Author:J.D. Glaess
Course:EEE 460
Date:Spring 2002

Nutritional Value of Irradiated Food

Irradiation on food products is just like any other "cooking" process in that some of the nutritional value of the food is lost. The purpose here is to outline those loses. Above are the symbols for irradiated food.

Why irradiate and approved FDA doses

How irradiation is done

How is the nutritional value effected

Other effects of irradiation of food

Food irradiation Game


Why irradiate and approved FDA doses.

Food irradiation is one way that food can be preserved longer. It does this by destroying insects, fungi or bacterial growth that can make the food inetable. By destroying the bacterial growth you also can delay the time that it take for fruits and vegetables to ripen, thus extending shelf life or the reduction of possible human disease caused by samonella, botulinum, or Tichinae. The table below outlines some of the purposes for different food products and the approved dose. See [6] for a more detailed summary of approved food irradiation processes.

Foods Permitted to be Irradiated Under FDA's Regulations [7]


Food
Purpose
Dose
Fresh pork
Control Tichinella spiralis
0.3 kGy min to 1 kGy max
Fresh foods
Growth and maturation inhibition
1 kGy max
Foods
Arthropod disinfection
1 kGy max
Dry Enzyme preparations
Microbial disinfection
10 kGy max
Dry spices/seasonings
Microbial disinfection
30 kGy max
Poultry
Pathogen control
3 kGy max
Frozen meats (NASA)
Sterilization
44 kGy min
Refrigerated meat
Pathogen control
4.5 kGy max
Frozen meat 
Pathogen control
7 kGy max

How irradiation is done

The food irradiation sources are either from a machine or radionuclides. X-ray machines and electron accelerators are the two types of machine sources and cobalt-60 or cesium-137 are the radionuclides that are used. Note that cobalt-60 is the primary radionuclide that is used. Below is a diagram of a typical irradiation facility. If cobalt-60 is used as the radiation source, it is contained in doubly encapsulated stainless steel pencil-like casings that are about 18 inches long by 3/8 inch in diameter. These casings may be stored in water in between uses. The facility itself is made up of thick concrete walls with a conveyer belt to carry the food products into the chamber that contains the radiation source and out again. The gamma rays produced from the Co-60 are highly penetrating and can be used to treat full boxes of food. Note that the pallets that the food is located on may be turned to provide uniform exposure over the route.

One could calculate the amount of Co-60 required to apply a dose to food. The equation for absorbed dose is H=fS Et/r. Picking the amount of dose that you want to absorbed desired you would know H. For example, from the table "Foods Permitted to be Irradiated Under FDA's Regulations" in the section Why irradiate and approved FDA doses, we saw that the maximum dose for fresh food with the purpose of maturation inhibition was 1kGy. If you were to apply half of this dose H=500Gy. You would need to know the macroscopic cross section (S) and density (r)of a fruit. The energies of the gammas released from Co-60 are 1.173 and 1.332 Mev. Then you would have to pick the speed of the conveyor belt going past the source and the average distance from the source, if you assumed that the food was being rotated. This would leave flux as the only unknown and treating it as a point source you could calculate the amount of Co-60 needed to generate that flux. Note that the half life for Co-60 is 5.3 years.
 
 

How is the nutritional value effected.

In this section we will look at the effects on the major food components: carbohydrates, proteins and lipids (or fatty acids), along with vitamins and minerals. Note that [2], [3] and to a lesser degree [4] and [5] are the best sources for this section.

Protein:

[2]There are at least three kinds of structures in proteins:

Primary: amino acids with peptide bonds

Secondary: arangement of polypeptide chains

Tertiary: spatial configuration in 3D of polypeptide chains

Irradiation can change the structure of a protein, primarily by breaking hydrogen bonds and other linkages in the long chains that make up proteins. A high dose will change the primary structure while moderate doses will affect the secondary and tertiary structures. In the table below you can see that there is not a lot of change to the amino acids from the irradiation. Thus the key components of proteins, amino acids, are primarily unaffected and thus so is protein in general.
 
