The impact of storage time and storage temperature on the quality of the oil of evening primrose (Oenothera biennis L.)

Evening primrose (Oenothera biennis L.) is a seed drug plant and a rich source of gamma-linolenic acid. The importance of gamma-linolenic acid as a food and a component of pharmaceutical products have long been established. The oil of evening primrose contains a high proportion of polyunsaturated fatty acids that makes it more susceptible to oxidative deterioration. This, coupled with the fact that there is variable demand for the product, means that the product sometimes needs to be stored for a long period. Therefore, creating suitable storage conditions that can preserve the seed oil quality and delay its oxidation (rancidity) is a vital factor in the evening primrose oil production system. Experiments were executed to clarify the effects of different storage temperatures and storage times on evening primrose seed and cold pressed oil and the percentage of gamma-linolenic acid and related fatty acids. The results showed that the oil percentage of the seed was significantly influenced by storage time. The lowest percentage of seed oil was observed in the last month of storage (24.3% comparison with 26.0% in control). The percentage of gamma-linolenic acid (GLA) and linoleic acid (LA) were influenced by storage time both in seed and oil samples. The amounts of these two fatty acids showed a decrease during storage. The free fatty acid (FFA) percentage and the peroxide value (PV) of the seed samples were significantly influenced by different storage times and temperatures. When seeds were stored under low temperature conditions the lowest percentage of FFA (2.7%) as well as the lowest peroxide value (18.6 meq peroxide/kg) were recorded. From the results obtained so far it can be concluded that the compositions of both seed and oil are more stable under low temperatures and the composition of seed oil appears to be more affected by temperature than that of extracted oil.