Abstract
Intermittent drying (ID) was applied to reduce soybean cracking because of the low moisture gradient and little thermal stress on soybeans during their tempering period. The drying temperature and relative humidity (RH) for the drying and tempering periods were 35°C and 20% and 25°C and 43%, respectively. The intermittency (α) of the drying was defined as the ratio of the drying period to the duration of the drying and tempering periods, and it varied at α = 1, 0.5, 0.4, and 0.25 to evaluate the drying characteristics and the soybeans’ quality. Intermittency processes redistributed the moisture in the soybean so that the low thermal stress was applied to the soybeans. The percentage of cracked grains increased with increasing the duration of drying period and decreasing tempering period. The moisture content and temperature changes during drying of soybeans were well fitted by reaction engineering approach (REA) modeling. Additionally, the physics that describe the soybeans’ drying behavior during ID were explained by the model parameters obtained from the REA modeling, such as the surface relative humidity and the surface water vapor concentration. ID showed the highest drying efficiency at α = 0.25 regarding the total drying time (13,800 s, i.e., the shortest drying time) and the lowest cracking ratio (<2.18%).
Highlights
Soybean is a well-known valuable resource because of its healthy nutrients, such as high-quality protein, oil, and phytochemicals, and for its wide application in processed foods like tofu, soybean sauce, and soy milks (Prachayawarakorn, Prachayawasin, & Soponronnarit, 2006; Wardhani, Vázquez, & Pandiella, 2008)
The sample temperature (T), the TA B L E 1 R2 and RMSE of moisture content and temperature profiles predicted by reaction engineering approach (REA) model
The relative activation energy of the REA model reflects the degree of difficulty in drying that correlates well with the drying rate
Summary
Soybean is a well-known valuable resource because of its healthy nutrients, such as high-quality protein, oil, and phytochemicals, and for its wide application in processed foods like tofu, soybean sauce, and soy milks (Prachayawarakorn, Prachayawasin, & Soponronnarit, 2006; Wardhani, Vázquez, & Pandiella, 2008). A time-dependent drying method with an intermittency of the heat flux, obtainable by controlling the drying conditions, including various air temperatures, flow rates, or humidities, has been applied in food drying (Kumar, Karim, & Joardder, 2014). Semi-empirical approaches with thin layer models (Rafiee et al, 2009), numerical simulations using the finite element methods (Rafiee, Keyhani, & Mohammadi, 2008), analytical solutions with coupled mathematical models (Hemis & Raghavan, 2014), and fractional- order kinetic models (Nicolin, Defendi, Rossoni, & de Matos Jorge, 2017) have been conducted to describe soybean drying characteristics at varied drying temperature and relative humidity, neither of them modeled the time-varying drying process to improve the quality of soybeans. The aims of this study were (a) to investigate the efficiency of the ID method on soybean drying compared with conventional hot-air-drying, (b) to determine the drying characteristics of soybeans using REA, and (c) to estimate the effect of intermittency during ID on the soybean quality after applying each drying process
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