The effect of nisin in preserving dairy products

As a natural peptide preservative, Nisin has become an important alternative to chemical preservatives in dairy product preservation due to its strong inhibitory effect on Gram-positive bacteria (especially heat-resistant pathogenic bacteria such as Bacillus and Listeria). Its efficacy can be comprehensively evaluated from three dimensions: microbial control, shelf-life extension, and quality maintenance.

I. Targeted Inhibition of Specific Microorganisms

Spoilage of dairy products (such as fresh milk, yogurt, and cheese) is mainly caused by excessive proliferation of lactic acid bacteria, germination of heat-resistant spore-forming bacteria (e.g., Bacillus subtilis), and contamination by pathogenic bacteria (e.g., Listeria monocytogenes). Nisin can efficiently inhibit the growth of these microorganisms by disrupting the cell membrane structure of Gram-positive bacteria (forming transmembrane channels, leading to leakage of intracellular substances).

In fresh milk, adding 0.03-0.05 g/kg of nisin can significantly reduce the survival rate of heat-resistant spore-forming bacteria, especially strongly blocking the germination of spores remaining after pasteurization. For example, in a 37°C accelerated experiment, the number of spore-forming bacteria in fresh milk without nisin increased to 10⁶ CFU/mL within 48 hours, while samples with nisin remained controlled below 10³ CFU/mL after 5 days under the same conditions. For fermented dairy products such as cheese, Nisin can inhibit the abnormal proliferation of spoilage lactic acid bacteria in the later stage, avoiding excessive acidification and flavor deterioration (e.g., bitterness, off-odor) of the product. In addition, targeting potential Listeria contamination in dairy products, the minimum inhibitory concentration (MIC) of nisin can be as low as 0.1-1.0 μg/mL, much lower than its conventional addition level in food, effectively reducing food safety risks.

II. Effect on Extending Shelf Life

The preservation effect of nisin is closely related to the type of dairy product, storage conditions, and synergistic treatment methods. In fresh milk stored at low temperatures (4-7°C), adding nisin alone can extend the shelf life from the conventional 7-10 days to 14-21 days; when combined with pasteurization (72°C/15s), through the synergistic effect of "heat+nisin" (heat damages bacterial cell membranes, enhancing the penetration efficiency of nisin), the shelf life can be further extended to more than 28 days. Moreover, it does not rely on ultra-high temperature sterilization (UHT), thus better retaining nutrients (such as B vitamins) and flavor substances in milk.

For fermented milk such as yogurt, the addition of nisin can inhibit the growth of miscellaneous bacteria during post-fermentation, reducing abnormal acidity increase and whey separation caused by fluctuations in storage temperature (e.g., cold chain interruption). For example, plain yogurt with 0.02 g/kg nisin added can have its shelf life extended from 14 days to 21 days at 10°C, with significantly better texture uniformity and flavor stability than the control group. In hard cheeses, it can reduce the risk of mold growth during maturation by more than 60% by inhibiting surface mold and internal spoilage bacteria, while reducing texture hardening caused by secondary sterilization.

III. Impact on Dairy Product Quality

The natural peptide property of nisin results in little impact on the sensory and nutritional quality of dairy products. With a small molecular weight (3354 Da), it can be degraded into amino acids by proteases during digestion, leaving no residual odor or toxicity. Adding it to whole milk does not affect the emulsification stability of milk fat, and the color and taste of the product show no significant difference from the non-added group; in low-fat milk, it can even reduce the oxidative loss of fat-soluble vitamins (such as vitamins A and D) by inhibiting the growth of bacteria related to fat oxidation.

However, the efficacy of Nisin is greatly affected by the dairy product matrix: high calcium content may reduce its activity through chelation, so the addition level needs to be appropriately increased in high-calcium milk (usually to 0.06-0.08 g/kg); the adsorption of milk proteins may weaken its antibacterial efficiency, requiring technologies such as microencapsulation to reduce loss. In addition, nisin has weak inhibitory effect on Gram-negative bacteria, so in practical applications, it is often compounded with other natural preservatives (such as lysozyme) to cover a broader spectrum of microorganisms, while avoiding excessive use of a single component that may affect product quality.

Nisin demonstrates a balance between high efficiency and safety in dairy product preservation by precisely inhibiting target microorganisms, significantly extending shelf life, and having little impact on quality. Maximizing its efficacy requires combining product characteristics, storage conditions, and synergistic technologies. In the future, through dosage form optimization (such as sustained-release microspheres) and upgraded compounding schemes, it is expected to achieve more precise preservation control in high-end dairy products (such as organic milk and fermented milk drinks).