Metal Chelators as Antioxidants

The Authors: Colin McCarthy and Eric Decker, Department of Food Science, University of Massachusetts, Amherst, USA

Introduction

Given the abundance of pro-oxidative metals naturally occurring in many foods, sequestration of free metals through chelation is critical for the mitigation of lipid oxidation and preservation of food quality. Free metals can reduce endogenous hydrogen and lipid hydroperoxides to generate free radicals which can propel further oxidation. Chelators, a class of secondary antioxidants, work by complexing metals and therefore marginalizing their reactivity. Additionally, some commonly used chelators may realize add-on effects that bolster food quality, for instance enhanced antimicrobial activity or increased water holding capacity within muscle foods [1-2].

Common chelators used in the food industry include acids- ethylenediaminetetraacetic acid (EDTA), citric acid and phosphates- (sodium tripolyphosphate and hexametaphosphate). Significant potential also exists for the use of proteins as chelators, and novel applications of chelator technologies outside of their traditional use as food additives are emerging, particularly within active packaging. In addition, an inchoate class of iron chelators derived from microorganisms, siderophores, may one day add another chelator class to those commonly used today.

Mechanism of Action

Metals catalyze non-enzymatic oxidation by redox cycling an electron from hydrogen or lipid hydroperoxides which generates lipid radicals and reactive oxygen species [3]. An example of this redox cycle begins with the oxidation of ferrous iron (II) into ferric iron (III) and reduction of hydrogen peroxide into a hydroxide anion and hydroxyl radical by way of the Fenton reaction. The cycle is completed with the reduction of ferric ions into ferrous ions via reaction with superoxide anion or other reducing agents [4]. This pathway for regeneration of iron atoms into their more reactive ferrous state means metal catalyzed lipid oxidation is especially deleterious when reducing conditions are present.

Chelators can inhibit the decomposition of hydroperoxides by binding and maintaining free metals in a single redox state. If oxidized ferric iron is sequestered the total oxidation rate decreases as ferric iron cannot be redox cycled to ferrous iron which has a significantly higher prooxidant activity. Sequestration of ferric iron will also shift the equilibrium and trigger the conversion of ferrous to ferric, further reducing oxidative stress [5].

Chelators, like phytate, can also form metal complexes that become insoluble thus removing metals from the reaction media.  Ferritin, an iron-binding protein found in many tissues, decreases iron-lipid interaction by binding iron in its core [6]. Chelators can also decrease lipid oxidation rates by driving the partitioning of metals away from the lipid.  For example, aqueous phase anionic proteins and polysaccharides like xanthan gum will bind metals within the continuous phase of oil-in-water emulsions and decrease their reactivity by decreasing metal-lipid interactions [7-8]. Chelator efficacy is highly dependent on the environmental conditions of the food matrix. Since many chelators bind metals through the charged acid groups, they will be most active when pH is greater than their pKa. Chelator concentrations are also critical to efficacy.  When chelator concentrations are similar to, or below the metal concentration, this can result in increased metal solubility.  If this solubilized chelator-metal complex is still redox active, the chelator can be pro-oxidative [9].  When chelator concentrations are greater than metal concentrations, the metal can be bound by multiple chelators thus tying up all metal coordination sites and thereby preventing redox cycling [10-11].

Food Grade Chelators

Common food grade chelators include EDTA, phosphates, proteins and organic acids. EDTA is very effective at inhibiting oxidation in oil-in-water emulsions like salad dressing and mayonnaise [10-11]. EDTA can promote oxidation if more iron than EDTA exists in the emulsion by increasing metal solubility and failing to prevent redox cycling [9]. In the United States, EDTA is allowable up to 75 ppm in dressings, mayonnaise and sauces; this concentration exceeds that of transition metals and strong antioxidant activity is observed (FDA, 2000). EDTA has two pKa’s below 3.0 (1.7 and 2.6) allowing it to bind metals in acid foods. The ferric dissociation content of EDTA is high, 1.2 x 1025 which helps maintain low levels of ferrous ions. EDTA-iron complexes have good bioavailability and are some used as effective iron dietary supplements [12]. The calcium and sodium salts of EDTA are most commonly used in foods due to their high-water solubility.

