Vol. 69, Nr. 9, 2004—JOURNAL OF FOOD SCIENCE S347Published on Web 10/28/2004

© 2004 Institute of Food TechnologistsFurther reproduction without permission is prohibited

S: Se

nsor

y & Nu

tritiv

e Qua

lities

of Fo

od

JFS S: Sensory and Nutritive Qualities of Food

Chlorophyll-bound Magnesium inCommonly Consumed Vegetables andFruits: Relevance to Magnesium NutritionT. BOHN, T. WALCZYK, S. LEISIBACH, AND R.F. HURRELL

ABSTRACT: Magnesium is bound as the central atom of the porphyrin ring of the green plant pigments chlorophylla and b. It has been suggested that chlorophyll-bound magnesium may play an important role in magnesiumnutrition because when iron is similarly bound in the porphyrin ring of heme, it is absorbed to a greater extent thannon-heme iron. We have analyzed 22 frequently consumed fruits and vegetables for the chlorophyll content byhigh-pressure liquid chromatography and for magnesium with atomic absorption spectroscopy. Chlorophyllconcentrations ranged from 6 �����g/g (grape) to 790 �����g/g (spinach) (median 63 µg/g). Magnesium concentrationsranged from 48 �����g/g (grape) to 849 �����g/g (spinach) (median 122 �����g/g). In the green leafy vegetables, such asspinach and lettuce, chlorophyll-bound magnesium represented 2.5% to 10.5% of total magnesium whereas othercommon green vegetables, pulses and fruits contained <1% chlorophyll-bound magnesium. The chlorophyllcontent of spinach was further decreased by about 35% on thawing frozen spinach or on chopping fresh spinach,and this degradation increased to about 50% after boiling and steaming. Based on the present results and pub-lished food consumption data, we estimate that chlorophyll-bound magnesium represents a very low fraction oftotal magnesium intake in industrialized countries, less than 1% in the case for data obtained from Switzerland.Thus, chlorophyll-bound magnesium is of little relevance to magnesium nutrition.

Keywords: magnesium, chlorophylls, chlorophyll-bound magnesium, vegetables and fruits, pigment degra-dation

Introduction

When iron is bound within the porphyrin ring of heme, it is ab-sorbed to a much greater extent than non-heme iron. In in-

dustrialized countries, heme iron from animal foods representsabout 10% to 15% of mean iron intake, but it provides up to 50% ofabsorbed iron (Allen and Ahluwalia 1997).

Magnesium is similarly bound to the central atom in the porphy-rin ring of the major green plant pigments chlorophyll a and b. Inplant leaves, up to 25% of total magnesium can be present as chlo-rophyll-bound magnesium (Marschner 1986), although this canincrease up to 60% under magnesium-deficient conditions (Doren-stouter and others 1985).

It has been suggested that chlorophyll-bound magnesium mayplay an important role in magnesium nutrition (Hazell 1985; Fair-weather-Tait and Hurrell 1996). Vegetables and fruits have beenreported to contribute 18% of the magnesium intake in Germany(Deutsche Gesellschaft für Ernährung 1996), 21% to 43% in Swit-zerland (De Rham 1991; Schlotke and Sieber 1998), 16% in the U.K.(Spring and others 1979), and 26% in the U.S.A. (Pennington andYoung 1991). The chlorophyll content of most common plant foodshowever is unknown, although there is limited data on some leafyvegetables (Kaur and Manjrekar 1975; Khachik and others 1986). Toevaluate the importance of chlorophyll-bound magnesium intakein industrialized countries, we have measured chlorophyll a and b

and magnesium in a variety of commonly consumed green vegeta-bles and fruits, and calculated the ratio of chlorophyll-bound mag-nesium to total magnesium. In addition, the influence of food prep-aration and cooking on the chlorophyll content was investigatedwith spinach as a model. Based on the chlorophyll-bound magne-sium fraction in the analyzed plant foods, and on published foodconsumption data, we then evaluated the importance of chloro-phyll-bound magnesium in magnesium nutrition.

