Mathematical modelling of drying kinetics of avocado peels and its influence on flavan-3-ols content and antioxidant activity

Avocado peel is one of the main by-products of avocado processing and is considered a promising source of phenolic compounds with various bioactivities. The drying step is essential for its storage at the industrial level, and it is the first step in the strategy of transforming by-products into functional ingredients. Therefore, this research evaluates the effect of the convective air-drying of avocado peels at three different temperatures (40, 60 and 80 °C) and airflows (0, 0.8 and 1.6 m/s) on the flavan-3-ols content and antioxidant activity. Moreover, the mathematical modelling of its drying kinetic was developed. A decrease in the flavan-3-ol and antioxidant content was found with increasing temperatures. However, a high impact of the airflow reducing the drying time and limiting the decrease in interesting compounds was found. Among the tested mathematical models, the Page model reported the highest values of R2 (from 0.9907 to 0.9973) and the lowest errors for most of the temperatures and airflows tested. However, at 80 °C with airflow, the Lewis model seemed to fit better (R2 = 0.9982). Finally, the drying conditions that showed the lowest decrease in procyanidin and antioxidants were 40 °C and an airflow of 1.6 m/s for 105 min.