The Ultimate Guide to Food Metal Detection

If you make food for a living, a metal detector is not a nice-to-have bolted to the end of the line — it is the physical guarantee behind your HACCP plan, your retailer approval, and your customer’s trust. Yet most plants buy it the wrong way: by datasheet bullet points, then spend months fighting false rejects, failed audits, or packaging they “didn’t know would be a problem.”

1. Product effect — why wet, salty or frozen food triggers false rejects, and how to stop them.

Each section is a short briefing with a deep-dive link to the full technical guide. SameGram has spent 16+ years building inline inspection systems — 5,000+ machines across 60+ countries, made in a 4,000 m² factory with in-house CNC, laser cutting and robotic welding. We wrote this from the questions we hear on real plant floors, not from a marketing deck.

Every metal detector projects an electromagnetic field through its aperture. When product passes, the field shifts; real metal creates a sharp spike the controller flags. The trouble starts with product effect: wet, salty, chilled or freshly cooked food is electrically conductive, so it shifts that field in a way that looks exactly like a metal fragment. The detector cannot tell the product from the contaminant, so good packs get ejected — or, if someone “fixes” it by lowering sensitivity, real metal slips through.

Temperature makes it worse. A chilled batch and a warm batch of the same recipe produce different baselines, so the signal drifts across a single shift. Single-frequency detectors — fine for dry crackers or flour — collapse here: keep sensitivity up and false rejects skyrocket; turn it down and you miss fragments. The engineering answer is multi-frequency detection. Multi-simultaneous systems fire high and low frequencies together and subtract the product signature, leaving a clean reading where ferrous, non-ferrous and stainless steel stand out even inside wet, salty product.

SameGram’s DD-390 dual-frequency conveyor metal detector self-learns each product’s profile and suppresses product effect automatically; the DD-720 high-sensitivity metal detector model reaches Fe 0.5 mm / SUS 0.7 mm, and the DD-500 digital metal detector series switches recipes on a touchscreen for mixed lines.

A detector is only as defensible as its paperwork. Retailers and certification bodies treat metal detection as a Critical Control Point (CCP): you must set critical limits (sensitivity per metal type and aperture), verify them with certified test pieces at start-up and every few hours, and keep records an auditor can read.

● Traceable data. Your system should log reject counts, test-piece results and operator access, and export to MES/ERP via RS-232/485, Ethernet or OPC/Profinet — so the record exists whether or not anyone filled in a clipboard.

Before you sign a purchase order, there are ten questions worth asking your supplier — from guaranteed sensitivity by metal type to auto-learn and remote support.

Two packages that look alike on the shelf behave completely differently inside a detection field, and confusing them is the most expensive mistake we see. Metallized film — the whisper-thin aluminium coating on most snack and coffee packs — is weakly and inconsistently conductive. An advanced multi-frequency detector profiles that packaging signal and suppresses it, so you can still inspect after sealing and catch all metal types.

Solid aluminum foil — lidding, foil laminate, foil-wrapped bars — is a continuous conductor. It shields the product and blinds a conventional balanced-coil head; you cannot tune your way out of it. The honest options are: inspect the product before it is sealed in foil, use a true foil-mode detector such as the SameGram DD-220 aluminum foil pack metal detector (Fe 0.6 mm / SUS 2.5 mm, 100+ auto-learn presets), or, where you also need glass, stone or bone, add an X-ray inspection system.

A trustworthy supplier tells you all three, not just the one they sell. SameGram designs the full ecosystem — multi-frequency heads for film, foil-mode heads for foil, and X-ray inspection systems where density matters — so the technology fits the pack instead of the other way around.

Most sensitivity complaints we hear have nothing to do with the detector head. The head is fine; the integration isn't. A conveyorized detector is a five-stage loop — convey, detect, process, reject, discharge — and weakening any link costs you both ways at once.

Start with placement: mark where metal can enter the product (mixers, slicers, fillers), then typically inspect twice — upstream in bulk and end-of-line after sealing. Size the metal-free zone before the conveyor is built: roughly 1.5× the aperture height of clear space each side for fixed metal, about 2× for moving metal such as rollers and reject arms. That is why an inspection conveyor runs 2–4 metres, not the width of the head.

Match the rejector to the pack, not the budget: air jet for light pillow packs, swing-arm flap for cartons, pusher for heavy rigid containers, drop-gate for sachets. The bin must be lockable and QA-controlled. Every SameGram detector ships with standard I/O, RS-232/485 and Ethernet, plus optional OPC/Profinet for direct MES/ERP tie-in. Where floor space is tight, the SCM-4530L2 combination unit weighs and inspects in one frame.

● Meat & Poultry — wet, salty, often chilled. Product effect is the enemy; the DD-390 dual-frequency metal detector with self-learning keeps good packs moving. IP65 washdown construction survives daily high-pressure sanitation.

Senior buyers weight hard numbers over “high sensitivity.” The table below lists representative performance at a stated aperture — your weakest metal type is your real detection limit.

* Sensitivity is aperture-dependent: larger apertures lower detectable size. Published figures at a stated aperture always beat vague “high sensitivity” claims. Verify on your actual product — see the free test below.