The selection of pots and pans can be a complicated affair. The shape of the cooking surface and handle(s), materials used in its construction, the intended purpose of the utensil's design, and its flexibility of use in the kitchen all are important factors in choosing cookware. Understanding the materials used is a good first step in understanding how cookware works and what factors may be important to your cooking style.
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The purpose of cookware is to impart energy to ingredients. In America, the energy comes mainly in two forms: burning natural gas or propane gas and electrical resistivity. In both methods, the source of the heat is not uniformly spread over the pan. In a gas stove, the gas come out at regular intervals and forms a ring of individual flames. The heating elements of an electric range are designed to cover as much area as possible, but still have patterns (usually spirals) where there is no heat. Because the heat is not applied evenly, the cook must be aware of this and either compensate with cooking technique or through cookware.
High quality cookware should not only be durable, but also take the energy from the heat source and effectively transmit this energy to the ingredients. There are several factors that affect this capability. The two most important factors are thermal conductivity and heat capacity. Almost all discussions concerning the materials used in cookware are focused on these two factors.
Thermal conductivity
In short, the thermal conductivity of a material is how readily that material absorbs and transmits (releases) energy. When the fire or heating element of a range comes in contact to a pan, the energy from the heat source is transmitted to the pan. This increases the internal kinetic energy of the pan (commonly called "heating up"). The heated material then transmits the energy to nearby materials that are at a lower average molecular kinetic energy level (at a lower temperature than the material). The higher the thermal conductivity of the material, the faster it will heat up and also, the faster the heated area will spread to unheated areas of the same piece of material.
For example, if we placed a large sheet of stainless steel (fairly low thermal conductivity as cooking materials go) on a burner and turned on the burner, the area directly under the burner would get hot while the rest of the sheet slowly heats up. The burner imparts heat quickly only to the region of steel directly over it. The rest of the pan heats up from the conduction of the heat from that spot. When the outer edges of the sheet have reached a hot temperature, the spot directly over the burner would be extremely hot. The figure below shows an example of the temperature of the sheet of steel over a gas burner. The hottest parts are shown in white, hot is red and cool is blue.
One solution to this problem is to make the sheet thicker. When heating a thick piece of steel (instead of a thin sheet), the bottom surface of the steel does not have the same temperature pattern as the top surface. Because the top surface is a greater distance from the heating element, the energy needs to conduct from the bottom to the top (just like the energy conducts outwards). The top surface of the steel is more evenly heated in this case. The figure below shows the thick sheet of steel after it has been sliced so the center of the front edge is where the burner heat touches the bottom of the sheet. The hot spot (white) is reduced by the time the heat conducts to the top surface of the sheet. Where the sheet is being heated, the temperature is more uniform now, but we still have uneven heating with this material.
For this reason, the thicker the steel, the less variation in temperature on the top surface. Unfortunately, low thermal conductivity means it a lot of energy needs to be imparted to the bottom of the steel in order to get the top hot. So a pan made of a low thermally conductive material will take a longer time to reach cooking temperatures. In fact, materials with low thermal conductivity take longer to react to any change in temperature, so the thermal response of the pan would also be slow. (Thermal response is how quickly the surface temperature of the pan reacts to when we increase or decrease the flame of the burner.)
In most cooking applications, it is desirable to have the utensil heat up quickly, not develop hot spots, and react to changes we make to the range controls. Materials with high thermal conductivity fulfill our needs because they transmit heat quickly resulting in fast response to thermal changes and even distribution of the internal kinetic energy.
Here is a list of some common materials used in cookware and their respective thermal conductivity:
| Material | Thermal conductivity |
|---|---|
| Copper | 401 W/m*K |
| Aluminum | 237 W/m*K |
| Cast Iron | 80 W/m*K |
| Carbon steel | 51 W/m*K |
| Stainless steel | 16 W/m*K |
Heat capacity
The amount of internal kinetic energy stored in a material can be referred to as it's heat capacity. This isn't the same thing as temperature, which is the average molecular kinetic energy within the material. For example, a kg of water at 100°F contains more energy than a kg of steel at 100°F. While thermal conductivity describes the materials ability to absorb energy, heat capacity is the amount of energy that is needed to raise or lower the temperature of the material. The molecular composition of some materials is such that as they absorb energy, much of it gets converted into potential energy and only a small amount increases the molecular kinetic energy (water is a common example). Other materials, like most metals, increase their molecular kinetic energy readily and do not store much of the absorbed energy as potential energy. The heat capacity of a material is proportional to its mass. So, a 2 kg piece of steel has double the heat capacity of a 1 kg piece of steel (make sense, right?).
