Technical field .of the invention The present invention relates to the use of special iron(III)phosphates for iron fortification of food prod¬ ucts, in particular grain products. Especially flour and flour products, breakfast cereals, milk-based bever¬ ages, broths, rice and fermented food products, such as bread, are here concerned.

Background of the invention Internationally seen, iron deficiency is one of the most frequent deficiency diseases which occurs in most developing countries and is also the main deficiency disease in industrial countries. Above all, iron defi¬ ciency condition is a problem to women of fertile age but is also to be found in children and young people. Iron fortification of certain food products is one way to prevent the occurrence of iron deficiency. Since grain products are an important source of consumer goods and are relatively cheap, they are usually chosen as carriers of iron additives. A suitable iron-fortifying agent must satisfy a number of requirements. First, it must be innocuous to the human body. Furthermore, it must be water-insoluble in neutral or moderately acid environment, which is decisive of good storage properties. It must further have high absorbability in the human body, i.e. good bioavailability, which means good solubility in the gastrointestinal tract at a pH value of about 1 (cor¬ responding to 0.1 M HC1). It must also be chemically definable and producible in a reproducible way, i.e. it must have guaranteed constant and controllable proper¬ ties.

Examples of substances used as iron-fortifying agents are iron powder (reduced iron), iron(III)phos-

phate, iron(III)diphosphate, iron(III)sodium diphos- phate, iron(II)sulphate and soluble organic iron(II)com¬ pounds. Soluble iron(II)compounds (e.g. iron(II)sulphate) have high bioavailability in the human body. Being easily soluble, they suffer, however, from the disadvantages of causing discolouration and changes in taste of the fortified products when they react with other components in the food, whereby coloured complex compounds and oxidation products are formed which turn the food product rancid. Iron sources which are water-insoluble (e.g. iron powder, iron(III) hosphate) are relatively inert and do not affect the food product to any appreciable extent. The lower solubility in water of these substances is, however, usually associated with low solubility at low pH and thus gives lower bioavailability as compared with soluble iron salts. To date, it has not been pos¬ sible to find an iron-fortifying agent which in a favour¬ able way combines the requirements for low solubility in water and solubility in the gastrointestinal tract (i.e. at low pH), respectively.

For iron fortification of flour and flour products, reference is made to, for example, US-A-3,803,292 which describes iron fortification of flour by using a combined iron(II)sulphate preparation. It appears that one has managed to combine the high bioavailability of iron

(II)sulphate with long storage life which has been ob¬ tained by the use of particles of iron(II)sulphate mono- hydrate having a surface coating of iron(II)sulphate heptahydrate. However, no bioavailability tests have been accounted for, only stability tests.

Furthermore, Canadian patent specification 1,044,945 describes an iron-fortified fruit porridge powder for infants, in which the fortification iron is electrolytic iron (iron powder having a small particle size). No bioavailability tests have been accounted for.

Reference is also made to US-A-3,876,813 which describes the bioavailability advantages of iron(III)poly-

3 phosphate as compared with iron(II)sulphate. These results are, however, not relevant to use on humans since they are based on animal (rat) experiments only. Iron absorp¬ tion in animals, such as for example rats, is about 100 times higher than in humans, calculated per unit of weight, and furthermore there are considerable distinc¬ tions with regard to the capability of absorbing different iron compounds. Thus, studies of bioavailability in humans are required in order to reach a relevant assess- ment.

The "Handbook of Food Additives", 2 Ed., T.E. Furia, Cleveland, Ohio (1972), p. 660, states that, inter alia, iron(III)orthophosphate and sodium iron pyrophosphate do not, in connection with the fortification of food products, cause the food product to turn rancid, but that their value as fortifying agents is questionable in respect to the bioavailability of iron.

Moreover, a paper by S. Rao and N. Rao, Nutrition Reports International, Vol. 29, No. 5, 1984, pp. 1101-1106, describes the fortification of food products by sodium tripolyphosphate which by chelate bonding with iron can increase iron absorption in the human body.

