Naked Science Forum
Non Life Sciences => Technology => Topic started by: pirunner on 07/12/2007 02:53:47
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I have heard that microwaving food "kills" many of the nutrients, vitamins, minerals, etc. in the food, therefore making it much less beneficial to eat. For example, microwaving vegetables to heat them up would be worse than steaming them.
Any truth to this?
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I am not aware that microwaves themselves will cause any direct effect on vitamins or nutrients. The only effect microwaves have is to heat the food, but the way it heats it may be different (it could get hotter, and the heat may penetrate deeper into the food), and the greater you heat any food, the more you are going to damage any complex molecules in the food.
The issue about minerals is a red herring, since minerals should not be effected by heat (at least not at the levels you are likely to be doing with domestic cooking).
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No truth at all.
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Microwave cooking, in fact, will involve lower peak temperatures and shorter times because, effectively, the heat source is nearer the centre of the food. You don't need to bake the surface so much. I assume that the same internal temperature and time are required to consider the food 'cooked'.
However, it is not totally obvious that em radiation has no effect on vitamins (double negative, I'm afraid) when you bear in mind that very low fields from the magnetic gizmos you buy to deal with hard water change the way in which limescale forms. They do work, although the result is subtle.
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Microwave cooking, in fact, will involve lower peak temperatures and shorter times because, effectively, the heat source is nearer the centre of the food.
Lower peek temperatures than what?
Maybe lower peek temperatures than a long period in the oven, but boiling food will inevitably be limited to the boiling point of water (which, unless you are using a pressure cooker, will be limited too 100C°).
If you microwave something with relatively low water content, you can certainly exceed this, and even with a higher water content, microwaves are capable of superheating water (which is why you have to be careful when taking out a mug of tea from the microwave, since despite looking calm on the surface, it can be superheated, and a little vibration can cause it to explosively boil).
The other point is that microwave cookers will generally cook faster, and so there will be a steeper rise in temperature. This is certainly one reason why cooking eggs in a microwave can be very problematic (the eggs, even if not in their shell, will easily explode unless cooked at a very low setting). It can also make a difference when cooking from frozen.
One other factor that that will differentiate the way that microwaves cook is that I suspect they will heat the interior of the cells and not the cell membranes, and so will easily cause the cellular structure to rupture (although I am speculating on that matter).
However, it is not totally obvious that em radiation has no effect on vitamins (double negative, I'm afraid) when you bear in mind that very low fields from the magnetic gizmos you buy to deal with hard water change the way in which limescale forms. They do work, although the result is subtle.
Those magnetic gizmos are somewhat controversial (and they certainly are anything but microwave). I have tried to use them without any benefit, but I know others have said they do work (but only on flowing water, not on standing water). I think there are about as many who would say there are rubbish as will say they are superb (as I said, somewhat controversial).
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But since the food ends up in our digestive systems, does it really matter if the cooking process has "damaged" the molecules? The food we eat gets torn to shreds in our bodies at a molecular level anyway. The only problem I can think of is that the microwaved molecules become compounds that the body isn't able to chemically process and utilize, or which are toxic.
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Microwaves don't penetrate very far into the food....few mm before becoming very weak. The centre is cooked by conduction.
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But since the food ends up in our digestive systems, does it really matter if the cooking process has "damaged" the molecules? The food we eat gets torn to shreds in our bodies at a molecular level anyway. The only problem I can think of is that the microwaved molecules become compounds that the body isn't able to chemically process and utilize, or which are toxic.
Vitamins and Minerals have no nutritional value. They are not broken up in your digestive system, but rather used as a whole for various bodily functions.
I guess the theory is that a vitamin molecule would be broken or structurally altered by a microwave, and would therefore not be a functional vitamin when ingested.
For example, if you decided to defrost a bag of frozen spinach (which is loaded in vitamin K) in a microwave oven, when complete, the spinach would still have the same amount o calories, carbs, fats, etc...but the levels of vitamin K would be significantly decreased, thus causing the spinach to loose much of its value.
