The preservative effect of Nisin in fried foods

Fried foods (e.g., fried potato chips, fried chicken, fried dough sticks, fried soybean products) are rich in fats, carbohydrates, and proteins. After processing, they are susceptible to contamination by Gram-positive bacteria, while also facing issues such as lipid rancidity and short shelf life. As a natural microbial-sourced antimicrobial peptide, nisin (lacticin) boasts advantages including good thermal stability, targeted antibacterial activity, and no impact on product flavor. It can effectively enhance the preservation efficacy of fried foods, meeting the demands for clean labels and healthy processing. Its specific functions are as follows:

I. Spoilage Challenges of Fried Foods and Nisin’s Adaptability

1. Core Spoilage Issues

During the processing of fried foods, high-temperature frying can kill some microorganisms but cannot completely eliminate heat-resistant spore-forming bacteria (e.g., Bacillus subtilis, Bacillus cereus) and Staphylococcus aureus. In addition, fried products are prone to secondary contamination during the cooling process, leading to spoilage phenomena such as mold growth, souring, and stickiness during storage. Furthermore, traditional fried foods often rely on high salt content and preservatives to extend shelf life, which does not align with the trend of healthy consumption.

2. Process Compatibility Advantages

Nisin exhibits excellent thermal stability. After short-term exposure to the high-temperature environment of frying (150–200℃), it can still retain 60%–70% of its activity, making it resistant to processing steps such as frying and sterilization. It is colorless and tasteless, and will not alter the crispy texture and unique flavor of fried foods. Meanwhile, nisin can form an antibacterial film on the surface of foods to resist microbial contamination during circulation, which is compatible with the production and storage characteristics of fried foods.

II. Preservation Mechanisms and Effects of Nisin in Fried Foods

1. Targeted Inhibition of Gram-Positive Bacteria to Extend Shelf Life

Nisin binds to lipid Ⅱ on bacterial cell membranes, disrupting cell wall synthesis and forming transmembrane pores, which causes the leakage of intracellular nutrients and leads to bacterial death. It has significant inhibitory effects on common spoilage and pathogenic bacteria in fried foods (e.g., Bacillus cereus, Staphylococcus aureus, Clostridium botulinum).

Application Cases: Adding 0.03–0.05 g/kg of nisin to the seasoning powder of fried potato chips can extend the product’s shelf life from 15–20 days to 30–45 days at room temperature, effectively inhibiting rancidity and mold growth caused by spore-forming bacteria. Spraying 0.08 g/kg of nisin solution on the surface of fried chicken pieces can prolong the refrigerated (4℃) shelf life from 3–5 days to 10–15 days, reducing the risk of Staphylococcus aureus contamination.

For fried soybean products (e.g., fried tofu puffs, fried yuba), nisin can inhibit the excessive proliferation of lactic acid bacteria that causes souring and spoilage, maintaining the product’s bean aroma and firm texture.

2. Synergistic Antioxidation to Delay Lipid Rancidity

Lipid rancidity is a key factor affecting the shelf life of fried foods. Although nisin has no direct antioxidant effect, it can reduce the decomposition of fats by inhibiting lipase-producing microorganisms (e.g., certain spore-forming bacteria), thereby indirectly delaying the formation of free fatty acids and peroxides. When compounded with natural antioxidants such as tea polyphenols and vitamin E, it can achieve dual effects of "antibacterial activity + antioxidation", significantly reducing the acid value and peroxide value of fried foods and extending the oxidative stability period of fats.

3. Supporting Salt and Preservative Reduction to Optimize Formula Healthiness

Traditional fried foods often use high salt content and chemical preservatives such as potassium sorbate for preservation. Nisin can work synergistically with low-salt formulas to reduce salt dosage by 20%–30%. At the same time, it can replace or reduce the amount of chemical preservatives added, and even completely replace chemical preservatives in some product categories, meeting consumers’ demand for clean labels. For example, adding 0.04 g/kg of nisin in the production of fried dough sticks can reduce the dosage of alum and salt by 25% while still ensuring the product’s preservation effect and texture stability.

4. Reducing Secondary Contamination for Ready-to-Eat Scenarios

Ready-to-eat fried foods (e.g., ready-to-eat fried chicken, fried snacks) are prone to secondary contamination during packaging, transportation, and sales. Nisin can be added via spraying, soaking, or mixing when the products are cooled to below 60℃ after frying, forming an antibacterial barrier on the food surface to inhibit microbial growth during circulation. This application is particularly suitable for ready-to-eat products distributed under cold chain or room temperature conditions.

III. Application Precautions

1. Dosage Control

Strictly comply with GB 2760 National Food Safety Standard for the Use of Food Additives. The maximum dosage of nisin is 0.5 g/kg (based on pure nisin) in fried meat products, and 0.1 g/kg in fried fruit and vegetable products as well as soybean products. Excessive addition does not enhance antibacterial effects but may increase costs.

2. Compound Application to Expand Antibacterial Spectrum

Nisin has weak inhibitory effects on Gram-negative bacteria (e.g., Escherichia coli) and no direct inhibitory effect on molds. It is recommended to compound nisin with the following substances:

Compound with EDTA or citric acid at a ratio of 5:1 to disrupt the outer membrane structure of Gram-negative bacteria and expand the antibacterial spectrum.

Compound with natamycin to inhibit mold growth in fried pastries and flour products.

Compound with natural antioxidants (tea polyphenols, rosemary extract) to meet both antibacterial and antioxidation needs.

3. Process Compatibility Key Points

It is recommended to add nisin during the cooling stage after frying to avoid activity loss caused by prolonged high-temperature frying. If it needs to be added before frying, nisin can be incorporated into batter or breading to reduce the duration of high-temperature exposure.

The pH value of fried foods typically ranges from 5.0 to 7.0, which is the optimal range for nisin stability. If the product is acidic (e.g., fried pickled foods), the dosage of nisin should be appropriately increased.

4. Storage Requirements

Store nisin in a sealed, light-proof container under refrigeration. Avoid contact with strong acids and alkalis. Nisin solutions should be used as soon as possible after preparation to prevent activity degradation.

Through targeted antibacterial activity, synergistic antioxidation, and support for salt and preservative reduction, nisin effectively addresses the problems of product spoilage, lipid rancidity, and short shelf life in fried foods, while maintaining the flavor and texture of fried foods, in line with the trends of clean labels and healthy processing. By rationally controlling the dosage, optimizing compound formulations, and improving process compatibility, the application effects of nisin in various fried foods can be further enhanced, providing technical support for the quality upgrade of the fried food industry.