Caution – hazardous chemicals!

…high consumption of processed foods may lead to an increased intake of saturated fats, cholesterol, salt, nitrite, haem iron, polycyclic aromatic hydrocarbons, and, depending upon the chosen food preparation method, also heterocyclic amines.” (Rohrmann and Linseisen, 2015).

Among potential dietary carcinogens, there are three groups of compounds that are not naturally present in meat but may develop during preservation or cooking:

  • N-nitroso-compounds (NOCs) are formed during food preservation
  • Polycyclic aromatic hydrocarbons (PAHs) are formed during cooking
  • Heterocyclic amines (HCAs) are formed during cooking  

It is difficult to estimate exactly how much of these harmful compounds are present in cooked and processed meat and therefore what a person’s intake might be, as they are not naturally present in meat and are not included in standard food composition tables (Jakszyn et al., 2004). 

NOCs

NOCs are a class of potent human carcinogens. Haem iron from meat can act as a catalyst in the production of NOCs in the gut. In addition to that, nitrates and nitrites found in processed meats (and smoked cheeses) contribute to the endogenous formation of NOCs in the gut (Abid et al., 2014).  Nitrite is used to preserve processed meat as it is extremely toxic to bacteria and serves, for example, as an antibacterial agent against the potential tummy bug Clostridium botulinum (Cross and Sinha, 2005). Nitrite also produces the characteristic red-pink colour of cured meats and gives cured meat its flavour. The difference in colour seen in raw cured meat like salami (dark pink, almost red) and cooked cured meat like frankfurters (paler pink) is due to the presence of nitrite. The red colour of raw cured meat is caused by nitrosylmyoglobin (a complex formed by the reaction of myoglobin with nitric oxide), but cooking denatures globin which then detaches from haem, yielding a pink mononitrosylheme complex, the colour of cooked cured meat (Santarelli et al., 2008).

NOCs are also detectable in food, especially in nitrite-preserved meat such as bacon and ham. One study found the faecal content of NOCs was 60 times higher in volunteers given cured meat than in volunteers given a vegetarian diet (Joosen et al., 2009). While some of this is due to endogenous production of NOCs in the gut that occurs in the presence of haem iron, nitrates and nitrites from red meat, some of the NOCs present are there simply because certain meats contain them.

HCAs

HCAs are formed from the reaction between creatine or creatinine (a compound found in the muscle of meat and fish), amino acids and sugars (found in muscle meats) at high cooking temperatures (Jägerstad and Skog, 1991). The three most abundant HCAs in food are:

  • 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)
  • 2-amino-3,8-dimethylimidazo(4,5-f)quinoxaline (MeIQx)
  • 2-amino-3,4,8-trimethylimidazo(4,5-f)quinoxaline (DiMeIQx)

(Abid et al., 2014).

Meat dripping is the animal fat produced either from the fatty unusable parts of a carcass or drained off from a roast piece of meat. It is similar to lard or tallow. Dripping and gravy made from dripping contains considerable amounts of HCAs (Rohrmann et al., 2015). In parts of Yorkshire, when spread on bread, it is known as a ‘mucky fat’ sandwich!    

PAHs

PAHs are formed when meat is cooked over an open flame; the breakdown of fats in meat at high temperatures (pyrolysis) generates PAHs, which then become deposited on the meat (Phillips, 1999). Seven PAH compounds are designated by the US Environmental Protection Agency as probable human carcinogens, including benzo(a)pyrene or BaP (Abid et al., 2014).

Of course these hazardous chemicals are just some of the many different reasons why meat is detrimental to health (evidence for the role of NOCs, HCAs and PAHs in different diseases is provided in later chapters).