Incorporation of hydroxytyrosol alkyl esters of different chain length as antioxidant strategy in walnut oil spray-dried microparticles with a sodium alginate outer layer

•Walnut oil (PWO) was spray-dried with Capsul and sodium alginate (SA, outer layer).•Different alkyl chain length hydroxytyrosol esters (HTEs) were used as antioxidants.•Antioxidant activity and alkyl chain length of HTEs showed a non-linear correlation.•HTE-C10 had the highest antioxidant activity and was located at oil:water interface.•The release of major fatty acids during digestion was delayed by the SA outer layer.