4. FEED ADDITIVES

Feed additives are substances which are added in trace amounts to a diet or feed ingredient either a) to preserve its nutritional characteristics prior to feeding (ie. antioxidants and mould inhibitors), b) to facilitate ingredient dispersion or feed pelleting (ie. emulsifiers, stabilisers and binders), c) to facilitate growth (ie. growth promotants, including antibiotics and hormones), d) to facilitate feed ingestion and consumer acceptance of the product (ie. feeding stimulants and food colourants), or e) to supply essential nutrients in purified form (ie. vitamins, minerals, amino acids, cholesterol and phospholipids). Reference will be made here only to the major feed additives; preservatives, binders, feeding stimulants, and food colourants. For information on growth promoters readers should refer to NRC (1983) and Matty and Lone (1985); whereas amino acids, vitamins and minerals have been dealt with previously (Tacon, 1987).

4.1 Preservatives

A major problem faced by the animal feed compounder is the susceptibility of individual feed ingredients and formulated feeds to oxidative damage (oxidative rancidity) and microbial attack on storage. For example, in the absence of natural antioxidant protection (ie. absence of vitamin E, selenium, soy lecithin, active β-carotene) feedstuffs rations rich in polyunsaturated fatty acids (ie. fish oils, fish meals, rice bran, and some expeller oil seed cakes) are highly prone to oxidative decomposition which in turn may cause a reduction in the nutritive value of the constituent lipids, protein and vitamins (Rumsey, 1980; NRC, 1983; Bell and Cowey, 1985). Similarly, feedstuffs and rations posessing an elevated moisture content (> 15%) are prone to microbial attack and decomposition with a consequent loss in nutritional value for non-ruminant animals and deleterious mycotoxin production (Chow, 1980; NRC, 1983; Jones, 1987). Table 31 summarises the major antioxidants and chemical preservatives used by the animal feed manufacturing industry to combat the development of oxidative rancidity and microbial infestation within stored agricultural feedstuffs and rations.

4.2 Binders

Binders are substances which are used within aquaculture feeds to improve the efficiency of the feed manufacturing process, to reduce feed wastage, and/or to produce a water-stable diet. For example, binders such as bentonites, lignosulphonates, hemicellulose and carboxymethylcellulose are used primarily within feed rations to improve the efficiency of the feed manufacturing process (ie. during pelleting by reducing the frictional forces of the feed mixture through the pellet dies and thereby increasing the output and horse power efficiency of the feed mill) and for the production of a durable pellet (ie. by increasing pellet hardness and reducing wastage in the form of ‘fines’ during the pelleting process and during handling/transportation). The dietary inclusion level of these binding agents generally varies between 1 and 2% of the dry diet (Reinitz, 1983; NRC, 1983). By contrast, for those aquaculture species which have a slow feeding habit and require to masticate their food externally prior to ingestion (ie. marine shrimp and freshwater prawns), it is essential that specific binders be used to delay the physical disintegration of the pellet or feed mash within the water until ingestion is complete. Under these circumstances additional dietary binding agents will be required such as starchy plant products (ie. sago palm starch, cassava starch, potato starch, bread or wheat flour, rice and maize; binding being achieved through heat treatment and consequent starch gelatinization), alginates (ie. salts of alginic acid extracted from seaweeds), carrageenin, plant gums (ie. guar gum, locust bean gum, gum arabic), agar, high-gluten wheat flour, chitosan, propylene glycol alginate and gelatin (Forster, 1972; Heinen, 1981; Balazs, 1973; Meyers, Butler and Hastings, 1972; Pascual and Sumalangcay, 1981; Pascual, Bandonil and Destajo, 1978; Murai, Sumalangkay and Pascual, 1981; Viola, Gur and Zohar, 1986). Alternatively, binding agents such as polymethylolcarbamide (ie. Basfin) and urea-formaldehyde/calcium sulphate mixtures (ie. Maxi-Bond) with dual binding properties may be used in conjunction with steam pelleting (ie. these binders being activated by the injection of steam during pelleting; the condensation reaction can be controlled during pelleting to give the required durability and water stability). The dietary inclusion level of these two binding agents is generally below 0.5% of the dry diet. For moist or semi-moist diets the use of alginates or salt should be mentioned. Alginates are valuable binding agents through their ability to produce water-stable diets. Although they can be equally used for moist or dry diets, they function by forming gels or highly viscous solutions on reacting with polyvalent metal ions such as calcium. Consequently, for diets containing ingredients such as fish meal and fish protein hydrolysates which have a high soluble calcium content, gelling occurs rapidly during mixing (ie. trash fish/fish silage moist diet preparations; Storebakken, 1985: Storebakken and Austreng, 1987). However, for dry diet preparations the premature gelling of the feed mixture prior to pelleting has to be controlled by the use of a sequestering agent such as sodium polyphosphates. The dietary level of alginates used in feed rations generally varies between 0.5 and 5%, with the level of sequestering agents (typically sodium hexametaphosphate) varying from 0.5 to 1.5% depending on diet composition. For those diets which have a low soluble calcium content for adequate gel formation by alginates, the addition of soluble calcium salt such as calcium sulphate, calcium carbonate or calcium phosphate will be required (Heinen, 1981; Storebakken and Austreng, 1987; Meyers, Butler and Hastings, 1972). Alternatively, salt can be used as an effective and low-cost binding agent for raw trash fish/moist diet combinations (1% dietary inclusion level).

