The effect of storage conditions on the stability of ...

Proceedings of the Nutrition Society of New Zealand, 2000, Vol. 25

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The effect of storage conditions on the stability of peroxide values of New Zealand grown walnuts Christine Stark1, 2, D.L. McNeil2 and G.P. Savage3 Fachhochschule Wiesbaden, Studienort Geisenheim, Germany, 2 Soils Plants & Ecological Sciences Division and 3Food Group, Animal and Food Sciences Division, Lincoln University, Canterbury 1

ABSTRACT Storage stability of three locally grown and produced walnut oils (wild, grafted, kernelz and Cultivar 149) was determined by laboratory testing of their peroxide values. Oils were stored at high (60oC) and moderate temperatures (room temperature, 24oC), either in the dark or in the light. Different coloured bottles and three levels of vitamin E addition (no addition; 60 mg addition per bottle; 180 mg addition per bottle) were used to give information about the effects of anti-oxidants and light on the oxidative rancidity of walnut oil. Peroxide value (PV) was used as an objective measurement of oxidation. The results indicated the tested walnut oils had a reasonably long shelf life probably due to their naturally high contents of antioxidants. Differences among oil types and storage conditions in storage stability could be determined by laboratory methods. Further studies to select a cultivar with optimum oil qualities and comparisons to hedonic and UV absorbence changes in the oil would be useful. INTRODUCTION Other than a previous report from this trial looking at hedonic testing of stability (Stark et al., 2000) there are no published studies on the stability of walnut oil during storage, although there have been studies about the oxidative stability of other edible oils such as olive oil. Stability in these other oils are affected by the fatty acid composition and the anti-oxidant content (Antoun and Tsimidou, 1997; Delacuesta et al., 1996; Cinquanta et al., 1997). The qualitative and quantitative composition of both walnuts and walnut oils, and the storage stability of walnut kernels have also been studied (Maté and Krochta, 1997; Zambon et al., 1993; Savage et al., 1998). These studies have shown that walnuts and their oils hold a positive nutritional advantage (Zambon et al., 1993), probably due to their large amount of polyunsaturated fatty acids (Hsieh and Kinsella, 1989; Savage et al., 1998), particularly linoleic acid, an essential fatty acid for humans. However, when the factors affecting storage stability and oxidative rancidity of the oils are considered the high levels of polyunsaturated fatty acids are likely to be a disadvantage as the autoxidation rate for oils increases with the degree of unsaturation (Lee, 1983). There are thus reasons why walnut oil may show a degree of instability during storage. Therefore, it is in the consumer’s, oil producers’ and sellers’ interests to purchase walnut oil with a storage life as long as possible. Equally important, they need to know what the likely storage life of the oil is and how that life may be extended. Two environmental characters have been shown elsewhere to affect oil breakdown these are light and temperature (Vaisey-Genser et al., 1994; Matthäus, 1996). It is therefore possible that reduced light inputs (e.g., through sale of the oil in different coloured bottles) may be a positive marketing strategy for the producers. Similarly recommendations of storage life under different temperature regimes would be of use to consumers. Levels of anti-oxidants may also be altered either genetically or by blending e.g. with hazelnut oil which is high in vitamin E (Savage et al., 1997) to improve vitamin E contents and thus these oils with enhanced vitamin E may have improved storage qualities.


