Qualitative Responses of “Nabali” Olive Oil to Harvesting Time and Altitudes at Sub-Humid Mediterranean

Abstract

To determine the effect of harvesting date and altitude on olive oil quality, a field experiment was done in two locations (400 and 700) meters above sea level. Olive fruits were sampled eight times and tested for oil quality characteristics. Oil content increased with delaying fruit harvest at high altitude. Oil content was greater at low altitude than high altitude. Oil quality was influenced by harvest date at both locations. Peroxide and acidity value increased significantly with late harvesting at both locations. Oil produced at lower altitude had higher acidities than oil produced at higher altitude. Saturated fatty acids (palmitic, stearic, and arachidic) and unsaturated (palmitoleic, oleic, linoleic, and linolenic) were influenced by harvesting dates and altitude, whereas palmitoleic acid was not affected at low altitude. Palmitic acid dominated the saturated fatty acids. The content varied with harvesting dates at both locations. Oleic acid was about 68% of the total fatty acids in oil produced from both locations. In general, a slight reduction in oleic acid was observed with advances in fruit ripening, whereas linoleic acid increased with fruit ripening at both altitudes. Linolenic and palmitoleic acids contents fluctuated among harvests. Oil produced from the higher altitude had a higher ratio of unsaturated to saturated fatty acid compare to oil produced at the lower altitude.

Keywords:

• Olives
• Cultivar Nabali
• Olive oil quality

INTRODUCTION

Nabali is an important olive cultivar successfully grown in Mediterranean. Fruits as a table olive can be stored longer time compared to other olive cultivars, and has an oil percent of a about 33% based on fresh weight. However, growers do not have a clue when to harvest their olives fruit for high oil quality and quantity. Olive oil plays a fundamental role in human nutrition in the Mediterranean.[1] Studies, have demonstrated the importance of olive oil quality in human health.[2] Olive oil quality is determined by both its chemical and organoleptic properties The variety of changes that occur in fruit constituents at ripening play a major role in determining the quality characteristics of olive oil.[3] Ripeness is a function of harvest time.[4] In general, riper olive fruits are more susceptible to hydrolytic and oxidative deterioration, which produces bitter oil.[5] However, olives harvested before or after fruits reach optimum weight and size suffer from a substantial loss in quantity and quality.[6]

Oil content of full mature olive fruit varies within the same cultivars in different locations. Nevertheless, the trend of oil accumulation is not affected by cultivars, rather by cultural and environmental condition.[7] Virgin olive oil is one of the few oils consumed without any further refining. Oil oxidation and hydrolysis causes unpleasant tastes and odors which will lower the quality and therefore price.[1] Moreover, oxidation may adversely affect oil nutritional value by destroying some essential fatty acids such as linoleic and linolenic and producing toxic substances.[1] Therefore, inhibiting rancidity in olive oil is of great value.[8] Fresh olive oil has a peroxide value below 10 meq O₂/kg oil, whereas rancidity develops when it exceeds 20 meq.[9] Oil acidity is considered as a basic oil quality characteristic for grading olive oil.[10] Olive oil is classified as edible or industrial, when the acidity is below or more than 3.3%, respectively.[11] Acidity of olive oils is determined by the quantity of free fatty acids—primarily oleic acid—expressed usually as percent oleic acid or gram of oleic acid/100 g of oil. In many countries, the acidity of olive oil varies between 0.2 to 10.22%, whereas in virgin olive oil it does not exceed 3.3% and if so olive oil would not be fit for human consumption and is recommended to be used for industrial purposes.[11] Cultivars, environmental conditions, degree of fruit ripeness, processing, storage,fruit health, some pigments, and phenols affect oxidative and hydrolytic rancidity.[12] Olive oil contains more oleic and less linoleic and linolenic acids than other oils, which makes olive oil more resistant to oxidation.[1,13] Furthermore, oil oxidation and hydrolytic processes increase slowly during fruit development.[14,15] An increase in oxidative rancidity as growth proceeds may result from the loss of natural antioxidants, particularly phenols, whereas the rise in acidity is attributed to the increase in endogenous lipase activity.[16] However, peroxide value and acidity did not change significantly during the fruit ripening cycle, but parasite infections as induced an increase in both parameters.[17] Low altitudes increased phenol content, which ultimately increased oxidative stability of the oil,[18,19] whereas oil acidity decreased at higher elevations.[18] Conversely, Tous and Romero[20] who detected that olive oil produced at higher altitudes displayed significantly higher stability, presumably due to the lower linoleic acid content in oils that developed at higher altitude. Infestation of olive fruits by D. oleae increased peroxide value to 6.9 meq O₂/kg oil.[21] Oil extracted from infested fruits with olive fruit fly was recorded to have 7 to 9 fold higher acidity than that of un-attacked fruits.[22]

