How to Make Acid Fish Silage: A Step-by-Step Guide

Every year, millions of tonnes of fish processing waste end up in landfills or pollute water bodies. But what if this waste could be turned into a high-value protein feed using a simple, low-cost method? That’s exactly what acid fish silage does. By adding organic acids – primarily formic acid – to minced fish waste, you can preserve it for months and create a nutrient-rich liquid feed suitable for livestock and aquaculture. This guide walks you through the entire process of making acid fish silage, from raw material selection to storage.

What is acid fish silage? 🔗

Acid fish silage is a liquid protein product made from whole fish or fish by-products that are liquefied by the action of natural enzymes present in the fish, in the presence of an added acid. When organic acids like formic acid are mixed into minced fish, they drop the pH to around 3.5-4.0. At this acidity level, harmful bacteria cannot grow, but the fish’s own digestive enzymes – particularly pepsin – remain highly active. These enzymes gradually break down the fish proteins into smaller peptides and free amino acids, eventually turning the entire mass into a homogeneous liquid.

The concept originated in the 1920s when A.I. Virtanen used mineral acids to preserve green fodder, and the technique was later adapted for fish in Scandinavia during the 1930s. Denmark began commercial-scale production in the late 1940s, and the technology has since spread to parts of Asia, Africa, and Latin America.

Why formic acid is the preferred choice 🔗

Several acids can be used for making fish silage – including sulphuric acid, hydrochloric acid, propionic acid, and acetic acid. However, formic acid is widely regarded as the best option for several reasons:

First, formic acid achieves preservation at a relatively higher pH (3.5-4.0) compared to mineral acids, which require a pH as low as 2.0 to prevent spoilage. This means less corrosion of equipment and containers. Second, formic acid has inherent antimicrobial and antifungal properties, acting as both a bacteriostat and bacteriocide. Third, silage preserved with formic acid does not need to be neutralised before feeding it to animals, which simplifies the process considerably. The typical dosage is 2-3% formic acid by weight of the fish material. While formic acid is costlier than mineral acids, the savings in processing steps and equipment make it more practical overall.

To reduce costs, some producers use a mixture of formic acid and sulphuric acid. In this approach, the inorganic acid brings down the pH enough for the organic acid to exert its antimicrobial effect. Research has shown that specific combinations of 85% formic acid and 50% sulphuric acid can produce silages with good preservation quality.

Step-by-step process of making acid fish silage 🔗

Step 1: Selecting and preparing the raw material 🔗

Start with fresh fish by-products – heads, viscera, frames, trimmings – or whole fish that is unsuitable for human consumption. Freshness matters significantly because fresh raw material liquefies faster and produces better-quality silage. Stale or partially spoiled fish leads to slower liquefaction and off-flavours in the final product.

Pass the raw material through a mechanical mincer or hammer mill grinder fitted with a screen of approximately 10 mm diameter holes. The goal is to achieve a uniform, small particle size that allows the acid to penetrate quickly and evenly. If the material isn’t properly minced, pockets of untreated fish will remain and these will putrefy, ruining the batch.

Step 2: Adding the acid 🔗

Immediately after mincing, add formic acid (85-90% concentration) at a rate of approximately 3-3.5% by weight of the fish. In practical terms, this works out to roughly 35 kg or about 30 litres of acid per tonne of fish. Pour the acid evenly over the minced fish and mix thoroughly.

Thorough mixing is critical. When acid is added to fresh fish mince, the muscle proteins tend to become rubbery and can form closed pockets where the acid cannot penetrate. This is why cutting or chopping the fish before acid addition is recommended. If you’re working with thawed or slightly older raw material, mixing is generally easier since the tissue is softer.

An antioxidant such as butylated hydroxytoluene (BHT) at around 200-250 mg per kg is often added at this stage to prevent oxidation of fish oils, especially when using fatty fish species. This step is particularly important in warm climates where oil oxidation occurs rapidly.

Step 3: Storage and agitation during liquefaction 🔗

Transfer the acid-fish mixture into acid-resistant containers – plastic drums, fibreglass tanks, polyethylene-lined steel drums, or even bitumen-coated concrete tanks for larger operations. Leave about 20% headspace in the container to accommodate expansion during the breakdown process.

The mixture now enters the liquefaction phase, driven by endogenous proteolytic enzymes (mainly from the fish gut). This is where regular agitation becomes essential. Stir or agitate the mixture daily during the first couple of weeks. You can use a pump, mechanical stirrer, or even a wooden paddle. Daily agitation serves multiple purposes – it redistributes the acid, prevents settling of heavier bone material, ensures uniform enzymatic activity, and helps release gases generated during protein breakdown.

Monitor the pH regularly using a pH meter or litmus paper. The target range should stay between 3.5 and 4.0. If the pH rises above 4.0, add more acid immediately. Protein breakdown naturally generates buffering compounds that can push the pH upward, so regular checking is necessary until the mixture stabilises.

Step 4: Monitoring the liquefaction process 🔗

The rate of liquefaction depends on three main factors: the type of raw material, its freshness, and ambient temperature.

Temperature is the single biggest variable. At 20°C, fresh white fish offal liquefies in about two days, but at 10°C the same material may take 5-10 days. In tropical climates, the process is faster – often completing within a few days. Optimal temperatures fall between 5°C and 40°C. Temperatures above 40°C risk inactivating the very enzymes that drive the process.

Raw material type also matters. Fatty fish species liquefy more quickly than lean fish offal. Sharks and rays are notoriously difficult to liquefy and should be mixed with other species. Fish viscera, rich in digestive enzymes, speeds up the overall process when included in the batch.

