The role of Nisin in maintaining the quality of thawed food

Thawing is a critical step in the processing, circulation, and pre-consumption stages of frozen foods. Microorganisms suppressed during low-temperature storage rapidly resuscitate and proliferate as temperatures rise; meanwhile, the food’s endogenous enzymatic and oxidative reactions accelerate, easily leading to spoilage, flavor loss, and texture deterioration. As a natural peptide preservative, Nisin can effectively maintain food quality during thawing through mechanisms such as targeted antibacterial activity and synergistic protection. Its functions focus on three core dimensions: microbial control, quality deterioration mitigation, and safety enhancement, with specific analyses as follows:

I. Core Function: Inhibiting Microbial Proliferation During Thawing to Block Spoilage Pathways

Under the -18℃ cold-chain condition, most microorganisms in frozen foods remain dormant, but spores or vegetative cells of Gram-positive bacteria such as Bacillus cereus, Listeria monocytogenes, and Staphylococcus aureus can survive low temperatures. When thawing raises the temperature to 0–10℃, these microorganisms quickly resuscitate and multiply, decomposing proteins, fats, and other components in the food to produce off-odors, slime, and even trigger foodborne illnesses.

Nisin exerts targeted inhibitory effects against such pathogenic bacteria:

Targeted Destruction of Bacterial Cell WallsNisin binds to lipid Ⅱ on bacterial cell membranes, interfering with peptidoglycan synthesis in the cell wall, causing membrane leakage and bacterial lysis. It exhibits particularly significant inhibitory effects on Gram-positive bacteria in the resuscitation phase. Adding 100–150 IU/g of Nisin to thawed frozen meat products (e.g., meatballs, bacon) can reduce the total bacterial count by 2–3 orders of magnitude after 24 hours of storage at 4℃ post-thawing, and decrease the detection rate of Bacillus cereus by more than 80%.

Inhibition of Spore GerminationUnlike conventional chemical preservatives, Nisin can penetrate the cortex structure of spores, preventing their transformation into vegetative cells and fundamentally reducing microbial contamination during thawing. For example, adding 80 IU/g of Nisin to frozen dumpling fillings can control the spore germination rate below 5% even when left at room temperature for 4 hours after thawing, which is far lower than the 40% rate observed in the non-added group.

Adaptation to the Microenvironment During ThawingThe pH of food slightly decreases (mostly ranging from 5.0 to 6.0) during thawing, an acidic environment where Nisin demonstrates enhanced activity, enabling it to exert its antibacterial effects fully. Additionally, the free water produced during thawing promotes the diffusion of Nisin in the food matrix, allowing it to uniformly cover both the surface and interior of the food and form a sustained antibacterial barrier.

II. Synergistic Function: Mitigating Food Quality Deterioration During Thawing to Maintain Texture and Flavor

During thawing, food is not only threatened by microorganisms but also suffers from texture softening, flavor fading, and color darkening due to water loss, enzymatic reactions, and oxidation. Although Nisin does not directly regulate enzymatic or oxidative reactions, it can synergistically delay quality deterioration through indirect means:

Reducing Quality Damage Caused by Microbial MetabolismMicrobial proliferation leads to the secretion of hydrolases such as proteases and lipases, which accelerate the degradation of meat proteins and fat oxidation, resulting in tough meat texture and rancid odors. By inhibiting microbial growth, Nisin significantly reduces the secretion of these hydrolases, thereby slowing down the decomposition rate of proteins and fats and preserving the tenderness and flavor of thawed food. For instance, adding Nisin to frozen surimi products can increase the gel strength of the thawed surimi by 15%–20% without causing any noticeable fishy odor.

Synergistic Protection with Other PreservativesNisin is often used in combination with natural preservatives such as chitosan, tea polyphenols, and ε-polylysine, forming a dual protection system of "antibacterial + antioxidant" in thawed food. For example, the combined treatment of frozen fresh-cut fruits and vegetables with Nisin and chitosan leverages chitosan’s ability to reduce water loss during thawing and inhibit polyphenol oxidase activity (preventing browning), while Nisin is responsible for inhibiting microorganisms. This synergy allows thawed apple slices and broccoli to retain their crisp texture and bright color even after 3 days of storage at 4℃.

