Detection of Physical Hazards

Food recalls can happen due to a variety of reasons, including the detection of foreign bodies in food products. In 2016, a very well-known chocolate manufacturing company has to withdraw its products from the market due to the incidence of finding a piece of plastic from it. The presence of foreign objects in food products is considered as one of the major reasons for consumer complaints in the food industry. They represent either a quality defect affecting the company and brand reputation or a food safety hazard, due to potential injury or choking. Foreign bodies are a sensitive issue for consumers and therefore require special attention when the product has to be either directly consumed from the packaging (e.g., ready to drink, ready to eat, confectionery, ice cream) or while feeding a third person (e.g., babies or the elderly).

Understanding Physical Hazards

Physical hazards are either foreign materials unintentionally introduced to food products (ex: metal fragments in ground meat) or naturally occurring objects (ex: bones in fish) that are hazardous to the consumer. A physical hazard contaminates a food product at any stage of production. Foreign bodies might be differentiated from the product by any of their physical characteristics: magnetic or electrical conductivity, density, color, shape, or dimension. Food processors should take adequate measures to avoid physical hazards in food. Physical hazards can be categorized into the following two groups based on the incidence of its occurrence:

Extraneous Foreign Bodies:

These can be defined as unwanted/unintentional material which is not product-associated and occurs in the product as the result of various agricultural, processing, transportation, and storage activities throughout the supply chain. e.g., glass, metal particles, plastic pieces, wood, hair, strings, stones, pebbles, etc.

Intrinsic Foreign Bodies:

These are defined as product-associated unwanted material which may present naturally in the raw material/product. E.g., eggshells, feathers, bone, hide (rind), cartilage, wood, leaves, larvae, eggs, etc.

Sources of Physical Hazards

Common sources of physical hazards in food include:

  1. Glass: light bulbs, glass containers, and glass food containers
  2. Metal: fragments from equipment such as splinters, blades, needles, utensils, staples, etc.
  3. Plastics: material used for packaging; fragments of utensils used for cleaning equipment
  4. Stones: incorporated in field crops, such as peas and beans, during harvesting
  5. Wood: splinters from wood structures and wooden pallets used to store or transport ingredients or food products
  6. Natural components of food: hard or sharp parts of food (ex: shells in nut products).

 Regulation for Controlling Physical Hazard

FDA declares any food item to be adulterated if it contains:

  1. a hard or sharp object between 7mm and 25mm in length in ready-to-eat food, whenever the product requires minimal preparation or further processing, which would not (or may not) eliminate the foreign object.
  2. any object of any length up to 25mm, in food intended for special groups such as infants, surgery patients, and the elderly.

The 7mm-threshold is based on several case studies related to foreign object contaminations, which states that objects below 7mm rarely cause injuries to individuals not in the special-risk group. Hard or sharp objects longer than 25 mm will also be investigated by the authorities, but in general, they do not lead to immediate product recalls.

 Physical Hazard Management Devices

Devices for preventing or minimizing physical hazard can be grouped into the following two categories:

A. Sorters:

This category of equipment is known to remove foreign bodies from a product identified as contaminates (e.g., agricultural material, nuts, cacao, or green coffee beans). For this equipment, no corrective actions are taken when foreign bodies are identified through these devices, except in the case of abnormally high contamination or if a non-conventional type of material is discovered. Generally, they do not have warning or reject mechanisms, for example, a filter or a magnet will not notify when a contaminant is collected. Sorters must be installed at locations such as:

  1. on incoming raw materials.
  2. after hazardous process steps, often involving metal-metal contact, like rotary valves, grinding, mixing, or cutting.
  3. before some processes where foreign bodies might be broken up into smaller parts or could damage the equipment itself, generating more contamination.
  4. upstream of a metal detector or X-ray as the last barrier to reduce the likelihood of an alarm.

       Following is the list of equipment that falls in the category of sorters:

Drawers Magnet:

It is ferrous metal removal equipment that can be either installed in the existing pipeline or outlet of various equipment like pulverizer, sifter, blenders, & classifier, or any other outlet location. They generally consist of rows of round magnetic tubes that are assembled into drawers. Drawer Magnets are recommended for fine cohesive materials such as powdered sugar, cornstarch, flour & protein powder. The drawer magnets are available with either Ferrite magnets or with high intensity, rare earth Nd-Fe-B Magnets.