Food grade phosphates include phosphoric acid, pyrophosphate, trisodium polyphosphate and hexametaphosphate. Phosphates have low pKa’s (0.8 and 2.0) and a high dissociation constant (1 x 1022). However, they are not effective antioxidants in oil-in-water emulsions ([13]).  Polyphosphates are strong antioxidants in muscle foods with the polymeric phosphates being the most effective. Phosphates also enhance the water holding capacity of muscle foods improving yield and quality ([1-14].

Organic acids including citric and tartaric acid are also effective chelating agents. These alpha hydroxy acids are impactful ligands as they are ringed with potentially anionic oxygens which can ensnare free metals. These acids are very food safe, with no limitations on use (FDA, 1994). As with other chelators, organic acids need to be adjusted to a concentration exceeding that of free metals in order to inhibit hydroperoxide formation [15].  Citric acid is the most common organic acid used as a chelator in products such as bulk oils and meats.  It is most effective at pH values higher than their pKa’s (citric acid = 3.1, 4.7 and 6.4).

Proteins exist within biological tissues that have specific iron binding capacity like ferritin, lactoferrin and phosvitin [11, 16-17]. While effective, the potential for using these as food additives is limited by their cost. Anionic portions of nonspecific iron binding proteins like whey protein isolate, casein, soy protein isolate and bovine serum albumin can bind with cationic metals and inhibit lipid oxidation. This inhibition occurs primarily by partitioning metals away from lipids within oil-in-water emulsion droplets [18]. However, their efficacy is limited by pH, with the pI of many chelating proteins ranging between 4.6-5.2.  Thus, these proteins become positively charged in acidic foods and consequently lose sequestration capacity [18-19]. Hydrolyzed proteins can be more effect chelators than intact proteins by exposing negatively charged peptides in the interior of the protein to the environment. Highly phosphorylated proteins like casein phosphopeptides from hydrolyzed casein and phosvitin from egg yolk can also bind iron very effectively [20-21].

Emerging Chelator Technologies

Understanding that two of the most impactful chelators used in food are synthetic (EDTA and polyphosphates), there is a desire for alternative technologies that would engender “clean” labeling. The incorporation of chelators into films is a potential emerging category for food packaging applications. Fixation of transition metal chelating ligands like carboxylic acid and iminodiacetic acid onto plastic packaging materials can effectively inhibit lipid oxidation in oil-in-water emulsions [22–24]. Chelating packaging could remove the need for the additive to be included in the ingredient declaration but would still necessitate regulatory approval as a food contact surface.

Chelators of microbial origin offer significant potential for high affinity iron binding food additives. Siderophores are a class of low molecular weight iron binding compounds originated from various microorganisms including bacteria and fungi. Isolated siderophores exhibit high antioxidant activity in oil-in-water emulsions. While siderophores are expressed by a vast multitude of microbes, siderophores originating from human gut microbiota or fermented foods may have a more straightforward path to approval as a food additive given their presence in the human body and the food supply. This is a developing area and much needs to be determined as to safety, labeling requirements and cost [25-26].

With any new chelation technology, there is a concern on how they could impact the bioavailability of iron in foods. Some chelators can increase iron solubility which in turn can increase iron bioavailability. Chelators could also be degraded in the gastrointestinal tract and release bound iron to make it bioavailable. However, if chelators decrease metal solubility and/or are not degraded in the gastrointestinal tract, they might not be suitable as food additives. Natural chelators like phosvitin, whey and oxalic acid can inhibit iron absorption in the gut [27] . In regards to metal chelating packaging, these might be limited to foods that are not a substantial source of dietary iron as they will remove iron from the packaged food.

Conclusion

Chelators are excellent antioxidants in food application such as bulk oils, oil-in-water emulsions and muscle foods.  Effective use of chelators depends on the solubility characteristics, pKa’s and mechanism of action. In addition, chelators can help improve the activity of other antioxidants such as free radical scavengers by decreasing free radical production.  Lastly, with the incessant and seemingly ever-increasing demand for clean label products, there will continue to be a demand for natural chelators that are effective in a variety of food applications.

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