A Step-by-Step Guide to Fish Meal Production

Fish meal is one of the most important products derived from the fish processing industry. It serves as a high-quality protein source used primarily in animal feed – for poultry, pigs, and farmed fish. But how exactly is raw fish converted into the dry, powdery product we call fish meal? The process involves carefully removing moisture and oil from fish through a series of mechanical and thermal operations. Two primary methods exist for this – wet reduction and dry reduction – each suited to different raw materials and scales of operation.

What is fish meal? 🔗

Fish meal is a protein-rich powder or cake produced by drying and grinding whole fish or fish by-products. It is made from wild-caught small marine fish (such as anchovies, menhaden, herring, and sardines) as well as from processing waste like heads, trimmings, and offal. High-quality fish meal typically contains between 60% and 72% crude protein, along with essential amino acids like lysine and methionine, omega-3 fatty acids, and minerals such as calcium and phosphorus. This rich nutritional profile makes it the preferred protein supplement in aquaculture feeds and livestock diets worldwide.

Why is fish meal production important? 🔗

The global aquaculture and livestock industries rely heavily on fish meal as a feed ingredient. According to the University of Florida IFAS Extension, adding fish meal to animal diets improves feed efficiency, growth rate, and nutrient absorption. It also enhances disease resistance and reproductive performance in animals. In aquaculture feeds, fish meal inclusion can range from 5-7% for species like carp and tilapia to as high as 40-55% for trout and salmon. With roughly 4 to 5 tonnes of raw fish needed to produce one tonne of fish meal, efficient processing is critical for both economic viability and resource conservation.

Two methods of fish meal production 🔗

Fish meal can be manufactured through two primary approaches: wet reduction and dry reduction. The choice depends largely on the type of fish being processed, the scale of operations, and the desired quality of the end product.

Wet reduction (wet rendering) 🔗

The wet reduction method is the dominant commercial process used across the world today. As documented by the Food and Agriculture Organization (FAO), this process is especially suited for processing oily fish species like menhaden, herring, pilchards, anchovies, and sardines, where simultaneous production of both fish meal and fish oil is required. The process is continuous rather than batch-based, making it ideal for large-scale industrial operations capable of handling hundreds of tonnes of raw material daily.

In this method, the raw fish is treated through a sequence of steps – cooking, pressing, liquid separation, evaporation, drying, and milling – to efficiently separate the three main fractions: solids (protein and bone), oil, and water.

Dry reduction (dry rendering) 🔗

The dry reduction method is a simpler process used primarily for non-oily (lean) fish and fish offal. In this approach, the oil produced during cooking is not separately removed. The raw material is coarse-ground, fed into a cooker-dryer, dried to about 10% moisture, and then pulverised. This method works well for batch operations and is suitable even when the quantity of raw material available is small. In its most basic form, fish can simply be sun-dried and ground – a practice still used in some regions. However, the oil recovered through dry rendering tends to be of inferior quality compared to the wet method, and removing oil is necessary for producing shelf-stable fish meal from fatty fish.

Step-by-step: the wet reduction process 🔗

Since the wet reduction method accounts for the majority of commercial fish meal produced globally, let’s break down each stage in detail.

Step 1: Cooking 🔗

The first and most critical step is cooking (or heating) the raw fish. The fish are fed into a commercial cooker – a long, steam-jacketed cylinder where they are moved along by a screw conveyor. Cooking serves to coagulate the protein, rupture fat depots, and release physically and chemically bound water and oil.

Getting the cooking temperature and duration right is essential. If the fish are undercooked, the liquids cannot be pressed out properly in the next step. If they are overcooked, the mass becomes too soft for effective pressing. Notably, no drying takes place during this stage – the purpose is purely to prepare the material for mechanical separation. Large fish are typically chopped into smaller pieces beforehand to ensure even heat distribution, as outlined in FAO’s technical documentation on fish meal.

Step 2: Pressing 🔗

After cooking, the coagulated mass is fed into a screw press (also called a twin-screw press), where it is compressed inside a perforated tube. This squeezes out most of the liquid – including water, oil, and dissolved solids – from the solid material.

