The 7-22mm standard comes from a compliance policy guide note which clarifies that the products will be considered adulterated and appropriate for seizure, basically giving inspectors a clear direction when those egregious cases are found at import or during other inspection events. It does not mean that other objects smaller are acceptable, only that those values may play a role in your risk assessment.
Added metal fragments of any size would fall under a definition of adulteration:
(a) Poisonous, insanitary, etc., ingredients (1) If it bears or contains any poisonous or deleterious substance which may render it injurious to health; but in case the substance is not an added substance such food shall not be considered adulterated under this clause if the quantity of such substance in such food does not ordinarily render it injurious to health.1 (2)(A) if it bears or contains any added poisonous or added deleterious substance (other than a substance that is a pesticide chemical residue in or on a raw agricultural commodity or processed food, a food additive, a color additive, or a new animal drug) that is unsafe within the meaning of section 346 of this title; or (B) if it bears or contains a pesticide chemical residue that is unsafe within the meaning of section 346a(a) of this title; or © if it is or if it bears or contains (i) any food additive that is unsafe within the meaning of section 348 of this title; or (ii) a new animal drug (or conversion product thereof) that is unsafe within the meaning of section 360b of this title; or (3) if it consists in whole or in part of any filthy, putrid, or decomposed substance, or if it is otherwise unfit for food; or (4) if it has been prepared, packed, or held under insanitary conditions whereby it may have become contaminated with filth, or whereby it may have been rendered injurious to health; or (5) if it is, in whole or in part, the product of a diseased animal or of an animal which has died otherwise than by slaughter; or (6) if its container is composed, in whole or in part, of any poisonous or deleterious substance which may render the contents injurious to health; or (7) if it has been intentionally subjected to radiation, unless the use of the radiation was in conformity with a regulation or exemption in effect pursuant to section 348 of this title
Metal is an added substance, it isn't unavoidable, it's not naturally occurring, is not an approved food additive at any usage level, and has the potential to render food injurious to health, even if it isn't in the range identified in the CPG as extremely likely to cause injury. FDA has enforced this position in various warning letters:
FDA went after this company for consumer complaints of metal shavings, not pieces: https://www.fda.gov/...4/ucm382794.htm
While this company had 7mm pieces in consumer complaints, FDA wrote a scathing warning letter outlining the level of validation and verification they expect from metal detection where it is a known hazard: https://www.fda.gov/...s/ucm349216.htm
Fraying pieces of metal mesh without control: https://www.fda.gov/...6/ucm506343.htm
CC's firms warning letter would read something like this:
Your firm failed to take effective measures to protect against the inclusion of metal or other extraneous material in food, as required by 21 CFR 110.80(b)(8). Specifically,
Our inspector noted consistent metal detector hits on your line which operators explained was common for equipment running slowly. While we recognize that you have implemented controls to prevent the inclusion of metal in your products, the use of equipment in a manner that results in the deterioration of metal surfaces demonstrates that your firm has failed to properly maintain plant equipment as to be adequately cleanable and properly maintained, as required by 21 CFR 110.40(a). 21 CFR 110.40(a) requires all plant equipment and utensils shall be designed and of such material and workmanship as to be adequately cleanable, and shall be properly maintained. The design, construction and use of equipment and utensils shall preclude the adulteration of food with lubricants, fuel, metal fragments, contaminated water, or any other contaminants. All equipment should be so installed and maintained as to facilitate the cleaning of the equipment and of all adjacent spaces.
So, ultimately, I would be concerned that if you're getting 50 hits that some stuff is making it through, and FDA warns about this as well in the seafood haccp guidance:
Alternatively, metal fragments may be detected in the finished food by an electronic metal detector. The use of electronic metal detectors is complex, especially with regard to stainless steel, which is difficult to detect. The orientation of the metal object in the food affects the ability of the equipment to detect it. For example, if a detector is not properly calibrated and is set to detect a sphere 0.08 inch (2 mm) in diameter, it may fail to detect a stainless steel wire that is smaller in diameter but up to 0.9 inch (24 mm) long, depending on the orientation of the wire as it travels through the detector.
