As a result of the growing demand for fish from consumers around the world, wild catch is unable to sustain the demand and in response an aquaculture industry is developing rapidly. In the meantime, fish meal and fish oil from unconsumed wild catch have become insufficient as the sole feed source for fish such as salmon, trout, shrimps, sea bass or sea bream, to mention only some species.

During the last decennia, fish feed formulations containing vegetable proteins and oils, as supplements for fish meal and fish oil, were continuously developed. The most popular vegetable protein sources encountered as fish meal substitutes in these formulations were selected among soybean meal, soy protein concentrate, corn gluten, pea protein and vital wheat gluten. These modern fish feed diets are most often prepared in the form of pellets. These pellets are made by first grinding and mixing together ingredients such as fish meal, vegetable proteins, and binding agents such as wheat flour. Then water is added and the resulting paste is extruded whereby the pellets obtained have dimensions which are controlled by the die of the extruder. These pellets are then dried and oils are added. Adjusting parameters such as temperature and pressure, enables the manufacturers to make pellets that suit different fish farming methods, for example feeds that float or slowly sink.

Among the vegetable proteins used as fish meal substitutes, vital wheat gluten is considered as a valuable source of protein and as a pellet binder. In a study made by Storebakken et al., published in Aquaculture (2000) p.1 15-132, it was shown that vital wheat gluten supported rapid growth and was highly digestible, did not reduce digestibility of fat or energy or availability of essential elements in Atlantic salmon. Thereby up to 50% of crude protein of fish meal was replaced with crude protein from wheat gluten. The study also showed that vital wheat gluten, at a level up to 35% of dietary protein, caused no pathological changes in the intestinal epithelium.

In a more recent study by S. Helland and Grisdale-Helland B., published in Aquaculture (2006) vol. 261, 4, p.1363- 1370, fish meal replacement by wheat gluten, in diets for Atlantic halibut was possible even at a high inclusion level of 20% of the diets. A further increase to 30% was also possible when supplementing the diet with lysine. The effect of wheat gluten incorporation on pellet morphology and physical and functional properties was studied by Draganovic et al. and reported in Aquaculture Nutrition Volume 19, issue 6, pages 845-859, dec. 2013. The inclusion of vital wheat gluten in fish feed pellets, under the conditions used during these experiments, showed a reduction in oil impregnation and oil uptake during coating but yielded strong, highly porous pellets.

The effects of ingredient composition and processing conditions on the physical qualities of extruded high energy fish feed were reviewed by Sorensen M. in Aquaculture Nutrition (2012) 18/3, p. 233-248. In the conclusions of this article it is stated that the production of pellets with predictable quality is, however, challenging because physical quality of the pellet is the result of the interaction between a broad range of processing conditions and the different ingredients. Among this broad range of processing conditions, energy input is one. This energy input is composed of two fractions: thermal energy which represents approximately 2/3 and mechanical energy which represents about 1/3 of that energy. This mechanical energy, reported as specific mechanical energy (SME) is generated by friction caused by the dough in the extruder barrel, as it is moved forward by the rotating screw. In the process of manufacturing fish feed pellets, reduction of energy consumption and increase of production capacity, without affecting the quality of the material produced, are both improvements which are desirable from an economical and technical point of view. At the same time pellet properties such as durability, hardness, expansion, fat absorption and water stability should remain optimal, in order not to affect negatively feed intake and digestion.

Thereby, pellet hardness and durability will determine storage and transportation stability of the pellet, while expansion and fat absorption will have an effect on sinking velocity. In the case of salmonids, the commercial grow-out diets typically contain up to 400g fat per kg of feed. From literature it is not always clear to what extent pellet hardness affects digestibility: it was mentioned in literature (Pillay & utty, 2005, Feed and nutrition. In: Aquaculture: Principles and Practices, 2 ^nd edn. Pp.105- 173, Blackwell Publishing) that too hard pellets may cause digestive disturbances. Thereby it is known that pellet hardness increases with the incorporation of plant ingredients. (M. Sorensen, Aquaculture Nutrition, 2012). In this publication, already cited earlier, values ranging between 40 and 80N are mentioned. Therefore, controlling hardness might be an advantage, when other properties are not changing too much.

