Solar Based Drying of Blanched Grapes for Efficient Raisin Production

229

In today’s world, where sustainability and health are at the forefront of global discussions, the way we grow, process, and preserve food is more important than ever.

A recent study by researchers Nitin Sharma and Namrata Sengar from the University of Kota, India, published in the journal Solar Compass (2024), offers a groundbreaking solution to one of agriculture’s oldest challenges: producing raisins quickly and safely without relying on harmful chemicals.

By combining a simple pre-treatment method called blanching with an efficient solar dryer, their research demonstrates how grapes can be turned into raisins in just three days—twice as fast as traditional methods—while preserving nutritional quality and eliminating toxic residues.

The Challenges of Traditional Raisin Production

Raisins are a popular snack and cooking ingredient, but their production has long been plagued by inefficiencies and risks. Traditionally, grapes are dried under the open sun, a process that can take anywhere from five days to several weeks, depending on weather conditions.

Open sun drying refers to the age-old practice of laying food products directly under sunlight to remove moisture. While low-cost, this method leaves the fruit exposed to dust, insects, birds, and unpredictable rain, which can spoil entire batches.

To speed up drying, many producers use chemical treatments like potassium carbonate or ethyl oleate. These chemicals break down the grape’s natural waxy coating, allowing moisture to escape faster. However, they leave behind residues that can harm human health and contaminate soil and water sources.

Potassium carbonate (K₂CO₃) is an alkaline compound often used to crack the grape’s skin, while ethyl oleate is an oily substance that accelerates moisture removal.

Both chemicals are cost-effective but pose risks: potassium carbonate can irritate the skin and eyes, while ethyl oleate residues may affect respiratory health.

Industrial dryers powered by fossil fuels offer a faster alternative, but they come with their own problems. These machines consume large amounts of energy, contribute to greenhouse gas emissions, and are often too expensive for small-scale farmers.

Enter solar dryers—devices that use sunlight to create controlled, heated environments for drying food. A solar dryer works by trapping solar radiation under a transparent cover (like glass or plastic), converting it into heat, and circulating warm air to dehydrate crops.

While solar dryers are cleaner and more efficient than open sun drying, most still rely on chemical pre-treatments to achieve optimal results. The University of Kota study challenges this norm by replacing chemicals with blanching, a natural and chemical-free step that not only speeds up drying but also improves the quality of the final product.

Blanching Grapes for Chemical-Free Raisin Production

Blanching is a simple process that involves briefly dipping fruits or vegetables in hot water or steam. For grapes, this step serves two critical purposes.

First, it inactivates enzymes that cause spoilage and browning, preserving the fruit’s color and nutrients. Enzymes are proteins that speed up chemical reactions in plants; in grapes, enzymes like polyphenol oxidase trigger browning when the fruit is cut or damaged.

By exposing grapes to boiling water (around 100°C) for two minutes, blanching denatures these enzymes, effectively “turning them off” and preventing decay.

Second, blanching softens the grape’s waxy outer layer, creating tiny cracks that allow moisture to escape more easily during drying. This process mimics the effect of chemicals like potassium carbonate but without the toxicity.

In the study, blanched grapes lost 3.6% of their weight immediately after treatment, indicating initial moisture loss.

When placed in the solar dryer, this pre-treatment reduced the drying time by nearly half compared to untreated grapes, all without leaving harmful residues.

The benefits of blanching extend beyond speed and safety. Unlike chemical treatments, which can alter the taste and texture of raisins, blanching preserves the fruit’s natural flavor and makes the final product softer and plumper when rehydrated.

During the experiments, blanched raisins soaked in water for four hours swelled more evenly and had a more appealing texture compared to raisins made with chemicals. This quality improvement could make blanched raisins more attractive to consumers seeking healthier, additive-free foods.

Solar Dryer Technology for Efficient Food Preservation

The solar dryer used in the study is a direct-type flat plate solar air collector, a device designed to maximize heat retention and airflow. Its key components include:

• Transparent Glass Cover: A sheet of glass covering the dryer allows sunlight to enter while trapping heat through the greenhouse effect. The greenhouse effect occurs when short-wavelength solar radiation passes through the glass, heats the interior surfaces, and is re-radiated as long-wavelength heat that cannot escape, raising the temperature inside.
• Absorbent Base Plate: Painted black to maximize heat absorption, this plate converts solar energy into warmth, heating the air inside the dryer to between 45°C and 69°C,
• Solar-Powered Fan: Connected to a photovoltaic panel, this fan regulates airflow to maintain consistent temperatures and humidity levels.

