Storage of fresh walnuts (Juglans regia L.) – Low temperature and phenolic compounds

Undried or fresh walnut (Juglans regia L.) kernels are a less widespread product than the dried kernels with no information concerning their storage. Weight loss, respiration, ethylene production, colour, total phenolics (TP) and total antioxidant capacity (TAC) were examined in fresh de-hulled walnuts of cv Franquette with or without extraction from the shell during storage in air at 1 °C and 90% RH for 20–40 d. The effect of low temperature on TP, TAC and individual phenolic compounds was also investigated during the storage of fresh shelled kernels at 1 °C and 8 °C for up to 30 d. Walnuts were harvested at the commercially mature stage. Respiration, as determined at 20 °C, decreased sharply in shelled kernels during the first 20 d at 1 °C, but gradually in whole walnuts during 40 d storage. No ethylene production was detected in samples. Before storage, the TP concentration in fresh kernels was already 1.2-times higher than in kernels dried at 36 °C for 24 h. Additionally, TP and TAC increased by 1.2- and 1.3-fold, respectively, in fresh kernels stored at 1 °C for 20 d and remained almost stable thereafter. Shelling increased kernel browning slightly at the lower temperature, but had no effect on TP and TAC values. At 1 °C, 4-hydroxybenzoic, protocatechuic, vanillic, ellagic, 2,4-dihydroxybenzoic and syringic acids exhibited major increases within the first 20 d and did not change subsequently. At 8 °C, no increases in TP, TAC and most phenolic compounds were observed apart from those in 2,4-dihydroxybenzoic and syringic acids. By contrast, protocatechuic acid ethyl ester decreased at both storage temperatures, the decrease being promoted by the higher temperature used. Ellagic acid was the major acid observed in all cases. Treatments with actinomycin D, cordycepin, cycloheximide and 2-aminoindane-2-phosphonic acid (inhibitor of phenylalanine ammonia-lyase or PAL inhibitor) inhibited TP increases at 1 °C, indicating the induction of at least the phenylpropanoid pathway in fresh walnuts at 1 °C.

► Mature fresh walnuts were stored in air at low temperature for up to 40 d.
► At 1 °C, browning was prevented less in shelled than in-shell kernels.
► Total phenolics and antioxidant capacity increased at 1 °C, but not at 8 °C.
► Ellagic acid was the major acid among seven phenolic measured.
► Inhibitors of mRNA, RNA, protein synthesis and PAL activity prevented total phenolic increases at 1 °C.