Nickel Pickup By Food Cooked In Stainless Steel Utensils
#3 -- 1994 Published by the Nickel Development Institute
Background Information
Cooking Real Foods
Simmering Acidic Solutions Simulating
Foodstuffs
Conclusions
Results of a recent study by the Laboratory of the Government Chemist, LGC, in the U.K. indicate that nickel pickup by meal-sized portions of real foods cooked in stainless steel utensils at normal domestic cooking temperatures and times was generally below the analytical detectable limits for nickel of 0.01 mg/kg, or l,ug in a 100 g serving of food. Undetectable nickel pickup is considered insignificant with respect to human health effects and reinforces the ongomg use of stainless steel by consumers as the material of choice for cookware.
Nickel in Foodstuffs
Like many other minerals, nickel is a natural constituent of foods. The concentration of nickel differs between foods and may even differ within the same food item, depending upon where the food is grown. Nickel intake from a normal daily diet generally average;s approximately 150 to 200 µg. However, some foods such as beans, lentils, soya products, certain nuts, oatmeal, and cocoa products are particularly rich in nickel. Because of this, vegetarians and "chocoholics" may consume more nickel in their daily diets. Nickel is an essential element for animals and plants and there is a general belief that nickel is likely essential to humans as well.
Media Reports and Health Implications
While people get most nickel intake from foods that naturally contain nickel, recent studies have focused on additional intake from foods cooked in stainless steel pots. An increasing number of media references, advising the public against the use of stainless steel pots, is generating growing concern by consumers. These warnings suggest that ingested nickel can cause nickel allergy or other health effects, even cancer. Such warnings are both needlessly alarming and unwarranted. To distinguish between fact and speculation, the facts are:
- There is no evidence that nickel ingested in food can cause a person to become sensitized. Ingested nickel may aggravate hand eczema, however, in persons previously sensitized to nickel via direct and continuous dermal contact with certain nickel-containing items(l).
- Most dermatologists agree that the quantity of ingested nickeI required to induce an allergic reaction in nickel-sensitized individuals is fairly high relative to normal daily intake. In reviewing the many studies done on nickel-sensitive patients, Menne and Maibach(2) concluded that only a minority of such patients are likely to react to oral doses below 1,250 µg of nickel. Gawkrodger et al(3) have noted that even doses as high as 5,600 µlg Ni have "failed to excite reactions more frequently than did a placebo in a double-blind study." These figures are many times the nickel intake in normal daily diets.
- The general consensus of the regulatory and scientific communities is that nickel does not appear to be an oral carcinogen(4).
Considering this background information and health facts, studies have been conducted with a view to understanding the significance of nickel pickup from stainless steel pots to human health.
The foods tested in the LGC study -- lemon marmalade, green tomato chutney, potatoes, rhubarb, and apricots -- were purposely selected because their acidity or chloride content make them more likely than other foods to be aggressive against stainless steel. With few exceptions, there was no detectable nickel pickup, even in the preparation of chutney, which involved two hours of simmering in vinegar. Some detectable nickel pickup did occur, however, with rhubarb and apricots cooked in new pots. This pickup was largely restricted to first use applications of the pots, with nickel pickup dropping substantially in subsequent cooking operations in the same pots. After several uses of the pots, nickel pickup by rhubarb was close to or below the detectable limit but with apricots some pickup persisted, although in small amounts relative to daily nickel intake. Twelve new pots were tested, three from each of four different manufacturers. All the pots were 304 type stainless steel. Nickel pickup by rhubarb from all 12 new pots was both higher and more variable in the first use of the pots than in subsequent uses, as shown in Table I*. After just the third cooking operation, the median nickel pickup was only 4 µg in a 100 g portion of rhubarb. By the fifth cooking operation, the pickup decreased to 3 µg in a 100 g portion.
* The difference in nickel pickup seen between new pots appear to be due, in part, to different finishing operations applied by manufacturers in producing their pots.
