Feature

Dangerous Drinks

Questionable practices in the technology of soft drink manufacture may be exposing people to dangerously high levels of aluminium.

By R. H. Molony

Aluminium compounds are common in treating the water used in beverage manufacturing, such as in soft drink production. Low levels of aluminium can be both toxic and undetectable by either the consumer or by the analytical tests used to check such levels at the factory.

Over recent years there has been a considerable amount of concern about health problems associated with dietary aluminium. Toxic effects of aluminium first became widely known after kidney dialysis patients received brain damage as a result of aluminium absorbed from dialysis water.

Later, some babies' deaths were associated with aluminium contamination of milk formula. More recently, a statistical association has been found between aluminum levels in drinking water and Alzheimer's disease.

There is now little doubt that many aluminium compounds are neurotoxic. In view of the toxicity of aluminium, a knowledge of the potential levels and sources of aluminium in food and drinks is important.

Any aluminium found in soft drinks could be derived either from the packaging or from the drink's ingredients. Aluminium packaging materials are normally coated with an acid-resisting lacquer. Metal pickup can only occur with faulty packaging in which the lacquer coating is perforated. The most probable source of aluminium is the water being used in product manufacturing.

Water Treatment

The domestic water supply is seldom suitable for soft drink manufacture. Further treatment is almost always required. The normal soft drink plant water treatment system consists of superchlorination (at 5 to 10 ppm), precipitation, sand filtration and activated carbon filtration. Some plants also include ultraviolet irradiation after the carbon filter immediately prior to use.

The carbon filter not only removes any remaining traces of flavour, but also catalytically dechlorinates the water. Precipitation both clarifies the water and removes traces of surface active materials which can inactivate the activated carbon, allowing chlorine into the product and interfering with gas transfer during the carbonation process.

Aluminium compounds are widely used as coagulation and flocculation agents during the precipitation process. When a simple aluminium salt, such as aluminium sulphate, is added to the precipitator, it hydrolyses, producing polyaluminium-polyhydroxy ions and colloidal aluminium hydroxide. These positively-charged compounds bind with and coagulate any negatively-charged material present in the water.

If insufficient negative colloids are present, some of the colloidal aluminium hydroxide and polyaluminium-polyhydroxy ions stay in solution. Carry-over of aluminium from the precipitator then occurs.

Colloidal aluminium hydroxide and polyaluminium-polyhydroxy ions can pass through the sand filter. As activated carbon has a negative charge it might be expected that the aluminium compounds would be adsorbed by the carbon filter.

Unfortunately, aluminium compounds tend to pass through the filter and end up in the product. Aluminium carry-over is often accentuated by operating upflow precipitators at a velocity too high for adequate precipitation. This commonly occurs when, due to financial considerations, a plant of inadequate size is used.

Undetectable Problems

The problems of carry-over from water treatment plants frequently go unnoticed due to analytical problems. Polyaluminium-polyhydroxy ions do not react well with the common colourimetric reagents used for aluminium determination.

These ions also tend to give low results for aluminium with the flame atomic absorption (AA) spectro-photometer. An initial acid digestion of the water sample prior to testing corrects this problem.

Aluminium can be detected spectro-photometrically to levels of 20 micrograms/litre, while flame AA has a detection limit of 0.1 mg/l after acid digestion. A more commonly used water treatment test kit for aluminium has a detection limit of 0.2 mg/l, and does not respond to polymeric alumin-ium ions or chelated aluminium.

Aluminium forms a wide variety of compounds, which vary in their bioavailability and hence toxicity. In nature, most aluminium is combined with silica to form alumino-silicates, which are very inert and unlikely to contribute significantly to dietary aluminium levels.

Aluminium in water treatment plant effluent consists primarily of colloidal aluminium hydroxide and poly-aluminium-polyhydroxy ions. These compounds are readily soluble in acids, such as the citric acid and citrates used in soft drinks, forming complex ions. These complex ions do not give any of the normal reactions associated with simple aluminium compounds.

Flocculation treatment of water used for soft drink manufacture is in most cases unavoidable. If aluminium coagulants are used, extra care should be taken that the water contains sufficient negative colloids for good flocculation.

Precipitation plants should be correctly sized, with a tendency to oversize rather than undersize the precipitator. Aluminium compounds can be replaced with iron compounds that are more stable and which allow plants to be operated at the higher upflow velocities desired. Unfortunately, the cost of using iron compounds is considerably greater than that of using aluminium compounds.

Evidence published recently indicates that the minimum toxic level of aluminium may be as low as 0.1 mg/l, and there is little doubt that levels of aluminium exceeding this are likely to go undetected in most soft drink plants.

Such levels have been found in beverage manufacturing water in New Zealand, and products overseas have been manufactured with water containing up to 10mg/l.

Soft drinks have the potential of being a serious toxic hazard due to aluminium. This hazard is likely to go unnoticed by the consumer, as aluminium compounds have little taste at toxic levels. Also, any toxic effects may be delayed and not noticed until years after consumption of the product.

R. H. Molony is a chemist and food technologist with twenty years in the beverage and fruit processing industries.