1 Introduction
Food is a human necessity. Like water and air, it should be made available for a hygienic living. Saffron and other spices are often used to impart yellow colour to various foods even since the ancient civilizations. Let alone children even adults have been attracted towards coloured foods.

Manufactures add food additives such as artificial colouring agents, flavouring agents and sweeteners for under mentioned reasons. Today our foods have several chemicals that may act as a carcinogenic chemical (Knudson, 1982). Of late our environment has turned into containment full of potential carcinogens (causative agents for cancer) such as UV rays, industrial pollutants pesticides food additives metal wastes tobacco products etc (Maron and Ames, 1980).

Among the most common mutagens, Heterocyclic Amines (HCAs) are known to be produced during frying, broiling or grilling over an open flame. HCAs are unique that they are naturally occurring (Nagao et al., 1977; Sugimura, 1988). The pan residues that remain after frying also have high mutagenic activity due to recurring oxidations and avoidance of exposure is also impractical and in that they induce tumours in breast, colon (Sana et al., 1991) and prostate (Shirai et al., 1997).

Non-organic foods (aspartame) an artificial sweetener found in sweets, soft drinks has been linkaged to large number of disease such as lupus, cancer, strokes, head ache, nervous system infection, chronic hepatitis and hyper sensitive (Blaylock, 1996).

Bacterial mutation assays are used in a large number of laboratories throughout the world. Several large scale trials are carried out to test the usefulness of these assays in detecting potential carcinogens and mutagens (Purchase et al., 1978; Mahon et al., 1979; Burtschs et al., 1980; De Serres and Ashby, 1981). Amino acid marker studies are widely applied in many fields. Some of the utility of amino acid are mentioned here as genetic, forensic and biomedical. It also serves as an effective test in detecting the carcinogenicity of compounds (Melcher et al., 1997).

Bacterial test system fall into 3 main classes namely those that detect backward mutation, those that detect forward mutation and those that rely on DNA repair deficiency. By far, the most widely exploited method is the indication of backward or reverse mutation in Salmonella typhimurium or less frequently E. coli (WHO, 1995).

R-factor plasmid (some strain) and multicopy plasmid (some strain) which contains error prone DNA repair system. The type strains also require histidine for growth due to mutation in the gene which control production of histidine (Ames et al., 1975).

2 Results
Man can not live without food. Foods are unavoidable one. Thus quality of the food is of major concern to public health authorities. Despite foods nutritious aspect, it plays a major role in affecting the health of people. This might be due to the adulteration and the nature of these adulterants. This study was carried out to assure the quality of oils and food substances. In this context, 55 samples (oils and foods) were analysed for their carcinogenic property using auxotrophic mutant and treated strains of Salmonella typhimurium.

Mutated strains were created by the physical and chemical mutagenic agent UV and NTG respectively. These results after the mutagenic treatment of wild are recorded in the Table 1 along with the calculated value of induced mutants.
 

Table 1 Calculated value of UV and NTG treatments in wild type Salmonella typhimurium MTCC 98

The treated and auxotrophic strains of S. typhimurium were used to test the carcinogenic chemicals in samples by reverse mutation assay. The results are recorded in Tables 2~5. It indicated the degree of carcinogenicity. Among the 55 samples, only one sample was found to be 2+ level while 22 sample were found to be 1+. The remaining 32 samples were found at the Safe level (raw oils, halwa and kesseri) (Figure 1). From the above results, it was clear that 41.8% of samples were positive for carcinogenicity test.

Table 2 Chemical carcinogenicity test by UV treated strain Ames spot test method using Salmonella typhimurium his MTCC 98

Table 3 Food sample tested for carcinogenicity using UV mutated strain of S. typhii MTCC 98

Table 4 Food sample tested for carcinogenicity using NTG mutated strain of S. typhii MTCC 98

Table 5 Food sample tested for carcinogenicity using Auxotrophic mutant strain of S. typhii MTCC 1251

Figure 1 Degree of carcinogenicity among the 50 food samples

The positive control (Induced mutants plus spontaneous mutants) and negative control (Spontaneous mutants) were tested by Spot test method (Ames, 1975). From these, the positive results were calculated. The negative and positive control which clearly showed that the pin pointed colonies and large colonies are represented in Plate 2 and Plate 3. Plate 3 clearly indicates the presence of revertants as large colonies. Large colonies were only calculated from the results since auxotrophic colonies turn into prototrophic.