I'm pretty sure I have heard of studies relating this theory to vitamin/mineral deficiencies and various diseases in the "TV dinner" generation.
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But since the food ends up in our digestive systems, does it really matter if the cooking process has "damaged" the molecules? The food we eat gets torn to shreds in our bodies at a molecular level anyway. The only problem I can think of is that the microwaved molecules become compounds that the body isn't able to chemically process and utilize, or which are toxic.
Not all molecules are broken down in the gut. If it were the case that complex molecules were to get broken down to basic amino acids, or simple sugars, or whatever, then a great many drugs (e.g. opiates) or poisons (e.g. ricin) or any vitamins, would have no effect upon us. They do have an effect upon us because there are various large molecules that do not get broken down, and some of them are still able to cross the lining of the intestines to enter the bloodstream.
Note: I see pirunner got there before me.
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Microwaves don't penetrate very far into the food....few mm before becoming very weak. The centre is cooked by conduction.
I would have thought they would have to penetrate at least half a wavelength, which would be about 6cm.
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6mm more like. You wouldn't even get 6cm at 27 MHz, let alone 2.45 GHz.
Depends on the food and the temperature. They don't define exactly what they mean by penetration depth. Loss in dB will be proportional to depth...
http://64.233.183.104/search?q=cache:ipk7b33xrmEJ:www.bsyse.wsu.edu/tang/tang69.pdf+Microwaves+%2Bpenetration+%2Bfood+%2BdB&hl=en&ct=clnk&cd=45&gl=uk
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6mm more like. You wouldn't even get 6cm at 27 MHz, let alone 2.45 GHz.
3x108/2.45x109 = 0.122m = 12.2cm (full wavelength), thus about 6cm half wavelength.
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Math looks right to me! But why would the microwave only penetrate half-way?
Question: What's new?
Answer: c over lambda!
Physics...never gets old.
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Depends on the food and the temperature. They don't define exactly what they mean by penetration depth. Loss in dB will be proportional to depth...
http://64.233.183.104/search?q=cache:ipk7b33xrmEJ:www.bsyse.wsu.edu/tang/tang69.pdf+Microwaves+%2Bpenetration+%2Bfood+%2BdB&hl=en&ct=clnk&cd=45&gl=uk
http://books.google.com/books?id=xqhCHespUwUC&pg=PA184&lpg=PA184&dq=microwave+temperature+penetration+depth+food&source=web&ots=2brfXqKZsN&sig=eHe2miBqCOj8Ke_XFe0MRBzbUAY gives a more numerical answer, but still hopeless without knowing the exact foodstuff involved.
There is no doubt that penetration is proportional to wavelength, but there is no cut off (as you say, loss in DB is proportional to depth, but ofcourse DB is a logarithmic scale).
There is also a comment the the centre of the food often does get hotter because it has overlapping radiation from all sides.
Then there is the problem of non-homogeneous foods that can have high absorption in one part, and low absorption in another part.
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Math looks right to me! But why would the microwave only penetrate half-way?
I have to admit that I was guessing, but you were suggesting there was a cutoff, and the implication of a cutoff would be some sort of surface wave, in which case it would have probably have had half its depth within the material.
In fact, as we have both found, there is no cutoff, so the assumption was wrong - there is no surface effect as such.
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I can't see microwaves penetrating very far into what is a very lossy dielectric. I don't see the significance of half a wavelength. Many websites suggest 6mm is about right for most foods at 2.45 GHz. I have seen figures between 2mm and 28mm quoted on scientific sites depending on the food. Some define penetration depth as being the point at which the RF falls to 1/e of initial power.. 37% or 4.3 dB down.
