Shortcomings of Twin Screw Fish Feed Extruders

What the Twin Screw Design Achieves and Where It Struggles

A twin screw fish feed extruder uses two intermeshing screws to mix, plasticize, and shape feed mash at high temperature and pressure, which gives excellent starch gelatinization, uniform pellets, and good water stability. These strengths make it the standard choice for high-quality floating feed. The trade-offs appear when the formula contains heat-sensitive nutrients: the very heat that creates the pellet structure can destroy vitamins, probiotics, and enzymes. Understanding these shortcomings helps a feed mill decide whether a twin screw line, a single screw line, or a post-processing technique fits the product being made.

High-Temperature Processing Destroys Heat-Sensitive Nutrients

To plasticize and sterilize the raw materials thoroughly, the plasticizing section of a twin screw fish feed machine is typically controlled at 110-130 degrees Celsius, and even when the forming section is reduced to 90-100 degrees Celsius, the overall processing temperature stays high. Heat-sensitive components such as probiotics, vitamin C, and vitamin E are largely inactivated or decomposed above about 80 degrees Celsius. In practice, vitamin C retention in twin screw processing can fall to roughly 30-40 percent, and probiotic survival is often below 20 percent. Functional feeds that must retain these components therefore cannot simply be processed at standard twin screw temperatures.

Lowering the Temperature Has Narrow Limits

Some producers try to reduce heat loss by lowering the processing temperature of the twin screw extruder, but the adjustment range is small. At lower temperatures, the raw material is insufficiently plasticized: starch gelatinization can drop from above 90 percent to below 70 percent, producing loose pellets with poor water resistance that disintegrate quickly and reduce digestibility for fish. The sterilization effect also weakens, with the sterilization rate falling from about 99 percent to below 80 percent, which increases the risk of intestinal disease in the stock. A post-coating technique, in which heat-sensitive components are sprayed onto the pellet surface after processing, avoids the heat but brings its own problems: uneven distribution of the additives and easy detachment during storage and transport.

Special Heat-Sensitive Ingredients Are Hard to Handle

Beyond common vitamins and probiotics, special functional ingredients such as plant extracts and enzyme preparations are also temperature-sensitive, and the high-temperature environment of the extruder can destroy their activity. Enzyme-based feeds are a typical case: processing an enzyme-containing diet in a standard twin screw line can reduce enzyme activity substantially, while switching to a lower-temperature extrusion process can retain a much larger share of the activity and preserve the feed efficacy. The practical conclusion is that a twin screw extruder is the wrong tool for such products unless a low-temperature capability is available.

Cost and Capacity Trade-offs of Low-Temperature Modules

Some high-end twin screw extruders are equipped with low-temperature extrusion modules that protect heat-sensitive ingredients. The trade-off is economic: these modules can raise the equipment price by a significant margin, and low-temperature processing reduces throughput, which adds production pressure for mills running high volumes. The decision to buy such a module should be based on whether the product portfolio actually contains heat-sensitive functional feeds, and on whether the higher unit cost of production can be passed on to the customer.

FAQ

Why does a twin screw extruder damage heat-sensitive nutrients?

Thorough plasticization requires barrel temperatures around 110-130 degrees Celsius. Heat-sensitive components such as vitamin C, vitamin E, and probiotics are largely inactivated above about 80 degrees Celsius.

Can I simply lower the extruder temperature to protect nutrients?

Only within narrow limits. Lower temperatures reduce starch gelatinization and water resistance of the pellets, and weaken the sterilization effect, so pellet quality and feed safety suffer.

Does post-coating solve the heat problem?

Partly. Spraying heat-sensitive components onto the pellet surface after processing avoids the heat, but distribution can be uneven and the coating may detach during storage and transport.

Are enzyme-based feeds suitable for twin screw extrusion?

Not at standard processing temperatures, because enzyme activity is largely destroyed. A lower-temperature extrusion process retains far more enzyme activity and preserves the feed efficacy.

Do low-temperature modules on twin screw extruders cost more?

Yes. Low-temperature modules raise the equipment price and reduce throughput, so they are only economical when the product mix actually includes heat-sensitive functional feeds.