Salmonella typhimurium MTCC 1251 and the treated strains of MTCC 98 were tested for amino acid and antibiotic markers (Table 6 and Table 7). In amino acids marker study, markers for the tested 12 amino acids were found to get transformed to the treated strains within one hour of incubation. No difference in amino acid markers were observed after 2 hrs and 3 hrs incubation studies. The auxotrophic mutant strain of MTCC 1251 showed his+ and argi+ character. There were no other notable changes.
 

Table 6 Antibiotic markers among Salmonella typhimurium strains

Table 7 Amino acid markers among Salmonella typhimurium strain

Table 7 represents the antibiotic marker changes among the strains used. Treated strains were formed to receive the resistant marker for Str+, Tet+ and kan+ (within one hr incubation) while auxotrophic mutant strain was able to develop resistance to tetracycline and ampicillin during one hour incubation. The wild type strain showed sensitivity towards all antibiotics.

The plasmids from wild, treated and reverse mutated strains isolated were electrophoresed in agarose gel. After electrophoresed, gel was taken and exposed to transilluminator under UV. Four bands were seen where the plasmids were found in the gel (Figure 2 and Figure 3).
 

Figure 2 Four bands plasmids in agarose gel electrophoresis

Figure 3 A : a representation of a LB agar plate; B: plates presenting a negative control and a positive control of treated strain of Saimonella typhimuriym by spot technique; C: plates showing variouslevels of carcinogenicity using Salmonella typhimuriym auxotrophic mutants by reverse mutations technique

3 Materials and Methods
3.1 Sampling area
Food samples were collected from several shops in and around Alwarkurichi, Tirunelveli District.

3.2 Sample selection
Samples tested for the presence of carcinogenic chemicals included. Oils varied types of raw oil (coconut oil, palm oil, sun flower oil, gingely oil and groundnut oil); Fried oil; Reheated oil. Coloured Foods: Jangiri; Halwa; Kesseri; Milk Sweets. Sweets like halwa, jangiri, milk sweets and kasseri were also collected under sterile conditions from different shops and brought within 4~6 hours to the lab. 

3.3 Treatment of strains
The Salmonella typhimurium MTCC 98 strain was mutated by UV and NTG as under mentioned.

3.4 UV mutant production
The overnight culture of S. typhimurium was centrifuged at 10 000 rpm for 10 minutes. The supernatant was discarded and the pellet was re-suspended in 1 mL sterile saline. The bacterial suspension was decimally diluted using sterile 9 mL saline blanks using sterile 9 mL saline banks upto 10^-8 dilution. From the dilutions 10^-2, 10^-3 and 10^-4, 100 μL was spread plated on LB agar for enumerating total viable counts. Similarly, plates were prepared for selected dilutions to quantify the spontaneous mutants using streptomycin incorporated LB agar plates (dilutions from 10^-2, 10^-3 and 10^-4). Four sets of LB str plates spreaded with 10^-2, 10^-3 and 10^-4 dilutions were exposed to UV for 5, 10, 15 and 20 minutes. The work was carried out under dim light (yellow or red). The plates were incubated in dark (to prevent photo reactivation) at 37℃ for 24 hours and to bacterial load in respective plates were enumerated.

