Cleaning and Washdown of Scales in Fish Processing
Clean scales in fish processing with a full cycle: remove residues, pre-rinse with cold water, apply an alkaline detergent for 10-15 minutes, rinse, disinfect and finish with a fresh-water rinse. Avoid hydrochloric acid and carbon steel brushes on stainless steel, keep pressure jets off seals, and check zero afterwards.
A scale in a fish processing line or on a sorting deck is a food contact surface, and it may be cleaned several times a day. Good cleaning protects food safety, keeps the zero stable and prevents the corrosion that shortens scale life. This article covers the legal hygiene requirements, hygienic design principles, the cleaning cycle, chemical compatibility with stainless steel and a routine that works at sea.
What EU hygiene law requires
EU food hygiene law requires equipment that touches fishery products to be corrosion-resistant, easy to clean and kept clean.
- All vessels: surfaces in contact with fishery products must be of suitable corrosion-resistant material that is smooth and easy to clean; equipment must be made of corrosion-resistant material that is easy to clean and disinfect (Regulation (EC) No 853/2004, Annex III, Section VIII, Chapter I, Part I.A, points 2-3).
- Landing: unloading and landing equipment in contact with fishery products must be easy to clean and disinfect and kept in good repair and cleanliness (Chapter II, point 1(a)).
- Primary production: operators must keep equipment, containers, crates and vessels clean and, where necessary after cleaning, disinfect them appropriately (Regulation (EC) No 852/2004, Annex I, Part A, point 4(b)).
- Establishments: food contact equipment must be effectively cleaned and, where necessary, disinfected at a frequency sufficient to avoid any risk of contamination, and installed to allow cleaning of the equipment and surrounding area (Annex II, Chapter V).
Hygienic design: a scale that can be cleaned
Cleaning cannot fix a design that traps soil. The European Hygienic Engineering and Design Group (EHEDG) sets out design criteria that apply directly to weighing equipment. The figures below are from EHEDG Document 8, 2nd edition (2004); the current Guideline 8, 4th edition (2025) is available free from EHEDG.
| Principle | EHEDG criterion | What to look for on a scale |
|---|---|---|
| No crevices | Avoid metal-to-metal joints other than welding; seals and gaskets must not leave crevices (6.2) | Continuous welds, no overlapping plates or exposed bolted joints in the product zone |
| Radiused corners | Internal corners preferably 6 mm radius or more, minimum 3 mm; avoid sharp 90° corners (6.2) | Rounded platform edges and frame corners |
| Surface finish | Large product contact areas Ra 0.8 µm or better, free from pits, folds and crevices (6.3) | Smooth, undamaged stainless surfaces |
| No threads in product | Eliminate product contact with screw threads (6.2) | Covered or relocated fasteners |
| Drainability | Equipment must be self-draining; horizontal surfaces should be avoided and should slope (6.4) | No pooling on housings or under platforms |
Material choice is part of hygienic design; see AISI 304 vs 316 vs 316L.
The cleaning and disinfection cycle
FAO guidance for seafood plants describes a complete cycle in which each step depends on the previous one (FAO Fisheries Technical Paper 444, section 7.2.2):
- Prepare. Remove products, bins and containers; dismantle or open equipment so that all surfaces are accessible; protect electrical installations and sensitive systems against water and chemicals.
- Remove gross soil. Scrape or brush off fish residues. Do not start by spraying floors and machinery with the pressure hose before all food products are removed.
- Pre-rinse. Rinse preferably with cold water, so that proteins do not coagulate on the surface.
- Clean. Apply the detergent and mechanical action. A moderately alkaline detergent, normally used for high-protein foods such as fish, typically needs 10-15 minutes contact time. Acid cleaners remove mineral scale.
- Rinse. Remove all detergent, because residues can inhibit disinfection.
- Disinfect. Use heat or a chemical disinfectant for the required contact time; effective disinfection is only possible after effective cleaning.
- Final rinse and dry. Rinse off disinfectant where required, reassemble and allow to dry.
FAO also notes that biofilm bacteria can be up to 1 000 times more resistant to common disinfectants than free cells, and that cleaning is often done at 60-80 °C where this is economical. For a scale, "reassemble" includes checking that the platform sits freely and the zero returns before weighing resumes; see routine checks on board.