A comparative study of bioavailability in humans has been carried out with regard to the commercially most frequent iron-fortifying agents which comprise iron(II)sulphate, iron powder and iron(III)phosphate, see Cook et al, "Absorption of fortification iron in bread" in the American Journal of Clinical Nutrition 26, pp. 861-872, August 1973, according to which iron(II)sul- phate is considered to have the highest absorbability (p. 864, col. 2, first complete par.). For these com¬ parisons, an isotope measuring technique was used for measuring the absorption of the thus marked fortification iron in the human body, a technique subsequently described in greater detail by Hallberg, "Food Iron Absorption",

Methods in Hematology, 1980, pp. 116-133, and by Hallberg et al., "Low bioavailability of carbonyl iron in man:

4 studies on iron fortification of wheat flour", the Ameri¬ can Journal of Clinical Nutrition 43, pp. 59-67, January 1986. This technique will be described in greater detail in the Examples below. The problem

Within the art, there is thus the problem of finding an iron-fortifying agent for food products, which both has high bioavailability in humans and insignificantly affects the appearance, the taste and the keeping quali- tites of the fortified food product. Thus, the desired agent shall be water-insoluble such that the food product is not discoloured and does not turn rancid, but shall still have sufficient solubility in the gastrointestinal contents so as to give good bioavailability. The common commercially used iron compounds have appeared to be either sufficiently soluble in water to cause technical problems or so difficult to dissolve that the absorb¬ ability in the human body is low.

Solution of the problem It has been found that common iron(III)phosphate compounds which are commercially used to date, have very low solubility at pH 1. By "common" are here meant the iron(III)phosphates which are stated in the Food Chemicals Codex, FCC, to be commercially used for forti- fication purposes.

We have now found that complex iron(III)phosphate compounds which include monovalent cations, have higher solubility at pH 1. The solubility has been increased

+ + + + in that NH. , H and/or Na and K have been added to the iron(III)orthophosphate. These compounds are comprised by the denomination iron(III)phosphates, but they dis¬ tinguish from the common iron(III)phosphates stated in the FCC, FePO. x H ₂ C mainly owing to a different mole fraction between phosphate and iron, which affects the solubility. Having made extensive systematic studies, we have further found a special type of complex iron(III) phosphate compound having a well-defined X-ray crystal-

lographic structure. Unexpectedly, this type has very good properties with respect to solubility at pH 1. Its bioavailability, particularly in flour products, also appeared to be unexpectedly high, simultaneously as it has the same favourable properties as pure iron(III)' phosphate compounds in respect of suitability as an iron- fortifying agent, i.e. it is almost colourless and has low reactivity in food products.

The invention thus relates to the use of complex iron(III)phosphate compounds for iron fortification of food products, the compounds used having the formula

Z [FeM _3y _ ₃ (P0 ₄ ) _y ].XH ₂ 0

wherein M is one of the the ions H +, Na+, K+, NH.+ or a combination thereof, 1.5 y 3.0, X > 0 and Z is an integer from 1 and upwards, Z -y being an integer.

Preferably, Z is an integer from 1 to 5.

If the chemical formula of the iron(III)phosphate compounds used according to the present invention is written such that merely one iron atom is included per unit of formula, it may be written

FeM _3y _ ₃ (P0 ₄ ) _y .XH ₂ 0

The invention also relates to a method of preparing iron-fortified food products and further to the iron- fortified food products.

The solubility of the iron(III)phosphates which are used according to the invention, has been increased in that the mole fraction between phosphate and iron has been increased. As a compensation for the increased negative charge, the compound thus comprises also mono- valent cations. The compounds mentioned above are per se known from, inter alia, J.F. Haseman, J.R. Lehr and J.P. Smith, Mineralogical Character of some iron and aluminium phos-

6 phates containing potassium and ammonium. Soil Science Society Proceedings 15(1951), pp. 76-84. The above com¬ pounds distinguish from iron(III)orthophosphates, -diphos- phates and -polyphosphates which are previously known in connection with fortification, by having a completely different structural composition.

Iron phosphates are composed of iron ions, divalent or trivalent, and phosphate ions. The phosphate ions and also the bonding between the phosphate ions and the iron ion can be very different in character. The most frequent type of iron phosphate is iron orthophos-

3_ phate. These phosphates contain the group PO. , as a discrete unit. Typical compounds of this type are

FePO. • 2 H~0 which as a mineral is called strengite, or Fe-.(PO.)„ • 8 H~0 which as a mineral is called vi- vianite.

The diphosphates or pyrophosphates contain a group P_ 0 ₇ 4- in which two phosphate tetrahedrons share a corner. An iron(III)diphosphate has the formula Fe.(P~0_,), • 9 H ₂ 0.

