**4. The effect of various cooking and freezing processes on contaminants in animal foods**

In the case of therapeutic drugs, before using the product, implementation of withdrawal time for the drug residues has been made mandatory. The obligation of drug applications to sick animals requires the disposal of the products containing residues of during this period, which means economic losses. Withdrawal time of drug residues in animal products is usually determined on unprocessed products. However, most of the animal products are consumed after certain treatments (such as cooking or storing in cooler at a certain time). Such processes may affect the drug residues in the products. Some previous studies have shown that processes applied to the product containing residue may result in changes in the level (quantity) of drug residues [87–90]. This suggests that, in inevitable conditions, the product containing residue is subjected to conditional consumption. Most of the researches on the subject are related to conventional animal products. The obtained results may vary depending on various factors such as quality of the animal products, the sample site on the same animal, the kind and duration of the applied processes. Studies have shown that tetracycline residues were decreased by 35–94% in muscle (cattle and sheep) and liver (cattle) through cooking (microwave, boiling, roasting, grilling and frying). Residues of penicillin (penicillin G-benzylpenicillin and cloxacillin) in milk have been reported to be decreased by the boiling and yogurt production (fermentation). On the other hand, since penicillinase released by microorganisms found in raw milk is deteriorated in the milk produced by UHT, benzylpenicillin is more stable (not disintegrated) in milk produced with this technique. Cooking cannot reduce the residues of oxolinic acid, flumequine, enrofloxacin and ciprofloxacin, which are belonging to Quinolone group, in fish. However, such residues can be removed by discarding the meat broth containing the residues, which are transferred into boiling water through boiling [87]. A similar situation has been observed in broilers concerning some drugs belonging to sulfonamides (sulfadiazine) and quinolone (danofloxacin) groups [88, 89]. Cooking decreases sulfamethazine residues in tissues (muscles and liver) of broiler at different rates. The most significant decrease occurs in boiling because during the boiling process drug in the tissue passes to water. Similarly, cooking (boiling and grilling, equally effective) may also be effective on sulfachloropyridazine-trimethoprim combinations in broiler tissues (muscle and liver) but these drugs cannot be transferred into boiled water in contrast to sulfamethazine [89]. Concentration of levamisole residues in broiler tissues (muscle, liver) can be diminished by different cooking processes (through disintegration and passing to water), whereas the effectiveness of deep freezing is time-dependent and the most losses occur on day 30th [90].

Especially washing as well as applications such as chlorine, chlorine dioxide, hydrogen peroxide, ozone, acetic acid, peracetic acid, hydroxy, iprodione can significantly reduce the pesticide residues in foods. Processes such as pasteurization, boiling, steaming and canning can reduce the levels of pesticide residues depending on the treatment type and time as in veterinary drug residues. In contrast, the implementation of food preservation techniques such as drying or dehydration increases the concentration of pesticides (due to a reduction in weight of product resulting from drying) [91].

Except the studies investigating the effects of processing on pesticide residues mostly in vegetables and cereals processing have diverse effects on pesticide residues in animal products such as milk (pasteurization) dairy products (cheese and yoghurt production) and eggs (boiling and scrambling). When reduction in pesticide residues in dairy products were compared, the reduction in foods made of sheep and goat's milk may be 50% less than in those made of cow's milk. Hexachlorocyclohexane (HCH) residues show a gradual decline by yoghurt production and by keeping at refrigerator [91]. Sausage making can lead to a significant reduction in organochlorine (hexachlorobenzene-HCB, α-, β-, γ-hexachlorocyclohexane-HCH and p,p'-DDE) pesticide residues [92].

Accumulation of organochlorine insecticides in fish is 10–10,000 fold higher than water [52]. Boling process is very effective in reducing DDT and heptachlor concentrations in dried fish (Bombay duck-loittya, ribbon fish-chhuri, shrimp-chingri, Chinese pomfret-rupchanda and Indian salmon-lakhua) [93]. It has been reported that frying process is effective in reducing α-HCH, β-HCH, γ-HCH, heptachlor, aldrin, heptachlor epoxide isomer B, pp′-DDE, endrin and pp'-DDT residues in commonly consumed fish (*Clupea harengus* L., *Salmo salar* L., *Cyprinus carpio* L., *Salmotrutta m. fario* L., *Platichthys flesus* L. and *Gadus morhua* L.) in Poland, and the most pronounced reduction is observed in β-HCH residues [94].