¹ Major in vitro synthetic antioxidants used in complete aquaculture feeds; themaximum level permitted by the U.S. Food and Drug Administration for BHA andBHT is 0.02% of the fat content and for ethoxyquin 150mg/kg feed (NRC, 1983).

² Major anti-mould agents used in complete aquaculture feeds c. 0.2-I.0% diet.

The effectiveness of individual binding agents will depend upon a variety of factors, including:

• Feed particle size - binding efficiency and pellet durability decreasing with increasing feed particle size (Hastings, 1971; Viola, Gur and Zohar, 1986).
• Manufacturing process - the binding capacity of starch based feed ingredients increasing with heat treatment and starch gelatinization (Hilton, Cho and Slinger, 1981; Murai, Sumalangkay and Pascual, 1981; Hastings, 1971; Viola, Gur and Zohar, 1986).
• Pellet diameter and die thickness - binding efficiency increasing with decreasing pellet diameter and increasing die thickness (Viola, Gur and Zohar, 1986).
• Diet composition - “Low-fibre foods present little yield to extrusion in a pellet die. High fat ingredients and added fat lubricate the food during extrusion limiting the work of compression in the die to form a solid pellet. Pellets which are formed with little compression are easily broken on handling and when wetted by water. Added fat also covers the surface of carbohydrate particles in a food, preventing proper starch gelatinization during the steam conditioning and extrusion process. Ingredients which are water repellent or contain little gelatinizable material (rice and oat hulls, alfalfa, bones in animal by-products) weaken a pellet. They are also often springy, and after being forced through a die hole, tend to expand into a soft, loose food. Some ingredients contain large components which are hard to grind and supply little or no binding power. Rice and oat hulls, bone in animal by-products, the ectoderm of dent corn, within the pellet and protruding through the pellet wall, make the pellet a target for moisture entrance and rupture on handling. Hydroscopic ingredients such as salt, sugar, molasses, urea, allow water into a pellet. These materials raise the air-dry water content of stored pellets to a level that softens them even before their use in water” (Hastings, 1971).

4.3 Feeding stimulants

Maximum benefit from feeding can only be achieved if the food provided is ingested. An understanding of the feeding behaviour of the fish or shrimp is, therefore, essential. The diet presented must have the correct appearance (ie. size, shape and colour), texture (ie. hard, soft, moist, dry, rough or smooth), density (buoyancy) and attractiveness (ie. smell or taste) to elicit an optimal feeding response (Mackie and Mitchell, 1985; Meyers, 1987a; Heinen, 1980). However, the relative importance of these individual factors will depend on whether the fish or shrimp species in question is mainly a visual feeder or a chemosensory feeder. For example, although marine fish held in captivity generally rely on sight to locate their food, they also rely on chemoreceptors located in the mouth or externally on appendages such as lips, barbels and fins; the feed being carefully ‘sensed’ before ingestion. A similar situation also exists with marine shrimp and freshwater prawns. The use of dietary feeding stimulants for these cultivated species is therefore essential to elicit an acceptable and rapid feeding response. The practical importance of feed attractants and diet palatability is particularly critical during the weaning of marine fish larvae from a live to a non-living diet. Similarly, as attempts are made to replace the fishmeal component of practical fish feeds with unconventional protein sources of an alien nature, the problem of diet texture and palatability will become even greater. In addition, by using feeding stimulants and improving feed palatability, the period of time the feed remains in the water can be reduced, thus minimizing nutrient leaching.