During extensive experimental work by the applicant, related to the substitution of other vegetable proteins by wheat gluten in fish feed compositions it has now been surprisingly observed that replacing these vegetable proteins by wheat gluten proteins in such formulations has a positive impact on energy consumption during the extrusion step, thereby resulting in an increased throughput even in composition containing a low content of fish meal (less than or equal to 10% of the total formulation). The applicant has further observed that pellet hardness could be controlled by combining the wheat gluten with a limited quantity of protein hydrolysate without seriously affecting durability, fat absorption and drying time of the pellets, in a negative way.

As a result the invention can then be formulated as to provide fish feed pellets containing fish meal, characterized in that the pellets comprise a protein hydrolysate and wheat gluten preferably, vital wheat gluten in a ratio wheat gluten: protein hydrolysate of 5: 1 to 20: 1, wherein the wheat gluten content in the pellet (preferably the vital wheat gluten content) varies between 15 and 35% w/w of the pellet. Preferably, the pellet referred to is the one already coated with oil or fat. Preferably, the fish feed pellet of the invention comprises less than 60, 50, 20 or 10% fish meal preferably between 1 to 10% of fish meal.

As used herein, the term "fish feed pellet" refers to a composition of fish feed that has been subjected to a heat treatment, such as a steam treatment, and extruded through a machine. The fish feed pellet generally contains at least a protein fraction, a binder, a lipid fraction and optionally, minerals and/or vitamins. Fish feed pellet is commonly made by extruding the protein raw materials, the binder and optionally the minerals and/or the vitamins to which water is added. Lipids can be added before extrusion but are commonly added into the porous pellets formed by extrusion (for example by coating). According to the invention, the lipid fraction may be vegetable or animal oil or fat such as soy oil, corn oil, palm oil, castor bean oil, maize germ oil or fish oil. The protein fraction comprises mainly fish meal and vegetable proteins. Indeed, the fish feed pellet may be coated or non-coated typically by vegetable or animal oil or fat. According to the invention, protein fraction comprises a protein hydrolysate and gluten preferably wheat gluten more preferably vital wheat gluten. The term "wheat gluten" means the protein fraction insoluble in water extracted from wheat flour wet and then dried. The wheat gluten principally comprises the Gliadins and the Glutenins. The wheat gluten is preferably vital wheat gluten which is the native form of gluten. The vital gluten has viscoelastic properties due to interaction between Gliadins and Glutenins in the presence of water. Preferably, the wheat gluten is a mixture of vital gluten and modified wheat gluten. The modification of gluten may be denaturation (eg. thermal, acid or enzymatic solubilization) of vital wheat gluten. Typically, the wheat gluten content varies between 7 to 40 wt% of the pellet, preferably, between 8 to 39, 10 to 38, 12 to 37, 14 to 36, 15 to 35, 16 to 34, 17 to 25, or 18 to 24 wt. % of the pellet. For example, the wheat gluten content varies between 15 to 35 or 15 to 25 wt. % of the pellet. The binder is usually in the form of a starch-containing raw material, such as wheat flour, potato flour, rice, rice flour, pea flour, beans or tapioca flour, to give the feed the desired strength and form stability.

Preferably, said fish feed pellet comprises 30 to 70% by weight of protein, 5 to 20% by wt of binder and, 10 to 50% of lipid. According to the invention the fish feed pellet size is between 0.5 to 30 mm depending on the age and type of fish. Thereby the wheat gluten proteins replace vegetable proteins selected among soy protein concentrate, soybean meal, corn gluten meal, pea protein concentrate, potato protein or other available protein sources such as lupin seed meal, cotton seed meal, linseed meal or sunflower meal in part or as a whole. This list is not to be considered as limiting, but merely as an illustration.

