Effect of packaging material headspace, oxygen and light transmission, temperature and storage time on quality characteristics of extra virgin olive oil

Food Control 21 (2010) 412–418

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Effect of packaging material headspace, oxygen and light transmission, temperature and storage time on quality characteristics of extra virgin olive oil G. Pristouri, A. Badeka, M.G. Kontominas * Laboratory of Food Chemistry and Technology, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece

a r t i c l e

i n f o

Article history: Received 5 February 2009 Received in revised form 17 June 2009 Accepted 19 June 2009

Keywords: Packaging Light Oxygen permeability Headspace volume Olive oil quality

a b s t r a c t The effect of packaging parameters (transmission to light and oxygen, headspace volume) and storage temperature on quality characteristics of extra virgin olive oil (EVOO) was studied as a function of storage time (0–12 months). Packaging materials tested included clear glass, clear polyethylene terephthalate (PET), clear PET + UV blocker, clear PET covered with aluminum foil and clear polypropylene (PP) bottles. Quality parameters monitored over the 12 month storage period included: acidity, peroxide value (PV), spectrophotometric indices (K232, K270) and color. Results showed that the best packaging material for olive oil packaging was glass followed by PET. PP proved to be unsuitable for such an application. Exposure of olive oil samples to light, high storage temperatures (35 °C) and large headspace volumes caused substantial deterioration in product quality parameters. The most pronounced effect was that of temperature and light while the smallest effect was that of headspace volume and packaging material permeability to oxygen. Olive oil color was not substantially affected by storage conditions with the exception of storage of olive oil at 35 °C exposed to light for 12 months. Shelf life of extra virgin olive oil was 6 months packaged in clear glass in the dark at temperatures up to 22 °C; 3 months in clear PET in the dark at 22 °C and less than 3 months in clear PP in the dark at 22 °C. When exposed to light, shelf life of olive oil was 9 months when packaged in PET + aluminum foil; 3 months in PET + UV blocker and less than 3 months in clear PET at 22 °C. Product shelf life was less than 3 months at 35 °C. Finally oxygen in the headspace of olive oil resulted in deterioration of product quality. The relative contribution of parameters studied to the retention of olive oil quality was: temperature  light > container headspace > packaging material oxygen permeability. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Olive oil is a major agricultural commodity for the European Union with ca. 75% of olive oil production coming from EU states; 95% of this amount comes from Spain, Italy and Greece (MorenoRojas, Reniero, Guillon, Klotz, & Giazzi, 2007) with Greece producing ca. 350,000 tons per year (13.5% of world production) (http:// en.wikipedia.org/wiki/Olive_oil, 2009). Olive oil quality is defined from commercial, nutritional and sensory perspectives (Duran, 1990). The nutritional value of olive oil is due to its high content of monounsaturated oleic acid and minor constituents such as phenolic compounds, tocopherols and carotenoids, while its sensory properties (mainly aroma) is the result of a complex mixture of volatile compounds (Angerosa, 2002; Tsimidou, Blekas, & Boskou, 2003). Oxidation constitutes a major factor for quality deterioration of olive oil. The rate of oxidation depends on a number of factors * Corresponding author. Tel.: +30 2651098342; fax: +30 2651098795. E-mail address: [email protected] (M.G. Kontominas). 0956-7135/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2009.06.019

including the availability of oxygen, presence of light and temperature. Auto-oxidation, that is oxidation in the absence of light, follows a free radical mechanism where initially absorption of oxygen results in the formation of hydroperoxides. These labile compounds further decompose to produce a complex mixture of volatile compounds such as aldehydes, ketones, hydrocarbons, alcohols, and esters responsible for the deterioration of olive oil flavor termed ‘‘oxidative rancidity” (Frankel, 2005; Morales, Rios, & Aparicio, 1997). In turn, when vegetable oils are exposed to light, photo-oxidation occurs through the action of natural photosensitizers (i.e. chlorophyll), which react with triplet oxygen to form the excited state singlet oxygen. Singlet oxygen then forms a free radical from unsaturated fatty acids leading to the production of hydroperoxides and eventually to carbonyl compounds resulting to the development of undesirable off flavors in oils (Skibsted, 2000). Thus protection from direct light is required for commercial edible oils (Bradley & Min, 1992; Khan & Shahidi, 1999). Methods used to evaluate olive oil quality include conventional as well as innovative techniques. Conventional methods include