The solid portion that remains after pressing is called the press cake. Through pressing, the moisture content of the material drops from about 70% to around 50%, and the oil content is reduced to roughly 3-4%. The liquid portion, known as press liquor, is collected and sent for further processing. Effective pressing is important not just for oil recovery, but also for reducing the energy needed during later drying stages.

Step 3: Fluffing (wet milling) 🔗

The press cake comes out of the press in dense, compacted form. Before it can be dried efficiently, it needs to be broken apart. This is done by passing it through a tearing machine (or wet mill), which disintegrates the cake into smaller, more uniform pieces. Fluffing increases the surface area of the material, allowing moisture to escape more easily during drying. It also facilitates thorough mixing with the concentrated stickwater that is added back to the press cake in later stages.

Step 4: Separation of press liquor 🔗

The liquid squeezed out during pressing (press liquor) contains oil, water, dissolved proteins, vitamins, and fine suspended solids. This liquid goes through a multi-stage separation process:

Decanting: The press liquor first passes through a decanter centrifuge, which removes the heavier fine solid particles (sludge). These solids are returned to the press cake. Oil separation: The remaining liquid is heated and passed through a centrifugal separator that splits it into crude fish oil and an aqueous fraction called stickwater. Oil polishing: The crude oil may go through a final polishing centrifuge to remove any remaining water and impurities before being pumped into storage tanks. The refined fish oil obtained is valuable and used in various industries, from aquaculture feeds to edible oil production.

Step 5: Evaporation of stickwater 🔗

Stickwater – the watery fraction left after oil separation – still contains about 8-9% solids by weight. These solids are mostly dissolved proteins, vitamins, and minerals, and they can account for as much as 20% of the total solids in the original fish. Simply discarding stickwater would mean losing a significant amount of nutritional value and creating a waste disposal problem.

To recover these nutrients, the stickwater is fed into multi-effect evaporators, which concentrate it into a thick syrup containing 30-50% solids. This concentrate is sometimes sold separately as condensed fish solubles, but more commonly it is mixed back into the press cake before drying. Adding concentrated stickwater back to the press cake produces what is known as “whole meal” – a nutritionally richer product because it retains the water-soluble proteins, vitamins, and minerals that would otherwise be lost.

Step 6: Drying 🔗

Drying is one of the most energy-intensive stages of fish meal production. The goal is to reduce the moisture content of the material (a mixture of press cake, decanter sludge, and concentrated stickwater) to below 12%, which is low enough to prevent microbial growth and ensure the meal is shelf-stable.

Drying is generally done in two stages. First, the press cake and decanter solids are partially dried. Then the concentrated stickwater is mixed in, and drying is completed to reach approximately 10% moisture. Two main types of dryers are used in the industry:

Direct dryers: Very hot air (temperatures as high as 500°C) is passed over the material as it tumbles inside a rotating cylindrical drum. This method is fast but carries a higher risk of heat damage if not carefully controlled. Indirect dryers (steam dryers): The material is tumbled inside a stationary cylinder fitted with rotating steam-heated discs or coils. The meal never comes in direct contact with flue gases. As explained by the FAO, the drying temperature should not exceed 90°C to avoid impairing the nutritional value of the protein. Indirect dryers also produce far less exhaust air, making odour control much simpler.

Getting drying right is a balancing act. Under-drying leaves enough moisture for moulds and bacteria to grow. Over-drying causes scorching, which reduces the digestibility and amino acid availability of the finished meal.

Step 7: Milling and screening 🔗

After drying, the fish meal is passed through a vibrating screen fitted with magnets to remove any foreign objects – hooks, plastic, metal fragments, and other debris. The screened material then goes through a hammer mill, which grinds it to a uniform particle size. This final grinding step removes lumps and bone fragments, producing the powdered or granular fish meal that is ready for packaging.

Before or during milling, an antioxidant (commonly ethoxyquin) is added to the meal. Fish meal contains polyunsaturated fatty acids that can oxidise and generate heat, which historically has even caused fires on factory ships. The antioxidant prevents this self-heating and preserves the nutritional integrity of the product during storage and transport.

