The biofortification of food crops for human consumption is a direct strategy increasing dietary intake of selenium (Se). The aim of this study was to evaluate the possibility of increasing the Se content of extra virgin olive oil (EVOO) by spraying the olive tree canopy with sodium selenate and the effect of the increase in Se on the chemical properties and sensory characteristics of the EVOO. Se treatments were up to 50 times more effective in enhancing Se content in the EVOO compared with the untreated controls. Se concentration in all the EVOO samples can be considered adequate and useful for providing the human diet with the correct dose of Se. Se-enriched EVOO showed a significant increase in pigment and phenol content. Also, Se treatment does not produce negative effects on fruit characteristics or the sensory quality of EVOO.
Selenium (Se) is a very important element for human health, and is involved in defensive mechanisms and the biosynthesis of hormones in adults and babies. Se is mainly involved in the production of the active thyroid hormone, in muscle function, in the reproduction process and in the immune response to some infections (Dhur
The essentiality of Se in higher plants is still under debate. Although it is harmful to plants in high concentrations, it exerts beneficial effects at low concentrations. Se increases the tolerance of plants to UV-induced oxidative stress, it delays senescence and promotes the growth of ageing seedlings. Recently, it has been shown that Se is able to regulate the water status of plants under drought conditions (Kuznetsov
The bio-fortification of food crops for human consumption is a direct strategy to increase dietary Se intake; moreover, Se-enriched crops can take advantage of its beneficial effect (Germ
The content of Se in plants can be increased in different ways: by adding Se to the soil, soaking seeds in an Se solution before sowing, hydroponic and aeroponic cultivation in a nutrient solution containing Se, and the foliar application of Se solutions to plants (Bittman
Gupta
Whereas many papers on the effects of fortifying herbaceous crops with Se are present in the literature, very few studies were aimed at investigating its effects on fruit trees. Nonetheless, it was ascertained that spraying peach and pear with sodium selenate affected the shelf-life of the fruit, delaying the reduction in flesh firmness and fruit ripening (Pezzarossa
Dugo
Normally, the Se content in olive oil is highly dependent on the amount of Se in the soil and on the ability of plants to take up and accumulate it (Terry
Mediterranean cuisine is widely considered a healthy and disease-preventive diet. Olives and olive oil, especially extra virgin olive oil (EVOO), are important components of this diet, since they contain a large variety of bioactive compounds (phytochemicals), which are considered to be beneficial for human health (Cicerale
EVOO contains a considerable amount of phenols that have a great effect on both the stability and the sensory and healthy characteristics of the oil. As water-soluble components, phenols play an important role since they have a wide range of biochemical and pharmaceutical effects, including anticarcinogenic, antiatherogenic, antimicrobial and antioxidant activities (Kohyama
Due to their antioxidant activity, the amount of phenols is correlated with the resistance of oil to oxidation and thus to oil stability (Boskov,
The aim of this study was to evaluate the possibility of increasing the Se content in EVOO by spraying the olive tree canopy with sodium selenate and the effect of the increase in Se content on the fatty acid composition, phenol and pigment content, oxidation stability and sensory characteristics of the oils. The effect of Se on fruit characteristics was also studied. Well irrigated and water-stressed trees were treated to investigate Se absorption by trees and if the effects of Se on the oil were influenced by soil water availability.
The research was carried out in Central Italy (Perugia, about 400 m a.s.l., 12°23′E longitude, 43°5′N latitude). Climatic conditions of the experimental site were monitored by a meteorological station. The rainfall pattern in the trial year (2011) was characterized by a scant rainfall in spring, summer and autumn (187 mm, 61 mm and 139 mm of rain from March to May, from June to August and from September to November, respectively).
During the experimental period, the highest average of the maximum daily temperatures was in August (33.6 °C), whereas the lowest one was at the beginning of November (15.7 °C).
