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FOOD IRRADIATION Ionizing radiation : takes the form of γ – rays from isotopes or from x- rays and electrons. It is currently used in more than 35 countries.
Table 2. Applications of food irradiation ApplicationDose range (kGy) Examples of foodsNotes Sterilization Sterilization of packaging materials Pathogen destruction Salmonella species Shigella species Campylobacter species Control of moulds 7-10 Up to Herbs, spices Long-term ambient storage of meat Frozen blocks, poultry, meat, frozen shrimps, spices Extended storage of fresh fruit Outside the permitted dose
Table 2. Applications of food irradiation (continued) ApplicationDose range (kGy) Examples of foodsNotes Extension of chill life from 5 days to 1 month Inactivation or control parasites (Trichinella species, Taenia species) Insect control, disinfesta- tion Decontamination of food ingredients Inhibition of sprouting Soft fruit, fresh fish and meat at 0-4 o C Pork Fruit, grain, flour, cocoa beans, dry fruits Dry food mixes Potatoes, onions, garlic Experimental
Main advantages of irradiation: (1) There is little or no heating of the food and therefore minor change to sensory characteristics. (2) Packaged and frozen foods may be treated. (3) Fresh foods may be preserved in a single operation, and without the use of chemical preservatives. (4) Energy requirements are very low. (5) Changes in nutritional value of foods are comparable with other methods of food preservation. (6) Processing is automatically controlled and has low labour costs.
Main disadvantage: *High capital cost of irradiation plant. Other problems/disadvantages: (1) loss of nutritional value. (2) the possible development of resistance to radiation in micro-organisms. (3) inadequate analytical procedures for detecting whether foods have been irradiated and (4) public resistance due to fears of induced radioactivity.
THEORY Summary of units used : Becqerel (Bq): one unit of disintegration per second Half-life: the time taken for the radioactivity of a sample to fall to half its initial value Electron volt (eV): energy of radiation (usually as mega-electronvolts (MeV) Grays (Gy): absorbed dose (where 1 Gy is the absorption of 1J of energy per kilogram of food) Previously rads (radiological units) were used, where 1 rad=100 ergs/g=10 -2 J kg -1 1 Gy therefore equals 100 rads.
γ – rays and electrons are exerting an ionizing ability (able to break chemical bonds). Products of ionization: electrically charged (ions) or neutral (free radical). Further reactions of ions and free radicals to cause changes in an irradiated product (radiolysis). These reactions are responsible for m/o, insects and parasites destruction. Water in fresh or high-moisture foods is ionized by irradiation.
Ionization of water and formation of radicals by irradiation H 2 O H 2 O + + e - e - + H 2 O H 2 O - H 2 O + H + + OH· H 2 O - H· + OH - Ionization of water H· + H· H 2 or OH· + OH· H 2 O 2 or H· + OH· H 2 O or H· + H 2 O H 2 + OH· or OH· + H 2 O 2 H 2 O + HO 2 · H· + O 2 HO 2 · Formation of radicals by irradiation
Radicals: extremely short lived (< sec), but sufficient to destroy bacterial cells. Fat-soluble components and essential fatty acids are lost during irradiation (development of rancid off – flavors) Therefore: some foods (e.g. dairy products) = unsuitable for irradiation.
EQUIPMENT High – energy isotope source to produce γ – rays ( 60 Co or 137 Cs) or machine source to produce a high – energy electron beam. Isotope source: cannot be switched off and is shielded within a pool of water. It is raised only in operation (packaged food is transported through the radiation field in a circular path). Isotope irradiation plant: It comprises (1) irradiation chamber, (2) control room, (3) infeed conveyor, (4) outlet conveyor, (5) raw food store, (6) irradiated product store, (7) concrete shielding wall.
EFFECT ON MICROORGANISMS Injury/destruction of m/o by changing cell membrane structure and affecting metabolic enzyme activity. More important effect: on DNA and RNA molecules in cell nuclei. Rate of destruction: depends on the rate of ion production. Reduction in cell numbers: depends on the total dose of radiation received. Theoretically: a logarithmic reduction in m/o numbers with increasing dose is expected. However, the destruction rate is not always linear with dose ( some m/o can repair damaged DNA or contain more than one DNA molecules).
