Pyrolysis products occurring in food

Decomposition of a compound into smaller, more reactive structures by the action of heat alone is known as pyrolysis. The fragmentation is usually followed by combination of the smaller structures to more stable compounds, provided the conditions do not allow the conversion to CO and CO2.

Since pyrolysis products may occur in all three of the macronutrient categories and the formation of these products proceeds largely by the same mechanism, this subsection deals with all three categories. The formation of pyrolysis products depends on the type of parent compound and the temperature. In the case of food, hazardous compounds are formed from about 300°C. In the following paragraphs, some well-known types of pyroly-sis products occurring in food are discussed.

Polycyclic Aromatic Hydrocarbons. Polycyclic aromatic hydrocarbons (PAHs) are likely to be formed from degradation products consisting of two- or four-carbon units, such as ethylene and butadiene radicals (Figure 6.9).

The most potent carcinogenic PAH is benzo[a]pyrene (3,4-benzpyrene). Benzo[a]pyrene has been demonstrated in pyrolysis productsoffood.Ithasbeenidentifiedinthecharred crusts of biscuits and bread, in broiled and barbecued meat, in broiled mackerel and in industrially roasted coffees. The levels found in broiled meat ranged from 0.17 to 10.5 ppb. Fat is an important "precursor" for the formation of PAHs (in meat and fish). Broiling of high-fat hamburgers led to the production of 43 ppb of PAHs, of which 2.6 ppb was benzo[a]pyrene. In the lean product, only 2.8 ppb of PAH were found, and no benzo[a]pyrene.

Starch may also undergo pyrolysis. On heating of starch from 370 to 390°C, 0.7 ppb benzo[a]pyrene was formed. The range of 370 to 390°C is readily reached in cooking procedures, e.g., at the surface of bread during baking and in boiling cooking fats.

The conditions during the preparation of food may affect the levels of PAH formed. In T-bone steaks cooked close to the charcoal and relatively long, benzo[a]pyrene levels up to 50 ppb have been detected. These levels could be considerably reduced by cooking at a larger distance from the charcoal. PAHs are abundantly found in smoked food (Table 6.5). They originate from the combustion of wood and other fuels.

Heterocyclic pyrolysis products from amino acids. Some potent mutagens are produced on pyrolysis of amino acids. Tryptophan has been shown to be the "precursor" of the benz[a]pyrene VII

tetralin V

X

Figure 6.9 Proposed routes for the formation of PAH.

Anthracene

Anthracene

Benz[a]anthracene

Benz[a]anthracene

7, 12 - Dimethylbenz[a]-anthracene

7, 12 - Dimethylbenz[a]-anthracene

3 - Methylcholanthrene

Benz[a]pyrene

3 - Methylcholanthrene

Dibenz[a,h]anthracene

Benz[a]pyrene

Dibenz[a,h]anthracene

Table 6.5 Benzo[a]pyrene content of smoked and other foods

Food

Benzo[a]-pyrene (ppb)

Food

Benzo[a]-pyrene (ppb)

Smoked fish Eel

Herring Sturgeon Chubs White fish Kippered cod Smoked meats Ham Mutton close to stove distant from stove Lamb

Sausage (with skin) cold smoked hot smoked Salami Bacon

Barbecued meats (Charcoal broiled) Hamburgers Pork chop Chicken Sirloin steak T-bone steak T-bone steak

(flame broiled) Ribs

Other steaks Other foods Spinach Kale Yeast Tea Coffee Cereals Soybean

mutagens 3-amino-1,4-dimethyl-5H-pyrido[4-fo]indole and 3-amino-1-methyl-5H-pyrido[4,3-fo]indole. Pyrolysis of phenylalanine may lead to the formation of the mutagenic substance 2-amino-5-phenylpyridine.

3 - Amino - 1 - methyl -5H - pyrido (4, 3 - b) indole

3 - Amino - 1 - methyl -5H - pyrido (4, 3 - b) indole

These mutagens and several structurally related substances have been isolated from the surface of protein-containing food cooked at 250°C and higher. Mutagens of the aminoimidazoazoarene type have also been isolated from different types of protein-rich foods heated at about 200°C. They include, for example, quinolines and quinoxalines:

2 - Amino - 3,8 - dimethylimidazole (4,5 - f) quinoxaline

2 - Amino - 3,8 - dimethylimidazole (4,5 - f) quinoxaline

Was this article helpful?

0 0

Post a comment