3.5 NTG treatment
The overnight culture of S. typhimurium MTCC 98 broth was subcultured again in LB broth (1:50 ratio) and incubated at 31℃ for 2-2 1/2 hours with agitation to get a midlog phase growth. From this, 1.5 mL was taken in eppendorf tubos and spun at 10 000 pm for 10 minutes. The pellet was washed twice with sterile saline (1 mL). The pellet resuspended in saline (1 mL) in three eppendorf tubes. Of the three eppendorf tubes the first tube was taken and the 1 mL sample was serially diluted to 10^-6, 10^-7 and 10^-8 with sterile saline and spreaded on air dried in plates (TVC). From dilutions 10^-1, 10^-2, 10^-3 and 10^-4, 0.1 was spread plated on LB with selective antibiotic plates to determine TSM. To the second and third tube, NTG (50 μg/mL) was added and incubated at 37℃ for 15~20 minutes. The tubes were spun at 10 000 rpm and to pellet was washed thrice with saline.

The pellet was resuspended in 10 mL LB broth and incubated at 37℃ for an overnight. Then it was serially diluted upto 10^-6 with sterile and from dilutions 10^-4, 10^-5 and 10^-6, 0.1 mL was taken and spread plated on air dried LB plates incorporated with streptomycin (TIM).

3.6 Maintenance of Salmonella typhimurium MTCC 1251 and treated strains of MTCC 98
Auxotrophic strain maintenance is very important. Being a mutant strain it could get easily reverted at any condition and at any time. Hence maintenance medium have the ability to maintain the character of an auxotrophy. Here, ampicillin (8 mg/mL in 0.02 N NaOH) / Tetracycline (8 g/mL in 0.02 N NaCl) and histidine (2 g/400 mL) was added to the medium. The lyophilized MTCC Cultrure 1251 and the treated strains of MTCC 98 were inoculated in maintenance medium and incubated at 37℃ for 24~48 hrs and after growth, it was stored in refrigerator.

3.7 Reverse mutation assay
9.9 mL, 9 mL, 7.5 mL and 95 mL of LB broths were prepared in required test tubes/conical flask and 0.1 mL of the test cultures were inoculated into the broths separately. To this, the selected samples (raw oils, fried oil and reheated oil (0.1 mL, 1 mL, 2.5 mL and 5 mL) and other foods (as 1 gm, 5 gms and 10 gms) were added in quantitative manner and incubated for 24~48 hrs at 37℃ with agitation. After incubation, 1 mL of the test sample mixed culture and 0.1 mL of histidine were added to the molten top agar and vortexed for a homogenous mixture.

The top agar was then poured on E-minimal glucose agar. The plates were incubated at 37℃ for 24 hrs. The growth was observed in the plates in which small pin pointed colonies were not to taken into account while large colonies were considered for enumeration (Cappuccino and Sherman, 2002). The number of chemically induced mutations were determinedby substracting the number of colonies on the negative control plate (representative of spontaneous mutations) from the number of colonies on each test plate. The relative mutagenicity of the test compounds were determined on the basis of the number of induced mutants. If below 10, (-); if greater than 10, (1+); if greater than 100, (2+) and if greater than 500, (3+).

3.8 Ames test and spot test method
A widely used method of Ames test (1975) was performed by spot test method.

3.9 Bacterial genetics of mutant strains
S. typhimurium auxotrophic mutant strain (UV and NTG) were taken up for the study of bacterial genetics on amino acid markers. The procedure followed was: From an overnight grown culture, one loopful was taken and inoculated into three sets of thirteen tubes containing 10 mL minimal broth and appropriate amino acids were given as selected concentrations. The tubes were incubated at 37℃ for 1 hr, 2 hrs and 3 hrs. After one hour of incubation, the first set of tubes was taken and a single set of tubes were taken and a single streak was made on the minimal plates. The plates were there incubated at 37℃ for 24 hrs. Repeated steps of 3 and 4 for each of the other two set of tubes were also carried oil. The growth was observed on the minimal agar plates.

3.10 Antibiotic marker studies using mutant strains and treated strains of S. typhimurium
Auxotrophic mutant strain of S. typhimurium MTCC 1251 and treated strains (UV and NTG) were taken up for the study on bacterial genotics for antibiotic markers. The procedure followed was from an overnight grown culture, a loopful inoculum was taken and inoculated into three sets of four tubes containing LB broth and approximate antibiotics at given concentrations. All the tubes were incubated at 37℃ for 1 hr, 2 hrs and 3 hrs.

