Taste matters: Unraveling the significance of feed additives in enhancing feed palatability

Aquaculture relies on costly fish meal for protein, but declining resources necessitate changes. Increasing feed intake enhances growth. Feed palatability is crucial in designing diets to enhance consumption. Adequate feed stimulants are necessary to attract fish to the feed. Exogenous feed additives, including amino acids, betaine, amines, sugars, and nucleotides, have shown potential in stimulating feeding responses, particularly in plant-based or fish meal-free diets. This highlights their practical significance in fish nutrition by enhancing palatability and promoting feeding.

Aquaculture is a rapidly growing sector in animal food production, meets half of the global seafood demand. The high cost of fish meal, contributes significantly to production costs. Fluctuating availability of fish meal due to declining marine fish production has made aquaculture essential for supplying marine protein. To address this, there is a growing demand for animal or plant protein sources in fish feed formulation. Attempts to replace fish meal have been challenging due to its unique growth factors and superior attractiveness compared to local protein ingredients. Although local ingredients offer high digestibility and favorable amino acid profiles, the taste and attractiveness of fish meal stimulate the consumption of less appealing balanced feed. Palatability plays a vital role in fish feed formulation with local ingredients, as unpalatable feed can result in waste, poor growth, satiation, and reduced consumption. Enhancing feed palatability with additives becomes crucial.

FEED ADDITIVES TO IMPROVE PALATABILITY
Feed additives are substances that are added in small amounts to fish diets to enhance feed consumption. They serve various purposes, including improving feed stability, enhancing color appearance, stimulating feed intake, and preserving feed quality. By incorporating specific ingredients or mixtures into the basic feed, the specific requirements of the cultured organism can be met. While plant or animal protein sources can be used as substitutes for fish meal from a nutritional standpoint, they may have limitations in terms of palatability. To address this, the inclusion of feed attractants and flavors in the diet can promote faster food intake and reduce the leaching of water-soluble nutrients by minimizing the food’s duration in the aquatic system. As a result, improved feed intake leads to enhanced feed efficiency, minimizing losses and maximizing utilization.

A. FREE AMINO ACIDS
Different fish species have varying feeding behaviors and consume a combination of prey and aquatic plants. These feeds naturally contain free amino acids that are highly soluble in water. Due to the sensitivity of fish taste receptors, these free amino acids are attractive and elicit electrophysiological responses related to taste. Aromatic amino acids, specifically non-polar ones, are particularly effective in stimulating feed intake. The mixture of free amino acids is more attractive than the whole preferred food organism. Certain free amino acids, such as L-alanine, L-glutamic acid, glycine, and L-arginine, possess attractive properties. Palatability of free amino acids varies among species, with neutral amino acids being more palatable than mixtures of acidic and neutral amino acids in European eels. In sea bream, L-alanine and L-serine were found to be more palatable. Various amino acids, including L-glutamine, L-tyrosine, glycine, L-arginine, L-histidine, L-threonine, L-valine, L-tryptophan, and L-aspartic acid, have been reported as feed stimulants. However, certain amino acids like L-cysteine, L-alanine, L-serine, L-lysine, L-norvaline, L-leucine, and L-isoleucine are less attractive and rarely act as stimulants. In rainbow trout, L-isoleucine and L-leucine are recognized as feed stimulants.

The essential amino acid requirements in fish species are similar to those in terrestrial animals. However, it has been observed that the essential amino acids themselves do not contribute to the palatability of the feed in common carp. In fact, some amino acids can act as deterrents, imparting an aversive taste to fish. Certain amino acids, including L-valine, L-phenylalanine, L-histidine, L-proline, L-cysteine, and L-methionine, have been found to decrease the palatability of the feed significantly. These deterrent amino acids stimulate an aversive taste response in fish, leading to complete rejection of the diet when formulated with these amino acids. The taste properties of amino acids are stereotypic, with D-isomers of amino acids being deterrents and lacking taste significance, while L-forms of amino acids are highly palatable in rainbow trout.

B. BETAINE
Betaine, a water-soluble quaternary ammonium compound, was first discovered in sugar beet juice. It is recognized by the olfactory bulb and gustatory system of fish, making it an attractive feed stimulant. Found naturally in various fish diets, betaine is commonly used as an additive in aquaculture to enhance feed consumption. When added to plant-based or non-fish meal diets, betaine increases diet attractiveness, although reduced feed intake has been observed in certain fish species. Betaine also acts as a flavor enhancer for other amino acids in the diet. Additionally, it plays a role in osmoregulation and methyl donation, protecting cells from osmotic pressure changes. Studies have shown positive effects of betaine on feed intake and growth in juvenile grouper, rainbow trout, cobia, and crustaceans like Macrobranchium rosenbergii, Peneus monodon, and P. indicus by improving food-searching behavior.