Olive oil containing more than 73% oleic acid and less than 10% linoleic acid is graded as good quality.[12] Chemical proprieties of virgin oil, in particular fatty acids, are influenced by cultivars, environmental conditions, harvesting date, diseases, and cultural practices.[23] The objective of this study was to determine the effects of harvesting dates and elevation on oil quantity and quality of “Nabali” olive fruits.

MATERIALS AND METHODS

Two field experiments were conducted during 2001–2002 growing season at 32°59′ N, 35°79′ E in Soum 400 meters above sea level (low altitude) and at 32°08′ N, 35°82′ E in Om Al- Dananeer, Jordan at 700 meters (high altitude). The climate has a Mediterranean pattern with cold-humid winters and hot dry summer with lower temperature of 2–4°C at high altitude compare to low altitude. Forty-eight uniform 20 year-old “Nabali” olive trees were randomly selected. Trees were spaced 10 m × 10 m, grown under rain-fed conditions and received traditional pruning and plowing. Eight harvesting dates at two weeks intervals were started 15 Sept, and ended 30 Dec 2001, and arranged in a randomized complete block design (RCBD) with three replicates. On each harvest date 100 fruit were randomly picked from each tree (plot) once throughout the harvest dates.

Oil Extraction

Oil was extracted according to Fedeli and Testolin procedures.[24] Oil samples used for fatty acid analysis were prepared according to Christie.[25] A gas liquid chromatograph (Varian Gc. 3300, U.S.A) with a flame ionization detector (FID) used to analyze the lipid classes of the olive oil. Standards with methyl esterification (Sigma Chemical Company kit ME-14 catalogue 2000–2001 page 2156, U.S.A) used were dissolved in n-hexane (10 ml, HPLC, grade Hexan) to create standard solutions of methyl ester for oleic, palmitic, linoleic, stearic, palmitoleic, arachidic and linolenic acids with ratio of 0.7269, 0.1532, 0.1496, 0.0335, 0.0236, 0.0087, and 0.0171, respectively. These standards were prepared according to the assumed fatty acid content present in olive oil. Standard solutions and samples were analyzed for retention times in the gas liquid chromatograph. Acidity and Peroxide value were determined according to AOAC.[26]

Statistical Analysis

Data were subjected to analysis of variance (ANOVA). Collected data were analyzed using SAS Software (Ver. 6, Vol. 2, 4^th ed. SAS Institute).[27] Means were compared using the least significant differences (LSD) test at 0.05 probability level.

RESULTS AND DISCUSSION

Changes in oil content of “Nabali” olive fruit as affected by harvest time and altitudes are shown in Fig. 1. With progressive fruit maturation, oil content based on dry weight significantly increased, reaching the maximum 53.14 on 15 December, and 61.6% on 30 December, at high and low altitudes, respectively. Meanwhile, no significant differences in oil percent were obtained among sampling date of November 1, 15, and last sampling date at low altitude. However, oil percent at low altitude was greater at each harvest date than high altitude. Accumulation of oil in olive fruit was reported to endorse by long sunny and warm summer and hampered by rainy, foggy, and cool weather. Delaying harvests increase oil content.[28] This suggestion is only in agreement with the current results obtained at high altitude, however, at low altitude did not, in accordance with Beltrán et al.[29] who showed that delaying the harvesting period did not increase the oil yield. Nevertheless, early or late harvest will negatively affect both quality and quantity of olive oil.[18] With respect to dry matter, different olive cultivars showed increase in oil content during ripening which varies from 35 to 70%.[14] However, oil reported to be deposited in olive fruit at slow rate during development and maturity of fruit, followed by a pronounced increase during the ripening stage. The oil content increases slowly at the final stage of fruit ripening with reduction for a second time in over-mature fruits.[30] Although, when olive fruits have reached a certain point of maturity, an increase in oil percent is not entirely resulted from continuous triacyglycerol synthesis, but as a result of progressive loss of water from fruit particularly at the end of the season reaching its maximum between late part of November and January.[1] Humeid et al.[31] estimated an overall increase of 223% in oil when picking of “Nabali” olive fruits were delayed from the beginning of October to the end of December. Oil contents in mature fruits have been recorded to vary between 20–70%, based on dry weight, depending on cultivars, geographical region, health soundness of the fruits and growing conditions.[7] Oil content of olive fruit increased continuously toward fruit maturation and followed a similar pattern at different altitudes.[18] However, lower altitude exhibited greater oil contents than higher altitude which confirms finding recorded in this study.[32] The different pattern of oil accumulation observed in both altitudes could be determined by cultural practices and environmental conditions in accordance with results obtained by Lavee and Wodner.[7]