G. Pristouri et al. / Food Control 21 (2010) 412–418

acidity determination, peroxide value, absorption coefficients (K232, K270), conductivity methods (Rancimat-OSI) and sensory evaluation. Innovative methods include Nuclear Magnetic Resonance (NMR), Isotope Ratio Mass Spectroscopy (IRMS), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR) and Solid-phase Microextraction–Gas Chromatography/Mass Spectroscopy (SPME–GC/MS) for the determination of volatiles (Kiritsakis, Kanavouras, & Kiritsakis, 2002; Sacco, Brescia, Sgaramella, & Sacco, 2005). Predictive modeling using advanced statistical techniques has been also used to evaluate olive oil stability (Kanavouras, Hernandez-Münoz, & Coutelieres, 2006; Zanoni, Bertuccioli, Rovelline, Marotta, & Mattei, 2005). Packaging can directly influence olive oil quality by protecting the product from both oxygen and light (Kanavouras, HernandezMünoz, Coutelieres, & Selke, 2004; Kiritsakis, 1998). Materials which have been used for olive oil packaging include glass, metals (tin-coated steel) and more recently plastics and plastics coated paperboard (Agricultural Cooperative Union of PEZA, 2008; Kiritsakis et al., 2002). Among plastics, PET has captured a large portion of the olive oil retail market due to its many advantages including clarity, chemical inertness, low oxygen permeability, and excellent mechanical properties (Papachristou et al., 2006). Incorporation of pigments and/or UV blocking agents or oxygen scavengers may improve plastics properties with regard to quality retention of olive oil. Besides PET, PE in the form of LDPE-coated paperboard/alufoil laminates, i.e. brick-type cartons and bag-in-box pouches and PP in the form of gallon or half gallon jugs are being used today for the packaging of vegetable oils including olive oil (Agricultural Cooperative Union of PEZA, 2008). Several studies have been carried out to determine the effect of package properties on olive oil quality including: glass, PET and PVC (Coutelieris & Kanavouras, 2005; Del Nobile, Bove, La Notte, & Sacchi, 2003; Kaya, Tekin, & Öner, 1993; Kiritsakis & Dugan, 1984; Méndez & Falqué, 2007; Tawfik & Huyghebaert, 1999). The present study was undertaken with the primary objective to investigate the effect of all three major packaging properties namely: oxygen permeability, light transmission rate and headspace volume, as well as storage temperature on olive oil quality as a function of storage time under realistic storage conditions. A second objective was to determine the relative contribution of each of the above parameters to olive oil quality. Quality parameters monitored over storage time included: acidity, peroxide value, absorption coefficients (K232, K270) and color.

2. Materials and methods 2.1. Materials EVOO samples (Liophori brand) were donated by EPIROTIKI ELAIOURGIA SA (Ioannina, Greece). The particular olive oil was produced in the Messinia region using ‘‘Koroneiki variety” olives. 2.2. Experimental set up – packaging materials To study the effect of container oxygen permeability, clear polyethylene terephthalate (PET), clear polypropylene (PP) and clear glass bottles, 500 mL in capacity were filled with olive oil and sealed with aluminum screw-type caps of negligible permeability to oxygen. Rectangular bottles were of dimensions (4 cm  6 cm) base  20.5 cm height (surface area/volume = 434 cm2/ 492 cm3 = 0.88). Containers were stored in a controlled temperature environmental chamber (Memmert, Binder model WTC, Schwabach, Germany) in the dark at 22 °C. To study the effect of container light transmission clear PET, clear PET + UV blocker and clear PET covered with aluminum foil of capacity 500 mL were

413

filled with olive oil and stored at ambient temperature on the lab shelf exposed intermittently (day–night) 12 h each to daylight. To study the effect of container headspace volume, clear PET bottles of capacity 1 L were filled to the top (zero headspace) with olive oil. At 3, 6, 9 months, 200 mL of olive oil were removed creating 200, 400 and 600 mL headspace, respectively. Containers were kept in a controlled temperature environmental chamber in the dark at 22 °C. Finally, to determine the effect of storage temperature, three temperatures, namely: 13, 22 and 35 °C were chosen to simulate (a) cellar temperature commercially used to store olive oil, (b) room temperature and (c) elevated ambient temperature encountered during the summer, respectively. 2.3. Reagents For the determination of acidity, ethanol and diethyl ether (pro analysis grade), potassium iodine, sodium thiosulphate and sodium hydroxide were purchased from Merck (Germany). For the determination of PV, chloroform and acetic acid were also purchased from Merck. Likewise for the determination of absorption coefficients (K232, K270) isooctane was purchased from Merck. 2.4. Methods 2.4.1. Determination of acidity, peroxide value and absorption coefficients All three parameters were determined according to the Official EU method 2568/91. 2.4.2. Determination of packaging material thickness Thickness was measured using a Thomas Scientific electronic micrometer model Positector 6000 (Swedesboro, NJ, USA). 2.4.3. Color measurement Olive oil color was measured using a HunterLab, model D25 L optical sensor (Reston, Virginia, USA). 2.4.4. Determination of oxygen transmission rates Oxygen transmission rates (OTR) for all bottles were measured using the Oxtran 2/20 oxygen permeability tester (Mocon Controls, USA) at a relative humidity (RH) of 60%, temperature of 22 °C (ASTM D-3985) and were expressed as mL/(package day). 2.4.5. Statistical analysis Data were subjected to analysis of variance using the Excel 97 software program (Microsoft, CA, USA). Where statistical differences were noted, differences among packages were determined, using the least significant difference (LSD) test. Significance was defined at P < 0.05.

