Effect of Microwave Pretreatment of Seeds on the Quality and Antioxidant Capacity of Pomegranate Seed Oil

Abstract

Microwave pretreatment of oilseeds is a novel technique used to enhance oil nutraceutical properties. In this study, the effect of microwave pretreatment of seeds was investigated on pomegranate seed oil quality attributes including oil yield, yellowness index, refractive index, peroxide value, ρ-anisidine value, total oxidation value, conjugated dienes, total phenolic content, total carotenoids content, phytosterol composition, fatty acid composition, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity, and ferric reducing antioxidant power (FRAP). The seeds of three different pomegranate cultivars ('Acco', 'Herskawitz', and 'Wonderful') were microwave heated at 261 W for 102 s. Pomegranate seeds microwave pretreatment enhanced oil yield, yellowness index, total carotenoids content, total phenolic content, FRAP and DPPH radical scavenging capacity, despite an increase in conjugated dienes, and peroxide value. Palmitic acid, oleic acid, linoleic acid, saturated, and monosaturated fatty acids were increased after pomegranate seeds microwave pretreatment, whilst the levels of punicic acid and β-sitosterol were reduced. Nevertheless, the refractive index, the ratio of unsaturated to saturated fatty acid of the extracted oil were not significantly (p > 0.05) affected by pomegranate seeds microwave pretreatment. Principal component analysis and agglomerative hierarchical clustering established that 'Acco' and 'Wonderful' oil extracts from microwave pretreated PS exhibited better oil yield, whilst 'Herskawitz' oil extracts showed higher total carotenoids content, total phenolic content, and antioxidant capacity.

Keywords: antioxidant capacity; microwave pretreatment; oil; pomegranate seeds; total phenolic content.

Figure 1

Oil yield from unmicrowaved and microwaved (261 W for 102 s) pomegranate seeds of three pomegranate cultivars. Within the same cultivar (unmicrowaved and microwave), columns followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test. Vertical bars indicate the standard deviation of the mean.

Figure 2

Representative scanning electron microscopy (SEM) micrographs show the effect of microwave pretreatment (261 W/102 s) on the pomegranate seeds microstructures. (a) Unmicrowaved pomegranate seeds, (b) microwaved pomegranate seeds, (c) parenchymal cells from unmicrowaved pomegranate seeds, and (d) parenchymal cells from microwaved pomegranate seed.

Figure 3

Yellowness index (YI) of pomegranate seed oil from unmicrowaved and microwave pretreated (261 W/102 s) seeds of three pomegranate cultivars. Within the same cultivar (unmicrowaved and microwaved), columns followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test. Vertical bars indicate the standard deviation of the mean.

Figure 4

Phytosterol composition of oil extracted from unmicrowaved and microwave pretreated (261 W/102 s) seeds of three pomegranate cultivars. Within the same cultivar (unmicrowaved and microwaved), columns representing the same phytosterol and followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test. Vertical bars indicate the standard deviation of the mean.

Figure 5

A typical gas chromatography-mass spectrometry (GC-MS) chromatograph of the major fatty acids identified in pomegranate seed oil and their retention times. C16:0 = palmitic acid, C17:0 = heptadecanoic acid (internal standard), C18:0 = stearic acid, C18:1 = oleic acid, C18:2 = linoleic acid, C18:3 = punicic acid, C20:0 = arachidic acid.

Figure 6

Principal component analysis data of pomegranate seed oil (PSO) quality attributes from unmicrowaved and microwaved (261 W/102 s) pomegranate seeds of three pomegranate cultivars. A = ‘Acco’, H = ‘Herskawitz’, W = ‘Wonderful’, AV = ρ-anisidine value, TOTOX = Total oxidation value, K232 = Conjugated dienes, RI = Refractive index, TPC = Total phenolic content, TCC = Total carotenoids content, PA = punicic acid, FRAP = Ferric reducing antioxidant power, DPPH = 2,2-diphenyl-1-picryl hydrazyl.

Figure 7

Agglomerative hierarchical clustering (AHC) of PSO extracts from unmicrowaved and microwaved (261 W/102 s) seeds. A = ‘Acco’, H = ‘Herskawitz’, W = ‘Wonderful’, TOTOX = Total oxidation value, TPC = Total phenolic content, TCC = Total carotenoids content, DPPH = 2,2-diphenyl-1-picryl hydrazyl, FRAP = Ferric reducing antioxidant power.