The Application of Nisin in convenience foods

Ready-to-eat (RTE) dishes, as a key category of convenience foods, have become a consumer hotspot in the fast-paced lifestyle due to their convenience of "ready-to-eat upon opening" or "ready-to-eat after heating." However, they are rich in protein, fat, carbohydrates, and moisture, making them vulnerable to microbial contamination after processing. During cold chain transportation (0–4℃), although low temperatures can inhibit most pathogenic bacteria, they cannot completely prevent the slow proliferation of psychrophilic microorganisms (such as lactic acid bacteria, Listeria, and spoilage yeasts). This leads to shortened product shelf life, sensory deterioration, and even food safety risks. Nisin (nisin), a natural antibacterial peptide, has emerged as a crucial technical means for cold chain preservation of RTE dishes, thanks to its efficient inhibitory effect on Gram-positive bacteria, low-temperature stability, and safety. By synergizing with the cold chain environment, it constructs a dual preservation system of "low temperature + antibacterial peptide," ensuring stable product quality during transportation and storage.

I. Microbial Spoilage Risks in RTE Dishes During Cold Chain Transportation

The processing flow (raw material handling, cooking, cooling, packaging) and cold chain links (warehousing, transportation, terminal display) of RTE dishes all pose potential microbial contamination risks. The dominant spoilage microorganisms in the cold chain environment are psychrophilic/cold-tolerant microorganisms, whose reproductive characteristics are highly compatible with the composition of RTE dishes and cold chain conditions:

1. Gram-Positive Pathogens and Spoilage Bacteria

These are the core sources of microbial risks in cold chain RTE dishes:

Listeria (Listeria monocytogenes): A typical psychrophilic pathogen that can reproduce slowly (with a generation time of approximately 8–10 hours) in the cold chain environment of 0–4℃. It has strong acid and salt tolerance and is prone to contaminating meat-based RTE dishes (e.g., sauced beef, braised chicken legs), soy products (e.g., braised dried tofu), and salad products. Its contamination may cause food poisoning, posing extremely high risks to the elderly, pregnant women, and immunocompromised populations.

Lactic acid bacteria (Lactobacillus plantarum, Lactobacillus brevis, etc.): Widely present in vegetable-based RTE dishes (e.g., cold cucumber salad, spicy lotus root slices) and mixed dishes (e.g., pre-prepared rice bowls with meat and vegetables). In the cold chain, they ferment carbohydrates in dishes to produce lactic acid and acetic acid, lowering the product’s pH value and causing a "sour spoilage taste." Meanwhile, they make the texture sticky (e.g., vegetables become mushy, meat loses elasticity), seriously impairing sensory quality.

Bacilli (Bacillus cereus): Their spores can withstand high cooking temperatures and germinate into viable bacteria during cooling and cold chain links. Although their reproduction rate is slow, they produce enterotoxins, leading to "bag swelling" and "off-odors" in products during shelf life. Moreover, the toxins are heat-resistant and may still cause food safety issues after heating.

2. Cold-Tolerant Yeasts and Molds

These are common in RTE dishes with high moisture and high carbohydrate content (e.g., sweet-tasting dishes, braised dishes containing starch):

Cold-tolerant yeasts (Candida, Pichia, etc.): Can slowly metabolize sugars at 0–4℃, producing ethanol, carbon dioxide, and organic acids. This causes dishes to develop a slight "alcoholic taste" or "fermented taste," and the aggregation of yeast cells forms white flocculent precipitates, damaging the appearance and homogeneity of dishes.

Molds (Penicillium, Aspergillus, etc.): Their spores easily contaminate products through packaging gaps or processing environment pollution. They germinate in the high-humidity cold chain environment (e.g., condensed water caused by temperature fluctuations during transportation), forming visible mold spots (green or black spots) and potentially producing mycotoxins (e.g., aflatoxin), which pose serious safety hazards.

The reproduction of these microorganisms intensifies with the extension of cold chain time: Typically, RTE dishes without effective preservation measures see microbial counts rise from an initial 10² CFU/g to over 10⁵ CFU/g after 1–3 days of cold chain transportation, reaching the spoilage threshold within 5–7 days—far exceeding food safety standards (microbial limits for RTE cooked meat products usually require ≤ 10⁵ CFU/g). This greatly limits the cross-regional transportation and sales radius of RTE dishes.