3. Results and discussion 3.1. Effect of packaging material oxygen transmission rate on olive oil quality Olive oil quality parameter values for glass, PET and PP are given in Table 1 as a function of storage time. Results show that for olive oil stored in glass in the dark at 22 °C the acidity and PV were within the set limit of 0.8% and 620 meq O2/kg, respectively (EU Regulation 1989/2003). K232 exceeded the limit of 2.50 after 6 months and K270 exceeded the limit of 0.22 after 9 months of storage. Acidity is mainly the result of triglyceride hydrolysis through the action of lipases present in olives and secondarily the result of microbial growth on the olive flesh. Such microorganisms include bacteria, yeast, molds, 70% of which exhibit lipolytic action (Suarez-Marti-

L = 51.73 a = 3.01 b = 35.20 L = 52.82 a = 2.80 b = 36.09 60.22 62.50 620 60.8% Adopted limits (EU Regulation 1989/2003)

0.83 ± 0.01 0.75 ± 0.01 12

0.79 ± 0.01

18.86 ± 0.10

20.61 ± 0.20

22.54 ± 0.10

2.82 ± 0.13

2.98 ± 0.13

3.22 ± 0.09

0.28 ± 0.02

0.32 ± 0.02

0.35 ± 0.01

L = 51.89 a = 3.02 b = 35.30

L = 52.14 a = 2.82 b = 35.49 L = 51.72 a = 2.72 b = 35.22 L = 52.43 a = 3.09 b = 35.69 0.26 ± 0.02 0.23 ± 0.03 0.22 ± 0.02 3.06 ± 0.01 2.90 ± 0.10 2.76 ± 0.11 21.57 ± 0.20 19.75 ± 0.40 0.79 ± 0.01 0.70 ± 0.02 9

0.79 ± 0.01

17.53 ± 0.10

L = 51.83 a = 2.82 b = 35.28 L = 51.44 a = 2.70 b = 35.01 L = 52.21 a = 2.92 b = 35.54 0.26 ± 0.04 0.22 ± 0.02 0.21 ± 0.04 2.85 ± 0.10 2.84 ± 0.10 2.54 ± 0.10 19.00 ± 0.10 17.95 ± 0.20 0.74 ± 0.01 0.68 ± 0.01 6

0.71 ± 0.01

16.57 ± 0.70

L = 51.86 a = 3.11 b = 35.30 L = 52.55 a = 3.16 b = 35.78 L = 51.65 a = 3.10 b = 35.17 0.25 ± 0.01 0.17 ± 0.04 0.15 ± 0.03 2.58 ± 0.20 2.57 ± 0.15 2.47 ± 0.16 15.93 ± 0.50 14.24 ± 0.54 0.70 ± 0.01 0.68 ± 0.01 3

