Evaluation of different storage environments of solar refrigerated evaporatively cooled structure on quality of leafy vegetables

Introduction

India ranks as the world's second-largest producer of vegetables, following China. In 2023-24, horticulture production was estimated at 354.74 million tonnes, with an expected increase to 362.08 million tonnes in 2024-25 (Anon., 2025) [1] . The demand for minimally processed foods has grown due to their convenience, nutritional value, and sensory appeal. These products are sanitized, ready for consumption, and packaged to maintain freshness. However, 30% of food produced for human consumption is lost or wasted due to factors such as inefficient input use, inadequate post-harvest storage, limited processing and insufficient transportation facilities (Rezaei & Liu, 2015) [7] . Leafy vegetables undergo continuous biological changes after harvest, including respiration, transpiration, and ethylene production, which can be either beneficial or harmful (Pinto et al., 2000) [6] . Carvalho (1999) [2] highlights that water loss affects the appearance, flavour, texture, and weight of plant-based foods. To minimize post-harvest losses, various preservation methods are applied, with refrigeration being the most effective in slowing undesirable changes. Factors such as temperature, relative humidity, and atmospheric composition influence the shelf life of leafy vegetables. Cooling methods vary depending on the product, with hydro-cooling using cold chlorinated water through dipping or sprayingbeing effective for preserving lettuce and spinach. Spinardi et al. (2009) [10] studied the post-harvest quality of spinach and lettuce baby leaves stored at 4-10℃. Their findings showed that carotenoids, chlorophyll, and phenols remained stable for up to six days at both temperatures. Manolopoulou et al. (2010) [4] examined passive modified atmosphere storage of fresh-cut Romaine lettuce at 0 °C and 5 °C using different packaging. They found that respiration remained stable at 0 °C for 10 days but increased at 5 °C. Schvambach et al. (2020) [8] studied hydroponic crisp lettuce storage under different packaging at 5 °C and 23 °C. Lettuce without packaging had the highest weight loss, reaching 53.6% at 5 °C and 43.7% at 23 °C. This study aimed to evaluate the impact of various storage conditions on the shelf life of lettuce and spinach leaves in a solar refrigerated evaporatively cooled structure.

Material and Methods

The research aims to determine the impact of different storage environments on maintaining the freshness and nutritional quality of leafy vegetables. The storage study was conducted in Solar Refrigerated Evaporatively Cooled (SREC) structure at Division of Agricultural Engineering, ICAR-IARI, New Delhi.

The Solar Refrigerated Evaporatively Cooled (SREC) structure:

The SREC structure is a hybrid cold storage system designed for agricultural produce, integrating evaporative and forced cooling. It has a 10 × 10 × 10 ft³ capacity and is powered by a 5-kW rooftop solar panel system connected to an inverter, which supplies energy to a 1.5-ton refrigeration unit (GREE Wall Mount Ductless Mini Split Air Conditioner & Heat Pump, Zhuhai, China) (Chopra et al., 2023) [3] . The refrigeration system utilizes refrigerantt coils to cool water in an internal tank, which is then circulated through PVC pipes, creating a thermal water battery. This system helps maintain a stable internal temperature, particularly during non-solar hours, ensuring efficient temperature regulation for prolonged cold storage of produce. The control circuit was integrated with the refrigeration system to analyze temperature reduction within the SREC structure. The study evaluated 12 treatment combinations involving two inverter types-off-grid/hybrid (X₁) and ongrid (X₂)-along with two coil thermistor resistance levels (A₁ = 45 kΩ, A₂ = 39.4 kΩ) and three air thermistor resistance levels (B₁ = 21 kΩ, B₂ = 18.91 kΩ, B₃ = 16.94 kΩ). Each treatment was replicated three times. Treatments were labeled T 1 to T 12 , with T 1 comprising an off-grid inverter (X₁), coil resistance A₁, and air resistance B₁, while T 4 included an off-grid inverter (X₁), coil resistance A₂, and air resistance B₁. Among the evaluated configurations, T 4 and T 8 demonstrated the highest cooling efficiency and were subsequently implemented within the SREC structure for a comprehensive storage study.

Storage Conditions

In the experiment, the storage study was conducted in two treatments in which the highest cooling efficiency improvement was found. So, the storage behaviour of spinach and lettuce was evaluated by storing them in ambient (AC), T 4 and T 8 storage conditions. During the course of storage, changes in physiological weight loss, total soluble solids, firmness and colour were analysed.