The rate of destruction varies with microbial species (figure 5) Fig. 5. Microbial destruction by irradiation: curve A, Pseudomonas; curve B, Salmonella; curve C, Bacillus cereus; curve D, Deinococcus radiodurans; curve E, typical virus.
D 10 value (dose reducing m/o population to 10% of its initial value): expresses the sensitivity of m/o to radiation. General rule: The smaller and simpler the organism the higher the dose of radiation needed to destroy it. Viruses: very resistant, unlikely to be affected by dose levels commercially used. Yeasts and molds: readily (at low doses) destroyed. Spore forming species: more resistant Food poisoning bacteria: less resistant and this is likely to become one of the most important applications of food irradiation.
The dose given to a food depends on: (a) The resistance of the m/o present (b) The objective of the treatment Combination of irradiation with heating is beneficial in causing a greater reduction in m/o numbers than would be achieved by either treatment alone. Also, enzymic spoilage is not entirely prevented by irradiation. Two potential problems of irradiation are: (1) that by destroying spoilage m/o and not destroying pathogenic bacteria a valuable indicator of wholesomeness is removed, and (2) that the destruction of toxins-producing bacteria after they have contaminated the food with toxins is a health hazard.
WHOLESOMENESS OF IRRADIATED FOODS The most important aspects of wholesomeness to be considered: (1) Can the process induce radioactivity? (2) Can a safety program prevent exposure of personnel to radioactivity during processing? (3) Do the changes in surviving microflora create a potential hazard? (4) Does the process result in nutrient losses and/or toxic product formation of public health significance? (5) Does the energy input by radiation result in chemical reactions producing toxic or carcinogenic compounds?
(1) Induced radioactivity Maximum recommended dose for foods: 15 kGy Average dose: < 10 kGy At this dose: 60 Co and 137 Cs do not induce radioactivity in the food. Machine sources of e and x-rays induce insignificant levels of radioactivity (far below the acceptable dose). (2) Radiolytic products Ions and radicals are capable of reacting with food components to produce radiolytic products. Extend of radiolysis: depends on food type and radiation dose However, no or little adverse effects have been found. (3) Toxicity and Carcinogenicity According to existing data: no clear indication of acute or chronic toxicity or carcinogenicity.
EFFECT OF FOOD PROCESSING AND PRESERVATION METHODS ON THE NUTRITIVE VALUE OF FOODS
1. BASIC PRINCIPLES Many unit operations, especially those that do not involve heat, have little or no effect on the nutritional quality of foods. Examples include mixing, cleaning, sorting, freeze drying and pasteurization. Heat processing is a major cause of changes to nutritional properties of foods.
Heat also destroys some types of heat-labile vitamin (Fig.1), reduces the biological value of proteins (owing to destruction of amino acids or Maillard browning reactions) and promotes lipid oxidation. Fig. 1. Stability of vitamins in food
Oxidation: is a second important cause of nutritional changes to foods. It occurs when food is exposed to air (for example in size reduction or hot-air drying) or as a result of the action of heat or oxidative enzymes (for example peroxidase or lipoxygenase). Main nutritional effects of oxidation: (1) the degeneration of lipids to hydroperoxides and subsequent reactions to form a wide variety of carbonyl compounds, hydroxy compounds and short chain fatty acids, and in frying oils to toxic compounds. (2) destruction of oxygen-sensitive vitamins.
The importance of nutrient losses during processing depends on the nutritional value of a particular food in the diet. In industrialized countries, the majority of the population achieve an adequate supply of nutrients from the mixture of foods that is eaten. Losses due to processing of one component of the diet are therefore insignificant to the long- term health of an individual. Variation in nutrient losses between cultivars or varieties can exceed differences caused by alternative methods of processing. Growth conditions, or handling and preparation procedures prior to processing, also cause substantial variation in nutrient loss.
2. EFFECT OF IRRADIATION ON NUTRITIVE VALUE & SAFETY OF FOODS At commercial dose levels, ionizing radiation has little or no effect on the digestibility of proteins or the composition of essential amino acids. At higher dose levels, cleavage of the sulphydryl group from sulphur amino acids in proteins causes changes in the aroma and taste of foods. Carbohydrates are hydrolyzed and oxidized to simpler compounds and, depending on the dose received, may become depolymerized and more susceptible to enzymic hydrolysis, without any reduction in nutritional value.
