Journal of Food Composition and Analysis 37 (2015) 38–43

Original Research Article

Comparison between the mineral profile and nitrate content ofmicrogreens and mature lettuces

Edgar Pinto a, Agostinho A. Almeida b, Ana A. Aguiar c, Isabel M.P.L.V.O. Ferreira a,*a REQUIMTE/Departamento de Ciencias Quımicas, Laboratorio de Bromatologia e Hidrologia, Faculdade de Farmacia – Universidade do Porto, Rua Jorge

Viterbo Ferreira, 228, 4050-313 Porto, Portugalb REQUIMTE/Departamento de Ciencias Quımicas, Laboratorio de Quımica Aplicada, Faculdade de Farmacia – Universidade do Porto, Rua Jorge Viterbo

Ferreira, 228, 4050-313 Porto, Portugalc REQUIMTE/Faculdade de Ciencias – Universidade do Porto, R. Campo Alegre, 4169-007 Porto, Portugal

A R T I C L E I N F O

Article history:

Received 21 March 2014

Received in revised form 21 May 2014

Accepted 24 June 2014

Available online 12 October 2014

Keywords:

Lettuce

Microgreens

Minerals

Nitrate

Estimated daily intake

Food safety

Food composition

Food analysis

A B S T R A C T

Microgreens are a new class of edible vegetables harvested when seed-leaves have fully expanded and

before true leaves have emerged. They are gaining increasing popularity as new culinary ingredients.

However, no scientific data comparing the mineral content of microgreens and mature plants are

available. Thus, the goal of this work was to perform a comparison between mineral profile and NO3–

content of microgreens and mature lettuces. Results showed that microgreens possess a higher content

of most minerals (Ca, Mg, Fe, Mn, Zn, Se and Mo) and a lower NO3– content than mature lettuces.

Therefore, microgreens can be considered as a good source of minerals in the human diet, and their

consumption could be an important strategy to meet children’s minerals dietary requirements without

exposing them to harmful NO3–.

� 2014 Elsevier Inc. All rights reserved.

Contents lists available at ScienceDirect

Journal of Food Composition and Analysis

jo u rn al ho m epag e: ww w.els evier . c om / lo cat e/ j fc a

1. Introduction

Mineral malnutrition is a common problem in both developedand developing countries. It is estimated that over 60% of theworld’s 7 billion people are Fe deficient, over 30% are Zn deficientand 15% are Se deficient. Additionally, Ca, Mg and Cu deficienciesare also common in several developed and developing countries(White and Broadley, 2009). Nowadays, mineral malnutrition isconsidered to be one of the most important global challenges tomankind that can be prevented (Miller and Welch, 2013).

Vegetables are a good source of vitamins and minerals and areknown to have protective benefits against oxidative stress-relateddiseases (Alarcon-Flores et al., 2014; Martinez-Tomas et al., 2012).Several studies have shown that vegetables consumption isassociated with a reduction in the development of chronic diseases,such as cancer and cardiovascular disease (Crowe et al., 2011; He

* Corresponding author. Tel.: +351 220428642; fax: +351 226093390.

E-mail addresses: edgarpinto7@gmail.com (E. Pinto), aalmeida@ff.up.pt

(A.A. Almeida), aaguiar@fc.up.pt (A.A. Aguiar), isabel.ferreira@ff.up.pt

(Isabel M.P.L.V.O. Ferreira).

http://dx.doi.org/10.1016/j.jfca.2014.06.018

0889-1575/� 2014 Elsevier Inc. All rights reserved.

et al., 2006). FAO/WHO stressed the need to increase consumption offruits and vegetables because they are important components of a

healthy diet (FAO/WHO, 2004). Therefore, their production, quality

and adequate consumption should be promoted.However, vegetables can also be an important source of nitrate

(NO3–) due to the use of agricultural fertilisers. NO3

– can

accumulate in plants and cause health problems (EFSA, 2008;

