Nutritional and physical quality of fish feeds extruded at various temperatures / Ernæringsmessig og fysisk kvalitet på fiskefôr ekstrudert ved ulike temperaturer
This thesis, which is based on four experiments, focuses on effects of extrusion temperature on nutritional and physical quality of feeds for rainbow trout (Oncorhynchus mykiss) and mink (Mustela vison).
Although extrusion technology has a broad application in production of pet food and fish feed, published literature dealing with extrusion temperature and effects on nutritional and physical quality is scarce. Hence, this study aimed to quantify effects of extrusion temperature in the range 100-150˚C on nutritional and physical quality of the feed.
Two experiments were carried out with rainbow trout to study effects of different extrusion temperatures on digestibility and feed utilization. In the first experiment apparent digestibility of energy, ash, protein and amino acids were measured in rainbow trout fed wheat/fish meal based diets extruded at the temperatures 100, 125 and 150˚C respectively (Paper I). In the second experiment apparent digestibility of energy, protein, ash and elements in addition to growth and nutrient retention were measured in rainbow trout fed fish meal/wheat/gluten based diets extruded at 100 and 140˚C (Paper II). True digestibility of protein, individual amino acids and starch were measured in mink (Paper III) fed a wheat/fish meal based diet extruded at the temperatures 100, 125 and 150˚C, respectively. Physical quality of the feeds used in paper II was evaluated using different mechanical tests (Paper IV).
Various processing conditions were used to achieve the target temperatures. The target temperatures in Paper I were achieved by changing the extrusion conditions primarily by two principal methods. By employing method 1, the temperature of the conditioner, feed rate, process water and screw speed were varied to change the temperature measured in front of the die. However, changes were also recorded in the dependent extrusion variables pressure and torque. In method 2, conditioner temperature and screw speed were kept constant, and the variation in process water input was restricted. The feeds used in the mink study (Paper III) were similar to those in Paper I, except that an untreated meal control was used and one additional processing method was studied.
The main findings were: 1) The apparent digestibility of protein, individual amino acids and energy in rainbow trout were not significantly affected by extrusion temperature in any of the two experiments. Apparent digestibility of ash was not affected by extrusion temperature in Paper I. In Paper II the apparent digestibility of ash and Mn was significantly lower at 140˚C compared to the 100˚C, whereas Fe showed the highest digestibility at the highest extrusion temperature. 2) Specific growth rate was significantly highest for the feed produced at 140˚C, while retentions of nitrogen and energy were not affected by extrusion temperature in rainbow trout fed diets extruded at 100 and 140˚C (Paper II). 3) True digestibilities of protein and total amino acids in mink were slightly lower for diets extruded at 125˚C and 150˚C than for the untreated control (Paper III). Digestibilities of protein, total amino acids, and the amino acids Cys, Asp, Glu, Ala, Val, Ile, Leu, His, Lys and Arg decreased with increasing extrusion temperature from 100˚C to 125˚C. However, increasing the temperature from 125 to 150˚C did not cause any significant subsequent reduction in these digestibilities (Paper III). Cys showed the most severe reduction in mink digestibility (6.8 percentage units) at elevated extrusion temperatures (Paper III). The starch digestibility in mink was increased by extrusion processing, but there was no further effect of increasing the temperature above 100˚C (Paper III). 4) Apparent digestibility of Cys in rainbow trout (Paper I) was significantly higher for the diets produced by the method employing the highest moisture addition during extrusion than for diets produced with restricted moisture addition. 5) Digestibilities of crude protein, total amino acids, Asp, Glu, Pro, Gly, Ala, Val, His, Lys, Arg and Trp in the mink study were affected by processing method (Paper III), which by multivariate analysis was unveiled to be explained mainly by the parameters; die temperature, feeding rate and conditioner temperature. 6) Digestibility of starch in the mink was unveiled by multivariate analysis to be explained mainly by water addition during extrusion (Paper III). 7) The lowest extrusion temperature (100˚C) resulted in the highest durability, breaking strength, bulk density and expansion ratio, whereas sinking velocity was not affected by extrusion temperature.
The main conclusion from the experiments with rainbow trout (Paper I and II) and mink (Paper III) was that increasing extrusion temperature from 100 to 150˚C caused surprisingly small effects in the measured digestibility coefficients. Thus, extrusion processing of feed represents a mild heat treatment of fish feed, even when the extrusion temperature is as high as 150˚C. Short retention time in the extruder and high moisture are the most plausible explanations to this. The results also indicate that process variables other than extrusion temperature, such as amount of process water, may contribute more to the digestibility of nutrients than the temperature in the extruder. The lowest extrusion temperature (100˚C) resulted in the best technical quality of the feed, compared to extrusion at 140˚C. This finding was probably explained by less expansion of the 100˚C feed.
