Stainless steel is a family of iron-based alloys widely recognized for its high resistance to corrosion and common use in commercial and household items. Consumers often wonder if this material, frequently in contact with food and drink, releases its constituent metals. Stainless steel does leach a small amount of metal into food and water, but this typically occurs in trace quantities. The release of these elements is dependent on specific environmental conditions and the chemical makeup of the steel itself.
The Elemental Composition of Stainless Steel
Stainless steel is defined by iron alloyed with a minimum of 10.5% chromium. This chromium content provides its signature corrosion resistance. Chromium reacts with oxygen to form an extremely thin, self-repairing layer of chromium oxide on the surface, known as the passive layer.
The two most common grades found in housewares and food processing are 304 and 316, both belonging to the austenitic class. Grade 304 stainless steel is often called “18/8” or “18/10,” signifying it contains approximately 18% chromium and 8-10% nickel. Nickel is included to stabilize the steel’s structure, enhancing its strength and durability.
Grade 316, frequently dubbed “marine grade,” has a similar composition but includes 2% to 3% molybdenum. The addition of molybdenum significantly boosts the material’s resistance to pitting and corrosion, particularly in environments exposed to chlorides. These alloying elements are crucial for the material’s performance, but they are also the primary metals that can potentially leach into food. The passive layer acts as a barrier, protecting the underlying iron and nickel from direct contact with the environment.
Factors Driving Metal Leaching
The protective passive layer is not impenetrable and can be compromised by environmental factors, leading to the release of trace metals. The primary driver of increased leaching is high acidity in the contacting food or liquid. Prolonged exposure to highly acidic foods, such as tomato sauce, vinegar, or citrus juices, facilitates the breakdown of the oxide layer and allows metal ions to escape into the solution.
The temperature of the contact material is another significant factor in accelerating the leaching process. Cooking or storing food at high temperatures for extended periods increases the kinetic energy of the system, making it easier for metal ions, particularly nickel and chromium, to be mobilized. Studies on acidic foods, like tomato sauce, have shown that metal concentrations can increase substantially after several hours of cooking.
The duration of contact between the stainless steel and the food is directly proportional to the amount of metal leached. Storing an acidic item in a stainless steel container overnight will typically result in more leaching than a brief cooking process. New stainless steel cookware tends to leach the highest amounts of metal during the first few uses. This leaching decreases with sequential cooking cycles, stabilizing after approximately six to ten uses. Physical damage, such as pitting, deep scratches, or abrasion, can also locally disrupt the passive layer, creating sites where corrosion and subsequent leaching are more likely to occur.
Health Impact of Trace Elements
The elements of primary concern in stainless steel leaching are nickel and chromium, but the resulting exposure is generally considered low for the average person. The concentration of leached metals is often well below the levels set by regulatory bodies for safe daily intake. For example, studies have found that a single serving of an acidic food cooked in stainless steel may contribute an average of approximately 86 to 88 micrograms of chromium and nickel.
The health consideration for chromium centers on its chemical form, or speciation, as it exists in two main states. Chromium(III) is considered an essential trace mineral for human metabolism and is the form predominantly released from stainless steel under normal use conditions. The highly toxic form, Chromium(VI), is not typically released in measurable amounts during food preparation.
Nickel presents a distinct concern for individuals with nickel sensitivity or allergies. Even low levels of orally ingested nickel can potentially trigger or worsen systemic contact dermatitis in highly sensitized individuals. While the amount of nickel leached is minor, it adds to the total daily intake, which is primarily sourced from various foods. Most experts agree that for the vast majority of the public, the minimal quantities of nickel and chromium released by stainless steel cookware do not pose a significant health hazard.