Figure 1 Percent oil (on dry weight basis) of Nabali olive fruits at 400 and 700 meters above sea level.

Tables 1 and 2 show the effect of altitude and harvest time on the fatty acid composition of “Nabali” olive oil. Olive fruit at ripening stage showed the following fatty acids: palmitic acids (C16:0), stearic acids (18:0), arachidic acids (20:0), palmitoleic acids (16:1), oleic acids (18:1), linoleic acids (18:2), and linolenic acids (18:3). Significant differences in palmitic acid were observed in oil produced from both altitudes at different harvests. Palmitic acid content (12.03 and 11.67 g/100 g oil) was highest in fruit from the third and first harvests at low and high altitudes, respectively. The response of palmitic acid synthesis to the degree of fruit ripening is variable. It is obvious that whenever palmitic acid decreased as linoleic acid increased. The content of palmitic acid tends to decline during fruit ripening, possibly as a result of dilution effect[14] and the increase in linoleic acid.[33,12] In contrast, delaying fruit harvest caused an increase in palmitic acid by the second half of Dec at high and low altitude.[18,34] In addition, a significant increase in stearic acid (2, 2.17) g/100 g oil was observed during the first half of December when palmitic acid (7.8, 7.83) was low, combined with a significant increase in linoleic acid (19.77, 19.33) and a decrease in oleic acid (67.3, 67.07) at high and low altitude respectively. Stearic acid increased at both altitudes as fruit maturity advanced. Marzouk and Cherif[28] reported the percentage of stearic acid increased 2–3% during fruit development. These results disagree with those obtained by Gutierrez et al,[14] who reported that stearic acid remained constant through fruit ripening. However, stearic acid has been demonstrated the effects of cultivar and elevation. It declined with increasing altitude, due to a rise in oleic acid content.[23] Variations in arachidic acid were also observed during fruit ripening. The maximum values of 0.67, 0.63, and 0.83, 0.87 g/100 g oil were recorded at the first and last harvest at high and low altitude respectively. Humeid et al.[31] reported an average of 1.1 g/100 g oil arachidic acids in “Nabali” olive oil that could be due to the differences in altitude. Palmitoleic acid content was significantly increased at high altitude as fruit matured, while it was not affected at low altitude, even though it followed the same trend as fruit at high altitude. On average, palmitolic acid was 1.5 and 1.45 g/100g oil at low and high elevation in accordance with results obtained by Humeid et al.[31] Marzouk and Cherif[28] observed stable palmitoleic acid content during fruit development. However, Cimato[34] reported differences in palmitoleic acid content among different altitudes.

