A food metal detector works by generating a balanced electromagnetic field. When a piece of metal passes through the aperture, it disturbs that field, and the system registers the change as a detection. Ferrous, non-ferrous and stainless-steel contaminants all disturb the field, though by differing amounts, which is why some metals are easier to find than others.

The product itself also passes through the field, and moist, salty or conductive products can create their own signal that the detector must work around. Effective detection therefore relies on distinguishing a genuine metal signal from the product's own response. This balance between sensitivity and stability is central to how the technology performs on a given food product.

Stainless steel and many non-ferrous metals are harder to detect than iron because they disturb the detector's electromagnetic field less strongly. Ferrous metals are both magnetic and conductive, giving a clear signal. Non-ferrous metals such as aluminium or copper are conductive but not magnetic, while common stainless steels are only weakly magnetic and relatively poor conductors, so their signal is weaker.

As a result, the detectable size for stainless steel is usually larger than for ferrous metal under the same conditions. This gap widens further with difficult products or packaging. Understanding which metals are the realistic risk helps set sensible expectations, since performance is defined by the hardest metal you need to find, not the easiest.

Achievable sensitivity cannot be quoted as a single figure in advance because it is set by the application, not the detector alone. Key influences include the product's moisture, salt and temperature; the aperture size relative to product dimensions; the metal type, with stainless steel usually the hardest; packaging such as foil; line speed; and the operating environment.

Product effect — the signal a moist or conductive product produces itself — often defines the practical limit, especially for wet, salty, hot or fresh products. This is why generic figures can mislead when applied elsewhere. The dependable way to establish realistic sensitivity is to test the actual product, in its true packaging, at the intended aperture and speed.

Aperture size and product dimensions have a strong influence on sensitivity. A metal detector is generally most sensitive at the centre of its aperture and less so near the walls, so a larger aperture than the product needs reduces achievable sensitivity. Matching the aperture closely to the product profile keeps contaminants nearer the most sensitive zone.

Product thickness and orientation matter too, since a contaminant's detectability changes with how the product presents it to the field. The practical guidance is to size the aperture to the product and pack, allowing for handling, rather than oversizing for convenience. Because these factors interact with the product itself, the realistic result is best confirmed for the specific application.

Product effect is the signal that the food itself produces in a metal detector, separate from any contaminant. It arises mainly from conductivity, which is driven by moisture and salt, and sometimes by temperature. Wet, salty, fresh or warm products — such as meat, cheese, bread or brine-packed items — tend to show strong product effect.

The problem is that this signal can resemble or mask a small metal contaminant, limiting how sensitive the detector can be without causing false rejects. It is a physical characteristic of the product, not a fault. Recognising which products carry significant product effect is the first step to setting the detector up realistically and choosing appropriate detection settings.

Product effect is managed during setup rather than eliminated. Modern detectors use techniques such as operating at a suitable frequency and 'phasing out' the product's signal, so the system focuses on the difference a metal contaminant would make. Careful setup with the real product, consistent presentation and stable line conditions all help.

The aim is to suppress the product's own response enough to detect realistic contaminants while keeping false rejects manageable. There are limits: very strong product effect still constrains achievable sensitivity. Because the right settings depend on the specific product, packaging and line, they are established during commissioning and confirmed by testing rather than assumed from a datasheet. In short, compensation reduces the impact of product effect but cannot remove it entirely for difficult products.

Metal detection can be used on wet, salty, hot and frozen products, but each affects performance in its own way. Wet and salty products are conductive and show strong product effect, which limits sensitivity to small metal, particularly stainless steel. Hot products can shift the signal as temperature changes. Frozen products are less conductive but dense and cold, which brings different considerations.

Detectors handle these cases with suitable frequency selection and setup, though there are realistic limits, and for some difficult products or packaging X-ray may be more appropriate. What is achievable depends on the specific product, packaging, dimensions and line speed, so results for these products are best confirmed by testing rather than assumed.

False rejects — where good product is rejected without any real contaminant — usually come from the product or the environment rather than actual metal. Common causes include strong product effect from moisture or salt, inconsistent product presentation, packaging such as foil, vibration, nearby electrical interference, and temperature changes. Settings that are too aggressive for the product also contribute.

Because several factors can combine, false rejects are best diagnosed by looking at the whole situation: product, packaging, setup, line conditions and surroundings. Some level of careful tuning is normal, especially with difficult products. Reducing false rejects without losing genuine sensitivity is a balance, and it depends heavily on the specific product and line conditions.