There are some important dietary metals in our food, such as iron, magnesium, selenium, zinc. calcium and copper. Unfortunately, however, traces of metals from polluted water sources are also increasingly present, the accumulation of which in the human body can, over time, cause harmful effects on health.
There is evidence of arsenic in rice, nickel in cereals and vegetables, lead in meat and fish, mercury in fish and seafood, cadmium in cereals, vegetables, potatoes, shellfish and molluscs. Metal contamination can occur at any stage of the product’s life, including processing, storage and cooking. Special attention is paid to aluminum, present in a large number of containers intended to come into contact with food, such as pots, wrapping films, disposable trays and coffee pots. Albeit generally modest, the migration of aluminum from containers becomes important when storing acidic foods (tomatoes), foods containing high amounts of salt, and especially during cooking.
In addition to the accumulation damage on the body, metals (both the desired ones and pollutants) promote the oxidation of organic substances in foods (especially lipids, but also proteins and carbohydrates), which can have negative effects on the nutritional and sensory properties of foods. With the formation of potentially toxic compounds which may have further negative consequences on the health of consumers. This problem is particularly present in food products that contain water, such as sauces and fruit juices.
Chelating agents
The sequestration of metals by suitable substances is the method used to hinder the activity of metals present in food, thus eliminating the initial cause of oxidation and preserving food quality for longer. Typical sequestrants are chelating agents, which form stable compounds in which metals are bound to two or more atoms present in the chelating agent, typically nitrogen, oxygen and sulphur. Thanks to the presence of these elements, metals are bound like the “claws” of a crab, hence the term “chelant”. A single metal can be coordinated by one or more chelating agents until it is completely surrounded by the chelator.
In this way, the reactivity of metals is inhibited both because the interaction of metal with the oxygen of air, which is the first step of oxidation reactions, is slowed down, and because metals are removed from organic molecules. In addition, some chelants such as polyphenols (tocopherol, etc.) may also inhibit free radical production and have an antimicrobial function.
The efficacy of chelants depends strongly on two factors: The characteristics of the food with which they interact, in particular its pH value, i.e. its acidity, and the concentration of the chelant used. In general, chelators act worse, that is, they bind metals worse from highly acidic foods, such as lemon juice and coffee, and they bind metals better under neutral/basic conditions. In addition, chelants generally need to be present at a higher concentration than the metals to be sequestered to have good antioxidant activity. The most used food chelants in the food industry are:
* Ethylenediaminetetraacetic acid (EDTA), which contains oxygen and nitrogen as chelating centers and has the property of permanently binding many metals (D. K. Thbayh et al., 2023). EDTA is very effective at inhibiting oxidation in oil-in-water emulsions, such as salad dressings and mayonnaise, and is an economical solution because it is effective even at low concentrations. Dow sells two chelating substances for food: the Dow VERSENETM CA (calcium disodium EDTA) and the Dow VERSENETM NA (disodium EDTA), both colorless, odorless, tasteless and authorized by the US Food and Drug Administration for direct contact with carbonated and alcoholic beverages, mayonnaise, sauces, margarine and spreads;
* Citric acid and tartaric acid, which contain oxygen as a chelating center, safe for food without restrictions on use (S. Begum et al., 2021). Citric acid is the most common organic acid used as chelant in products such as bulk oils and meat;
* Phosphates for food use, such as phosphoric acid, pyrophosphate, trisodium polyphosphate and hexamethaphosphate (S. Begum et al., 2021), which contain oxygen as a chelating center. Phosphates are strong antioxidants in food for muscular effort, in which they improve water retention and thus quality.
Although citric acid and phosphates are sometimes proposed as metal chelating agents in food, these materials are generally less effective than EDTA.
How to use chelants in food?
The use of chelants as food additives, i.e. their direct addition to food, is the most used 0method. However, new ways of using active packaging are emerging. Chelants are already incorporated into plastic packaging materials from which active components are slowly released into food. For example, tocopherol embedded in low-density polyethylene (LDPE) forms antioxidant and antimicrobial active films (D. K. Thbayh et al., 2023). It is true, however, that these films act by direct addition, because chelants must be transferred to packaged food in order to act.
To meet the right demand for reduction of additive use, some researchers have studied new packaging films in which the active agent can react, trap or modify the substances involved at any stage of the oxidation process without being released to food. More precisely, chitosan (CS) and sodium alginate (SA) multilayers have been assembled on the surface of polypropylene (PP) bound to polyacrylic acid (PAA) to produce active packaging films and metal chelating named PP-g-PAA/(CS/SA) (Z. Yu et al., 2022). This material has been prepared by binding polyacrylic acid on the surface of the PP by ultraviolet (UV) radiation. PP has no chelating capacity, PP-g-PAA films contain polyacrylic acid, which already has chelating capacity, but above all, which allows the further modification of the material with chitosan and alginate.
The final material has been prepared using the layer-by-layer assembly (LBL) method, which involves the self-assembly of electrostatically attracted chitosan polyanions and sodium alginate polycations. Both chitosan polymers and sodium alginate polymers are of biological origin and from renewable sources: Chitosan is obtained from chitin, which in turn is extracted from the exoskeleton of crustaceans, while sodium alginate is extracted from algae. Both chitosan and alginate are biodegradable, low-cost, non-toxic and contain functional groups in their molecular chains (hydroxyl, amino and carboxyl groups) that function as active sites capable of binding metals.
Studies have shown that PP-g-PAA/(CS/SA) films can effectively chelate both copper and iron, which are the most easily studied metals to determine the effectiveness of chelation. It has been found that the concentration of chitosan and sodium alginate in the material and the pH value are important parameters not only for the chelating capacity of films, as seen above, but also for the morphological characteristics of multi-layer films: The chelating capacity of the films increases with pH and with the concentration of chitosan used, although the concentration of chitosan cannot be increased indefinitely, because an excessive value leads to the formation of a film with a rough surface.
In summary, the optimal parameters for the preparation of PP-g-PAA/(CS/SA) films are a concentration of 1.0 mg/ml for chitosan and 5.0 mg/ml for sodium alginate and a pH value of 5.0. The chelating capacity of these films is approximately 8.57% better than PP-g-PAA films. The antioxidant protection effect of PP-g-PAA/(CS/SA) films has been studied on an oil-in-water emulsion that has been used as a model of mayonnaise and an antioxidant effect similar to that of EDTA has been observed.
Perspectives
For sure, chelators are good antioxidants in foods such as bulk oils and oil-in-water emulsions. In addition, the development of antioxidant active packaging opens a new window to preserve food quality without releasing agents to food, and this concept is useful for many packaging materials. However, it should be stressed that chelators can bind not only to polluting metals, but also to essential metals in food, starting with iron. This property can be exploited as a natural therapy to cure excess iron. In other cases, however, this property can be a problem because it alters the bioavailability of iron in food. In such cases, the use of chelating agents may be restricted to the packaging of food which is not a source of iron or other nutrient metals, or chelating agents which selectively bind polluting metals and leave essential metals unchanged.
References: K. Thbayh et al., Heliyon 9, 2023, 16064; Begum et al., Carbohydrate Polymers, 259, 2021, 117613; Yu et al., Food Packaging and Shelf life 32, 2022, 100846.