After one hour of incubation, the first set of tubes was taken and a single streak was made on air dried LB agar plates. The plates were then incubated at 37℃ for 24 hrs. The steps 3 and 4 were repeated for each of the other two set of tubes. The growth was observed in LB agar plates.

3.11 Isolation of plasmids
Plasmids are extrachromosomal, covalently closed, circular and automatically replicating DNA elements. There are known to carry genes that specify resistance like drug resistance, degenerative fertility, virulence, tumour inducing and for the production of restriction enzymes. Plasmids were isolated from strains by alkaline lysis method.

3.12 Agarose gel electrophoresis
Before the electrophoretic run, agarose gel (0.7%) was prepared along with ethidium bromide at a concentration of 0.5 μg/mL. The boat (carrying the gel) was placed in to the eletrophoresis tank carefully and the tank was filled with 1 × TBE buffer which was just enough to immerse the gel upto 1 mm top.

The plasmids and maker (10 μL) were loaded into separate wells. The gel loading was carefully done without touching the walls of the well but allowing the samples to fall into the well white pipetting. The eletrophoretic apparatus was than connected to the power pack and electrophoresed at 50 mV using 1 × TBE as the running buffer. The migration of the plasmid samples from the negatively charged to the positive charged ends was carefully observed. The electrophoretic run was stopped when the taking dye had migrated and the distance judged as sufficient for separation of the plasmid fragment (before 1 cm at anodic end). The gel was taken carefully using a X-ray film /OHP sheet/ scoop and transferred onto a transilluminator and viewed for the occurrence of bands and compared to the marker for molecular weight determination.

4 Discussion
Ames et al. (1990) estimated that about 99.9% of the chemicals ingested by humans are natural. The amount of pesticide residues in the plant food are insignificant as compared to the amount of pesticides produced by plants themselves against fungi, insects and animal predators.

Ames et al. (1990) estimate state that the daily average American exposure to burnt material in the diet is about 2000 mg and to natural pesticide about 1500 mg. Review findings of epidemiological literature Steinmetz and Potter (1991) and Brock et al. (1992) from nearly 200 studies show that the lack of adequate consumption of fruits and vegetables were consistency related to cancer.

Thun et al. (1992) observed the colon cancer rates that are probably due to addition of diet, to difference of physical activities which are inversely related to colon cancer risks. Ames et al. (1993) also suggested the role of antioxidants such as Vitamins C, E and carotenoids in fruits and vegetables account for a good part of their beneficial effects in present epidemiological studies and related the role of greater antioxidants consumption in human cancer prevention.

In comparison, the total daily exposure to all synthetic pesticides residues combined is about 0.09 mg based on the sum of residues reported by the US Food and Drug Administration (USFDA) (1993). Doll and Peto (1981) were able to estimate that 35% of cancer was due to dietary factors and the plausible contribution ranged from 10% to 70% due to food intake.

Doll and Peto (1981) work on breast cancer reflected a dietary contribution of around 50% and reported that it may not be avoidable in a practical sense. Rapid growth rates are the most important underlying nutrition factor found responsible. Ames and McCann et al. (1975) using a combination of bacterial and mammalian microsomal enzymes. In this study, around 300 chemicals were tested and approximately 9% were checked for carcinogenicity especially bacterial mutagens and 9% of non-carcinogens failed to show mutagenic activity.

5 Conclusion
They were not aware of the treat of dangers awaiting them in the form of food increasing with modernization in food along with the development new diseases many desirable factors are involved in foods. We take responsibility for our health. We have to depend on more organic foods wherever possible preferably freshly cooked and ought to learn avoiding foods, those that contain harmful ingredients and carcinogenic elements. If these situations persist, that would affect the future generations.