Table 1 Effect of harvest date on fatty acids composition of “Nabali” olive oil grown at 700 m above sea level

	Fatty acid composition (g/100 g oil)
	Saturated				unsaturated
Date of harvest	Palmitic acid 16:0	Stearic acid 18:0	Arachidic acid 20:0	Total of saturated fatty acids %	Palmitoleic acid 16:1	Oleic acid 18:1	Linoleic acid 18:2	Linolenic acid 18:3	Total of unsaturated fatty acids %	Saturated:unsaturated fatty acids ratio Sat./unsat.
Sept.15	11.67 a	1.73 b	0.67 a	14.07 a	1.13 c	71.44 ab	12.43 f	1.00 b	86.00 c	0.16 a
Oct.1	8.57 bc	1.47 e	0.37 c	10.41 c	1.23 bc	73.33 a	13.90 e	1.10 ab	89.56 a	0.12 bc
Oct.15	8.90 bc	1.53 de	0.53 abc	10.96 bc	1.13 c	72.00 a	14.77 d	1.13 ab	89.03 ab	0.12 bc
Nov.1	9.80 ab	1.60 cd	0.53 abc	11.93 bc	1.77 a	68.70 bc	16.13 c	1.53 a	88.13 ab	0.14 abc
Nov.15	9.37 bc	1.57 cde	0.43 bc	11.37 bc	1.70 a	67.67 c	18.13 b	1.10 ab	88.60 ab	0.13 bc
Nov. 30	8.67 bc	1.67 bc	0.37 c	10.71 c	1.40 abc	68.67 bc	18.23 b	0.97 b	89.27 ab	0.12 bc
Dec.15	7.80 c	2.00 a	0.53 abc	10.33 c	1.60 ab	67.30 c	19.77 a	1.03 b	89.70 a	0.11 c
Dec.30	10.23 ab	1.93 a	0.63 ab	12.79 ab	1.60 ab	67.03 c	17.70 b	1.00 b	87.33 bc	0.15 ab
Average	9.38	1.69	0.51	11.58	1.45	69.52	16.38	1.11	88.45	0.13
Means followed by the same letter(s) within each coloumn are not significantly different at P ≤ 0.05 according to LSD test.

Table 2 Effect of harvest date on fatty acids composition of “Nabali” olive oil grown at 400 m above sea level

	Fatty acid composition (g/100 g oil)
	Saturated				Unsaturated
Date of harvest	Palmitic acid 16:0	Stearic acid 18:0	Arachidic acid 20:0	Total of saturated fatty acids %	Palmitoleic acid 16:1	Oleic acid	Linoleic acid 18:2	Linolenic acid 18:3	Total of unsaturated fatty acids	Saturated:Unsaturated fatty acids ratio Sat./unsat.
Sept.15	10.10 bc	1.93 bc	0.83 a	12.86 abc	1.27 a	69.47 ab	14.87 c	1.53 a	87.14 bcd	0.15 ab
Oct.1	8.50 cd	1.73 d	0.40 d	10.63 d	1.37 a	72.10 a	14.87 c	1.00 bc	89.34 a	0.12 c
Oct.15	12.03 a	1.83 cd	0.63 b	14.49 a	1.47 a	68.90 bc	14.23 c	0.97 c	85.57 d	0.17 a
Nov.1	11.47 ab	1.87 bcd	0.67 b	14.01 ab	1.63 a	67.07 bc	16.35 b	0.97 c	86.02 cd	0.16 a
Nov.15	10.03 bc	2.00 b	0.53 bcd	12.56 bc	1.63 a	66.10 c	18.33 a	1.37 a	87.43 bc	0.15 ab
Nov. 30	8.30 d	1.93 bc	0.47 cd	10.70 d	1.33 a	68.37 bc	18.37 a	1.27 abc	89.34 a	0.12 c
Dec.15	7.83 d	2.17 a	0.60 bc	10.60 d	1.63 a	67.07 bc	19.33 a	1.33 ab	89.36 a	0.12 c
Dec.30	8.43 cd	2.20 a	0.87 a	11.50 cd	1.63 a	66.67 bc	18.77 a	1.50 a	88.57 ab	0.13 bc
Average	9.59	1.96	0.63	12.17	1.50	68.22	16.89	1.24	87.84	0.14
Means followed by the same letter(s) within each coloumn are not significantly different at P ≤ 0.05 according to LSD test.

