Food and Chemical Toxicology 47 (2009) 571–577

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Food and Chemical Toxicology

journal homepage: www.elsevier .com/ locate/ foodchemtox

A simple pre-treatment of aluminium cookware to minimizealuminium transfer to food

Rim Karbouj *,1, I. Desloges, P. NortierLGP2 UMR 5518 (CNRS, Grenoble INP, AGEFPI, CTP), 461 rue de la Papeterie, BP. 65, 38402 Saint Martin d’Hères cedex, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 August 2008Accepted 15 December 2008

Keywords:AluminiumLeachingFoodTreatment

0278-6915/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fct.2008.12.028

Abbreviations: AD, Alzheimer disease; Al, alumcoupled plasma-mass spectrometry; S.H.E, Standard H

* Corresponding author. Tel.: +33 4 76 49 01 85.E-mail address: karbouj_rim@hotmail.com (R. Kar

1 Present address: 72 Avenue Rhin et Danube, 38100

In this work, we studied aluminium leaching from cookware to food under the effect of citric acid that iscommonly found in foods and beverages. The authors showed that boiling the cookware in water prior tocooking is suitable for the decrease of aluminium leaching into food by a factor up to sixty (with a cor-responding decrease of the aluminium intake by consumers). The effect of the pre-treatment has beenstudied by scanning electron microscopy and X-Ray diffraction and the effect has been attributed tochanges in the structure and morphology of the passivation layer, from an initial heterogeneous layerto a surface uniformly covered with fine needles of Boehmite (a-AlOOH).

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminium has been used extensively in foodstuffs, utensils andpackaging (Baxter et al., 1989; Karbouj, 2007). Depending on themeal, the cooking in aluminium cookware can expose humans tothe ingestion of important quantities of aluminium. In our humanbody, aluminium ion can inhibit different metabolism processes bycompetition reactions with other ions such as iron, magnesium,calcium, phosphor, fluoride and others (Mahieu et al., 2004; Aikohet al., 2005; Kaur and Gill, 2005). Aluminium (Al) is associated withanaemia, osteomalacia, and a neurologic syndrome known as dial-ysis encephalopathy (Malluche, 2002; Savory et al., 1986). Then,both in experimental animals and in human, aluminium is clearlyidentified as a potent neurotoxicant (Flaten et al., 1996; WHO,1997).

The World Health Organisation considered in 1986 that humansconsumed about 30 mg of Al/day on average, through water, foodsand drugs (WHO, 1986). In some conditions, the daily intake fromfood can be much larger than this average value (Karbouj, 2007).The Joint FAO/WHO Expert Committee on Food Additives (WHO,1989) has established the Provisional Tolerable Weekly Intakeof 7 mg/kg body weight (equivalent to 60 mg/day for an adultman).

ll rights reserved.

inium; ICP-MS, inductivelyydrogen Electrode.

bouj).Grenoble, France.

The cooking in utensils like aluminium skillets, pressure cook-ers, roasting pans, pots, saucepans, frozen dinner trays, foils andwrappers provide an important amount of aluminium, especiallyin the case of acidic dishes as tomato sauce for example (Ranauet al., 2001; Šcancar et al., 2004; Karbouj, 2007).

1.1. Dissolution of aluminium

In spite of the low redox potential (�1.66 V versus S.H.E.), thecorrosion of aluminium is inhibited at 4 < pH < 8.5 by the presenceof a barrier layer of aluminium (III) (oxy)-hydroxide (Hollings-worth and Hunsicker, 1983). Transfer of aluminium to food duringcooking can arise from two competitive mechanisms:

– slow uniform dissolution of the (oxy)-hydroxy layer at thesolution interface, itself being regenerated by slow oxidation atthe metal interface. Under cooking conditions, uniform dissolutionof the aluminium (oxy)-hydroxide is kinetically controlled bythe detachment of the hydrated aluminium ion, itself governedby the surface proton adsorption. The dissolution occurs atconstant rate per unit area (Dietzel and Böhme, 2005; Karbouj,2008).– pitting corrosion consists in the localised formation of pitsacross the passivation and metal layers. It is promoted by differentfactors, including the presence of precipitates of some secondaryphases (e.g. Al12Fe3Si2) in the aluminium matrix (Hollingsworthand Hunsicker, 1987).

In this paper, we describe a simple treatment that can drasti-cally decrease the leachability of aluminium cookware. For

Assemblage of the study of the aluminium transfer

1. Heating circulator bath 2. Peristaltic pump 3. Reactor 4. Aluminium sample 5. Sample Handler 6. Thermostat 7. Watson-Marlow Bredel Bioprene tubing 8. Solution 9. Refrigerant 10. pH-meter 11. Electrode 12. Temperature probe

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Fig. 1. Experimental device.