Acknowledgement
Authors are thankful to Dr. A.J.A. Ranjit Singh, Principal, Sri Paramakalyani College, Alwarkurichi, Tirunelveli, Tamilnadu, Lab facilities and encouragement. We are thankful for supporting agency of TNSTC, Govt. of Tamilnadu.

Reference
Ames B.N., 1984, Cancer and diet, Science, 224: 668-670, 757-760
http://dx.doi.org/10.1126/science.224.4650.668 PMid:17780577

Ames B.N., and Gold L.S., 1990, Chemical carcinogenesis: too many rodent carcinogens, Proc. Natl. Acad. Sci. USA, 87(19): 7772-7776
http://dx.doi.org/10.1073/pnas.87.19.7772 PMid:2217209 PMCid:54830

Ames B.N., Magaw R., and Gold L.S., 1987, Ranking possible carcinogenic hazards, Science, 236(4799): 271-280
http://dx.doi.org/10.1126/science.3563506 PMid:3563506

Ames B.N., McCann J., and Yamasaki E., 1975, Methods for detecting carcinogens and mutagens with the Salmonella / Mammalian Microsome Mutagenicity Test, Mutation Research, 31(6): 347-364
http://dx.doi.org/10.1016/0165-1161(75)90046-1

Ames B.N., Dursten W.E., Yamasaki E., and Lee F.D., 1973, Carcinogens are mutagens a simple test system combining liver homogenates for activation and bacteria for detection, Proc. Natl. Acad. Sci. USA, 70(8): 2281-2285
http://dx.doi.org/10.1073/pnas.70.8.2281 PMid:4151811 PMCid:433718

Ames B.N., Magaw R., and Gold L.S., 1987, Ranking possible carcinogenic hazards, Letters, 237: 1283-1284 and 235: 1399–1400

Ames B.N., 1983, Dietary carcinogens and anti-carcinogens: Oxygen radicals and degenerative diseases, Science, 221(4617): 1256-1264
http://dx.doi.org/10.1126/science.6351251 PMid:635125

Ames B.N., Shigenaga M.K., and Hagen T.M., 1993, Oxidants, antioxidants and the degenerative diseases of aging, Proc. Natl Acad. Scad. Sci. USA, 90(17): 7915-7922
http://dx.doi.org/10.1073/pnas.90.17.7915

Amstrong B., and Doll R., 1975, Environmental factors and cancer incidence and mortality in different countries, with special reference to dietary practices, Int. J.Cancer, 15(4): 617-631
http://dx.doi.org/10.1002/ijc.2910150411

Ashby J., De Serres F.J., Draper M., Jr. Ishidate M., Margolin B.H., Matter B.E., and Shelby M.D., eds., 1985, Evaluation of short term test for carcinogens. Report of the International Programme on Chemical Safety’s Collaborative Study on In Vitro Assays, Amsterdam, Oxford, New York, Elsevier Science Publishers, Progress in Mutation Research, Vol.5

Blaylock R.L., ed., 1996, Excitotoxins: the taste that kills, Health press NA

Cappuccino and Sherman, eds., 2002, Microbiology - A Laboratory Manual, Sixth edition, Pearson Education, Inc., UK

Chaxton L.D., Allen J., Auletta A., Mortelmans K., Nestmann E., and Zeiger E., 1987, Guide for the Salmonella typhimurium/ mammalian microsome tests for bacterial mutagenicity, Mutation Resarch, 189(2): 83-91
http://dx.doi.org/10.1016/0165-1218(87)90014-0

Clarke R.J., and Macrae R., eds., 1988, Coffee, Elseiver, New York

Cohen S.M., and Lawson T.A., 1995, Rodent bladder tumors do not always predict for humans. Cancer Lett., 93(1): 9-16
http://dx.doi.org/10.1016/0304-3835(95)03785-U

De Serres F.J., and Ashby J., eds., 1981, Evaluation of short-term tests, for carcinogen, Amsterdam, Oxford, New York, Elsevier Science, Publishers. Progress in Mutation Research, Vol.1