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From ...
http://books.google.com/books?id=xqhCHespUwUC&pg=PA184&lpg=PA184&dq=microwave+temperature+penetration+depth+food&source=web&ots=2brfXqKZsN&sig=eHe2miBqCOj8Ke_XFe0MRBzbUAY#PPA45,M1
Thus, MW radiation at 915 MHz penetrates more than the 2450 MHz radiation, ie, 30 versus 10 mm.
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From ...
http://books.google.com/books?id=xqhCHespUwUC&pg=PA184&lpg=PA184&dq=microwave+temperature+penetration+depth+food&source=web&ots=2brfXqKZsN&sig=eHe2miBqCOj8Ke_XFe0MRBzbUAY#PPA45,M1
Thus, MW radiation at 915 MHz penetrates more than the 2450 MHz radiation, ie, 30 versus 10 mm.
Not sure about the relevance of this. The document seems to refer to the effects of food packaging, and nothing about how the food itself reacts to microwaves.
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Some define penetration depth as being the point at which the RF falls to 1/e of initial power.. 37% or 4.3 dB down.
But if you have 1KW of microwave energy at the surface, then 37% of that is still 370watts of energy (and don't forget that the 1KW is distributed across the top surface, whereas the 370W is distributed across an inner surface that is almost certainly smaller than the top surface - i.e. if one has a spherical object, then reducing its radius, even by 6mm, will also reduce its surface area in proportion to the square of its radius, and so provide a proportionately higher energy density for the same overall energy).
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Looks like it was the wrong link...
Thus, MW radiation at 915 MHz penetrates more than the 2450 MHz radiation, ie, 30 versus 10 mm.
http://books.google.com/books?id=OycSdO1F0_sC&pg=PA111&lpg=PA111&dq=%22thus+mw+radiation+at+915+mhz+penetrates+more+than+the+2450+mhz+radiation+ie+30+versus+10+mm%22&source=web&ots=A4dcYlAnRj&sig=PWzwx-A1-gXr7Y25Qx7o5vUvcVY
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But after another 6mm (12 mm total) the 1 kW is down to 140 Watts. By 18 mm it is down to 51 Watts and by 24mm ..19 Watts.. So essentially very low at 24mm (bit less than an inch).
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But as regards if there is any 'damage', (which many conclude is rubbish) to food from microwave cooking many foods may have been processed anyway in the factory with RF possibly with the deeper penetrating 915 MHz.
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Looks like it was the wrong link...
Thus, MW radiation at 915 MHz penetrates more than the 2450 MHz radiation, ie, 30 versus 10 mm.
http://books.google.com/books?id=OycSdO1F0_sC&pg=PA111&lpg=PA111&dq=%22thus+mw+radiation+at+915+mhz+penetrates+more+than+the+2450+mhz+radiation+ie+30+versus+10+mm%22&source=web&ots=A4dcYlAnRj&sig=PWzwx-A1-gXr7Y25Qx7o5vUvcVY
That makes more sense, but the 10mm and 30mm are only examples. Possibly realistic examples, but examples of a situation where there is a very high degree of variability, not only with the original composition of the food, but it also varies with the temperature of the food (thus you can get a situation where the outer portion of the food is cooked, thus causing a reduction of dielectric, and thus exposing inner layers to greater heat - this is probably the more common scenario, but the converse is also possible, and is likely when defrosting food).
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But after another 6mm (12 mm total) the 1 kW is down to 140 Watts. By 18 mm it is down to 51 Watts and by 24mm ..19 Watts.. So essentially very low at 24mm (bit less than an inch).
But you are assuming a constant distribution of energy.
Say you have a ball of food that is 24mm in diameter (a reasonable bite sized piece of food), in a 1KW oven. This gives a surface area of 0.00181m2, which gives a power density at the surface of 552621Wm-2.
6mm below the surface of this sphere, you have a virtual sphere of only 12mm diameter. This virtual sphere 0.000452m2. You also have a residual power of 370W spread across this surface. This gives a power density of 817879Wm-2. Thus, the power density at 6mm below the surface is 47% higher than at the surface.
