Studies on convective drying of ‘Ameclyae’ Opuntia ficus-indica seeds and its effect on the quality of extracted oil based on its α-tocopherol content

Abstract

The aim of this work is to model the thin layer convective drying kinetics of ‘Ameclyae’ prickly pear seed variety and to evaluate the effects of drying conditions on the quality of extracted seed oil, specifically α-tocopherol content. Drying experiments were carried on following a full 2³ factorial design using a vertical drying tunnel. The study is restricted to a particular particle size and a particular seed bed of height 0.5 cm, of effective porosity around 0.4 and of initial moisture content on dry basis equal to 1.2 (± 0.01) kg/kg. The temperature range was 45 to 70 °C, relative humidity range was 15–30% and air velocity was 1 and 2 m/s. The experimental drying curves were fitted to different semi-theoretical drying models proposed in the literature. The Midilli-Kucuk model was found with satisfaction describing the seed air drying curves with a correlation coefficient of 0.999 and a standard error of 0.01. For each drying condition, the extraction of fixed oil seeds was performed at cold using liquid/solid separation method. The oil quality was evaluated on the basis of the α-tocopherol content. The α-tocopherol was identified and quantified by high-performance liquid chromatography (HPLC-UV). According to the experimental results, it was found that drying air velocity is the most important factor influencing the concentration of α-tocopherol, whereas the effects of temperature and relative humidity were lower. The increase of the velocity from 1 m/s to 2 m/s reduced the α-tocopherol concentration by about 25%. The convective drying of thin layer of seeds at soft air conditions, drying temperature of 45 °C, relative humidity of 15% and air velocity of 1 m/s give the optimal quality of extracted oil in terms of α-tocopherol content. Characterization of the morphologic structure of seeds was also performed by SEM.

Appendix

$$ r=1-\frac{\sum_{i=1}^n{\left({MR}_{i,\exp }-{MR}_{i, pre}\right)}^2}{\sum_{i=1}^n{\left({\overline{MR}}_{i,\exp }-{MR}_{i, pre}\right)}^2} $$

$$ s=\frac{\sum_{i=1}^n{\left({MR}_{i,\exp }-{MR}_{i, pre}\right)}^2}{n-z} $$

where MR_{i, exp.} is the ith experimental moisture ratio, MR_i,_pre is the ith predicted moisture ratio, n is the number of observations (is the number of experimental points), z is the number of constants in the drying model and ${\overline{MR}}_{i,\exp }$is the average value of experimental moisture ratio was calculated by using the following relation:

$$ \overline{MR}=\frac{1}{n}\sum_{i=1}^n{MR}_{\exp, i} $$