Materials and Methods

Plant foodsFrequently consumed vegetables and fruits, expected to con-

tain relevant amounts of chlorophyll, were selected based on Swissfood consumption data (Erard and Sieber 1991; Grüter and others1998). Twenty-one fresh plant foods in amounts of 50 to 500 g werebought between July and August at local supermarkets, weighed,and immediately frozen at –70 °C until analysis. Green peas werepurchased already frozen. Only the edible parts of each plant foodwere analyzed with the exception of artichoke where whole leaveswere measured and cucumber, which was analyzed unpeeled.

Quantification of chlorophyllsChlorophyll a and b were measured in duplicate by high-perfor-

mance liquid chromatography (HPLC) as described by Bohn andWalczyk (2004). In short, the chlorophylls were extracted from theuntreated frozen plant material by N,N-dimethylformamide, pu-rified by solid-phase extraction, and chlorophyll a and b were quan-tified by an HPLC fluorescence technique, using zinc-phthalocya-

MS 20040209 Submitted 4/5/04, Revised 6/17/04, Accepted 7/23/04. Authorsare with Inst. of Food Science and Nutrition, Laboratory for Human Nutri-tion, Swiss Federal Inst. of Technology Zurich, Seestrasse 72, 8803 Rueschlikon,Switzerland. Direct inquiries to author Bohn (E-mail: bohn.22@osu.edu).

S: Sensory & Nutritive Qualities of Food

S348 JOURNAL OF FOOD SCIENCE—Vol. 69, Nr. 9, 2004 URLs and E-mail addresses are active links at www.ift.org

Chlorophyll-bound magnesium in foods . . .

nine as an internal standard. Pigments were identified by retentiontimes. The degradation products chlorophyll a� and b� (the C132

epimerization products), and pheophytins (a, b, a�, b�) were used asmarkers for degradation and were detected by comparing reten-tion times with literature values (Khachik and others 1986; Zapataand others 1987; van Breemen and others 1991; Brotas and Plante-Cuny 1996). All reagents and solvents were of analytical grade orsuperior, and only 18 M� water (Milli Q water system, Millipore, Zu-rich, Switzerland) was used.

Quantification of total magnesium and chlorophyll-bound magnesium

Frozen uncooked plant material (10 to 20 g) was thoroughlycrushed and mixed at room temperature in a mortar. A 1- to 3-g al-iquot was weighed into a Teflon vial, mineralized in a microwavedigestion system (MLS 1200 Mega, MLS GmbH, Leutkirch, Germa-ny) in a mixture of 65% HNO3 and 30% H2O2 and quantified by flameatomic absorption spectroscopy (SpectrAA 400, Varian, Mulgrave,Australia) by external calibration using aqueous standards (Titrisol,Merck, Darmstadt, Germany). All measured solutions containedLa(NO3)3 at 5000 mg La/L to suppress matrix effects. A certified ref-erence material (wheat flour 1567 a, NBS, Natl. Bureau of Stan-dards, Gaithersburg, Md., U.S.A.) was analyzed in parallel to mon-itor accuracy of analysis. All samples were analyzed in duplicate.

Chlorophyll-bound magnesium of each plant food was calculat-ed based on the chlorophyll a and b content determined by HPLCand the theoretical magnesium content (in % mass) of chlorophylla (2.72%) and chlorophyll b (2.68%).

Food preparation and cooking proceduresAs most plant foods are not consumed raw, the effect of food

preparation and cooking procedures on the chlorophyll content wasestimated. The influence of thawing and chopping, followed by

boiling and steaming, was investigated with spinach, a vegetablewith a high chlorophyll content.

