Dry matter in apples

The preliminary results of a three-year Hortgro-funded project assessing the relationship of dry matter to other fruit characteristics. By Anna Mouton.

Modern shoppers are spoilt for choice when it comes to fruit. Supermarket shelves are brimming with a rainbow offering of everything from blueberries to dragon fruit. So, how can apple growers and marketers ensure their product is competitive?

Consumers will buy apples once if the fruit looks good, but repeat purchases require quality that’s more than skin-deep. Studies have shown that flesh firmness is the number one determinant of eating experience in apples. This is why maintaining firmness is a priority during postharvest handling and storage.

But crispiness is not enough for apples to outshine other fruit — consumers also expect excellent taste. In apples, taste is affected by sugars, acidity, and aromatic compounds. Whereas analysing aromatics is still confined to research laboratories, measuring sugars and acidity to assess fruit quality is commonplace.

More recently, scientists have begun exploring the relationship between dry matter content, other fruit characteristics, and consumer preferences. New Zealand research has shown that consumer liking for Royal Gala apples increases with an increase in dry matter content. The taste panel also reported better taste in fruit with higher dry matter content.

Perhaps the most interesting finding of the New Zealand study was that taste panellists reported being more willing to buy fruit with a higher dry matter content.

These results caught the attention of researchers at ExperiCo Agri-Research Solutions, who proposed investigating dry matter content in Bigbucks and Cripps Red apples in South African apple orchards. Dr Kenias Chigwaya, now Quality Assurance Manager at Fruitways, initiated and led the project.

Deconstructing apple composition

Dry matter is precisely what the term suggests: everything in the apple except water. “You remove a piece of the apple and dry it,” elaborates Ineke de Jong. “You weigh it before and after drying. What remains after drying is expressed as a percentage of the mass before drying.”

Previously at ExperiCo and currently Technical Researcher at KiwiKo, De Jong led the project during its final season. She explains that dry matter is measured on a sample of the fruit flesh and doesn’t include the skin. Other researchers have reported that 71%–82% of dry matter in fresh apples consists of sugars, starch, and organic acids.

Since apples with a higher dry matter percentage contain more carbohydrates, they could potentially achieve higher sugar levels as they mature, and starch is converted to sugars. “For example, in kiwifruit, early season dry matter content indicates later sugar levels,” says Daniël Viljoen, Manager of Research at ExperiCo.

De Jong adds that dry matter content is considered a quality metric in kiwifruit production, and some companies pay growers a premium for fruit with high dry matter levels.

A New Zealand study found that dry matter content at harvest correlated with total soluble solids after storage for several apple cultivars. Total soluble solids consist of sugars, organic acids and inorganic salts, but provide a good approximation of sugar levels and sweetness.

The New Zealand researchers also noted that the dry matter includes compounds or the building blocks for compounds that contribute to flavour. They speculated that consumers preferred apples with higher dry matter not only because such fruit are crisper and sweeter but also because the fruit tastes better.

Dry matter in the orchard

The Hortgro-funded study focused on Bigbucks and Cripps Red. Bigbucks was sampled in 2022, and Cripps Red in 2023 and 2024. The researchers identified 12 commercial full-bearing orchards of each cultivar in the EGVV.

They sampled fruit weekly from three weeks before to two weeks after harvest. They also analysed fruit samples after cold storage and shelf life.

“For the Cripps Red, we visited all the orchards with Dr Nigel Cook to look at vigour, crop load, and other orchard characteristics,” says Viljoen. “He provided guidance on which preharvest factors we should consider.”

Dry matter content measured before harvest didn’t show a consistent pattern in the Bigbucks or the Cripps Red orchards.

“According to the literature, dry matter content doesn’t necessarily follow an upward curve,” says Viljoen. “After the cell division phase, it seems to stabilise. I suspect that after this, the fruit is gaining dry matter, but it’s still becoming bigger, so the percentage remains the same.”

