Effect of packaging and storage conditions on quality of shelled walnuts

The present study investigated the effect of packaging and storage conditions on quality of raw shelled walnuts. Walnut kernels were packaged in: (a) low density polyethylene (LDPE), 55 μm in thickness in air, (b) polyethylene terephthalate||polyethylene (PET||PE), 70 μm in thickness under N2, and (c) PET-SiOx||PE pouches, 62 μm in thickness under N2. Samples were stored either under fluorescent light or in the dark at 4 or 20 °C for a period of 12 months. Quality parameters monitored were peroxide value (PV), hexanal, 2-thiobarbituric acid (TBA), odor, and taste of product. PV ranged between 0.3 for fresh walnut kernels and 31.4 meq O2/kg oil for walnuts packaged in PE pouches exposed to light after 12 months of storage. Respective values for hexanal were <28.5 μg/kg and 36.0 mg/kg and for TBA ca. 0.2 and 11 mg MDA/kg. Values for odor ranged between 0.2 for fresh walnut kernels and 5.7 for walnut kernels packaged in PE exposed to light after 12 months of storage at 20 °C. Respective values for taste were 0.7 and 6.8. Taste proved to be a more sensitive attribute than odor. Based on shelf life (taste) values and PV data it is proposed that the upper limit value for PV is close to 10.0 meq O2/kg walnut oil. Respective limit values for hexanal are 1–2 mg hexanal/kg walnut and for TBA is 1–2 mg malondialdehyde/kg walnut. Walnuts retained acceptable quality for ca. 2 months in PE-air, 4–5 months in PET||PE-N2 and at least 12 months in PET-SiOx||PE-N2 pouches at 20 °C, with samples stored in the dark retaining slightly higher quality than those exposed to light. The effect of parameters investigated followed the sequence: temperature > degree of O2 barrier > lighting conditions.