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Food irradiation is a step in either food processing or food packaging in which food and beverages are subjected to ionizing radiation to greatly reduce the pathogens such as bacteria, microorganisms, viruses or insects in food that may account for much severe foodborne illness. For instance, many bacteria such as E. coli, Listeria and Campylobacter can be eliminated almost up to 99% through irradiation process. The sources of ionizing radiation used in food irradiation include X-rays, Gamma rays and electron beams with high energy. Besides the purpose as the preservative method, there are many other advantages and some limitations of using these radiations on food and beverages. These include, inhibiting sprouting in seeds, delay ripening of fruits, increased juice yield of pulps and improved re-hydration of meat tissue (Alam & Abrahem, 2010).
The effect of radiation and type of food being radiated also varies according to the applied dosage of irradiation. Generally, a low dosage below 1kGy is used to control insects and food parasites, besides to delay the physiological processes such as sprouting or ripening of fresh vegetables or fruits. Whereas, medium dosages of 1 to 10kGy is used to decrease the pathogenic microbe counts thus reducing the spoilage of food and extend the shelf-life. Besides that, this dosage can also be used to enhance the technological properties of food in which the cooking period of dehydrated foods can be significantly reduced. In addition, high dosage of food irradiation is usually applied to sterilize meats, poultries and seafood. Mild heating is applied together with this high radiation to inactivate enzymes besides disinfecting the foods or ingredients such as spices and specific enzymes (Arvanitoyannis et al., 2009).
In this review, only the effects of gamma radiation on food and beverages are analyzed. Based on the 19 peer reviewed primary scientific journals and three review articles studied, the effects of gamma radiation on food and beverages were determined as follow. The foods and beverages that are exposed to gamma radiation are usually free from microbial activities, have extended shelf- life and storage capacity, besides having improved chemical properties where the quality of the content remains unchanged. However, gamma radiation also causes the formation of some harmful substances like furan and sulphur, degrades some essential compounds of the food, besides reducing the enzyme activity and causing the loss of nutritive and aesthetic value of the food.
There are many advantages of radiating food and beverages with gamma ray. However, only a few of them were analyzed in this review. These advantages include the prevention of microbial activity, extended shelf-life and storage capacity of food and enhancement of chemical properties in which the quality of the content is maintained.
An experiment conducted by Badr (2005), has proved that a medium gamma radiation with the dosage 3kGy applied at room temperature and accompanied by the cold storage at 4 +/- 1Â°C is the optimum condition to treat food products such as liquid egg white and yolk to improve the microbial safety of the food. This is because, gamma radiation could effectively influence the synthesis of three heat shock proteins (Hsps) which are known as DnaK, GroEL and GroES of both gram positive and gram negative bacteria . Thus, high gamma radiation on these bacteria will significantly inhibit the synthesis of such proteins, hence prevents the microbial activity on the food products (Caillet et al., 2008). Besides, Onyenekwe and his colleagues (1996) has also discovered that a specific dosage of gamma radiation could effectively eliminate the natural microflora (fungus and bacteria) found on Ashanti pepper. They have successfully established that a 2.5kGy dosage of gamma radiation could reduce the bacterial and fungal load by 2 log cycles; a 7.5kGy dosage could completely eliminate the population of fungus in Ashanti pepper whereas a 10kGy dosage would decontaminate the sample regardless of its forms. In addition, the experiment conducted by Paster et al, (1984) also confirms the above fact in which 200krad gamma irradiation accompanied by an exposure to the temperature of 40Â°C for 15 to 30 minutes, had successfully inhibited the fungal (mycelial) growth and decreased the ochratoxin production by the fungus.
Gamma irradiation is an effective method to extend the shelf-life of food products. This is because gamma radiation could inhibit the after ripening and senescence effect which could destroy thermophilic anaerobic spores. Heat sensitization followed by gamma radiation usually helps to eliminate the surviving spores using less severe canning process. Thus, it is also ensured that the quality loss of the food product is minimized (Kim et al., 1987). Besides that, gamma irradiation on stored onions could reduce the sprouting of fresh bulb and the characteristic pungent smell of the onion. In addition, it also lowers the content of ascorbic acid and free amino nitrogen found on the onion, thus increasing the storage capacity of the onion to 2 months when maintained at 40Â°C (Salem, 1973). Furthermore, a research conducted by Brito and his colleagues in 2011 reveals that the storage capacity of meats can be extended until 7 days if its irradiated with 3kGy of gamma radiation. This is established as the psychrotropic bacterial count on the meat only significantly increased one week after irradiation and produced very weak intensity of oxidized or irradiated odor.
Many sugar containing food products are sterilized by gamma radiation nowadays. Thus, it is important to know that gamma radiation in sugar containing products initiates browning reaction. This is confirmed by the production of reductones, enols or enediols, hydrogen, methanol, and a small amount of formaldehyde and carbon dioxide in the crystalline sugars. However, hexoses were not seen in the gamma irradiated sugar crystalline, indicating an increased reducing power of the product (Liggett et al., 1959). Besides, the gamma radiation treatment on the liquid egg white and yolk showed no change in the chemical and organoleptic properties of the egg, proving that the quality of the egg is maintained (Badr, 2005). In addition, an experiment conducted by Dixit and his colleagues in 2010 also revealed that gamma radiation could successfully induce enhancement of total phenol, isoflavones and anthocyanin content and improves the antioxidant property of soybeans with colored seed coat. However, the extent of the effect depends on the dosage of radiation applied and also the genotype of the three Indian soybean used (Hara soya, Kalitur and NRC37).
