Understanding Extrusion Technology in Food Processing

Extrusion technology is one of the most versatile and efficient methods used in modern food processing. It combines multiple operations – mixing, cooking, kneading, shearing, and shaping – into a single, continuous process. Whether it’s breakfast cereals, snack foods, or protein-rich fish noodles, extrusion plays a central role in transforming raw ingredients into shelf-stable, uniform, and consumer-ready products. In the context of fishery by-products and waste utilization, extrusion has opened up exciting possibilities for converting low-value fish into high-quality, nutritious foods like fish noodles.

What is extrusion technology? 🔗

At its core, extrusion involves forcing a mixture of raw materials through a specially shaped opening called a die under controlled conditions of heat, pressure, and mechanical shear. The machine that performs this task is called an extruder, and the material being processed is known as the extrudate. The extruder consists of one or two large rotating screws that fit tightly inside a stationary barrel. As the screws rotate, they push the raw material forward, compressing and heating it along the way. By the time the material reaches the die at the end of the barrel, it has been thoroughly mixed, cooked, and transformed into a semi-molten mass that takes on the shape of the die opening as it exits.

This process is often described as a high-temperature, short-time (HTST) operation. It cooks the food rapidly while retaining more nutrients compared to traditional cooking methods. The intense pressure inside the barrel – typically between 10 and 20 bar – generates significant friction and heat, causing physical and chemical changes in the raw ingredients. Starch granules gelatinize, proteins denature, enzymes are inactivated, and microbial loads are reduced, all within a matter of seconds.

Key components of an extrusion system 🔗

An extrusion processing line involves more than just the extruder itself. According to NC State Extension, the system generally consists of several interconnected units that work together to produce the final product.

Feed hopper and preconditioner 🔗

Raw materials are first ground to the correct particle size – usually to the consistency of coarse flour – and then fed into the system through a live bin at a constant rate. Before entering the extruder, the material passes through a preconditioner where it is uniformly hydrated with water or steam, and additional ingredients like fats, flavours, or meat slurries can be added. This step initiates partial cooking and helps ensure a consistent feed into the extruder.

The extruder barrel and screw 🔗

The extruder barrel houses one or two screws that convey the material forward. As it moves along the barrel, the screw compresses the food, and smaller screw flights restrict the available volume. This builds resistance, forcing the material to fill every gap between the screw and barrel wall. The combination of friction, compression, and external heating transforms the material into a plasticized, semi-solid mass. If temperatures exceed 100°C, the process is referred to as extrusion cooking or hot extrusion.

Die and cutting assembly 🔗

At the discharge end of the barrel, the cooked material is forced through the die, which determines the shape of the final product – whether thin strands, flat ribbons, tubes, or more complex shapes. A rotating knife assembly mounted in front of the die cuts the extrudate to the desired length. When the material exits the die, the sudden drop in pressure causes trapped moisture to flash off as steam, which can create the characteristic puffed or expanded texture seen in many snack foods.

Types of extruders 🔗

Food extruders come in two main configurations, each suited to different applications.

Single-screw extruders (SSE) 🔗

These were the earliest type adopted by the food industry, beginning in the 1930s for breakfast cereals and dry pasta. A single-screw extruder uses one rotating screw inside the barrel. It is simpler in design, more affordable, and works well for straightforward formulations with consistent particle sizes.

Twin-screw extruders (TSE) 🔗

Introduced in the 1970s, twin-screw extruders use two intermeshing screws that can co-rotate or counter-rotate. They offer superior mixing, conveying, and handling capabilities, especially for formulations that are difficult to process in SSEs – such as those with high moisture content, variable particle sizes, or complex ingredient profiles. Twin-screw extruders are particularly important in the production of fish noodles from surimi, where moisture content can exceed 70%.

Critical process parameters 🔗

The quality of an extruded product depends heavily on how well the process parameters are controlled. Even small changes can significantly affect the texture, appearance, colour, and nutritional value of the output.

Moisture content 🔗

Moisture is arguably the most important variable in extrusion. It acts as a plasticizer, affecting the viscosity of the extrudate. Increasing moisture reduces viscosity, torque, and product temperature while increasing the bulk density of the final product. Most food extrusion processes operate at moisture levels below 40%, which is considered low to intermediate moisture extrusion. High-moisture extrusion (above 40%) is used for specialized products like meat analogues and fish noodles, and typically requires twin-screw extruders with better conveying capabilities.

