Trends in Food Science & Technology 19 (2008) 94e101

Review

* Corresponding author.

0924-2244/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.tifs.2007.08.002

Technological issues

associated with

iodine fortification of

foods

Ray J. Wingera,*, Jurgen Konigb

and Don A. Housec

aInstitute of Food, Nutrition and Human Health,

Massey University, New Zealand

(e-mail: r.j.winger@massey.ac.nz)bDepartment of Nutritional Sciences,

University of Vienna, AustriacUniversity of Canterbury, Christchurch, New Zealand

With a decrease in the addition of salt to food, iodine defi-

ciency diseases are re-emerging globally. Public Health offi-

cials are considering the addition of iodine salts to processed

foods to redress this situation. However, iodate is a powerful

oxidising agent and iodide, a powerful reducing agent. Their

use in foods may result in redox reactions which in turn may

influence food characteristics, shelf life and stability. Very little

research has been conducted with respect to the reactions of

iodine and its salts in food matrices. Thus, the impact of forti-

fying processed foods with iodine salts is unclear.

IntroductionIodine has been recognised by the World Health Organi-

zation to be one of the most critical nutrients world-widewith nearly two billion people, or 35.2% of the global popu-lation, having inadequate iodine nutrition (United NationsSystem, 2004). The spectrum of iodine deficiency disorders(IDD) is exhibited across the entire life span (Table 1). Iodinestatus in developed countries has decreased significantlyover the past 10 years (Aitken, 2001; Hynes, Blizzard,Venn, Dwyer, & Burgess, 2004). While there is generalagreement on the benefit of salt iodisation for the improve-ment of iodine status of the population, this appears to be

inadequate to further secure sufficient iodine supply for theprevention of IDD. Although the usage of salt for householdpurposes is decreasing, it is undesirable to increase salt in-take for other health reasons. There is also a move towardsusing non-iodised salts at home.

The majority of food companies use non-iodised salt forprocessed foods. Iodised salt is used in several countriesaround the world in baked goods (mainly bread) instead ofnon-iodised salt. There have been no reported technologicalissues associated with this level of use. However, the use ofiodine as an ingredient in a wider range of processed foodshas received little research attention.

Iodine salts may be volatile and lost through food pro-cessing operations. The World Health Organization(2001) suggests there may be 20% loss of iodine throughprocessing and another 20% through cooking and foodpreparation practices. The justification for this, however,is not clear.

Iodide is a strong reducing agent and iodate, a strong ox-idising agent. Thus, it is theoretically possible that reactionsin foods involving iodine and its salts may potentially:

� cause colour reactions� increase oxidative reactions, hence reducing shelf life� decrease bioavailability of iodine� reduce bioavailability of other nutritionally important

substances.

This report provides a review of the literature related tothe addition, or use of iodine and its salts in processedfoods. It summarises fundamental iodine chemistry andidentifies the potential reactivity of these compounds.

Iodine intake and dietary sourcesInsufficient iodine intake is the major cause of low io-

dine status. This led to universal salt iodination (USI) tothe general public. USI is considered to be the most effi-cient way to improve iodine intake since both availabilityof iodine rich foods, such as marine fish and the iodine con-centration in those foods are decreasing world-wide. Dairyproducts substantially contribute to iodine intake only ifmilk is produced with iodised fodder (Dahl, Johansson,Juishamm, & Meltzer, 2003).

Currently, the approved iodine salts to be added to food(salt) are normally sodium or potassium iodate or iodide.

Table 2. Levels of salt iodisation in selected countries (ICCIDD,2007)

Country Amount and form of salt iodisation(mg iodine/kg salt, compound)