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The objective of this study was to determine the stability of walnut oil during storage by measuring the PV. Other studies looked at the hedonic stabilities of the oil (Stark et al., 2000) and UV absorbance changes of the oil (Stark, 1999) under the same storage conditions. Ultimately a comparison will be undertaken among all of these characteristics. Five different factors were examined: 1) the influence of light by using different coloured bottles; 2) the influence of temperature by storing at room temperature and at 60oC; 3) the influence of an additional anti-oxidant by adding of two different amounts of α-tocopherol; 4) the influence of the oil origin by testing three types of oil; and 5) the influence of time (the oils were stored for up to four months and tested at intervals). MATERIALS AND METHODS The oils and storage conditions Three different walnut oils were obtained as the first cold press of ground walnut kernels purchased from “A Cracker of a Nut”, West Melton, Canterbury. The oils used were: 1) kernelz, oil extracted from grafted walnuts selections (McNeil, 1999), 2) wild, oil extracted from seedling tree walnuts, and 3) Cultivar 149, oil extracted from this specific line. The oils were pressed at the end of August 1998 and stored in the dark until the beginning of the trial in October 1998. The oils were stored on the windowsill in five different coloured bottles, for 115 days from the14th of October 1998 until the 5th February 1999. The bottles were 2/3 full (230 ml). The bottle colours were green, amber, brown, clear and dark brown. A set of three bottles was kept in the dark at 24oC for the same storage period. The window was on the east side of the building giving six hours exposure to the morning sun. Three bottles of each oil, each with different additions of vitamin E ((+) α-tocopherol mixed isomers (Sigma Chemicals, USA), 1000 IU vitamin E/g; 670 mg d-α-tocopherol/g, nonα-tocopherol-content 5-20 mg/g) were stored under accelerated storage conditions at 60oC for 92 days from 6th November 1998 until 5th February 1999. The sets either contained no additional vitamin E, an addition of 60 mg/bottle or 180 mg/bottle in addition to a natural content of the oil of approximately 60 mg/bottle. These amounts refer to the approximate amount of vitamin E originally contained in walnut oil assuming a content of 250 µg vitamin E/g walnut oil (Savage et al., 1998). Sampling The oils on the windowsill and the set stored in the dark were sampled into Eppendorff tubes (1.8 ml) every 14 days during the first six weeks and every three weeks for the rest of the storage period. From the oils stored at 60oC, samples were taken three times a week for 21 days, once a week for 34 days and after that every two weeks. The Eppendorff tubes were stored in the freezer at -18oC until analysis for peroxide value, could commence. Temperature The temperature on the windowsill was recorded every 30 minutes by a Tinytag® miniature datalogger (Gemini Data Loggers (UK), Ltd.) and the data were processed using OTLM 1.41 (©1994-8, Gemini Data Loggers (UK), Ltd.) for Windows. Peroxide value The peroxide value was determined following the International Dairy Federation standard method (74A, 1991) based on Labuza and Schmidl (1988). An oil sample of 0.3 g was diluted with chloroform/methanol (70:30) to a 1:100 dilution. From this dilution, 0.1 g, 0.2 g and 0.3 g were weighed into 15 ml Kimax test tubes covered with aluminium foil and made up


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to 10 ml with chloroform/methanol (70:30) respectively. Following this, 0.05 ml ammonium thiocyanate solution (30 g/100ml) was added and the entire solution mixed. The absorbance was measured at 500 nm (Unicam 8625 UV/VIS spectrometer) against a chloroform/methanol control. After adding 0.05ml of iron-II-chloride solution (0.35 g FeCl3. 4H2O in 100ml distilled water and 2 ml 10mol/l HCl) the mixed solution was allowed to stand for 5 minutes in the dark. The increase in absorbance was measured against chloroform/methanol as blank solution at 500 nm. The absorbance of a reagent blank (9.9 ml chloroform/methanol, 0.05 ml ammonium thiocyanate solution and 0.05 ml iron-II-chloride solution plus waiting time of five minutes) was also measured against chloroform/methanol solution at 500 nm. The peroxide value of the oil was expressed as milliequivalents (meq)oxygen/kg oil. Statistics The data were analysed using correlation and general linear model analysis of variance in Minitab® for Windows Release 9.2 (©1993, Minitab Inc., USA). RESULTS Temperature Figure 1 shows minimum and maximum temperatures on the windowsill as well as the temperature in the dark storage over a period of 115 days as recorded by the datalogger. While the temperature in the dark stayed constant at 24oC the maximum temperature on the windowsill varied from 22 to 42oC and the minimum temperature from 13 to 24oC. The oil temperature measured inside the bottles varied from 20 to 56oC depending on time and weather conditions, but not on bottle colour. Figure1: Temperature recorded on the windowsill and in the dark over the storage period. Temperature in degree C 45.0

maximum temp.

40.0 35.0

minimum temp.

30.0

temperature in the dark

25.0 20.0 15.0 10.0 5.0 0.0 0

20

40

60 Storage period in days

80

100

120


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Peroxide values The different oils started with similar peroxide values (between 4.7 and 6.2) and the slope of the initial change with time was also similar (Figure 2). The peroxide values of Cultivar 149 and wild reached their peaks after 60 days (Cultivar 149: 24 meq oxygen/ kg oil; wild: 36 meq oxygen/kg oil) then dropped. However, the peroxide value of kernelz kept rising slowly up to 48 Meq oxygen/ kg oil at 115 days. The peroxide values of the oils stored in the clear bottles had lower peaks (kernelz: 26; Cultivar 149: 15; wild: 12 meq oxygen/kg oil) but similar trends (Figure 3). Figure 2: Peroxide values of three different cultivars over 115 days storage in dark brown bottles on the windowsill. PV in meq Oxygen/kg oil 60.0 kernelz cultivar

