Effects of drying temperature and relative humidity on the quality of dried onion slice

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%.

Keywords: Degradation kinetics; Food science; Food technology; Onion slice; Phenolic compounds.

Figure 1

The schematic diagram of research.

Figure 2

The schematic of laboratory tray dryer system: 1.Blower; 2. Heater; 3. Anemometer; 4.Temperature and relative humidity sensor (T-RH); 5. Dehumidification column and 6. Tray dryer.

Figure 3

Moisture ratio versus time for drying temperatures 40, 50, 60 and 70 °C.

Figure 4

The estimated drying time at various temperatures and relative humidity.

Figure 5

Reaction of a phenolic compound with oxygen from air forming quinone.

Figure 6

Degradation of total phenolic compounds during drying at 40, 50, 60 and 70 °C.

Figure 7

FTIR spectra of fresh and dried onion at frequency of 400–4000 cm⁻¹

Figure 8

The two dimensional plot (a) and three dimensional plot (b) of total phenolic compounds degradation.