The experiment used six-year-old potted olive trees of the Maurino cultivar, trained according to the open-center training system and spaced 2.0×1.5 m between and along the rows (North-South). The trees were about 2.0 m tall and the canopy had a diameter of approximately 1.0 m and a height of about 0.9 m. The soil in the pot (approximately 70 kg) was medium textured and had a pH of 7.7 and a content of 0.2% of total nitrogen, 55 and 190 mg kg−1 of assimilable phosphorus and exchangeable potassium, respectively. Each olive was fertilized every year in mid April with the controlled-release fertilizer “Osmocote” (“Scott Italia” 16:8:12 N:P:K) at 100 g per pot. From mid-May to mid-September 2011, 27 trees were irrigated (well irrigated trees: WI) to 80% of the substrate available water (substrate water content approximately 24% on a dry weight basis), whereas the other 27 trees (water stressed trees: WS) were irrigated to 25% of the substrate available water (substrate water content approximately 15% on a dry weight basis). Irrigation was carried out every day in the morning using drip lines (two drippers per pot, each with a flow rate of 4 L h–1). The pots were covered with aluminium foil in order to prevent both overheating and the supply of rain water.
At the beginning of July, the trees (both WI and WS) were sprayed with a solution containing two different Se concentrations, i.e. 50 and 150 mg L−1 (respectively 50 Se and 150 Se), obtained by dissolving the correct amount of sodium selenate in water. (“Sigma-Aldrich” cod. S0882-25g). For each treatment, 0.5% of the wetting agent “Albamilagro” was added. The controls included in the experiment were sprayed with a solution containing only the wetting agent.
At harvest (beginning of November) the yield per tree, the fruit ripeness indexes (detachment force, flesh firmness and fruit pigmentation) of the fresh fruit and dry weight and oil content were determined.
The yield per tree was determined by weighing the harvested olives for each tree (9 trees per treatment).
Detachment force was measured using the “Carpo” hand dynamometer, and flesh firmness was determined by means of an “Effe.gi” dynamometer DT 05 (with 1.0 mm diameter tip), on about 20 olives per tree. Fruit pigmentation was evaluated using the maturity index (MI) according to the method developed by the Agronomic Station of Jaén (Uceda and Hermoso,
The fresh fruit fresh and dry weight (the latter determined by drying three samples of fresh olives per thesis at 90 °C) and the oil content (using the “SpectraAlyzer ZEUTEC” – NIR: Near Infra Red on two samples of olives per thesis) were determined.
The oil samples from approximately 2.5 kg of olive for each sample (two per thesis) were extracted using a mini olive-mill one day after harvesting (beginning of November), and carried out in the following stages: hammer crushing, 25 minutes rippling at room temperature (about 22 °C), centrifugation of the paste to separate the oil with a centrifugation basket without using water, oil filtration with cotton wool and sodium sulphate anhydrous to remove the water and impurities. The oil samples were stored in closed, dark glass bottles in a fridge at 4 °C until analysis (a week after extraction).
The Se content in the oil samples was determined as follows (US EPA Method 3031,
Taking into account the low concentration of Se in vegetable oils, we used a highly sensitive, analytical method, which adopted a graphite furnace for use with the atomic absorption spectrophoto-meter, Shimadzu AA-6800 (GF-AAS; GFA-EX7, Shimadzu Corp., Tokyo, Japan), with a deuterium lamp background correction and a matrix modifier (Pd(NO3)2, 0.5 mol·L−1 in HNO3). The accuracy of the analytical procedure was obtained by spiking a suitable, known amount of the analyte into a test portion of the oil sample, having a known concentration of the added analyte (100 µg·L−1), and by analyzing the spiked test portion along with the original sample. The mean percent recovery of Se for 4 samples of spiked olive oil was 92.4%.
The Fatty Acid Methyl Ester
Phenol content was determined as described by Vazquez Roncero
Pigments (chlorophyll a and b, and total carotenoids) were determined in 0.5 g oil samples dissolved in 25 mL of 95% diethyl ether. The solution was filtered through a double layer of cheese cloths and absorbances at 662, 646 and 470 nm were determined using a Genesys 10 BIO spectrophotometer (Wellburn,
Where
The acidity and peroxide number of the oil were determined according to the EEC 2568/91 (
Oxidation induction time (h) was determined using a Rancimat 679 apparatus (Metrohm Co., Herisau, Switzerland) according to Mateos
A flow of air (20 mL·h−1) was bubbled through 5.0 g of oil heated to 110 °C, 120 and 130 °C. The volatile oxidation products were stripped from the oil and dissolved in cold water, thus increasing its conductivity. The time taken to reach an inflection point at the induction curve was measured.