II. Mechanism and Application of nisin in Cold Chain Preservation of RTE Dishes

The value of nisin in cold chain preservation lies in its targeted inhibition of dominant spoilage microorganisms (especially Gram-positive bacteria) in the cold chain environment and its "synergistic antibacterial effect" with low temperatures, making up for the shortcomings of single cold chain preservation. Its mechanism and application methods need to be designed based on the category characteristics and processing flow of RTE dishes:

(I) Core Antibacterial Mechanism: Targeted Inhibition of Gram-Positive Bacteria and Synergistic Enhancement of Low-Temperature Effects

Direct Killing and Proliferation Inhibition of Gram-Positive Bacteria

As an antibacterial peptide, nisin targets the cell membrane of Gram-positive bacteria. For viable bacteria such as Listeria and lactic acid bacteria, nisin damages the cell membrane through two pathways:

Transmembrane pore formation: Amino acid residues in nisin bind to phospholipids on the bacterial cell membrane and cell wall precursor substances (lipid II), forming transmembrane pores (with a diameter of approximately 1–2 nm). This increases membrane permeability, causing massive leakage of intracellular nutrients such as potassium ions and amino acids, and imbalance of osmotic pressure, leading to cell death.

Interference with cell wall synthesis: The binding of nisin to lipid II competitively inhibits the involvement of lipid II in peptidoglycan synthesis, preventing bacterial cell wall assembly and division, fundamentally cutting off the microbial "reproductive chain."

This effect is more advantageous in the cold chain environment: Although low temperatures slow down bacterial metabolism,nisin’s molecular activity is not affected by low temperatures (retaining over 90% of its antibacterial activity at 0–4℃). It can continuously act on bacterial cell membranes, putting originally "slowly reproducing" bacteria into a "growth stagnation phase" or even gradual apoptosis.

For spores of Bacillus,nisin cannot directly kill spores but can inhibit the growth of viable bacteria after spore germination. When spores germinate into vegetative cells during the cold chain,nisin can quickly bind to and damage their cell membranes, preventing further proliferation and toxin production—this is particularly crucial for RTE dishes that have undergone high-temperature cooking (which kills viable bacteria but not spores).

Synergistic Enhancement with Cold Chain Environment

The limitation of single low-temperature preservation is that it "can only slow microbial reproduction, not stop it." Nisin can enhance the inhibitory effect of low temperatures by reducing microorganisms’ "cold tolerance." Studies have shown that after adding nisin, the generation time of Listeria at 0℃ extends from 8 hours to over 24 hours, and the growth lag phase of lactic acid bacteria extends from 2 days to 5–7 days. Additionally, nisin reduces microorganisms’ adaptability to cold chain temperature fluctuations: When short-term temperature rises occur during transportation (e.g., temperature rises to 10℃ during unloading), microorganisms in dishes without nisin may "recover" and reproduce rapidly, while those in nisin-treated groups remain inhibited, significantly reducing spoilage risks caused by temperature fluctuations.

(II) Application Methods Adapted to RTE Dishes

The application of nisin must balance antibacterial efficacy and product quality (e.g., taste, flavor, color), and appropriate addition methods and dosages should be selected based on the processing technology and category characteristics of RTE dishes:

Timing and Methods of Addition

Addition during post-cooking cooling: After RTE dishes undergo high-temperature cooking (usually 85–100℃ for 10–15 minutes), most initial microorganisms are killed. When cooled to 40–50℃ (to avoid damaging nisin activity due to high temperatures), dissolve nisin in a small amount of sterile water or dish soup, then evenly spray or stir it into the dishes. This ensures uniform distribution of nisin and reduces loss during processing (e.g., addition during raw material handling may lead to degradation by proteases in raw materials).

Addition during stewing/curing: For sauce-braised RTE dishes (e.g., sauced beef, braised eggs), nisin can be directly added to the braising broth. During stewing (temperature ≤ 80℃), it penetrates into the dishes with the broth, inhibiting residual microorganisms in the broth and forming an "antibacterial protective layer" inside the dishes to enhance preservation.