What this means is that cookware made of materials with high heat capacity, will take longer to heat up, but will also have a significant amount of energy stored up when it is hot. When energy is pulled out of the material, the temperature of the material will lower slowly when compared to materials with low heat capacity. Cast iron is often cited as an example of a high heat capacity cookware material. The specific heat (the heat capacity of a material for a given mass) of cast iron is half of aluminum's specific heat, but because cast iron cookware is generally several times the mass of aluminum cookware, it has a much higher heat capacity.
The thickness of metals used in the construction of cookware are often sited by the manufacturer (for example, 3 mm aluminum), but since heat capacity is a function of the mass of the material, density must be known to make comparisons between cookware of different materials.
| Material | Specific Heat | Density |
|---|---|---|
| Aluminum | 910 J/kg*K | 2600 kg/m3 |
| Stainless Steel | 500 J/kg*K | 7500 - 8000 kg/m3 |
| Carbon Steel | 500 J/kg*K | 7500 - 8000 kg/m3 |
| Cast Iron | 460 J/kg*K | 7900 kg/m3 |
| Copper | 390 J/kg*K | 8900 kg/m3 |
Looking at the table above, if you multiply specific heat with density, you'll find that the heat capacity per unit volume of steel, cast iron, and copper are about 1.5 times that of aluminum. This means, to achieve the same heat capacity in an aluminum pan as in stainless steel pan, the aluminum pan needs to be 1.5 times as thick (assuming the other pan dimensions are the same).
Pulling it together: thermal diffusivity
If you've been paying attention, you'll realize that I've misled you when I discussed thermal conductivity. Thermal conductivity alone does not determine how fast the pan will heat up (and also how evenly it will heat). In fact, the heat capacity plays a role in determining this as well. Wouldn't it be great if we had a single number that told us at what rate heat would transfer through and spread out in the material? There is, it's called the thermal diffusivity of a material and is simply the thermal conductivity divided by the unit heat capacity (specific heat times density). Let's take a look at how the materials stack up:
| Material | Thermal diffusivity |
|---|---|
| Copper | 120 * 10-6 m2/s |
| Aluminum | 100 * 10-6 m2/s |
| Cast Iron | 22 * 10-6 m2/s |
| Carbon Steel | 14 * 10-6 m2/s |
| Stainless Steel | 4.3 * 10-6 m2/s |
Without additional calculations based on the heat conduction equation, there is very little that we can do with this table of values, except compare the materials against each other. It is clear, however, that the best performing materials (in terms of both holding and dishing out energy) are copper and aluminum. This leads us to our final consideration: reactivity.
Reactivity
Not only do we have to concern ourselves with the thermal properties of materials, but we need to make sure that the materials we use in our cookware do not harm us or adversely affect the taste of our food (you decide which is worse). For this reason, in addition to the high thermal diffusivity, we would also like a non-reactive material. Unfortunately, both copper and aluminum react readily to foods. (Copper, when ingested in quantity or consistently, can cause liver, stomach, and kidney problems as well as anemia. Also, aluminum has long been suspected of contributing to Alzheimer's disease. Oh, every cookbook mentions, at this point in the discussion, that the occasional foamed egg white whipped in a copper bowl is not enough to harm you - but refrain from cooking every day on exposed copper.) Stainless steel, the least reactive of all popular materials used in cookware, also has the worst thermal diffusivity.