To estimate the effect of thawing on fresh frozen vegetables,about 200 g fresh spinach was frozen (–70 °C) for 24 h and thenthawed at room temperature (4 h) before analysis (n = 6). The effectof chopping on chlorophyll degradation (n = 6) was investigated bycutting fresh leaves into about 10 cm2 pieces with a sharp knife. Toallow for homogenous sampling, a large amount of spinach (1 kg) waschopped, mixed, and then divided into 3 equal portions for analysisof chopped, boiled, and steamed spinach. Fresh chopped spinach(50 g) was weighed into 250 mL glass beakers (n = 6), 1 g sodium chlo-ride, and 100 g tap water were added, and the beaker was coveredwith a lid and brought to boiling on a heating plate. After boiling hadstarted, the beaker was stirred for about 5 s every min for 3 min, af-ter which the heating plate was switched off. After a further 2 min, thebeaker was removed from the plate, allowed to cool for 10 min atroom temperature, and the boiling water was decanted. The boiledspinach and the decanted water were weighed and analyzed sepa-rately for their chlorophyll content. A similar steaming procedure wasfollowed with 5 g tap water without stirring. All samples obtainedafter thawing, chopping, boiling, and steaming were frozen to –70 °Cand analyzed for chlorophyll content as described previously.

Statistical analysisCalculations were done using commercial spreadsheet software

(Excel 97, Microsoft, Chicago, Ill., U.S.A.; and SPSS 10.0, SPSS Inc.,Chicago, Ill., U.S.A.). Means are expressed as arithmetic means ±standard deviation (SD). Level of significance was defined asP < 0.05. Paired Student’s, 2-tailed t-tests were used to comparechlorophyll a and b degradation, and unpaired, 2-tailed t-testswere used to compare the different food preparation procedures.Normal distribution of absorption values was confirmed by skew-ness and Kolmogorov-Smirnoff test.

Table 1—Magnesium (Mg), chlorophyll (chl), chlorophyll-bound magnesium (chl-Mg), and chlorophyll-bound magnesiumas % of total magnesium (chl-Mg/total-Mg) of commonly consumed, uncooked vegetables, fruits, and pulsesa

Plant food Mgb (�����g/g) Chlc (�����g/g) Ratio Chla/chlbd Chl-Mge (�����g/g) Chl-Mg/total-Mg (%)

Artichoke 347 49 3.2 1.33 0.38Broccoli 139 21 3.5 0.58 0.41Celery 264 23 3.4 0.64 0.24Chinese cabbage 115 58 2.7 1.59 1.38Cress 271 311 2.6 8.49 3.13Cucumber 85 36 2.5 1.00 1.17Endive 90 104 3.1 2.85 3.16Fennel 81 6 5.4 0.16 0.19Grape 48 6 5.3 0.17 0.34Green bean 246 75 3.5 2.05 0.83Green pea 243 50 3.3 1.36 0.56Iceberg lettuce 51 29 3.7 0.81 1.58Kiwi 128 15 2.7 0.41 0.32Lettuce 103 245 4.3 6.67 6.49Green paprika 82 38 2.8 1.03 1.26Parsley 112 632 3.1 17.26 15.41Prickly lettuce 101 388 3.4 10.61 10.50Leek 66 87 3.2 2.38 3.60Rocket salad 310 408 3.6 11.15 3.60Spinach 849 791 2.9 21.59 2.54Sugar pea 266 76 4.2 2.07 0.78Zucchini 225 68 2.6 1.85 0.82Mean (± SD) 192.± 173 160.± 217 3.4 ± 0·8 4.37 ± 5·91 2.67 ± 3·75Median 122 63 3.3 1.72 1.22aData are reported as the mean of 2 independent analyses.bMagnesium content as determined by flame atomic absorption spectroscopy.cTotal chlorophyll content as determined by HPLC.dRatio of the amount (µg) chlorophyll a/chlorophyll b.eCalculated based on measured magnesium and chlorophyll content and the theoretical magnesium content of chlorophyll a (2.72%) and chlorophyll b (2.68%)by mass.

Vol. 69, Nr. 9, 2004—JOURNAL OF FOOD SCIENCE S349

S: Se

nsor

y & Nu

tritiv

e Qua

lities

of Fo

od

URLs and E-mail addresses are active links at www.ift.org

Chlorophyll-bound magnesium in foods . . .

Results

Magnesium, chlorophyll, and chlorophyll-boundmagnesium

The amount of magnesium, chlorophyll, and the fraction ofmagnesium present as chlorophyll are shown in Table 1. The mag-nesium content of the commonly consumed, uncooked vegetablesand fruits varied from 48 �g/g (grape) to 849 �g/g (spinach) with amedian of 122 �g/g.

