Metal Detection vs X-Ray Inspection: How Food Factories Catch Foreign Bodies

How a food metal detector works

The detector head contains a transmitter coil flanked by two receiver coils in a precisely balanced arrangement. With nothing in the aperture, the two receivers cancel each other exactly. When metal passes through, it disturbs the electromagnetic field — conductive metals by carrying eddy currents, magnetic metals by distorting the field — and the balance shifts by an amount the electronics can detect, triggering the reject mechanism. Ferrous metal is the easiest catch; non-ferrous metals like aluminium and brass come next; non-magnetic stainless steel, the most common metal in a food plant, is the hardest.

The complication is the product itself. Wet, salty, or dense products — meat, cheese, brine-packed goods, warm bread — are slightly conductive and produce a "product effect" signal of their own, which forces sensitivity down. This is why operating frequency matters: modern detectors such as A&D's current metal detection line offer multiple selectable frequencies with automatic selection per product, learn each product's effect into memory, and can compensate for oxygen-absorber sachets that would otherwise trigger false rejects in dried-food packs.

Sensitivity is also a function of aperture size: the smaller the opening the product passes through, the smaller the fragment the detector can resolve. The published figures for A&D's AD-4971 range show both effects clearly — note how much larger a stainless sphere has to be before it is detected, compared with iron of the same aperture.

How X-ray food inspection works

An X-ray system works like a continuous X-ray photograph of the line: a generator above the belt transmits a fan-shaped beam through the product onto a detector array below, and the system builds a greyscale image where darkness corresponds to how much each point absorbed. Dense contaminants — metal, glass, stone, bone, hard rubber, dense plastics like PVC and PTFE — absorb more than the surrounding food and appear as dark spots the image-processing algorithms flag and reject.

Because it reads density rather than conductivity, X-ray sees straight through aluminium foil and metallised film — the packaging that blinds a metal detector — and it inspects more than contamination: the same image verifies component count, estimates mass, and catches broken or missing pieces and trapped product in seals. A&D's AD-4991 ProteX X-ray line covers belt widths from 250 mm to 600 mm and products up to 25 kg, with IP66 construction and a guided automatic setup that learns a new product in three passes.

What each one cannot see

A metal detector finds only metal. Glass, stones, bone, plastics, and wood pass through untouched, and metal hidden inside foil or metallised packaging is masked by the packaging signal. Very fine stainless wire and small stainless spheres in a high-product-effect application are its hardest legitimate targets — which is exactly what auditors probe with certified test pieces.

X-ray finds only what is denser (or much less dense) than the surrounding product. Low-density contaminants — hair, insects, paper, cardboard, string, most soft plastics, thin films — do not image reliably, and very small or very thin glass fragments can fall below the resolution the belt speed and product depth allow. Neither technology replaces upstream controls: sieves, magnets, glass-and-brittle-plastic registers, and supplier controls remain part of the foreign-body programme. An inspection machine is the last line of defence, not the whole defence.

Metal detector or X-ray: how to choose

Let the packaging and the risk assessment decide. Product in plastic, paper, or glass-free packaging with metal as the dominant risk: a metal detector is the economical, proven control — the standard choice for most UAE lines. Product in foil or metallised film: X-ray, because the metal detector cannot see into the pack. Glass jars and bottles: X-ray, which handles glass-in-glass inspection with defined blind-zone caveats at the neck and base. Bone risk in poultry and fish, or a retailer code that demands detection of glass and stone: X-ray.

Cost still separates them — a metal detector is typically a fraction of the price of an X-ray system, smaller, and simpler to run — so many factories fit metal detection on every line and reserve X-ray for the high-risk or customer-mandated lines. High-assurance operations sometimes run both: metal detection early (protecting downstream machinery) and X-ray at end of line as the final CCP.

The table below is the short version. Find the row that matches your packaging or your dominant hazard, and the technology follows from it.

Running inspection as a HACCP critical control point

Whichever technology you choose, the machine becomes a critical control point with defined critical limits: the certified test-piece sizes (typically ferrous, non-ferrous, and stainless spheres for metal detection; glass, metal, and ceramic pieces for X-ray) that the system must reject at your line speed. Verification means running those test pieces at defined intervals — commonly at start-up, at product changeover, and every one to two hours — confirming the reject actually diverted the test pack, and recording the result.

A&D's inspection range is built for this regime: stored per-product settings, reject confirmation, and logged inspection history that your auditor can trace, with password control over settings changes. A&D Gulf supplies the matching certified test pieces and provides traceable calibration and service from JAFZA, so the CCP evidence chain — machine, test piece, calibration record — comes from one supplier.

The A&D inspection line in the Gulf

A&D builds the full end-of-line set: metal detection, AD-4991 ProteX X-ray, and AD-4961 series in-motion checkweighers that verify declared weight on every pack — all hygienic stainless, washdown-rated, and designed to integrate via simple digital I/O without complex PLC work. A&D Gulf FZE supplies, integrates, calibrates, and services the range across the UAE and GCC from Jebel Ali, including line reviews for factories specifying their first inspection CCP. See the full food factory line on our food inspection systems page.