Wet reduction vs. dry reduction: a quick comparison 🔗

Raw material suitability: Wet reduction is designed for oily fish (herring, menhaden, sardines), while dry reduction works best with lean fish and fish offal. Scale of operation: Wet reduction is a continuous process suited for large-scale factories; dry reduction is batch-based and suited for small-scale or irregular operations. Oil recovery: Wet reduction efficiently separates and recovers high-quality fish oil; dry reduction does not remove oil separately, resulting in lower-quality oil trapped in the meal. Product quality: Wet reduction, especially when combined with stickwater recovery, yields a more nutritionally complete “whole meal.” Dry reduction produces a simpler product but one that retains all the water-soluble nutrients within the meal itself. Cost and complexity: Wet reduction plants require significant capital investment in equipment (cookers, presses, centrifuges, evaporators, dryers); dry reduction setups are simpler and cheaper. Processing flexibility: Dry reduction offers greater flexibility in adjusting cooking and drying times, while wet reduction requires more rigid operating conditions to maintain product quality.

Quality considerations in fish meal production 🔗

The nutritional value and safety of the final fish meal depend heavily on how each processing step is managed. Several factors play a key role:

Freshness of raw material: Processing should begin as soon as possible after the fish are landed. Delays lead to protein degradation and the formation of biogenic amines (especially histamine), which reduce meal quality. Temperature control during drying: As highlighted in a review published in the Journal of Animal Physiology and Animal Nutrition, overheating during drying decreases protein digestibility and amino acid availability. Maintaining drying temperatures below 90°C for the material is crucial. Moisture content: The finished meal should have moisture content at or below 10-12%. Higher moisture invites microbial growth; too little means the product was over-dried and likely nutritionally compromised. Stickwater recovery: Recovering and adding concentrated stickwater back to the meal increases the yield by 20% or more and adds valuable water-soluble vitamins and proteins. Skipping this step wastes raw material and creates pollution issues. Antioxidant addition: To prevent fat oxidation and self-heating during storage, antioxidants must be added promptly after drying.

Applications of fish meal 🔗

Fish meal is used primarily as a protein supplement in compounded animal feeds. As of recent estimates, around 56% of global fish meal goes into aquaculture feeds, about 20% into pig feed, roughly 12% into poultry feed, and the remainder into other applications including fertilisers. Its balanced amino acid profile – particularly its richness in lysine and methionine – makes it an effective complement to plant-based protein sources like soybean meal, which are often deficient in these amino acids. Beyond nutrition, fish meal improves feed palatability, enhances nutrient absorption, and supports immune function in animals.

Challenges and the road ahead 🔗

Fish meal production faces growing scrutiny due to sustainability concerns. With approximately 4-5 tonnes of raw fish required to produce just one tonne of fish meal, the pressure on wild fish stocks is significant. The search for alternative protein sources – including soybean meal, insect meal (from black soldier fly larvae and mealworms), and single-cell proteins – is accelerating. At the same time, advances in processing technology, including more energy-efficient dryers, better evaporation systems, and centrifugal separation replacing traditional presses, are helping manufacturers improve product quality while reducing environmental impact.

What do you think? Given the sustainability concerns around wild fish harvesting, can improved processing technology and better by-product utilisation reduce the industry’s ecological footprint? And as alternative protein sources gain ground, do you see fish meal continuing to hold its dominant position in aquaculture and livestock feeds in the coming decades?

References

1. https://en.wikipedia.org/wiki/Fish_meal
2. https://edis.ifas.ufl.edu/publication/FA122
3. https://www.fao.org/4/x6899e/x6899e04.htm
4. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/fish-meal
5. https://www.fao.org/4/x5926e/x5926e01.htm
6. https://www.feedipedia.org/node/208
7. https://www.fao.org/4/X6899E/X6899E04.htm
8. https://onlinelibrary.wiley.com/doi/10.1111/j.1439-0396.2010.01109.x
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC9475308/