The vegetable protein hydrolysate may be from soy protein, soybean protein, corn protein, pea protein, potato protein, lupin protein, cotton protein, linseed protein or sunflower protein and/or a mix thereof. Among those vegetable protein hydrolysates, wheat protein hydrolysates are the most preferred. The protein hydrolysates can be obtained by means of standard chemical or enzymatic processes, which are well illustrated in the prior art. Typically, protein hydrolysates are purified protein hydrolysates.

The pellets are further characterized in that they have a hardness of less than 40N, preferably less than 35, 30, 25N, in combination with a Pellet durability Index (PDI) of greater than 94, preferably greater than 95, 96, 97, 98 or 99.

Hardness was analysed by diametrical compression using a Schleuninger tablet tester described in the literature by van der Poel in: Physical quality of pelleted animal feed: 1. Criteria for pellet quality (1996). PDI was determined by means of the Holmen Pellet Tester NHP100, as described on httpi/A Av.tekpro.com/hohiien-portable-nhpl 00. html. Thereby the analysis was done over a period of 60 sec. Controlled hardness, in combination with low dust formation (high PDI) is advantageous with regard to feed uptake and digestibility of the fish feed pellet. At the same time expansion ratios between 20 and 30% were realized, thereby providing a good quality sinking pellet.

In a preferred embodiment, the ratio wheat gluten: protein hydrolysate varies from 8: 1 to 14: 1, more preferably from 9: 1 to 12: 1. The wheat gluten protein content in the pellet preferably varies between 15 and 25% w/w of the pellet, more preferably between 18 and 22% w/w of the pellet.

The invention also concerns a process for preparing fish feed pellets or a process for reducing the Specific Mechanical Energy (SME) in the extrusion plant for the preparation of fish feed pellets wherein said method comprises the following steps:

grinding and/or mixing of wheat gluten, fish meal and vegetable protein hydrolysate, wherein the ratio of wheat gluten: vegetable protein hydrolysate is between 5: 1 - 20: 1 ;

adding water and homogenizing the obtained mixture until a paste with a moisture content of 28-32 % is formed;

extruding the paste obtained by an extrusion installation comprising a mold and a number of mixing and kneading zones, composed of a plurality of alternately forward and backward kneading screw elements, wherein the temperature in the extrusion plant is increased from 50°C at the inlet to 150°C in the final kneading zone to the die;

cutting of the extruded material exiting the die, into porous pellets of a suitable length;

drying of the porous pellets to a moisture content of 6 to 10%;

absorbing of a preheated vegetable or animal oil or fat in the porous pellet under vacuum, and

cooling of the pellets thus obtained at room temperature. According to the invention, the use of a combination of a vegetable protein hydrolysate and wheat gluten in a composition of fish feed pellets preferably in a ratio wheat gluten: protein hydrolysate of 5: 1 to 20: 1, provides the reduction of SME in a process of preparing said fish feed pellets.

Grinding of the different ingredients is performed in any suitable mill such as, e.g. a pin mill, a roller mill, a turbo mill or a hammer mill, a hammer mill being preferred. The water added during the preparation of the paste has a temperature of between 10 and 30°C, preferably, no additional steam was used.

According to the invention "SME" or "specific mechanical energy" represents the energy transferred from the main drive motor of the extruder through frictional heating for melting, mixing and die pressurization in the compounding process. The SME will thus vary, depending on the screw used (high shear or low shear screw) and the composition processed (resistance of the composition to the rotation of the screw). The calculation of SME is performed using the extruder motor load (e.g. shaft torque), screw speed and total throughput to provide energy input on a unit mass basis. Typically, the SME value is calculated using the formula disclosed by Levine in Cereal Foods World, 1997, p.22 and recalculated from kWh/kg into kJ/kg. Indeed, the reduction of the SME value reflects a reduction of energy consumption during processing or the opportunity to increase production capacity with identical energy consumption.

During the processing of the compositions disclosed above it has been surprisingly observed that the throughput of the extruder was increased by at least 17%, preferably at 25%, and more preferably by at least 30%, as the result of the use of the wheat gluten: protein hydrolysate combinations as claimed. Thereby a formulation containing 10% wheat gluten was taken as the reference (see reference example 1). Thus, the invention further concerns a process for improving the production of a fish feed pellet, said process comprising the step of adding to the composition of said fish feed pellets a combination of a vegetable protein hydrolysate and wheat gluten, preferably in a ratio wheat gluten: protein hydrolysate of 5: 1 to 20: 1 and the step of extruding said composition and thereby improving the throughput of the extruder.

