Effects of Processing on the Nutritional Value of Fish Feeds

Various ingredients are used in the preparation of supplementary food to meet the nutritional needs of fish. Foods and prepared feeds are processed to improve the nutritional value of these ingredients. The reasons why food is processed are: (1) to facilitate the movement of food; (2) can be shaped like a pellet; (3) the quality of food can be increased by increasing digestibility; (4) Special growth inhibitors can be inactivated. 

Some food processing methods increase the quality of food while some methods reduce the quality of food and make the food inedible. Therefore, food processing should be done in such a way as to improve the quality or nutritional value of the food. Below are the different methods of food processing:

Use of temperature in food processing (Heat Treatment): Heat is applied to dry some plant foods and to increase the quality of pellet food. Sometimes heat is applied to remove excess oil from oily foods. The application of heat destroys the naturally occurring trypsin inhibitor in soybeans and increases the value of proteins, amino acids, lipids and sugars present in food. The application of heat increases the digestibility of food. The application of such heat increases the nutritional value of cereal grains. Heat is applied through a stream of hot water to make high quality pellet food.

Crushing: Crushing reduces the shape of food items which helps in increasing the nutritional value of the food and reduces the surface area of the food consumed as it helps in digestion of food. This method also eliminates the lack of molecular minerals by adding metallic substances to the food from the crushing machine. Iron (Fe), Zinc (Zn), Copper (Cu), Manganese (Mn) and Sodium (Na) are all metals or minerals that are added to food from crushing machines.

Pelleting and Crumbling: Pelleting and crumbling methods increase the amount of nutrients in the food by mixing the food ingredients firmly. It increases the acceptability and taste of food to some species. Pelleting also increases the digestibility of food and destroys the toxins naturally present in some vegetable foods. It increases the amount of phosphorus in wheat bran.

Effects of processing on the destruction of naturally occurring toxins and nutrients in plant foods

Cotton Seed Meals

Cotton seed meal contains a substance called gossip which reduces the quality of nutrients. Such free gossip is integrated with the pigment gland. The free gossip emitted from the pigment gland mixes with other components of the cotton seed to form bound gossip. Such gossips are less harmful than free gossips. If there is a high level of gossip in the food, the growth of fish is hindered. The combination of gossip with lysine reduces the availability of amino acids in fish. As a result, the growth of fish is reduced. It adversely affects the liver and kidneys of fish.

Cyclopropionic fatty acids are found in all types of cotton seeds. It is not possible to remove it through the oil extraction process. Its effects increase wound healing in the body, increase the level of glycogen and increase the level of soluble fatty acids in the liver. Such substances often destroy the liver by reducing the activity of enzymes in the liver. This type of material can be used in the preparation of fish food by heat treatment.

Food processing is done to remove oil from gossip-containing cotton seed meals. Hydraulic pressure causes high levels of gossip from cotton seeds. Glandless cotton seeds contain low levels of gossip. Mixing cotton seed meal with mineral salts reduces the toxicity of gossip but results in food coloring which is unacceptable to food producers and fish farmers. Mixing phytase with cotton seed meal increases phosphorus levels, reduces gossipol toxicity, and increases the amount of some proteins.

Soybean Meal

Plants, especially soybeans and mustard, contain a variety of protease inhibitors that vary in molecular weight, structure and function. Five types of protease inhibitors have been found in soybeans. Such protease inhibitors are associated with the enzymes chymotrypsin, trypsin, which interferes with its effectiveness. As a result, the digestibility of food decreases and the growth of fish decreases. When most protease inhibitors or their sources are properly heated, their effectiveness is reduced.

Like protease inhibitors, soybeans contain another type of nutrient inhibitor, called hemagglutinin or lectin. It is a proteineous substance with a special addiction to sugar. Apart from oilseeds, such material is also found in tree roots and tubers. It reduces the absorption of nutrients in the intestines of the fish, prevents red blood cells from clotting, and slows down the growth of the fish by causing internal bleeding. As such nutrients are prone to heat, they can be deactivated through heat treatment.


Erucic acid is found in rapeseed oil seeds. It contains 22-carbon monounsaturated fatty acids. 20-55% of rapeseed oil is erucic acid. According to Parker and Hendricks, 3-6% of erucic acid causes fish death in various parts of the body, especially in the skin, gills, kidneys and heart. Such ingredients must be inactivated by heat treatment before use in food.

Other Feed Ingredients

Phytic acid is present in different types of oilseeds and grains. Phytic acid is converted to insoluble phosphate by combining with various bipolar organic matter such as calcium, magnesium, zinc, phosphorus etc. taken with food. As a result, the organic matter mentioned in the intestines of the fish cannot be properly absorbed. This causes malnutrition in fish. High levels of phytate cause zinc deficiency in fish. One study found that 0.5% phytic acid reduced fish growth by 10%. Phytic acid is often associated with dietary protein and reduces the digestibility of meat. As a result, the growth of fish is reduced. Most plant food products contain about 80% phosphorus, which causes fish poisoning in the form of phytates. The harmful effects of phytic acid can be reduced by crushing food and adding iron salts to food.