50.0

wild 40.0 30.0 20.0 10.0 0.0 0

20

40

60

80

100

120

140

Storage period in days

(±SD = 4.221; P = 0.008) The peroxide values of the oils stored in the dark show a different tendency (Figure. 4). The graphs indicate a linear increase of peroxide from the initial values of 5 (kernelz) and 6 (wild and Cultivar 149) meq oxygen/kg fat to 15 (kernelz), 16 (wild) and 20 (Cultivar 149) meq oxygen/kg fat. Figure 5 shows how the peroxide values changed under accelerated storage conditions without additional vitamin E. Both types of oil behaved similarly and the peroxide values drop down to 26 meq oxygen/kg fat (Cultivar 149) and 23 meq oxygen/kg fat (kernelz), respectively after reaching a peak of approximately 29 meq Oxygen/kg fat. The peroxide values of the oils with 60 mg vitamin E addition show similar trends but Cultivar 149 increased more strongly than kernelz (Figure 6). The peroxide value for Cultivar 149 rose to 41 whereas kernelz only reached a value of 25 meq oxygen/kg fat.


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The comparison of the peroxide value of the kernelz and Cultivar 149 stored at 60 C with 180 mg vitamin addition is shown in Figure 7. It shows a very similar development of peroxide in both oil types as was also shown in Figure 5. Figure 3: Peroxide values of the three different cultivars over 115 days storage in the clear bottle on the windowsill.

PV in meq Oxygen/kg oil

50.0

kernelz

40.0

cultivar wild

30.0 20.0 10.0 0.0 0

20

40

60

80

100

120

140


(±SD = 3.050; P = 0.179) Figure 4: Peroxide values of the three different cultivars over 115 days storage in the dark at 24oC. PV in meq Oxygen / kg oil 50.0

o

45.0

kernelz

40.0 35.0

cultivar

30.0

wild

24 oC

25.0 20.0 15.0 10.0 5.0 0.0 0

20

40

60

80


(±SD = 1.148; P = 0.005)

100

120

140


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The peak in peroxide value was reached after 57 days (45 meq oxygen/kg fat) with final values of 37 meq oxygen/kg fat (kernelz) and 34 meq oxygen/kg fat (Cultivar 149). Figure 8 shows how the different vitamin E contents influenced the peroxide values. All peroxide values behaved similarly (rising then falling). However, the oil with the highest vitamin E content experienced the most drastic changes in peroxide value (up to 45 meq oxygen/kg oil; drop down to 37 meq oxygen/kg oil). The peroxide value of the oil with low or no vitamin E addition stayed on a lower level (about 24 meq oxygen/kg fat). Figure 5: Changes in peroxide values of kernelz and Cultivar 149 stored at 60oC without vitamin E addition. PV in meq oxygen / kg oil 50.0 45.0 40.0

kernelz

35.0

cultivar

30.0 25.0 20.0 15.0 10.0 5.0 0.0 0

10

20

30

40

50

60


(±SD = 0.772; P = 0.001)

70

80

90

100


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Figure 6: Changes in peroxide values of kernelz and Cultivar 149 stored at 60 C over 92 days with a vitamin E addition of 60 mg per bottle. PV in meq oxygen / kg oil 50.0 45.0 kernelz

40.0

cultivar

35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0

(±SD = 3.571; P = 0.034) Figure 7: Changes in peroxide values of kernelz and Cultivar 149 stored at 60oC over 92 days with a vitamin E addition of 180 mg per botttle PV in meq oxygen / kg oil 50.0 45.0 40.0 35.0 30.0 25.0 20.0 kernelz

15.0 10.0

cultivar

5.0 0.0 0

10

20

(±SD = 1.099; P = 0.000)

30

40 S

50

60

70

80

90

100


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Figure 8: Changes in peroxide value of kernelz oil stored at 60 C with three different vitamin E contents.