Table I: Nickel Pickup by Rhubarb from 12 Stainless Steel Pots
| Number of Cooking Operations | Nickel Pickup from 12 pots, mg/kg | |
| Range | Median | |
| 1 (new) | 0.07 - 0.62 | 0.20 |
| 3 | 0.01 - 0.15 | 0.04 |
| 5 | 0.01 - 0.04 | 0.03 |
To determine nickel pickup upon more extended use of the pots, three pots from one of the manufacturers were tested further for 20 operations with rhubarb. The results are shown in Table II. After only the eighth use of all three pots, nickel pickup was near to or below the detectable limit of 0.01 mg/kg and remained so thereafter. Similar tests done with apricots showed marked reduction in nickel pickup after the first operation but some pickup persisted in subsequent operations. The amount in a meal-sized portion, however, was only about lOµg, a small quantity relative to an average daily dietary intake of 150-200µg.
Table II: Nickel pickup by rhubarb from three stainless steel pots
| Number of Cooking Operations | Nickel Pickup, mg/kg | ||
| Pan 1 | Pan 2 | Pan 3 | |
| 1 | 0.08 | 0.27 | 0.07 |
| 2 | 0.07 | 0.10 | 0.05 |
| 3 | 0.04 | 0.03 | 0.01 |
| 5 | 0.03 | 0.02 | 0.03 |
| 8 | N.D. | 0.02 | N.D. |
| 16 | N.D. | N.D. | N.D. |
| 20 | N.D. | 0.02 | 0.01 |
N.D. Not detected
Simmering Acidic Solutions Simulating Foodstuffs
In a separate study by the BNF-Fulmer laboratory in the U.K., an investigation was made of nickel pickup from stainless steel by hot solutions of citric acid, acetic acid, sodium chloride, and others chosen to simulate cooking of acidic fruits and other foods containing vinegar or salt. Various such synthetic food media, adjusted to pH3, which is quite acidic, pH5, which is less acidic, and pH7, which is neutral, were simmered for one hour in three new stainless steel pots. There was no detectable pickup of nickel in any of the solutions at pH5 and 7. Even at pH3, the only detectable nickel pickup occurred in the citric acid/sodium citrate solution, where a nickel level of 0.1 mg/l or l0µg Ni in a 100 ml portion was measured.
Assuming 100 ml of this solution are equivalent to a l00g portion of "real food", the nickel pickup would be only 10µg which is small compared to average dietary nickel intake of approximately 150-200µg/day. And even this small nickel pickup decreased to undetectable levels in subsequent uses of the same pots.
Contrary to some recent media suggestions that stainless steel pots should be avoided, scientific studies have confirmed that nickel pickup from such pots is generally undetectable, even for particularly aggressive foodstuffs. For those foods that may cause leaching, the amount of nickel likely to be released from the pots upon first use would be modest relative to daily dietary intake. And even that modest level of nickel pickup decreases significantly in subsequent uses of the pots. In summary, the potential pickup of nickel from stainless steel cookware relative to normal daily intakes of nickel is generally insignificant and should not cause either allergic reactions or other adverse health effects. Stainless steel thus remains the material of choice for modern cookware and hygienic food preparation and handling.
1. Nickel and nickel alloy articles that come into contact with the skin. NiDI Status Report, September 1992.
2. Menné and Maibach. Systemic contact-type dermatitis. Dermatatotoxicology, 4th ed., Mazulli and Maibach, edltors, Hemisphere Publishing Corp., p. 453-472, 1991.
3. Gawkrodger et al. Nickel dermatitis in the reaction to oral nickel challenge. British J of Dermatology, 115, p. 33-38, 1986.
4. California Air Resources Board. Proposed identification of nickel as a toxic air contaminant. Technical Support Document, Part B, June 1991.
Note:
mg/kg = milligrams per kilogram
µg = microgram
g = gram