Pinto et al., 2014a,b). Nitrate itself is relatively non-toxic but about

5% of ingested NO3– is reduced in the gastrointestinal tract to the

more toxic nitrite anion (NO2–). In fact, it is the NO3

– metabolites,

e.g., NO2–, nitric oxide (NO) and N-nitroso compounds, that are

considered toxic and can lead to human disorders, for example

methaemoglobinaemia, particularly in infants and children (EFSA,

2008; Santamaria, 2006). Several reports have documented the

development of methaemoglobinaemia in children after the

consumption of vegetables (Sadeq et al., 2008; Sanchez-Echaniz

et al., 2001; Savino et al., 2006). Thus, reduction of NO3– intake

associated with vegetables consumption is advisable.Microgreens are a new class of edible vegetables harvested at

the soil level when cotyledons (seed-leaves) have fully expandedand before true leaves have fully emerged, which usually occurs

E. Pinto et al. / Journal of Food Composition and Analysis 37 (2015) 38–43 39

within 7–14 days after germination (Sun et al., 2013). These greensare produced from the seeds of vegetables and their size is usuallybetween 3 and 10 cm in height, depending on the plant species.Over the past few years, microgreens have gained increasingpopularity as new culinary ingredients due to their wide range ofintense flavours, attractive colours and tender texture. Micro-greens can be served in salads, soups, sandwiches and main dishes(Xiao et al., 2012).

Lettuce (Lactuca sativa) is one of the most consumedvegetables in the world with a mean daily consumption of22.5 g per person in Europe, representing around 6.5% of thetotal dietary intake of vegetables (WHO, 2003). Lettucecontains several elements (e.g., Ca, Mg, K, Fe, Mn, Cu and Zn)that are considered essential for humans. Lettuce is also knownto be an excellent source of phytonutrients that play a key rolein human nutrition, such as chlorophylls and carotenoids(Pinto et al., 2014a).

Until now, the few studies performed with microgreens haveonly addressed a limited set of phytonutrients. Sun et al. (2013)showed that microgreens from Brassica species are good sources ofpolyphenols. Recently, Xiao et al. (2012) measured ascorbic acid,carotenoids, phylloquinone and tocopherols in 25 commerciallyavailable microgreens. Comparison between nutritional composi-tion of mature plants and microgreens indicated that the latter hadhigher nutritional densities. Younger leaves of baby spinach(Spinacia oleracea L.) possessed higher levels of phytonutrients(ascorbic acid, carotenoids, folate, a-tocopherol and phylloqui-none) than more mature leaves (Lester et al., 2010). To ourknowledge, no scientific data are available comparing the mineralcontents of microgreens and mature plants. The aim of this studywas to compare the mineral profile and NO3

– content of micro-greens and mature lettuces.

2. Material and methods

2.1. Lettuce cultivation, sampling and sample preparation

Lettuce (L. sativa L. var. capitata) seeds (n = 100) were germinat-ed in three greenhouse experimental fields – A1 (418 26.991 N,88 46.335 W), A2 (418 25.249 N, 88 44.936 W) and A3 (418 27.435 N,88 45.377 W) – located in NW Portugal. All plants were exposed tothe same light intensity (a total of 334 h of sunshine duration),photoperiod (on average 11.16 h) and temperature (average min.temperature: 7.6 8C; average max. temperature 20.5 8C) fromDecember to February. Inorganic fertilisers were dissolved in 50-Lwater tanks and injected by electric pumps in each of the six 25-mpipes,fromwheretheplantswerefertigated.Thenutrientelementsprovided per fertigation to lettuces were as follows:0.127 g N plant�1 fertigation�1, 0.041 g K plant�1 fertigation�1,0.029 g P plant�1 fertigation�1, 0.212 mg Fe plant�1 fertigation�1,0.212 mg Fe plant�1 fertigation�1,0.052 mg Mn plant�1fertigation�1and0.014 mg Zn plant�1 fertigation�1.

One kilogram of microgreens was randomly harvested fromeach field 2 weeks after germination and 10 units of mature lettuceweighing around 1 kg were randomly harvested from each field10 weeks after germination, according to Directive (2002/63/EC). Sample collection was always performed during the sameperiod of the day (09:00–12:00).