0.68 ± 0.01

13.07 ± 0.40

L = 53.04 a = 2.8 b = 36.09

PET Glass

L = 53.04 a = 2.80 b = 36.09 0.14 ± 0.02

PP PET

0.14 ± 0.02 0.14 ± 0.02

Glass PP

2.25 ± 0.05 2.25 ± 0.05

PET Glass

2.25 ± 0.05 12.92 ± 0.44

PP PET

12.92 ± 0.44 0.63 ± 0.01

Glass PP

0.63 ± 0.01

PET

0.63 ± 0.01

Glass

nez, 1975). Peroxide value measures the primary oxidation products of lipids (hydroperoxides) while K232 measures conjugated dienes and their oxidation products which absorb at k = 232 nm. In turn K270 measures conjugated trienes and secondary products of oxidation (carbonyl compounds) which absorb at k = 270 nm. For olive oil stored in PET the acidity values were within the limit even after 12 months of storage. PV values exceeded the limit after 9 months, K232 after 3 months and K270 after 6 months of storage. For olive oil stored in PP the acidity values exceeded the limit after 9 months, PV values after 6 months, K232 after 3 months and K270 for a period less than 3 months of storage. Color was unaffected (P > 0.05) both by packaging material oxygen transmission rate and storage time (data not shown). A first conclusion to be drawn is that olive oil will retain its quality characteristics packaged in clear glass if stored in the dark at 22 °C for ca. 6 months. Respective shelf life for PET and PP packaged olive oil is 3 months and <3 months. Given the OTR of glass (negligible), PET [0.9 mL/(package day)] and PP [15.6 mL/(package day)] and excluding the effect of light, the difference in packaging material OTR is reflected in product shelf life. Of course it has to be stressed that sources of oxygen besides packaging wall permeability are (a) the amount of oxygen dissolved in the oil and (b) the headspace which in the present study was less than or equal to 10 mL. Given that the above two parameters were the same for all containers used their relative contribution may be neglected. Sacchi et al. (2008) evaluated quality parameters of EVOO packaged in PET, PET + oxygen scavenger and glass bottles and found differences in initially dissolved oxygen (higher in glass which was related to different bottling lines used for glass and PET bottles). Oxygen concentration, however, remained constant (ca. 0.4 ppm) between 3 and 6 months of storage for all packaged samples leading to the conclusion that the use of oxygen scavenger did not give rise to significant differences in quality parameters of EVOO during the above period of storage. Dissolved oxygen was not measured in the present study but given the fact that all containers were filled manually it is logical to expect the same amount of dissolved oxygen in all samples. Quality parameter values (acidity, PV, K232, K270) of the present study are in general agreement to those of Sacchi et al. (2008) for storage up to 6 months, given the differences in experimental conditions used by these authors. Present results with regard to glass and PET are in general agreement with those of Kiritsakis and Dugan (1984), Del Nobile et al. (2003) and Coutelieris and Kanavouras (2005). They are also in general agreement with those of Gambacorta et al. (2004) who evaluated quality parameters of EVOO packaged in glass, PET, PET + 1% oxygen scavenger (OS), PET + high barrier coating (BC) and PET + OS + BC and showed that acidity, PV, K232 and K270 maintained lower values in high barrier containers than in PET in the dark both at 22 and 37 °C. They differ, however, than those in the present study in that quality parameters tested remained within the set limits up to one year of storage. Such differences may due the fact that initial values of quality parameters tested in the present study were higher than those of the above study. 3.2. Effect of packaging material light transmission on olive oil quality

0

Color K270 K232 PV (meq O2/kg) Acidity (%) oleic acid Month

Table 1 Change of acidity, PV, absorption coefficients and color of olive oil stored in the dark at 22 °C as a function of packaging material, oxygen transmission rate and storage time.

12.92 ± 0.44

PP

G. Pristouri et al. / Food Control 21 (2010) 412–418 L = 53.04 a = 2.8 b = 36.09

414

Olive oil quality parameter values for clear, PET, PET + UV absorber and PET + aluminum foil are given in Table 2 as a function of storage time. Results show that for olive oil packaged in clear PET at 22 °C and intermittently exposed to daylight the acidity exceeded the limit of 0.8% after 6 months, PV exceeded the limit of 20 meq O2/kg after 9 months; while K232 and K270 exceeded the limit 2.50 and 0.22, respectively, for a period less than 3 months. For olive oil packaged in PET + UV blocker the acidity and PV did not exceed the set limits even after 12 months; while K232 and K270 exceeded the limits after 3 months of storage. Finally, for olive

L = 53.45 a = 4.16 b = 36.34 L = 60.85 a = 2.58 b = 41.64 L = 60.93 a = 2.76 b = 41.51 0.24 ± 0.01

60.22

0.37 ± 0.02

62.50

2.88 ± 0.01

620

19.24 ± 0.30

Adopted limits (EU Regulation 1989/2003)

