The effects of air temperature and pretreatments on drying kinetics and color change in convective drying of green beans

Drying is one of the widely used methods in the preservation of agricultural products such as cereals, fruits and vegetables, medicinal and aromatic plants. The important aim of drying is to reduce the moisture content and thereby increase the lifetime of products by limiting enzymatic and oxidative degradation. In this study, it was aimed to determine the best drying conditions for Gina variety fresh bean samples grown in Tokat province. The experiments were conducted in a laboratory type hot air experimental dryer, in open sun and shade. Fresh bean samples were kept in boiling water for 3 minutes before drying, whole and sliced and dried in hot air dryer at 50, 60 and 70 ℃ air temperatures. The drying time of the green beans, effects of pretreatment, color change of dried product were determined. In addition, to determine the drying kinetics, the curves of moisture ratio-drying time obtained by experiments were modelled by using three semi-theoretical models, namely the Modified Page, Midilli-Küçük and Diffision Approach model. The results have showed that the increase in air temperature and pretreatment reduced the drying time. Moreover, it was observed that the temperature and pretreatments affect the colour values of the dried products. According to the results of drying experiments, the fastest drying took place at 70 oC. The closest color values according to fresh product were determined in laboratory type dryer at 50 ℃ drying temperature and in boiled samples. Midilli-Küçük model gave better prediction than other models, and satisfactorily described drying characteristics of green beans.