You’ll notice the mixture progressively changing – from a chunky paste to a smoother, more liquid consistency. The colour will darken to brown or amber, which is normal. Properly made silage develops a sharp, acidic smell. Any putrid or ammonia-like odour indicates bacterial contamination and that the batch may be compromised.

De-oiling the silage 🔗

In warmer climates, fish oil naturally separates and floats to the surface of the silage. If the oil content exceeds about 2% of the wet weight, de-oiling is recommended. Excess oil in silage can undergo oxidation and create nutritional problems when fed to animals. The oil can be removed by simple decanting, or for larger operations, by heating and centrifuging.

The separated oil is itself a valuable feed ingredient, particularly for aquaculture feeds, so it should not be discarded. If an antioxidant was added during preparation, oil quality is better preserved during storage.

Shelf life and storage 🔗

One of the most attractive features of acid fish silage is its long shelf life without refrigeration. When preserved with formic acid at the correct pH, silage remains stable for at least two years at room temperature without putrefaction. Under typical tropical conditions, a shelf life of up to one year is reliably achieved.

Several factors influence how long the silage remains in good condition. Initial freshness of the raw material is perhaps the most important – silage made from fresh fish lasts longer than silage from stale material. Adequate acid concentration and proper pH control throughout the process are equally critical. Storage in clean, covered, acid-resistant containers away from direct sunlight helps maintain quality.

During storage, protein continues to slowly hydrolyse into smaller peptides and free amino acids. The silage becomes smoother over time and develops what has been described as a malty odour. The acid environment also makes the product nearly sterile, effectively killing pathogens like Salmonella.

Nutritional composition and quality 🔗

The nutritional profile of acid fish silage closely mirrors that of the raw material used to make it. A typical analysis of silage made from white fish offal shows approximately 80% water, 15% protein, 4.5% ash, and 0.5% fat. Whole fatty fish produces silage with higher protein and fat levels. On a dry matter basis, the nutrient composition is comparable to fishmeal.

What makes silage nutritionally distinctive is that its proteins are already pre-digested – broken down into free amino acids and short-chain peptides. Research published in Trends in Food Science & Technology highlights that these hydrolysed proteins are more bioavailable and easier for animals to absorb than intact proteins in fishmeal. Studies on Atlantic salmon showed improved growth when 10% of dietary fishmeal was replaced with silage protein hydrolysate.

Essential amino acids like lysine, methionine, and cystine are generally well-retained in acid silage. However, prolonged storage can lead to excessive protein hydrolysis, which may reduce nutritional value – so using the silage within a reasonable timeframe (a few months) is advisable for optimal results.

Applications of acid fish silage 🔗

In aquaculture feeds 🔗

Acid fish silage is increasingly used as a partial replacement for fishmeal in aquaculture diets. It can be incorporated into moist pellets, extruded feeds, or mixed with dry ingredients. Research with species like tilapia, catfish, and rohu has shown that silage-based feeds can perform comparably to fishmeal-based diets at appropriate inclusion levels. The formic acid present in the silage may also provide additional health benefits by suppressing harmful gut bacteria in farmed fish.

In livestock feeding 🔗

Pig farming has been the most established use of fish silage, particularly in Denmark, where it is widely integrated into liquid feeding systems. Feeding trials have confirmed that pigs grow well on silage-based diets with good feed conversion and acceptable meat quality. Poultry and dairy cattle have also been fed fish silage successfully, with no reported tainting of eggs, milk, or butter.

As fertiliser 🔗

When silage does not meet feed-grade quality standards, it can be used as an organic fertiliser. It provides nitrogen from hydrolysed proteins along with phosphorus, potassium, calcium, and trace elements derived from fish bones and tissue.

Advantages over fishmeal production 🔗

The key advantage of acid fish silage is its simplicity and low capital investment. Unlike fishmeal production, which requires expensive drying and rendering equipment, skilled technical staff, and large volumes of raw material to be economical, silage can be made at virtually any scale – from a single oil drum to industrial tanks holding several tonnes. Energy requirements are very low compared to fishmeal processing, and the process produces minimal odour (unlike fishmeal plants, which are notorious for smell problems).

The main disadvantage is the high water content (~80%), which makes silage bulkier and more expensive to transport than dry fishmeal. To address this, some producers mix silage with dry agricultural by-products like rice bran or cereal flour, then sun-dry the mixture to create a solid feed that is easier to handle and store.

Safety precautions 🔗

Working with formic acid requires proper safety equipment – rubber gloves, goggles, protective clothing, and rubber boots. Acid storage tanks should be made of resistant material and kept secured. All mixing and storage containers must be acid-resistant; ordinary steel corrodes rapidly at low pH. Good ventilation in the working area is important, especially when handling concentrated acid. Despite these precautions, the final silage product itself is quite safe to handle – with a pH comparable to household vinegar.

What do you think? Could acid fish silage be a viable solution for managing fish processing waste in your region? And given that it requires minimal equipment and no refrigeration, how might small-scale fishing communities benefit from adopting this technology?

References

1. https://www.fao.org/4/x5937e/x5937e01.htm
2. https://www.fao.org/4/s5347e/S5347E08.htm
3. https://v4.infofish.org/index.php/fishbytes-fish-silage
4. https://openknowledge.fao.org/server/api/core/bitstreams/9b83c384-975b-49f7-9710-745d2cc201a0/content
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/fish-silage
6. https://www.sciencedirect.com/science/article/abs/pii/S092422441630348X
7. https://www.tandfonline.com/doi/full/10.1080/09712119.2015.1021811
8. https://www.fao.org/4/v4440t/v4440T0d.htm