Mealiness in Forelle

Mealiness was probably a Forelle issue long before it was first planted in South Africa in the 1800s. How is modern science solving this old problem? By Engela Duvenage.

Research on Forelle was already underway by the 1990s, with work on whether intermittent warming, and controlled- and regular-atmosphere storage intervals could help.

In the early 2000s the effects of harvest maturity, chilling injury, ethylene treatments, different rootstocks, and mineral nutrients were studied, and climatic and ripening models were drawn up. At the Department of Horticultural Science at Stellenbosch University, researchers such as Prof. Karen Theron, and Drs Marius Huysamer, Deirdre Holcroft and Elmi Lötze were involved.

Maturity parameters and storage regimes to obtain Forelle pears of an acceptable eating quality were published in 1993 in a Unifruco research report by Pieter de Vries and Richard Hurndall. Within ten years, researchers had discovered that prolonged cold storage reduces mealiness upon ripening.

This research contributed to the introduction of a mandatory 12-week post-harvest storage period at -0.5 °C to allow enough time for ethylene production to stimulate the ripening of Forelle. The 12-week period includes shipping and distribution.

Figuring out floury Forelle

The initial work on cold storage and mealiness was done by Dr Elke Crouch as part of her MSc, and she continued to work on the topic for her PhD. Nowadays, Crouch holds the Chair in Post-Harvest Deciduous Fruit Physiology Research at Stellenbosch University.

In the introductory chapter of her thesis, Crouch reported that harvest maturity only seemed to affect mealiness in Forelle when fruit were stored for less than 12 weeks at -0.5 °C. Other factors did not appear to play a role or explain the development of mealiness in Forelle.

Back then, experts were all but certain that mealiness related to high temperatures before harvesting in most pear cultivars, but not in Forelle. Mealiness was also not caused by too-long periods of cold storage, as is the case with, for example, D’Anjou.

Crouch started her investigations with the idea that all soft, dry textural disorders, whether induced by chilling injury or other factors, relate to a difference in cell-wall breakdown when compared with juicy fruit.

She took her cue from research on apples, which showed that textural disorders like mealiness are related to calcium that pastes cells together, and to cell size and shape. Mealiness in apples usually increases as the fruit ripens.

Her results showed that mealiness in Forelle is associated with low adhesion of neighbouring cells. It further forms when the minute cavities or air spaces between adjacent cells become larger.

Factors that affect mealiness

Over the past decade, Crouch and others in academia and industry have guided further research on Forelle mealiness by postgraduate students, most of which has been funded by Hortgro and initiatives such as the Post-Harvest Innovation Fund, which is a public-private partnership between the Department of Science and Innovation and the Fresh Produce Exporters’ Forum.

“We now understand mealiness better,” says Crouch. “We know for instance that its development starts in the neck of the fruit, extending downwards through its flesh.”

Light and fruit size are direct influences. Mealy fruit are generally larger with higher sugar content and sugar:acid ratios, and lower juice area, juice weight, and firmness.

Thanks to greater light exposure, outer-canopy pears and those on open training or trellis systems tend to be more blushed, and have higher total soluble solids, which are characteristics preferred by consumers. However, the cells in the neck of outer- canopy pears tend to be less dense, with more cavities between them.

“Such fruit get mealy easier if not kept in cold storage long enough, or if FEMA [Forelle Early Market Access] guidelines are not followed,” Crouch points out.

Shade-netting can reduce the occurrence of mealiness. A study by postdoctoral researcher Dr Laetitia Schoeman showed that fruit under black 60% netting had the least mealiness compared with white 20% netting. Unfortunately, netting reduces red blush.

“It’s important to harvest pears at the right time. If you wait too long, pears will just become bigger and more mealy,” Crouch adds. “At least three of my students, Tavagwisa Muziri, Rudolph Cronje, and Patricia Carmichael have confirmed this.”

Bonus: Does pollination matter?

A lack of pollination is one of the factors that can influence the development of mealiness by changing the porosity of the fruit flesh. Dr Laetitia Schoeman, at the time a postdoctoral researcher in the Department of Horticultural Science at Stellenbosch University, took a detailed look at the three- dimensional microstructure of 20 pollinated and 20 unpollinated Forelle pears.

She used X-ray micro- and nano-computed tomography to create three-dimensional images. Careful examination revealed microstructural differences in the tissue and pore network of the two types of pears. Unpollinated pears are less dense and more porous and had a significantly lower juice weight and a higher mealiness score.

Schoeman speculated that pollinated pears have higher hormone levels due to their higher number of viable seeds, resulting in more cells, denser flesh and lower mealiness. This suggests that growers should strive to create optimal conditions for pollination.

Bonus: Seeing inside mealy pears

There was a time when one had to bite into a Forelle to find out whether it was mealy or not. Thanks to science, different scanning methods can now provide snapshots of the inner workings of intact fruit.

In the mid-2010s, Dr Tavagwisa Muziri, then a doctoral student at Stellenbosch University, did ground-breaking research demonstrating that non-destructive near- infrared scanning can detect mealiness immediately after the fruit has been taken out of cold storage.

Using X-ray computed tomography, Muziri found that mealiness starts developing in the neck of the fruit. His work and, more recently, that of Dr Laetitia Schoeman, showed that this technology can also identify mealiness after cold storage or when it has already fully developed.

Theoretically, it should be possible to add these technologies to pack lines for quality control, to ensure that no mealy fruit reach the consumer.

In practice, however, says Dr Elke Crouch, this is still quite difficult to do. Extended cold storage or using the FEMA protocol are still recommended ways to avoid mealiness.

Image: Mealiness in a Forelle pear.

Supplied by Elke Crouch | Stellenbosch University.