Two types of feeding stimulants may be considered for use within aquaculture feeds; natural ingredient sources which exhibit attractant or feeding stimulant properties or the use of the purified or synthetic chemical derivatives which are responsible for the attractant property of natural ingredient sources. For example, feed ingredients which have been found to impart specific attractant properties for shrimp and marine fish include squid meal, mussel flesh, shrimp meal and waste, short-necked clam flesh, marine polychaete worms, blood worms, certain terrestrial oligochaete worms, marine fish oils, fish meal, fish solubles, fish protein hydrolysates and soybean protein hydrolysates (Meyers, 1987a; Heinen, 1980; Appelbaum, 1980; Stafford, 1984; Tacon, 1983; Harada and Matsuda, 1984; Fuke, Konosu and Ina, 1981; Cadena Roa et.al., 1982; Harada, 1985; Mackie and Mitchell, 1985; and Shewbart, Mier and Ludwig, 1973). Purified or synthetic substances which have been found to act as dietary feeding stimulants include mixtures of L-amino acids (particularly amino acid mixtures including glycine, alanine, proline and histidine: Salmonids-Adron and Mackie, 1978; Mearns, 1986; eel-Takeda, Takii and Matsui, 1984; Takii et.al, 1986; european sea bass - Mackie and Mitchell, 1985; yellowtail - Harada and Matsuda, 1984; marine shrimp - Deshimaru and Yone, 1978; and possibly the freshwater prawn - Farmanfarmaian et.al., 1979), mixtures of L-amino acids and the quaternary amine glycine betaine (marine shrimp - Meyers, 1987a; Carr, 1978; dover sole - Mackie and Mitchell, 1985; Cadena Roa et.al, 1982; Metailler Cadena-Roa and Person-Le Ruyet, 1983; yellowtail - Harada and Matsuda, 1984), the nucleosides inosine and inosine-5-monophosphate (Turbot - Mackie and Mitchell, 1985; Person-Le Ruyet et.al., 1983; yellow tail - Mackie and Mitchell, 1985), the nucleotide uridine-5-monophosphate (eel - Takeda, Takii and Matsui, 1984) and trimethyl ammonium hydrochloride (freshwater prawn-Costa-Pierce and Laws, 1985). Good dietary sources of betaine and soluble nucleotide bases include mussels, polychaetes, squid, shrimp waste and shrimp blanch water, fish products, polychaetes, respectively (Meyers, 1987a; Heinen, 1980). Figure 4 shows the effect of various potential dietary feeding stimulants on the food intake of rainbow trout fed a ‘bland’ soybean based ration.

Satiation food intake relative to soybean control diet (0)

Figure 4. Effect of various dietary supplements on the satiation food intake of rainbow trout fed a soybean meal based trout ration (Tacon, 1983).

Notes: Composition of soybean based diet: soybean meal (hexane extracted) 35%, White fish meal (Icelandic) 27%, soybean: fish oil (1:1) 7%, meat meal 4%, meat and bone meal 10%, brewers yeast 5%, corn (expanded) 5%, wheat middlings 5%, vitamin and mineral premix 2% (diet contained 45% protein, 10.5% lipid)

Sprayed shrimp meal extract-100g of dried shrimps made up to 350ml with water, allowed to stand overnight, 150ml of supernatant decanted, and 100ml sprayed on to 2.5kg of dry diet with air nozzle, and the pellets dried to 10% moisture.

Sprayed fish silage-5kg of flatfish minced with sufficient formic acid to lower ph to 4, stored for 2 weeks, homogenized, and 300ml of silage mixed with 2.5kg dry diet, and the pellets dried to 10% moisture.

Meat aroma-H and R Nr 94830, Haarmann and Reimer. GmbH; natural dry aroma, type: roasted meat. ‘Maggi’ soup aroma - Nestles hydrolysed vegetable protein, plus-flavours and herbs. Fish aroma - Dry fish-ade, Feed Flavors Incorp.

Vinasse - Concentrated product remaining after fermentation and distillation of beet molasses, Zuid Nederlandse Spiritus Fabrick (rich source of betaine).

