In December 2025, a meat company recalled nearly 2,000 pounds of holiday kielbasa after a consumer found a 1.25-inch metal strip in their sausage. The recall made national news and serves as a reminder that metal contamination remains a persistent threat in food manufacturing.
The scale of this problem is significant. According to USDA (FSIS) data analyzed by Sedgwick Brand Protection, foreign material contamination accounted for 58.03 million pounds of recalled meat and poultry products in Q3 2025 alone. That figure represents the highest quarterly FSIS recall volume in over 13 years.
Setting up your first metal detection system comes with a critical decision: choosing between gravity-fed and flow-through configurations. Pick the wrong one, and you could face poor detection sensitivity, excessive false rejects, or equipment that simply does not fit your production line.
Both systems serve the same core purpose (identifying metal contaminants before products reach consumers), but they are engineered for very different applications. Understanding those differences helps you invest in the right equipment from the start.
Let’s give you a simple framework for identifying the right fit for your operation. If you need a refresher on metal detection fundamentals, start with our guide to food metal detector basics.
What Is a Gravity-Fed Metal Detector?
A gravity-fed metal detector (also called a gravity fall, freefall, or vertical fall system) inspects dry, free-flowing products as they drop vertically through the detection aperture. The product falls from a hopper or conveyor discharge, passes through an electromagnetic detection coil, and continues to the next stage of production.
When the system identifies metal contamination, an automatic reject mechanism activates. Most gravity-fed units use a flap diverter or Y-valve that redirects only the contaminated portion of the product stream, minimizing waste while keeping good product moving through the line.
Best suited for:
- Powders (flour, spices, cocoa, powdered milk, protein supplements)
- Granules (sugar, salt, rice, coffee beans, pet food kibble)
- Dry bulk ingredients (nuts, seeds, cereals, dried herbs, tea leaves)
Manufacturers typically position these systems after blending or mixing stages and before final packaging. They integrate well with vertical form-fill-seal (VFFS) machines where product drops directly into bags or pouches. For a deeper look at how these systems operate, see our article on how gravity-fed metal detectors work.
What Is a Flow-Through Metal Detector?
A flow-through metal detector (commonly called a pipeline metal detector) is designed for liquids, pastes, and semi-liquid products that move through sealed pipes. The product is pumped through a non-metallic pipe section surrounded by the detection coil. When contamination is detected, an auto-reject valve diverts the affected product from the main flow.
These systems handle the unique challenges of wet, conductive products. Moisture, salt content, and temperature variations can create signal interference (known as product effect) that makes detection more difficult. Flow-through detectors are built to compensate for these factors while maintaining high sensitivity.
Best suited for:
- Liquids (beverages, juices, oils, milk, liquid flavorings)
- Pastes and slurries (sauces, soups, tomato paste, peanut butter, baby food)
- Viscous products (meat slurries, ground meat, sausage fillings, cheese spreads)
Flow-through systems are typically installed inline with existing piping, positioned before filling, canning, or depositing stages. Understanding product effect and its impact on metal detection is especially important when working with wet or conductive products.
Key Differences at a Glance
The table below summarizes the core distinctions between gravity-fed and flow-through metal detectors. Use it as a quick reference when evaluating which system matches your production requirements.
| Factor | Gravity-Fed | Flow-Through |
| Product type | Dry, free-flowing (powders, granules, bulk solids) | Liquids, pastes, slurries, viscous products |
| Product movement | Falls vertically under gravity | Pumped through sealed pipe |
| Installation | Vertical (requires headroom) | Inline with piping (horizontal or vertical) |
| Reject mechanism | Flap diverter or Y-valve | Auto-reject valve in pipeline |
| Throughput | High-volume bulk flow (varies by aperture) | Continuous flow (limited by pump speed) |
| Best placement | After blending, before packaging or VFFS | Before filling, canning, or depositing |
| Common industries | Bakery, snacks, spices, coffee, grains, pet food | Dairy, sauces, beverages, meat processing, soups |
The fundamental difference comes down to product consistency. If your product flows freely when poured, a gravity-fed system is likely the right choice. If your product requires pumping, flow-through detection is the better fit.
How to Choose the Right System
Use this decision framework to quickly identify which metal detector configuration fits your production needs.
Decision Flowchart
START: What is your product’s physical state?
DRY (powder, granules, pellets, whole pieces)
► Is it free-flowing when poured?
YES → GRAVITY-FED METAL DETECTOR
NO (sticky, clumpy, high moisture) → Consider conveyorized system or consult a specialist
WET (liquid, paste, slurry, viscous)
► Is it pumped through pipes in your process?
YES → FLOW-THROUGH METAL DETECTOR
NO (conveyed in trays, containers) → Consider conveyorized system or x-ray inspection
Keep compliance in mind as well. Major retailers require metal detection in supplier food safety programs, and GFSI certifications (BRC, SQF) evaluate your detection program during audits. When metal detectors serve as Critical Control Points in your HACCP plan, proper system selection directly affects audit performance.
For more guidance on optimal detector placement, review our article on where metal detectors can be used in food production.
What If Your Product Doesn’t Fit Either Category?
Not every product falls neatly into the dry or wet category. High-moisture powders, chunky sauces with solid pieces, or sticky granules can present challenges for both gravity-fed and flow-through systems.
Packaged products (bags, boxes, pouches, trays) typically require conveyorized metal detectors rather than gravity-fed or flow-through configurations. Products with strong product effect (high salt content, significant moisture, or temperature variations) may need multi-frequency detection technology to maintain sensitivity without excessive false rejects.
When products are packaged in metalized film or foil, metal detection becomes extremely difficult. In these cases, x-ray inspection often provides a more reliable solution for identifying contaminants.
The right answer is not always obvious from specifications alone. Product validation testing helps confirm which system delivers the best sensitivity and fewest false rejects for your specific application.
Choosing the Right Partner for Your Metal Detection Needs
At TDI Packsys, we help manufacturers identify the right inspection solution for their specific products and processes. Our team provides gravity-fed metal detectors for bulk dry products and flow-through metal detectors for liquid and viscous applications, along with the expertise to help you choose between them.
Not sure which configuration fits your product best? TDI Packsys offers free product validation testing to determine the right metal detector for your application. Our team will test your actual product samples and recommend the system that delivers the sensitivity and performance your line requires. Request a product validation test today.