Oleic acid was statistically higher during the second harvest, followed by a slight decrease towards fruit ripening at both altitudes. These results are in accord with those obtained by Osman et al.[18] The reduction in oleic acid coincides with an increase in linoleic acid. Gutierrez et al.[14] found that the increase in linoleic acid is due to the continuing biosynthesis of triglycerides, and with the formation of oleic acid, the enzyme oleate desaturase is actively transforming oleic acid into linoleic acid, explaining the increase in linoleic acid as fruit ripens at both altitudes. Several studies showed an increase in linoleic acid with advanced fruit maturation.[33] Nabali cultivar exhibited an average oleic acid of 74.5 g/100 g oil among different harvest dates.[31] Oil produced from high altitude had relatively greater oleic acid than that of low altitude, which agrees with several previous results[19,18] and less linoleic acid.[20] Oleic acid content depends on cultivar, growing season, and altitude.[35]

Fluctuation in linolenic acid was noticed as a result of harvest time, but it was more pronounced at low elevation. Linoleic and linolenic containing more than two double bonds are polyunsaturated fatty acids and are required for normal human growth.[36] In addition, several reports indicate the importance of the polyunsaturated fatty acids in reducing cholesterol levels in human blood and thereby reducing the risk of heart diseases.[37] However, significant increases in linolenic acid were obtained from the final harvest of low altitude. Linolenic acid content varied among harvesting dates in both locations. Very little linolenic acid (0.4) was found in “Nabali” olives.[31] Nevertheless, it has been reported that linolenic acid decreased in olive fruits during ripening.[38,14] However, percentages of all fatty acids determined in this study were within the international standards.[39] The ratio of saturated-unsaturated fatty acids tends to decrease during fruit ripening in accordance with those of Gutierrez et al.[14] and Uceda et al.[38] However, oils produced at higher altitude have higher ratios of unsaturated-saturated fatty acid. Similarly, Mousa et al.[19] and Osman et al.[18] reported similar results with low temperatures at higher elevations.

The impact of harvest time on acidity of olive oil produced from both locations is presented in Fig. 2. Harvesting time had an obvious effect on olive oil acidity at both altitudes. Nevertheless, early and/or late harvesting negatively affects oil quality.[40,18] Acidity was significantly reduced by 1 Oct to mid Nov followed by a gradual increase as fruit ripened. High levels of acidity in oil produced from early stage fruit could be ascribed to the higher chlorophyll and water content of the fruit, which facilitate lipolysis, dilute antioxidant enzymes, and stimulate growth of microorganisms.[1] Subsequently, a significant increase was observed with advancing fruit maturity at both altitudes, which is in consonance with previous studies.[19,40] In addition, acidity of olive oil was greater at low altitudes through the entire investigation relative to olive oils produced at the higher altitude. This finding agrees with the results of Osman et al.[18] Percentages of oil acidity fluctuated between 0.28–0.82% and 0.32–0.95%, at high and low elevation, respectively, with the highest value on 30 Dec at both elevations.

Figure 2 Acidity of “Nabali” olive oil as affected by harvest date and altitudes.

Harvesting time exhibited a significant influence on peroxide value (Fig. 3). Changes in peroxide value followed the same trend at both altitudes. Higher values were obtained in oils produced from the higher altitude in all harvests compared to that produced at the lower altitude, agreeing with results obtained by Mousa et al.[19] In both locations, a reduction in peroxide level was noticed right after the first harvest. This behavior can be explained by a decrease in lipoxygenase enzyme activity as reported by Gutierrez et al.[14] and agreeing with the results obtained by Uceda et al.[38] It declined to 4.94 by first part of Nov at high altitude, and to 4.12 by 15 Oct at the lower altitude, followed by a gradual and a significant increase towards fruit ripening at both altitudes. Similarly, Mousa et al.[19] and Kiritsakis et al.[8] reported that oxidation is initiated in the fruit and increases slowly during fruit maturation, and this also may be related to lipoxygenase enzyme activity.[14] The greatest values of 12.88 and 10.17 meq O₂/kg oil was recorded at the end of December at high and low altitudes respectively, which were within the international standards (≤ 20 meq. O2) for virgin olive oil.[11] However, fresh oil usually has a peroxide value below 10 meq O₂/kg oil.[9] Harvest time and altitudes appears to have a significant influence on quality of “Nabali” olive oils. These results suggest that knowing the proper harvesting time at a particular altitude would greatly help the growers to produce good quality oil, and for high oil production harvesting should be delayed at higher elevation.

Figure 3 Peoxide value of “Nabali” olive oil as affected by harvest date and altitudes.