C. NUCLEOTIDES AND NUCLEOSIDES
Fish acquire nucleotides through diet or synthesis pathways. These nucleotides act as feed stimulants. Nucleotides such as IMP, inosine, and their breakdown products enhance feed intake in various aquatic animals. Krill synthetic extract and yeast RNA extract increase feeding rates. Marine organisms with higher IMP show improved performance in soybean meal-based diets. Nucleotide supplementation improves feed intake, growth, immune responses, and disease resistance in species like Litopenaeus vannamei, Oreochromis mossambicus, Epinephelus malabaricus, and Rainbow trout. Synthetic mixes of IMP and AMP attract crabs and lobsters, increasing feed intake.

D. AMINES
Scavenging aquatic species, such as Indian River prawn and other fishes, are known to consume decaying matter. Biogenic amines like cadaverine and putrescine found in decaying material attract these species due to their resemblance to parent amino acids. These amines act as chemoattractants, incitants, and stimulants for feeding. However, trimethylamine oxide, creatinine, and guanidino compounds like glycocyamine and creatine have limited feed stimulation activity in certain species. A mixture of organic bases, including betaine, choline chloride, homarine-HCl, TMAO, TMA-HCl, and DMA-HCl, stimulates feeding when incorporated into feed pellets. The odor of amines in the feed or decaying material mimics food intake, resulting in increased feeding activity. Dimethylthetin acts as a taste stimulant in Dover sole, while other quaternary amines like homarine, betaine aldehyde, and trigonelline have no significant taste effect.

E. SUGARS
Sugars in fish can have varying effects on feed intake, acting as stimulants, deterrents, or indifferent taste substances. Certain sugars like D-glucose, sucrose, D-ribose, and D-fructose are highly palatable for rainbow trout, while others like D-galactose and D-xylose have no significant effect. However, in red sea bream, maltose and glucose were found to be ineffective in stimulating feed intake. A study involving different poecilids (Platy and Molly) tested 15 sugars and found that sucrose supplementation increased feed intake. Interestingly, there was a species-specific taste preference for sugars between Platy and Molly, indicating that taste preferences are highly specific to each fish species.

F. ORGANIC ACIDS
Organic acids are organic compounds with carboxyl groups and are produced through carbohydrate fermentation by bacteria under different conditions and metabolic pathways. In aquatic animals, organic acids have been observed to act as feed stimulants. For example, reactive amino-caproic acid has been found to increase feed intake in rainbow trout. Gamma amino butyric acid and L-Alpha-amino butyric acid were active at a 10-fold dilution in seabream, whereas succinic acid, malic acid, and citric acid were ineffective in stimulating feed intake. In tilapia, malic acid had no effect on feed consumption stimulation, while citric acid stimulated feed intake at a concentration of 0.01 M. Some organic acids like malic acid, alpha-keto-glutaric acid, and oxalic acid were highly palatable in certain fish species. However, the taste response to carboxylic acids varies among different fish species. Monocarboxylic acids with 3 and 5 carbons elicited the maximum taste response in common carp, while dicarboxylic acids showed a peak response with compounds containing 5 and 6 carbons. However, Bitterlings and Tench displayed different taste responses to carboxylic acids. Monocarboxylic acids like propionic acid, n-butyric acid, and n-valeric acid were indifferent taste substances for Bitterlings, while dicarboxylic acids like alpha-keto-glutaric acid and adipic acid acted as deterrents. In Tench, adipic acid was less palatable compared to malonic acid and succinic acids with two, three, and four carbon atoms.

CONCLUSION
The gustatory system of fish plays a critical role in their food intake, and taste preferences differ among species. To attract fish to the feed, it is crucial to include adequate feed stimulants in their diet. Supplementary feed additives like amino acids, betaine, amines, sugars, and nucleotides have significant importance in enhancing feed consumption. The use of exogenous feed additives shows promise in stimulating the feeding response when using plant-based ingredients or fish meal-free diets. This highlights the practical application of these additives in fish nutrition.

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About Kalaiselvan Pandi
Kalaiselvan Pandi is a Research scholar in the Department of Fish Nutrition and Feed Technology at Institute of Fisheries Post Graduate Studies of Tamil Nadu Dr. J. Jayalalithaa Fisheries University (TNJFU), India. He has been honored with the prestigious Bayer Fellowship “Medha”.

About Dr. Amit Ranjan
Dr. Amit Ranjan is an Assistant Professor in the Department of Fish Nutrition & Feed Technology at the Institute of Fisheries Post Graduate Studies of Tamil Nadu Dr. J. Jayalalithaa Fisheries University (TNJFU), India. His research focus is on fish and shrimp nutrition, where he conducts both strategic and applied research. With his extensive experience in commercial culture of shrimp and freshwater fish, he has published several research papers in international peer-reviewed journals and serves as a reviewer for over 30 international peer-reviewed journals.