Sensory evaluation of the oil was carried out by nine panel tasters. The tasters were fully trained in the evaluation of virgin olive oil according to the EEC 2568/91 (
The experimental design was a completely randomized 2-factor factorial with 9 trees per treatment, though samples were collected at harvest time from three trees and mixed together for the analyses, so that the final number of true-replicates was three.
All data, with the exception of those of the sensorial evaluation, were submitted to 2-way ANOVA and the means were compared using the Fisher Least Significant Difference at p = 0.05. A graphical analysis of residuals was used to make sure that the basic assumptions for ANOVA were met and, whenever necessary, data was square-root transformed prior to analyses.
In general, as expected, yield per tree was higher in WI than in WS (
Yield per tree and fruit characteristics in olives treated with Se or not, water stressed (WS) or not (WI)
Sample | Yield per tree (g) | Fruit fresh weight (g) | Fruit dry weight (g) | Maturity index (MI) | Fruit detachment force (N) | Flesh firmness (g) |
---|---|---|---|---|---|---|
|
610±16d | 1.91±0.15b | 0.76±0.08b | 2.98±0.45 |
3.39±0.40 |
386±16d |
|
461±11 |
1.38±0.17 |
0.61±0.11 |
4.96±0.32 |
3.25±0.42 |
300±11 |
|
606±15d | 2.06±0.15bc | 0.85±0.08bc | 3.07±0.25 |
3.52±0.52 |
380±18d |
|
581±15c | 1.55±0.16 |
0.68±0.07 |
4.13±0.25b | 3.44±0.48 |
363±13c |
|
620±16d | 2.22±0.16c | 0.90±0.05c | 2.25±0.52 |
3.53±0.49 |
400±16d |
|
501±12b | 1.84±0.16b | 0.83±0.16b | 5.04±0.65b | 3.43±0.43 |
340±13b |
In each column, means followed by different letters are significantly different (
The fruits of the WI trees showed a lower pigmentation and higher detachment force and flesh firmness than those of stressed trees. These effects indicate, as expected, that in trees with a higher crop load ripening is slowed down. No differences due to Se treatments were found.
Se concentration in extra virgin olive oil from treated with Se or not, water stressed (WS) or not (WI) olives
Sample | Mean Se content ( |
---|---|
|
15.0±1.5 |
|
22.3±2.1b |
|
430.8±7.6c |
|
458.6±47d |
|
850.3±20.2e |
|
956.6±8.5f |
Mean and standard deviation are based on three replicate analyses. Anova and multiple comparising testing were performed on square root transformed data.
The U.S. Recommended Dietary Allowance (RDA) (National Research Council,
