Effect of Citric Acid Pretreatment on Drying Kinetics of Tamarillo in a Greenhouse Solar Dryer

Abstract

The production and consumption of tamarillo (Solanum betaceum) is gaining momentum due to its immense nutritional, health, and commercial potential. However, like many other horticultural products, high postharvest losses are incurred in tamarillo production. In particular, decay is a major problem. Drying is a viable option that can address this problem and significantly increase the shelf life of tamarillo. Consequently, the objective of this study was to evaluate the effect of citric acid pretreatment on the drying kinetics and quality of tamarillo slices. The tamarillos were dried in a passively operated even-span greenhouse solar dryer (8 m long, 4 m wide, and 3.6 m high to the ridge) covered with a 200 µm thick ultraviolet-stabilised polyethylene film. Solar radiation, temperature, relative humidity, moisture content, colour, firmness, and vitamin C content were periodically measured, and the data were analysed statistically. The citric acid-pretreated and control (untreated) tamarillo slices were dried from an initial moisture content of 655% (db) to a final moisture content of 16.25% and 22.50% (db), respectively, in 10 hours. In addition, the pretreated slices attained a higher average drying rate of 42.01±0.23 g/g/hr compared to 40.23±0.12 g/g/hr attained by the control slices. Further, the Page model best described the drying kinetics of both tamarillo samples with R², χ2, and RMSE values of 0.9975, 0.0004, and 0.0005, respectively. The pre-treated samples had better colour retention with a total colour change of 15.07±1.12 compared to 35.99±0.98 attained in the control samples. Moreover, the pretreated tamarillo slices attained a lower percentage firmness increment of 281.82±3.21% compared to 337.5%±1.54 attained in the untreated samples. Further, the pretreated tamarillo slices had 42.94±0.41% vitamin C retention compared to 40.17±0.67% retention in the control samples. Generally, pre-treating tamarillo slices with citric acid improves their drying kinetics and both physical and nutritional qualities.