60.8%

0.75 ± 0.02

21.98 ± 0.31 0.80 ± 0.03 0.85 ± 0.01 12

415

oil packaged in PET + aluminum foil the acidity and PV did not exceed the limit even after 12 month; K232 exceeded the limit after 6 months while K270 exceeded the limit after 9 months of storage. Color was affected (P < 0.05) in samples of olive oil packaged in both clear PET and PET + UV bottles after 6 months of storage. A first observation to be made is that olive oil exposed intermittently to light at 22 °C will retain its quality characteristics packaged in clear PET for a period of less than 3 months. Respective shelf lives of samples packaged in PET + UV blocker and PET + aluminum foil is 3 and 9 months. It is noteworthy to mention that the UV blocker used in the PET bottles provided only partial protection to the product from light while complete protection from light (i.e. using aluminum foil) provided an adequate retention of olive oil quality for at least 9 months. The discrepancy between PV and K232 values is probably related to the fact that the former measures hydroperoxides while the latter measures hydroperoxides plus conjugated dienes. According to the literature (IOOC, 1996) PV and K232 correlate rather well during the early stages of oxidation, something that was not shown in this study. Similar findings on the effect of light on olive oil quality were reported by Coutelieris and Kanavouras (2005) and Kanavouras, Hernandez-Münoz, and Coutelieres (2004). They are also in agreement with those of Coltro, Padula, Segantini Saron, Borghetti, and Penteado Buratin (2003) who reported that the addition of 0.08% of Tinuvin 326TM UV absorber is probably enough to provide the studied PET bottles with the light barrier characteristics necessary to maintain stability of olive oil for 6 months at 25 °C. 3.3. Effect of storage temperature on olive oil quality

18.81 ± 0.59

3.18 ± 0.09

2.90 ± 0.06

0.37 ± 0.02

L = 51.94 a = 3.15 b = 35.35 L = 57.80 a = 2.86 b = 36.72 L = 57.50 a = 2.68 b = 36.19 0.19 ± 0.03 0.31 ± 0.03 2.71 ± 0.22 9

0.73 ± 0.01

17.53 ± 0.10 0.79 ± 0.01 0.82 ± 0.01

19.20 ± 0.20

17.47 ± 0.10

2.90 ± 0.10

2.87 ± 0.10

0.39 ± 0.01

L = 51.36 a = 3.12 b = 34.95 L = 53.38 a = 2.46 b = 36.19 L = 53.82 a = 2.73 b = 36.63 0.16 ± 0.05 0.29 ± 0.10 2.69 ± 0.20 6

0.69 ± 0.01

16.90 ± 0.52 0.73 ± 0.01 0.73 ± 0.01

17.35 ± 0.20

16.25 ± 0.20

2.85 ± 0.10

2.93 ± 0.10

0.31 ± 0.01

L = 50.77 a = 3.47 b = 34.56 L = 54.61 a = 2.52 b = 36.44 L = 54.01 a = 2.48 b = 36.86 0.14 ± 0.01 0.27 ± 0.05 2.66 ± 0.22 3

0.65 ± 0.01

14.70 ± 0.50 0.70 ± 0.02 0.69 ± 0.01

15.45 ± 0.50

14.00 ± 0.20

2.82 ± 0.10

2.65 ± 0.20

0.31 ± 0.01

L = 53.04 a = 2.8 b = 36.09 L = 53.04 a = 2.8 b = 36.090 L = 53.04 a = 2.80 b = 36.09 0.14 ± 0.02 0.14 ± 0.02 2.25 ± 0.05 0

0.63 ± 0.01

12.92 ± 0.44 0.63 ± 0.01 0.63 ± 0.01

12.92 ± 0.44

12.92 ± 0.44

2.25 ± 0.05

2.25 ± 0.05

0.14 ± 0.02

Clear PET PET + UV blocker Clear PET PET + Alum. foil PET + UV blocker

K270 K232

Clear PET PET + Alum. foil PET + UV blocker

PV (meq O2/kg)

Clear PET PET + UV blocker

PET + Alum. foil Acidity (%) oleic acid

Clear PET

Month

Table 2 Change in acidity, PV, absorption coefficients and color of olive oil packaged in PET bottles at 22 °C as a function of packaging material, light transmission rate and storage time.