For Bigbucks, the average dry matter content at harvest varied from 13.2%–16.5% for the different orchards. Within orchards, between-tree variation in average dry matter was 0.2%–1.9%.

For Cripps Red in 2023, the average dry matter content at harvest varied from 14.1%–16.6% for the different orchards. Within orchards, between-tree variation was 0.9%–2.9%.

The researchers also collected fruit from the canopy’s top and bottom. “In the Bigbucks and the Cripps Red, fruit from the top of the tree had more dry matter than fruit from the bottom,” says De Jong.

This is to be expected, as apples obtain most of their carbohydrates from adjacent leaves, so fruit close to leaves that receive more light will benefit from more photosynthetic output. Similarly, more vigorous orchards generally produced fruit with lower dry matter content.

“We also saw a trend for orchards with higher crop loads to have lower dry matter content,” says Viljoen. “But this wasn’t a nice straight line.”

Fruit-quality correlations

Where one part of the project examined the development of dry matter in the orchard, another searched for correlations between dry matter content and other fruit-quality parameters. The researchers measured firmness, colour, total soluble solids, titratable acidity, starch breakdown, size, and mass.

The Bigbucks were stored in regular atmosphere at minus 0.5 °C for 10 weeks, followed by seven days of shelf life. The Cripps Red were stored in controlled atmosphere for 30 weeks at minus 0.5 °C, followed by seven days of shelf life.

For Bigbucks, dry matter content measured at harvest didn’t correlate with total soluble solids at harvest, but it did correlate with total soluble solids after storage and shelf life. Dry matter content at harvest better predicted total soluble solids after storage and shelf life than total soluble solids measured at harvest.

Total soluble solids after storage also correlated with dry matter content measured one and two weeks before harvest.

In addition, dry matter content measured at harvest correlated with titratable acidity at harvest. Dry matter content and titratable acidity at harvest correlated with firmness after storage and after shelf life.

However, dry matter content didn’t correlate with storage disorders. “But there weren’t many defects,” recalls Viljoen. “There were too few to draw conclusions.”

For Cripps Red in 2023, dry matter content measured 2–3 weeks before harvest correlated with firmness and total soluble solids 1–2 weeks after harvest. Fruit with higher dry matter tended to be firmer and to have higher total soluble solids. They also tended to have better blush colour and higher titratable acidity, although the correlations were weaker for these variables.

The storage data for Cripps Red are still being analysed.

What comes next?

Besides the assessment of Cripps Red fruit-quality data, the determination of eating quality is also still in process. This involves descriptive sensory analysis conducted by the Department of Food Science at Stellenbosch University.

“Descriptive sensory analysis is done by a small panel of trained people. Everyone tastes a slice of the same apple,” says De Jong. The panellists rate firmness on a scale and compare sweetness and acidity to standard solutions.

Although these analyses may confirm previous findings of consumers’ preference for fruit with high dry matter content, Viljoen and De Jong would have liked additional correlations.

“I hoped that dry matter content could predict internal browning,” says De Jong. “There was internal browning in some of the trial orchards, but glancing at the data, it doesn’t look as if those with lower dry matter content have higher internal browning.”

One of the challenges with the project was the impossibility of controlling for all variables. “There are temperature and other stressors impacting the fruit,” says Viljoen, “which is why you don’t always see clear-cut differences between treatments.”

However, growers can expect to hear more about dry matter in future. The importance of this fruit characteristic was already emphasised in a presentation at the Hortgro Technical Symposium in 2022.

For more on dry matter and fruit quality, read the summary of Prof. Stefano Musacchi’s presentation in Fresh Quarterly issue 18.

What does industry say about this research?

“The modern thinking around fruit quality says that the higher the dry matter content of an apple, the better the quality. It reflects orchard factors like light distribution — everything that takes place long before you start picking the apples.

“In my view, most of the maturity indexing parameters that we monitor tell us when to pick and how to store. They are better storage-potential than quality predictors. So, it would be good if dry matter content could help us identify our better-quality apples.”

Horticulturist Dr Nigel Cook. Prophyta.