The central premise of RWD is elegantly simple. A thin layer of food puree or slices is spread on a transparent plastic film that floats on the surface of circulating hot water, typically at temperatures near the boiling point. Heat conducts through the film into the wet product, but here the physics takes an unusual turn. Because the food layer is moist and the film is transparent, most of the incoming energy is transmitted directly into the water inside the food, driving rapid evaporation from the exposed surface. As the product dries and its water content falls, its ability to transmit infrared radiation collapses. The plastic film then acts as a barrier, reflecting radiant heat back into the water bath and effectively switching off further heating. The result is a self-regulating system in which the product never lingers at high temperatures once most of the moisture is gone.

This built-in safety valve is what makes RWD so attractive for heat-sensitive nutrients. Ascorbic acid degrades through both thermal pathways and oxidative reactions, and its loss accelerates sharply at elevated temperatures and prolonged drying times. Conventional hot-air drying, the workhorse of the food industry, exposes material to heated air for hours, giving oxygen and heat ample opportunity to dismantle the vitamin. Freeze-drying avoids heat but is expensive and energy-hungry. RWD sits in a sweet spot: drying times are far shorter than hot-air methods, product temperatures remain moderate for most of the cycle, and the thin product layer limits oxygen diffusion deep into the matrix. The Indian review synthesises evidence across fruits, vegetables and herbal products and concludes that RWD and its hybrid variants generally achieve superior ascorbic acid retention compared with conventional techniques.

The evidence base is broad. Early landmark work on strawberry and carrot purees dried with the original Refractance Window system showed quality retention approaching that of freeze-drying at a fraction of the cost. Subsequent studies on acerola pulp, one of nature’s richest vitamin C sources, found that RWD preserved a striking share of the ascorbic acid that hot-air drying destroyed. Guava, pineapple, passion fruit, aonla, orange and peach products have all been dried successfully with the technique, with researchers consistently reporting better colour, flavour and nutrient profiles alongside competitive drying kinetics. Leafy greens and herbs such as asparagus, Malabar spinach, broccoli, kale and spinach have also benefited, and even probiotic bacteria like Lactobacillus plantarum have survived conductive hydro-drying through the refractance window, a testament to how gentle the process can be.

Underlying these outcomes are well-characterised heat and mass transfer phenomena. During the constant-rate period, evaporation from the free surface keeps the product near its wet-bulb temperature, which is low even when the water bath beneath is hot. Moisture migrates from the interior to the surface by diffusion and capillary flow, and the thin spreading of the material ensures that this pathway stays short. Quantitative modelling work has confirmed that the fraction of energy transferred by conduction through the film is modest compared with the radiant and evaporative fluxes, and that the dramatic drop in infrared transmittance of the drying film is the key to the falling-rate behaviour. In practical terms, the product spends its final minutes of drying in an increasingly protected microclimate rather than in a blast of hot air.

The review also examines hybrid RWD systems, which are where much of the current excitement lies. Far-infrared-assisted RWD adds radiant energy that penetrates the product and speeds internal heating, cutting drying times for apple slices and banana leather while preserving nutrients and aroma compounds better than hot-air drying. Microwave-assisted RWD, demonstrated recently for fruit dehydration, combines volumetric microwave heating with the gentle conductive base, reducing processing time further and improving bioactive compound retention. Ultrasound-assisted conductive hydro-drying uses acoustic cavitation to disrupt boundary layers and enhance moisture removal from apple slices and okra, and vacuum-coupled RWD lowers the boiling point of water to dry apple pomace powder at even gentler temperatures. Researchers have even built IoT-enabled, infrared-assisted RWD dryers with real-time monitoring, and solar photovoltaic-thermal versions that cut energy costs and emissions compared with conventional dryers.

Pre-treatments add another layer of control. Cold plasma treatment of apple slices before RWD has been shown to modify drying kinetics and quality attributes, while microwave pre-treatment of orange pestil, the traditional fruit leather, improves subsequent RWD performance. Optimisation studies using response surface methodology have mapped the interplay of water bath temperature, product loading thickness and drying time for tomato slices, jackfruit pulp, papaya, sapota bars and Tarhana dough, some of the latter even dried under vacuum to explore texture and quality trade-offs. Across these studies, the recurring theme is that RWD tolerates a wide processing window without catastrophic nutrient loss, giving food engineers genuine flexibility in tailoring products.

