Since I receive so many questions on this blog pertaining to products made from stainless steel, I thought I’d give you some background about what stainless steel is and how it leaches into various foods and beverages.
Steel is used to make a wide variety of industrial, construction, and consumer products. The largest use by far for consumer products is to make auto bodies and “tin cans” (actually made of steel with a very thin protective coating of tin), although it is also used to make appliances, cutlery, flatware, razor blades, cookware, water bottles, and many other items.
What Steel is Made From
Steel, by definition, is made primarily from iron. Iron is a naturally-occurring metallic element. It is rarely found on the surface of the earth because it oxidizes readily in the presence of oxygen and moisture, and disintigrates into rust. Iron used in products, such as cast iron, is made from the iron ore hematite, from which oxygen has been removed by heating to high temperatures. So when a product is made primarily from iron, it will naturally rust.
Pure single crystals of iron are very soft, so other metals are added to strengthen it. These are called “alloys”. Alloying iron with small amounts of other metals and carbon produces steel, which can be 1,000 times harder than pure iron.
There are two basic types of steel: carbon steel and stainless steel.
The type of steel used to make auto bodies, cans, cutlery, and woks is plain carbon steel, produced by the Basic Oxygen Furnace process. You can easily tell that it is carbon steel because it turns black over time and easily rusts when exposed to air and moisture. Made simply of iron with 0.1 to 1.2 percent carbon and even less manganese, carbon steel can be recycled and often contains a minimum of 25 percent recycled content whether it is so labeled or not. The Electric Arc Furnace process, which is used to produce steel shapes such as railroad ties and bridge spans, uses virtually 100 percent recycled steel.
Most consumer products made from steel, however, such as cookware and bakeware, cooking utensils, and flatware, are made from stainless steel, which has a special ability to resist stains and corrosion (so it doesn’t rust or turn black). The average stainless steel object is made of about 60% recycled material of which about 40% originates from products consumers have recycled and 60% comes from manufacturing processes. (The Recycling of Stainless Steel” International Stainless Steel Forum. 2006)
In metallurgy, stainless steel is defined as a steel alloy with a minimum of 11% chromium content by mass. The addition of chromium prevents the corrosion that causes rust, and also prevents stains, thus the name “stainless steel.”
The chromium–and this is an important point with regards to metal toxicity–forms a layer of chromium oxide over the steel when exposed to oxygen. This layer is impervious to water and air, protecting the metal beneath. This layer quickly reforms when the surface is scratched, so when a food or beverage or your body comes in contact with stainless steel, what it is actually contacting is chromium.
There are more than 150 grades of stainless steel, of which fifteen are most common. In addition to chromium, nickel and manganese are added to some alloys.
More than 70% of stainless steel production is of the 300 series, which produces a particular crystalline structure called “austenitic.” These contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain their crystalline structure.
These stainless steels are named by their chromium and nickel content. The common composition of 18% chromium and 8% nickel is known as 18/8 stainless. 18/0 and 18/10 are also available.
The most common grade is Type 304, which is 18/8 stainless steel.
The second most common is Type 316, called “surgical stainless steel” for food and surgical stainless steel uses. In addition to chormium and nickel, surgical stainless steel also contains molybdenum to prevent specific forms of corrosion and help maintain the cutting edge. Three hundred sixteen surgical steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination.
Type 440 is used to make knives, as it holds a sharp edge well.
|SAE||% Cr||% Ni||% C||% Mn||% Si||% P||% S||% N|
Leaching of Metals from Stainless Steel
Despite the fact that stainless steel has an outer protective layer that reforms almost immediately when the metal is scratched, iron and nickel do leach through this layer, resulting in the leached of all three metals into food and beverages.
I searched for studies that would show leaching of metals from stainless steel and found one that I could purchase for $34, so I bought it: Bulletin of Environmental Contamination and Toxicology: Leaching of Heavy Metals (Chromium, Iron, and Nickel) from Stainless Steel Utensils in Food Stimulants and Food Materials.
It’s from 1994, but stainless steel is stainless steel.
