilitate rapid cooling.

Moelich highlighted the value of maturity indexing. “Most postharvest specialists believe that maturity indexing is an essential tool to optimise keeping quality over extralong distances.” He believes that product development in this area is lacking in SA.

“This is an indication that SA table grape stakeholders have not been serious enough about longer-term cold storage of table grapes.”

Comparisons to Chile and Australia

The packaging used by Chile is designed for good ventilation to facilitate fumigation, which is a phytosanitary requirement for export of their product to the US. “The rest of their system is designed around this requirement,” clarified Moelich.

The advantages of highly perforated bags are less sulphur dioxide bleaching, less splitting, and more rapid cooling. The disadvantages are lower humidity, which can lead to more rachis dehydration and lower sulphur dioxide levels, which can compromise decay control.

Chileans mitigate the risk of rachis dehydration by an intense focus on removing field heat as soon and as rapidly as possible. Moelich contrasts this with South Africans, who have managed the vapour pressure deficit by a greater reliance on higher humidity created by less-perforated or non-perforated liner bags during the storage and transportation phases.

Australians pack the grapes in the vineyard and transport the open cartons to the cooling facility. Cartons are left stacked on racks overnight in the cold store. “This is obviously the most energy-efficient way to remove field heat,” said Moelich. “When the grapes reach the target temperature, the final packing and palletisation is done in the cold store.” For this reason, the Australian system relies on a significant investment in refrigerated facilities.

Moelich pointed out that Australian packaging is non-perforated to insulate the grapes against potential moisture loss and temperature changes after the grapes have been pre-cooled to target temperature, and packed and palletised.

The strongest tool available to slow down the inevitable loss of quality is temperature.

Success with a systems approach

“Postharvest quality is not only determined by logistics,” said Moelich. “Of course the logistical chain needs to be optimally managed, but many aspects thereof are standardised and lie outside your control.”

Moelich stressed the contribution of growing and harvesting practices to the intrinsic quality of the grapes. A systems approach enables producers to identify areas where they need to apply more effort. It requires analysis of the system as a whole, including both successes and failures.

“You need accurate information,” said Moelich. “Identify people in your company who specialise in your own postharvest system approach to work on this continually.”

Moelich invited table grape producers to contact SATI for assistance in implementing a holistic quality assurance approach to postharvest quality.