Doll R., and Peto R., 1981, The causes of cancer quantitative estimates of avoidable risks of cancer in the United States today, J. Natl. Cancer Inst., 66(6): 1191-1308
PMid:7017215

Garner R. C., Miller E.C., Miller J.A., Garner J.V., and Hanson R.S., 1971, Formation of a factor lethal for S. typhiumurium TA 1530 and TA 1531 on incubation of aflatoxin B1 with rat liver microsomes, Biochem. Biophys. Res. Common., 45(3): 774-780
http://dx.doi.org/10.1016/0006-291X(71)90484-0

Gold L.S., De Veciana M., Backman G.M., Magaw R., Lopipero P., Smith, M., Blumenthal M., Levinson R., Bernstein L., and Ames B.N., 1986, Chronological supplement to the carcinogens potency database standardized results of animal bioassay published through December, 1982, Environment Health Perspect, 67: 161-200
http://dx.doi.org/10.1289/ehp.8667161 PMid:3530736 PMCid:1474421

Green M.H., Muriel W.J., and Bridges B.A., 1976, Use of a simplified fluctuation test to detect low levels of mutagens. Mutaion Research, 38(1): 33-42
http://dx.doi.org/10.1016/0165-1161(76)90077-7

Hunter D.J., and Willet W.C., 1993, Diet, body size and breast cancer, Epidemiol. Rev., 15(1): 110-132
PMid:8405195

Malling H.V., 1971, Dimethlnitrosamine: formation of mutagens compounds by interaction with mouse liver microsomes, Mutat. Res., 13(4): 425-429
http://dx.doi.org/10.1016/0027-5107(71)90054-6

Maron C., and Ames B.N., 1984, Revised methods for the Salmonella mutagenicity test, In: Kilbey, B.J., Legator, M., Nichols, W. and Reamel, C, (eds.), Handbook of Mutagenicity Test Procedures, Amsterdam, Oxford, New York, Elsevier Science Publishers, pp. 93-141

Maron D.M., and Ames B.N., 1983, Revised methods for the Salmonella mutagenicity test, Mutation Research 113(3-4): 173-215
http://dx.doi.org/10.1016/0165-1161(83)90010-9

Matshushima T., Takamoto Y., Shivai A., and Sugimuts T., 1981, Rreverse mutation test on H2-coded compounds with the code WP2 system, In: De Serres F.J., and Ashby J., (eds.), Evaluation of short-term tests for carcinogens, Amsterdam, Oxford, New York, Elsevier Science Publishers, Progress in Mutation Research, 1: 287-395

Mc Mahon R.E., Cline J.C., and Thompson C.Z., 1979, Assay of 855 test chemicals in ten tester strains using a new Modification of the ames test for bacterial mutagens, Cancer Res., 39(3): 682-693

Mccann J., Choi E., Yamasaki E., and Ames B.N., 1975, Detection of Carcinogens as mutagens in the Salmonella / microsome test assay of 300 chemicals, Proc. Natl. Acad. Sci. USA., 72(12): 5135-5139
http://dx.doi.org/10.1073/pnas.72.12.5135 PMid:1061098 PMCid:388891

Melcher U., Ye F., and Fletcher J., 1997, Molecular characterization of a gene encoding a membrane protein of Spiroplasma citri, gene, 189(1): 95-100

Miller E.C., and Miller J.A., 1966, Mechanisms of chemical carcinogenesis nature of proximate carcinogens and interactions with macromolecules, Pharmacol. Rev., 18(1): 805-838
PMid:5325210

Miller E.C., and Miller J.A., 1971, The mutagenicity of chemical carcinogens correlations, problems and interpretation, In: Haddrect A., (eds.), Chemical Mutagens Principles and Methods for their Detection, New York, London, Plenum Press, pp.83-119
http://dx.doi.org/10.1007/978-1-4615-8966-2_3

Nagao M., Honda M., Seino Y., Yahaqi T., and Sugimura T., 1977, Mutagenicities of smoke condensates and the charred surface of fish and meat, Cancer Lett., 2: 221-226
http://dx.doi.org/10.1016/S0304-3835(77)80025-6