Albania 25, KIAustria 20, KI, KIO3

Bosnia/Herz 20e30, KIBulgaria 19e32, KICroatia 20e30, KICzech Rep. 20e34, KIO3

Denmark 8e13, KIFinland 21e26, KIFrance 10e15, NaIGermany 20, KIO3

Greece 40e60, KIHungary 15, KIO3

Ireland 25, KIItaly 30, KIMacedonia 20e30, KIO3

Netherlands 50, KIPoland 20e40, KIPortugal 20, KIRomania 15e25, KIO3

Slovak Rep. 19, KIO3

Slovenia 20e30, KISpain 51e69, KI, KIO3

Sweden 50, KISwitzerland 20, KI

Australia 25e65, KI, KIO3

New Zealand 25e65, KI, KIO3

USA 75, CuI, KI

Algeria 30e50, KIO3

Benin 20e40, KIO3

Botswana 50, KIO3

Burkina Faso 40, KIO3

Burundi 50, KIO3

Cameroon 100, KIO3

Congo (DR, Kin) 80e100, KIO3

Cote d’Ivoire 30e50, KIO3

Egypt 50e80, KIO3

Eritrea 100, KIO3

Ghana 20e40, KIO3

Guinea 30e50, KIO3

Guinea-Bissau 100, KIO3

Kenya 100, KIO3

Table 1. The spectrum of iodine deficiency disorders (WorldHealth Organization, 2004)

Life stage Symptom of deficiency disorder

Foetus AbortionsStillbirthsCongenital anomaliesIncreased perinatal mortalityEndemic cretinismDeaf mutism

Neonate Neonatal goitreNeonatal hypothyroidismEndemic mental retardationIncreased susceptibility of thethyroid gland to nuclear radiation

Child and adolescent Goitre(Sub-clinical) hypothyroidism(Sub-clinical) hyperthyroidismImpaired mental functionRetarded physical developmentIncreased susceptibility of thethyroid gland to nuclear radiation

Adult Goitre, withits complicationsHypothyroidismImpaired mental functionSpontaneous hyperthyroidismin the elderlyIodine-induced hyperthyroidismIncreased susceptibilityof the thyroid gland to nuclear radiation

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The use around the world varies from country to country, asshown in Table 2.

The contribution of iodised salt to total iodine intakehowever is difficult to determine, as salt consumption ingeneral and usage of iodised salt in particular are frequentlyunclear. In addition, salt used in foods consumed awayfrom home is often neither iodised nor declared as such. Es-timates on the dietary contribution of iodine from iodisedsalt can be as high as 50% for specific groups of the popu-lation (Elmadfa & Koenig, 1998), indicating that iodine de-ficiency disorders would be a considerable health problemwithout food fortification.

Iodine fortification levels of salt differ substantially. InEuropean countries, levels range from 8 to 69 mg iodineper kg salt, while in some African countries the levels goup to 100 mg iodine per kg salt (Table 2). The level of io-dine added to salt is adjusted normally to provide 100 mg ofiodine per day, in addition to iodine from natural sources,taking into account average consumption levels of salt.

Although toxicity of iodine from natural sources andfrom iodised salt is uncommon, excessive intakes maylead to complications such as iodine-induced hyperthyroid-ism. This has been reported in almost all iodine supplemen-tation programs in their early phases (Stanbury et al., 1998).Iodine-induced hyperthyroidism is transient and, with

proper identification and treatment, reverts to normal after1 to 10 years (Todd, 1999).

Iodine bioavailabilityThere are no recent data on the bioavailability of iodine

from the diet. The methods to test bioavailability of iodineall require the use of radioisotopes. As there is only one sta-ble isotope for iodine, isotope tracer studies are not possiblefor ethical reasons (Hurrell, 1997). However, earlier litera-ture provides a useful guide to iodine availability.

There are two aspects to the bioavailability of iodine:

� absorption of iodine in the gastrointestinal tract, and� absorption of iodine from the bloodstream by the thyroid

gland.

96 R.J. Winger et al. / Trends in Food Science & Technology 19 (2008) 94e101

Inorganic iodide is readily and completely absorbedfrom the gastrointestinal tract (Keating & Albert, 1949).It is believed that iodate and protein-bound iodine are re-duced to iodide for absorption. However, neither the reduc-ing agent nor the site of reduction is clear. While iodineabsorption from the gastrointestinal tract has been reportedto be virtually 100%, the absorption of iodine by the thyroidgland was estimated to be around 10e15% under normalintakes (Lamberg, 1993). Using a 100 mg radioisotopeiodine dose, about 20% was taken up by the thyroid gland(Keating & Albert, 1949).

There are virtually no data on the bioavailability ofiodine from foods. Very early human balance studies ofvon Fellenberg (1926) demonstrated high iodine absorp-tion from most foods (ca. 90%) except water cress. Therewas some evidence that iodine from plants may showa lower absorption rate, apparently caused by the poor re-lease of iodine from the plant structure during digestion.Prolonged cooking of the seaweed ‘‘hijiki’’, which isused in Japan, has led to an increase of urinary iodine ex-cretion in men to 53% of dose compared to only 10% forthe raw material uncooked (Katamine et al., 1987). How-ever, as with other nutrients, mere balance techniques areimprecise and the assessment of iodine bioavailability isfurther complicated by analytical difficulties in measuringtrace quantities in the diet and by possible contaminationwith atmospheric iodine.