It seems that today, physics is not our friend. But, through the magic of cookware companies wanting to find ways to charge us lots of money, solutions have been devised to enable us to enjoy cookware made of materials with high thermal diffusivity and low reactivity. By combining the non-reactive surface of stainless steel with the thermal properties of copper or aluminum, you get the best of both worlds. There are several variations on this theme: steel- or tin-lined copper, stainless steel with aluminum or copper disk, stainless steel cladded aluminum, and stainless steel cladded copper. The table below summarizes my subjective assessment of the effectiveness of various material combinations (they are listed in order from most effective to least):
| Rank | Composition | Comments |
|---|---|---|
| 1 | Copper with tin lining | Highest response; tin lining can be finicky can be susceptible to melting; copper exterior requires more care |
| 2 | Copper with stainless steel lining | Copper exterior requires more care but imparts the utensil with copper's excellent thermal properties |
| 3 | Aluminum with stainless steel lining | Thick aluminum provides excellent thermal response to thin steel interior |
| 4 | Copper fully clad by stainless steel | Copper layer may be thinner than copper with stainless steel lining; exterior and interior are durable and easy to maintain |
| Aluminum fully clad by stainless steel | Aluminum layer may be thinner than aluminum with stainless steel lining; exterior and interior are durable and easy to maintain | |
| Aluminum with stainless steel lining and copper exterior | Same performance as cladded aluminum, but with the difficulties in maintaining copper | |
| 5 | Stainless steel with copper disk | Curved edge of the bottom causes the disk to not come into full contact with the complete bottom of the pan resulting in inferior heat conduction as compared to cladded copper |
| Stainless steel with aluminum disk | Same as stainless steel with copper disk |
Previously, I mentioned that cast iron has a large heat capacity as compared to the other materials (mostly because of the mass used when making the cookware). Because of this attribute, cast iron gets a special place in the kitchen. When the cooking task requires the ability to maintain consistent heat (and ample amounts of it), nothing beats cast iron. Because cast iron can react with acidic foods and ingredients that are cooked for a long time, cast iron cookware is seasoned - a process by which layers of fat are slowly cooked into the porous iron until the fat polymerizes forming a protective barrier (and makes the utensil relatively non-stick).
Common materials and how they compare
Now that we've looked at the important properties in selecting cookware material, let's take a look at each of the common materials used in cookware.
| Copper | |
|---|---|
| Description | Copper is a soft (scratches easily) but durable (will last a lifetime) material that has great thermal properties. The material is prone to oxidation but with care, will retain its beauty indefinitely. |
| Pros |
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| Cons |
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| Best uses |
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| Care |
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| Examples |
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| Aluminum | |
|---|---|
| Description | Plain aluminum utensils are low-cost, light-weight, and thermally responsive - but it's reactive. Teflon coated aluminum utensils are low-cost and both non-stick & non-reactive. Anondized aluminum has been treated to develop an aluminum oxide (extremely hard and non-reactive) coating on the surface of the utensil. Clad or lined aluminum has had stainless steel bonded to the interior of the utensil to form a non-reactive surface. |
| Pros |
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| Cons |
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| Best uses |
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| Care | Hand-wash with a mild detergent and washcloth or sponge. |
| Examples |
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| Cast iron | |
|---|---|
| Description | Cast iron is composed on iron, carbon (more than carbon steel), and trace elements found in common clays. The iron is melted down and poured into a sand or clay mold to form the utensil. Enameled cast iron has a thin but durable nonreactive layer of glass fused to the surface of the utensil. |
| Pros |
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| Cons |
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| Best uses |
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| Care | Plain cast iron should be seasoned before first use and as needed. A seasoned utensil should receive minimal contact to soap or detergent. Wash by soaking in warm water for a few minutes and repeatedly scrubbing with salt and rinsing until salt remains white (usually one scrubbing is does it). Dry with a cloth and heat over low heat briefly to evaporate all moisture. For enameled cast iron, hand wash in hot soapy water. |
| Examples |
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| Carbon steel | |
|---|---|
| Description | Carbon steel contains less carbon than cast iron and is formed and pressed from sheets instead of being casted. It can be annealed (heating the metal until its molecular structure realigns to alleviate internal stresses and then specially cooled to preserve the new structure) to form blue steel (or black steel), a harder and less reactive material. Carbon steel can also be enamel coated. |
| Pros |
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| Cons |
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| Best uses |
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| Care | Should be seasoned before first use. Care for as if it was cast iron. If desired, pan can be washed in soapy water, scoured, and reseasoned quickly (15 minute seasoning) because of its less porous nature than cast iron. |
| Examples |
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| Stainless steel | |
|---|---|
| Description | Mixing steel with chromium and nickel (18/8 stainless steel is 18% chromium and 8% nickel while 18/10 has 10% nickel) produces a corrosion resistance steel that is both hard and easy to maintain a shine. Disks of copper or aluminum can be fused to the stainless steel cookware to enhance its thermal properties. Stainless steel can also be used to line copper or aluminum utensils as well as cladding aluminum or copper (see aluminum and copper cookware summaries above). |
| Pros |
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| Cons |
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| Best uses |
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| Care | Hand wash with mild detergent. Use gentle abrasives as needed. |
| Examples |
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My personal favorites for cookware materials are stainless steel clad aluminum or copper and cast iron (for skillets and woks). The stainless steel clad utensils perform well, are easy to clean, and look beautiful. Of course, not all stainless steel clad aluminum (sometimes called tri-ply or five-ply depending on construction) are the same. All-Clad has definitely earned their reputation as quite possibly the best general use cookware money can buy, but it's a lot of money to be spending. All-Clad rarely goes on sale, but other reputable brands, such as Calphalon, have clad lines as well - and they are more likely to have their product lines go on sale. Keep checking the Cooking For Engineers Deals Blog to see when deals do come up.
