Cooking Chicken Breast Reduces Dialyzable Iron Resulting from Digestion of Muscle Proteins

The purpose of this research was to study the effect of cooking chicken breast on the production of dialyzable iron (an in vitro indicator of bioavailable iron) from added ferric iron. Chicken breast muscle was cooked by boiling, baking, sautéing, or deep-frying. Cooked samples were mixed with ferric iron and either extracted with acid or digested with pepsin and pancreatin. Total and ferrous dialyzable iron was measured after extraction or digestion and compared to raw chicken samples. For uncooked samples, dialyzable iron was significantly enhanced after both extraction and digestion. All cooking methods led to markedly reduced levels of dialyzable iron both by extraction and digestion. In most cooked, digested samples dialyzable iron was no greater than the iron-only (no sample) control. Cooked samples showed lower levels of histidine and sulfhydryls but protein digestibility was not reduced, except for the sautéed sample. The results showed that, after cooking, little if any dialyzable iron results from digestion of muscle proteins. Our research indicates that, in cooked chicken, residual acid-extractable components are the most important source of dialyzable iron. 1. Introduction Iron is an essential micronutrient. Most of the iron in the diet is in the nonheme form and is poorly absorbed [1]. Iron absorption can be influenced by a variety of dietary components but it is well accepted that the most effective enhancers of absorption are ascorbic acid and muscle tissue [2, 3]. The effect of muscle has become known as the “meat factor.” The mechanism of the meat effect remains controversial even after decades of research, largely with in vitro systems. Much research suggests that peptides, derived from enzymatic digestion of muscle proteins, chelate iron that would otherwise be insoluble in the upper intestine. In vitro studies have indicated that cysteine and histidine residues in peptides could act as iron chelators [4–6]. Peptides could also reduce ferric iron to the more soluble and bioavailable ferrous form through the action of cysteine residues [7]. Both of these mechanisms can lead to increased levels of dialyzable iron. Some studies have suggested that a nonprotein muscle component may be involved [8]. All research involving human subjects (and most of the in vitro studies) have used cooked meat or fish as the source material. However, the effect of cooking per se has received little attention despite the knowledge that heat causes oxidation of muscle protein sulfhydryls [9] and that sulfhydryls are the structures most often thought to be