The polyphosphates comprise chains or rings of PO. tetrahedrons. The chain length may vary most con¬ siderably. As a rule, the compounds are non-crystalline and are composed in various manners. The polyphosphates form strong bonds with iron(III)ions.

Complex iron phosphates contain, in addition to iron, also other cations. The compounds used according to the present invention are composed of orthophosphates

+ + + and contain one or more of the cations H , Na , K and NH. . In common iron phosphate, all oxygen atoms in the orthophosphate groups are bonded with iron, whereas the complex iron phosphates contain oxygen atoms which are not bonded Vith iron. If the compound contains H ions, they are bonded with these oxygen atoms. Since the complex phosphates contain other cations than iron, the mole fraction between phosphate and iron is larger than in a pure iron phosphate. The mole fraction between

7 phosphate and iron as well as the composition of the remaining cations are very important to the chemical properties of the compounds.

Figs. 1 and 2 show different spatial arrangements for different types of phosphate ions which are all based on a tetrahedron unit. Figs. 3 and 4 show atomic models for a common phosphate compound and a compound according to the invention having such phosphate ion tetrahedrons inserted in the space lattice. It will be appreciated from Figs. 3 and 4 that the complex compounds according to the invention distin¬ guish materially from common phosphates which are pre¬ viously known in connection with fortification.

A large number of substances having the above-men- tioned general formula may be prepared (see the Table below). Their solubility and stability properties are, to a certain extent, affected by the value of y and by the composition of the cations in the structure. Most of the compounds are very stable at room temperature and are not hygroscopic.

Table. Examples of compounds according to the present inven y = 1.5; Z = 2 ^Fe ₂ ^Na 3 ^(P0 4 ■ 3" ³ H ₂ 0, Fe ₂ Na ₃ (P0 ₄ ) ₃ ^• 4 H ₂ 0 y = 1.8; Z = 5 Fe ₅ HgNa ₄ (P0 ₄ ) _g • 10H ₂ O y = 2.0; Z = 1 FeH ₃ (P0 ₄ ) ₂ ^■ 2 H ₂ 0

FeH ₂ Na(P0 ₄ ) ₂ FeH ₂ Na(P0 ₄ ) ₂ - 0.5 H ₂ 0 FeH ₂ Na(P0 ₄ ) ₂ • H ₂ 0

FeH ₂ Na(P0 ₄ ) ₂ • 3 H ₂ 0 FeNa ₃ (P0 ₄ ) ₂ -H ₂ 0

Z = 3 Fe ₃ H ₈ K(P0 ₄ ) ₆ 6 H ₂ 0

FeH ₂ K(P0 ₄ ) ₂

FeH ₂ K(P0 ₄ ) ₂ H ₂ 0

Z = 3 Fe ₃ Hg(NH ₄ )(P0 ₄ ) ₆ 6 H ₂ 0

FeH ₂ (NH ₄ ) (P0 ₄ ) ₂ FeH ₂ (NH ₄ ) (P0 ₄ ) ₂ ^• 0.5 H ₂ 0 y = 2.67; Z = 3 Fe ₃ H ₁₄ K(P0 ₄ )g ^• 4 H ₂ 0 y = 3.0; Z = 1 FeH ₆ (P0 ₄ ) ₃ FeH ₅ Na(P0 ₄ ) ₃ - H ₂ 0

FeH ₄ a ₂ (P0 ₄ ) ₃ • 3 H ₂ 0

All iro (III)phosphates in the Table are soluble in 0.1 M HC1. The dissolving rate is affected more by the particle size than by the composition. Solubility examinations for representative compounds from the Table above are shown in the experimental part of the speci¬ fication, and regarding the most preferred compounds, an extensive absorption and bioavailability test has also been carried out.

The compounds used according to the invention are prepared by reaction between iron(III)salts and H-.P0 ₄

+ + + - ^{3 4} in the presence of NH. , Na or K .