Excellent post. I've been wondering recently whether to upgrade my cookware... and to what. I'm using a Viking saute pan at the moment, which i believe is very thick stainless steel with an aluminium base... but i find it takes so long to heat up that i often prefer cooking with my cheap K-mart brand frying pan...
After reading your post, it's confirmed a number of thought's i've had... in regards to heat transfer and capacity. I'm thinking a nice Mauviel saute pan would look great on my stove top.
Cheers :)
Matt
I was hoping to see something about Teflon (Or any kind of non-stick coating), as well.. How it works, care it requires, etc. Will you write something about it in the future?
If you live in a metropolitan area you can often find them at garage sales -- and at 50 cents to a couple bucks per pan, this excellent cookware is a steal.
Often you can find the older stuff on eBay, as well. I recently puchased a huge skilllet, with lid, of this vintage Farberware for about 15.00.
In the future, I will have a (shorter) discussion on non-stick cookware. My current problem is that I can't seem to get enough info on the newer non-stick solutions (Scanpan, etc.) and I'd like to include those as well.
I have a few cybernox pans, and they're great, but I have no idea what they actually are.
Of course, I don't really like the taste of aluminum in my foods...
However, many of the non-stick coatings outgas irritating or toxic gases when heated strongly (which those of us who stir-fry a lot tend to do). These gases can sicken people and kill birds.
Teflon is a persistent chemical.
Are these types of pans still available?
How does the cookware feel when you handle it? Does the handle get Hot? Will it take hours a week to keep clean/looking nice? The easiest way to answer the question is to recommend any verison of the "All Clad" cooking tools. They come in both regular and nonestick, handles stay cool, they are comfortable, conduct heat well and evenly considering the Stainless Steel quotient, and will be the last set of pots & Pans you'll ever buy. Oh yea, "all Clad" is always rated in the top two by Consumer Reports/Cook's Illustrated etc., not to mention most major cooks without a cookware deal use it also, just watch the food network/TV.
Don't get me wrong, you can go with Faberware, and TFal and so on and so on, but you'll replace it every 5 to 7 years, and eventually pay more in a lifetime than if you bought "All Clad" up front.
Check out the egullet understanding stovetop cookware for another excellent take on this topic.
Cookware Safety
FDA article
Might be a bit out of date, but still handy
inch casserole pan from Nordic Ware. It has
badly applied non stick that I am pleased to see
is rapidly flaking off. It is steel so will
work on my induction cook top. The greatest
use is for baking between the smooth hot
upper and lower plates of my Griddler panini griddle.
It makes beautiful casseroles, bakes a roast, or heats frozen entres.
If I need a lid
another pan inverted and held on by two
one inch paper clamps that squeeze easily around
the two rims.
It has many uses except for microwaving.
This type of cookware is thin aluminium coated with enamel inside and out. Used by our Grandmothers when cooking and still around. Relatively cheap. Usually found in camping stores or where you find home canning supplies for the larger pots.
Excellent if you want to boil anything. Horrible if you want to cook something slowly.
Will there be another article on what works best in an oven?