PV in meq oxygen / kg oil 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0

no vitamin E addition

10.0

60 mg addition 180 mg addition

5.0 0.0 0

20

40

60

80

100


(±SD = 2.677; P = 0.001 DISCUSSION The fatty acid composition of walnut oil suggests that it is likely to oxidise and go rancid fairly rapidly (Zwarts et al., 1999, Savage et al., 1998; Fox and Cameron, 1995). However, this study proved that walnut oil was reasonably stable and not subject to PV deterioration over four months when stored under optimum conditions in the dark at room temperature. It is likely that the high natural anti-oxidant (vitamin E and sterol) contents of walnut oil plays an important role in achieving high storage stability (Savage et al., 1998). Oil stability is more influenced by anti-oxidants and the position of fatty acids within the triglycerides than by the fatty acid composition (Kaijser, 1998; Neff, et al., 1992). While the PV is commonly used to determine oxidative rancidity of oils (Satue et al., 1995; Antoun and Tsimidou, 1997) this study shows that the PV was not necessarily easy to interpret as a quality measure. Difficulties in interpretation arose mainly because few other studies on walnut oil are available. Thus a complete picture within which to place our data does not exist. More analytical determinations are needed alongside other methods to understand the relationship of PV to “quality”.


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Development of oxidative rancidity Figure 9: The three stages of oxidative rancidity (Fox and Cameron, 1995). 1) Initiation: formation of unstable free radicals (derived from triglycerides molecules from which the H-atom is removed under the influence of light, heat or a catalyst such as copper traces in the oil) Initiator ⇒ R. ⇑ heat, light catalyst 2) Propagation: The free radicals react with atmospheric oxygen to peroxy free radicals which react with unsaturated oil molecules to form unstable hydroperoxides and free radicals reacting with atmospheric oxygen (self-generated process). R. + O2 ⇒ RO2 . ⇒ R. + ROOH RO2 . + RH 3) Termination: hydroperoxides are very unstable and break down to ketones and aldehydes, causing off-flavours The reactions shown in Figure 9 progress until all oxygen is used or all free radicals are removed. Anti-oxidants inhibit the reaction by reacting with the peroxy free radicals so that they are no longer available. The reaction chain is dependent on the amount of oxygen available and eventually stops with low oxygen (Maté et al., 1996). The oils in this experiment were stored in sealed 2/3 full bottles under the same oxygen conditions. Taking samples every three days lead to a constant oxygen supply in the bottle; while the intervals between sampling got longer the headspace in the bottles increased which maintained oxygen supply. Influence of oil type Overall the results suggested that the three oils (kernelz, wild, Cultivar 149) have different storage stabilities although all three oils had the same initial peroxide values (these varied by only 1.5 units or 20%). Throughout all tests, the biggest differences were between kernelz and wild. As Cultivar 149 was a selected grafted variety it is reasonable that its oil was closer to the oil extracted from a mixture of selected grafted trees (kernelz). However, kernelz showed a different autoxidation rate to Cultivar 149 (Figure 2), probably due to different contents of anti-oxidants that have been shown to influence UV absorbance (Chen-Xiao Ying et al., 1998). It seems that Cultivar 149 has a higher oxidation rate than kernelz. The PV changes in Figures 3 and 4 also point in that direction: the peroxide value of Cultivar 149 rose to a higher value in one case and showed a stronger drop in the other. All oil types did, however, have similar initial slopes (Figures 2 and 5), which suggests that the three oils are influenced by light and heat in the same way during the initial stages of oxidation (Figure 9). Only after about 35 days did the peroxide value of Cultivar 149 experience a different development with lower values, which might indicate a lower content of free radicals, probably due to a different content and composition of fatty acids and anti-oxidants in this oil. The oxidation reaction therefore slows down and stops earlier. Other studies have reported differences in fatty acid composition, vitamin E and sterol contents in the different walnut cultivars, which went into the kernelz blend (Savage et al., 1998). International studies have also indicated differences among walnut oils (Greve et al., 1992; Garcia et al., 1994). These differences found for the oils suggest it may be possible to select a specific cultivar for oil, which has optimum properties for storage stability.