Why does ascorbic acid fare so well under RWD? The review points to the convergence of several protective factors: short exposure times, moderate product temperatures, limited oxygen contact in the wet product layer, and the rapid self-limiting temperature drop once drying progresses. Ascorbic acid is water-soluble and highly reactive, degrading to dehydroascorbic acid and then to irreversible breakdown products when conditions are harsh. By minimising the time-temperature integral, RWD preserves not only the vitamin itself but also the broader antioxidant capacity of the food, including phenolic compounds and carotenoids that often degrade in parallel. For consumers, that means dried fruit powders, leathers and snack products that deliver more of the nutritional promise of the fresh ingredient.

Challenges remain before RWD can displace conventional drying at scale. The technology requires thin, evenly spread product layers, which favours purees and slices but complicates handling of whole or particulate foods. Capital costs exceed those of simple hot-air dryers, and continuous industrial lines demand careful control of water bath temperature, film integrity and residence time. The review notes that standardised comparisons across studies are hampered by differences in product preparation, initial solids content and analytical methods for measuring vitamin retention, making it difficult to draw universal quantitative conclusions. Energy efficiency, while generally favourable, still depends on system design, and hybrid configurations add complexity that must be justified by measurable quality gains.

Nevertheless, the trajectory is clear. As demand surges for nutrient-dense dried fruits, vegetable powders, herbal products and functional food ingredients, drying technologies that respect molecular fragility are moving from laboratory curiosity to commercial reality. Refractance window drying, especially when paired with infrared, microwave, ultrasound or vacuum assistance and smart digital control, offers a rare combination of speed, gentleness and self-regulation. The Indian team’s critical appraisal suggests that for preserving ascorbic acid across diverse food matrices, the refractance window is less a niche trick than a genuinely transformative platform, one that could reshape how the food industry thinks about the humble act of removing water.

Subject of Research: Refractance window drying for preserving ascorbic acid in food matrices

Article Title: A critical appraisal of refractance window drying (RWD) for preserving ascorbic acid in diverse food matrices

Article References: Nayanthara, B., Nema, P. K., Gakhar, V., & Malakar, S. (2026). A critical appraisal of refractance window drying (RWD) for preserving ascorbic acid in diverse food matrices. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02320-8

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02320-8

Keywords: refractance window drying, ascorbic acid, vitamin C retention, food drying, hybrid drying, heat and mass transfer, bioactive compounds, food processing, fruits and vegetables, infrared drying, microwave drying, food engineering

The team, led by Traiphop Phahom of Suranaree University of Technology together with Kamolwan Soubsub, Gaelle Roudaut, and Singhanat Phoungchandang, harvested flowers from a farm in Buriram province, Thailand, in March and April 2024. Petals were separated by hand, cut into half-centimeter strips, frozen at minus 20 degrees Celsius, and thawed slowly overnight to ensure a uniform starting material. They then compared three approaches: conventional convective hot-air drying at 50, 60, and 70 degrees Celsius; intermittent microwave drying at 450, 720, and 900 watts; and freeze drying, which served as the gold-standard control because its low temperatures and vacuum conditions minimize thermal and oxidative damage.

The speed difference was staggering. Convective drying took between 6,120 and 12,420 seconds, meaning up to nearly three and a half hours of continuous heat exposure. Intermittent microwave drying finished in just 220 to 340 seconds, roughly a 97 percent reduction in processing time. Raising the air temperature from 50 to 70 degrees Celsius cut convective drying time by about half, while increasing microwave power from 450 to 900 watts shortened it by roughly a third. Drying rates under microwaves ran about twenty times higher than under hot air, a consequence of volumetric heating: microwaves generate heat inside the tissue itself, creating strong internal vapor pressure gradients that drive moisture outward far faster than surface convection ever could.

