pubs.acs.org/JAFCPublished on Web 09/22/2010
©2010 American Chemical Society
J. Agric. Food Chem. 2010, 58, 10323–1032910323
Effects of Organic and Conventional Growth Systems on the
Content of Flavonoids in Onions and Phenolic Acids in
Carrots and Potatoes
MALENE SØLTOFT,†,‡JOHN NIELSEN,‡KRISTIAN HOLST LAURSEN,§SØREN HUSTED,§
AND PIA KNUTHSEN*,†
†Department of Food Chemistry, National Food Institute, Technical University of Denmark, Mørkhøj
Bygade 19, DK-2860 Søborg, Denmark,‡Department of Basic Sciences and Environment, Faculty of Life
Soil Science Laboratory, Department of Agriculture and Ecology, Faculty of Life Sciences, University of
Biotechnology, Faculty of Agricultural Sciences, Aarhus University, Research Centre Foulum, Blichers
Alle ´ , DK-8830 Tjele, Denmark
The demand for organic food products is steadily increasing partly due to the expected health
benefits of organic food consumption. Polyphenols, such as flavonoids and phenolic acids, are a
group of secondary plant metabolites with presumably beneficial health effects, and contents in
plants are affected by, for example, plant nutrient availability, climate, pathogen infection, and pest
attack. In the current study, onions, carrots, and potatoes were cultivated in two-year field trials in
three different geographical locations, comprising one conventional and two organic agricultural
systems. The contents of flavonoids and phenolic acids in plants were analyzed by pressurized
liquid extraction and high-performance liquid chromatography-ultraviolet quantification. In onions
and carrots, no statistically significant differences between growth systems were found for any of the
analyzed polyphenols. On the basis of the present study carried out under well-controlled conditions,
it cannot be concluded that organically grown onions, carrots, and potatoes generally have higher contents
of health-promoting secondary metabolites in comparison with the conventionally cultivated ones.
KEYWORDS: Carrots (Daucus carota L.); flavonoids; onions (Allium cepa L.); organic agriculture;
potatoes (Solanum tuberosum L.); phenolic acids
Plant production in organic agricultural systems relies on
organic manures and biological pest control, excluding the use
of inorganic fertilizers and synthetic pesticides. Organic agricul-
ture has developed rapidly in most parts of Europe since the
1990s, especially in the Scandinavian and Mediterranean coun-
tries (1), and the market shares were around 5% in Denmark at
the end of 2007 (2). Improved animal welfare, environmental
protection, human health, and taste and freshness are the most
important reasons for the increasing demand of organic food
products (3). However, the possible health benefits of organic
food consumption are still controversial and not considered
scientifically well-documented (4).
The fundamental differences in organic and conventional
agricultural systems, particularly regarding fertilization strategy
and soil fertility management, affect the nutrient composition in
plants (5). However, previous studies on the nutrient content of
organically and conventionally grown plants have generated
and conventional plants, important variation caused by several
factors such as geographical location and growth season must be
included to ensure that the possible differences found are sys-
tematic and reliable, but this has not been the case in most
Little consideration has previously been given to bioactive
secondary plant metabolites (8), which play an important role in
the growth, development, and defense system of plants (9).
Despite being nonessential to humans, various secondary plant
metabolites have been proposed to prevent numerous dis-
eases (10). Polyphenols have one or more phenol units in their
chemical structure (11) and are a group of secondary plant
metabolites, frequently speculated to have positive health effects
including a decreased risk of heart diseases, dementia (12), and
the plant kingdom (15). Onions are one of the main sources of
polyphenols in the daily food intake (16,17) and a rich source of
to 80% of the total content of flavonols inonions, but kaempferol
found (Figure 1).
*To whom correspondence should be addressed. Tel: þ45 35 88 74
32. Fax: þ45 35 88 74 48. E-mail: email@example.com.
10324J. Agric. Food Chem., Vol. 58, No. 19, 2010Søltoft et al.
Caffeoylquinic acid (5-CQA) (Figure 1) is the predominant phenolic
the total content of phenolics.
as cultivar (24), pathogen infection and pest attack (25), time of
harvest,and storage and processingprocedures(26).Thecontent
of nutrients and secondary plant metabolites in food products is
climate (27), and plant nutrient availability (28).
