Abstract
Onion, a very common season ingredient, is useful as an antioxidant and optimal conditions are required for its drying while ensuring the best quality is retained. This study evaluated the effect of drying temperatures and relative humidity on both drying rate and onion quality. Onions with an average diameter of 20.125 ± 0.025 mm were peeled and sliced into a thickness of 1.233 ± 0.029 mm. They were then dried for 120 min under various temperatures ranging from 40 to 70 °C. Both moisture content and total phenolic compounds were measured and analyzed as responses, and the data obtained were used for estimating the kinetic parameters of drying rate and total phenolic compounds degradation. The results show that the drying kinetics followed Fick's model. Moreover, the total phenolic compounds degradation can be properly expressed using a first-order reaction model, and the optimization using response surface method revealed that the optimum conditions of onion slice drying were achieved at 49.6 °C and relative humidity of 0.65%. These conditions can significantly reduce drying time with phenolic compounds retention of up to 96%.
Highlights
Onion, which is primarily used as a seasoning ingredient in several countries, is one of the most popular vegetables containing various beneficial chemical compounds such as fibers, vitamins, organic acids, phenolic compounds, and other antioxidants (Mitra et al, 2012)
The moisture content in onion was measured every 10 min (600 s); the total phenolic compounds expressed as gallic acid equivalent were analyzed every 30 min (1800 s)
For some food items such as apple, apple peel, and encapsulated yacon juice, total phenolic compounds degradation is expressed as a pseudo-first or first-order reaction (Arora et al, 2018; Henríquez et al, 2014; Lago and Noren, 2017), in which different process treatments and material condition and properties result in different degradation rates
Summary
Onion, which is primarily used as a seasoning ingredient in several countries, is one of the most popular vegetables containing various beneficial chemical compounds such as fibers, vitamins, organic acids, phenolic compounds, and other antioxidants (Mitra et al, 2012). Phenolic compounds are highly influenced by temperature change; to retain these compounds, appropriate postharvest treatments and onion storage are necessary. Drying is one of the postharvest treatments for onion; for this process, water content is removed by introducing heat (Djaeni et al, 2014; Sharma et al, 2005). Studies investigated onion drying characteristics and its kinetics using different methods and under various treatments (Asiah et al, 2017; Bebartta et al, 2014; Demiray et al, 2017; Revaskar et al, 2014). At higher temperature, components such as phenolic compounds can be potentially degraded As per these facts, a comprehensive study to determine the optimum drying condition of onion is important by investigating the effect of drying temperature and relative humidity on drying time and retention of onion components such as total phenolic compounds. This study's results are beneficial for postharvest treatment of onion
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