Optimization of subcritical water fluid extraction of phenolic compounds from pistachio hull: evaluation of radical scavenging properties of extracts

Phenolic compounds are known as a base of many functional foods due to their antioxidant properties and salutary effects. Phenolic compounds mainly found in plant sources. Agricultural wastes are one of the cheapest sources for getting phenolic compounds. Pistachio is considered one of the most important agricultural products in Iran. Study on the different parts of solid wastes generated from pistachio processing is worthy due to containing valuable compounds. On the other hand, using novel techniques such as supercritical CO2, subcritical water, pulsed electric fields, etc., have gained more attention in the past few years. These technologies do not have side effects on the raw materials and final products; do not use chemical solvents such as hexane, methanol, acetonitrile, etc., in comparison to traditional extraction methods with the long time needed and using chemical solvents with hazardous effect on consumers. In this study, the subcritical water extraction method as a green technology was used for the extraction of phenolic compounds from pistachio hull using y response surface methodology.

In this study antioxidant properties of phenolic compounds extracted from pistachio hull by subcritical water fluid were evaluated. The extraction process is done at 120– 180 Celsius degree, pressure 10– 50 bar and ratio 1:10– 1:30 (sample to solvent) by using the subcritical water fluid method. The total phenolic compound, flavonoid compounds, reduction power, radical scavenging, and stability power of soybean edible oil were determined for each treatment. The stability power of extracts in soybean measured with rancimat test at 110 Celsius degree and airflow of 20 litter per minute. Comparison and statistical analysis for optimization of results accomplished using response surface methodology (RSM) by design expert software 7.0.0 version. After optimization, the results for optimum treatment were compared with BHT, ascorbic acid, and alpha-tocopherol. This comparison was done for reduction power, stability power, and radical scavenging ability.

The results showed that the total phenolic compounds changed from 7671.43 to 8903.57 mg Gallic acid equivalent per 100 g dry matter. Reduction power increased by increasing temperature from 120 to 150 Celsius degree and after that decreased. The effect of pressure on the phenolic compounds wasn’t remarkable. The amount of flavonoid compound was between 142.87 and 290.21 mg equivalent catechin per 100 g dry material. The most effective parameter among investigated parameters (temperature, pressure, and ratio), was temperature. The pressure of extraction in subcritical water extractions did not have any significant effect and every change in laboratory results was very small The best results were observed at 147 Celsius degrees, pressure 10 bar, and 1:19 ratio. In these optimum conditions, the amount of total phenolic compounds, total flavonoid compounds, stability power, radical scavenging power, and reduction power was 2497.8 mg gallic acid per 100 g dry material, 267.99 mg flavonoid compounds catechin equivalent per 100 g dry material, 7.44 h, 1370.77 ppm, and 362.94 ppm respectively. Reduction and stability power with radical scavenging ability of pistachio hull extracts were very near to BHT as well as a synthetic antioxidant. Results showed that the extracts obtained from pistachio hull by subcritical water had high amount of phenolic compounds and had the same antioxidant properties as the synthetic antioxidant BHT