How does Fuji respond to cooling?

A Hortgro-funded study investigated the effect of different cooling strategies on common storage disorders of Fuji apples. By Anna Mouton.

Study highlights summarised

The main finding of this study is that room cooling is superior to stepwise cooling for Fuji. However, industry experience suggests that stepwise cooling may still be the preferred option after rain, as it helps to mitigate water core.

Room cooling is associated with less bitter pit expression. “But preharvest factors determine whether or not bitter pit develops postharvest,” cautions MSc student Anmari Kriegler.

Although internal browning in Fuji is a CO₂ injury, Fuji’s response to high CO₂ concentrations was unexpected. “We thought that storing fruit for medium-term at 5% CO₂ would result in significantly higher concentrations of CO₂ damage, but that was not the case,” says project leader Dr Anél Botes.

“The recommendation for Fuji is to store at 0.5% CO₂ because we think it’s sensitive to CO₂,” she reflects. “But clearly, it’s not only the high CO₂ during storage that induces browning. A combination of factors, especially harvest maturity, plays a role.”

The study underscored the importance of harvest maturity. “The correct maturity is a big factor with all disorders,” says Kriegler. “Post-optimum fruit get more browning, and pre-optimum fruit get more bitter pit. You have to find the sweet spot because then you eliminate many problems.”

Kriegler and Botes found that bitter pit was more prevalent after four months than six months in controlled atmosphere, suggesting that Fuji apples at risk of bitter pit should be stored for at least four months in controlled-atmosphere storage. However, fruit were not stored for more than six months in this work.

Optimum harvest maturity for Fuji is 30%–40% starch breakdown, 8.5 kg firmness, 13.5 °Brix total soluble solids, and 0.45% total titratable acidity.

“I realise that harvesting at the right maturity is a challenge, especially when picking windows overlap or it rains,” says Kriegler. “But at the end of the day, harvest maturity determines how you manage fruit after harvest and how much money you can make from them.”

Introduction

Fuji is the fifth-largest apple cultivar in South Africa by hectares planted and the sixth-biggest by cartons passed for export in 2023, but until recently, there was little experimental data on the best way to handle it after harvest.

“With Cripps Pink, stepwise cooling is recommended to prevent internal browning,” says Dr Anél Botes, researcher in the Postharvest and Agro-processing Technologies Division at the ARC Infruitec-Nietvoorbij. “So, the industry suspected stepwise cooling would have the same effect on Fuji.”

To improve our understanding of best practices for Fuji, Botes led a Hortgro-funded project assessing how different cooling strategies affect internal browning and bitter pit in this cultivar. Botes collaborated with Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit in the Department of Horticultural Science at Stellenbosch University.

Anmari Kriegler, the MSc student on the project, was co-supervised by Botes, Crouch, and Dr Mariana Jooste, postharvest-programme manager at Hortgro Science.

The treatment protocols

Fuji apples for all the trials came from commercial orchards in the Koue Bokkeveld. In the 2021 and 2023 seasons, the fruit were harvested closer to optimum maturity (42% and 29% starch breakdown, respectively), and in the 2022 season, at a more post-optimum maturity (52% starch breakdown).

No 1-MCP (1-methylcyclopropene) was applied to any of the trial fruit.

“We wanted to see what the effect of cooling was on the occurrence of the disorders,” explains Botes. “If we had used 1-MCP, we would have needed to replicate all the treatments with and without 1-MCP.”

To evaluate the different cooling strategies, fruit were either room-cooled in a cold room at 0.5 °C or subjected to one of two stepwise cooling protocols (Table 1). Cooling occurred in regular atmosphere for 49 days before commencing low-oxygen storage.

Table 1. Room and stepwise cooling protocols tested on Fuji apples.

Room cooling: coldest treatment	Stepwise cooling 1: warmest treatment	Stepwise cooling 2: intermediate treatment
Directly in 0.5 °C	7 days at 4 °C then 14 days at 3 °C then 28 days at 2 °C then remainder at 1 °C	7 days at 3 °C then 7 days at 2 °C then remainder at 1 °C

The fruit were then stored in controlled atmosphere (1.5% O₂ and 0.5% CO₂) for either four or six months, followed by six weeks of simulated shipping in regular atmosphere at 0 °C, and seven days of shelf life at 20 °C.

Fruit quality was assessed at harvest, after cold storage, after simulated shipping, and after shelf life.

Stepwise versus room cooling

When fruit were harvested close to optimum maturity, no internal browning was found, but the apples cooled with the stepwise protocol developed bitter pit during storage. Bitter pit manifested more in fruit cooled with the warmer stepwise protocol (Table 1: stepwise cooling 1) than the intermediate stepwise protocol (Table 1: stepwise cooling 2).

