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Post-harvest quality: slowing down the ‘ticking clock’

Industry Best Practice

Nutrition, fruit maturity and climatic conditions are among the pre-harvest factors which can have an impact on the quality of apples post-harvest, according to Dr Hannah James, AgroFresh Research and Development Manager Australia/New Zealand .

AgroFresh’s Dr Hannah James says the process of getting an apple from the orchard through to the consumers’ hands should be thought of as a journey by all in the supply chain.

Hannah says the process of getting an apple from the orchard through to the consumers’ hands should be thought of as a journey by all in the supply chain including growers, transporters, packhouse employees and wholesalers.

“There are a lot of different things that can happen on this journey which all have different impacts on the quality of the fruit, and it comes down to what happens pre-harvest just as much as what happens post-harvest,” she says.

“When the fruit is picked that is the best it’s going to be, so it’s a matter of how we can maintain that quality for as long as we can.”the fruit is picked that is the best it’s going to be, so it’s a matter of how we can maintain that quality for as long as we can.”

The journey of an apple

The journey of an apple starts with flowering, which is followed by fruitlets. During the course of this period there will most likely be some applications of crop protection products.

Permanent or temporary netting will change the environment of where the fruit is growing. How and when irrigation is applied and whether or not the fruit gets sunburnt are also factors for growers to consider when thinking about post-harvest quality.

These factors, Hannah says, all play a role in post-harvest quality.

“Eventually we will have a nice piece of fruit which will be harvested and transported,” she says.

“The more times you move and bump a piece of fruit will further change the respiration of the fruit. Respiration is all the energy that fruit has got. It’s like a ticking clock and if you can slow that ticking down, that piece of fruit is going to last longer.

“When we take that apple off the tree it’s still a living, breathing thing and we want to keep it that way, but we just want to slow down that ticking so we can keep it in the best quality we can for as long as we can.”

Post-harvest practices such as drenching, pre-sizing and storage also play a role in the quality of the fruit.

“Drenches may have fungicides, calcium or DPA in them, all of which in their own right or in combination can have an impact on the post-harvest quality of the apple,” Hannah says.

How the fruit is handled in the packing shed as well as the type of storage can both affect fruit quality.

“A pre-sizer can have benefits for growers in terms of inventory management and reducing some sorting, but there can also be some negative impacts. There are sanitisers and soaps in there, water and water temperature which can all impact on lenticels and lenticel quality.

“We can also get some issues from pre-sizing when that fruit comes out of storage months and months later.”

Storage type and duration and how the fruit is packed will also affect its quality.

“They will all have different impacts on quality,” Hannah says. “Then the fruit will be transported somewhere either on a truck or a boat.

“What we think is going on in that shipping container isn’t necessarily what is actually happening in the shipping container in terms of how the temperature is managed and how the fruit quality is coming out the other end.

“At the end of the day this fruit is then going to go to a consumer but the journey doesn’t even end there. You have the distribution centre behind the supermarket, then the consumer buys the fruit and they could have it for a week or two before they consume it.

“There is no doubt that we are going on a very long journey with this piece of fruit.”

The nutrition factor

There are a number of different minerals in fruit which have different functions and their ratios to each other can have significant outcomes in how that piece of fruit is going to store, as well as the disorders which might develop in storage.

Hannah says boron is a particularly important element as it relates to cell stability and is linked to internal browning.

“Calcium is another important element as it very important for cell wall strength,” she says.

“It is linked to some internal browning, very much linked to storability and very much linked to bitter pit.

“But getting that calcium onto the plant and where it needs to be is a challenge because applying it doesn’t mean it gets to where it needs to be.

“We have seen examples of calcium deficiency in Pink Lady® apples where the higher the levels of calcium were, the lower the incidence of browning was. That is not an atypical relationship we expect to see with calcium.”

Hannah says young trees are generally lower in calcium and generally the only way that can be managed is to wait for the tree to get older.

Low cropping trees also generally have lower calcium and managing the crop load and making sure it is under control is going to help ensure the mineral content of the fruit has the right balance for optimum post-harvest quality.

Water availability will also help with calcium levels in the fruit as calcium is moved through transpiration.

“If there is no irrigation and no water moving, you’re not going to have calcium moving through the plant into the fruit,” Hannah says. “Irrigation, especially early when calcium is mobile, is incredibly important.

“Hot weather will shut down transpiration of a tree and will therefore slow calcium uptake.