Cyanogen is found in large amounts in some grains, root crops, and fruit kernels. It is also found in some oil plants and they are not toxic but hydrolysis themselves, providing thiocynate, isocyanate, goitin and nitrite, and in such hydrolysis process mainly one kind of enzyme mirosinase is responsible. All of them act as anti-thyroid ingredients and prevent the secretion of thyroid gland. Such enzymes can be inactivated by applying heat treatment. The cyanogen found in mustard oil is soaked in water for 24 hours to stop its poisoning.

Table: Destruction of toxins and nutrient inhibitor through processing

Feed ingredients

Growth inhibitors

Neutralization Process

Cotton Seed Meal

Gossip Cyclopropane fatty acids; Phytate; Protease inhibitor

Adding of mineral salts; Rupture of the pigment gland; Soaking in water; Apply a small amount of heat; Use in controlled doses.

Soybean meal

Trypsin inhibitor; Phytic acid; Antivitamin; Hemagglutinin

Applying heat treatment; Autoclaving; Soak in water.

Lynn Seed Meal

Crystalline water-soluble matter

Washed by water

Raw fish


Applying heat treatment

Alfalfa Meal

Saponins; Pectin methyl estrogen

Feeding small amounts of food


5-N-alkaline resorcinols

Feeding small amounts of food

Sprouted wheat


Applying heat treatment


Isothiocyanate; Thyroactive substances


Epil epil


Soak in water and apply heat.


Protease insulation; Phytic acid; hemagglutinin

Soak in water and apply heat and add ferrous salt.

Rice husk

Protease insulation; Phytic acid;

Soak in water and apply heat treatment and add ferrous salt.


Protease insulation; Phytic acid;

Soak in water and apply heat treatment and add ferrous salt.

Effects of processing on fungi, bacteria and other harmful substances

Fungi and Mycotoxins: The fungus Aspergillus flavus produces a toxic substance called aflatoxin in some foods that cannot be inactivated by the usual pelleting process. Preventing the growth of fungi is a significant way. It is also possible to keep other foods free from the fungus by removing the substance damaged by the fungus. Some foods such as corn, peanut meal, cotton seed meal and fish meal can be easily contaminated by aflatoxin. As a result the presence of aflatoxin should be checked regularly.

Bacteria (Salmonella): Pelleting kills bacteria (Salmonella). Meat meal is a type of food ingredient that is infected by bacteria. Therefore, regular testing can reduce bacterial (Salmonella) infections.

Effects of processing on nutritional value and adequacy of vitamins

Prolonged exposure to heat destroys fat-soluble vitamins and water-soluble vitamins such as thiamine, pantothenic acid, folic acid, and biotin. This type of heat is applied during drying of proteins obtained from plants and animals. As a result of such processing, the thiamine in the meat and bone is completely destroyed. Vitamin B dissolved in water is removed during fish meal processing. Unprocessed fish contains more vitamin B than fish meal.

The pelleting method changes the amount of vitamins in the diet. If the food contains insufficient antioxidants, the pelleting method can destroy vitamins A, E and K. Pelleting on the other hand can increase the amount of nicotinic acid and biotin. Nicotinic acid and biotin are added to food.

Some trace minerals act as pro-oxidants in food and destroy vitamins A, D, and K. Such trace elements are iron and manganese. Nutritious food additives such as bentonite binding agents are added to the diet which cause side effects with riboflavin and zinc.

Processing of various oil seed meal reduces the level of vitamins in the diet. For example, coarse wheat flour makers reduce the amount of vitamin E than fine wheat flour makers. Sprouted wheat seeds have lower levels of vitamin E than 30% of germinated wheat seeds. Removing oil from soybean meal reduces Vitamin E and Vitamin K, which in turn reduces the quality of the product.

Effects of Processing on the Adequacy and Nutritional Quality of Secondary Minerals

Processing usually has no effect on the adequacy of minerals. Adequacy of zinc in the pellets diet is accelerated. Pellets and crumbling have no effect on trace mineral levels in the diet. Almost all the anti-nutrient or anti-growth ingredients present in various foods used to make fish supplements are sensitive to heat. When the supplementary food is prepared, the effectiveness of the anti-nutrient or anti-growth product is lost if the food is half-cooked or heat is applied. As a result the quality of food increases. Harmful levels of these anti-growth or anti-growth products vary depending on the amount of their presence in the supplementary diet, fish species, fish size, age and physiological conditions.