Given the presence of various reducing and oxidisingagents in foods there is a wide range of possible interac-tions between iodide and iodate with respect to their bio-availability. The various studies with iodate as iodinesource for salt iodisation suggest that iodate bioavailabilityis extremely high. However, these studies have not includedinteractions within the food matrix, or in vivo changes dur-ing digestion and absorption.

One example of a potential problem was the reaction be-tween iodate and ferrous salts used for dual fortification ofsalt with iodine and iron. The water-soluble, highlybioavailable ferrous compounds reacted with moisture

Table 3. Physical properties of some iodine compounds (Greenwood & Ea

Compound Molar mass(g/mol)

Density(g/cm3)

Melting point/boilingpoint (�C)

Sol(g/

I2 253.8 4.93 50 Sublimes 0.0116 Decomposes 0.0

NaI 149.9 3.667 651 11304 3

KI 166.0 3.13 686 1271330 2

NaIO3 197.9 4.277 Decomposes

KIO3 214.0 3.93 560 4.>1000 32

and impurities in the salt, causing colour changes and io-dine losses (Mannar & Diosady, 1998). Even encapsulationof ferrous sulphate with partially hydrogenated vegetableoil resulted in yellow colour changes in dual fortified saltwhen salt moisture content was high (Zimmermann et al.,2003).

In conclusion, there is almost no information on iodinebioavailability, either from pure iodine sources or from un-processed and processed foods. All present data are basedon limited studies using radioiodine in very small samplesand specific foods from some decades ago. There areknown reactions, particularly with mineral salts, where bio-availability appears to be significantly reduced.

Iodine chemistryIodine (symbol I, molar mass 126.9) is the heaviest, nat-

urally occurring, member of the halogens. Chemical no-menclature does not distinguish between the iodine atom(I) and the di-iodine molecule (I2) e both are ‘‘iodine’’and this can sometimes cause confusion.

Free iodine (as I2) does not occur in nature. Iodine is the60th most abundant element in the earth’s crust (compara-ble with thallium) and today, most iodine is extracted fromnatural brines in Japan or Midland, Michigan, USA. Theiodine concentration in seawater is only 0.05 ppm butsome seaweeds can concentrate iodine up to 0.45% of theirdry weight.

Table 3 lists some of the key physical properties of iodineand its salts. A number of radioactive isotopes of iodine canbe produced artificially, including: 129I (t½¼ 1.6� 107 years,from uranium fission), 125I (t½¼ 60.2 days), 131I (t½¼ 8.04days) and 128I (t½¼ 24.99 min). Those with shorter half-lives(especially 131I) are used in medicine to follow metabolicpathways.

Redox reactions in aqueous solutionIodate, iodine and iodide are all quite reactive species

and the following is a discussion of the redox chemistryof these substances (Vogel, 1978).

rnshaw, 1984)

ubility in water100 mL) at (temp. �C)

Solubility in other solvent

30 (25) Very soluble in most organic solvents andin aqueous KI solution.78 (50)

84 (25) Very soluble in ethanol02 (100) Very soluble in acetone

.5 (0) Soluble in ethanol08 (100) Soluble in acetone

9 (20) Insoluble in organic solvents34 (100)

75 (0) Insoluble in organic solvents.3 (100)

97R.J. Winger et al. / Trends in Food Science & Technology 19 (2008) 94e101

The standard half-reactions for iodate and iodine are

2IO�3 þ 12Hþ þ 10e� / I2þ 6H2O Eo¼ 1:194 V ð1Þ

I2 þ 2e� / 2I� Eo ¼ 0:536 V ð2Þ

Indicating that both IO3� and I2 are oxidising agents and

I� is a reducing agent. We can combine (1) and (2) directlyto give

IO�3 þ 5I� þ 6Hþ / 3I2 þ 3H2O ð3Þ

Combining the Eo values require incorporation of thenumber of electrons transferred.