To describe this behavior mathematically, the researchers fitted their data to five classical thin-layer drying models, including Newton, Page, and Fick’s second law. The Midilli-Kucuk model emerged as the clear winner, achieving coefficients of determination of at least 0.999 with the lowest error statistics across every condition. Effective moisture diffusivity, calculated from Fick’s law, ranged from about 1.7 times ten to the minus seventh square meters per second under hot air to nearly 7 times ten to the minus sixth under microwaves, roughly a twenty-five-fold difference. Arrhenius-type analysis yielded an activation energy of 28.05 kilojoules per mole for convective drying and 16.05 watts per gram for microwave drying, parameters that quantify how sensitively water movement responds to temperature or power, respectively.

Structure and color told their own story. Using image analysis, the team tracked shrinkage in two phases: a rapid collapse early in drying, accounting for 83 to 95 percent of total shrinkage, followed by a slower plateau. Microwave-dried petals shrank less overall, between 51.1 and 54.1 percent, than hot-air samples, which lost up to 62.6 percent of their original area. The explanation lies in heat transfer physics. Slow convective drying lets a dry, hard crust form at the surface while the interior stays above its glass transition temperature longer, promoting cellular collapse. Rapid microwave heating, by contrast, generates internal vapor pressure that can temporarily prop up the cellular structure and even create porosity. Freeze-dried petals kept the highest lightness and the strongest red-purple hue, while both heat-based methods pushed the petals toward brown and dark-brown tones through nonenzymatic browning, with the mildest hot-air treatment at 50 degrees Celsius showing the smallest total color difference.

Antioxidant assays revealed a nuanced hierarchy. Freeze drying and the hottest convective treatment at 70 degrees Celsius produced the highest total phenolic content, at up to 139.54 milligrams of gallic acid equivalents per gram of dry weight, likely because rapid moisture removal curtailed enzymatic oxidation while cell wall damage improved extractability. For flavonoids, moderate microwave power at 450 watts actually beat freeze drying, reaching 148.09 milligrams of quercetin equivalents per gram. Radical-scavenging and ferric-reducing activities followed a similar pattern: freeze drying led, low-to-moderate microwave power came close, and prolonged hot-air drying scored lowest. The culprit for convective drying is cumulative exposure, which enables enzymatic oxidation by polyphenol oxidase, thermal degradation of phenolic structures, and oxidative polymerization. Excessive microwave power at 900 watts also backfired, as localized overheating degraded heat-sensitive compounds, confirming that lower power offers the best balance between speed and preservation.

The most technically ambitious part of the study was metabolite profiling by liquid chromatography coupled to high-resolution quadrupole time-of-flight mass spectrometry. Across seven drying conditions analyzed in triplicate, the team annotated 125 phenolic compounds, about half of all detected metabolites, divided into 81 flavonoids, 25 phenolic acids, 14 tannins, and 5 stilbenes, all at confidence level 2 of the Metabolomics Standards Initiative. Compounds such as ellagic acid, quercetin, kaempferol-3-glucoside, delphinidin-3-glucoside, and isorhamnetin correlated strongly with antioxidant activity. Intriguingly, the phenolic acid class correlated negatively with the classic Folin-Ciocalteu total phenolic measurement, a finding the authors attribute to the assay’s nonspecific response to any reducing compound, including Maillard reaction products formed during thermal drying. The lesson is that colorimetric assays measure total reducing capacity, not true phenolic content, and only compound-specific profiling can resolve what drying actually does to individual molecules.

Moisture sorption behavior, critical for shelf life, was measured with a dynamic vapor sorption analyzer at 25 degrees Celsius across relative humidities from 0 to 90 percent. The isotherms showed the classic Type II sigmoid shape, with more than half of the total moisture uptake occurring above a water activity of 0.75, driven by hydrophilic carbohydrates, proteins, and fiber. Among four sorption models tested, the Peleg model fit best, while the BET model estimated monolayer moisture contents of roughly 4.1 to 4.8 percent dry basis, the level at which dried foods are most stable. By setting water activity to 0.6, the conventional upper limit for safe storage of dried foods, the researchers calculated a critical moisture content of 10.75 to 11.40 percent dry basis, meaning the petals remain stable as long as moisture is kept below about 10 percent.