4.4 Food colourants

Food colourants are substances which are added in trace amounts to a diet or feed mixture to facilitate its ingestion (through improved visibility of feed particles) or to impart a desired colouration within the carcass of the cultured fish or shrimp. For example, Dendrinos, Dewan & Thorpe (1984) were able to improve the feeding efficiency of larval and post larval sole (Solea solea) by staining Artemia salina nauplii with different food colourings (staining achieved by keeping nauplii in a 0.5% (v/v) solution of the food dye for a 48h period) so as to improve contrast perception of the food in relation to the illumination of the background (see Figure 5). Table 32 lists the permitted dietary food colourants within the United Kingdom (MAFF, 1973)

Figure 5. Effect of staining Artemia salina nauplii with different coloured food dyes on the feeding efficiency of larval sole in glass tanks (Dendrinos et.al, 1984).

¹ ‘colourant’ means any substance, other than a basic feed ingredient, which is added to a feeding stuff only to impart colour to the feeding stuff or to an animal product, but does not include a substance which is added to material only for the purpose of rendering that material fit only for animal feeding (MAFF, 1973).

An important factor governing the consumer acceptance and market value of many cultivated fish and shrimp species is the pink or red colouration of their flesh or boiled exoskeleton. In the wild this colouration is derived through the ingestion of carotenoid pigments contained within invertebrate food organisms; the characteristic pink colouration of salmonids and red sea bream, and the red colour of the boiled crustacean exoskeleton, being mainly due to the carotenoid pigment astaxanthin (Johnson, Conklin and Lewis, 1977; Ibrahim, Shimizu and Kono, 1984). The carotenoid pigments represent a widespread group of plant-synthesized polyene pigments, which vary in colour from yellow and orange to red. The chemical structure of some important carotenoids are shown in Figure 6. Although animals, including fish and shrimp, are believed to be unable to synthesize carotenoids de novo, certain aquaculture species (ie. crustaceans, omnivorous/herbivorous fish) are capable of transforming ingested carotenoids such as Β-carotene and depositing the resulting end products, usually astaxanthin, in their tissues (Simpson and Chichester, 1981; NRC, 1983). By contrast, carnivorous fish species such as salmonids and red sea bream are believed to be incapable of carotenoid transformation; the ingested carotenoid being deposited in its unaltered state within the body tissues (Simpson, 1979; NRC, 1983; Ibrahim, Shimizu and Kono, 1984). Since this natural carotenoid pigmentation is usually lacking within intensively farmed fish and shrimp with no access to live (carotenoid containing) food organisms, it is necessary to fortify practical rations with carotenoid - rich ingredient sources (ie. marine animal by-products such as shrimp waste, fish and crustacean oils, marine yeast and algae; Table 33) or with purified carotenoid preparations (ie. canthaxanthin powders) if the farmer is to achieve a marketable product with the desired carcass pigmentation. For example, in Scandinavian countries (ie. Norway), shrimp offal meals are commonly used as natural carotenoid sources within commercial salmonid rations (Hildingstam, 1976). For those countries where natural carotenoid-containing feedstuffs are in short supply, adventitious artificially produced carotenoids must be added to the ration; commercially available canthaxanthin being the predominant pigment source for salmonid fish in the United Kingdom. Although these industrially produced microbial pigments are costly, they usually impart the same visual colouration of muscle as natural astaxanthin sources, and as a dry powder can be included within rations at controllable levels. Within the United Kingdom, canthaxanthin (in the form of carophyll red) is routinely added to salmonid rations at a dietary level of 50–100mg carotenoid/kg for a two or three month feeding period prior to harvesting or restocking. However, the success of the pigmentation programme will depend upon a variety of factors, including: dietary pigment source and concentration, length of feeding and continuity of feeding the pigmented diet, size of the fish or shrimp, water temperature, developmental status of the fish or shrimp (ie. sexual maturity), hormonal status of the animal, chemical form of the carotenoid fed, photoperiod regime, dietary lipid/emulsifier concentration, and the availability of natural food organisms (Johnson, Conklin and Lewis, 1977; Foss et.al., 1984; Torrisen et.al., 1981; Torrisen, 1985; Choubert, 1979, 1986; Choubert and Luquet, 1983). In addition to the role of carotenoids in carcass pigmentation and as dietary sources of provitamin A, these compounds may play other important biological functions within the animal body (for review see Choubert, 1986; Gerber and Erdman, 1982; Peto et.al, 1981 and Tacon, 1981).

Figure 6. Structure of some important carotenoids

¹ Data obtained from NRC (1983), Simpson, Katayama and Chichester (1981); Choubert (1979), Choubert and Luquet (1983), Ibrahim, Shimizu and Kono (1984) and Foss et.al., (1984).