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Influence of light and temperature Storage in the different coloured bottles and in the dark affected the oils differently. The oil stored in the dark at 24oC experienced the least changes. The temperature profile in Figure 1 shows that the bottles on the windowsill were not only exposed to light but also to heat. Consequently the oil in the dark brown bottle, which blocked out the light, behaved similarly to the oils that were stored in the dark of the oven. However, some changes, such as the fall in PV, that occurred in the heated bottles did not happen to the oil in the dark brown bottle (Figure 2), probably due to less constant, shorter duration and milder temperatures than in the oven (maximum of 56oC for the windowsill compared with constant 60oC in the oven). The oils stored in the dark at a constant 24oC changed very little in PV (Figure 4). The oil in the clear bottle on the windowsill was the only one exposed to light and hence the only one exposed to photo-oxidation, which is influenced by UV radiation, as well as O2 dependent lipid oxidation. Photo-oxidation takes place in the presence of light and a sensitiser, eg. chlorophyll or riboflavin, (Gunstone and Norris, 1983). At the end of the trial the oils stored in the clear bottle appeared much lighter, almost colourless, similar to the oils that have been stored in the oven at 60oC (data not shown). Only the oils stored in the dark at room temperature and in the dark brown bottle kept their original colour. This suggests that high heat and particularly light have a colour destroying effect. Influence of additional antioxidant The different additional vitamin E contents did not have much effect on the storage stability of walnut oils (60 mg vitamin E = addition of approximately 250 ppm; 180 mg vitamin E = addition of approximately 750 ppm). The peroxide values of the oils with vitamin E addition were not significantly lower than those of the oil without additional vitamin E. Figure 8 shows a stronger dropdown for the oil with double vitamin E addition, which suggests that the reactions of oxidative rancidity have progressed further. The generally higher peroxide values of the oils with 180 mg of additional tocopherol might be explained by the fact that αtocopherol can have pro-oxidant effects in high concentrations. Satue et al. (1995) found out that a concentration of 500 ppm inhibits the anti-oxidant activities of α-tocopherol and has a pro-oxidant effect on olive oils measured using the peroxide value. An addition of 60 mg per bottle seems to have resulted in the same anti-oxidant properties as the oils without additional vitamin E. The PV stays at a high level and does not drop down. This suggests that the natural amount of vitamin E contained in walnut oil is already high enough to protect the oil from oxidation and an addition that doubles this natural content does not improve nor deteriorate the antioxidant effect. The larger addition may, however, be excessive. The peroxide values for the oil without vitamin E addition and with 180 mg addition behave similarly both for the oils of Cultivar 149 and kernelz. This indicates that the oil type does not affect the oxidation rate at these vitamin E levels. However, the oil with the 60 mg addition of vitamin E per bottle shows a different development between kernelz and Cultivar 149. For Cultivar 149 the peroxide value increases more strongly indicating a higher peroxide production. This suggests that the addition of 60 mg of vitamin E per bottle contributes positively to the natural amount of vitamin E contained in the kernelz-oil. Cultivar 149 has the lowest peroxide value of the three oils without any additional vitamin E. The graph of oil breakdown with time (Figure 5) does not show a drastic fall for Cultivar 149 which suggests that the amount of naturally contained anti-oxidants inhibits the autoxidation best in Cultivar 149. Consequently, the addition of vitamin E has prooxidant effects at both, high and low additions in this oil. There thus appear to be differences among the different oil types, especially in the content of anti-oxidants.


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CONCLUSIONS The peroxide values have indicated a range of environmentally induced effects as well as effects of oil types and time. They have also indicated that under reasonable storage conditions (room temperature in the dark) walnut oil is very stable by this measure in spite of its high level of polyunsaturated fatty acids. The major question of the stability will, however, only be answered when the peroxide values are compared to hedonic and UV absorbance data for these oils REFERENCES Antoun, N. and Tsimidou, M. (1997). Gourmet olive oils: stability and consumer acceptability studies. Food Research International 30 (2), 131-136; Elsevier, New York. Chen -Xiao Ying, Ahn, D.U. and Chen, X.Y. (1998). Antioxidant activities of six natural phenolics against lipid oxidation induced by Fe2+ or ultraviolet light. Journal of the American Oil Chemists’ Society 75 (12), 1717-1721. Christophersen, A.G. (1992). Storage life of frozen salmonoids. Effect of light and packaging conditions on carotenoid oxidation and lipid oxidation. Zeitschrift fuer Lebensmittel Untersuchung und Forschung 194 (2), 115-119. Bocca, A., Fabietti, F. and Pagano, M.A. (1990). Analytical characteristics of extra virgin olive oil in canned vegetables. Revista della Societa Italiana di Scienza dell’Alimentazione 19(1-2), 39-43. Cinquanta, L., Esti, M. and La Notte, E. (1997). Evolution of phenolic compounds in virgin olive oil during storage. Journal of the American Oil Chemists Society 74 (10), 12591264. Delacuesta, P.J.M., Martinez, E.R. Chaparro, M.G. (1996). Alpha-tocopherol antioxidant role in rancidity of edible oils from vegetable sources. Anales de Quirnica 92 (6), 370-374. Fox, B.A. and Cameron, A.G. (1995). Food Science, Nutrition & Health, 6th ed.; Edward Arnold; London, UK. Garcia, J.M., Agar, I.T. and Streif, J. (1994). Lipid characterisation in kernels from different walnut cultivars. Turkish Journal of Agriculture and Forestry 18, 195-198. Greve, C., McGranahan, G., Hasey, J., Snyder, R., Kelly, K., Goldhamerer, D. and Labavich, J. (1992). Variation in polyunsaturated fatty acids composition of Persian walnuts. Journal of American Society of Horticulture Sciences 117, 518-522. Hsieh, R.J. and Kinsella, J.E. (1989). Oxidation of polyunsaturated fatty acids. Advances in Food and Nutrition Research 43, 232-341. Kaijser, A. (1998). Stability and lipid composition of oils in macadamia nuts (Macadamia tetraphylla and M. integrfolia) grown in New Zealand. Institutionen för Livsmedelsvetenskap, nr. 86, Swedish University of Agricultural Sciences (Uppsala). Labuza, T.P. & Schmidl, M.K. (1988). Use of sensory data in the shelf life testing of foods: principles and graphical methods for evaluation. In: Vaisey-Genser, M., Malcolmson, L.J., Ryland, D., Przybylski, R., Eskin, N.A.M. and Armstrong, L. (1994). Consumer acceptance of canola oils during temperature-accelerated storage. Food Quality and Preference 5, 237-243. Lee, F.A. (1983), Basic food chemistry; 2nd ed.; AVI Pub. Co.; Westport, Conn. Maté, J.I. and Krochta, J.M. (1997). Whey protein and acetylated monoglyceride edible coatings: effect on the rancidity of walnuts. Journal of Agricultural and Food Chemistry 45 (7), 2509-2513. Maté, J.I., Saltveit, M.E.and Krochta, J.M. (1996). Peanut and Walnut rancidity: Effects of oxygen concentration and relative humidity. Journal of Food Science 61 (2), 465-472.