Eo ¼ ð1:194� 5Þ þ ð0:536� 1Þ5

¼ 1:301 V

Iodide ionIodide is a strong reducing agent and reacts with molec-

ular oxygen (O2)

4I� þ 4Hþ þ O2 / 2I2 þ 2H2O ð4Þ

This is very slow in neutral media, but the rate increaseswith increasing acidity (decreasing pH). It is also acceleratedby sunlight and by various metal ion catalysts (Kimura,Tokuda, & Tsukahara, 1994). This reaction is very slow atroom temperature but is catalysed by nitrite (NO2

�) and cupric(Cu2þ) ions (Kimura, Sato, Murase, &Tsukahara, 1993).

Iodate ionIodate is a strong oxidising agent, especially in acid

media.

IO�3 þ 6Hþ þ 4e� / Iþ þ 3H2O ð5Þ

At lower acid concentrations, IO3� is first reduced to iodide,

and the latter is subsequently converted to I2. This ‘‘LandoltReaction’’ is a spectacular ‘‘clock’’ and oscillating chemicalreaction, dependent on [Hþ] and the relative concentrationsof reacting species through three linked reactions:

IO�3 þ 3SO2�3 / I� þ 3SO2�

4 ð6Þ

IO�3 þ 5I� þ 6Hþ / 3I2 þ 3H2O ð7Þ

I2 þ SO2�3 þ H2O / 2I� þ 2Hþ þ SO2�

4 ð8Þ

Step 6 is slow and I2 (as indicated by the blue starcheiodine complex) will appear when all the SO3

2� is used up.Ascorbic acid reacts rapidly with iodate to give iodide

(Samios, Karayannis, & Hadjiioannou, 1977).Unfortunately, there is a lack of kinetic data for the

actions of potential reducing agents such as nitrite, sugars,aldehydes, ketones, thiols and amino acids.

IodineIodine (I2) is a reasonable oxidising agent in acid solution.

At low pH, air oxidation (Eq. (4)) can interfere and at higherpH (pH> 8), I2 undergoes disproportionation reactions:

I2 þ 2OH� / I� þ IO� þ H2O ð9Þ

3IO� / 2I� þ IO�3 ð10Þ

There are over 70 oxidising agents capable of oxidising

I� to I2 and 70 reducing agents capable of reducing IO3� to

I� (or I2). These I2 producing reactions are popular becausethe product is easily detected, either spectrophotometrically(lmax (I2)¼ 460 nm: 3¼ 740 L mol�1 cm�1: lmax (I3

�)¼350 nm: 3¼ 25,000 L mol�1 cm�1) under dilute condi-tions, or with starch (Coumes, Vargas, Chopin-Dumas, &Devisme, 1998).

It should be noted that in all these I2 producing reac-tions, excess iodide ion rapidly converts I2 to triiodide(I3�) as the reaction proceeds. This generates a new reactant

(I3�) which generally reacts more slowly than I� with the

oxidising agent.

I2þ I�#kf

kr

I�3 ; K¼ kf=kr ð11Þ

At 25 �C, the rate of the forward reaction for Eq. (11) is6.2� 109 mol�1 s�1 and K¼ 729 (Palmer, Ramette, &Mesmer, 1984).

Iodide, iodate and elemental iodine can undergo oxida-tion and reduction cycles in a food system. Lipid oxidation,degradation of ascorbic acid (vitamin C) to dehydroascor-bic acid (or reverse), reduction of ferric ions to ferrousions (or oxidation of ferrous ions to ferric ions), andchanges in protein functions due to formation of disulfidelinkages between amino acids are some examples that the-oretically could be mediated through the presence of iodinein food systems. Iodine and its salts could cause changes incolour, flavours, odours, texture, stability, and nutritionalvalues of food matrices. It is interesting to postulate the im-pact of food systems containing both iodine and ascorbicacid: the iodate/iodide pairing and ascorbic acid/dehydroas-corbic acid pairing could have major implications for thestability of the food system.

In summary, elemental iodine does not occur naturally,but it is a key intermediate in iodine reaction chemistry.Potassium and sodium iodates are strong oxidising agents,while the iodide salts are reducing agents. As such, theypotentially are important reactants in food systems. Thechemistry of the reversible interconversion of iodate toiodide involves elemental iodine and thus a study of iodineand its salts in food systems must take into account all threeforms.