The dryer was equipped with advanced sensors, including a Kipp & Zonen CMP 10 pyranometer (a device that measures solar radiation) and a digital anemometer (which tracks wind speed). These tools ensured precise monitoring of environmental conditions during experiments.

Experiments were conducted on the rooftop of the University of Kota, located in Rajasthan, India—a region known for its hot, sunny climate. The dryer maintained temperatures between 45°C and 69°C, ideal for drying grapes without cooking them.

Humidity control played a critical role: levels inside the chamber dropped from 21–24% in the morning to 10% by evening, creating perfect conditions for moisture removal. Even after sunset, residual heat continued to dry the grapes overnight, further speeding up the process.

Blanching and Solar Drying Reduce Raisin Production Time

The study compared two groups of grapes: one dried without any pre-treatment and another blanched before drying. The results were striking. Untreated grapes took five to six days to dry into raisins using the same solar dryer. In contrast, blanched grapes achieved the same result in just three days.

On the first day alone, blanched grapes lost 36% of their weight, shrinking from 964 grams to 565 grams. By the third day, their weight dropped to 257 grams—a total moisture loss of 74.3%.

The dryer’s thermal efficiency, which measures how well it converts solar energy into heat for drying, peaked at 17% on the first day when moisture loss was highest.

This efficiency dropped to 5% by the second day and 2% by the third as the grapes dried out. While these numbers may seem low, they are comparable to other solar drying systems and highlight the potential for further optimization.

Benefits of Solar-Dried Raisins for Health and Farming

For farmers, the implications of this research are profound. Faster drying times mean less risk of spoilage and higher yields, which can translate to increased profits. Spoilage—the decay of food due to microbes, insects, or environmental factors—is a major issue in open sun drying.

Solar dryers mitigate this risk by providing a controlled environment. Additionally, solar dryers are cheaper to operate than fossil fuel-powered machines, making them accessible to small-scale farmers in developing regions.

For consumers, blanched raisins offer a safer, healthier alternative to chemically treated products. The absence of toxic residues reduces the risk of long-term health issues, while the improved texture and flavor make the raisins more enjoyable to eat.

Rehydration—the process of soaking dried food to restore moisture—is also more effective with blanched raisins, as their porous structure absorbs water evenly.

The environmental benefits are equally significant. Solar dryers produce zero emissions and reduce reliance on non-renewable energy sources. By eliminating chemical treatments, this method also prevents groundwater contamination and soil degradation, contributing to more sustainable agricultural practices.

Challenges and Future Directions

While the study’s findings are promising, there are challenges to overcome. Solar dryers depend heavily on consistent sunlight, which may limit their use in cloudy or rainy regions. Scaling up the technology for commercial use will also require investment in larger dryers and training for farmers.

Additionally, the study focused on short-term results; long-term research is needed to assess the shelf life (how long a product remains usable) and nutritional stability of blanched raisins over months or years.

The researchers suggest several avenues for future work. Integrating thermal energy storage materials like soapstone (a heat-resistant mineral) could allow dryers to operate overnight or during cloudy weather. Testing the method on other crops, such as mangoes or tomatoes, could expand its applications.

Finally, comparing blanching with other physical pre-treatments, such as ultrasound (high-frequency sound waves) or infrared heating (radiant heat), could uncover even more efficient drying strategies.

Sustainable Agriculture Through Solar Drying Innovations

The University of Kota study is more than just a technical achievement—it’s a blueprint for a healthier, more sustainable future.

By combining ancient techniques like blanching with modern solar technology, the researchers have shown that it’s possible to produce high-quality food without compromising human health or the environment. For farmers, this means better tools to combat post-harvest losses (crop waste after harvesting).

For consumers, it means safer, tastier products. And for the planet, it’s a small but meaningful step toward reducing the environmental footprint of agriculture.

As the world grapples with climate change and resource scarcity, innovations like this solar dryer remind us that solutions often lie in blending simplicity with science. The sun, a resource available to all, has the power to transform how we grow, process, and preserve food—if we harness it wisely.