The chlorophyll content also varied considerably. The lowest totalchlorophyll concentrations were found in grape and fennel (both 6�g/g), the highest in spinach (791 �g/g). The median of total chlo-rophyll content was 63 �g/g. Chlorophyll-bound magnesium as %of total magnesium varied between 0.2% (fennel) and 15.4% (pars-ley) with a median of 1.2%. The ratio of the amount chlorophyll a tochlorophyll b varied from 2.5 to 5.4 with a median of 3.3. No, or onlyminor indications, of degradation products were detected in theanalyzed fresh food samples. Pheophytin a and b and the C132

epimerization products pheophytin a� and b� and chlorophyll a�

and b� were detected in the peas, which were bought frozen.

Food preparation and cooking proceduresChopping was the major cause of chlorophyll degradation in the

food preparation procedures. Chopping of spinach resulted in38.7 ± 6.1% total chlorophyll degradation in spinach (Figure 1).Chopping together with boiling and steaming caused only smalladditional chlorophyll degradation of about 6% and 13%, respec-tively (total degradation was 44.2 ± 5.9% and 51.5 ± 4.4%). Chop-ping and boiling resulted in significantly lower chlorophyll degra-dation than chopping and steaming (P < 0.05). The boiling andsteaming water contained <1% of the original chlorophyll content(about 0.6% and 0.3%). Thawing of frozen fresh spinach also result-ed in high chlorophyll degradation (34.6 ± 4.2%). Mean chlorophylla degradation was significantly higher than chlorophyll b degrada-tion when the results for all preparations and cooking procedureswere combined (45.6 ± 8.0% compared with 32.8 ± 9.8%, P < 0.0001,n = 24), and also for each of the 4 individual food preparation pro-cedures (P < 0.05, n = 6).

Pheophytin a and b and the C132 epimerization products pheo-phytin a�and b� and chlorophyll a� and b� were detected for spinachafter thawing, chopping, and additional boiling and steaming ofspinach.

Discussion

The magnesium content of the 22 common plant foods rangedfrom 48 to 849 �g/g. This range is similar to that reported in

food databases (Haenel 1979; Holland and others 1994; Souci andothers 1994). The chlorophyll content in the plant foods rangedfrom 6 to 791 �g/g, a larger variation when compared with the rangeof the magnesium content. This is to be expected, as chlorophyll isprimarily needed for photosynthesis whereas magnesium hasmany functions and would therefore be expected to be more even-ly distributed.

A high chlorophyll content was found in leafy vegetables suchas spinach and lettuce. As photosynthesis takes place mainly inleaves, leafy vegetables have a much higher chlorophyll contentthan other vegetables or fruits. In previous studies (Kaur andManjrekar 1975; Khachik and others 1986), the chlorophyll con-tent of 5 plant foods (spinach, cabbage, Brussels sprouts, kale,and broccoli) was measured. The chlorophyll content in spinachreported by these authors was found to be around 130 mg/100 g,which is somewhat higher than in the present study (80 mg/100g), while chlorophyll content of cabbage was reported to be 1.7mg/100 g, which is slightly below the value which was obtained forChinese cabbage in the present evaluation (5.8 mg/100 g). Thesedifferences are not unexpected as magnesium and chlorophyllcontent would be expected to vary with growing site, agriculturalpractices, time point of crop, weather, soil composition, and differ-ent sub-species. The amount ratios of chlorophyll a/chlorophyll b(2.5 to 5.4) were similar to the previously reported range of 2.8 to4.7 (Kaur and Manjrekar 1975; Khachik and others 1986; Rousosand others 1986).

No chlorophyll degradation to their corresponding pheophyt-ins and epimerization products was observed during pigment ex-traction and purification during analysis of the untreated plantfoods, as indicated by HPLC analysis. The presence of degrada-tion products in the frozen peas however indicated chlorophylldegradation during the industrial process, probably during theinitial blanching step prior to freezing (Schwartz and Vonelbe1983). Blanching is usually done to inactivate enzymes, includingchlorophyllase, thus preventing further degradation of the chlo-rophylls. Detection or quantification of chlorophyll degradationproducts has been suggested as an indicator for food processing(Clydesdale and Francis 1968; Ashgar and others 1978; Mangosand Berger 1997).