Plant samples were placed into plastic containers, previousrinsed with diluted nitric acid (10% v/v) and deionised water, andkept at 4 8C on the way to the laboratory. At the laboratory, plantsamples were cleaned with a brush and thoroughly washed withultra-pure water to remove soil contamination, frozen at �80 8Cand then freeze-dried. The dried samples were homogenised bygrinding in a blender and sieved through a nylon sieve of 150 mmmesh size.

2.2. Apparatus and reagents

A Telstar (Terrassa, Spain) Cryodos-80 freeze-dry system wasused to lyophilise the lettuce samples. The moisture content wasdetermined by using an HR73 Moisture Analyzer from MettlerToledo (Greifensee, Switzerland). An MLS 1200 Mega highperformance microwave digestion unit (Milestone, Sorisole, Italy)equipped with an HPR-1000/10 S rotor was used for acid digestionof lettuce samples. A K-424 digestion unit and a KjelFlex K-360distillation unit coupled to a B-414 scrubber, all from Buchi(Postfach, Switzerland), were used to perform the N-Kjeldahldetermination. A GFL (Burgwedel, Germany) 1083 shaking waterbath was used to carry out NO3

– extraction. Only plastic labware –pipette tips (VWR, Radnor, PA), volumetric flasks (Kartell, Milan,Italy) and centrifuge tubes (TRP, Trasadingen, Switzerland) – wasused during the work. All solutions were prepared using ultrapurewater (>18.2 MV cm at 25 8C) obtained with a Milli-Q RG(Millipore, Billerica, MA) water purification system.

A chromatographic system consisting of a Jasco (Tokyo, Japan)PU-2089Plus gradient pump, a Waters (Milford, MA) IC-PAKTM anion-exchange column (4.6 � 150 mm) and a Waters model 431 conduc-tivity detector was used for NO3

– determination. Data acquisitionwas done with Borwin PDA Controller Software (JMBS Develop-ments, Grenoble, France). The eluent was a 1.3 mM sodiumgluconate/1.3 mM borax solution adjusted to pH 8.5. A standardstock solution of NO3

– was prepared from its analytical reagent gradesodium salt.

A VG Elemental (Winsford, UK) PlasmaQuad 3 (quadrupole-based) instrument (ICP–MS) equipped with a Meinhard1 Type Aconcentric glass nebuliser, a water-cooled glass spray chamberwith impact-bead, a standard quartz torch, a nickel skimmer andsampling cones was used for elemental analysis. A Minipuls3 peristaltic pump (Gilson, Villiers le Bel, France) was used forsample introduction. The plasma source was supplied by argon of99.9% purity (Alphagaz 2TM, supplied by Air Liquide, Maia,Portugal). The ICP–MS instrumental conditions were: argon flowrate: 13 L min�1; auxiliary argon flow rate: 0.7 L min�1; nebuliserflow rate: 0.8 L min�1; RF power: 1350 W; scan regions dwell time:200 ms; detection mode; pulse counting. The elemental isotopes(m/z ratios) 31P, 55Mn, 65Cu, 66Zn, 82Se and 95Mo were monitoredfor analytical determinations; 45Sc, 89Y and 115In were used asinternal standards. The instrument was tuned daily for maximumsignal sensitivity and stability using 115In as the target isotope.Internal standards and tuning solutions were prepared byappropriate dilution of the corresponding AccuStandard1 (NewHaven, CT) solutions (ICP-MS-200.8-IS-1: 100 mg mL�1 of Sc, Y, In,Tb and Bi, and ICP-MS-200.8-TUN-1: 10 mg mL�1 of Be, Mg, Co, Inand Pb). Calibration standards were prepared from AccuStandard1

10 mg mL�1 multi-element ICP–MS standard solution (ICP-MS-200.8-CAL1-1). High purity HNO3 (65% w/w, TraceSELECT1 Ultra,Fluka, L’Isle d‘Abeau Chesnes, France) and H2O2 (30% v/v,TraceSELECT1, Fluka, Seelze, Germany) were used as received.

