Six characteristics of extruded feed

Our country is the world’s largest aquatic feed producer, and its aquatic feed production accounts for about half of the world’s total production. In recent years, among the rapidly developing aquatic feed varieties, extruded feed has developed most rapidly and has become a bright spot and growth point in the aquatic feed industry. However, under the conditions of puffing processing, the selection of raw materials, determination of nutritional parameters, formula design, processing technology, feeding technology, etc. are somewhat different from traditional pellet feeds. However, there is still a lack of relevant research. Based on some research carried out by our laboratory in the field of extruded feed in recent years, this article intends to review several issues of concern in the production and use of extruded feed, and provide some thoughts and guidance for the reasonable application of extruded feed in the aquaculture industry.

1.The effect of puffing on the digestion and utilization of nutrients

Puffing processing of feed has been found to primarily improve starch and energy digestibility, with minimal impact on protein digestibility. Studies have shown that puffing can lead to improved protein deposition efficiency, although the absolute increase in protein digestibility may not be significant. Researchers have also investigated the effects of extrusion temperature and pre-conditioning on the digestibility of nutrients in compound feed for fish, highlighting the importance of pre-conditioning in puffing processing. Overall, the findings suggest that excessively high temperatures and pressures may not be necessary for effective puffing, and emphasize the significance of pre-conditioning in the process.

2.Comparison of the effects of extruded feed and pellet feed on fish growth performance

Extruded feed, compared to pellet feed, can improve the digestion and utilization of nutrients but may lead to the loss of heat-sensitive nutrients, potentially impacting fish growth performance. There are conflicting reports on the overall production performance impact of extruded feed, with domestic studies in aquaculture often indicating improved growth and fish quality, while foreign studies show mixed results. These differences might be attributed to varying fish species and their sensitivity to feeding adjustments. Additionally, for certain farmed fish, such as yellow catfish and channel catfish, differences in intake of extruded feed and pellet feed not only affect growth performance but also body color. Some individuals fed with extruded feed exhibit abnormal body color, possibly due to the loss of heat-sensitive nutrients during extrusion. Strategies like increasing heat-sensitive nutrients, alternating between extruded and pellet feed, or using both types intermittently may help mitigate these issues to some extent.

3.Effect of puffing on protein and fat requirements

The laboratory conducted experiments to investigate the impact of extrusion processing on the protein and fat requirements of tilapia. In Experiment 1, varying protein levels in pellet and extruded feeds were fed to juvenile tilapia, revealing that the suitable protein requirement for extruded feed was lower than that for pellet feed. Experiment 2 involved adding different levels of fat to feeds and feeding them to tilapia, indicating that the optimal fat addition level for both pellet and extruded feeds with a 28% protein level was 2%. These findings suggest that extrusion processing reduces the protein requirement but does not decrease the fat requirement for tilapia.

4.Application of amino acids in extruded feed

The use of cheap animal and plant proteins in aquatic feeds has led to increased attention on balancing amino acid composition through the addition of amino acid additives, particularly in extruded feed. High temperature and pressure conditions during extrusion can denature proteins, aiding in their digestion and absorption, but can also lead to the loss of effective amino acids through the Maillard reaction, reducing the feed’s nutritional value. The stability of microencapsulated amino acids under extrusion conditions is higher compared to crystalline amino acids. While the effectiveness of supplementing crystalline amino acids varies among fish and shrimp species, a study found that supplementing crystalline methionine in extruded feed improved fish growth performance. This may be attributed to the puffing process fully gelatinizing the starch, leading to the coating of crystal methionine by gelatinized starch, which slows down the absorption process in the intestine, resulting in a sustained release effect and improved growth performance.

5.Loss of vitamins during puffing processing

In feed processing, especially during puffing processing, vitamins can undergo significant damage, with sensitive vitamins such as VA, VE, VC, VB1, and folic acid being particularly vulnerable. To counteract vitamin loss during extrusion, some producers add 20% to 50% more vitamins to the pellet feed formula, which can lead to both wastage and deficiencies, resulting in imbalanced vitamin levels. A study found that reinforcing the vitamin addition in fishmeal and soybean meal-based feeds significantly improved growth performance in rainbow trout, highlighting the practical significance of this approach in aquaculture. To address this issue, it is recommended to increase the overall vitamin dosage by 25% when producing extruded feed, with special consideration for adding VA, VE, VC, VB1, and folic acid to mitigate vitamin loss and maintain balance, taking into account both economic and practical factors.

6.Effect of puffing on mineral availability

Limited research exists on the effects of extrusion processing on mineral digestibility in aquatic feed. A study using rainbow trout found that apparent digestibility of certain minerals in raw materials, such as iron, zinc, phosphorus, copper, and magnesium, was significantly reduced after puffing treatment. The impact on the digestibility and utilization of added mineral elements after extrusion processing is yet to be fully explored, highlighting the need for further research in this area.

As a relatively new variety of aquatic feed in China, extruded feed presents distinct characteristics in terms of nutritional requirements, nutrient changes during processing, and formula adjustments compared to pellet feed. However, research in this area is currently lacking. Strengthening basic research on extruded feed and exploring strategies such as increasing the amount of heat-sensitive nutrients added or using a combination of extruded feed and pellet feed could provide valuable insights in improving its efficacy and performance.