PET + Alum. foil

Color

PET + UV blocker

PET + Alum. foil

G. Pristouri et al. / Food Control 21 (2010) 412–418

Olive oil quality parameter values for PET as a function storage temperature and time are given in Table 3. Results show that for olive oil packaged in clear PET, stored in the dark at 13 °C the acidity and PV did not exceed the adopted limits of 0.8% and 20 meq O2/kg oil, respectively, even after 12 months of storage; K232 exceeded the limit of 2.5 after 6 months while K270 exceeded the limit 0.22 after 9 months of storage. For olive oil stored at 22 °C both the acidity and PV did not exceed the respective limits even after 12 months of storage; K232 exceeded the limit after 6 months while K270 exceeded the limit after 9 months of storage. Finally for olive oil stored at 35 °C both the acidity and PV exceeded the respective limits after 9 months while both K232 and K270 exceeded the respective limits for a period less than 3 months of storage. By plotting the change in PV over time the peroxidation reaction rate constant (k) was calculated. Then by plotting ln(k) as a function of 1/T the activation energy for peroxidation of EVOO was calculated to be 8.004 kJ/mol (Fig. 1). Color was not substantially affected (P > 0.05) by storage temperature given that olive oil was stored in the dark. Thus olive oil stored in clear PET in the dark will retain its quality characteristics for a period of 6 months at 13 °C and 22 °C and for less than 3 months at 35 °C. Similar effect of storage temperature on olive oil quality were reported by Gambacorta et al. (2004) and Kanavouras and Coutelieris (2006) who showed that elevated temperatures caused substantial deterioration in olive oil quality but less than that caused by light. 3.4. Effect of container headspace volume on quality of olive oil Olive oil quality parameter values for clear PET stored in the dark at 22 °C are given in Table 4 as a function of headspace volume and storage time. The experiment was designed so as to remove the first 200 mL of olive oil after 3 months of storage (resulting headspace equal to 200 mL), the following 200 mL of oil after 6 months (resulting headspace equal to 400 mL) and final-

60.22 62.50

0.32 ± 0.03 0.25 ± 0.02 0.25 ± 0.02 3.27 ± 0.01 3.00 ± 0.11

620

0.73 ± 0.01

60.8%

12

Adopted limits (EU Regulation 1989/2003)

0.77 ± 0.02

0.83 ± 0.01

16.81 ± 0.50

20.60 ± 0.20

23.61 ± 0.40

3.10 ± 0.02

0.28 ± 0.02 0.24 ± 0.03 0.21 ± 0.01 3.16 ± 0.02 3.00 ± 0.02 0.71 ± 0.01 9

0.73 ± 0.01

0.78 ± 0.02

16.00 ± 0.40

18.43 ± 0.40

19.66 ± 0.30

3.00 ± 0.05

0.26 ± 0.05 0.22 ± 0.01 0.21 ± 0.01 3.04 ± 0.20 2.50 ± 0.14 0.71 ± 0.01 6

0.72 ± 0.01

0.73 ± 0.02

15.40 ± 0.14

17.53 ± 0.22

18.48 ± 0.10

3.00 ± 0.20

0.24 ± 0.01 0.20 ± 0.01 3.03 ± 0.07 2.30 ± 0.10 0.70 ± 0.01 3

0.70 ± 0.02

0.71 ± 0.01

14.38 ± 0.10

15.60 ± 0.20

15.90 ± 0.20

2.50 ± 0.20

0.19 ± 0.10

L = 53.04 a = 2.80 b = 36.09 L = 52.44 a = 2.34 b = 35.67 L = 52.75 a = 2.46 b = 35.89 L = 51.79 a = 2.87 b = 35.21 L = 52.84 a = 2.95 b = 35.51 L = 53.04 a = 2.80 b = 36.09 L = 51.32 a = 2.68 b = 34.82 L = 51.18 a = 2.40 b = 33.42 L = 52.59 a = 2.72 b = 35.39 L = 51.78 a = 2.88 b = 34.80 0.14 ± 0.02 0.14 ± 0.02 2.25 ± 0.05 2.25 ± 0.05 12.92 ± 0.44 0.63 ± 0.01

Table 4 Changes in acidity, PV, absorption coefficients and color of olive oil packaged in clear PET at 22 °C in the dark as a function of headspace volume and storage time.

0.63 ± 0.01

0.63 ± 0.01

Fig. 1. ln(k) versus 1/T plot for peroxidation o EVOO packaged in PET bottles.

0

22 °C

12.92 ± 0.44

12.92 ± 0.44

2.25 ± 0.05

0.14 ± 0.02

22 °C Color

13 °C 35 °C 22 °C 13 °C 35 °C 22 °C

K270 K232

13 °C 35 °C 22 °C

PV (meq O2/kg)

13 °C 13 °C

35 °C Acidity (%) oleic acid Month

Table 3 Change in acidity, PV, absorption coefficients and color of olive oil stored in PET bottles in the dark as a function of temperature and storage time.