No Impact on the Intrinsic Texture and Flavor of FoodNisin is colorless, tasteless, and has a small molecular weight, so it does not bind to components such as proteins and starches in food. Therefore, it does not alter the texture (e.g., tenderness of meat, elasticity of pasta) or inherent flavor of food during thawing, meeting consumers’ demand for "original taste and flavor".

III. Auxiliary Function: Reducing Chemical Preservative Dosage to Enhance the Safety of Thawed Food

To extend shelf life, traditional thawed foods are often supplemented with chemical preservatives such as potassium sorbate and sodium dehydroacetate. Excessive addition of these chemicals can cause abnormal flavors and fail to meet the "clean label" consumption trend.

Nisin has distinct advantages in this regard:

Replacing or Reducing Chemical PreservativesNisin can be used alone or in combination to replace more than 50% of chemical preservatives. For example, in thawed frozen prepared dishes (e.g., braised pork, Kung Pao chicken), a compound system of Nisin (120 IU/g) and citric acid (0.1%) can completely replace sodium dehydroacetate, while still meeting food safety standards even after 6 hours of room-temperature storage post-thawing.

Avoiding Side Effects of Chemical PreservativesSome chemical preservatives may migrate and accumulate with water during thawing, affecting food flavor. In contrast, Nisin is degraded into amino acids by proteases in the human body, leaving no residues or cumulative toxicity. Even appropriate addition to thawed food poses no harm to human health.

Reducing Cross-Contamination Risks of Thawed FoodMicroorganisms on the surface of thawed food can easily spread through contact, causing cross-contamination. The antibacterial barrier formed by Nisin reduces the number of viable bacteria on the food surface, lowering the likelihood of pathogenic bacteria spreading during processing and consumption.

IV. Key Factors Affecting the Efficacy of Nisin

Addition Timing and Method

Recommended Addition Before Freezing: Dissolve Nisin and distribute it uniformly in food through dipping, spraying, or stirring. During freezing, Nisin becomes immobilized in the food matrix along with ice crystals and is rapidly released during thawing to exert its effects. If added after thawing, the concentration needs to be increased by 20%–30%, and thorough stirring is required to ensure uniform dispersion.

Avoid High-Temperature Treatment: If the food requires heating and cooking after thawing, the temperature should be controlled below 100℃, as high temperatures reduce Nisin activity and compromise its antibacterial efficacy.

Food Matrix Characteristics

High-Protein and High-Fat Foods (e.g., frozen meat and aquatic products): Proteins and fats can bind to Nisin, reducing its free concentration. Thus, the addition dosage needs to be appropriately increased (30%–50% higher than that for ordinary pasta products).

Alkaline Foods (e.g., frozen soybean products): Acidity regulators such as citric acid should be added to adjust the pH to 5.0–6.0, enhancing Nisin activity.

Optimization of Compounding SchemesNisin alone has limited inhibitory effects on Gram-negative bacteria (e.g., Escherichia coli). Combining it with EDTA can disrupt the outer membrane structure of Gram-negative bacteria, broadening the antibacterial spectrum. The combination of Nisin and natamycin can inhibit both bacteria and molds, making it suitable for maintaining the quality of mold-prone frozen baked foods (e.g., bread dough, egg tart shells) after thawing.

The core value of Nisin in maintaining the quality of thawed food lies in the precise control of microbial risks during thawing, as well as the mitigation of texture and flavor deterioration through synergistic protection, while also boasting natural and safe properties. To maximize its efficacy, the addition timing, concentration, and compounding scheme need to be optimized based on food type. In the future, with the application of technologies such as microencapsulation, Nisin can achieve intelligent release during thawing, further improving the quality stability and safety of thawed food.