Filter Bags for Fiber and Fat Analysis: What to Check Before Switching Suppliers

If your laboratory determines crude fiber, NDF/ADF or crude fat with the filter-bag method, you already know the arithmetic: the instrument is bought once, but the bags are bought forever. A feed laboratory reporting twenty fiber results a day consumes roughly fifty bags every working day — more than twelve thousand a year. Over a five-year period the bags cost more than the instrument's maintenance, and in many labs more than the instrument itself.

That is why procurement teams keep asking the same question: is there a reliable alternative to the imported original bags? The answer is yes — but only if the replacement is checked properly. A filter bag is not a commodity plastic part. It sits inside the measurement: any mass the bag loses, any ash it contributes, and any particle it lets through shows up directly in your reported percentage. This note lists the five checks we recommend before switching, the real cost math, and a validation protocol you can run in a week.

Five things to check before switching suppliers

1. Pore size — and pore-size uniformity

Pore size is set by the method, not by preference. Fiber analysis (NDF, ADF, crude fiber) is built around a 25 µm bag — the specification used by the widely deployed ANKOM® F57. Crude-fat extraction by the Soxhlet-type bag method (AOCS Am 5-04) needs a much finer bag, 2–3 µm, to hold back fine sample particles while letting solvent and dissolved fat pass — the class of the ANKOM XT4. Our FB25 fiber bag is made at 25 µm as standard, with 15 µm and 10 µm available on request at otherwise identical specifications; our GB3 fat bag is made at 3 µm.

Uniformity matters as much as the nominal number. A bag whose pores wander between 20 and 35 µm will pass fine particles on one sample and retain reagent on the next, and you will see it as scatter between replicates. Ask the supplier how pore size is controlled across a production lot, not just what the nominal figure is.

2. Mass loss during acid and alkali treatment

Fiber methods boil the bag in acid and alkali solutions, and the lignin step (ADL) exposes it to 72% sulfuric acid. Whatever the bag loses in those baths is weighed as if it were sample residue — a direct positive bias on your result. Specify a maximum treatment loss and put it in the purchase specification. The FB25 is specified at below 1% mass loss after the full acid–alkali treatment sequence.

3. Ash residue

Crude-fiber and ADL methods end with ashing at 525–600 °C, and the bag is ashed with the sample. Any inorganic residue the bag leaves behind is weighed as ash and inflates the result. A good bag is essentially ash-free; check the specification for ash content rather than accepting "low ash" as a marketing phrase.

4. Temperature and solvent resistance

Bag-method extraction runs hot: drying at 100–105 °C is part of every fiber and fat procedure, and fat extraction works with petroleum ether, hexane or diethyl ether. The bag material must hold its structure and its mass through all of it. Both the FB25 and the GB3 are specified for continuous exposure at 105 °C; the GB3 uses a hydrophobic polymer matrix designed for organic-solvent extraction.

5. Batch-to-batch weight consistency

Every result is computed from a tare weight: empty bag, bag plus sample, bag plus residue. If empty-bag weight drifts between production batches, your blanks drift with it and your control charts fill up with noise that has nothing to do with your samples. Ask for the empty-bag weight tolerance, and weigh ten bags from each new lot before putting them into service — it takes five minutes and catches bad lots before they touch data.

What a bag actually costs you per reported result

A reported result does not consume one bag. Standard practice adds blank bags and duplicate determinations, so each reported figure consumes two to three bags. Run the numbers for a mid-size feed laboratory:

At twelve thousand bags a year, the unit price difference between suppliers is no longer a rounding error — it is a budget line. Sonnen fiber and fat filter bags are priced at roughly half the level of the imported originals, at specifications our customers validate against the same check samples. For the laboratory in the middle row, that difference alone funds an analyst's instrument budget for the year.

How to validate a new bag without re-validating your whole method

You do not need a formal method revalidation to change a consumable supplier. You need evidence that results do not move. The protocol our customers use:

1. Request trial bags — we supply FB25 and GB3 trial quantities free of charge for exactly this purpose.
2. Pick ten to twenty routine samples that bracket your usual range, plus your in-house control sample if you have one.
3. Run every sample in parallel: current bag against trial bag, same batch, same operator, same instrument.
4. Include blank bags from both lots in every run.
5. Compare the paired results: the mean difference should sit inside your normal repeatability, and the RSD of the trial-bag results should be no worse than your current baseline.
6. If the paired data agree, switch. Keep the paired dataset on file — it answers any auditor who asks why the consumable changed.

In our home market this is how the large feed groups switched: a trial run, a paired dataset, then standing monthly orders. Most laboratories that complete the trial reorder.

The two bags we make

Both bags run on our own instruments — the FB25 in the F20/F22 fiber analyzers, the GB3 in the G100/G350 fat extractors — and both are manufactured to the pore-size specifications the imported reference bags made standard. If your laboratory works to those methods, the validation protocol above will tell you in a week whether the switch is safe. The trial bags are free; the paired dataset is yours to keep.