A Perkin Elmer (Uberlingen, Germany) 3100 flame (air-acetylene) atomic absorption spectrometer (AAS) was used forthe determination of Ca, Mg, K and Na. Multi-element calibrationstandards were prepared from 1000 mg L�1 single-elementstandard stock solutions (Sigma–Aldrich, St. Louis, MO) of thoseelements.

2.3. Lettuce analysis

The extraction of NO3– and NH4

+ from lettuce samples wasperformed according to Pinto et al. (2010) with some modifica-tions. Freeze-dried plant samples (0.1–1 g) were transferred into a125-mL conical flask, about 100 mL of hot water (70–80 8C) wereadded and the mixture was heated for 15 min in a boiling water

Table 1Concentrations obtained for minerals determination in BCR 679 (certified reference

material), expressed as mg g�1 (n = 5).

Certified value � uncertainty Experimental value

Mean RSD (%)

Ca 7768 � 655 7998 0.51

Mg 1362 � 127 1305 0.54

P 3307 � 241 3341 0.81

Fe 55.0 � 2.5 54.7 2.56

Mn 13.3 � 0.5 13.0 0.77

Cu 2.89 � 0.12 2.80 2.14

Zn 79.7 � 2.7 80.5 0.62

Mo 14.8 � 0.5 15.1 1.32

E. Pinto et al. / Journal of Food Composition and Analysis 37 (2015) 38–4340

bath with reciprocal shaking. After cooling, the content wastransferred to a 200-mL volumetric flask and diluted to the markwith ultra-pure water. Plant extracts were further analysed by ionchromatography for NO3

– content. Ammonium was determined bya spectrophotometric method (ISO, 1984). Nitrogen was deter-mined by the Kjeldahl method (ISO, 2009).

Total metal content was determined in plant samples accordingto the following microwave-assisted digestion procedure. Approx-imately 500 mg of lettuce samples were weighed into PTFE vessels,5 mL of 65% (w/w) HNO3 and 2 mL of 30% (v/v) H2O2 were added toeach vessel and the mixture was digested. The microwave heatingprogramme was: 250 W for 1 min, 0 W for 2 min, 250 W for 5 min,400 W for 5 min, and 600 W for 5 min. After cooling, samplessolutions were transferred to 25-mL volumetric flasks and thevolume made up to the mark with ultra-pure water. Afterwards,solutions were analysed by AAS and ICP-MS for total metal content.All lettuce samples were analysed in quintuplicate. Results wereexpressed on a fresh weight (fw) basis.

2.4. Soil analysis

Soil pH and electrical conductivity (EC) were measured in thesupernatant of a 1:5 (w/v) suspension prepared with ultra-purewater (ISO, 2005). Organic matter (OM) content was determinedthrough an oxidation procedure with a potassium dichromate(0.27 M) and sulphuric acid (98% v/v) solution at 135 8C (ISO, 1998b).Cation exchange capacity (CEC) was determined using the ISO(1994). Particle size distribution was determined according to ISO(1998a). Al-, Fe- and Mn-oxides content were determined accordingto ISO (2012). Extraction of NO3

– from soil samples was conductedfollowing the ISO/TS (2003) with some modifications. Briefly, soilsamples were extracted with ultra-pure water at a ratio 1:5 (w/v) at20 8C. After 1 h of extraction, the solution was centrifuged at 3000 � g

for 10 min and the supernatant was used for the measure of NO3� by

ion chromatography. All soil samples were analysed in quintuplicate.

2.5. Estimated daily intake of major and trace elements from lettuce

The estimated daily intake (EDI) of minerals resulting fromlettuce consumption was calculated. The EDI, which is dependenton both the mineral content of the edible part of the plants(Cmineral; mg g�1 fw basis) and the average daily consumption oflettuce (DClettuce), was calculated using the following formula:

EDI ¼ DClettuce � Cmineral (1)

where EDI is expressed as mg day�1 and DC was assumed to be22.5 g day�1 for adults (WHO, 2003).

2.6. Quality control

For the quality control of total metal determination, thecertified reference material BCR 679 (white cabbage, supplied

Fig. 1. Comparison between the mineral content of microgreens and mature lettuces. D

experimental fields. Different letters on the bars of each mineral indicate a significant dif

by EC Institute for Reference Materials and Measurements, Geel,Belgium) was analysed in triplicate after being subjected to thesame pretreatment as samples; the values obtained proved theaccuracy of the analytical procedure.