Ofcourse, where you are talking about a super large microwave oven, containing spherical food that has a diameter of 1m, then the power density at the surface of that sphere (for a 1KW oven) would be 318Wm-2, but at 6mm beneath the surface (assuming a 37% drop in power) the power density would be only 120Wm-2. Thus the power density at 6mm below the surface is indeed 37% below that at the surface.
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I have just tried putting wet paper sheets between a satellite dish and the LNB. 3 wet sheets is enough to 'kill' the signal. I think it has to be 4 - 6 dB down before it can't decode the digital TV. So that suggests a 4 - 6 dB loss through only 0.3mm of wet paper. I need the whole of a (dry) 20 mm catalogue to for the receiver to fail. This is at 12GHz of course rather than 2.45.
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Most things I cook are in flat rectangular trays.
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Most things I cook are in flat rectangular trays.
There are two issues here.
Firstly, what matters is not the shape of the tray, but the shape and size of the individual pieces of food (you can put round meatballs into a flat tray, but they still remain round meatballs).
But the real key issue is not the shape, but the size. Shape will have an effect on the evenness of the heat (the corners of a rectangular piece of food will heat more than the flat sides). Whatever the shape, so long as the shape remains the same, the surface area will increase (or decrease) in proportion to the square of any one dimension, and that is the key issue. So, if you half the dimension, you will reduce by a quarter the surface area, so if 12mm (6mm from each side, as there will be two opposite sides) represents 50% of the depth of the item, then the surface area will have been reduced to a quarter of what it was, but the overall power only reduced by 37%, so the average power density will still increased 47% (although, as I said, if one is dealing with more realistic shapes, then this only represents average power distribution, and the actual power distribution will be very much more uneven).
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But as regards if there is any 'damage', (which many conclude is rubbish) to food from microwave cooking many foods may have been processed anyway in the factory with RF possibly with the deeper penetrating 915 MHz.
For processed foods, I have no doubt of that, but the premise here was with regard to loss of nutrients from fresh vegetables, where I doubt that would be applicable.
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I will have to try putting sliced veg in front the satellite LNB. Do I have to wait for a cooking programme?
There are claims that microwave cooking is actually better......
In studies at Cornell University, scientists looked at the effects of cooking on water-soluble vitamins in vegetables and found that spinach retained nearly all its folate when cooked in a microwave, but lost about 77 percent when cooked on a stove.
http://www.nytimes.com/2006/10/17/health/17real.html?_r=1&oref=slogin
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Others conclude that overcooking or overboiling is the problem and nowt to do with microwaves per se.
http://news.bbc.co.uk/1/hi/health/3188558.stm
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Yes, I think the problem on the stove comes from boiling in the veggies. This supposedly sucks all the nutrients out. Steaming is supposed to be better.
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Others conclude that overcooking or overboiling is the problem and nowt to do with microwaves per se.
http://news.bbc.co.uk/1/hi/health/3188558.stm
Certainly, that would be my preferred theory. The only real question in my mind is whether the nature of microwaves makes overcooking more probable than with some other forms of cooking.
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I have just tried putting wet paper sheets between a satellite dish and the LNB. 3 wet sheets is enough to 'kill' the signal. I think it has to be 4 - 6 dB down before it can't decode the digital TV. So that suggests a 4 - 6 dB loss through only 0.3mm of wet paper. I need the whole of a (dry) 20 mm catalogue to for the receiver to fail. This is at 12GHz of course rather than 2.45.
Bear in mind that wet paper would be thicker than dry paper (the main issue is that the paper present a matrix to absorb the water, but the water also has volume of its own).
But more importantly, you are not measuring the loss of signal in the wet paper, you are measuring the reduction of transmission through the paper (i.e. one has to ask how much of the signal was reflected, rather than absorbed?).
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Points agreed but I was surprised that only 3 wet sheets did the trick. What is that about ..three sheets to the.... ?
I might do some more scientific tests one day.. I have a lot of RF test gubbins here.