The research team harvested fully ripe fruits of three ornamental and culinary Sorbus varieties: Sorbus × arnoldiana ‘Copper Glow’, Sorbus aucuparia ‘Rosina Aurea’, and ×Sorboaronia fallax ‘Titan’, collected from research stations in Lower Silesia, Poland. Each batch was then split across six dehydration routes: conventional convective drying in hot air at 60 °C, freeze-drying under vacuum at −60 °C, vacuum-microwave drying, microwave-convective drying, and two hybrid protocols in which fruits were first pre-dried convectively for three or six hours before being finished in a vacuum-microwave field at reduced power. Every dried sample was then subjected to a battery of ultra-performance liquid chromatography measurements targeting five polyphenol classes, carotenoids, and L-ascorbic acid, alongside three complementary antioxidant assays — ABTS, FRAP, and ORAC — plus colorimetry, water activity analysis, and, in a first for this fruit group, X-ray micro-computed tomography of the fruit interior.

The drying kinetics alone tell a striking story. Convective drying, the industrial workhorse, required roughly 950 minutes — nearly sixteen hours — to bring the berries below ten percent moisture. Freeze-drying took a full 24 hours. Vacuum-microwave drying, by contrast, accomplished the same dehydration in just 34 to 46 minutes, a speed-up of more than twentyfold. The reason lies in the physics of volumetric heating: instead of waiting for heat to conduct slowly from the skin inward, microwave energy is absorbed directly by water dipoles throughout the tissue, generating internal vapor pressure that drives moisture toward the low-pressure chamber surface. The hybrid CPD/VMFD protocols cut total processing time severalfold relative to hot-air drying while keeping the microwave exposure short and gentle. The modified Page model fitted all of the drying curves with coefficients of determination between 0.976 and 0.999, allowing fair comparison across the very different heating regimes.

But speed is worthless if it destroys the chemistry, and here the species-dependence of the results becomes the study’s central finding. No single drying method preserved every quality attribute in every fruit. Freeze-drying generally retained the most bioactive compounds and the highest antioxidant capacity, as expected from its low-temperature sublimation mechanism, but in Sorbus aucuparia ‘Rosina Aurea’ it paradoxically produced the highest water activity of any treatment — likely because the intact, rigid skin left behind by gentle freeze-drying acted as a diffusion barrier that trapped residual vapor inside the fruit. Microwave-assisted methods, which tended to crack the peel, released that moisture more efficiently in this variety, flipping the usual hierarchy.

The vitamin C data are perhaps the most dramatic illustration of how much the response varies by genotype. In ‘Copper Glow’, vacuum-microwave drying actually raised the measured L-ascorbic acid concentration by nearly 35 percent relative to fresh fruit, yielding 326.8 mg per 100 g of dry matter — an outcome plausibly linked to concentrating effects and improved extractability. In ‘Titan’, the same family of microwave treatments was catastrophic: microwave-convective drying erased 98.9 percent of the vitamin C, and even the hybrid methods destroyed more than 86 percent. For that variety, only freeze-drying kept losses to a negligible 1.6 percent. Prolonged convective drying was consistently the worst option for ascorbic acid, cutting it by 25 percent in ‘Copper Glow’, 55 percent in ‘Rosina Aurea’, and a devastating 93.8 percent in ‘Titan’, reflecting the compound’s well-known sensitivity to heat and oxygen over long exposure windows.

Polyphenols followed their own species-specific logic. The dominant fraction in every Sorbus fruit was polymeric procyanidins, ranging from roughly 4,100 to 8,775 mg per 100 g of dry matter depending on variety and treatment, followed by phenolic acids, flavonols, and flavan-3-ol dimers. Anthocyanins were detected exclusively in the dark-red ‘Titan’, and hot-air drying wiped out 74 percent of them, while freeze-drying preserved 98.4 percent. Most intriguingly, in ‘Rosina Aurea’ microwave-based methods increased the measured content of phenolic acids, flavan-3-ols, and flavonols by 50 to 160 percent over fresh fruit. This apparent gain likely reflects a combination of thermal conversion of precursors, Maillard-type chemistry, and microwave-induced disruption of cell walls that releases bound phenolics into the extractable pool — a reminder that ‘content’ in dried plant foods is a moving target shaped by both destruction and liberation.

