Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Likewise, extract prepared with 70% aqueous acetone from 500 mg ground lyophilized rose hip and seed (the supernatants were dried and the solid residue dissolved in methanol) containing 13 mg of gallic acid equivalents inhibited the oxidation of methyl linoleate in vitro (Kähkönen et al., 1999).
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