“Some of these things we can’t do anything about but being aware of when they might happen can help us to have some understanding of how the fruit might behave once it goes into storage.”

Phosphorus is another important element because, like calcium, fruit with higher levels of phosphorus tends to have less instance of disorders.

Harvest maturity

Early-maturing fruit may be better equipped to store longer, but it will have less sugars and less flavour.

While early-maturing fruit – or fruit that is pre-climacteric with little or no ethylene production and less than two on the starch scale – may be better equipped to store longer, it will have less sugars and therefore less flavour when it is eventually consumed.

“That piece of fruit is never going to catch up to a lot of those flavour characteristics that it would have had if it stayed on the tree for longer,” Hannah says. “Early maturity will usually mean less blush, because it comes off the tree sooner and doesn’t have the time to develop the blush.

“There is also the risk of superficial scald and external carbon dioxide injury. Those disorders are impacted by harvest maturity.”

Commercial maturity, or fruit that is just prior to climacteric and between two to four on the starch scale, carries the lowest risk of developing disorders post-harvest.

Late maturity fruit – post-climacteric with a score of above four for starch – will carry an increased risk of developing senescent disorders.

“Fruit can’t always be harvested at commercial maturity because growers may be waiting for a blush or some other market quality and therefore might be forced into having fruit of a late maturity going into storage,” Hannah says.

“It’s about knowing how to manage that and knowing if the piece of fruit is going to have less storability so decisions can be made as to when that piece of fruit is marketed.

“Once that fruit comes out of storage there can be some pretty significant impacts on the firmness of the fruit with every delay in harvest maturity.”

One of the biggest challenges facing growers is how to measure harvest maturity.

Ethylene is a good indicator of physiological maturity of apples but requires chromatography, which is expensive and has to be done through a laboratory.

However, Hannah says there are continuing developments in the area of chromatography which should see it become portable, cheaper and easier to use.

“Genomics is another area where there are some new tools being developed which can measure gene changes associated with cell wall softening or other ethylene systems,” she says.

“It’s a good tool but it takes some time. It’s not something where we can go out in the orchard and get instant feedback because it takes a couple of days.

“There is also some experimentation happening with DA metres where you do get some instant feedback, but it doesn’t necessarily relate to starch or ethylene in the way we want it to and we definitely need regional variations.

“It isn’t something you can take out of the box and use, it needs to be calibrated for particular conditions.”

While measuring starch is an easy and cheap way to gauge maturity which usually correlates to ethylene, there are some challenges because there is not always such a correlation.

Hannah says the success of all these techniques depends on the sample size and collection.

“If you’ve got someone skilled going out to the orchard and picking the right things, you can get a nice uniform measurement, but if you just take a random sample of 50 pieces of fruit then you’re going to get a number that doesn’t necessarily make sense for what you’re trying to do,” she says.

“Starch can be disassociated from ethylene due to stress. If you have a stress event, such as a hot spell, ethylene can go up which can start some starch degradation but starch doesn’t require that ethylene to continue to be there.

“After that stress event the ethylene will come back down but the starch will continue with its degradation.

“In this instance, we can get growers measuring starch which looks like it is static and okay to harvest, but there are quality changes happening within the fruit.”

Climatic conditions

Cool temperatures early in the season – the first 50 days after flowering – can increase cell division and lead to fruit with a higher density, which has been shown in research conducted in New Zealand.

Hannah says fruit with higher density going into storage tends to be more prone to carbon dioxide type disorders, such as internal browning.

Warm temperatures early in the season can also be a challenge as there is less cell division. Fruit with larger cells tends to have less minerals and a lower shelf life.

“When we talk about the late season, or the last 60 days before harvest, cool temperatures can delay maturity and as the grower waits and leaves the apple on the tree to try and get colour, the risk of chilling injury can increase,” Hannah says.

“Warm temperatures late in the season mean we don’t get that diurnal temperature shift. Sometimes if we have warm nights and if we get hot enough temperatures maturation can stop altogether.

“When we look at the whole season, warm temperatures are related to superficial scald, which is something we see in fruit from the Goulburn Valley in Victoria.

“In regions with cool temperatures over the whole season there are links to internal browning disorders.”

While there is nothing growers can do about climatic conditions, Hannah says being aware of what is happening with the climate can help to give some clues as to what may happen to the fruit during storage.

More information

Dr Hannah James, AgroFresh:  [email protected]

nutrition and irrigation post-harvest quality soils

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