 

Amino Acid Content of Dry Gelatin after Irradiation[2] * from [9]

   
Dose (kGy)
Amino Acid
Control (no irradiation)
10
25
35
Hydroxyproline
11.9
10.6
11.0
13.0
Aspartic acid
5.4
6.0
5.7
5.5
Threonine
1.65
2.12
1.70
1.70
Serine 
3.13
3.14
3.15
3.30
Glutamic acid
10.1
10.5
10.3
10.0
Proline 
13.55
13.3
13.2
14.4
Glycine 
22.3
21.2
21.3
21.5
Alanine 
8.6
9.35
9.2
8.6
Methionine
0.53
0.90
0.53
0.40
Isoleucine
1.15
1.23
1.16
1.00
Leucine 
2.80
2.80
2.74
2.70
Tyrosine
traces
traces
traces
traces
Phenylalanine
2.04
1.90
1.87
1.90
Hydroxylysine
0.90
0.91
0.77
1.10
Lysine 
3.45
3.87
3.48
2.70
Histidine
0.77
0.73
0.64
0.60
NH4
1.12
1.01
0.77
0.90

 

Carbohydrates:

None of my references cite any actual data for the effects of irradiation on carbohydrates. In [2] the author goes into some detail about how the different bonds are broken and states "generally the radiation-induced changes in carbohydrates are too small to be very important in food irradiation." This statement is agreed upon in the other sources. One note to make is that irradiation breaks the glycosidic bond which is the primary cause of the reduction in firmness and texture (see Other effects of irradiation on food)

Lipids:

In general, fats are not carriers of insects or parasites and they do not decay from microbial action. For these reasons there is no real desire to irradiate fats other than the fact that they are a part of foods that contain carbohydrates and/or protein. [3] reports the following test on polyunsaturated fatty acids (PUFAs):

substance
Dose (kGy)
PUFAs lost
herring fillets
59
none observed
whole grains of rye, wheat, and rice
0.1-1
none observed
whole grains of rye, wheat, and rice
63
only small losses

Also: "[3] No significant effects on the fatty acid composition of the lipids of chicken meat sterilized by gamma-ray or electron irradiation for the Raltech feeding studies were observed." These results seem to point that the loss of fatty acids is negligible in the FDA approved range of dose (see table Foods Permitted to be Irradiated Under FDA's Regulations.)

Vitamins:

Irradiation of food, like cooking (thermal treatment), causes a loss of vitamins. The presence of oxygen and temperature greatly effect the amount lost. For example a beet that was electron-irradiated with a dose of 10kGy had a loss of thiamine that "[3] was 65% at room temperature, 24% at -10° C, 12% at -20° C, and 5% at -75° C." Note that the beet was in a sealed can of nitrogen. For an oxygen example beef was irradiated to 30 kGy in Nitrogen and it had no loss of Vitamin E while in the presence of air the loss was 37%. The table below compares non-irradiated, heat sterilized and irradiated chicken meat. From it you see that the thermal and irradiation losses track rather closely with a couple exceptions.
 
 

Vitamin content of frozen, thermally processed, gamma-irradiated and electron-irradiated enzyme-inactivated chicken meat [3]


 
Process
Vitamin Frozen Control Heat-sterilized Gamma-irradiated (59 kGy at -25° C) Electron-irradiated (59 kGy at -25° C)
Thiamine hydrochloride (mg/kg) 2.31 1.53a 1.57 a 1.98
Riboflavin (mg/kg) 4.32 4.60 4.46 4.90 b
Pyridoxine (mg/kg) 7.26 7.62 5.32 6.70
Nicontinic acid (niacin) (mg/kg) 212.9 213.9 197.9 208.2
Pantothenic acid (mg/kg) 24.0 21.8 23.5 24.9
Biotin (mg/kg) 0.093 0.097 0.098 0.103
Folic acid (mg/kg) 0.83 1.22 1.26 1.47 b
Vitamin A (IU/kg) 2716 2340 2270 2270
Vitamin D (IU/kg) 375.1 342.8 354.0 466.1
Vitamin K (mg/kg) 1.29 1.01 0.81 0.85
Vitamin B12 (mg/kg) 0.008 0.016 b 0.014 b 0.009

a significantly lower than frozen control

b significantly higher than frozen control
 
 

Minerals:

"[3] Mineral and trace elements are not affected by irradiation, and there is no evidence that the bioavailability of these elements might be adversely affected by irradiation."