2.7. Statistical analysis

Data exploration was performed to check the normal distribu-tion of data. Student’s t-test was carried out to comparecomposition of microgreens and mature lettuces. ANOVA followedby Tukey’s test was used to search for differences between soils.Statistical treatment was done using IBM SPSS Statistics forWindows, version 22.0 (IBM Corp, Armonk, NY). Statisticaldifferences were assumed at p < 0.05.

3. Results and discussion

3.1. Validation of the analytical procedures

To ensure the precision and accuracy of the analyticalprocedures, a BCR 679 certified reference material was analysed.Results are presented in Table 1. The obtained values were alwayswithin the certified range, proving the accuracy of the analyticalprocedures used. The low RSD values proved the high precisionobtained (Table 1). The limit of quantification (LOQ) for Ca, Mg, Kand Na (AAS determination) was <5 mg g�1. For the remainingelements (P, Fe, Mn, Cu, Zn, Mo, Se), which were determined byICP-MS, the LOQ was <25 ng g�1.

3.2. Mineral content of microgreens and mature lettuces

The moisture content of microgreens and mature lettuces wassimilar (94.8 � 0.3% and 94.3 � 0.5%, respectively). No significantdifference was also observed in moisture content of lettuces from thedifferent fields. The average weight of microgreens and maturelettuces was 1.96 � 0.12 and 176 � 7 g fw plant�1, respectively. Fig. 1shows the mean mineral content in both microgreens and maturelettuces from the three fields. Except for Mg, Na and Cu, t-test

ata are presented as mean � SD of the results obtained for the samples from three

ference at p < 0.05.

Table 2Estimated daily intake (EDI) of nutritional minerals resulting from microgreens and

mature lettuces consumption based on an average daily intake of 22.5 g person�1.

Element Microgreen

lettuces

Mature

lettuces

Ratio microgreen/

mature

lettuces

RDA/AIa

Men Women

EDI (mg day�1)

N 82.1 98.1 0.8 – –

P 21.7 41.5 0.5 700 700

Ca 34.8 17.2 2.0 1000 1000Mg 6.5 4.5 1.4 420 320

K 82.1 98.1 0.8 4700 4700

Na 12.4 14.1 0.9 1500 1500

Fe 0.32 0.17 1.9 8 18

Mn 0.28 0.03 9.3 2.3 1.8Zn 0.13 0.08 1.6 11 8

EDI (mg day�1)

Cu 9.6 8.5 1.1 900 900

Se 1.49 0.28 5.3 45 55

Mo 2.58 1.04 2.5 45 45

a RDAs (recommended dietary allowances) and AIs (adequate intakes) may both

be used as goals for individual intake. RDAs are in regular type and AIs in bold type.

E. Pinto et al. / Journal of Food Composition and Analysis 37 (2015) 38–43 41

indicated significant differences between microgreens and maturelettuces for all the other minerals. Mature lettuces possessedsignificantly higher N, P and K content. All the other elementsdetermined (Ca, Fe, Mn, Zn, Se and Mo) were present at significantlyhigher levels in microgreens. The mean content of all the elements inmature lettuces were within the typical range described in theliterature (Ekholm et al., 2007; Kawashima and Soares, 2003; Lisiewskaet al., 2009). The mean Na content in mature lettuces(628 � 29.3 mg g�1) was almost 13-fold higher than the value(50 mg g�1) reported by Kawashima and Soares (2003) for butterheadlettuce. Regarding microgreens, few data are available on their mineralcontent, which limits data comparison. Recently, Santos et al. (2014)quantified several minerals (P, K, Ca, Mg, Na, Fe, Mn, Zn and Cu) inready-to-eat ‘‘baby leaf’’ vegetables. The results obtained in our studyare in good agreement with the values reported by those authors.

