Re: effect of far infrared lamp in a domestic refrigerator

Alejandro,

Food Irradiation is a very important form of sanitization and I, for one, hope that it will become much more common in the future. However, it seems to be horribly misunderstood by many people (as exemplified by your refrigerator manual) and so there is some foot-dragging about using it. If more people were as interested as you are and were willing to investigate it, perhaps the situation would be changed!

As you probably know, light can be divided into different categories based on frequency (or the inverse: wavelength). The energy of the orresponding light is E = hv (in a vacuum) where h is Plank’s constant and v is the frequency. It is the energy of the light that determines the chemical response of matter. For example, ultraviolet light is often used for sanitization because there is enough energy to move electrons around and so alter chemical bonds (or one can say that the frequency is fast enough to interact with electrons, either way). UV light causes DNA to cross- link and cause skin cancer in humans and outright death to microorganisms!

However, infrared IR light does not have this much energy; it merely causes the vibration of bonds in a compound. So can these mere vibrations really preserve food? According to some sparse literature, yes!

They suggest that it is heat that kills the microorganisms. Remember that heat is just molecular movement caused by the bond vibrations. The wavelength of IR is rather short (around 10 micrometers) so the depth of penetration of the light is also of this order. So essentially only the very surface of the food is actually heated by IR and there is really not much competition between your refrigerator cooling the meat and the light warming it. (Do a quick geometry calculation and compare the amount of meat actually heated to the total amount to see for yourself!) And also consider that only a fraction of the surface is facing the light so the dark side is continually being cooled. You could even compare the mass of the thermometer to the mass of the point the thermometer touched to see that a rise in 1 degree C means that the very thin surface must be quite hot!) It is also important to note that the IR would heat and kill any spores or microorganisms that float into your refrigerator hoping to land on some meat. It seems to me that both of these heating effects are important.

There may also be something more subtle occurring. IR light is absorbed by certain chemical groups, particularly nitrogen-hydrogen (N-H) bonds and oxygen-hydrogen (O-H) bonds. Many biological compounds contain these groups and their functionality is dependent upon their shape. For example, a protein involved in transporting nutrients across a bacterial membrane may have three or four different, but very specific, shapes that it must go through in order to complete the task. If certain bonds are vibrating all over the place (and for a large molecule like a protein there are hundreds of modes of vibration) it may not be able to stay in a certain shape long enough to finish that portion of its task. If enough proteins (and other bio-molecules) are unable to function then the organism dies!

You performed an interesting experiment! The fact that the aluminum- foiled piece of meat spoiled faster makes sense because the aluminum would just scatter the IR and so the surface of the meat would not be heated and the microbes on the meat would not have their bio-molecules affected. Since only the heat-killing-floating-things effect is occurring, we know that it must be a minor preservative. (You could try a piece of meat in aluminum foil in the light and one just without the light to demonstrate this.) The piece in the glass bottle rotted also but IR should pass through clear glass. But whenever light passes from air to glass it is refracted (it bends so that the incident angle becomes smaller) according to Snell’s law. The light would also partly be reflected and scattered. The end result is that not much of it actually reaches the meat! (You could try a piece of flat, thin glass over the top to lessen these effects!) But many plastics absorb IR, so why did that piece keep so well? Perhaps the type of plastic you used is IR transparent so that most of it passed through (it is thin and often has a smaller index of refraction so refraction, reflection, and scattering probably happened less). But there is another question you must consider! The plastic was probably very clean (if it was new). Was the bottle clean, too? What was in it before that might have some microbes?

Congratulate your wife for purchasing an interesting refrigerator, but disregard the manual: There is no way that IR could promote amino acid formation and neither would amino acid formation preserve meat.

I hope this information helps and good luck with future experiments!

Jason

some references:
Kalashnova; [Quality of meat products treated with IR radiation]; Izvetiya- Vysshikh-Uchebnykh- Zavedenii, -Pishchevyaya-Tekhnologiya, no. 1, 128-129; 1989

Inagaki; Sterilizing method for treatment of fresh fruits and apparatus used for the method; United States patent #US 4 875 407 (US4875407)

http://www.wvu.edu/~agexten/v3i1/irradiat.htm#Food

Skoog/Leary; Principles of Instrumental Analysis; Harcourt Brace; 1992