The worst cases have required some soaking with water and dish detergent but that's it. And there's no seasoning required, unlike cast iron.
The only thing that really stuck to the All-Clad was a melted $3 plastic steam basket. (don't ask)
Freezing, WD-40, Simple Green and Wright's Silver Cream cleaned the pan up nicely. My only hope is that I didn't damage the corrosion resistance of the stainless steel.
Does anyone know if I could have permanently damaged the stainless steel somehow?
Here's a fun place to start inquiy into stainless:
http://www.corrosion-doctors.org/MatSelect/corrstainsteel.htm
I had read about the chromium-oxide "layer" on stainless steel (see below) and wondered if I had damaged the corrosion resistance of the All-Clad, or any of its inherent non-stick properties.
I'm really out of my element here so my questions might seem a bit "daft." ;)
Is the chromium-oxide layer important for cookware? If so, does it require maintenance or special care?
Thanks.
"Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film."
"The purpose of passivation of the surface is not only to clean and remove free iron, but to maximize the chromium content of that top, very thin "layer" of chromium oxide. (Other metals in the alloy also greatly affect this.) This gives the best corrosion resistance,"
"As you are no doubt aware, one of the attributes of stainless steel is the fact it has a built in oxide layer over it. This is in the form of chromium oxide and it is this that gives it its corrosion resistance. If you want to enhance this, you have to be careful how you do it; if you simply anodically polarise it, you will run the risk of breaking down the existing layer and dissolving out the metals. You could try putting it in hydrogen peroxide and leaving it, or better still, try electropolishing it."
geo
Regarding the possible health risks of using teflon, there are none.
http://www.cei.org/gencon/019,04820.cfm
It is true that there may be some health risks caused by a chemical used to <b>make</B> teflon (namely perfluorooctanoic acid, or PFOA), but that is a problem for chemical factory workers and perhaps people who live near the factories, not people who use teflon. There is no PFOA in actual teflon.
http://www.planetark.com/dailynewsstory.cfm/newsid/29003/story.htm
Teflon itself is chemically unreactive; in fact, it is one of the least reactive substances ever discovered. If we ate some teflon, it would pass through our bodies unchanged. None of it would enter our blood stream. But even if it did, it would would not affect any of our biochemical processes, since it is unreactive. It would interact with the body about as much as a stone reacts to me shouting at it. In fact, that unreactivity is part of why it is so slippery. Maybe the subject of another article in Cooking for Engineers?
People pay lots of attention to health scares, as they should, but they often don't take time to look at it carefully. PFOA is not an "ingredient" of teflon, it is one of the chemical precursors of teflon, like oil is to polyurethane. If oil is bad for you, it doesn't mean that polyurethane is. (And in fact, high-density polyurethane is totally inert in the body just like teflon, so it is used in artificial joints and the like.)
Like EMF radiation from powerlines, this one appears to have been cooked up by trial attorneys looking to make a buck.
Karl
http://news.bbc.co.uk/2/hi/science/nature/3441255.stm
Admittedly, canaries are not humans, but Teflon seems to break down under high temperatures according to one group of researchers (absolutely not! says the other side - Teflon is unreactive).
It's always like this - everyone argues and there are strong motives for lying and exaggeration - but at the end of the day we're built out of the same basic stuff as canaries so I think I will give Teflon a miss - if it's killing them then it's most likely partially killing me...
(from the viewpoint of an economist in his 20s) < always important for evaluating info i.m.o - how much do they really know about x, y or z after all... perhaps not too much in my case, but I do like to think critically - I dislike selective evidence - now you have both sides to consider. ^.^
[edit] from that link posted earlier:
Although nonstick pans will wear away with hard use and particles may chip off, the Food and Drug Administration has stated that these particles would pass unchanged through your body and pose no health hazard. A coated pan heated for long periods at high temperatures will give off fumes, but these are less toxic than fumes given off by ordinary cooking oils.
Do these people ordinarily cook with crude oil or something!??!
Runner up is stainless clad aluminum, but it is harder to clean and not as nonstick, and much more expensive.
To add a data point:
After much research and reading here and else where, I picked up the member's mark tri-ply cookware at Sam's club. I could never afford an All-clad set right off so this get's me the same performance at a great price. So far the quality and performance is right up there with All-Clad. I know I'll probably pop for something from the big "A" just for the fun of it but I'm very happy with my selection.
