Hi-tech postharvest research

Everyone wants to save energy

One way SA and Belgium have converged since Nicolaï first visited is energy – or the lack thereof. His recent sabbatical coincided with the worst loadshedding in South African history, but he was prepared by the energy crisis back home in Europe.

"Energy reduction is a big issue," he reports. "Europe is heavily hit by the war in Ukraine. At one point, the gas price had increased by 400 – 500%, and the electricity price is coupled to the gas price."

Belgian fruit cooperatives can have hundreds of cold rooms, placing them among the largest electricity users. "They are not going to make any investments for the next year unless they're related to energy savings," says Nicolaï.

According to Nicolaï, all the cooperatives already have roofs covered in solar panels, thanks to a stimulus programme by the Flemish government. "But it's not nearly enough to run their refrigeration equipment," he adds.

Other infrastructure investments – improved insulation, electronically commutated fans, and energy-efficient refrigeration – can all lead to further energy savings. But Nicolaï has found management interventions such as tweaking storage temperatures to be less successful.

He previously had an energy-savings project that investigated allowing fruit to warm a little, followed by cooling to slightly below normal. No energy savings resulted because higher temperatures increased fruit respiration rates and heat production – heat which required energy for its removal.

Seeing inside fruit with X-ray CT

Nicolaï is working with Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit at SU, on gas transport in pome fruit. Understanding gas transport in fruit is essential because many storage disorders result from low oxygen or high carbon dioxide levels – or both – in the fruit.

Fruit continually consumes oxygen and produces carbon dioxide to generate energy, which they need for fundamental processes such as membrane repair. When membranes break down, enzymes and substrates that are usually in different compartments in the cell are no longer separated, so they react. The reaction of polyphenol oxidases with polyphenols leads to browning.

Low oxygen and high carbon dioxide levels may also cause fruit cells to die, forming cavities.
Gas transport in fruit is strongly related to porosity. "If you have low-porosity fruit, then you may have more risks of browning," says Nicolaï. "Or, if you could measure porosity, then maybe you could predict storability – you might be able to estimate the storage potential for different batches of fruit from different orchards."

Nicolaï has been using X-ray CT – computed tomography – to build three-dimensional images of the fruit microstructure that can be used to model gas transport and quantify gas concentration gradients.

"There was a student of mine who visited Stellenbosch a couple of months ago to do some of the X-ray CT scanning," he says. "We combined the measurements obtained here with modelling in Leuven. During my sabbatical, I fine-tuned the models and discussed the way forward with Elke."

Models of fruit metabolism

"We're trying to focus not only on gas transport but also on the metabolism of the fruit," says Nicolaï. Once harvested, fruit must completely reconfigure their metabolism to survive – they express different genes and form different proteins.

Understanding fruit metabolism creates opportunities for better maintaining quality. "This technology has been around for fermentations for many years," says Nicolaï, "For example, microorganisms are used to produce compounds like penicillin and other antibiotics."

Biochemical engineers model the metabolism of microorganisms so that they can manipulate the pathways that produce the desired product. They can optimise yields by changing conditions such as oxygen concentrations or temperatures.

"Microorganisms are much simpler than plants or fruit," says Nicolaï, "and in metabolic engineering, these techniques have been developed much better because that's where the big money is. In plants, we're lagging behind, but we can use the technology that they've developed and translate it to our systems."

He has been mapping out the biochemical pathways in fruit exposed to different oxygen concentrations. "You basically translate the metabolic reactions into differential equations. The next step is to couple it with the gas transport model, and then we can predict what is going on inside the fruit."

Real-time aroma measurement

Fruit aroma is an aspect of postharvest metabolism that intrigues Nicolaï. The volatile compounds of fruit such as apples and pears are formed from metabolic products generated during ripening. Nicolaï thinks that metabolic modelling techniques can also be applied to better understand how volatile compounds are formed and how storage affects these processes.

Volatile compounds have traditionally been measured using time-consuming gas chromatography-mass spectrometry. But the newer SIFT – selected-ion flow-tube – mass spectrometry provides real-time aroma measurement.

"It's so fast that you can stick a tube in your mouth while you consume an apple, and you can see how the aroma changes in your mouth because of oxidation," says Nicolaï. "It does not give you analytical information, but it gives you a fingerprint."

He is working with the Leuven experimental station to incorporate fruit aroma in cultivar selection. "Farmers don't want to look at aroma profiles and which esters are important – we want to combine production and postharvest data into a cultivar ID that's easy to interpret by cooperatives and farmers."

According to Nicolaï, postharvest research in Belgium now happens sooner rather than later in cultivar-selection programmes. Basic trials of a small range of storage conditions are done once cultivars pass the initial screening.

"We need decision-support systems that enable the industry to choose new cultivars, because if you choose the wrong cultivar, you lose five years. It's a very expensive mistake, and you need all the information that's available to avoid it."

As a postharvest technologist, Nicolaï says his goal is to keep fruit quality as good as possible. But he stresses that fruit quality is largely determined by what happens before harvest. "Of course, storage conditions are crucial, but then again, if you don't produce a good fruit, you will not improve it after harvest."

Featured Image: Prof Bart Nicolaï is working with Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit at Stellenbosch University, on gas transport in pome fruit