Effect on lipids: similar to that of autoxidation – Foods having high lipid concentrations are generally unsuitable for irradiation. Water soluble vitamins vary in their sensitivity to irradiation. The extent of vitamin loss also depends on the dose received and the type and physical state of food under investigation. The effects on thiamine in meat and poultry are likewise inconsistent but other vitamins of the B group are largely unaffected.
Fat soluble vitamins vary in their susceptibility to radiation. Vitamins D and K are largely unaffected whereas vitamins A and E undergo some losses, which vary according to the type of food examined. In conclusion: At commercial dose levels, irradiation causes no greater damage to nutritional quality than other preservation operations used in food processing.
RECENT CONCERNS ABOUT THE SAFETY OF IRRADIATED FOODS - Recently, many in the Scientific Community are opposed to irradiation. They allege that: (a) γ-rays can kill harmful bacteria in food, but one big problem is they kill the helpful microflora, too. (b) Food irradiation increases the levels of mutagens and carcinogens in the food. As a consequence, in two or five decades in the future, the incidence of cancer will increase in direct proportion to the amounts of irradiated food consumed. (c) Irradiation causes a host of unnatural and sometimes unidentifiable chemicals to be formed within the irradiated food. The number, kind, and permanence of these depend on the food itself and the irradiation dose. (d) Mutagenic doses of formaldehyde are formed during irradiation of carbohydrate.
A new class of chemicals can be formed during food Irradiation. These chemicals, called cyclobutanones, do not occur naturally anywhere on Earth. They recently were found to cause genetic damage in rats, and genetic and cellular damage in human and rat cells. This class of chemicals can be formed in many irradiated foods including beef, pork, chicken, lamb, eggs, tropical fruits (mangoes and papayas). Moreover: From a nutritional perspective, exposing food to irradiation depletes vitamins, often significantly. Especially vulnerable are A, the B-complex, C, E, and β-carotene. From a chemical perspective, irradiation blows apart bonds, resulting in the formation of hundreds of new compounds, some of them being known or suspected to cause cancer or birth defects. Examples of these compounds are:
Benzene, Ethanol, Hexane, Methyl ethyl ketone, Toluene. From an aesthetic perspective, irradiation can destroy the flavor, odor and texture of food. - Another concern is that irradiation will eventually be used to mask filthy slaughtering and food processing practices. Substandard food could be “treated” with high-dose radiation in unlicensed and dirty facilities. This fact could jeopardize the quality and safety of food sold to the consumers.
Χρηματοδότηση Το παρόν εκπαιδευτικό υλικό έχει αναπτυχθεί στα πλαίσια του εκπαιδευτικού έργου του διδάσκοντα. Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα στο Πανεπιστήμιο Ιωαννίνων» έχει χρηματοδοτήσει μόνο τη αναδιαμόρφωση του εκπαιδευτικού υλικού. Το έργο υλοποιείται στο πλαίσιο του Επιχειρησιακού Προγράμματος «Εκπαίδευση και Δια Βίου Μάθηση» και συγχρηματοδοτείται από την Ευρωπαϊκή Ένωση (Ευρωπαϊκό Κοινωνικό Ταμείο) και από εθνικούς πόρους.
Σημείωμα Ιστορικού Εκδόσεων Έργου Το παρόν έργο αποτελεί την έκδοση 1.0. Έχουν προηγηθεί οι κάτωθι εκδόσεις: Έκδοση 1.0 διαθέσιμη εδώ.
Σημείωμα Αναφοράς Copyright Πανεπιστήμιο Ιωαννίνων, Διδάσκοντες: Κ. Ακρίδα, Π. Δεμερτζής, Κ. Ρηγανάκος, Ι. Σαββαΐδης. «Προχωρημένα Μαθήματα Επεξεργασίας και Συντήρησης Τροφίμων. Συντήρηση με ακτινοβόληση (FOOD IRRADIATION)». Έκδοση: 1.0. Ιωάννινα Διαθέσιμο από τη δικτυακή διεύθυνση:
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