Use of iodine in processed foodsThe incorporation of iodates or iodides directly into

foods has received very little research attention. The major-ity of work published in this area has involved the

98 R.J. Winger et al. / Trends in Food Science & Technology 19 (2008) 94e101

replacement of non-iodised salt with iodised salt in thestandard food formulations.

South Africa requires mandatory iodisation of table salt(40e60 ppm) using potassium iodate as the fortificant. Thisis not a mandatory requirement for processed foods, however.Harris, Jooste, and Charlton (2003) surveyed the use and an-alysed the iodine content of salt as an ingredient used in 12South African companies (six bread manufacturers, twobread premix manufacturers, two margarine manufacturersand two flavour houses producing salty snack flavours).From the returned questionnaires, only one company (breadmanufacturer) claimed to use iodised salt as an ingredient.Three companies stated that they believed iodine would af-fect product stability, taste and the stability of flavours (wheresalt was used as a carrier). The authors measured the actualiodine content of the salt these manufacturers used and notedthat four companies (one bread company, one margarinecompany and both flavour houses) actually used salt witha significant level (39e69 ppm) of iodine. The unintentionaluse of iodised salt as an ingredient in these foods suggeststhere is no adverse effect on these products.

Hard evidence for adverse reactions in foods is difficultto justify from published literature. Kojima and Brown(1955) found no undesirable effects of iodised salt oncanned tomato juice, canned green beans, yellow sweetcorn, bottled olives and canned or bulk sauerkraut. Thisstudy included storage for 2e3 months.

El Wakeil (1958) evaluated iodide/iodine mixtures, iodidein salt, and an iodophor on the quality of canned sweet corn,canned tomato juice and canned sauerkraut. Apart from a fla-vour change in tomatoes for high concentrations (200 ppm)of iodine/iodide mixture, there were no identifiable qualitychanges. It was noted that sodium thiosulphate, commonlyused then for stabilising iodide in the salt, caused substantialcorrosion of the cans. This is no longer an issue as thiosul-phate is not an approved additive for iodised salt.

West and Merx (1995) cited reports that iodised salt (con-taining potassium iodide) used in Switzerland for manufac-turing cheese (particularly Emmenthaler and Gruyere) hadno discernable impact on flavour or quality. The authorsthemselves experimented with rice and potatoes boiled inwater with and without iodised salt. They used four timesthe level of salt recommended by WHO. They found no sig-nificant difference in sensory tests (flavour and appearance).

A study in Germany by Wirth and Kuhne (1991) evalu-ated the use of iodised salt in a wide range of processedmeat products. They could find no changes in sensory prop-erties, no influence on the curing characteristics of nitriteand no additional nitrosamine formation.

These papers suggest that the replacement of non-iodisedsalt with iodised salt in the standard formulations for manyfood products will not cause any noticeable changes.

Skudder, Thomas, Pavey, and Perkin (1981) found12.7 ppm iodine from added potassium iodate induced pro-teolysis in casein during the UHT processing of milk. Thisdid not occur with 6.3 ppm iodine from the iodate. This

concentration of iodate was considerably higher than thoseanticipated from adding iodised salt. In addition, Sevenantsand Sanders (1984) found a reaction between iodide and thecresol component of lemon flavouring used in a cake mix.The resulting flavour was unacceptable and the authors rec-ommended non-iodised salt be used to avoid the problem(as cresol is a key component of lemon flavour).

It has also been suggested that a decline in the use ofiodophors in the dairy industry may result in lower levelsof iodine in dairy foods. However, Skeaff, Thompson, andGibson (2002) refer to a government report that found nochange in the iodine content of dairy products between1988 and 1998. Cressey (2003) completed analysis of differ-ent types of dairy products which had been analysed between20 and 30 years earlier, in an attempt to compare iodine con-tents before and after iodophors usage. These included 13products manufactured in New Zealand and 3 from overseas.Wherever possible, Cressey used the same methodology asthat used in those earlier studies. There was no clear trendin iodine content over time, with one exception e butter.The iodine content of butter had clearly decreased over time.

