New research on untreated green olives has found that products with a stated shelf life of 2-3 years can be 'unacceptable' long before their sell-by date. The study, published in the Journal of the Science of Food and Agriculture, looked at the growing trend towards using polyethylene pouches, which are vacuum-packed, filled with brine or packed in 'modified atmospheres'.
According to the research, only the vacuum pouches gave promising results, producing a shelf life of nearly two years, while those packed in ordinary air had a true shelf life of only nine months. The author of the study, Dr Efstathios Panagou, points out that the stated shelf life on new packaging "is not clearly defined…. quite arbitrary and is not supported by relevant studies".
Dr Panagou's research looked at different packing methods over a six-month period, testing the olives for microorganisms, acidity, colour and firmness. A ten-member expert panel then rated the product on smell, taste, and general acceptability.
The study looked at olives in different kinds of brine, in air, in a vacuum and in a modified atmosphere of 40 per cent carbon dioxide, 30 per cent nitrogen and 30 per cent oxygen.
No harmful bacteria was found in any of the olives after six months storage at room temperature, but the overall quality of taste, smell, colour and firmness was found to be "unacceptable" in the air-packed olives and those packed in a modified atmosphere, and only "medium acceptability" in brine.
About a million tonnes of olives are eaten each year worldwide, and most are produced in the EU. Untreated green olives are valued for their superior flavour, but very little research has been done on the way their quality declines after packing.
This study follows a recent report by Spanish researchers on the storage of olive oil. They found that the chlorophyll, carotenoids, and total phenol content of virgin olive oils decreases sharply after a 12-month storage period.
The food scientists at the university of Lleida, also found an increase in oleic acid percentage in the fatty acid composition.
Oxidative rancidity development has been recognised as the leading cause of oil deterioration during storage. The two compositional factors of oils that determine their susceptibility to oxidation are the fatty acid composition and inherent antioxidant compounds. Virgin olive oil holds a rich source of natural antioxidants including carotenoids, tocopherols and phenolic compounds.
According to the scientists, this is evidence that the antioxidant fighting power of extra virgin olive oil has a limited shelf life. The researchers say that they found a 30 per cent loss of chlorophyll content during storage. Carotenoid content followed a similar trend to that of chlorophylls.
Alpha-tocopherol levels fell by a massive 100 per cent after 12 months storage leading the scientists to suggest that alpha-tocopherol plays a significant role as an antioxidant in the induction period of oxidation. Among the phenolic compounds, lignans were found to be the more stable.
In recent years extra virgin olive oil has been the central element for a host of studies on the Mediterranean diet as researchers investigate why the dietary regime in the Mediterranean basin - olives, tomatoes, fish, olive oil - appears to combat heart disease. According to the World Health Organisation (WHO), heart disease and strokes are the major causes of death and disability in ageing women, accounting for close to 60 per cent of all adult female deaths.