Reading about material and construction/performance here was a great help in my selection.
THanks!
I am very close to picking up a set of Copper-clad cookware from All-clad. It is copper-lined w/ stainless steel coating so it should have the best of both worlds- good heat properties and low reactivity with food. But, it will not work as good with an induction cooktop as good, old-fashioned steel or cast-iron.
There are several things I like about induction cooktops (stays cool to touch), but the biggest is it's rated efficiency. Can anyone compare the efficiency of an induction cooktop to a high-end ceramic cooktop? What about the heat properties of a good stainless steel set on an induction stove to a aluminum/copper clad set on a decent stove?
~Thanks
The oxide layer on stainless steels is (a) self-maintaining, and (b) very diffficult to remove. Unless you're performing electrochemical experiments in your cookware, or cooking with concentrated acids, you won't damage the stainless. If you are doing those things, the condition of your cookware is probably not your biggest problem.
What makes stainless steel so much nicer than regular steels is that chromium oxide is not air-permeable, so only a very thin layer at the surface reacts. Iron-oxide, which developed when normal steel reacts in air, is much less attractive and is air permeable. As a result, a piece of mild steel can corrode all the way through.
I was wondering a while back, while thinking of excuses not to clean the copper bottom of my cheap stainless steel pan, whether the better radiative heat transfer that I'm bound to get from the non-shiny, more darkly colored surface of my discolored copper is worth decrease in conductivity that I assume I will get due to the oxide layer that is causing the discoloration.
Obviosly this is assuming that I'm not concerned with the asthetic aspects of the pan, since shiny things are definitely prettier.
Anybody happen to know?
As far as the Alzheimers thing goes. Studies have shown that the original premise was false. Doctors had been finding elevated levels of aluminum in the brains of Alzheimers patients so they naturally linked it to the disease. Now though, studies are showing that the Alzheimers may cause the elevated aluminum levels rather than the other way around, and that aluminum causes no harm to the body.
Also, I have a horrible cook top in my apartment and often find myself doing dishes requiring searing. I can get around this using cast iron which holds enough heat that I can finish searing before the thing noticeably cools down.
http://www.healthy.net/asp/templates/article.asp?pagetype=article&id=1958
With use of aluminum pots and pans and aluminum foil, some aluminum leaches into food, especially with acid foods such as tomatoes or rhubarb. Cooking with fluoridated water in aluminum cookware increases the aluminum in the water and the food; still, the amounts we obtain in this manner are small in comparison with those from additives. Aluminum salts used in antiperspirants are not a major contaminant either, unless these products are overused. (Aerosol sprays, however, should be avoided for environmental toxicity reasons.) Antacids containing aluminum hydroxide can be a big source if they are taken regularly or abused, as antacids sometimes are. Some children's aspirins have been found to contain aluminum as well.
http://www.doctoryourself.com/alzheimer.html
A single aluminum coffee-pot was shown to have invisibly added over 1600 mcg aluminum per liter of water. This is 3,200% over the World Health Organizations set goal of 50 mcg per liter. Aluminum is known to build up in the bodily tissues of persons with Alzheimers disease, Parkinsons disease, and amyotrophic lateral sclerosis. Aluminum is a known neurotoxin. Aluminum is also a component of so- called silver amalgam dental fillings. Composite (white) fillings do not contain aluminum (or mercury, for that matter.) Most baking powder contains aluminum. Rumford brand baking powder does not, however. Neither does baking soda, which is a different substance entirely.
FACT: More than half of nursing home beds are occupied by AD [Alzheimers Disease] patients.
FACT: Alzheimers disease is the Number 4 Killer of Americans, causing over 100,000 deaths each year in the USA alone.
In new tests conducted by a university food safety professor, a generic non-stick frying pan preheated on a conventional, electric stovetop burner reached 736°F in three minutes and 20 seconds, with temperatures still rising when the tests were terminated. A Teflon pan reached 721°F in just five minutes under the same test conditions (See Figure 1), as measured by a commercially available infrared thermometer. DuPont studies show that the Teflon offgases toxic particulates at 446°F. At 680°F Teflon pans relea