The Tasmanian Iodine Supplementation Program beganin October 2001 to encourage bakeries to use iodised saltin preference to regular salt. Four of the six major bakerychains, along with a number of independent bakeries, agreedto participate. Estimates suggest that participating bakeriesproduced about 80% of the bread available for consumption.Salt manufacturers agreed to supply iodised salt at pricescomparable to regular salt. A survey in July 2003 foundabout 70% had changed to using iodised salt and none hadreverted to regular salt. Of the non-participating bakeries,most baked from premixes, or used frozen dough formulatedinterstate, to which salt was already added. The researchersconcluded that the Program has high acceptance amongsmall-medium sized bakeries with little impact on business,including time, cost, or consumer acceptance. Unfortu-nately, there are no published results on the true iodine con-centration of the bakery products (Turnbull, Lee, Seal,Johnson, & Shaw, 2004). Personal communication indicatedthat iodine levels of bread baked with iodised salt werebelieved to be about 35 mg per 100 g (range from 20 to100 ignoring outliers). The Supplementation Program isalso monitoring milk, where iodine levels are about200 mg per litre (range 100e420) (Judith Seal, Senior Scien-tific Officer (Iodine), Public Health Service, Department ofHealth and Human Services, Hobart, Tasmania).

Volatility of iodine in foodsThe retention of iodine in processed foods has received

limited attention. The World Health Organization (2001)recommends:

‘‘in typical circumstances, where:

� iodine lost from salt is 20 percent from production tohousehold,

99R.J. Winger et al. / Trends in Food Science & Technology 19 (2008) 94e101

� another 20 percent is lost during cooking before con-sumption, and

� average salt intake is 10 g per person per day,

iodine concentration at the point of production should bewithin the range of 20e40 mg of iodine per kg of salt (i.e.20e40 ppm of iodine) in order to provide 150 mg of iodineper person per day. The iodine should be added as potassium(or sodium) iodate. Under these circumstances median uri-nary iodine levels will vary from 100 to 200 mg/litre’’

It is unclear what evidence was used to support the indi-cated losses.

Kuhajek and Fiedelman (1973) found retention was

� between 70 and 80% during the processing and 10 days’frozen storage of white bread

� 48e73% in potato chips after 13 weeks’ storage� 41e61% in frankfurters after refrigerated or frozen storage.

In an extensive study using 50 different Indian recipes,prepared in a hospital kitchen using different cooking pro-cedures, Goindi, Karmarkar, Kapil, and Jagannathan (1995)found the range of losses between 3 and 67%. The meansranged from 6 to 37%, though clearly the variation in re-sults was very large. Some foods (Kadhi, carrot, spinach)had losses in excess of 50%. The cooking method had a sig-nificant impact also:

� boiling (37% average loss)� steaming (20% loss)� pressure cooking (20% loss)� shallow frying (27% loss)� deep frying (20% loss)� roasting (6% loss).

The methodology involved drying at 100 �C, then dry�

ashing at 600 C and colorimetric analysis of iodine. There

was no evidence the researchers assessed iodine losses dur-ing these analytical procedures. Triplicate samples (250 mgeach) were taken before and after cooking one batch offood and this level of sampling is of questionable valuefor complex food materials. However, the results do indi-cate that iodine is labile and lost from food systems duringroutine preparation processes.

In summary, the use of iodine salts in processed food sys-tems has been studied predominantly in terms of the replace-ment of non-iodised salt with iodised salt in a range ofprocessed foods. Most studies reported no significant impactof this iodine fortification on the sensory or other food quali-ties. However, there were reactions with milk proteins under-going a UHT treatment and lemon flavouring. There weresome reactions observed in canned products when iodine con-centrations were above 100 ppm. In most reports the amountof iodine present in the food products was calculated from theamount of iodine in the added salt, rather than by direct mea-surement. Where studies did measure the iodine levels, it wasclear that a considerable loss of iodine during processing was

possible e sometimes as much as 70%. However, these stud-ies lacked sufficient controls and used methods which wouldbe considered inappropriate today. One of the issues whichprecludes unambiguous conclusions from these publishedstudies is the limitation of adding iodine to foods via salt. Un-less salt levels were normally high, the levels of iodine wouldbe low and the background levels naturally present in thefoods may confound the results (the controls may have similarlevels of natural iodine, for example). The published researchon iodine in processed foods has not involved systematic andcarefully designed experiments on the addition of iodine toprocessed foods, in order to be able to clearly establish therole that iodine may play in food chemistry.