Quality Parameters

The PLW of fruit was calculated on the initial weight basis and expressed in percent. Burst strength of Spinach and Lettuce leaf was measured by using a texture analyser (TA-XT plus of Stable Microsystems, Ltd., Surrey, UK) in compression mode with a 5 mm spherical ball probe to penetrate the leaf at a pre-test speed of 2 mm/s, a test speed of 1 mm/s as the probe contacted the leaf and a post-test speed of 10 mm/s (More et al., 2014) [5] . Firmness was measured as the maximum force recorded in a force-time curve obtained during the compression of test material by the probe. The colour of the vegetables in terms of L*, a*, b* values were determined using AMTAST 8MM Digital Precise Color Analyzer. The chromaticity dimension a* depicts redness with +ive, greenness when-ive, and b* measures yellowness when +ive and blueness when-ive. The total soluble solid (TSS) were determined by following the procedure described by Seyoum et al. (2011) [9] using handheld refractometer (Model Misco) with a range of 0 to 32 °Brix and a resolution of 0.2 °Brix and expressed as percent TSS after making the temperature correction at 20 °C. The experiments consisted of 6 treatments and 3 storage condition and laid out in a completely randomized design with five replications. The physiological loss of weight (PLW) varied significantly across storage conditions in lettuce and spinach. PLW in spinach increased sharply under ambient conditions (AC), reaching 31.63% in six days, while T 4 had the lowest PLW (22.80% over 14 days) and T 8 recorded 24.08% over 12 days. PLW in lettuce increased rapidly under ambient conditions (AC), reaching 29.74% in six days, while T 4 had the lowest PLW (20.52% over 14 days) and T 8 recorded 21.53% over 12 days. These results indicate that T 4 storage extended spinach and lettuce shelf life by two days compared to T 8 , demonstrating superior storage efficiency. The rise in TSS in spinach correlated with PLW, with TSS reaching 6.36°B after 14 days in T 4 and 6.44°B after 12 days in T 8 . The TSS in lettuce increased with storage duration, reaching 4.88°B after six days under ambient conditions. TSS in lettuce increased proportionally to PLW, reaching 5.18°B in T 4 after 14 days and 5.44°B in T 8 after 12 days. The slowest increase in TSS was observed in T 4 , aligning with PLW trends, indicating that T 4 effectively preserved solids content for a longer duration in both spinach and lettuce. The bursting strength of stored spinach varied significantly. Initially recorded at 1.74 N, it reached 3.49 N, 2.28 N, and 2.75 N on day 6 under ambient, T 4 and T 8 conditions, respectively. The bursting distance of spinach ranging from 2.81 mm to 5.01 mm. The highest increase was observed in ambient conditions, followed by T 8 and T 4 . On the final storage day, it reached 5.01 mm in T 4 (day 14) and 4.75 mm in T 8 (day 12). The bursting strength & bursting distance indicates better structural retention in T 4 storage. The bursting strength of stored lettuce varied significantly, initially recorded at 1.73 N. On day 6, it reached 4.09 N (ambient), 2.68 N (T 4 ), and 3.14 N (T 8 ). On the final storage day, it was 4.31 N in T 4 (day 14) and 4.20 N in T 8 . The bursting distance of lettuce increased from 2.84 mm to 3.63 mm during storage, with the highest rise in ambient conditions, followed by T 4 and T 8 . On day 6, it reached 3.86 mm (ambient), 3.14 mm (T 4 ), and 3.32 mm (T 8 ). On the final storage day, it was 3.63 mm in T 4 (day 14) and 3.35 mm in T 8 (day 12). The bursting strength and bursting distance of stored lettuce indicating better structural retention in T 4 storage.

Total Soluble Solids (TSS)

Bursting strength and bursting distance of spinach

Bursting strength and bursting distance of lettuce

Colour change in spinach

Conclusion

The physicochemical properties of the selected horticultural produce were significantly influenced by storage temperature and relative humidity. The SREC structure demonstrated the lowest physiological loss in weight (PLW), while ambient storage resulted in the highest PLW due to increased transpiration and respiration. Total soluble solids (TSS) initially increased due to desiccation and later declined due to sugar consumption for respiration, with the rate of change being faster under ambient conditions. Higher temperatures accelerated respiration, leading to greater shrinkage and structural weakening in spinach and lettuce. Moreover, ambient conditions induced faster colour loss, whereas the SREC structure better preserved colour, with T 4 storage conditions proving superior to T 8 . Overall, T 4 storage conditions in the SREC structure were the most effective in maintaining the quality and shelf life of lettuce and spinach.

References

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