La type of packaging plays a fundamental role in storage of extra virgin olive oil and its chemical compositions. And although the green glass bottle is considered the container par excellence, we must be careful. A recent study has highlighted that this is certainly not the most suitable material, especially in the supermarket, even if it is ultraviolet grade green glass.
In fact, an extra virgin olive oil packaged in a dark green glass bottle only needs 5 months of shelf light exposure of a supermarket to be damaged by photo-oxidation and thus modify its flavour, progressively losing even its healthy properties. After 8 months it even loses the requirements to be able to define itself as extra virgin. Therefore, if it is believed that the oil will not be consumed in a decidedly shorter period of time (regardless of whether it is on the shelf of a supermarket or in any case subject to prolonged light over time), better to choose black coated or colored bottles with multilayer coating, which offer strong protection, respecting a glass culture that is dominant in the olive-oil sector. Other forms of packaging are also good such as tetrapack, bag-in-box or l 'aluminum.
The study was conducted by the Department of Agricultural, Food and Environmental Sciences, University of Perugia guided prof. Maurice Servili and had as its object three different packaging materials: a green glass bottle, one of ultraviolet grade green glass and a container in multilayer material (plasticized cardboard with aluminum foil lamination, tetrapack type).
“The impact of light on extra virgin olive oil – explains Servili – it is crucial because of one of its components, chlorophyll, a pigment that generates photo-oxidation, modifying the quality profile of a bottle over time. This does not happen with any other oil, because it is practically absent in most other fats, such as sunflower or soybean oils”.
The simulation
The study focused on three different packaging materials: one bottle made of green glass, one bottle made of ultraviolet grade green glass and one bottle made of multilayer material (plasticized cardboard with aluminum foil lamination, tetrapack type).
In such containersi (15 for each type) they have been inserted two types of oil – one with a high polyphenol content, the other with a medium content – to then be exposed to the artificial light typical of a supermarket (two LED tubes) 12 hours a day for 300 consecutive days, with a constant temperature of 25°. The various containers were arranged in rows and moved within each row every two weeks to ensure that each one received the same exposure over the duration of the experiment.
Tests were carried out on the samplesand periodic control analysis which they indicated as free acidity, peroxides and ethyl esters of fatty acids increased slowly over time for both oils and in all three types of packaging. In particular, the rate of increase, determined at 20-day intervals, was, for free acidity, 0,002 g of oleic acid/100 g of oil every 20 days; for the peroxide value, 0,05 meq. O2/kg oil; for ethyl esters of fatty acids, 0,37 mg/kg.
The effect on polyphenols
“The packaging material – we read in the research publication – had a significant effect on the concentration of polyphenols during long-term exposure to light. In the extra virgin olive oils stored in the two glass containers, polyphenol losses of 79% and 75% respectively occurred, while for the oils stored in the multilayer container the loss was 32%, indicating the superiority of this container compared to the previous ones in terms of protection from light radiation. The oleuropein derivatives, which constituted on average 77% of the initial polyphenols in both types of oil used, showed a decrease in concentration similar to that observed for polyphenols, confirming their antioxidant role”.
The effect from a sensory perspective
Even from a sensorial point of view, the quality of the packaging materials had a significant influence. The characters of hay e rancid they were the ones who showed the greatest correlations with the preservation in glass material. In reverse, the positive attributes of 'bitter', 'spicy', 'fruity' and 'grassy/cut grass’, were i most frequent sensory descriptors for extra virgin oils contained in the multilayer material. This material it has proven to be the most effective in reducing the negative effects of the oxidative phenomenon also from a sensorial point of view, delaying the appearance of the 'rancid' defect and the loss of the 'green' color and limiting the reduction in the perception of bitter and spicy over time.
A confirmation of the fact that light, therefore, like high temperature, remains one of the enemies of extra virgin olive oil. As a final clarification: in Italian supermarkets, oil hardly stays on the shelf for more than a few weeks, given that it is a widely consumed commodity. Rather, the question may concern those oils destined for the foreign market where exposure to light can occur both during transport and during the period of exposure on the shelf of a shop or market.