Acidity, peroxide number, K232 and K270 were not influenced either by the irrigation regime or by the Se treatments, and they were at an optimal level and considerably lower than the limits established by the Commission Regulation (EU) N°61/
Acidity, peroxide value and fatty acid composition of extra virgin olive oil from treated with Se or not, water stressed (WS) or not (WI) olives
WI | WS | 50 Se WI | 50 Se WS | 150 Se WI | 150 Se WS | |
---|---|---|---|---|---|---|
Acidity (% oleic acid) | 0.23±0.03 |
0.22±0.02 | 0.19±0.01 | 0.20±0.02 | 0.22±0.02 | 0.19±0.01 |
Peroxide value (meq O2 kg−1 of oil) | 12.1±0.4 | 18.3±0.5 | 13.6±0.1 | 13.2±0.2 | 11.4±0.4 | 11.1±0.5 |
K270 | 0.12±0.03 | 0.13±0.05 | 0.12±0.03 | 0.11±0.02 | 0.16±0.01 | 0.14±0.02 |
K232 | 1.89±0.30 | 1.90±0.21 | 1.78±0.51 | 1.95±0.31 | 1.54±0.44 | 1.95±0.41 |
C16:0 |
12.99±0.33 | 13.35±0.25 | 12.89±0.28 | 12.91±0.22 | 12.57±0.31 | 13.31±0.29 |
C16:1 | 1.22±0.01 | 1.26±0.01 | 1.20±0.02 | 1.25±0.03 | 1.14±0.02 | 1.20±0.01 |
C17:0 | 0.03±0.01 | 0.03±0.01 | 0.03±0.01 | 0.03±0.01 | 0.03±0.01 | 0.04±0.01 |
C17:1 | 0.1±0.05 | 0.09±0.05 | 0.09±0.02 | 0.09±0.02 | 0.08±0.01 | 0.09±0.02 |
C18:0 | 1.37±0.12 | 1.84±0.09 | 1.48±0.08 | 1.65±0.07 | 1.49±0.05 | 1.69±0.04 |
C18:1 | 71.94±0.34 | 71.05±0.45 | 71.83±0.48 | 72.12±0.21 | 72.45±0.38 | 71.79±0.24 |
C18:2 | 9.91±0.40 | 9.7±0.25 | 9.65±0.33 | 9.05±0.25 | 9.34±0.21 | 9.1±0.21 |
C18:3 | 0.75±0.03 | 0.71±0.02 | 0.67±0.02 | 0.62±0.03 | 0.68±0.02 | 0.71±0.03 |
C20:0 | 0.25±0.04 | 0.28±0.03 | 0.24±0.02 | 0.26±0.04 | 0.25±0.03 | 0.27±0.03 |
C20:1 | 0.26±0.03 | 0.25±0.03 | 0.24±0.03 | 0.23±0.04 | 0.27±0.02 | 0.24±0.02 |
C22:0 | 0.07±0.01 | 0.1±0.01 | 0.06±0.01 | 0.07±0.02 | 0.07±0.03 | 0.08±0.03 |
Σ SFAs | 13.34±0.51 | 15.60±0.39 | 14.7±0.55 | 14.92±0.37 | 14.41±0.43 | 15.39±0.40 |
Σ PUFAs | 10.66±0.43 | 10.41±0.27 | 10.32±0.35 | 9.67±0.28 | 10.02±0.23 | 9.81±0.24 |
Σ MUFAs | 73.52±0.43 | 72.65±0.54 | 73.36±0.55 | 73.69±0.30 | 73.94±0.43 | 73.32±0.29 |
Unsat/Sat | 5.75 | 5.36 | 5.72 | 5.61 | 5.85 | 5.43 |
MUFAs/PUFAs | 6.90 | 6.98 | 7.11 | 7.62 | 7.38 | 7.47 |
C18:1/C18:2 | 7.26 | 7.32 | 7.44 | 7.97 | 7.76 | 7.89 |
Mean ± standard deviation (
C16:0 palmitic, C16:1 palmitoleic, C17:0 margaric, C17:1 margaroleic, C18:0 stearic, C18:1 oleic, C18:2 linoleic, C18:3 linolenic, C20:0 arachidic, C20:1 gadoleic, C22:0 behenic, SFAs saturated fatty acids, PUFAs polyunsaturated fatty acids, MUFAs monounsaturated fatty acids.