Temperature and pressure 🔗

Barrel temperature directly influences the degree of starch gelatinization and protein denaturation. As noted in research published in ScienceDirect, temperature, pressure, and screw design are among the most critical parameters affecting nutrient retention and product quality. Higher temperatures promote expansion and crispness, while lower temperatures yield denser products.

Screw speed and configuration 🔗

The speed at which the screws rotate affects the shear rate – higher speeds increase shear and mechanical energy input. The geometry of the screw, including the number, pitch, and diameter of flights, as well as features like kneading discs or throttle rings, can be adjusted to fine-tune mixing intensity and residence time for different formulations.

Die design 🔗

The shape, number, and size of openings in the die determine the final form of the extruded product. Combined with the speed of the cutting blade, die design controls both the shape and length of the output.

What happens during extrusion cooking? 🔗

The transformations that occur inside an extruder barrel are complex and happen simultaneously. Understanding these changes is important for producing high-quality extruded foods.

Starch gelatinization is one of the primary changes. When starch granules encounter heat and moisture inside the extruder, they absorb water, swell, and lose their crystalline structure. This gelatinized starch acts as a binder in the extruded product, giving it structural integrity and a characteristic chewy texture.

Protein denaturation occurs as heat unfolds protein molecules, altering their functional properties. In fish-based products, the denaturation of myofibrillar proteins is critical for gel formation, which gives fish noodles their distinctive elasticity.

The process also leads to enzyme inactivation and microbial reduction, improving product safety and shelf life. As described by the Institute of Food Technologists, the product is cooked by three separate energy sources during extrusion: mechanical energy from screw shear, thermal energy from the heating system, and self-heating caused by the viscosity of the material in the barrel.

Additionally, extrusion can destroy naturally occurring antinutritional factors such as trypsin inhibitors, tannins, and phytates, thereby improving overall protein digestibility and mineral availability in the final product.

Application of extrusion in fish noodle production 🔗

Fish noodles are an excellent example of how extrusion technology can be used to add value to underutilized fish species and fishery by-products. Traditional noodles are primarily carbohydrate-heavy products made from wheat flour with limited protein content. By incorporating fish protein into the formulation, manufacturers can produce a product with significantly higher nutritional value – typically 12-18 grams of high-quality protein per serving – along with beneficial omega-3 fatty acids and minerals like phosphorus and selenium.

How fish noodles are made using extrusion 🔗

The production process begins with preparing fish paste or surimi from low-cost or underutilized fish species. The fish mince is washed to remove water-soluble components like fats and enzymes, concentrating the myofibrillar proteins that are essential for good texture. This surimi is then blended with starch (such as wheat flour or cassava starch), water, and other functional ingredients.

A typical formulation might include 20-30% fish paste, 60-70% starch or flour, and 10-15% water, though ratios vary depending on the target product. The blended mixture is fed into the extruder, where it encounters temperatures typically ranging from 80°C to 120°C. During this process, the starch gelatinizes and the fish proteins denature and reorganize, forming a cohesive, elastic matrix. The extrudate is then forced through a noodle-shaped die and quickly cooled to set the structure.

The role of high-moisture extrusion 🔗

Research published in the Journal of Food Engineering has investigated the use of twin-screw, high-moisture extrusion specifically for producing fish noodles from surimi as the primary ingredient. The study found that the extrusion process can be divided into distinct zones – transport, cooking, mixing, and chilling – each playing a unique role in the final product structure. In the mixing zone, disulfide bonds form in large numbers due to turbulent flow, while the chilling zone restores hydrogen bonds and gradually organizes the protein structure into the final noodle shape.

However, producing fish noodles with surimi as the main ingredient is challenging because surimi typically contains more than 70% moisture, which can cause problems like spraying and failure to form properly. Researchers have addressed this by adding functional ingredients like sodium alginate and xanthan gum to reduce water activity and improve processability.