Iodine methodologyThe determination of different iodine species in foods

(and other biological samples) is extremely complicatedand requires both substantial investment in technical equip-ment and day-to-day operating expenses. Two methods arepresently used for this determination: neutron activationanalysis or a combined ion-chromatography/inductivelycoupled plasma-mass spectrometry (ICP-MS) (Fecher,Goldman, & Nagenat, 1998). However, there has been nowork published which establishes the efficiency of iodineand its salts recovery from complex solid food matrices(Chai et al., 2004; Leiterer, Truckenbrodt, & Frank, 2001).Determination of iodine species (iodate, iodide and elemen-tal iodine) is not possible by these routine measurements.

Conclusion and future workThere is almost no information in the literature on the

true absorption rate of iodine from the gastrointestinal tract.The present information, which suggests 100% absorptionfrom the gastrointestinal tract, is based on studies con-ducted in the first half of the last century using smallsample sizes and only basic analytical methodology. Radio-isotope techniques cannot be used (ethically) for the assess-ment of iodine availability in humans and therefore noaccurate figures exist. There is less information on the inter-actions between iodine and food components in respect totheir impact on bioavailability from processed foods. Thereare known reactions, particularly with mineral salts, wherebioavailability appears to be significantly reduced.

The existing literature tends to suggest that iodine andits salts, when used as iodised salt to replace added non-iodised salt, will have limited impact on food quality. How-ever, these studies have focused on very few food systemsand at low iodine concentrations, often not directly mea-sured. While the use of iodised salt to replace non-iodisedsalt is one method of adding iodine to foods, the levels ofadded iodine are very low. There are no data to predictthe outcome of the use of higher levels of iodine (eg,25e65 mg iodine per kg food, as is required for salt).

Literature evidence suggests there will be reactivity be-tween iodine and some food components when the level ofiodine is increased above 100 ppm. This suggests, clearly,

100 R.J. Winger et al. / Trends in Food Science & Technology 19 (2008) 94e101

that consideration should be given to the fortification of foodsbeing limited, in the first instance, to the use of low concen-trations of iodine. There are insufficient data to establish what‘‘low’’ means and a suitable threshold level cannot be definedat this stage, but must clearly be lower than 100 ppm.

In short e for food systems such as bread, processedmeats, salty snack foods, some canned foods and somecheeses e the use of iodised salt to replace non-iodised saltin the formulations does not appear to create any observabletechnological issues. There are examples, such as milk forUHT processing, some flavourings, foods containing highlevels of unsaturated fatty acids and some foods (eg, bread)where iodine concentrations of 100 ppm and above wereused, where there is an observable reaction between iodineand the food matrix. Clearly, there are some technologicalissues surrounding the use of iodine in these food systems.

There are insufficient data available in the literature tobe able to describe the range of foods which will be suit-able, inert vehicles for the fortification of processed foodswith iodine. It appears from world-wide use of iodisedsalt in bread that this is a suitable food system to beused, but this result cannot be extrapolated to all foods.

While modern techniques are available to detect lowlevels of iodine, sample preparation is inadequately devel-oped and insufficiently robust to be confident in measuringlow levels of iodine and those covalently bound forms ofiodine present in food matrices. It is also unclear whetherthe different forms of iodine can be detected in real timewithin a food matrix. This is an important area of work re-quired to further iodine studies on food.

Iodine is a volatile material that can be readily lost dur-ing food processing. The assumption that iodine concentra-tion can be calculated mathematically in foods is flawed.Iodine losses depend on the salt used.

References

Aitken, E. (2001). Iodine status in New Zealand: is history repeatingitself? Journal of the New Zealand Dietetic Association, 55, 4e5.

Chai, Z. F., Zhang, Z. Y., Feng, W. Y., Chen, C. Y., Xu, D. D., &Hou, X. L. (2004). Study of chemical speciation of trace elementsby molecular activation analysis and other nuclear techniques.Journal of Analytical Atomic Spectrometry, 19, 26e33.

Coumes, C. C. D., Vargas, S., Chopin-Dumas, J., & Devisme, F. (1998).Reduction of iodine by hydroxylamine within the pH range 0.7e3.5.International Journal Chemical Kinetics, 30(11), 785e797.

Cressey, P. (2003). Iodine content of New Zealand dairy products.Journal of Food Composition and Analysis, 16, 25e36.

Dahl, L., Johansson, L., Julshamm, K., & Meltzer, H. M. (2003). Theiodine content of Norwegian food and diets. Public HealthNutrition, 7, 569e576.

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