Finding the best long-term storage strategy for Granny Smith apples. By Grethe Bestbier.
For acceptance by international markets, not even 1% of a shipment of Granny Smith apples may be affected by superficial scald. Unfortunately, this cultivar is especially prone to superficial scald, which causes unappealing brown disfiguration of fruit peels. An extensive research project on superficial scald in Granny Smith apples is looking into long-term storage protocols and treatments, hoping to identify ways to prevent the problematic disorder.
Granny Smith is the apple cultivar that is exported the third-most commonly from South Africa. To ensure prolonged availability, fruit are often stored for many months after harvest.
“If you look at the price per volume, you’ll see that prices increase between November and February. The longer we can store the fruit, the better our income,” says post-harvest physiologist and project leader Dr Elke Crouch.
However, prolonged storage can increase the risk of several physiological disorders. One such disorder is superficial scald, characterised by uneven browning of the fruit peel. Superficial scald is not always immediately detected and, as Crouch explains, can be a very expensive problem.
“Often, by the time you notice superficial scald, you have already suffered all the costs of shipping the fruit through the entire logistics chain. It is a massive risk, for both international exports and the local market,” she says.
To prevent superficial scald, industry has investigated a variety of techniques and developed their own protocols over the years. Unfortunately, the lack of rigorous research has left some gaps in best-practice knowledge.
“Industry tried many different protocols and to a certain extent they were successful. However, they never had a control group, or something to compare their results with. So, we never knew whether it was the protocol’s effect or if the particular fruit were just less sensitive to superficial scald,” says Crouch.
The protocols included the use of the antioxidant diphenylamine which was historically applied to limit scald. Due to toxicity concerns, international markets have now set maximum residue levels that effectively preclude the use of diphenylamine.
The ethylene inhibitor 1-methylcyclopropene — 1-MCP — is also a well-known treatment against superficial scald, although in some seasons it has not been completely effective. It binds to the ethylene receptors in cells, thereby blocking the action of ethylene. Ethylene is a ripening hormone and plays a role in the development of superficial scald.
“There have been various studies on superficial scald, but there remains a lot of uncertainty,” says Crouch. “Although we know that 1-MCP is a good chemical treatment, it needs to be combined with other treatments.”
Protocols for preventing superficial scald
With no consensus on the best approach to prevent superficial scald, it became necessary for the research team, consisting of Dr Elke Crouch, doctoral student Monja Gerber and post-doctoral researcher Dr Pandian Thirupathi, together with partners Anél Botes from the Agricultural Research Council, Daniël Viljoen and Dr Ian Crouch from ExperiCo, and Dr Erika Moelich from the Department of Food Science at Stellenbosch University, to tackle two central questions. Firstly, what protocols are available and how effective are they in preventing superficial scald? Secondly, how do these protocols work?
To answer the first question, the researchers tested various treatments and storage protocols individually, and in combination — see table 1. They evaluated the apples after different lengths of time in cold storage, and again after shipping and storage conditions conducive to scald development.
Table 1: Treatments and protocols tested in the project
Also included in the evaluation was maturity indexing — testing for ethanol and ethylene content, other biochemical compounds associated with superficial scald, and very importantly, sensory qualities.
“A few characteristics are important in Granny Smith apples,” says Crouch. “We don’t want any scald, apples must not be yellow or greasy, and must of course taste good.”
While quite a few of the tested treatments did reduce the appearance of superficial scald and produced good colour and sensory qualities, there were some stand-out protocols. The best treatments were a single application of 1-MCP immediately after harvest followed by storage in either dynamic controlled atmosphere based on chlorophyll fluorescence — DCA-CF — or static controlled atmosphere — CA — with an additional 1-MCP treatment applied after 24 weeks. In the first season, fruit treated with 1-MCP after harvest and held in static controlled atmosphere did not show scald after 34 weeks cold storage, shipping, and shelf life, but in the second season slight scald did develop. Therefore, 1-MCP application at the end of storage is recommended.
Many factors can influence treatment choice, and all have pros and cons. DCA-CF, for example, is not inexpensive and application may depend on how high the risk is for that season and how long you plan to store. Using DCA-CF at the end of storage seems unnecessary whereas adding 1-MCP at the end is critical. On plus side, apples stored with DCA-CF did have excellent eating quality.
For any protocol against superficial scald, timing is crucial. Treatment with 1-MCP must be applied as soon after harvest as possible and definitely within five days of harvest, while the second round of 1-MCP is applied 24 weeks later.
“The first seven days after harvest is incredibly important for superficial scald prevention. That is when ethylene, which is a ripening hormone that may cause scald, can be induced. This means that if you inhibit ethylene — via one of the protocols — later on, it might be too late,” Crouch explains. It has also been noted in other DCA trials that a low oxygen concentration is critical in the inhibition of superficial scald.
Figuring out early warning systems
To better understand how the mechanisms behind these different protocols work, Crouch recruited the help of doctoral student Monja Gerber and post-doctoral researcher Dr Pandian Thirupathi. Together, they are hoping to find biochemical risk indicators for the early detection of superficial scald in Granny Smith apples. Superficial scald is often not noticeable after harvest or even cold storage, but appears during shelf life.
“Practically, when the storeroom is opened at the pack house, everything looks in order. Then, when fruit reach the consumer after shipment and shelf life, after all the expenses have already been made, the fruit is in a poor condition due to superficial scald,” explains Crouch.
“We are trying to find something that will help us to detect it quicker, but it is very complicated,” adds Gerber.
These warning signals could possibly present at any stage of the value chain and Gerber is looking at different stages during cold storage, shipment, and shelf life.
Both biochemical and protein-related information are needed to find a reliable warning system. Therefore, the project also has a proteomic section handled by Drs Ashwil Klein and Lizex Husselman at the University of the Western Cape under the guidance of Dr Patricia Mathabe from the Technology Innovation Agency. They are examining how proteins react to the different protocols and treatments against superficial scald.
“Just like we are looking at the biochemical side of superficial scald, they are looking at the proteomic side of it,” says Gerber. “There are many new proteomic studies that have surfaced. Biochemical tests are great, but we still can’t exactly identify the correct mechanisms behind the protocols without protein knowledge.” What are people saying about this research?
Jaco Moelich | Fruitways Group
“Superficial scald is a progressive defect, and we can’t always foresee how the fruit will react,” explains Moelich. Therefore, clients have a very conservative approach to the defect and readily turn down any fruit with signs of superficial scald. “With South Africa exporting large quantities of fruit to the Northern Hemisphere, any uncertainty over the control of scald is risky and can lead to great financial losses.”
According to Moelich, this research project is vital for understanding the processes behind superficial scald. “Without such knowledge, we would not be sure why and under which circumstances some treatments can successfully control the problem. For fruit handlers and marketers, certainty around the control is crucial to the long-term marketing of fruit.”
Image supplied by Karen du Plessis.
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