Enriching noodles with fish protein 🔗

Studies on incorporating fish-derived ingredients into noodle products have shown promising results. Research reviewed in the Journal of Aquatic Food Product Technology highlights that fish processing by-products – such as heads, bones, skins, and viscera – can be transformed through extrusion into functional ingredients rich in protein, calcium, and omega-3 fatty acids for noodle formulations. This approach not only improves the nutritional profile of the noodles but also helps reduce waste in the fish processing industry.

Advantages of extrusion technology 🔗

Extrusion offers several compelling advantages that make it the method of choice for large-scale food processing.

Efficiency and continuous operation: Extrusion is a continuous process, meaning raw materials go in at one end and finished products come out the other without interruption. This results in high throughput with minimal downtime. As noted by The Pharma Innovation Journal, extrusion is one of the most cost-effective methods in food processing, offering a platform for developing products from various food groups by modifying ingredients and processing conditions.

Versatility: By adjusting the formulation and process parameters, the same extruder can produce foods with vastly different textures, shapes, and densities. For aquatic products alone, extrusion can create floating feeds, sinking feeds, fish noodles, meat analogues, and textured protein products.

Nutrient retention: Because extrusion cooking operates on an HTST principle, it retains nutrients better than conventional batch cooking methods. The short processing time minimizes losses in heat-sensitive vitamins and amino acids when moderate conditions are used.

Food safety: The combination of high temperature, pressure, and low water activity in extruded products reduces microbial contamination and inactivates enzymes that could cause spoilage. This results in products with longer shelf life and improved microbiological safety.

Waste utilization: Extrusion provides an economical way to incorporate food processing by-products and residues back into the food stream, turning what would otherwise be waste into valuable, nutritious products.

Challenges and considerations 🔗

Despite its many advantages, extrusion technology does come with challenges. Controlling the process requires precise management of numerous interacting variables. As the FAO notes in its technical guidance on fish feed processing, extrusion cooking is difficult to control due to its many parameters, and the underlying mechanisms are not always fully understood.

For fish noodles specifically, maintaining the right balance of moisture is critical – too much water makes the dough unprocessable, while too little results in brittle, crumbly noodles. The fish proteins are also more prone to spoilage than plant-based ingredients, so proper processing conditions and packaging are essential. Additionally, achieving consumer-acceptable colour and appearance requires careful selection of fish species or the use of natural colourants.

Equipment costs can also be significant. Twin-screw extruders, which are often necessary for high-moisture fish-based products, have approximately twice the capital and maintenance costs of single-screw machines. This can be a barrier for small-scale processors, particularly in developing countries where fishery by-product utilization could have the greatest impact.

The future of extrusion in fish product development 🔗

Extrusion technology continues to evolve rapidly. Newer developments such as supercritical CO₂-assisted extrusion, 3D food printing combined with extrusion, and advanced twin-screw configurations are expanding what is possible. In the fish noodle industry, standardized large-scale production through twin-screw extrusion is seen as an inevitable trend, moving beyond the small family workshop model that currently dominates.

Research is also ongoing into optimizing formulations for different fish species, exploring blends of plant and fish proteins, and incorporating additional functional ingredients like seaweed, probiotics, or vegetable extracts. As consumer demand for high-protein, convenient, and sustainably produced foods continues to grow, extrusion technology is well-positioned to meet these needs while helping reduce fishery waste.

What do you think? Could extruded fish noodles become a mainstream protein-rich alternative to traditional wheat noodles in your region? How might extrusion technology be further leveraged to reduce waste and improve food security in fishing communities?

References

1. https://en.wikipedia.org/wiki/Food_extrusion
2. https://content.ces.ncsu.edu/extrusion-processing-a-versatile-technology-for-production-of-foods-and-feeds
3. https://www.fao.org/4/s5347e/S5347E09.htm
4. https://www.sciencedirect.com/science/article/abs/pii/S2949824425001764
5. https://www.ift.org/news-and-publications/food-technology-magazine/issues/2017/july/columns/processing-extrusion-and-applications-in-food-industry
6. https://www.sciencedirect.com/science/article/abs/pii/S0260877423004545
7. https://www.tandfonline.com/doi/full/10.1080/10498850.2022.2120376
8. https://www.thepharmajournal.com/archives/2020/vol9issue2/PartD/9-1-59-634.pdf