3.3. Estimation of the mineral daily intake from microgreens and

mature lettuces

The mean EDI values obtained for microgreens and maturelettuces are shown in Table 2. Microgreens showed higher EDIs for

Fig. 2. Comparison between NO3– and NH4

+ content of microgreens and mature

lettuces. Data are presented as mean � SD of the results obtained for the samples of

the three experimental fields. Different letters in the bars of each compound indicate

significant difference at p < 0.05.

Ca, Mg, Fe, Mn, Cu, Zn, Se, Mo (EDIs for N, P, K and Na were higherfor mature lettuces). Overall, microgreens could provide a higherdaily intake of most minerals compared with mature lettuces.Specifically, microgreens consumption can provide twice as muchCa and Fe, almost three-fold as much Mo, five-fold as much Se andnine-fold as much Mn as mature lettuces. This information is abreakthrough in the field of human nutrition. The consumption ofmicrogreens can be a cost-effective solution to meet daily mineralrequirements.

3.4. Nitrate content of microgreens and mature lettuces

Both NO3– and NH4

+ contents were determined in microgreensand mature lettuces from the three fields. As shown in Fig. 2, higherNO3

– and NH4+ contents were observed in mature lettuces. It is

relevant to know which form of inorganic N (NO3– or NH4

+) ispresent in plant foods, since NO3

– can accumulate in vegetables,leading to the toxic effects mentioned above. A maximum tolerablelevel of NO3

– is set at 4500 mg kg�1 by EU Regulation (2006). NO3–

content of both microgreens and mature lettuces was below thislimit; however, mature lettuces provides four-fold as much NO3

–

than microgreens. The accumulation of NO3– in leaf crops (e.g.,

cabbage, spinach, lettuce) is reported in the literature (Pinto et al.,2010; Santamaria, 2006; van Velzen et al., 2008). Since infants andchildren are particularly susceptible to methaemoglobinaemia andthere are reports describing the development of methaemoglobi-naemia in children after the consumption of vegetables (Sadeqet al., 2008; Sanchez-Echaniz et al., 2001; Savino et al., 2006),microgreens are a good alternative to reduce the intake of NO3

–

while increasing the intake of essential minerals.

3.5. Soil physicochemical properties and elemental phytoavailability

The physicochemical characteristics pH, OM, EC, CEC, oxides(Al, Fe and Mn) content and particle size distribution of the threesoils are shown in Table 3. A2 soil is different when compared withA1 and A3 soils, it presented lower pH, and significantly higher CEC(17.2 cmol kg�1), EC (633 mS cm�1) and OM (72.8%). A2 soil alsoshowed a significantly higher Al-, Fe- and Mn-oxides content (1.58,0.40 and 4.77 mg g�1, respectively). Regarding particle sizedistribution, A2 soil was classified as loamy sand soil while A1

and A3 soils were classified as sandy soils.With regard to element phytoavailability, A2 soil showed a

significantly higher phytoavailable content of almost all elements,namely Ca, Mg, K, Na, NO3

–, PO43–, Mo and Se. The only exceptions

were Mn, Cu and Zn (Table 4).The mineral content of lettuces can significantly vary depend-

ing on the soil characteristics (physicochemical properties andcomposition) where they grow (Pinto et al., 2014a). Microgreensand mature lettuces studied in this work were grown in threedifferent greenhouse experimental fields in order to take into

Table 3Main physicochemical properties of soil samples collected from the three

experimental fields (A1, A2 and A3).

A1 A2 A3

pH 6.8 � 0.1a 6.7 � 0.2b 6.8 � 0.2a

CEC (cmol kg�1) 7.9 � 0.4a 17.2 � 0.6b 7.9 � 0.5a

OM (%) 56.3 � 3.9a 72.8 � 2.3b 20.7 � 1.6c

EC (mS cm�1) 283 � 40a 633 � 69b 196 � 27c

Al oxides (mg g�1) 1.11 � 0.07a 1.58 � 0.09b 1.12 � 0.08a

Mn oxides (mg g�1) 0.23 � 0.01a 0.40 � 0.02b 0.20 � 0.01c

Fe oxides (mg g�1) 2.20 � 0.10a 4.77 � 0.17b 2.16 � 0.11a

Soil classification Sandy soil Loamy sand soil Sandy soil

Data presented as mean � SD (n = 5 time points during the study period). Differences

were tested according to ANOVA followed by Tukey’s test. In a row, different letters (a,

b and c) indicate significant differences (p < 0.05) between the experimental fields.

