Feed processing and extrusion

1.5.1 General

Most commercial fish feeds are processed by compression pelleting or extrusion and other manufactured forms include moist and semi-moist, microencapsulated and micropulverized feeds (NRC, 1993). Processing techniques such as heat treatment, solvent extraction, flaking and grinding can improve the nutritional value of the diet ingredients and this in turn could be very beneficial for several fish species (Gouveia and Davies, 2000)

1.5.2 Processing Methods

Processing of the feed mixture through grinding, steam conditioning and extrusion is currently a big issue for fish feed companies, as these processes inevitably affect both the physical and chemical characteristics of a feed (Hilton et al., 1981). These characteristics include water stability and durability, pellet hardness, nutrient availability and digestibility (Hardy, 1989). Other factors influenced by processing such as the palatability and organoleptic properties of a diet may affect the amount of feed consumed by a target species (Mackie and Mitchell, 1985).

Since the method of pelletisation has a major influence on the nutritional characteristics of feeds it is important to ensure that ingredients identified as having potential in aquaculture feeds are incorporated into experimental diets that reflect current commercial processing (Oliva-Teles et al., 1994). There are a lot of different processing methods to produce pellets, but there are just few general techniques (Hardy and Barrows, 2002) described below .

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CHAPTER 1.GENERAL INTRODUCTION

1.5.2.1 Pelleting

Cold pellet extrusion involves no thermal activity and the wet mixture is forced

through a plate with holes drilled into it. The resulting noodles are cut into appropriate lengths by an external cutting blade as they leave the die (Hardy and Barrows, 2002).

Compressed pelleting is a process which forces a feed mixture through holes in a

metal die by the action of a roller located inside the die. Before this, the mixture can be exposed to dry steam for some seconds to increase the temperature to about 85ºC and the moisture to about 16% (Hardy and Barrows, 2002), but compressed pellets can also be produced without any steam as well (Thomas et al., 1997).

Extruded pellets are made in the same general way as compressed pellets, but the

use of different dies and physical conditions results in a very different product (Hardy and Barrows, 2002) as described below.

1.5.3 Extrusion

Extrusion requires more elaborate equipment and higher inputs of moisture, heat and pressure than simple pelleting. Usually, mixed ingredients are finely ground before entering the pre-conditioner where steam treatment takes place until the mixture is converted into a mash that may or may not be pre-cooked (pre-conditioning) before entering the extruder. The mash, which contains around 25% moisture, is compacted and heated to 104º - 148ºC under pressure in the barrel of the extruder. As the material is squeezed through die holes at the end of the barrel, and external pressure decreases, part of the water in the superheated dough immediately vaporizes and causes expansion of the feed particles (NRC, 1993) leaving a large number of air-filled interstices. The extruded particles have high moisture content and require external heat for drying.

Thus, after extrusion the particles must pass through a drying process to reduce

CHAPTER 1.GENERAL INTRODUCTION

moisture to a safe level for storage (NRC, 1993). The air trapped inside the particles enables, when needed, the coating under vacuum with oil and other heat-sensitive materials giving an important advantage to this method.

Extrusion processing can impart a wide range of different physical characteristics to the final product due to the different settings in pre-conditioning, the types and number of screws (single or twin-screw) in the extruder, the screw speed, the temperature, the mechanical shear and the duration of every application (Thomas et al., 1997). This manufacturing method has the advantage of short cooking time and high productivity, but high operating costs.

1.5.3.1 Effects of extrusion on feed nutrients

Manufacturing processes can influence the utilization of nutrient ingredients and feed intake in fish (Booth et al., 2000). The effects of processing can differ for different species and it is important to apply the appropriate processing technique to maximize production efficiency at the lowest possible cost (Tacon, 1990). The use of extrusion increases nutrient availability of plant meals especially in relation to the amount of DE available through starch gelatinization. Dias et al. (1998) reported that inclusion of extruded wheat in the diet had an advantage over raw wheat improving energy digestibility of the diet for European seabass. More specifically extrusion processing gelatinizes starch and improves the digestion of starch in salmonids (Thodesen and Storebbaken, 1998).

Mild extrusion processing usually enhances the digestibility of plant proteins (Håkansson et al., 1987), while Booth et al. (2002) found increased feed efficiency and DP of extruded diets compared with cold and steam pelleted diets for silver perch.

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CHAPTER 1.GENERAL INTRODUCTION

Extrusion seems not only to improve protein and carbohydrate digestibility, but also lipid digestibility. The influence on lipid digestibility could be related to a decrease in indigestible carbohydrates (Storebakken et al., 1998).

Extrusion effects have been investigated in some ingredients, and resulted in increased nutritional value of rapeseed and peas for rainbow trout (Gomes et al., 1993), improvement of digestibility of canola meal for chinook salmon Oncorhynchus tshawytscha (Satoh et al., 1998), and increased nutritional value of dehulled pea seed

meal for European seabass (Gouveia and Davies, 2000). However, Oliva-Teles et al.

(1994) reported that extrusion had no effect on the nutritional value of full fat soybean in rainbow trout diets and, according to Sorensen et al. (2002), differences in extrusion temperatures (100, 125 and 150°C) caused no significant differences in ADC of crude protein, individual amino acids or energy in fish meal and wheat meal based diets for rainbow trout.

While processing techniques can improve the nutritional and physical qualities of diet ingredients, the heating process can also have detrimental effects. Damage to proteins and losses in nutritional value with destruction of IAA and reduction in amino acid availability, with lysine being the most sensitive, may be observed during processing (Papadopoulos, 1989). Allan et al. (2000) also reported that excessive heat during the rendering process could reduce the digestibility of proteins and amino acids, damage lysine and contribute to low nitrogen digestibility in animal meals. Generally, severe heating in combination with low moisture content have resulted in reduction of digestibility of most amino acids in fishmeal (Ljokjel et al., 2000). In addition, heat labile vitamins can also be lost at elevated temperatures, for example ascorbic acid had been shown to be unstable during heat treatments such as steam conditioning and extrusion (Slinger et al., 1979).

CHAPTER 1.GENERAL INTRODUCTION

1.5.3.2 Effects of extrusion on feed ANF

Various processing procedures can decrease levels of a number of ANFs and, concurrently, increase the protein and starch availability of the plant seeds as has been shown by several authors (Gomes et al., 1993; Oliva-Teles et al., 1994; Pfeffer et al., 1995; Abd El-Hady and Habiba, 2003; Wang et al., 2004). Specific protease inhibitors, lectins, antivitamins and α-amylase inhibitor are heat labile factors (Abd El-Hady and Habiba, 2003; Wang et al., 2004), which means that extrusion processing can reduce or minimize their activity. According to the same authors, saponins, non-starch polysaccharides, antigenic proteins, phytoestrogens and some phenolic compounds that are classified as ANFs are heat resistant.

Legume extrusion cooking may allow reduction of ANF and therefore improve the nutritional quality at a cost lower than other heating systems (baking, autoclaving, etc.) due to more efficient use of energy and better process control with greater production capacities (Alonso et al., 2000a). The digestibility coefficients of legumes increased after extrusion, compared to raw legumes both in vitro and in vivo, namely in rats (Alonso et al., 2000b), but also in rainbow trout (Cheng and Hardy, 2003a; Cheng and Hardy, 2003b) and silver perch (Allan and Booth, 2004).

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CHAPTER 1.GENERAL INTRODUCTION