Magnet Traps:

These are ferrous traps providing magnetic protection for liquid lines and processing equipment. They preserve product purity by removing small particles of magnetic scale, rust, and fine iron contamination. They capture fine ferrous particles that are created by the wear of upstream processing equipment and often pass undetected through electronic metal detectors. These are typically placed in front of pumps, screens, and mills to protect vital plant equipment from costly metal damage and system downtime. These are installed in pipelines conveying liquid.


Sifters are specifically designed to remove oversize contamination from liquid or powder. These are available in a wide variety of sizes and options. These can be customized to fit into exact process specifications and is ideal for high-capacity safety screening of liquid slurries and powders. Mesh Size is one of the most important process parameters for designing and selecting of the sifter. The term ‘Mesh’ is used to describe the size of an abrasive particle whereas mesh size defined as the number of openings in one square inch of a screen. For example, a 36-mesh screen will have 36 openings while a 150-mesh screen will have 150 openings.

Simplex Filter:

Simplex filters are the basic model of filtration technology. They have a compact design and are extremely easy to clean. The simplex strainers are preferably used where systems or the filter can be switched off with additional shut-off devices. They serve to protect the system parts from soiling and siltation.

Duplex Filter:

These are designed for applications where the flow cannot be shut down to service the strainer screen. Changeover is accomplished by the use of butterfly valves. They are very economical because they are fabricated to meet process-specific requirements.

Optical and Laser Sorters:

They are complex systems adapted to bulk sorting, using various types of lights and cameras to segregate contaminants according to their shape, size, and color. Of the two sorts, laser sorters are more complex and expensive, allowing for structure recognition. We can separate two objects having similar colors by using this technology. Even chlorophyll content, water content, and biological characteristics may be distinguished. Both, optical and laser sorters are surface scan only. They are used on items such as vegetables, fruits, seafood, or dry foods (e.g., nuts, hazelnuts).

B. Detection Equipment

Detection Systems are usually installed at the last stage of the process to control foreign body hazards prior to dispatch to consumers. Detection devices should be installed at the end of the process, at or after packing, when no contamination is expected. These can be also installed at important intermediate process steps also for example at the filling or discharging of intermediate bulk containers. Following is the list of Detection System which is used in the food industry:

Metal Detectors:

Metal detectors are the most common and widely used equipment to detect foreign bodies in the food industry, but they cannot control a large spectrum of non-metal materials. When using this type of equipment, metal objects within a food product produce an unbalance in a balanced radio frequency or magnetic field. This typically generates an electrical signal alarming the equipment operator of the situation. This method can detect very thin metallic objects such as metal rust and aluminum foil. However, certain factors can affect the sensitivity of metal detectors. The type, shape, and orientation of metal play a role in the effectiveness of detection. The equipment’s operating frequency also affects the sensitivity. Other factors like the position of the metal in the aperture, type of product scanned, packaging material, and environmental conditions such as vibration are often referred to as limitations to appropriate detection. Also, non-metallic contaminants, such as rubber and glass, are not detectable since they do not affect the magnetic field.

X-Ray System:

In the case of X-ray detection, the equipment produces an image of the food product that can be analyzed for contamination. Due to their high energy, X-rays can penetrate solid bodies. When going through the inspected product X-rays are attenuated according to the density of matter present. If an object is present with a different density from that of the food product, it will be represented in the form of an image. The ability to spot different densities adds another feature to the equipment i.e. the capacity to identify product defects like missing products in packages, products without filling, or incorrect shape. X-ray systems can detect several foreign bodies such as metals, non-ferrous metals, stones, glass, PVC plastic, bones, ceramic, Teflon plastic, sugar, or flavor clumps. On the other hand, X-ray machines cannot detect some contaminants such as paper, cardboard, wood, hair, insects, thin glass, low-density plastics, and low-density stones.

The application of good manufacturing practice (GMP) and hazard analysis (HACCP) through the whole food supply chain, “from plant to plate,” is the most effective way to prevent and reduce contamination and thereby protect the consumer; this includes, for example, hygienic design of buildings and machinery, training of factory employees, eradication of pests or certification of raw material suppliers. In addition, separators, and sorters (e.g., filters, sieves, magnets, lasers) must be placed on production lines to improve foreign body reduction.


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