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0 100 200 300 400

Dis

solv

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tal [

Al]

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Time (min)

abcd

Fig. 2. Dissolved total [Al] (mg/L) as a function of the time and temperature ofleaching (Citric Acid 10 mmol/L). (a) Without pre-treatment (leaching: 84 �C); (b)With pre-treatment 5 h at 20 �C (leaching: 84 �C); (c) With pre-treatment 5 h at94 �C (leaching: 84 �C); and (d) With pre-treatment 5 h at 94 �C (leaching: 100 �C).

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Fig. 3. Dissolved total [Al] (mg/L) as a function of the time of pre-treatment at 84 �C(Citric Acid 10 mmol/L). (a) Without pre-treatment (leaching: 84 �C) and (b) Withdifferent pre-treatment’s time at 94 �C pre-treatment.

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Fig. 4. Photographic images by SEM of the foil surface.

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economical reasons, one cannot prevent peoples to usealuminium kitchen utensils such as pots, pans, and coffee perco-lators and others, but one can educate users to simple gesturesthat can contribute to reduce the total dietary intake ofaluminium.

2. Materials and methods

2.1. Reagents

Citric acid monohydrate (crystallized), was purchased from SIGMA–ALDRICH (St.Quentin Fallavier, France). Samples (2.5 � 15 cm2) were cut out of Aluminium Foil

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nsity

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Fig. 5. Spectres by XRD of the foil surface.

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(99.5% pure), from ALUPLUS� d’ALFAPAC (Paris, France), purchased in a departmentstore, verifying that all packages were from the same batch. For all leaching experi-ments, deionised water was used (Elga, Villeurbanne, France).

2.2. Instrumentation

The experimental device included: a peristaltic pump (Sci-Q 323 E, Watson-Marlow, Falmouth, Cornwall, England), a pH-meter (Haana 211, Tanneries, France)and a heating circulator bath (Model C1-B3, HAAKE, Germany). Polypropylene alu-minium-free bottles (WVR International GmbH, Darmstadt, Germany) were usedfor storage of all samples. (See Fig. 1)

2.3. Pre-treatment

The aluminium foil was used either as such or pre-treated. Pre-treatment con-sisted in dipping the foil for a given time (0, 15, 30, 60 or 300 min) into water atambient (20 �C) or near-boiling temperature (94 �C).

2.4. Dissolution experiments

Samples of aluminium foil (2.5 cm � 15 cm) were tested by the contact in areactor together with 275 ml of citric acid 1.0 � 10�2 mol/L at near-boiling temper-ature (84 or 100 �C), for 10, 30, 60 or 300 min. These times of reaction with citricacid were selected because they are different times of preparation or cooking ofcommonly consumed foods and beverages, like dairy products, berries and citrusfruits (i.e. lemon, black-currant, red-currant). The temperature was chosen as closeas possible to the boiling point, considering practical limitations of the device. Theflow rate in the reactor was set to 1.8 L/min, as preliminary experiments showedthat the dissolution is not affected by mass transport at the corresponding velocity(Karbouj, 2008).

2.5. Analysis

Aluminium content in water was measured at the Central Analytic Departmentof the National Centre for Scientific Research (Service Central d’Analyse, CNRS,Vernaison, France) using inductively coupled plasma-mass spectrometry (ICP-MS)analyses with an X7 series quadrupole instrument (Thermo Electron Corporation,Cergy-Pontoise, France).

Scanning electron microphotographs were performed at the LGP2 on a QUANTA200 from FEI, fitted with an X PGT probe (Sahara SDD) for chemical analysis, datawere treated using Spirit software.

Crystalline phases on aluminium foil were determined by XRD at the ScientificResearch Laboratory (Consortium des Moyens Technologiques Commun, CMTC) ofGrenoble Institute of Technology using a PanAnalytical X’Pert Pro MPDDiffractometer.

Additional chemical analysis were also realised at the CMTC using a Jeol 6400SEM fitted with Brüker SDD EDS probe.

3. Results and discussion

The amount of aluminium in solution after contact of non pre-treated and pre-treated (20 or 94 �C during 5 h) aluminium foil dur-ing typical ‘‘cooking times” at 84 and 100 �C is depicted on Fig. 2.

The Fig. 2 shows a very important effect of the pre-treatment at94 �C, which decreases the amount of dissolved aluminium by afactor from 20 to about 60, while the pre-treatment in cold wateris ineffective.

This protection of the foil by the pre-treatment is kept duringleaching at higher temperature (100 �C).

The amount of aluminium in solution after contact of non pre-treated and pre-treated aluminium foil for a given time (0, 15, 30,60 or 300 min) near-boiling temperature (94 �C) during typical‘‘cooking times” at 84 �C is depicted on Fig. 3.