The objective of our study was to compare the content of
selected flavonoids and phenolic acids in organically and con-
the ability of the crops to synthesize selected secondary metabo-
lites is systematically affected by growth systems across different
growth years as well as geographic locations (soil types).
MATERIALS AND METHODS
graphy (HPLC)-grade, Rathburn Chemicals Ltd., Scotland] and Milli-Q
water (18 MΩ, Millipore, United States) were used for standards and
were purchased from Merck (Germany). Purchased were the following
standards: quercetin (Q) from Extrasynthe ´ se (France), quercetin-3,40-
diglucoside (Q-3,40-diglu) from Polyphenols (France), quercetin-7,40-di-
(Q-40-glu) from Plantech (United Kingdom), and quercetin-3-glucoside
(Q-3-glu, purity >90%), 5-CQA (purity >95%), and caffeic acid (CA,
purity >99%) from Sigma-Aldrich (Germany).
Individual standard stock solutions were prepared by dissolving the
solid compounds in DMSO (100 μg/mL). Working solutions were
solutions were stored at -80 ?C (<1 year), while the working solutions
were prepared shortly before analyses and kept at -20 ?C until analyses.
Samples. Potato, carrot, and onion samples were obtained from field
trial studies undertaken in 2007 and 2008 (year 1 and 2). The potatoes
(Solanum tuberosum cv. sava) were grown in the long term CropSys crop
rotation experiment (29) at three different geographical locations
(Flakkebjerg, Foulum, and Jyndevad) (http://www.cropsys.elr.dk/uk/);
Table 4 in the Supporting Information, and planting time, harvest time,
and irrigation are shown in Table 5 in the Supporting Information. The
grown at one location (Aarslev) in the VegQure rotation experiment
(http://www.vegqure.elr.dk/uk/). The crops were grown in three different
agricultural systems at all locations: one conventional system (C) and two
organic systems (OA and OB). The systems were all based on stockless
cash crop production systems with an identical sequence of main crops
(8 year rotation in Aarslev and 4 year rotation at the other locations). In
the conventional system, pesticides and inorganic fertilizer were used like
OA system relied on the import of animal manure. In the OB system, the
nutrient supplywas based onthe use of cover crops (mainly legumes), but
animal manure was also applied to onions to satisfy the high nitrogen
crops were grown. The organic systems were managed in full compliance
(36 plots in total), while carrots and onions were grown with three
replicates of each growth system resulting in nine plots per year (18 plots
in total). Table 1 in the Supporting Information presents details on field
trial characteristics such as geographical locations, soil types, fertilizer
The crops were harvested at the same day for all systems. A 15 kg
sample was collected from each plot using the following inclusion criteria
both years (marketable quality): potatoes, 35-60 mm diameter; carrots,
50-250 g fresh weight (fw); and onions, 40-80 mm diameter. Represen-
tative sampling was ensured by stepwise mass reduction as described in
the bulbs of the onions were usedfor analysis by removing the shoots and
outer leaves and the shoot of the carrots was discarded. The samples were
washed in Milli-Q water, peeled (only carrots and potatoes), cut into
0.5 cm thick slices, and freeze-dried at 0.08 kPa for 1-2 days at a
commercial freeze-drying company (Danish Freeze-Dry A/S, Kirke Hyl-
preparation by wrapping them in aluminum foil and storing them in a
Chemical Analysis of Flavonoids and Phenolic Acids. The princi-
be seen in Søltoft et al. (31). Extraction of flavonoids and phenolic acids
was performed by pressurized liquid extraction using an ASE-200
(Dionex, Sunnyvale, CA). The sample material (0.5 g) was added to
5 mL extraction cells together with 0.5 g of C18-material (Sepra E-C18,
methanol. The volume of the extracts was adjusted to 20 mL and filtered
(0.20 μm, Sartorius Minisart, Aubagne, France) before quantification by
5 μm, 30 ?C) with 0.1% formic acid in Milli-Q water (v/v) and methanol
(90:10, v/v, A eluent) and 100% methanol (B eluent). The flavonoids and
Figure 1. Chemical structures of selected flavonoids and phenolic acids. Systematic names:a2-(3,4-Dihydroxyphenyl)-3,5,7-trihydroxychromen-4-one;
phenyl)chromen-4-one;e2-propenoic acid, 3-(3,4-dihydroxyphenyl)-;f2-propenoic acid, 3-(4-hydroxyphenyl)-, (2E)-;g2-propenoic acid, 3-(4-hydroxy-
3-methoxyphenyl)-; andhcyclohexanecarboxylic acid, 3-[[3-(3,4-dihydroxyphenyl)-1-oxo-2-propen-1-yl]oxy]-1,4,5-trihydroxy-, (1S,3R,4R,5R)-.
