Azo dyes are synthetic colours that contain an azo group, -N=N-, as part of the structure. Azo groups do not occur naturally. Most azo dyes contain only one azo group, but some contain two (disazo), three (trisazo) or more.

Azo dyes account for approximately 60-70% of all dyes used in food and textile manufacture.  In theory, azo dyes can supply a complete rainbow of colours, but yellow/red dyes are more common as blue/brown dyes. 

The different, mainly aromatic, side groups around the azo bond help to stabilise the N=N group by making it part of an extended delocalised system. This also has the effect of making many azo compounds coloured, as delocalised or conjugated systems often absorb visible frequencies of light. Aromatic azo compounds (R = R' = aromatic) are usually stable and tend to produce strong vivid colours.

The general formula for making an azo dye requires two organic compounds- a coupling component and a diazo component.  Since these can be altered considerably, an enormous range of possible dyes are available, especially as the starting molecules are readily available and cheap.  Furthermore, the simplicity of the reactions mean that the process can be scaled up or down very easily.  Energy requirements for the reaction are low, since most of the chemistry occurs at or below room temperature.  The environmental impact is reduced by the fact that all reactions are carried out in water, which is easy and cheap to obtain, clean and dispose of.  All these factors make azo dyes very cheap to produce.

Azo dyes are much more stable than most of the natural food dyes. Azo dyes are stable in the whole pH range of foods, are heat stable and do not fade when exposed to light or oxygen. This makes azo dyes applicable in nearly all foods. The only disadvantage is that azo dyes are not soluble in oil or fat. Only when azo dyes are coupled to a fat soluble molecule, or when they are dispersed as very fine particles, oils can be coloured.

Toxicity

The acute toxicity of azo dyes, as defined by the EU criteria for classification of dangerous substances, is rather low. Direct toxic levels of azo dyes will never be reached by consuming azo dye coloured food. The majority of azo dyes (food and textile) have LD50 values between 250-2,000 mg/kg body weight, indicating that for a lethal dose many grams of azo dyes have to be consumed in a singe dose. As azo dyes are highly water soluble, they do not accumulate in the body, but are metabolised in the liver and excreted in the urine. As azo dyes are very strong colour, foods normally are coloured with dyes in levels of mg dye/kg food. To reach a lethal dose an average adult person thus need to consume over 100 kg of azo coloured food in a single day.

Nevertheless some azo dyes have been banned for food use due to toxic side effects. These are not due to the dye itself, but to degradation products of the dyes.

The azo linkage is the most labile portion of an azo dye molecule and may easily undergo enzymatic breakdown in mammals, including man. The azo linkage may be reduced and cleaved, resulting in the splitting of the molecule in two parts. This reaction is carried out by an enzyme named azo-reductase. It is a non-specific enzyme, found in various micro-organisms (such as in intestinal bacteria) and in all tested mammals.

In mammals azo-reductases are, with different activities, present in various organs like liver, kidney, lung, heart, brain, spleen and muscle tissues. The azo-reductase of the liver, followed by the azo-reductase of the kidneys possesses the greatest enzymatic activity.

After cleavage of the azo-linkage, the component aromatic amines are absorbed in the intestine and excreted in the urine. However, the polarity of azo dyes influences the metabolism and consequently the excretion. Sulphonation of azo dyes appears to decrease toxicity by enhancing urinary excretion of the dye and its metabolites. Sulphonated dyes, mainly mono-, di- and trisulphonated compounds are world-wide permitted for use in foods, cosmetics and as drugs for oral application.

As several of the degradation products of the dyes have been found to be mutagenic or carcinogenic and subsequently, some dyes were no longer permitted as food dyes.

Hyperactivity

It is claimed that some food colours increase, or even cause, hyperactivity in children. This is especially claimed for azo dyes. Since the late 1970s the effects of azo dyes on hyperactivity have been studied. Most studies were non conclusive and several studies are contradictory. A main drawback for these studies is that there are no clear indicators for hyperactivity and that in many cases parental reports were used. Often parental reports are biased, which makes interpretation difficult. Also, several studies that have shown an effect on hyperactivity based on parental reports, fail to show an effect based on physiological parameters. A typical example thereof is the recent article of Bateman et al. (see references below) from 2004 and the reactions on this article published in the Lancet afterwards (references see below). In nearly all studies the azo dyes itself had no effect, but the strongest effects were observed in children receiving azo dyes and benzoic acid combinations.

It is still not proven whether food colours (either azo dyes, synthetic colours or natural colours) have an effect on hyperactivity and ADHD, however, it has also not been proven that there is no effect. The strong effects which were claimed in the 1980s do not seem to be valid. But effects on certain sensitive groups of children can not be excluded.

Allergies

Azo food dyes do not cause allergic reactions as such. Many azo textile dyes can cause skin hypersensitivity and allergy, but these are not used as food dyes.

Azo dyes are too small to have a direct effect on the immune system and thus do not cause a direct allergic reaction. However, some azo dyes, especially tartrazine, may increase allergic reactions towards other substances (among these many drugs). Similarly tartrazine (and probably some other azo dyes) may cause increased symptoms in people with asthma and similar disorders.

The exact mechanism why tartrazine increases allergic reactions or asthma is still not fully understood.

Azo dyes used in food

Table 1 shows the azo dyes used in foods.

Table 1