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Matthäus, B.W. (1996). Determination of the oxidative stability of vegetable oils by Rancimat and conductivity and chemiluminescence measurements. Journal of the American Oil Chemists’ Society 73 (8), 1039-1043. McNeil, D.L. (1999). The Rex Baker walnut trials at Lincoln University: Summary of growth and yields across 13 years. Southern Nut Growers Association: Health in a Shell 34: 3-7. Neff, W.E., Selke, E., Mounts, T.L., Rinsch, W, Frankel, E.N. and Zeitoun, M.A.M. (1992). Effect of triacylglycerol composition and structures on oxidative stability of oils from selected soybean germplasm. Journal of the American Oil Chemists' Society 69, 2, 111-118. Satue, M.T., Shu-Wen Huang and Frankel, E.N. (1995). Effect of natural antioxidants in virgin olive oil on oxidative stability of refined, bleached, and deodorized oil. Journal of the American Oil Chemists’ Society 72 (10), 1131-1137. Savage, G.P., McNeil, D.L. and Dutta, P.C. (1997). Lipid composition and oxidative stability of oils in hazelnuts (Corylus avellana L.) grown in New Zealand. Journal of the American Oil Chemists’ Society 74, 755-759. Savage, G.P., McNeil, D.L. and Dutta, P.C. (1998). Vitamin E content and oxidative stability of fatty acids in walnut oil. Proceedings of the Nutrition Society of New Zealand 23, 8190. Stark, C., Savage, G.P. and McNeil, D.L., (2000). Subjective preference tests on the storage stability of New Zealand walnut oils. Southern Nut Growers Association: Health in a Shell 37, 3-10. Stark, C. (1999). The storage stability of New Zealand walnut oils. Thesis submitted to the Fachhochschule Wiesbaden, Studienort Geisenheim in partial fulfilment of the degree Dipl. Ing. Gartenbau. 41pp. Vaisey-Genser, M., Malcolmson, L.J., Ryland, D., Przybylski, R., Eskin, N.A.M. and Armstrong, L. (1994). Consumer acceptance of canola oils during temperatureaccelerated storage. Food Quality and Preference 5 (4), 237-243. Zambon, D., Sabate, J., Munoz, S., Campero, B., Casals, E., Merlos, M., Laguna, J.C., Ros, E. (2000). Substituting walnuts for monounsaturated fat improves the serum lipid profile of hypercholesterolemic men and women - A randomized crossover trial Annals of Internal Medicine 132(7), 538-546. Zwarts, L., Savage, G.P., McNeil, D.L. (1999). Fatty acid content of New Zealand grown walnuts (Juglans regia L.). International Journal of Food Sciences and Nutrition 50(3): 184-194.