The computed tomography scans gave the chemical data a physical explanation. Fresh fruits showed a dense, finely porous mesocarp with no internal voids. Freeze-dried berries kept a structure closest to the original, with visible pores from ice sublimation but the pulp still filling the interior. Convective drying produced severe tissue shrinkage and large internal cavities as the flesh contracted, while the microwave treatments carved out extensive voids and frequently cracked the skin, a consequence of rapid internal pressurization. These structural fingerprints mapped cleanly onto the chemical outcomes: berries that retained continuous tissue architecture — especially after freeze-drying — also preserved more vitamin C, carotenoids, and polyphenols, whereas ruptured skins and collapsed tissue presumably opened the door to oxygen, accelerating oxidative degradation of the most labile compounds. Carotenoids told a complementary story, with ‘Titan’ — the richest source at over 7,300 mg per 100 g fresh — keeping 98.4 percent after freeze-drying but losing more than two-thirds under microwave-convective conditions, while ‘Copper Glow’ actually gained about 12 percent after convective drying, consistent with its comparatively intact pulp structure in the CT images.

A principal component analysis pulled the entire dataset together and delivered the study’s clearest takeaway: two components explaining 85.6 percent of the variance separated the fruits primarily by species, not by drying method. ‘Titan’ clustered with carotenoids, anthocyanins, and flavonols; ‘Copper Glow’ aligned with vitamin C, phenolic acids, and antioxidant assays; and ‘Rosina Aurea’ sat apart with its distinctive flavan-3-ol profile. Drying treatment modulated the intensity of each fruit’s signature, but the botanical identity dictated the direction of the response. In practical terms, there is no universal recipe: a processor chasing vitamin C should treat ‘Copper Glow’ and ‘Titan’ completely differently, and pigment preservation demands its own protocol in each case.

The authors are careful to frame the work as preliminary — single-season harvests, three varieties, and no cost or scale-up analysis — but the direction is clear. Freeze-drying remains the gold standard for quality, yet its energy appetite and 24-hour cycle are hard to justify for a niche crop. The hybrid convective pre-drying followed by vacuum-microwave finishing offers a defensible compromise, cutting processing to a fraction of the time while retaining much of the nutritional and functional value, particularly for phenolic-rich products. For a genus of trees that lines European streets and parks largely as ornament, the message is quietly radical: rowan berries are a serious functional-food candidate, and with the right dehydration strategy — chosen species by species — they could graduate from bird food to supermarket shelf. The next step is engineering: optimizing microwave power profiles, quantifying the economics, and proving that what works in a Wrocław laboratory can survive an industrial dryer line.

Subject of Research: Effects of six drying technologies on bioactive compounds, antioxidant activity, color, water activity, and microstructure of three underutilized Sorbus fruit species, assessed by chromatography and X-ray computed tomography.

Article Title: Drying of underutilized Sorbus fruits species: effects on quality attributes, bioactive compounds and microstructure

Article References: Siniawska, M., Lech, K., Masztalerz, K., Bąbelewski, P., & Wojdyło, A. (2026). Drying of underutilized Sorbus fruits species: effects on quality attributes, bioactive compounds and microstructure. Journal of Agriculture and Food Research, 31, Article 103265. https://doi.org/10.1016/j.jafr.2026.103265

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103265

Keywords: Sorbus, rowan, fruit drying, freeze-drying, vacuum-microwave drying, polyphenols, vitamin C, carotenoids, antioxidant activity, computed tomography, food processing, functional foods