Here are additional references below, however they're all guidance, which means FDA does not have to abide by them if you use them to release product, they only have to abide by the adulteration standard and equipment GMP's referenced above, which would clearly consider any metal you add to the food may render it injurious to health and is a result of poor design, construction, and use of equipment.
FDA has some notes on metal inclusion in the seafood haccp guidance:
Once introduced into a product, metal fragments may be removed from the product by passing it through a screen, magnet, or flotation tank. The effectiveness of these measures depends on the nature of the product. These measures are more likely to be effective in liquids, powders, and similar products in which the metal fragment will not become imbedded. Alternatively, metal fragments may be detected in the finished food by an electronic metal detector. The use of electronic metal detectors is complex, especially with regard to stainless steel, which is difficult to detect. The orientation of the metal object in the food affects the ability of the equipment to detect it. For example, if a detector is not properly calibrated and is set to detect a sphere 0.08 inch (2 mm) in diameter, it may fail to detect a stainless steel wire that is smaller in diameter but up to 0.9 inch (24 mm) long, depending on the orientation of the wire as it travels through the detector. Processing factors, such as ambient humidity or product acidity, may affect the conductivity of the product and create an interference signal that may mask metal inclusion unless the detector is properly calibrated. You should consider these factors when calibrating and using this equipment. Finally, the hazard of metal inclusion may also be controlled by periodically examining the processing equipment for damage that can contribute metal fragments to the product. This measure will not necessarily prevent metal fragments from being incorporated into the product, but it will enable you to separate products that may have been exposed to metal fragments. Visually inspecting equipment for damaged or missing parts may only be feasible with relatively simple equipment, such as band saws, small orbital blenders, and wire mesh belts. More complex equipment that contains many parts, some of which may not be readily visible, may not be suitable for visual inspection and may require controls such as metal detection or separation.
Recent preventive controls guidance goes over the CPG guidance again as well:
Metal: Metal-to-metal contact during processing can introduce metal fragments into products. For example, metal fragments can break off during mechanical cutting and blending operations, and some metal equipment has parts that can break or fall off, such as wire-mesh belts. FDA’s Health Hazard Evaluation Board (FDA, 2005e; Olsen, 1998) has supported regulatory action against products with metal fragments of 0.3 inches (7 mm) to 1.0 inches (25 mm) in length. Such fragments have been shown to be a hazard to consumers. Metal hazards can be controlled by the use of metal detection devices or by regular inspection of at-risk equipment for signs of damage.
The Juice HACCP guidance also includes the CPG reference:
1.2 Metal Fragments
We can recommend several possible ways to establish control measures for metal fragments in juice.
One way involves the use of on-line metal detection equipment. With this method, the equipment continuously monitors the product after the last step at which metal inclusion is reasonably likely to occur (e.g., after bottling and sealing of the juice) at a process step designated for metal detection. The critical limit might be designated as "no metal fragments in the finished product." The following illustrates some of the elements that might be entered into your HACCP plan.
- What is the critical limit? No metal fragments in finished product (Note: FDA's Health Hazard Evaluation Board has supported regulatory action against product with glass fragments of 0.3" (7 mm) to 1.0" (25 mm) in length. See also FDA Compliance Policy Guide 555.425).
- What will be monitored? The presence of metal fragments in containers passing the CCP.
- How is monitoring done? By the use of metal detection equipment.
- How often? Continuously. Each container is subjected to detection. We recommend that you confirm that the device is operating correctly at least at the start of each production day.
- Who should perform the monitoring? Monitoring is performed by the equipment itself. We recommend that a check be made at least once per day to ensure that the device is operating correctly.
A second way to control metal fragments involves the use of a separation device such as a screen after the last step at which metal inclusion is reasonably likely to occur, at a process step designated for screening. For this approach (see example HACCP plans for Pasteurized Refrigerated Apple Juice and Not-from-concentrate Orange Juice in section VII):
- The critical limit might be designated as "screen is functional."
- Monitoring may be done by a daily visual check for screen integrity.
- We recommend that verification include periodic calibration testing to ensure that the screen retains its separation capability for metal particles of a specific size. In establishing this size, we recommend that you consider that FDA's Health Hazard Evaluation Board has supported regulatory action against product with glass fragments of 0.3" (7 mm) to 1.0" (25 mm) in length. (See also FDA Compliance Policy Guide 555.425).