The fatty acid composition was similar in all the oils, with percentages of total saturated fatty acid (SFAs), total polyunsaturated fatty acids (PUFAs) and total monounsaturated fatty acids (MUFAs) ranging from 13.34 to 15.60%, from 9.67 to 10.66% and 72.65 to 73.94%, respectively. The unsaturated/saturated acid ratio (ranging from 6.90 to 7.62) and the mono-unsaturated/poly-unsaturated acid ratio (ranging from 5.36 to 5.85) were also uninfluenced by irrigation and the Se treatments (
All the EVOO samples obtained from the WS Se-treated and control trees, showed concentrations of phenols significantly higher (P < 0.05) than those from the WI trees (
Phenol, chlorophyll and carotenoid contents of extra virgin olive oil from treated with Se or not, water stressed (WS) or not (WI) olives
WI | WS | |
---|---|---|
Total phenolic compounds (expressed as µg of gallic acid g−1) |
||
|
473±12 |
626±15c |
|
490±20 |
663±16c (5.8) |
|
586±19b (23.9) | 686±21d (9.5) |
Total carotenoids (µg g−1) |
||
|
8.08±0.32 |
10.37±0.42b |
|
12.14±0.43c (50.2) | 12.68±0.52c (22.1) |
|
18.64±0.41e (129.6) | 16.52±0.62d (58.6) |
Total chlorophylls (µg g−1) |
||
|
12.70±0.41 |
14.9±0.56b |
|
20.10±0.48c (107.2) | 22.60±0.42c (51.6) |
|
31.57±0.56e (225.4) | 27.93±0.51d (87.4) |
Means followed by different letters are significantly different (
The high phenol content in EVOO could be considered a positive factor, since the higher the phenol content, the greater the antioxidant activity of the oil. Furthermore, they positively influence the oil sensory profile, giving tastes of bitterness and pungency which consumers appreciate at the correct level. In the EVOO samples obtained from the Se-treated trees there was a significant increase of the phenol content compared with the control. The increase in WI was approximately 4 and 24% for 50 Se and 150 Se, respectively, and in WS it was approximately 6 and 10% for 50 Se and 150 Se, respectively. This means that Se-treatments could cause a further increase in phenol contents in the oils from water stressed trees. As reported by Turlo
In the EVOO samples from Se-untreated trees, the pigment (chlorophylls and carotenoids) concentrations in the oils from WS plants were significantly higher than in the oils from WI trees (
The positive effect of Se in increasing pigment content in oil concurs with what is reported by Chen
The color of the EVOO, which depends exclusively on biological compounds such as chlorophylls and carotenoids, is very important, since it is one of the factors that influence consumers’ choice; moreover, these pigments play an important role in the oxidative stability due to their antioxidant nature in the dark and pro-oxidant activity in the light (Minguez-Mosquera
With respect to the EVOO samples from the controls, the induction times in the WS plant oils was higher (30.7 h at 110 °C) than in those from the WI oils (21.1 h at 110 °C), which could be related to the higher content in phenols of the WS oil (
Induction time (h)
Sample | Induction time at 110 °C | Induction time at 120 °C | Induction time at 130 °C |
---|---|---|---|
|
21.1±1.2 | 10.6± 1.1 | 5.0±0.1 |
|
30.7±2.1 | 13.9±0.5 | 6.7±0.2 |
|
20.8±1.3 | 11.1±1.2 | 5.2±0.2 |
|
28.2±1.2 | 13.5±1.2 | 6.4±0.5 |
|
26.1±1.5 | 12.9±1.0 | 6.1±0.2 |
|
32.0±2.1 | 15.2±0.5 | 6.6±0.5 |
Each Rancimat value is listed with precision of one tenth of an hour and is the average of 3 determinations; data are expressed as mean values ± standard deviation, Mean ± SD (
The sensory oil characteristics did not substantially differ between WI and WS and Se treated and untreated trees; the only exception was the bitter and pungent sensations that tend to be higher in WS compared with WI and in Se treated, especially at 150 Se, compared to untreated trees (
Sensory profile of extra virgin olive oils from treated with Se or not, water stressed (WS) or not (WI) olives
WI | WS | 50 Se WI | 50 Se WS | 150 Se WI | 150 Se WS | |
---|---|---|---|---|---|---|
|
2.5 | 2.5 | 2.0 | 3.0 | 2.5 | 3.0 |
|
2.0 | 3.0 | 2.0 | 3.5 | 3.0 | 3.5 |
|
2.5 | 3.0 | 2.0 | 3.0 | 3.0 | 3.0 |
|
2.0 | 1.5 | 2.5 | 2.5 | 2.0 | 2.5 |
|
green | green/mature | mature | green | green | green |
|
8.0 | 8.0 | 7.5 | 8.5 | 8.0 | 8.5 |
In conclusion, the technique of fortifying olive tree canopies with sodium selenate, in addition to not adversely affecting the fruit characteristics, is a good way to increase the content of Se in EVOO, adding a beneficial element to this important food in the Mediterranean diet which affects human health. Another advantage is the fact that increased Se in EVOO positively changes some oil properties, such as color intensity, stability and sensory quality.