The iron(III)phosphates used according to the in¬ vention can be added as an iron-fortifying agent to e.g. flour in the usual manner, whereupon the flour can be used for baking bread. The fact that fortification and absorption results of equivalent values are obtained when the fortifying agent is used in non-fermented product has been shown in comparative tests where fortified flour has been made to porridge. In analysing the bio- availability in humans, use is made of radioactive iron labelling in the synthesis of the iron(III)compound used. Examination of bioavailability The examination was carried out by the technique described in the above-mentioned paper by Hallberg et al. The iron compound to be tested is labelled with radioactive iron, 55Fe, m the synthesis of the iron compound used. This iron is used to fortify flour from

9 which buns are made by standard procedures, for example as follows:

Ingredients Parts by weight wheat-flour 100 sugar 3.5 salt 1.0 yeast 6.5 water 83 iron source 6.5 The iron occurring naturally (native iron) in wheat- flour is labelled by adding tracer amounts of 59Fe- labelled iron(III )chloride. The buns are thus made of flour which is labelled with two different radioactive isotopes. The ingredients are mixed, and the dough is allowed to rise for 30 minutes. After the usual kneading, the dough is placed in small aluminium baking tins where the dough pieces are allowed to rise for another 30 min¬ utes. The dough pieces in the baking tins are subsequently baked at a temperature of 250 C for 10 minutes.

These double-labelled buns are served a group of test subjects, for breakfast with coffee, margarine and marmalade two successive mornings after a night's fast. No food or beverage is allowed for three hours after breakfast. Two weeks after the last breakfast, blood tests are made to measure the occurrence of 55Fe and 59Fe in blood. Furthermore, the totally absorbed amount of 59Fe in the body is measured by a so-called total-body counter. Via the ratio of 59Fe to 55Fe in blood samples, also the totally absorbed amount of 55Fe can be determined.

Subsequently, a reference solution is administered, consisting of 59Fe-labelled iron(II)sulphate and ascorbic acid at two successive breakfasts after a night's fast. No food or beverage is allowed for three hours thereafter.

Another two weeks later, a new total-body measurement is made to measure the absorption of the reference dose.

The reference solution is administered to make it possible to carry out corrections with regard to differences in the iron absorbing .capability of the different test subjects.

59 59 The amount of Fe which is added as ^FeC 3 ^to the wheat-flour, isotope-labels by so-called isotopic exchange all biologically available nonheme iron (=non- blood iron) in the meals analysed, i.e. on the one hand the native iron and, on the other hand, the part of the fortification iron which is dissolved. However, that part of the fortification iron which is not dissolved in the gastrointestinal tract, is not labelled. Since the fortification iron added to the bread is labelled with another isotope (= 55Fe), the ratio of the 55Fe portion to the 59Fe portion as absorbed will be a direct measure of that portion of the examined, Fe-labelled iron compound which has been absorbed in the so-called pool of nonheme iron in the gastrointestinal tract and which thus is potentially available to absorption. This portion is a measure of the bioavailability of the ex¬ amined iron compound in relation to an easily soluble iron compound, such as iron(II)sulphate. When iron(II)- sulphate is added as an iron-fortifying agent, it is, in fact, completely mixed with the pool of nonheme iron. Thus, a measure of the bioavailability in relation to a known, easily available compound is obtained, viz. the ratio of 55Fe to 59Fe or the relative bioavailability.

In explanation of the terminology used above, it may be mentioned that so-called heme iron which occurs in meat and blood products and is present in the form of hemoglobin and myoglobin, is a relatively small portion of the total intake of iron together with food. So-called nonheme iron which mainly occurs in bread and other cereal products, fruit and vegetables, constitutes the main part of iron in food. It may be mentioned that the composition of the meal is most important to the absorption of nonheme iron in the body.

Example 1 below accounts for the examination of bioavailability of the compound Fe,H _g (NH ₄ ) (P0 ₄ )-- •6H-0 according to the invention, since this compound can be reproducibly prepared in crystalline form with very small particle size. However, the selection of this particular compound constitutes no restriction whatso¬ ever since one of the other compounds mentioned having the formula FeM _{3 3} (P0 ₄ ) -XH ₂ 0 could have been studied just as well. As a comparative Example 1, an analogous examination of iron powder is described, which to date is the most frequently used type of fortification iron. Many types of iron powder have, however, very low absorbability, and only if the particle size is small enough, relatively good absorption values can be obtained. In the comparative Examples below, the absorption of so-called carbonyl iron is measured, which is considered one of the better commercial iron powders. Its particle size was 1-10 μm.

As a comparative Example 2, an analogous examination of common iron(III)phosphate is described.