L = 53.04 a = 2.80 b = 36.09 L = 52.41 a = 2.87 b = 35.32 L = 51.98 a = 2.95 b = 35.38 L = 52.40 a = 2.77 b = 35.63 L = 53.04 a = 2.90 b = 36.08

G. Pristouri et al. / Food Control 21 (2010) 412–418

35 °C

416

Months/ headspace (mL)

Acidity (% oleic acid)

PV (meq O2/kg)

K232

K270

Color

0/0

0.63 ± 0.01

12.92 ± 0.44

2.25 ± 0.05

0.14 ± 0.02

L = 53.04 a = 2.80 b = 36.09

3/0

0.70 ± 0.02

15.60 ± 0.20

2.50 ± 0.20

0.20 ± 0.01

L = 52.36 a = 2.97 b = 35.64

6/200

0.72 ± 0.02

18.48 ± 0.22

3.14 ± 0.10

0.22 ± 0.01

L = 52.35 a = 3.16 b = 35.64

9/400

0.77 ± 0.02

32.20 ± 0.20

3.24 ± 0.10

0.27 ± 0.01

L = 52.14 a = 2.92 b = 35.50

12/600

0.85 ± 0.02

37.19 ± 0.50

3.26 ± 0.10

0.33 ± 0.02

L = 53.39 a = 3.61 b = 36.35

ly the following 200 mL of oil after 9 months (resulting headspace volume equal to 600 mL). Results show that for olive oil packaged in clear PET held in the dark at 22 °C the acidity exceeded the adopted limit of 0.8% after 9 months (headspace 400 mL); PV exceeded the limit of 20 meq O2/kg oil after 6 months (headspace 200 mL); K232 exceeded the limit of 2.5 after 3 months (headspace 0 mL) and K270 exceeded the limit of 0.22 after 6 months (headspace 200 mL). Color was not affected (P > 0.05) by headspace volume and storage time under the present experimental conditions. Thus even with no headspace auto-oxidation proceeded to such an extent that K232 reached its upper acceptable limit (2.50) for extra virgin olive oil after 3 months of storage. This most probably may be attributed to oxygen permeation through the PET bottle walls and possibly to the action of oxygen dissolved in the oil (Del Nobile et al., 2003). What should be stressed is the fact that PV increased exponentially after 9 months of storage when the headspace volume was P400 mL. This has also been reported by Bauer-Plank and Steenhorst-Slikkerveer (2000). Present data on the effect of headspace volume on the quality of olive oil are in general agreement with those of Del Nobile et al. (2003) who showed that by reducing either the bottle volumetric capacity or the oxygen partial pressure in the bottle headspace the quality decay kinetics of olive oil slowed down. Cecchi, De Marco, Passamonti, and Pucciarelli (2006) evaluated EVOO quality packaged in PET bottles by filling up to one third of their volume and exposing the product to daylight for 2 months at 20 °C. They

G. Pristouri et al. / Food Control 21 (2010) 412–418

reported substantial losses in quality beyond the acceptable level from both the analytical and sensory points of view. 3.5. Comparison of relative effect of parameter investigated on quality of olive oil 3.5.1. Acidity Acidity is used as a criterion for the categorization of olive oil. Despite this fact acidity is not a good indicator of olive oil quality since for instance an olive oil with a relatively high acidity may possess a highly desirable aroma whereas an olive oil with a low acidity may lack in aroma (Kiritsakis, 1998). The smallest increase in acidity after 12 months of storage was observed in glass and clear PET at 13 and 22 °C stored in the dark. Alternatively, similarly low values for acidity were recorded for PET + UV and PET covered with aluminum foil at 22 °C in the presence of light. Increase in temperature (from 22 to 35 °C) in the dark, increase in headspace (from 0 to 600 mL) in the dark, use of PP containers in the dark at 22 °C, or clear PET under light at 22 °C resulted in the highest acidity values (0.83–0.85) after 12 months of storage. Results with regard to high storage temperatures and the presence of headspace are in agreement with those of Gutiérrez and Fernández (2002). Plastics (PP and to a lesser degree PET) exhibit a higher tendency for hydrolysis of triglycerides which can be justified by their higher oxygen permeability as compared to glass. A high oxygen concentration results in a high rate of hydroperoxide formation and decomposition leading to the formation of carboxylic acids, responsible for an increase in acidity (Velasco & Dobarganes, 2002). Of the two plastics PET provided a better protection to olive oil than PP due to its significantly lower OTR. These results are in agreement with those of Tawfik and Huyghebaert (1999). Despite the negligible OTR of glass its permeation to light enhanced decomposition of triglycerides increasing olive oil acidity as postulated above. This phenomenon was inhibited by storing glass containers in the dark. A similar effect was reported by Méndez and Falqué (2007). Olive oil deterioration in the presence of light is enhanced by trace constituents such as chlorophyll which are excited through the absorption of light. Subsequently they transfer this excess energy to ground state triplet oxygen to form the excited state singlet oxygen which readily reacts with free fatty acids (Hamilton, Kalu, Prisk, Padley, & Pierce, 1997). 3.5.2. Peroxide value (PV) Reactions contributing to an increase in PV are auto-oxidation and photo-oxidation the former occurring in the absence of light while the latter occurs in the presence of light. Initial PV value of olive oil was 12.92 meq O2/kg. During the initial stages of storage in the dark, oxidation proceeds in the product due to the oxygen dissolved in the oil (Del Nobile et al., 2003). PV values after 12 months of storage were 18.86 for glass, 20.61 for clear PET and 22.54 meq/kg for PP in the dark at 22 °C. Increase in PV values is solely due to auto-oxidation. Respective PV values in presence of light at the same temperature were 21.98 (for clear PET), 19.24 (for PET + UV) and 18.81 meq O2/kg (for PET + aluminum foil). Thus for PET the effect of light is more pronounced than that of OTR. After 12 months of storage PV values for PET in the dark were 16.81, 20.60 and 23.61 meq O2/kg at 13, 22 and 35 °C, respectively. That is, the effect of temperature under specific experimental conditions was more pronounced than that of both light and oxygen. Finally, large headspace volumes (i.e. 400–600 mL) resulted to a drastic increase of PV values after 12 months of storage (i.e. 32.20 and 37.19 meq O2/kg, respectively) and indicate that under extremely high concentrations of oxygen, olive oil quality drastically deteriorates. At such high oxygen headspace concentrations the effect of this parameter becomes the most critical compared to the other three parameters.