ArticleJ. Agric. Food Chem., Vol. 58, No. 19, 2010 10325
for analyses of bioactive compounds (32). The chemical structure of the
flavonoids and phenolic acids was elucidated by mass spectrometry (MS)
and MS/MS analysis and by comparison of retention times, UV, and MS
data with available standards (Q, Q-3,40-diglu, Q-7,40-diglu, Q-40-diglu,
Q-3-glu, 5-CQA, and CA) according to the principles detailed in Søltoft
et al. (31), which also presents the validation parameters used.
Quality assurance was performed by including an in-house sample as a
of 6.2% for all quantified polyphenols (n = 4). Furthermore, duplicate
measurements (including weighing and extraction) of randomly selected
samples in each series of analyses were included, yielding an average RSD
of 6.8, 11, and 2.7% for potatoes, carrots, and onions for all quantified
for carrots and onions).
Elemental Nitrogen Analysis. Nitrogen was measured using isotope
Approximately 4 mg of pulverized material was weighed in tin capsules
and introduced to the MS via a combustion interface. Quality assurance
was performed using certified reference material, frequent quality control
samples, and duplicate measurements of all samples.
Statistical Analysis. The responses yysbfrom the Aarslev location
weremodeledas:yysb=μþ Rbþ βyþ δsþ εysþ εybþ εysb, where μisthe
year 2 is the effect of year; and δs, s = C, OA, OB is the effect of growth
system. The responses yyslbfrom the other locations were modeled as:
yyslb= μ þ βyþ δsþ γlþ Rlbþ εysþ εylþ εslþ εylbþ εyslb, where μ isthe
generalized intercept; βy, y = year 1, year 2 is the effect ofyear; δs, s = C,
OA, OB is the effect of growth system; γlis the effect of location, l =
Foulum, Jyndevad, Flakkebjerg is the location; and Rlb, b =1, 2 is the
effect of the blocks within each location.
Errors (ε) are considered independently and normally distributed and
represent corresponding variance components of interaction. The pair
wise comparisons and their confidenceintervals between the systems were
adjusted to obtain a family wise error rate of 5%. The model was fitted
using the proc mixed procedure in the SAS/STAT software packages
(Version 9.2, SAS Institute Inc., Cary, NC).
RESULTS AND DISCUSSION
Flavonoids in Onions. The flavonoid profile of onions was
similar to profiles previously identified in southern Italian red
onions (20) and white onions (33) with a high abundance of
Q-3,40-diglu and Q-40-glu, but traces of quercetin-3,7,40-triglyco-
side (Q-3,7,40-trigly), Q-3-glu, isorhamnetin-3,40-diglycoside (I-
3,40-digly), and isorhamnetin-40-glycoside (I-40-gly) were also
detected (Figure 2). Q-7,40-diglu was only present in concentra-
Figure 2. Average concentrations of flavonoids in onions (μg quercetin-equivalents/g fresh weight) in three different growth systems (C: conventional,
OA: organic using animal manure, and OB: organic using cover crops) and in two harvest years (A: year 1 and B: year 2). Error bars describe the standard
deviation of replicates from the field (n = 3).
10326J. Agric. Food Chem., Vol. 58, No. 19, 2010Søltoft et al.
