The stability of Nisin in condiments

As a natural peptide preservative, the stability of Nisin (nisin) in condiments directly affects its bacteriostatic effect and application value. Its stability is mainly regulated by factors such as the physicochemical environment of condiments, processing technology, and storage conditions. Specific studies can be carried out from the following aspects:

I. Influence of pH on Nisin Stability

Nisin's molecular structure contains multiple amino acid residues (such as lysine and histidine), and its charge state changes with pH, thereby affecting stability:

In acidic conditions (pH 2.0-6.0), Nisin molecules are positively charged and their structure is relatively stable. This is because the acidic environment can inhibit the hydrolysis of intramolecular amide bonds, and the repulsive force between positive charges reduces the risk of molecular aggregation and precipitation. For example, in low-pH condiments such as vinegar and acidic soy sauce, Nisin can retain high bacteriostatic activity after long-term storage.

In neutral to alkaline conditions (pH>7.0), the stability of Nisin decreases significantly. The alkaline environment promotes the hydrolysis of its peptide bonds, leading to molecular 断裂，and the increase in negative charges may cause intermolecular aggregation, further reducing activity. Therefore, in neutral or weakly alkaline condiments such as broad bean paste and oyster sauce, the half-life of Nisin is short, and other means are needed to enhance its stability.

II. Influence of Temperature and Heat Treatment on Nisin

The thermal stability of Nisin is closely related to the pH of the medium it is in:

In acidic media, Nisin has a certain degree of heat resistance. For example, in vinegar with pH 3.0, after heating at 80°C for 30 minutes, its activity retention rate can reach more than 70%; even after 121°C high-pressure sterilization, it can still retain part of its activity. This characteristic allows it to adapt to the pasteurization or sterilization processes of condiments such as soy sauce and vinegar.

In neutral or alkaline media, Nisin is extremely sensitive to heat. For example, in broad bean paste with pH 7.0, heating at 60°C for 10 minutes can cause more than 50% loss of its activity. High temperatures will accelerate peptide bond hydrolysis and conformational damage. Therefore, in neutral condiments that require high-temperature processing, direct addition of Nisin during the heat treatment stage should be avoided; instead, it can be added after cooling to reduce activity loss.

III. Interference of Condiment Components on Nisin Stability

Components such as salt, oil, and protein in condiments may affect the stability of Nisin:

A high-salt environment (such as soy sauce with a sodium chloride content of 15-20%) can change the molecular conformation of Nisin through osmotic pressure, exposing some active sites and accelerating degradation. However, a moderate salt concentration (5-10%) has little effect on it, and may even indirectly extend its effective action time by inhibiting microbial reproduction.

Oil components (such as oil in mayonnaise and seasoning sauce) may cause Nisin to distribute at the oil-water interface. If the interfacial tension is too high, it will cause Nisin molecules to aggregate and precipitate, reducing their effective concentration in the aqueous phase. In addition, free radicals generated by oil oxidation may oxidize amino acid residues of Nisin (such as methionine), destroying its structure.

Macromolecular substances (such as polypeptides in soy sauce and monosodium glutamate in MSG) may non-specifically bind to Nisin to form complexes. Although they do not directly destroy its structure, they will reduce the concentration of free Nisin, indirectly affecting the bacteriostatic effect.

IV. Long-Term Influence of Storage Conditions on Nisin Stability

Temperature, light, and oxygen during storage are key factors affecting Nisin stability:

Low-temperature storage (such as 4-10°C) can significantly delay the degradation of Nisin. In acidic condiments, after 3 months of storage at 4°C, its activity retention rate can reach 60-80%; while at room temperature (25°C), the activity may drop to below 30% within the same period.

Light (especially ultraviolet light) can trigger photooxidation reactions of Nisin molecules, destroying their spatial conformation and leading to activity decline. Therefore, condiments containing Nisin need to use light-proof packaging (such as brown glass bottles) to reduce the impact of light.

In the presence of oxygen, Nisin is easily oxidized, especially under high temperature or alkaline conditions, the oxidation rate accelerates. Vacuum packaging or adding antioxidants (such as vitamin C) can reduce the damage of oxygen to Nisin and extend its validity period.

V. Strategies to Improve Nisin Stability

In response to the complex environment in condiments, Nisin stability can be enhanced through the following methods:

Compound with other natural preservatives (such as lysozyme, plant extracts), which can not only improve the bacteriostatic effect through synergistic effects but also reduce the usage concentration of single Nisin, reducing activity loss during degradation.

Adopt microcapsule embedding technology to wrap Nisin in polysaccharide or protein carriers, isolating the direct effect of the external environment (such as high salt, alkaline medium) on it, and controlling its slow release to extend the effective action time.

Optimize the addition timing: add Nisin in the later stage of condiment processing (such as the cooling stage) to avoid exposure to harsh processes such as high temperature and high shear, maximizing activity retention.

The stability of Nisin in condiments is comprehensively affected by multiple factors. It is relatively stable in acidic, low-temperature, and low-oxygen environments, but easily degrades under alkaline, high-temperature, high-salt, or oil-rich conditions. By targeted regulation of environmental parameters or adopting auxiliary technologies, its stability can be effectively improved, providing support for its wide application in condiment preservation.