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The above results are in general agreement with those of Min (1998) who reported higher losses in olive oil quality stored under light as compared to those stored in the dark. Kiritsakis and Dugan (1984) also reported that PV values were higher for olive oil packaged in plastic containers as compared to those packaged in glass bottles in the dark. According to Interesse, Ruggiero, and Vitagliano (1971) photo-oxidation does not occur in olive oil stored in the dark at relatively low temperatures (i.e. 13–20 °C). Under such conditions natural pigments of olive oil (i.e. chlorophyll) act as antioxidants along with polyphenols protecting the product from oxidation. Contemporary trends in olive oil packaging include dark colored glass bottles and PET bottles which have incorporated oxygen scavangers (Del Nobile et al., 2003). 3.5.3. Absorption coefficients (K232, K270) The initial values for K232 and K270 were 2.25 and 0.14, respectively. K232 increased from 2.25 to 3.22 after 12 months of storage in the dark at 22 °C as a result of auto-oxidation caused primarily by oxygen transmission through the packaging material and secondarily by oxygen initially dissolved in the oil. K232 also increased to 3.18 in the presence of light, to 3.27 at 35 °C and to 3.26 in the presence to excess oxygen in the headspace (headspace volume 600 mL after 12 months of storage). Thus the most pronounced changes in K232 were caused by an excess of oxygen and temperature followed by container OTR and container light transmittance. K270 increased from 0.14 to 0.35 after 12 months of storage in PP in the dark at 22 °C as a result of auto-oxidation due to oxygen transmission through the package. It also increased to 0.37 in the presence of light, to 0.32 at 35 °C and to 0.33 in the presence of excess oxygen in the container headspace (HS = 600 mL) after 12 months of storage. Thus all four parameters: oxygen transmission, light transmission, temperature and HS volume resulted in similar changes of K270. Present results with regard to K270 are somewhat different than those of Kanavouras and Coutelieris (2006) who reported that light had a more pronounced effect on deterioration of olive oil quality (production of higher amounts of hexanal) than elevated temperatures. Lastly with regard to olive oil color there were no statistically significant (P > 0.05) changes observed under specific experimental conditions of storage with the exception of olive oil exposed to light. Similar results were reported by Morelló, Motilva, Tovar, and Romero (2004) regarding the color parameter b (yellowness) of olive oil. Changes in olive oil color are related to the decomposition of chlorophylls during photo-oxidation (Kiritsakis, 1998).

4. Conclusions Based on the above data the following conclusions may be drawn: (a) Containers with high OTRs such as PP, PE are not suitable for the packaging of olive oil. (b) UV filters in PET bottles do not substantially contribute to the retention of olive oil quality. (c) Packaging olive oil in low OTR bottles such as PET does not effectively protect the product beyond 3 months in the presence of light. (d) Large HS volumes should be avoided indicating the need for consumption of olive oil within a given container as soon as possible. (e) The most appropriate material for olive oil packaging is glass followed by PET and preferably a dark colored container to be stored in the dark at temperatures less than or equal to 22 °C. Under such conditions the shelf life of extra virgin olive oil is 6 months.

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(f) The relative contribution of parameters studied to the retention of olive oil quality are: temperature  light > container headspace > packaging material oxygen transmission rate. This sequence changes when HS volume becomes P400 mL (headspace volume/total volume P 4/5 = 0.8).

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