Since a complete bioavailability test on humans is very complicated and time-consuming, the bioavail¬ ability of further compounds according to the invention has been tested schematically in solubility examinations in artificial gastric environment (Examples 2-6), the compound examined in Example 1 serving as a reference substance. Here, solubility tests for some commercially used iron(III) hosphates have also been included.

Example 1 From flour fortified with Fe from the compound Fe ₃ H ₈ ( H ₄ ) (P0 ₄ ) _fi -6H ₂ 0 and with 59Fe from FeCl ₃ , buns weighing 40 g were made in the above-mentioned manner. Each bun contained 0.2 mg native iron and 2.2 mg forti¬ fication iron. After measurements conducted in the manner described above on a test group of ten persons, the administered reference solution consisting of 10 ml 0.01 M HCl con-

taining 3 mg 59,Fe in the form of FeS0 ₄ and 30 mg ascorbic acid, the results summarised in the Table below were obtained.

TABLE 1 Compound according to the invention

Comparative Example 1 From flour fortified with 55Fe from carbonyl iron and with 59Fe from FeCl_,, buns weighing 60 g were made in the above-mentioned manner. Each bun contained 0.4 mg native iron and 1 mg fortification iron.

After measurements conducted in the manner described above on a test group of ten persons, the administered reference solution being the same as above, the results summarised in the Table below were obtained.

TABLE II

Carbonyl iron

Fig. 5 is a staple diagram illustrating a comparison between the relative bioavailability of carbonyl iron and of iron-fortifying agents according to the invention when iron-fortified bread was included in different meal combinations, viz. I: broth and bread; II: breakfast with bread and milk (alternatively bread, sour milk, cereals and coffee); and III: breakfast with bread and coffee.

Comparative Example 2

From flour fortified with Fe from common iron(III)- phosphate and with 59Fe from FeCl ₃ , buns weighing 60 g were made in the manner stated above. Each bun contained

0.5 mg native iron and 3 mg fortification iron.

After measurements conducted in the manner described above on a test group of ten persons, the administered reference solution being the same as above, the results summarised in the Table below were obtained.

TABLE III

Common iron(III)phosphate

It will be appreciated from the Examples above that the compound used according to the invention has a far better bioavailability than both the commercial fortifying agent which is to date most frequently used, i.e. iron powder, and the known commercial fortifying agent which, in structural respect, is its closest relation, i.e. common iron(III)phosphate. Regarding the effect on storage life, appearance and taste of the fortified food products,

14 the three products examined are approximately equivalent.

Examples 2-6 The percentage solubility per unit of time was measured for a number of representative compounds ac- cording to the invention in artificial gastric environmen (0.1 M HC1 ; pH = 1 ; 37°C). The compound examined in Example 1 was also measured as a reference substance (compound A) .

The compounds examined are: Compound A: Fe ₃ H _g (NH ₄ ) (P0 ₄ ) ₆ ^• 6H ₂ 0

Compound B: FeH ₅ a(P0 ₄ ) ₃ ^• H ₂ 0

Compound C: Fe.-H _g Na ₄ (P0 ₄ ) ₉ ^• 10H ₂ O

Compound D: PeH ₂ a(P0 ₄ ) ₂ ^• 0.5H ₂ O

Compound E: Fe ₃ H _g K(P0 ₄ )g ^• 6H ₂ 0

It may be noted regarding compound E that the struc¬ ture of the compound prepared for the test did not confor entirely to the microcrystallinity of the remaining com¬ pounds, which to some extent explains its deviating solubility values.

For comparison, some iron compounds stated in the Food Chemicals Codex (FCC), viz. amorphous FePO. and microcrystalline FeP0 ₄ , were also tested in the same manner with respect to solubility.

The results are shown in Table IV below and are also graphically shown in Fig. 6.

TABLE IV

It appears from the Table that compounds B, C and D which contain sodium, have approximately the same good solubility properties as the ammonium-containing compound A which has been tested for bioavailability. Compound E containing potassium has a low initial solu¬ bility but reached, after some time, relatively good values.

The iron phosphates known have very low values under the same test conditions. The amorphous compound is the one most frequently used in connection with iron fortification, since a compound having an amorphous structure is generally more easily soluble because of its larger surface. The comparison also to the micro¬ crystalline compound is, however, necessary since the compounds used according to the present invention are present in microcrystalline form so as to obtain a repro¬ ducible and well-defined crystal structure.

Thus, the compounds used according to the invention have surprising and highly valuable properties with respect to solubility in gastric environment.