Bitter pit was more prevalent after four months than six months in controlled atmosphere, regardless of cooling protocol, suggesting that Fuji apples at risk of bitter pit benefit from at least four months in controlled-atmosphere storage.

“We didn’t store fruit for longer than six months,” says Botes. “So we don’t know what happens after six months.”

When fruit were harvested more post-optimum, they didn’t develop bitter pit but instead showed internal browning due to CO₂ damage. However, the prevalence of browning was very low, and there were no significant differences between cooling treatments. Fruit stored for four months had more browning than those stored for six months, regardless of cooling treatment.

Based on their results, Botes and Kriegler don’t recommend stepwise cooling as a standard practice for Fuji. Industry experience suggests that stepwise cooling might be beneficial if harvesting immediately after rain, when Fuji is susceptible to developing water core.

“The stepwise cooling helps resolve the water core,” says Kriegler. She wanted to test this in her study, but the weather was uncooperative, and she didn’t have fruit harvested immediately after rain.

Spotlight on Fuji physiology

Kriegler wasn’t only interested in fruit quality and storage disorders. She wanted to understand the physiological basis for Fuji’s response, so she measured internal O₂, CO₂, ethylene, reactive oxygen species, antioxidant capacity, and phenolics at different cooling stages.

Reactive oxygen species are unstable molecules that form during cellular processes. When too many of these molecules accumulate, they can damage cells through oxidation. Antioxidants protect cells by neutralising reactive oxygen species.

Phenolic compounds can also act as antioxidants. When phenolics are oxidised, the brown pigment melanin is produced. Melanin is responsible for the discoloration seen in internal browning. Unoxidised phenolics do not show up as browning.

At harvest, the physiological metrics differed between optimum and more post-optimum fruit. Optimum fruit generally had lower internal O₂ and higher internal CO₂ concentrations, lower internal ethylene, lower reactive oxygen species, and higher phenolics than the more post-optimum fruit. Antioxidant concentrations were similar in optimum and post-optimum fruit.

More post-optimum fruit tended to have higher internal ethylene concentrations than optimum fruit throughout cooling. Although more post-optimum fruit had higher concentrations of reactive oxygen species at harvest, these almost halved within the first week of cooling and remained low. More post-optimum fruit also tended to maintain slightly higher concentrations of antioxidants.

In contrast, optimum fruit had lower concentrations of reactive oxygen species at harvest, but their concentrations increased within the first week of cooling and remained high.

When comparing room- and step-cooled fruit, the room-cooled fruit showed increased internal ethylene during the first 14 days, followed by a decline, so that the concentrations were lower than those in step-cooled fruit by the end of cooling. In step-cooled fruit, ethylene spiked at the end of cooling.

“When you use stepwise cooling and store for a shorter time in controlled atmosphere, the ethylene concentrations in the fruit are much higher,” says Botes. “That’s why the bitter pit appeared.”

Start now or start later

The Fuji project included the assessment of delayed cooling and controlled-atmosphere establishment on fruit quality. Fruit was harvested closer to optimum (42% starch breakdown) and more post-optimum (52% starch breakdown) maturity and held for 0, 4, 12 or 28 hours at ambient temperature before room cooling to 0.5 °C.

The fruit was then kept at 0.5 °C in regular atmosphere for three weeks before being stored in controlled atmosphere (1.5% O₂ and 0.5% CO₂) at 0.5 °C for four or six months, followed by six weeks of simulated shipping in regular atmosphere at 0 °C, and seven days of shelf life at 20 °C.

Fruit quality was assessed at harvest, after cold storage, after simulated shipping, and after shelf life. Physiological parameters were measured at harvest and after cooling delays.

Delaying the cooling of closer to optimally mature fruit by 28 hours significantly increased internal ethylene concentrations. Delayed cooling didn’t appear to be associated with increased internal ethylene concentrations in more post-optimum fruit.

Bitter pit in fruit harvested closer to optimal maturity manifested in the fruit where cooling was delayed by 28 hours, but only in fruit stored for four months. Fruit stored for six months had no bitter pit.

Lastly, Kriegler tested whether a short-term CO₂ exposure after harvest could elicit CO₂ injury and be used to predict the risk for a batch of fruit. She concluded that the short-term exposure didn’t accurately predict the development of internal browning during storage.

What does industry say about this research?

“We go to Hortgro when we have a problem with a cultivar like Fuji. Whenever you have a new cultivar, you get surprises like internal browning or lenticel disorders. Then we approach Hortgro to help us develop best practices for that cultivar. So this research is initiated at the pack-house level, when we struggle to understand a cultivar.”

Technical adviser Nico Ferreira. Fruitmax Agri.