What is Lactic Acid and Why is it Important for Fish Quality?

Fish is a popular and healthy food that many people enjoy. However, fish is also very sensitive to spoilage and quality loss after harvest. One of the factors that affects the quality of fish is the level of lactic acid in the muscle tissue. In this blog, we will explain what lactic acid is, how it is formed in fish, how it influences the quality of fish, and how it can be controlled or reduced.

What is Lactic Acid?

Lactic acid is a natural substance that is produced by the cells of living organisms, including fish, humans, and bacteria. Lactic acid is formed when the cells break down glucose (a type of sugar) for energy without enough oxygen. This process is called anaerobic metabolism or fermentation.

Lactic acid has different roles and effects in different organisms. For example, in humans, lactic acid is produced by the muscles during intense exercise, when the oxygen supply is not sufficient to meet the energy demand. Lactic acid can cause muscle fatigue and soreness, but it can also be used as a fuel source by the liver and other tissues. In bacteria, lactic acid is produced by some species that can ferment carbohydrates, such as lactobacilli and streptococci. Lactic acid can act as a preservative and a flavor enhancer in some fermented foods, such as yogurt, cheese, and sourdough bread.

How is Lactic Acid Formed in Fish?

In fish, lactic acid is produced by the muscle cells during normal metabolism, when the oxygen supply is adequate. The lactic acid is then transported by the blood to the liver, where it is converted back to glucose. This process helps to maintain the pH balance and energy supply of the fish. However, after harvest, the blood circulation stops and the oxygen supply is cut off. The muscle cells continue to break down glucose for energy, but without oxygen, they produce more lactic acid than normal. The lactic acid accumulates in the muscle tissue, causing the pH to drop and the muscle to become acidic.

The amount of lactic acid formed in fish depends on several factors, such as the species, the size, the freshness, the handling, and the storage conditions of the fish. Some fish species, such as tuna and salmon, have higher metabolic rates and higher glycogen (stored glucose) levels than others, such as cod and haddock. This means that they produce more lactic acid after harvest. The size of the fish also affects the lactic acid production, as larger fish have more muscle mass and more glycogen than smaller fish. The freshness of the fish is another factor, as fresher fish have higher glycogen levels than older fish. The handling and storage conditions of the fish also influence the lactic acid formation, as stress, injury, temperature, and time can affect the metabolic rate and the oxygen availability of the fish.

How Does Lactic Acid Influence the Quality of Fish?

The level of lactic acid in fish has a significant impact on the quality and shelf life of fish. Some of the effects of lactic acid on fish quality are:

• Texture: The low pH caused by lactic acid weakens the connective tissue and causes the muscle fibers to contract and shrink. This results in a loss of water-holding capacity, firmness, and elasticity of the fish flesh. The fish becomes soft, dry, and prone to gaping (separation of muscle blocks).
• Color: The low pH caused by lactic acid affects the stability and solubility of the pigments in the fish flesh. This results in a loss of brightness, transparency, and freshness of the fish color. The fish becomes pale, dull, and discolored.
• Flavor: The low pH caused by lactic acid alters the chemical composition and activity of the enzymes and proteins in the fish flesh. This results in a loss of natural flavor, aroma, and taste of the fish. The fish becomes sour, bitter, and rancid.
• Microbial spoilage: The low pH caused by lactic acid creates a favorable environment for the growth of spoilage bacteria, especially lactic acid bacteria, which further produce more lactic acid and other metabolites. This results in a rapid deterioration of the fish quality and safety. The fish becomes slimy, rotten, and contaminated.

How Can Lactic Acid be Controlled or Reduced in Fish?

The level of lactic acid in fish can be controlled or reduced by applying the following methods:

• Proper handling and stunning: The fish should be handled gently and stunned quickly and effectively before slaughter to avoid stress and muscle exhaustion, which can increase the lactic acid production. The fish should also be bled properly to remove the blood and lactic acid from the muscle tissue.
• Rapid cooling and icing: The fish should be cooled and iced as soon as possible after harvest to slow down the metabolic and enzymatic activities and reduce the lactic acid accumulation. The fish should be stored in ice or refrigerated at temperatures below 4°C to maintain the low temperature and prevent bacterial growth.
• Salting and acidification: The fish can be treated with salt or organic acids, such as citric acid or acetic acid, to increase the pH and inhibit the lactic acid bacteria. The salt or acid can be applied as a dry rub, a brine solution, or a marinade to the fish surface or flesh. The fish can also be smoked, dried, or fermented to preserve the quality and extend the shelf life.

Conclusion

Lactic acid is a natural substance that is produced by the cells of living organisms, including fish, humans, and bacteria. Lactic acid is formed when the cells break down glucose for energy without enough oxygen. Lactic acid has different roles and effects in different organisms, but in fish, it is a major factor that affects the quality and shelf life of fish. Lactic acid causes the pH to drop and the muscle to become acidic, which leads to texture, color, flavor, and microbial deterioration of the fish. To control or reduce the level of lactic acid in fish, proper handling, stunning, cooling, icing, salting, and acidification methods should be applied. By following these methods, the quality and safety of fish can be maintained and improved. 😊