The percentage of total magnesium that is chlorophyll-boundwas relatively low and ranged from 0.2% to 15.4%. It was highest inleafy vegetables and herbs. These results are similar to the rangesof chlorophyll-bound magnesium of total magnesium reported byMichael (1941) for maize leaves (0.3% to 19%), Scott and Robson(1990) for clover leaves (6% to 35%), and the range (6% to 25%) givenin a review by Marschner (1986) including Norway spruce needles.Under more extreme conditions, such as for magnesium-deficientplant leaves that are in the shade, chlorophyll-bound magnesiumas percentage of total magnesium can rise to nearly 60% (Doren-stouter and others 1985).

The amount of chlorophyll-bound magnesium can be expectedto further decrease after food preparation and cooking. We foundchlorophyll degradation as a result of thawing and chopping ofabout 35%, while boiling or steaming of chopped fresh spinach leadonly to small additional chlorophyll degradation (Figure 1). This canbe explained by breakdown of the cell structures and the release ofchlorophyllase, leading to saponification of the phytol chain of chlo-rophyll to form the respective chlorophyllides. During heat treat-ment, the chlorophyllase is inactivated, and the small amount offurther degradation observed in the present evaluation may have

Figure 1—Degradation of chlorophyll in spinach by differ-ent preparation and cooking procedures (n = 6 for eachprocedure, 100% = fresh frozen spinach, error bars rep-resent standard deviation).

S: Sensory & Nutritive Qualities of Food

S350 JOURNAL OF FOOD SCIENCE—Vol. 69, Nr. 9, 2004 URLs and E-mail addresses are active links at www.ift.org

Chlorophyll-bound magnesium in foods . . .

been caused by organic acids released by cell rupture or heat (Ha-isman and Clarke 1975). A study by Khachik and others (1986) in-dicated chlorophyll degradation of 37% and 39% for kale and Brus-sels sprouts by cooking alone.

All food preparation and cooking procedures caused a signifi-cantly higher degradation of chlorophyll a than chlorophyll b(45.6 ± 8.0% compared with 32.8 ± 9.8%), which is in line with earlierreports on chlorophyll degradation (Mackinney and Joslyn 1940;Schanderl and others 1962; Siefermann-Harms and Ninnemann1983). However, as chlorophyll content and enzyme activities dif-fer between plant parts and plant species, degradation pathwaysand rate of chlorophyll degradation may differ.

To evaluate the nutritional importance of chlorophyll-boundmagnesium, it is necessary to evaluate whether chlorophyll-boundmagnesium could be absorbed to a different extent than otherfood magnesium. When iron as heme iron is bound within a similarporphyrin structure, its absorption is up to 10 times higher thannon-heme iron (Weintraub and others 1968; Bothwell and others1989). The heme molecule is absorbed intact and iron absorptionis not inhibited by food components such as phytic acid. Althougha similar effect has been suggested for chlorophyll (Hazell 1985;Fairweather-Tait and Hurrell 1996), this seems unlikely because atthe acidic pH of the gastric juice, a rapid and irreversible degrada-tion of the chlorophylls to their corresponding pheophytins is likelyto occur. During this degradation, magnesium is released from theporphyrin ring and replaced by 2 protons (Schanderl and others1962; Siefermann-Harms and Ninnemann 1983).