Hierarchical cluster analysis, validated by a cophenetic correlation coefficient of 0.93, grouped the treatments into three distinct clusters: freeze drying alone, all hot-air conditions, and all microwave conditions. Crucially, the microwave cluster sat closer to freeze drying in the quality space than the hot-air cluster did, indicating that microwaves better preserve the physicochemical and bioactive character of the petals. Within the microwave family, 450 and 720 watts offered the most favorable compromise between quality retention and processing speed. The practical implication is significant for the nutraceutical, functional food, and natural colorant industries: intermittent microwave drying could serve as a fast, energy-efficient alternative to expensive freeze drying for heat-sensitive botanical materials. The authors note that metabolite identities remain tentative until confirmed with authentic standards, and they call for long-term storage stability studies and formulation trials to carry red water lily petals from the pond to the product shelf.

Subject of Research: Effects of drying technologies on metabolite profile, drying kinetics, quality retention, and moisture sorption of red water lily petals

Article Title: Shaping metabolite profile, dehydration kinetics, quality retention, and sorption properties of red water lily ( Nymphaea x rubra ) petals through drying technology

Article References: Phahom, T., Soubsub, K., Roudaut, G., & Phoungchandang, S. (2026). Shaping metabolite profile, dehydration kinetics, quality retention, and sorption properties of red water lily (Nymphaea x rubra) petals through drying technology. Journal of Agriculture and Food Research, 31, Article 103337. https://doi.org/10.1016/j.jafr.2026.103337

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103337

Keywords: red water lily, Nymphaea x rubra, microwave drying, freeze drying, convective drying, antioxidants, phenolic compounds, flavonoids, metabolite profiling, LC-MS, moisture sorption isotherm, food drying

The central insight running through the collection is that foods rarely sit comfortably on either side of the fluid-solid divide. Instead, they travel continuously through liquid-like, viscoelastic, plastic and solid-like regimes as their internal microstructure evolves. In some materials, a network is born through gelation or crystallization; in others, molecular mobility is frozen out by cooling or a glass transition. In yield-stress materials such as pastes, flow destroys a pre-existing structure that then rebuilds. During drying, cooking and frying, changes in moisture content can simultaneously alter mechanical properties and generate internal stresses and deformation. This continuous, mechanism-dependent journey between states is precisely why food structuring has historically been treated empirically, with process-property relationships assembled through trial and error rather than derived from first principles.

A major theme of the Special Issue is the tight coupling between transport processes, changing material properties and mechanical deformation, particularly during drying. Hu and colleagues present a multiphase, multiscale mechanistic model for hot-air drying of shiitake mushroom, capturing how water removal reshapes the material while it shrinks and stresses. Veser and co-workers predict cabbage-seed drying at laboratory and industrial scales using a non-equilibrium sorption-isotherm approach, while Rizki and colleagues track material-property changes during electrohydrodynamic drying with a close look at the falling-rate period, the stage where moisture loss slows and the material stiffens most dramatically. Shah and Takhar push the coupling even further in microwave frying, combining unsaturated transport based on hybrid mixture theory with electromagnetic equations. Together, these studies make a forceful case that heat and mass transfer cannot be modeled independently of the evolving physical state of the food.

Deformation itself can become a tool rather than a nuisance, and several contributions exploit this deliberately. van der Sman, Curatolo and Teresi investigated buckling during the drying of edible soft matter with a cylindrical core-shell geometry, showing how drying-induced mechanical instabilities generate intricate, predictable deformation patterns. In a companion study, the same team demonstrated programmable shape morphing during drying through symmetry breaking, effectively using moisture gradients to sculpt foods into designed shapes rather than accepting warping as a processing defect. Grasa, Teresi and van der Sman extended the coupled transport-mechanics framework to large-strain anisotropic behavior of meat during cooking using finite-element modeling. These works signal a shift in the field: instead of treating shrinkage, curling and buckling as quality problems to suppress, researchers are beginning to engineer them for structure design.