(q-eq)/g fw (μg q-eq/g fw)]. The average concentrations [(stan-
dard deviation (SD), n = 9] of Q-3,40-diglu and Q-40-glu in year1
were 262 ( 38 and 208 ( 41 μg q-eq/g fw, respectively, and the
concentrations were 9 and 8% lower in year 2, respectively. The
concentrations correspond to 530 ( 77 μg Q-3,40-diglu/g fw and
330 ( 65 μg Q-40-diglu/g fw, and the concentration levels of
No significant differences (p > 0.05) in the content of any
of the flavonoids in onions between the three growth systems
(C, OA, and OB) were observed. However, a large variation
(average RSD for all quantified flavonoids: 16%, n = 3) within
growth systems was seen in most cases, despite the replicates
effects of differences in soil fertility and microclimate.
significantly higher content in year 2 (p < 0.05). The variation in
flavonoid content ofonionsbetween growthyearshaspreviously
been shown by Mogren et al. (34,35). A significant difference in
growth years was also found (Table 2 in the Supporting In-
formation). The observed year-to-year variation could, for in-
stance, be related to the weather conditions since year 1 was a
growth year with higher temperatures and more precipitation in
comparison with year 2.
The use of inorganic nitrogen in conventional agriculture often
has previously caused lower accumulation of flavonoids in toma-
study (Table 1 in the Supporting Information). Thus, the applica-
resulted in the highest harvest yield in both years. However, no
significant differences between the conventionally and the organi-
callygrown onions in the content ofnitrogenandflavonoids, size,
at harvest werewithinnorm values[1.2-1.4% in drymatter(37)],
the yield plateau as is often the case in organic farming (36).
studies, where neither the source (organic or inorganic) nor the
levels of nitrogen fertilization affected the content of flavo-
noids (34, 38). In contrast, a higher content of myricetin and
of the growth system was difficult to determine as the crops were
grown at two different farms (39).
Phenolic Acids in Carrots. The only polyphenol found in
carrots was 5-CQA, which is in accordance with previous
observations (23, 40), where it accounted for up to 80% of the
total phenolic acid content (23). The average concentration
((SD, n = 9) of 5-CQA in year 1 was 15 ( 4.8 μg q-eq/g fw,
which corresponds to 55 ( 18 μg 5-CQA/g fw (refer to Figure 3),
and the concentration levels were in agreement with previous
levels reported for carrots (23, 40). The concentrations were on
average 28% lower in year 2, but no significant year-to-year
variation was found across the growth systems (p > 0.05).
The very different amounts of fertilizer and fertilizer types
the Supporting Information). However, no significant diff-
erences in the harvest yield, dry mattercontent,or theconcentra-
tions of 5-CQA were found (p > 0.05). Similar results have been
levels (0-300 kg N/ha) (41). Furthermore, a large average
variation within growth systems was observed for 5-CQA in
carrots (RSD, 12%) (Figure 3) as was also seen for flavonoids in
Phenolic Acids in Potatoes. 5-CQA was the most abundant
phenolic acid identified in potatoes in agreement with previous
studies (22, 41), but also small quantities of 4-O-caffeoylquinic
acid (4-CQA) were found (Figure 4). The average concentrations
28.2 ( 1.5 μg q-eq/g fw at the three different locations Foulum,
Jyndevad, and Flakkebjerg, respectively, which corresponds to
concentrations in year 2 were 22% lower at the Foulum location
and 17% higher at both the Jyndevad and the Flakkebjerg
locations. The concentration levels were in accordance to ref 42
Figure 3. Average concentrations of 5-CQA in carrots (μg q-eq/g fw) in three different growth systems (C, conventional; OA, organic using animal manure;
and OB, organic using cover crop) and in two harvest years (gray square, year 1; and black square, year 2). Error bars describe the standard deviation of
replicates from the field (n = 3).
Article J. Agric. Food Chem., Vol. 58, No. 19, 201010327
and onions (RSD, 16%). The concentration of 4-CQA consti-
higher than previously found for the sum of 3-O-caffeoylquinic
acid (3-CQA) and 4-CQA in potatoes of cv. Ranger russet and
in the potatoes, but the concentrations were below the limit of
quantification (4 μg q-eq/g fw; data not shown).