The relevance of chlorophyll-bound magnesium-to-magne-sium nutrition can be evaluated by estimating the percent of totalmagnesium intake provided by chlorophyll-bound magnesium.Fruits and vegetables have been reported to contribute between16% and 43% of the total magnesium intake in industrializedcountries (Spring and others 1979; De Rham 1991; Penningtonand Young 1991; Deutsche Gesellschaft für Ernährung 1996;Schlotke and Sieber 1998). The amount of consumed green fruitsand vegetables containing chlorophyll can be expected to bemuch lower. Based on recent Swiss food consumption data (Erardand Sieber 1991; Grüter and others 1998) we estimated that chlo-rophyll-bound magnesium represents less than 1% of total mag-nesium intake. This estimation is based on the assumption that43% of total dietary magnesium intake in Switzerland comes fromfruits and vegetables, that 60% of the fruits and vegetables con-sumed contain chlorophyll, and, from the present investigation,that an average of 2.7% of total magnesium in green fruits andvegetables is chlorophyll-bound magnesium. Furthermore, cook-ing and food preparation will cause additional degradation, espe-cially if the plant foods are chopped.

In other industrialized countries, the chlorophyll-bound magne-sium intake can be expected to differ to some extent from the esti-mation based on the data obtained from Switzerland, as differentgrowing conditions in other countries might alter the percentage oftotal magnesium that is bound to chlorophyll. An even lower chlo-rophyll-bound magnesium intake seems also possible, as lowerpercentage of total magnesium intake coming from vegetables andfruits (16% to 26%) have been reported for England, the U.S., andGermany (Spring and others 1979; Pennington and Young 1991;Deutsche Gesellschaft für Ernährung 1996).

Conclusions

It can be concluded from the present results that chlorophyll-bound magnesium contributes a small and nutritionally insignif-

icant part of total magnesium intake in industrialized countries.

ReferencesAllen LH, Ahluwalia N. 1997. Improving iron status through diet. Washington,

D.C.: USAID. 83 p.Ashgar A, Sami M, Nadeeem MT, Sattar A. 1978. Effect of some pre-drying unit

operations on the chlorophyll stability and nutritional quality of dehydratedpeas. Lebensm Wiss Technol 11:15–8.

Bohn T, Walczyk T. 2004. Chlorophyll analysis in plant samples by HPLC usingzinc-phthalocyanine as an internal standard. J Chromatogr A 1024:123–8.

Bothwell TH, Baynes RD, MacFarlane BJ, MacPhail AP. 1989. Nutritional ironrequirements and food iron absorption. J Intern Med 226:357–65.

Brotas V, Plante-Cuny MR. 1996. Identification and quantification of chlorophylland carotenoid pigments in marine sediments. A protocol for HPLC analysis.Oceanol Acta 19:623–34.

Clydesdale FM, Francis FJ. 1968. Chlorophyll changes in thermally processedspinach as influenced by enzyme conversion and pH adjustment. Food Tech22:135–8.

De Rham O. 1991. Berechnungen der täglich verbrauchten und verzehrten Men-gen an Energie und Lebensmittelinhaltsstoffen. In: Stähelin HB, Lüthy J, Casa-bianca A, Monnier N, Müller HR, Schutz Y, Sieber R, editors. Dritter Schweiz-erischer Ernährungsbericht. Bern: EDMZ. p 41–7.

Deutsche Gesellschaft für Ernährung. 1996. Ernährungsbericht 1996. Frankfurt:Deutsche Gesellschaft für Ernährung. 380 p.

Dorenstouter H, Pieters GA, Findenegg GR. 1985. Distribution of magnesiumbetween chlorophyll and other photosynthetic functions in magnesium defi-cient “sun” and “shade” leaves of poplar. J Plant Nutr 8:1089–101.

Erard M, Sieber R. 1991. Verbrauch und angenäherter Verzehr von Lebensmit-teln in der Schweiz. In: Stähelin HB, Lüthy J, Casabianca A, Monnier N, MüllerHR, Schutz Y, Sieber R, editors. Dritter Schweizerischer Ernährungsbericht. Bern:EDMZ. p 31–40.

Fairweather-Tait S, Hurrell RF. 1996. Bioavailability of minerals and trace ele-ments. Nutr Res Rev 9:295–324.

Grüter R, Schmid I, Sieber R. 1998. Verbrauch an Lebensmitteln in der Schweiz.In: Bundesamt für Gesundheit, editor. Vierter Schweizerischer Ernährungs-bericht. Bern: EDMZ. p 5–16.