Furthermore, a study conducted by Jimenez et al, (2011) states that gamma irradiation on green onion causes no change in its DNA, thus leads to the DNA integrity of the food product besides maintaining the quality and originality of the food. This is because; gamma irradiation increases the phenol concentration and ascorbic acid content of the onion which has significant role against the damage of nucleic acid. Moreover, gamma radiation also used to detoxify food products. For instance, a 100krad of gamma radiation in the presence of 5% hydrogen peroxide could deactivate 50Âµg of aflatoxin Î²1 in groundnuts. A fourth fold increase in the dosage of radiation results in the double degradation of the toxin under the same condition, thus significantly reduces the toxin levels of the groundnuts (Patel et al., 1988). Lastly, aroma compounds found on the food products such as vanillin beans are not affected by gamma radiation and are maintained though no enhancement could be made to the aroma quality due to the highly stable oxygen-carbon linkage between the glucose and vanillin molecule (Kumar et al., 2010).
There are some noticeable disadvantages of radiating food and beverages with gamma ray and a few of them were pointed in this review. These disadvantages include the formation of harmful substances, degradation of essential compounds, reduced enzyme activity and loss of nutritive and aesthetic qualities.
Gamma radiation up to the dosage of 4.5kGy can induce a carcinogenic substance, known as furan formation in aqueous solutions of Na-erythorbate, Na-ascorbate, honey, corn syrup and glucose that are found in ready-to-eat products. However, this could be either completely eliminated or significantly reduced, depending on the amount, through the addition of Na-nitrite into the above solutions immediately after irradiation (Fan & Sommers, 2006). Besides that, during food packaging when the packed foods are irradiated with gamma rays, there are high chances for the two harmful plasticizers from the PVC film known as acetyltributyl citrate (ATBC) and di-(2-ethylhexyl)adipate (DEHA) to migrate into the food simulant isooctane. The extent of migration of these two substances is directly proportional to the contact time of food with the PVC film and the dosage of the gamma radiation supplied. However, the migration of ATBC into the isooctane is significantly higher than the migration of DEHA and the migration of these two substances into the food molecules causes significant effect on the human health (Zygoura et al., 2007).
Gamma radiation on fruits shows significantly great impact in the degradation of essential compounds found on the fruits. For instance, an experiment conducted to study the effect of gamma radiation on orange juice has revealed that there is a significant loss of a volatile compound known as acyclic monoterpenes such as neral, geranial, linalool 1 and myrcene one week prior to irradiation. The reduction of this compound were found to have a positive linear relationship with the dosage of gamma radiation applied, and proven to be correlated with an increase in the content of thiobarbituric acid reactive substrates (TBARS) found on the orange juice (Fan & Gates, 2001). On the other hand, a study conducted by Massey and Parsons (1964) on the effect of gamma radiation on keeping the quality of apple has evidenced that a gamma radiation with the dosage of 10krad results in immediate softening of the fruit followed by stimulation of oxygen consumption to cause browning and experience some loss of grains. Besides, it was also observed that gamma radiation could induce injury to the skin and impairs the flavor of the apples where these effects were specific to the variety of apples available.
Gamma radiation on meats such as beef also has some negative effects on the meat. For instance, a fraction of proteolytic enzyme extracted from beef muscle in the pH of 9.6 was irradiated with 1.6 x 106 rep. of gamma rays and resulted in the reduction of proteinase activity by 50%. Proteolytic enzymes are essential to weaken the beef meat structure and improve the functional properties of the meat. Thus, the loss of this enzyme activity hardens the meat and makes it unfavorable (Doty & Wachter, 1955). Besides, gamma radiation on beef meat also causes the accumulation of sulphur compounds in the meat, resulting in the release of undesirable odors from the meat (Batzer & Doty, 1955). In addition, gamma radiation on milk and milk products causes the loss nutritive quality upon the irradiation. This is proven through a research conducted by Kung et al., (1953) which revealed that there is a significant loss of vitamins such tocopherol, ascorbic acid and vitamin A, followed by riboflavin and carotenoids and a slight loss of phosphatase enzyme due to the gamma radiation applied on milk products.
Gamma irradiation is essential in food processing and food packaging. It is used as the traditional method of sanitation and pasteurization to protect us against the foodborne illness caused by harmful pathogens (Tauxe, 2001). Despite all the advantages and limitations discussed above, it is important to realize that gamma radiation is applied on food mainly to prevent microbial activity and extent the shelf-life. Keeping those two points in mind, it is essential to examine the extent of this fact. However, many research conducted earlier were not successful in reaching this aim.
Researches should keep these two factors as a control to conduct the experiment. For instance, it is important to state the aim of the experiment as the effect of gamma radiation on a particular food, when the food is completely hygiene (free from microbes) and have extended shelf. There is no use of having improved qualities and pointing out any small loss of nutrition in the food, when the two main purposes are not established. Therefore, an experiment should be conducted to analyze if there is any severe loss of nutrition or induction of the harmful substances in the food products when the food is irradiated with gamma ray, provided the food is pathogen free and have extended shelf life due to the irradiation process.
In conclusion, it is deduced that gamma irradiation is an essential pathogen killing step of food processing method. This review has successfully analyzed the advantages and limitations of irradiating food and beverages with gamma rays besides for the purpose of preventing microbial activity and extending shelf life of the food. It is clearly proved that gamma radiation on food does help to enhance the chemical properties and maintain the quality of the food, though there are also some formations of harmful substances, degradation of essential compounds and loss of enzyme activity and nutritive value seen. However, it is essential to examine whether these properties (advantages and limitations) exist when the two main purposes of gamma irradiation on food are reached.