Plant compressed air: sampling and contaminants
Compressed air is used during production processes by most companies in the food industry. In compressed air, pollutants such as microorganisms, mineral oils, and particles can be present, which could contaminate the final product and cause huge damage to the company. Compressed air monitoring reduces this risk and is very important to protect consumers and ensure a safer process.
Air can contain contaminant traces such as viruses, bacteria, fungi, yeasts, mineral oils, solid particles, or gases. When the air is compressed, the number of these contaminants rises significantly. The use of not enough clean compressed air can cause machines to malfunction during years and contaminate food, for example by giving undesired tastes, smells or moisture, with a resultant loss of quality.
Typical applications of compressed air in the food industry are processes such as cleaning, filling, mixing, spraying, cutting, transportation, and packaging. During these different stages, and depending on the application, direct and/or indirect contact takes place between compressed air and the product.
Contaminants that are normally present in compressed air are solid contaminants (dust), liquid contaminants (oils), and microorganisms.
The group of solid contaminants includes impurity particles such as pollen, dust fibres, soot, and heavy metals that may come from the distribution line. These contaminants are independent from the compressing technology used. Depending on the workplace, these particles can also have chemical features that cause the corrosion of balance tanks and of the distribution system or other side damages.
To generate compressed air, oil-free compressors or lubricated compressors are used. In the first case, the outcoming compressed air cannot be automatically oil-free, because the inlet air contains hydrocarbons or oil in a gas form. In the lubricated compressors mineral oil is used for cooling and sealing. The outcoming compressed air is so inevitably contaminated by oil. Factors determining oil dragging are to be attributed to mechanical stresses within the compressor, which form vapours, mists, and micro-mists so thin that separators are not able to stop them.
After ambient air compression by the compressor, the outcoming air is hot and saturated by moisture: these are ideal conditions for germs and bacteria to proliferate. Microorganisms that could affect compressed air are fungi spores, bacteria spores, and vegetative forms defined as microbiological particles.
During dust sampling, the air coming from the line is deviated within a filter holder containing a proper filter to the following assessment. Determination of dust quantity on filters is performed by a gravimetric method with differential weighing (it must always be taken into consideration that on the filter, in addition to the dust, there may be drops of aerosol which must be deducted from the gravimetric weighing).
The sampling of aerosols and gas contained in the stream of compressed air is performed by capture on glass fibre filters and adsorption on a specific solid support (coal).
The air stream coming from the line is then conveyed on the plate surface containing the nutrient medium that is chosen based on microbiologic research to be performed (microorganisms at 30 °C - TSA Tryptic Soy Agar; moulds and yeasts – Sabouraud Dextrose Agar).
The sampling speed is always regulated so as to have a steady and well-defined flow rate.
Depending on the analyte, sampling results are expressed as mg/m3 (dust and oils) or CFU/m3 (microorganisms).
There are no specific, internationally valid laws regulating the use of compressed air in the food industry. Nevertheless, the quality of compressed air has a direct impact on food safety and therefore companies must monitor it.
The BRC (British Retail Consortium) standard and the British Compressed Air Society (BCAS) developed a conduct code setting minimum purity levels (quality) for compressed air and defining the allowable levels for impurities, water, and oil, with reference to the levels specified in the ISO 8573-1 Standard. This international standard provides some general information and specifies the allowable quantity for the three main contaminants (solid particles, water, and oil) per cubic metre of compressed air, and sets some quality classes based on these contaminants.
According to the conduct code, the risk of microbiological contamination must be defined by HACCP analyses. Microorganism level must be within the reference values given in the ISO 8573-7 Standard.