Haenel H. 1979. Energie-und Nährstoffgehalt von Lebensmitteln. Berlin: VEBVerlag Volk und Gesundheit. 896 p.

Haisman DR, Clarke MW. 1975. The interfacial factor in the heat-induced conver-sion of chlorophyll to pheophytin in green leaves. J Sci Food Agric 26:1111–26.

Hazell T. 1985. Chemical forms and bioavailability of dietary minerals. WorldRev Nutr Diet 46:19–21.

Holland B, Welch AA, Unwin ID, Buss DH, Paul AA, Southgate DAT. 1994. Thecomposition of foods. 5th ed. Cambridge: Xerox Ventura. 462 p.

Kaur B, Manjrekar SP. 1975. Effect of dehydration on the stability of chlorophylland beta carotene content of green leafy vegetables available in northernIndia. J Food Sci Technol 12:321–3.

Khachik F, Beecher GR, Whittaker NF. 1986. Separation, identification, and quan-tification of the major carotenoid and chlorophyll constituents in extracts ofseveral green vegetables by liquid-chromatography. J Agric Food Chem 34:603–16.

Mackinney G, Joslyn MA. 1940. The conversion of chlorophyll to pheophytin. JAm Chem Soc 62:231–2.

Mangos TJ, Berger RG. 1997. Determination of major chlorophyll degradationproducts. Z Lebensm Unt Forsch A - Food Res Technol 204:345–50.

Marschner H. 1986. Mineral nutrition of higher plants. London: Academic Press.277 p.

Michael G. 1941. Über die Aufnahme und Verteilung des Magnesiums und des-sen Rolle in der höheren grünen Pflanze. Bodenkunde und Pflanzenernährung25:5–120.

Pennington JA, Young BE. 1991. Total diet study nutritional elements, 1982-1989.J Am Diet Assoc 91:179–83.

Rousos PA, Harrison HC, Palta JP. 1986. Reduction of chlorophyll in cabbage leaf-disks following Cu-2+ exposure. HortSci 21:499–501.

Schanderl S, Chichester CO, Marsh GL. 1962. Degradation of chlorophyll andseveral derivatives in acid solution. J Org Chem 27:3865–8.

Schlotke F, Sieber R. 1998. Berechnung des Verbrauchs an Nahrungsenergie,Energieträgern, Nahrungsfasern, Vitaminen, Mineralstoffen und Spurenele-menten. In: Bundesamt für Gesundheit, editor. Vierter SchweizerischerErnährungsbericht. Bern: EDMZ. p 18–27.

Schwartz SJ, Vonelbe JH. 1983. Kinetics of chlorophyll degradation to pyropheo-phytin in vegetables. J Food Sci 48:1303–6.

Scott BJ, Robson AD. 1990. Changes in the content and form of magnesium in thefirst trifoliate leaf of subterranean clover under altered or constant root sup-ply. Aust J Agric Res 41:511–9.

Siefermann-Harms D, Ninnemann H. 1983. Differences in acid stability of thechlorophyll protein complexes in intact thylakoids. Photobiochem Photobio-phys 6:85–91.

Souci SW, Fachmann W, Kraut H. 1994. Food composition and nutrition tables. 5thed. Stuttgart: CRC Press. 1091 p.

Spring JA, Robertson J, Buss DH. 1979. Trace nutrients. Magnesium, copper, zinc,vitamin B6, vitamin B12, and folic acid in the British household food supply. BrJ Nutr 41:487–93.

van Breemen RB, Canjura FL, Schwartz SJ. 1991. Identification of chlorophyllderivatives by mass-spectrometry. J Agric Food Chem 39:1452–6.

Weintraub LR, Weinstein MB, Huser HJ, Rafal S. 1968. Absorption of hemoglobiniron: the role of a heme-splitting substance in the intestinal mucosa. J ClinInvest 47:531–9.

Zapata M, Ayala AM, Franco JM, Garrido JL. 1987. Separation of chlorophylls andtheir degradation products in marine-phytoplankton by reversed-phase high-performance liquid-chromatography. Chromatographia 23:26–30.