Underlying this shift is a broader theoretical realignment. Food rheology has traditionally concentrated on materials in their flowing state, while the mechanics of foods undergoing large deformations in a more solid-like condition received comparatively little attention. Meanwhile, advances in soft-matter mechanics and poromechanics now allow deformation, viscoelasticity and moisture transport to be described within thermodynamically consistent frameworks. A particularly striking conceptual bridge is the mathematical correspondence between the configuration tensor used in advanced rheological models and the Cauchy-Green tensor used in large-deformation mechanics. This equivalence offers a common language for materials that alternate between flowing and solid-like states, and its implications for elasto-viscoplastic food materials and stress-driven moisture migration are explored in a recent review by van der Sman in Current Opinion in Food Science. The practical consequence could be a new generation of models that predict, rather than merely reproduce, how foods behave through their whole processing journey.

At the opposite end of the fluid-solid transition lie processes in which a material must first flow in a controlled manner and then preserve its generated shape, and nowhere is this clearer than in three-dimensional food printing. Kim and colleagues examined the printability and structural properties of plant-based scallop adductor-muscle analogues, relating performance to amylose content and the pasting behavior of different rice cultivars. Liu and co-workers studied edible three-dimensionally printed emulsion gels, showing how inulin incorporation modifies both mechanical and sensory properties. Both studies converge on a key conclusion: printability is not simply a question of viscosity. A printable food must respond appropriately during extrusion, then recover or develop enough structural integrity after deposition to hold its printed geometry, a dual requirement that demands careful control across the entire fluid-solid spectrum.

The mechanisms by which foods acquire that structural integrity vary enormously across systems, and the Special Issue maps this diversity down to the molecular scale. Renzetti and colleagues investigated cereal and tuber starches, identifying hydrogen-bond density and glass-transition temperature as governing factors in gelatinization and gel rheology, thereby linking molecular interactions and thermal transitions directly to macroscopic mechanical behavior. On a different front, Holian, Bolton and Wilson explored how structure can be generated in soft solids to produce heat-stable milk chocolate, a formulation challenge that hinges on managing the solid-fat network so the product survives warming without collapsing. Both studies underscore that transitions between fluid-like and solid-like response ultimately originate at smaller length scales, from molecular mobility and intermolecular interactions up to the formation of mesoscopic networks that carry load.

Structured lipid systems offer further illustration of the same soft-matter principle. Shuai and colleagues investigated rice-bran-wax-structured macadamia oleogels and temperature-responsive water-in-oil emulsions, showing how organization of the lipid phase provides a route to tuning mechanical and functional properties, including delivery of bioactive compounds such as astaxanthin. Rasouli Pirouzian and co-workers, in their study of sucrose-free probiotic dark chocolate, addressed formulation optimization in relation to rheological characteristics and the viability of Saccharomyces boulardii. Although oleogels and functional chocolates sit far from drying mushrooms or printed gels on the supermarket shelf, they obey the same underlying rule: macroscopic material behavior emerges from an evolving internal structure whose formation is dictated by composition and processing conditions. This unity of principle is what makes the cross-disciplinary framing of edible soft matter so productive.

Taken together, the contributions point to a future in which food structuring moves from empirical process-property relations toward a genuinely mechanistic and predictive science. Achieving that will require closer integration of experimental characterization and modeling across length and time scales, with rheological and mechanical measurements increasingly combined with techniques that track microstructure, moisture distribution and deformation in real time during processing. Constitutive models, in turn, must evolve beyond reproducing behavior under narrow conditions toward formulations that capture the evolution of internal structure and its coupling to deformation, temperature and moisture transport. The Wageningen symposium and the resulting collection demonstrate the value of bringing food scientists, rheologists, physicists and solid-matter mechanicians to the same table. If that cross-fertilization continues, the next generation of foods may be designed, on paper, from the flow and setting behavior of their molecular ingredients upward.

Subject of Research: The fluid-to-solid state transitions of edible soft matter during food processing

Article Title: Edible soft matter in between fluid and solid states

Article References: Edible soft matter in between fluid and solid states. (n.d.). https://doi.org/10.1016/j.crfs.2026.101561

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101561

Keywords: edible soft matter, food rheology, food drying, 3D food printing, gelatinization, glass transition, oleogels, moisture transport, large-deformation mechanics, food structure, buckling, chocolate structuring