Why it happens and how to prevent it

By Anna Mouton

We need to know more about the rachis physiology, but there is limited information.

Harvest at the right time – thinking of the berry and the rachis – and know the storage potential of your variety.

“The rachis is one of the key freshness indicators in grapes,” explained Dr Bruno Defilippi, post-harvest specialist at the Institute of Agricultural Research in Chile. Consumers may reject grapes with rachis browning even though the berries are in good condition. Defilippi presented research results on the causes and control of rachis deterioration.

Rachides are like lettuces

“First, we have a very long wish list for the consumer in terms of quality,” said Defilippi. Consumers are concerned about many aspects of appearance, fl avour and texture, and the trick is fi nding the right balance. “Sometimes, having a good fl avour, for example, means we’re not going to have the best texture or the best rachis appearance.”

Defilippi pointed out that, while attributes such as soluble solids may remain stable after harvest, rachis quality tends to deteriorate. This is exacerbated by long shipping times to distant markets – a problem shared by Chilean and South African producers.

The proliferation of new cultivars adds further complexity, as postharvest management needs to be optimised for each one. “We are dealing with more than fi fty varieties that we have available right now,” said Defilippi.

He listed further challenges as the lack of knowledge about rachis physiology, the tendency to harvest based only on sugar content, the failure to follow correct harvest and postharvest practices, the impact of environmental conditions such as drought, and the shortage of technologies to extend rachis quality.

“We know that the rachis contributes 3 – 8% of the total fresh weight of the bunch,” stated Defilippi. He emphasised that the rachis has a high respiration rate, a high surface area to volume ratio, a large number of stomata and lenticels, and a permeable cuticle. Defilippi compared the rachis to a lettuce in terms of its sensitivity to the environment.

Cumulative water loss from the rachis and the berries leads to visible dehydration of the rachis. Rachis quality is impacted when water loss from the bunch exceeds 2%.

Water loss of 2 – 4% will have an adverse effect on berry texture, and levels above 5% will result in visible dehydration of berries.

Rachis dehydration is usually accompanied by browning. Browning is a process of age-related deterioration involving oxidative stress. Nearly all cultivars are susceptible to rachis dehydration and browning after 30 days of storage.

The key to keeping it green

Defilippi stressed the necessity of knowing the optimum harvest maturity, harvest window, harvest index, and storage potential for individual cultivars. He presented data on harvest maturity and rachis quality that shows that rachis quality is worse for later than earlier harvests. Harvesting later improves aspects like flavour, but rachis deterioration is greater. This effect increases with longer storage times.

“The take-home message is, harvest at the right time, thinking of the berry and the rachis,” said Defilippi, “and of course you need to know the storage potential of your variety.”

Harvest and postharvest practices are very important. “The knowledge is there. We know the temperature management and the specifications of the cooling process,” Defilippi pointed out. “We need to do the right thing at the right time and in the right way.”

Plant growth regulators offer promise for maintaining fruit quality during the postharvest period. Defilippi first discussed ethylene. It can be used to improve berry colour in some cultivars, but it also speeds up ripening and deterioration. Ethylene does not affect rachis dehydration.

As for ethylene inhibitors, – such as 1-MCP – when applied at véraison they’ve been shown to improve rachis quality. “But, we didn’t get a consistent effect among seasons,” said Defilippi.

Cytokinins are another growth regulator, mainly used to manage berry size. Cytokinins also help pr event agerelated deterioration of plant tissues.

Research has demonstrated that cytokinin application results in greener rachides. Excessive cytokinins can lead to rachis rigidity, stem thickening, and berry drop, so the correct dosage and time of application are important.

Controlled atmosphere storage and modified atmosphere packaging help to slow fruit metabolism, and reduce deterioration and decay. Higher relative humidities contribute to improved fruit quality by limiting dehydration. Both controlled atmosphere storage and modified atmosphere packaging help keep rachides green and hydrated.

Defilippi concluded by reminding participants that the principles which apply to rachis quality are the same as for grape quality. “Water vapour pressure deficit control is a must,” he said. He also underlined the need for better information about new cultivars, and cautioned that alternatives to sulphur dioxide may increase the risk of rachis browning.

“We need to know more about the rachis physiology. We know the physiology of the berries – the véraison, the changes in sugar and acid, the metabolism, the changes in respiratory rate. But there is little information about rachis physiology.”