A significantly higher content of 5-CQA was found in the OB
as compared to the conventional growth system (C) across
locations and growth years (p < 0.05), despite the fact that no
significant differences in nitrogen content were found at harvest
(Table 3 in the Supporting Information). This is in agreement
with a previous study, where no correlation between nitrogen
observed (41). Instead, the significantly higher content of 5-CQA
in the OB system could be related to a lower potassium fertiliza-
tion level in comparison with the conventional growth system
(Table 1 in the Supporting Information), which has previously
been observed for polyphenols in potatoes (44, 45). Hajslova
et al. (46) also found a higher level of 5-CQA in organically as
comparedtoconventionallygrown potatoes.However,the crops
were grown at two different farms, which could have induced a
located relatively close to each other.
The potato sizes and dry matter contents did not differ, but a
positive correlation between potato harvest yield and nitrogen
application rate was found. Hence, the yield was significantly
system (p < 0.05), C > OA > OB, Table 3 in the Supporting
optimum in the organic growth systems. Furthermore, a signifi-
acids in potatoes has previously been observed (46), but no
significant location differences were found in the present study
for any of the analyzed parameters. Furthermore, no significant
year-to-year variation was seen in the content of phenolic acids
in potatoes in contrast to a previous farm study by Hajslova
et al. (46).
as one and three phenolic acids in carrots and potatoes, respec-
tively. A significant year-to-year variation was observed for
Q-3,7,40-trigly in onions, but no significant differences in the
content of flavonoids and phenolic acids between the conven-
tional and the two organic growth systems were found. In the
Figure 4. Average concentrations of phenolic acids in potatoes (μg q-eq/gfw) in three different growth systems (C, conventional; OA, organic using animal
manure; and OB, organic using cover crops) and in two harvest years (A, year 1; and B, year 2) at three different locations (Foulum, Jyndevad, and
Flakkebjerg). Error bars describe the standard deviation of replicates from the field (n = 2). Gray square, 5-CQA; and black square, 4-CQA.
10328 J. Agric. Food Chem., Vol. 58, No. 19, 2010Søltoft et al.
organically grown potatoes fertilized with cover crops, a signifi-
cantly higher content of 5-CQA was found as compared to the
conventional system, which overruled variation caused by geo-
graphical location and growth year. On the basis of the present
study carried out under well-controlled conditions, it cannot be
concluded that organically grown onions, carrots, and potatoes
generally have higher contents of health-promoting secondary
metabolites in comparison with the conventionally cultivated
ones. The ability of crops to synthesize selected secondary
metabolites was not systematicallyaffected bythe growth system
across different growth years and geographical locations.
C, conventional growth system using pesticides and inorganic
fertilizer; CA, caffeic acid; 4-CQA, 4-O-caffeoylquinic acid; 5-
CQA, 5-O-caffeoylquinic acid; fw, fresh weight; I-3,40-digly, iso-
rhamnetin- 3,40-diglycoside; I-40-gly, isorhamnetin-40-glycoside;
MS, mass spectrometry; MS/MS, tandem mass spectrometry;
OA, organic growth system using animal manure; OB, organic
growth system using cover crops; Q, quercetin; q-eq, quercetin
equivalents; Q-3,7,40-trigly, quercetin-3,7,40-triglycoside; Q-3,40-
diglu, quercetin-3,40-diglucoside; Q-7,40-diglu, quercetin-7,40-di-
glucoside; Q-40-glu, quercetin-40-glucoside; Q-3-glu, quercetin-
3-glucoside; RSD, relative standard deviation; SD, standard
We thank Emese Ka ´ polna for assistance with the sample
preparation. We also acknowledge the research groups in the
CropSys and VegQure rotation experiments, coordinated by
Jørgen E. Olesen and Kristian Thorup-Kristensen, for carrying
out the field trials.
Supporting Information Available: Tables of field trial char-
acteristics, onion and carrot harvest yields, potato harvest yields,
average temperature and precipitation for periods of potato
growth,and plantingand harvesttimeand irrigationofpotatoes.
This material is available free of charge via the Internet at http://
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Received for review March 22, 2010. Revised manuscript received
June 29, 2010. Accepted July 12, 2010. This work was financially
supported by the Ministry of Food, Agriculture and Fisheries,
Denmark and coordinated by the International Centre for Research in
Organic Food Systems (ICROFS) via the OrgTrace project (http://