When the disturbance exceeds the threshold value (that has been predetermined during calibration), the contaminated product is “rejected” (full automatic rejection or after stoppage of the production line). The determination of the threshold value is a critical phase: in fact, the lowest threshold value might appear the best solution, but this is often cause of an excess of rejected packages, because the metal detector detects even tiniest metal particles, as those present in the inks used for printing labels. Last generation metal detectors use variable-frequency generators, to best adapt the generated signal to the electromagnetic properties of the tested product. Metal detectors detect ferrous (Fe), non-ferrous (NFe) metals, and stainless steel (SS). Location of ferrous objects is easier; non-ferrous metals are non-magnetic metals (such as brass, aluminum, or copper) are almost as easy to detect as ferrous metals. Stainless steels (among which the most common types are AISI304 and AISI316) are not magnetic and have low electric conductivity, and for this reason they are by far the most diffi cult of metals to detect. Some of the latest developments (see image by courtesy of the company CO-EL based in Modena) operate at very high frequencies, and can detect both ferrous and non-ferrous metals. Small spheres of the different metals and with certifi ed diameter are available for testing the sensitivity of the installed metal detector. Metal detector sensitivity can be affected by many factors, as: type of product (dry, humid, wet, frozen, etc.); product size; product variability; head aperture; environmental conditions (vibrations, electric and electro-magnetic disturbances, disturbances in the conveying system); correct use by the operator (training); user-friendly interface; reliable control and management software; automatic product identifi cation; variable frequency (from 40 to 70 frequencies); DSP technology (digital signal processing); IP69 protection (in humid environments); head with self-diagnostic function; The food industry mainly implements in-line metal detectors (i.e. installed on the conveyor belt); the assembly can include also automatic rejection unit with wastes collecting drawer and a variety of alarms and signal lights. Metal detectors can be combined also with checkweighers. Powder or granulate products and bulk material can be checked by means of free-fall gravity metal detectors, for mechanical or pneumatic conveying systems. In these devices, the product passes through the detecting head and, if contaminated, the rejection system deviates it to a rejection line. There are different apertures for product passage, as well as “Slim Line” confi gurations for applications in limited spaces. Pipeline metal detectors are used to inspect a variety of food products that travel through process pipelines.
Origin of metal fragments in the meat processing industry
Special attention must be devoted to the meat sector; the high humidity of meat, combined with its complex texture (water, fats, and proteins) and the variable quantity of product on the conveyor, make the detection of metal foreign bodies extremely diffi cult. In this sector, the danger of metal foreign bodies is very high (exactly the contrary of what one might think), and is due to a variety of reasons, some of which are not quite obvious, as for instance: • Pork tripe (utilized in large amounts for the production of Mortadella): farm animals are fed with compound feed products from different sources that are not chewed for a long time; hence it is not infrequent that they swallow metal objects such as chain links, wire etc. present in the feedstock as well as in the farm fencing. Often these metal objects are not expelled and remain in the gastrointestinal apparatus of the animals until slaughter. • Beef, horse, pork, cattle meat: the needles used for injecting medications can break and remain in the tissue; if farmer does not signal this problem when the animal is sent to the slaughter line, the needle fragment can defi nitely end up in a steak; the same can occur with broken or forgotten scalpels, used for the sterilization of animals (mainly pork), that are enclosed by the growing animal and fi nally end up in bacon; • Various products, processed and unprocessed: next to the dangers originating in the farms, one should not forget those coming from industrial processing, including the slaughtering line: loosened nuts and bolts falling out from the processing machines due to vibrations; objects coming from on-site emergency maintenance; pins of order sheets (which should be banished from the processing area), etc. Speaking of meat, the most involved products certainly are ground meat and its derivatives, that is sausages, hamburgers, skewers, “ready to cook” products, etc., because they are potentially susceptible of a variety of contamination sources.