They tested a variety of stainless steel tumblers and bowls used in India, some new and some used.
Foods of various pH were tested for the leaching of iron, chromium, and nickel.
|5% Sodium carbonate||11.50|
|5% Acetic Acid||2.11|
- Iron, chromium and nickel were all found to leach into both alkaline and acidic foods and beverages, while none of the metals leached into distilled water.
- Leaching of iron, chromium and nickel was observed from both new and old utensils.
- Leaching of iron occurred in all foods.
- Leaching of nickel occurred in curd, fruit juice and pickle (more acid)
- Leaching of chromium occurred in milk, coffee, and tea (only slightly acid)
Thus, the study concluded “stainless steel utensils may put reasonable amount of iron and chromium trace element in diet. The concentration of nickel leach out in food products probably do not constitute hazard to consumer as the amount of nickel eleaced out is lower to that of recommended values of EPA (0.02 mg/day).”
It also noted, “There are many factors which probably effect the release of iron, chromium and nickel in food. These will include stainless steel surface area of contact, physical nature of surface area, pH of food products, its temperature, time and contact, agitation, chemical composition of steel alloy and presence of organic chelating constituents like citric acid, tartaric acid and oxalic acid.”
So there is a wide spectrum of possible leaching that could occur.
My philosophy is to apply the “precautionary principle” and avoid potentially toxic chemicals whenever possible. Since cookware and water bottles exist that do not leach these metals, I prefer to use and recommend those that do not leach toxic substances.
Here is a whole list of other studies that have data on the leaching of metals into food and beverages:
Brittin Helen, C and Nossaman, Cheryl E (1986) Iron content of food cooked in iron utensils. J Am Diet Assoc 86:897-901
Buhler DR (1973) Environmental contamination by toxic metals, heavy metals in environment: Seminar Conducted by Water Resources Research Institute, Oregan. EHC, Nickel (1991), WHO, Geneva, 108
Inoue I, Ishiwala H and Yoshihira K (1988) Aluminium levels in food-simulating solvents and various food cooked in Alpans. J Agric Food Chem 36:599-601
Joel Kuligowski and Kopal M Halperin (1992) Stainless steel cookware as a significant source of Ni, Cr and Fe. Arch Environ Contam Toxicol 23:211-215
Krishnamurti CR, Vishwanathan Pushpa (1991) Toxic metals in Indian environment. Pub. Tata MCGraw Hill Publishing Co. Ltd., New Delhi, India.
Mosironi R (1973) International Studies on Trace Elements in etiology of cardiovascular diseases. Nutr Rep Int 7: 51-59.
Offenbacher Esther, G Pi-sunyer, F Xavier (1983) Temperature and pH effects on the release of Cr from stainless steel into water and fruit juices. J Agric
Food Chem 31:89-92
Ohkubo Noboru, Kate Takashi, Koshiola Kyoko, Miyazaki Genichi (1983) Dissolution of Cr from stainless steel tablewares. Hokuriku Koshu Eisei Gakkaishi 10:
O’Neill NC, Tanner MS (1989) Uptake of Cu from brass vessels by bovine milk and its relevance to Indian Childhood Cirrhosis. J Pediatr Gastrointerol Nutr
Reilly C (1985) The dietary significance of adventitious Fe, Zn, Cu and Pb in domestically prepared food. Food Additives and Contaminant 2:209-215
Stoewsand GS, Stanner JR, Kosikowski FV, Morse RA,
Bache CA and Lisk DJ (1979) Cr and Ni in acidic food and by product contacting stainless steel during processing. Bull Environ Contam Toxicol 21: 600-603.
U.S. Department of Health and Human Services (1991)
Toxicological profile for nickel, Agency for toxic substances and disease Registry,Atlanta, Georgia.
Van-Schoor O, Deelstra H (1986) The influence of home preparation and eating habits on daily Cr intake. Trace Elem Anal Chem Med Biol Proc Int Workshop 4th
(Pub 1987, 165-168)
Venugopal B and Luekey TD (1978) Metal toxicity in mamnals I I . Chemical toxicity of metals and metalloids. Plenum Press, New York.