Summary of Nutritional value:

Proteins, carbohydrates, fats, and minerals are primarily unaffected by irradiation from a nutritional point of view. Vitamins on the other hand have various levels of loss with thiamin and Vitamin E & C being the most sensitive. See the table below for vitamin sensitivity.

Relative radiation sensitivity of vitamins[3] (Note this was reproduced from [8])



Most sensitive least sensitive
Fat-soluble vitamins
Vit. E -> Carotene -> Vit. A -> Vit. D -> Vit. k
 
Water-soluble vitamins
Vit. B1 (thiamin) -> Vit. C -> Vit. B6 -> Vit. B6 -> Folate, nicontinic acid (niacin) -> Vit. B12

Other effects of irradiation of food.

From above it can be seen that the nutritional effects are not that great, unfortunately there are some other undesirable affect that occur. One of them is that the food can develop off flavors. These flavors have "[1] been variously described as 'goaty', 'wet-dog', etc. It resembles a scorched flavour." According to [2] this off flavors are most likely to occur in eggs and dairy products. "[1] Beef is known to be the most prone among the common meats to development of the characteristic irradiation off-flavour." If the doses for meat are kept in the 4Mrad (40 kGy) region there is little or no off-flavor. The loss of texture (softening) is the main draw back from a qualitative point of view relative to fruits and vegetables, though discoloration is also an issue. Overall it seems that with judicious use of the amount of radiation and other techniques (such as irradiating at low temperature, the absence of oxygen and chemical additives) allow most food to be treated with minimal negative effects.

Food irradiation game by Mike Gobster courtesy of Moltar Productions

The purpose of the game is to eliminate the germs before the nutritional value of the food goes to zero. Each time you fire the x-ray gun to kill germs, the nutritional value of the food goes down. The graphic below shows the concept of killing germs with an irradiation source.

 FoodZapper.exe
 

Fruit being irradiated by x-ray machine to kill germs on the fruit.



References

1. W.M. Urbain, Food Irradiation: Benefits and Limitations, Factors influencing the economical application of food irradiation. Proceedings of a panel ... organized by the Joint FAO/IAEA Division of Atomic Energy in Food and Agriculture and held in Vienna, Austria, 14-18 June 1971, pp. 101-115.

2. Walter M. Urbain, Food Irradiation, Academic Press, 1986.
3. Joint FAO/IAEA/WHO Study Group, High-Dose Irradiation: (Wholesomeness of Food irradiated with Doses Above 10kGy), 1997, Geneva, Switzerland.
4.T.K. Murray, Nutritional aspects of Food Irradiation, Recent advances in Food Irradiation, edited by P.S. Elias, A.J. Cohen, Elsevier Biomedical, 1983, pp.203-216.

5. Charlotte P. Brennand, Food Irradiation, http://www.physics.isu.edu/radinf/food.htm, Idaho State University, 1995.

6. Elizabeth L. Andress, Keith Delaplane, George Schuler, Food Irradiation, http://www.fcs.uga.edu/pubs/current/FDNS-E-3.html, University of Georgia, 1998.

7. Kim M. Morehouse, Food Irradiation: The treatment of foods with ionizing radiation, http://vm.cfsan.fda.gov/~dms/opa-fdir.html, US Food and Drug administration, updated 1999.

8. JF Diehl, Food irradiation: is it an alternative to chemical preservatives? Food additives and contaminants, 1992, 9:409-416.

9. S Bachman, H. Zegota, Physicochemical changes in irradiated (gamma 60Co) inulin. Improvement of Food Quality by Irradiation. Intl. Atomic Energy Agency, Vienna, 1974.




 

by J.D. Glaess for EEE591/460

Arizona State University
 

Last Updated: April 30, 2002