Color plays a crucial role in the sensory appeal and quality evaluation of food, and aquatic products are no exception. Consumers associate the color of seafood with freshness, safety, and overall quality. Understanding how color is formed, how it deteriorates, and how it can be preserved or enhanced is vital for the seafood industry. This article delves into the mechanisms of natural coloration in aquatic products, factors leading to discoloration, and methods used to maintain or enhance the color of these products.
The natural coloration of aquatic products is primarily due to skin chromatophores, pigment proteins, and carotenoids like astaxanthin. Chromatophores are pigment-containing cells located in the dermis of aquatic animals, responsible for their varied skin colors. The most common types of chromatophores are melanophores (black-brown), xanthophores (yellow), erythrophores (red), and iridophores, each contributing to the color variation based on the distribution and concentration of pigments. For example, melanophores, which synthesize melanin, are the most influential in giving fish their characteristic colors.
Endogenous pigment proteins, such as myoglobin (Mb), hemocyanin (Hc), and crustacyanin, also significantly impact aquatic product color. Myoglobin, found in the muscle tissue of aquatic animals, plays a vital role in oxygen storage and transport. The state of myoglobin—whether deoxygenated (purple-red), oxygenated (bright red), or oxidized (brown)—affects the overall color of the fish muscle. Hemocyanin, a copper-containing respiratory protein in arthropods like shrimp, imparts a blue color when oxygenated, but deoxygenated hemocyanin is colorless. Crustacyanin, another pigment protein in the shells of shrimp and crabs, binds with astaxanthin to produce a blue color. Upon cooking, the complex denatures, releasing free astaxanthin, turning the shell red.
Carotenoids like astaxanthin are critical for the vivid orange-red color observed in many aquatic species, especially salmon. Astaxanthin is stored in muscle tissue and is responsible for the vibrant coloration of seafood like shrimp and salmon. However, free astaxanthin is unstable and often forms esters for better storage.
Discoloration in aquatic products can result from several factors, including enzymatic browning, lipid oxidation, and the degradation of pigments. A key factor influencing discoloration is myoglobin oxidation. In the presence of oxygen, myoglobin oxidizes into metmyoglobin (brown), which leads to an undesirable color change, particularly in tuna and swordfish during storage.
Lipid oxidation is another primary cause of color deterioration in seafood. The fats in aquatic species, especially polyunsaturated fatty acids, are highly susceptible to oxidation. This process generates free radicals that damage cell membranes, leading to browning. Moreover, lipid oxidation accelerates the oxidation of myoglobin, creating a feedback loop that worsens discoloration.
In crustaceans, enzymatic browning due to polyphenol oxidase (PPO) can also cause undesirable darkening, particularly melanosis or “black spot” formation in shrimp. PPO catalyzes the oxidation of phenolic compounds into quinones, which polymerize into black pigments, especially during storage.
Several external factors influence discoloration in seafood. These include:
Genetics and environment: The natural coloration of fish is heavily influenced by genetics and environmental factors such as water temperature, quality, and light exposure. For instance, prolonged exposure to high temperatures can increase melanocyte production, leading to darker skin.
Slaughter methods: The methods used to slaughter and process aquatic animals impact their coloration. Techniques such as electrical stunning or bleeding can reduce hemoglobin and myoglobin content, thereby preventing discoloration during storage.
Storage conditions: Temperature and oxygen availability during storage play critical roles in preventing or accelerating discoloration. Freezing at ultra-low temperatures (-40 to -60°C) can preserve the bright red color of fish by slowing lipid oxidation and myoglobin degradation.
Processing methods: High-temperature cooking, smoking, and drying can cause irreversible chemical reactions, such as Maillard reactions, leading to browning.
Due to the negative economic impact of discoloration, various technologies have been developed to maintain or enhance the color of aquatic products. These include physical, chemical, and emerging methods.
Ultra-high pressure (UHP): UHP is used to maintain the quality of seafood by inactivating spoilage microorganisms and enzymes without excessive heat. It can enhance the whiteness of fish by denaturing myoglobin, reducing its red color.
Low-temperature plasma: This non-thermal technology inactivates PPO, preventing enzymatic browning in shrimp. It can also be combined with other techniques for better results.
Irradiation: This method uses gamma or electron beam irradiation to control microbial contamination and delay discoloration. However, irradiation can oxidize lipids and proteins, causing color changes in some seafood.
Microwave heating: Microwave-assisted drying and cooking have been shown to preserve or enhance the brightness and whiteness of fish fillets by limiting oxidative reactions.
Lactates: Sodium or potassium lactates are commonly used to maintain the color stability of seafood. Lactates reduce myoglobin oxidation, thus preserving the bright red color of fish.
Natural preservatives: Extracts from plant sources such as rosemary, basil, and tea polyphenols are gaining popularity as natural preservatives that inhibit lipid oxidation and maintain the color of seafood. Chitosan, a natural polysaccharide derived from crustacean shells, has been effective in preventing melanosis and improving the shelf life of seafood.
Nitrites: Nitrites help maintain the red color in seafood by forming stable nitrosomyoglobin. However, due to health concerns, their use is regulated, and alternative natural preservatives are being sought.
Nanotechnology: The use of nanomaterials for packaging can inhibit oxidation and microbial activity, helping to preserve the natural color of aquatic products during storage.
Edible coatings: Coatings made from proteins, polysaccharides, and lipids provide a physical barrier to oxidation and can incorporate color-protecting agents such as antioxidants.
Maintaining the natural color of aquatic products is essential for ensuring their marketability and consumer appeal. While discoloration is inevitable due to biological and environmental factors, advances in technology offer promising solutions. By combining physical treatments, chemical preservatives, and emerging technologies like nanotechnology, the seafood industry can effectively combat discoloration and enhance product quality. Understanding the complex interplay between pigment chemistry, oxidation processes, and external factors is crucial for developing tailored approaches to color preservation in the future.
Citation: Zhang, K., et al. (2024). "The natural color of aquatic products is derived from skin chromatophores, endogenous pigment proteins, and astaxanthin" (Food Chemistry, 434, 137495)
