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Focus on metal detectable plastic bristles

06 November 2016

Debra Smith reports on the findings of an investigation into the detectability, durability and functionality of metal detectable plastic bristles on cleaning equipment used in the food sector. 

Foreign body contamination of foods poses safety and quality issues which can be costly and damaging for food processors.

Metal detection is a well-established method used in the sector to reduce the risk of metal fragments as well as controlling the presence of metal detectable plastics. A recent addition to the metal detectable plastic product range offered to the food industry is the metal detectable plastic brush, incorporating metal detectable bristles. Detectability of these bristles will depend on a number of factors, including the;
• metal content of the bristle, both quantity and type of metal,
• diameter/thickness of the bristle,
• size of the bristle fragment,
• orientation and position of the bristle in the food product,
• conveyor speed,
• food product,
• detector used,
• detector calibration,
• and vibration.

The influence of these factors is variable and accumulative which will affect the detection threshold. Consequently, the ability of a metal detector to detect very small metallic objects is limited.

To determine the metal detectability of metal detectable plastic bristles, as well as looking at their durability and functionality, Vikan, in collaboration with Mettler Toledo, set out to investigate standard metal detectable bristles with regard to their detectability using a Profile Advantage multi-frequency metal detector, with and without the presence of either a wet or dry food product. 

This joint research found that metal detectable plastic bristles were not detectable in the presence of food and packaging. Additionally, to achieve a similar detection to that of a ferrous ball with a spherical diameter of 1.5mm, metal wire lengths of between 3mm and 9mm would be required, as shown in Table 2.

Currently brushes with metal detectable plastic bristles are only available with bristle diameters of 0.35mm, 0.50mm and 0.60mm – much thinner than the metal wires assessed in Table 2.  It can be concluded, therefore, that even longer lengths of metal detectable plastic bristles would be required to achieve the same level of detection.

Using the data from the research, a metal detectable bristle with a cross-sectional diameter of 0.50mm and a length of 50mm gives an equivalent detection to that of a 1.8 mm ferrous ball.

Tests to assess the break strength and elasticity of metal detectable and plastic bristles were performed by Zwick Roell. Plastic (PBT) bristles were found to be 68% stronger and more than twice as elastic as metal detectable bristles. The ability of metal detectable bristled brushware to clean a surface of a wet (tinned chopped tomatoes) and a dry (mix of milk powder and ground coffee) food soil, was compared with that of a standard plastic bristled brush. Based on visual assessment, the metal detectable plastic bristled brush was found to be no more effective at cleaning than the standard plastic bristled brush.

So, it seems unlikely that metal detectable plastic bristles would be detectable in a food product, especially given detector and product accumulative variances, and the fact that the plastic fragments are likely to be small. To detect these small fragments the metal detector sensitivity would need to be set so high that most products would be rejected. The use of metal detectable plastic bristled brushes may, in fact, increase the risk of bristle contamination of food, due to their reduced strength and elasticity, and a perception that any metal detectable bristles will be controlled via the metal detector.

Additionally, metal detectable plastic bristled brushware offer no advantage over plastic bristled brushes with regard to cleaning efficacy. Only relatively thick metal detectable plastic bristles are currently available. However, fine bristled brushes were found to be more effective at removing fine powders, including some allergens. Consequently, the use of thicker bristled brushes may result in poor cleaning efficacy and therefore, increase the risk to the business/consumer.

As an alternative to the use of metal detectable plastic bristled brushware, it is suggested that brushes are regularly inspected and replaced in order to minimise the risk of foreign body contamination from this source, and that brushes of a contrasting colour to the food product be used to enable the plastic bristles to be seen more easily in the product. The use of a brush that minimises bristle loss through good construction is also advised. 
Debra Smith is global hygiene specialist at Vikan.

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