National Research Council, 1987, Regulating Pesticides in Food, The Delaney Paradox. National Academy Press, Washington, DC

National Research Council, 1994, Science and Judgement in Risk Assessment, National Academy Press, Washington, DC. Committee on Risk Assessment of Hazardnous Air Pollutants

Ochiai M., Masotoshi W., Hiromi K., Wakabayashi K., Sugimura T., and Minako N., 1996, DNA adduct formation, cell proliferation and aberrant crypt focus formation induced by PhIP in male and female rat colon with relevance in carcinogenesis, Carcinogenesis, 17(1):95-98
http://dx.doi.org/10.1093/carcin/17.1.95 PMid:8565143

Omenn G.S., Stuebbe S., and Lave L.B., 1995, Prediction of rodent carcinogenicity testing results. Interpretation of the Lave- Omenn value of information model, Mol. Crcinog., 14: 37-45
http://dx.doi.org/10.1002/mc.2940140108 PMid:7546223

Purchase I.F., Longstaff E., Ashby J., Styles J.A., Anderson D., Lefevre P.A., and Westwood F.R., 1978, An Evaluation of six short term tests for detectiong organic chemical carcinogens, Br. J. Cancer, 37(6): 873-903
http://dx.doi.org/10.1038/bjc.1978.132 PMid:354672 PMCid:2009661

Shirai T., Sano M., Tamano S., Takahashi S., Hirose M., Futakuchi M., Hasegawa R., Imarida K., Matsumoto K., Wakabayashi K., Sugimura T., and Ito N., 1997, The prostate: a target for carcinogeniciy of 2-amino-1-methyl-6-phenylimidazo [4, 5-b] pyridine (PhIP) derived from cooked foods, Cancer Res., 57(2):195-198
PMid:9000552

Steinmetz K.A., and Potter J.D., 1991, Vegetables, fruit and cancer. I. Epidemiology, Cancer Causes Control, 2(5): 325-357
http://dx.doi.org/10.1007/BF00051672 PMid:1834240

Sugimura T., Nago M., Kawachi T., Honda M., Yahgi T., Semon Y., Sanes S., Matsukura N., Matsushima T., Usni A., Swannama M., and Matsumotao H., 1977, Mutagen-carcinogens in foods, with special reference to highly mutagenci pyrolytic products in broiled foods, In: Hiatt H.H., Watson J.D., and Winstern J.A., (eds.), origins of human cancer, Cold Spring Harbor Laboratory, Cold Spring Harber, New York, NY, pp.1561-1577

Sugimura T., 1986, Studies on environemtal chemical carcinogenesis in Japan, Science, 233(4761): 312-318
http://dx.doi.org/10.1126/science.3088728 PMid:3088728

Thun M.J., Calle E.E., Namboodhri M.M., Flanders W.D., Coates R.J., Byers T., Boffetta P., Garfinke L., and Jr Health C.W., 1992, Risk factor for fatal colon cancer in a large prospective study, J. Natl Cancer Inst., 84(19): 1491-1500
http://dx.doi.org/10.1093/jnci/84.19.1491 PMid:1433333

Tsugane S., Akabune M., Inumi T., Matshshima S., Ishibashi T., Ichimowatari Y., Miyagima Y., and Watanbe S., 1991, Urinary salt excretion and stomach cancer mortality among four Japanese populations, Cancer Causes Control, 2(3): 165-168
http://dx.doi.org/10.1007/BF00056209 PMid:1873446

Vebytt S., and Crofton-Sleigh C., 1981, Mutagenicty of 42 coded compounds in a bacterial assay using Escherichia coli and Salmonella typhimurium, In: De Serres F.J., and Ashby J., (eds.), Evaluation of short –term tests for carcinogens, Amstertam, Oxford, New York , Elsevier Science Publishers, Progress in Mutation Research, 1: 351-360

WHO, 1995, Environmental Health Criteria, Geneva: World Health Organization, Vol.165