Cleaning agents and stainless steel: what is compatible
| Agent or tool | Use on stainless scales | Source |
|---|---|---|
| Alkaline detergents | Standard for protein and fat soils from fish | FAO 7.2.2 |
| Chlorine disinfectants (hypochlorite) | At about 200 ppm free chlorine and ambient temperature, non-corrosive to high-quality stainless steel; more corrosive at low pH; rinse thoroughly, since chlorine bleaches left on a surface can lead to corrosion | FAO 7.2.2; Nickel Institute 9001 |
| Hydrochloric acid cleaners | Avoid for routine cleaning: they generate ferric chloride, a strong pitting agent | Nickel Institute 9001 |
| Acid cleaners in general | Rinse chlorides away with ample, preferably hot, water before applying | Nickel Institute 9001 |
| Carbon steel brushes, steel wool | Avoid: embedded particles can lead to rusting | Nickel Institute 9001 |
| Soft cloth, sponge, fibre brushes, plastic or stainless pads | Recommended; rub along the polish lines | Nickel Institute 9001 |
The Nickel Institute handbook on cleaning and descaling stainless steels adds three general rules: use the mildest procedure that works, rinse thoroughly with fresh water after every cleaning operation, and wipe dry to avoid water marks. On a vessel the fresh-water rinse matters twice over, because it also removes seawater salts. Always follow the detergent supplier's instructions and ask the equipment supplier before using chlorine or other aggressive disinfectants, as FAO recommends.
High-pressure hot water: useful, with limits
Hot high-pressure water removes soil quickly, but its use has to respect both the enclosure rating of the equipment and the cleaning sequence.
- Know the rating. The IEC 60529 IPX9 test uses water at 80 °C, 15 l/min and a 175 mm distance; cleaning closer, hotter or for longer on one spot goes beyond what was tested. See IP67, IP68 and IP69K explained.
- Aim at surfaces, not joints. Avoid pointing the jet directly at cable glands, connector caps, keypad membranes and gasket lines.
- Follow the sequence. Pressure hoses should not be the first step while products are still present (FAO).
- Protect the load cell path. Keep the jet off the gap between platform and frame, where debris can wedge and cause weighing errors.
An example cleaning routine for a marine scale
The frequencies below are an example to adapt to your HACCP plan and the equipment manual, not a legal requirement.
| When | Task |
|---|---|
| Between hauls or batches | Remove fish residues and ice; rinse with clean water; check zero |
| End of each shift or production day | Full cycle: gross soil removal, pre-rinse, alkaline detergent, rinse, disinfect, fresh-water rinse, dry; lift or open platforms where the design allows |
| Weekly | Inspect welds, gaskets and cable glands for damage or trapped soil; check for rust spots or discolouration |
| Monthly or as scale forms | Descale with a suitable non-chloride acid cleaner, followed by thorough rinsing |
| After cleaning | Verify zero and a check weight before regulated weighing |
How WPL approaches this
The M2 Series has smooth, rounded edges for washdown. The M5 platform scale flips up for cleaning underneath, and the M6 platform scale has a fully sealed AISI 316 construction for daily washdown. The hygiene and materials hub covers construction in more depth.
Frequently asked questions
Can I use seawater to clean a scale on board?
EU hygiene law allows clean seawater for some purposes on vessels, but for stainless steel equipment the final rinse should be fresh water. The Nickel Institute recommends rinsing thoroughly with fresh water after every cleaning operation, and chloride deposits left to dry on stainless surfaces promote pitting and crevice corrosion over time.
Is chlorine bleach safe on stainless steel scales?
At the concentrations used for disinfection, about 200 ppm free chlorine at ambient temperature, FAO describes chlorinated disinfectants as non-corrosive to high-quality stainless steel. Problems arise with higher concentrations, low pH, heat or residues left on the surface, so follow the supplier's dilution and contact time and rinse thoroughly afterwards.
Why does a scale read differently after cleaning?
Water pooled on the platform, ice or debris wedged between platform and frame, or a platform not correctly reseated all change the reading. Temperature changes from hot water can also cause a temporary zero shift. Let the scale drain, reseat the platform, zero it and confirm with a check weight before weighing again.
How often must weighing equipment be cleaned in fish processing?
EU law does not set a fixed interval. Regulation 852/2004 requires food contact equipment to be cleaned and, where necessary, disinfected at a frequency sufficient to avoid any risk of contamination. In practice the frequency is set in the operator's HACCP-based procedures, typically with residue removal between batches and a full cycle at the end of each shift.
Sources
- Regulation (EC) No 853/2004 laying down specific hygiene rules for food of animal origin (Annex III, Section VIII)
- Regulation (EC) No 852/2004 on the hygiene of foodstuffs
- EHEDG Document 8: Hygienic Equipment Design Criteria, 2nd edition (April 2004)
- EHEDG: Guideline 8 Hygienic Design Principles, 4th edition (December 2025)
- FAO Fisheries Technical Paper 444 (2004) Assessment and Management of Seafood Safety and Quality, Chapter 7 Prerequisites to HACCP
- Nickel Institute / AISI: Cleaning and Descaling Stainless Steels, Designers' Handbook Series No 9001
- IEC 60529:1989/AMD2:2013 Degrees of protection provided by enclosures (IP Code), Amendment 2 (preview)
Written and reviewed by WPL Industries weighing engineers. Technical and regulatory content is checked against the cited sources. Editorial policy