Table 4Phytoavailable content (mg g�1) of elements in soil samples collected from the three

experimental fields (A1, A2 and A3) at the end of the experiment.

A1 A2 A3

NO3– 127.2 � 6.7a 329.5 � 8.2b 50.5 � 7.7c

PO43– 51.3 � 3.2a 101.4 � 6.2b 63.8 � 4.4c

Ca 780 � 111a 1293 � 161b 897 � 102c

Mg 90.2 � 10.1a 204 � 11b 119 � 17c

K 168 � 17a 401 � 39b 108 � 22c

Na 76.5 � 4.9a 194 � 23b 40.3 � 13.7c

Fe 0.32 � 0.03a 0.31 � 0.03a 0.29 � 0.05a

Mn 0.22 � 0.07a 0.13 � 0.05b 0.27 � 0.07c

Cu 0.077 � 0.006a 0.055 � 0.004b 0.476 � 0.031c

Zn 0.082 � 0.011a 0.030 � 0.002b 0.065 � 0.022c

Mo 0.021 � 0.004a 0.038 � 0.006b 0.020 � 0.003a

Se 0.023 � 0.007a 0.028 � 0.008b 0.018 � 0.007a

Data presented as mean � SD (n = 5). Differences were tested according to ANOVA

followed by Tukey’s test. In a row, different letters (a, b and c) indicate significant

differences (p < 0.05) between the experimental fields.

E. Pinto et al. / Journal of Food Composition and Analysis 37 (2015) 38–4342

account the soil mineral variability. For microgreens, the influenceof soil physicochemical properties and composition is somehowinsignificant because changes due to soil properties/compositionusually occur throughout all plant growth period. For maturelettuces, soil physicochemical properties/composition clearlyinfluences their mineral and nitrate content. A2 soil is clearlydifferent from the other two soils (A1 and A3) regarding theirphysicochemical properties. Since these properties have a majorinfluence in elements phytoavailability, it is expected that A2 soilshowed the highest phytoavailable content of almost all thestudied elements. This, in turn, greatly influences the maturelettuces mineral content as shown in Fig. 1. It is widely describedthat NO3

– accumulation in plants is the result of several factors,such as soil NO3

– phytoavailability, nitrate reductase (NR) activity,temperature, light intensity, water availability, time of the yearand harvest date (Chen et al., 2004; Ierna, 2009; Pinto et al., 2014b).In our study, since temperature, light intensity, water availability,harvest date and time of the year were the same or kept constantthroughout the all study period, it is proposed that NO3

–

accumulation in mature lettuce occurred mainly due to the highsoil NO3

– phytoavailability and the low capacity of NR to reducethis anion in the latter stages of lettuce growth (Pinto et al., 2014b).

4. Conclusions

This is the first study that compared the mineral profile andNO3

– content of microgreens and mature lettuces. Microgreens hadhigher content of Ca, Mg, Fe, Mn, Zn, Se and Mo than maturelettuces, although the former possessed higher N, P and K content.Thus, microgreens can provide a significantly higher intake ofessential minerals than mature lettuces. They can be considered agood source of minerals. Additionally, the NO3

– content ofmicrogreens was very low. Thus, microgreens can be safely usedin the human diet, in particular for children, to fulfil their dailymineral requirements, while reducing their exposure to harmfulNO3

–. In conclusion, this work provides important data on thenutritional value of microgreens.

Acknowledgements

Edgar Pinto thanks to FCT (Portuguese Foundation for Scienceand Technology) for his PhD grant (SFRH/BD/67042/2009). Thiswork received financial support from the European Union (FEDERfunds through COMPETE) and National Funds (FCT) throughproject Pest-C/EQB/LA0006/2013. The work also received financialsupport from the European Union (FEDER funds) under the

framework of QREN through Project NORTE-07-0124-FEDER-000069. To all financing sources the authors are greatly indebted.

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