Refractance Window Drying Emerges as a Gentle Powerhouse for Preserving Vitamin C in Foods

Vitamin C is one of the most fragile nutrients in the human diet, and one of the most valuable. Also known as ascorbic acid, it underpins antioxidant defences, collagen synthesis and anti-inflammatory pathways in the body, yet it is notoriously easy to destroy. Long exposure to heat, oxygen and metal ions strips it from fruits, vegetables and herbs with ruthless efficiency, which is precisely what happens during conventional drying. A new review published in Food Science and Biotechnology by B. Nayanthara, Prabhat K. Nema, Vidya Gakhar and Santanu Malakar of the National Institute of Food Technology Entrepreneurship and Management in Sonipat, India, takes a hard look at an emerging alternative: refractance window drying, or RWD, a technique that promises to remove water quickly while keeping product temperatures surprisingly low.

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