The Fig. 3 illustrates the effect of the time of a pre-treatment at94 �C on dissolution. We see that a short pre-treatment, of someminutes, already assures a decrease of the dissolved amount (‘‘Alleachability”) by a factor of 3. Then, a pre-treatment of half an hourensures a decrease of ‘‘Al leachability” by a factor of 9.

3.1. Characterisation of the aluminium passivation layer

The most important feature from Fig. 2 is the drastic hindranceof dissolution by the pre-treatment in near-boiling water. The rateof dissolution of aluminium changes from 5.5 mg m�2 min�1 for noor cold water pre-treatment to 0.13 mg m�2 min�1 for a near-boil-ing water pre-treatment of 300 min.

Fig. 6. Photographic images by SEM of the foil surface.

Fig. 7. Photographic images by SEM of the foil surface.

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Fig. 8. Spectres by XRD of the foil surface after treatment and leaching exhibiting bayerite.

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This result is original, to our knowledge. Nevertheless, it isnot so surprising: Alwitt (Alwitt, 1976) showed that the waterborn hydroxylated film structure and texture undergo a transi-tion around 90 �C: below this temperature, the film is made oftwo layers, the inner (close to the metal) layer being pseudo-boehmite, the outer layer being bayerite. At a higher tempera-ture, the film is made of pseudo-boehmite only, and the porosityis lower (by a factor of about 20) than the porosity of low tem-perature films.

3.1.1. Surface of the aluminium foil before acidic leachingScanning electron microscope (SEM) observations of the foil

surface state before and after pre-treatment and before acidic con-tact, are shown in Fig. 4a–e. The virgin foil (Fig. 4b) shows an het-erogeneous relief, rather smooth but including some macropores(diameter about 0.5 lm). On the contrary, the Fig. 4d and e showsthe aspect of the surface of the Al pre-treated foil, exhibiting awallpapered surface with fine curved needles (about 0.2 lm intheir length and less than 50 nm in their thickness) delimitingmesopores. This morphology is classical of the boehmite or thepseudo-boehmite AlOOH polymorphs.

As depicted in Fig. 5, the X-ray diffraction (XRD) only discerns Aland Al4.01MnSi0.74 crystallized phases on samples without pre-treatment and with (5 h at 20 �C) pre-treatment. On the contrary,the hot water pre-treated (5 h at 94 �C) sample introduces thecharacteristic lines of the boehmite.

3.1.2. Surface of the aluminium foil after acidic leachingNon pre-treated surface. After leaching by citric acid (5 h, 84 �C,

10 mmol/L), we observe in the Fig. 6 that the surface roughnessof the non pre-treated foil (Fig. 6a and b) and the pre-treated foil(5 h in 20 �C) (Fig. 6c) seems very high. The corrosion figures de-picted on Fig. 6 are typical of pitting corrosion.

Hot water pre-treated surface. The hot water pre-treated foilexhibits very few pittings and keeps on the whole the same mor-phology (paperwall of pseudo-boehmite needles) as the unleachedfoil (Fig. 7a).

We also notice in the Fig. 7b, on this pre-treated foil in 94 �Cduring 5 h and subjected to sour leaching in 84 �C during 5 h, thegrowth of crystals of which morphology is typical of the bayerite.The bayerite was also identified by XRD as depicted on Fig. 8.

4. Conclusion

We studied the corrosion of alimentary aluminium foils by anaqueous solution of citric acid, simulating the conditions of cook-ing acidic ingredients as tomato sauce, lemon juice. . .

Pitting corrosion was detected and can be favored by the pres-ence of precipitates of a secondary phase Al4.01MnSi0.74. The pre-treatment of the foils in boiling (or near boiling) water stronglymodifies the passivation layer, giving rise to the growth of a wall-paper of boehmite needles. During a subsequent cooking simula-tion, this layer inhibits the formation of pits and the mainmodification is the crystallisation of bayerite.

Our results indicate that for reducing the leachability of alumin-ium from aluminium cookware into foods and per consequence,also to decrease the daily aluminium intake during the preparationof food, we must boil water in aluminium kitchen utensils (such aspots, pans, and coffee percolators and others) used for cooking thefood. Per consequence, the daily aluminium intake will decrease.

The pre-treatment of aluminium utensils for a certain time isvery important to protect the public health.

In conclusion, we recommend a change in foods and beveragescooking practices. This could also apply to storage materials.

Conflicts of Interest statement

The authors declare that there are no conflicts of interest.

Acknowledgements

The authors wish to acknowledge collaboration of Dr. StéphaneCoindeau at the CMTC (Grenoble INP) for EDX-SEM and XRD deter-

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minations, and of Mrs. Bertine Khelifi at the LGP2 ESEM facility forSEM photographs.

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