Hereafter the invention will be illustrated by means of a number of examples. These examples are to be considered as an illustration of the invention and should not be considered as limiting the scope of the invention.

Examples All experiments were performed on a ZS 25-type twin screw extruder from Coperion using a screw configuration composed of a number of elements. The screw speed was set at 130 rpm and the six compartments of the barrel were heated respectively, starting from the inlet, at 50°C, 70°C, 90°C, 120°C, 150°C and 150°C.

When necessary, the different ingredients were milled using a hammer mill, to provide a fine powder. Then all ingredients were mixed and homogenized together with water in order to obtain a paste containing 30% moisture.

This paste is then extruded under the conditions mentioned before. The expanded pellets thus obtained are then dried to a moisture content of about 9%, before the coating step.

The coating with oil is then performed in a pellet vacuum coater (PG-10VC, Dinissen BV, Sevenum, the Netherlands), using vacuum conditions of 0.2 bar, and a heated oil at 60°C. Thereby pellets having an oil content of 30.6 -31% w/w on the pellet are prepared. Following reference compositions and compositions according to the invention were used for preparing fish feed pellets;

Reference examples 1-3:

Reference formulations contain no protein hydrolysate, be it wheat based or originating from another source.

Table 1: Reference formulations containing 10, 20 and 30% vital wheat gluten (VWG) on commercial product

Pellets prepared, using these compositions of table 1 , using the processing conditions disclosed above, did show the following properties:

Table 2: Hardness and PDI of reference pellets containing 10, 20 and 30% wheat gluten on commercial product.

Table 2 clearly shows that the physical characteristics of the pellets such as hardness and PDI are not altered by the addition of vital gluten. The SME- values for the reference examples 1-3 were respectively 189, 167 and 142kJ/kg.

Examples 4-7:

The following formulations, according to the invention, contain vital wheat gluten in combination with a vegetable protein hydrolysate. Thereby VWG stands for vital wheat gluten, HWG stands for hydrolysed wheat gluten, and HSP stands for hydrolysed soy protein. The hydrolysed wheat gluten protein used is SOLPRO 508, commercialised by TEREOS SYRAL and the hydrolysed soy protein is ESTRIL 75, from SOPROPECHE.

Table 3: Pellet compositions according to the invention, ingredients being expressed as % on commercial product Pellets prepared, using these compositions of table 3, using the processing conditions disclosed above, did show the following properties:

Table 4: hardness and PDI of pellets according to the invention

The SME-values for the examples 4-7, according to the invention were respectively 160, 164, 160 and 128 kJ/kg.

It is clear from the above that the incorporation of hydrolysed vegetable proteins results in comparable hardness and PDI values versus a reference formulation containing 10% V WG, when increasing gluten content.

Other properties, such as bulk density (of coated pellets), OAC (oil absorption capacity) and sinking velocity are represented in the following table. Thereby these values are compared with those of a reference formulation containing 10% VWG (reference example 1).

The OAC is in fact the "maximum oil infusion" into pellets as disclosed in V. Draganovic et al. in Aquaculture Nutrition (2012) DOI: 10.1111/anu.12029.

Sinking velocity is determined according to the method disclosed in Aquaculture Nutrition (2012) vol. 18, p.233-248.

10% 18/2 18/2 19/1 27/3 WG WG.-HWG WG.-HSP WG.-HWG WG.-HWG

Bulk 604 597 598 601 606 density(g/l)

OAC 41 ,8 40,2 43,6 43,5 42,9

Sinking 8,5 6,2 ND ND ND velocity

cm/s)

Table 5

The table 5 clearly confirms that the principal physicochemical characteristics of the pellets are maintained although their SME are clearly reduced by the addition of vegetable protein hydrolysate and vital gluten has here above mentioned.