Surface treatment before packaging: For solid-form RTE dishes (e.g., fried chicken legs, roasted chicken wings), nisin can be made into a 0.05%–0.1% solution. The dish surface is treated by spraying or soaking to form an "antibacterial film," preventing microbial attachment and reproduction in the packaging air—this is particularly suitable for products using modified atmosphere packaging (MAP).

Dosage Control and Category Adaptation

According to China’s National Food Safety Standard for the Use of Food Additives (GB 2760), the maximum dosage of nisin in pre-prepared meat products, aquatic products, and soy products is 0.5 g/kg (approximately 500 IU/g). In practical applications, the dosage should be adjusted based on dish categories:

High-risk categories (meat and soy product RTE dishes): Dishes such as sauced beef and braised dried tofu, which are prone to contamination by Listeria and Bacillus, require a dosage of 0.2–0.3 g/kg (200–300 IU/g), which can effectively extend the cold chain shelf life from 3–5 days to 10–15 days.

Medium-risk categories (vegetable and mixed RTE dishes): Dishes such as cold kelp shreds and rice bowls with meat and vegetables, where lactic acid bacteria and yeasts are the main targets, require a dosage of 0.1–0.2 g/kg (100–200 IU/g), extending the cold chain shelf life from 2–4 days to 7–10 days.

Low-risk categories (high-acid and high-salt RTE dishes): Dishes such as spicy radish and pickles (pH < 4.0 or salt content > 5%), where the acidic or high-salt environment already inhibits some microorganisms, only require 0.05–0.1 g/kg (50–100 IU/g) to avoid affecting taste (high-concentration nisin may cause a slight bitter taste).

III. Synergistic Application of nisin with Other Cold Chain Preservation Technologies

Using nisin alone cannot cover all spoilage microorganisms in RTE dishes (e.g., it has weak inhibitory effects on yeasts and molds). It needs to be combined with other cold chain-compatible preservation technologies to build a "multi-dimensional synergistic preservation system" and further enhance preservation effects:

1. Synergy with Modified Atmosphere Packaging (MAP)

MAP changes the gas environment inside the packaging by filling inert gases such as nitrogen (N₂) and carbon dioxide (CO₂) (usually 20%–40% CO₂ and 60%–80% N₂), inhibiting microbial respiratory metabolism. The synergistic advantage of nisin and MAP lies in: CO₂ enhances nisin’s inhibitory effect on Gram-positive bacteria (by lowering the pH of the bacterial cell membrane, making it easier for nisin to penetrate), while nisin compensates for CO₂’s insufficient inhibition of yeasts and molds. For example, in the cold chain preservation of sauced beef, using "Nisin (0.2 g/kg) + MAP (30% CO₂ + 70% N₂)" extends the cold chain shelf life from 10 days (using Nisin alone) to 18–20 days, with no sour spoilage or bag swelling.

2. Synergy with Natural Plant Extracts

To meet consumers’ demand for "clean labels," nisin can be compounded with natural plant extracts with antibacterial activity (e.g., tea polyphenols, rosemary extract, cinnamaldehyde). These extracts not only inhibit yeasts and molds (e.g., cinnamaldehyde has an inhibition rate of over 90% on mold spore germination) but also have antioxidant effects, delaying fat oxidation in RTE dishes (e.g., browning and off-odors in meat dishes). For example, adding "Nisin (0.15 g/kg) + tea polyphenols (0.02%)" to cold chicken shreds not only extends the cold chain shelf life to 10 days but also maintains the tender color and flavor of the chicken, avoiding a "rancid taste."

3. Synergy with Low-Temperature Plasma (LTP) Pretreatment

LTP technology generates reactive oxygen and nitrogen species to kill microorganisms (including bacteria, yeasts, and mold spores) on the surface of RTE dishes, reducing the initial number of contaminating bacteria. Combining it with nisin forms a closed-loop of "pretreatment to reduce bacteria + full-process antibacterial": First, LTP treatment (30–60 seconds) reduces the number of surface microorganisms from 10³ CFU/g to below 10¹ CFU/g; then nisin is added to inhibit the reproduction of residual microorganisms during the cold chain. This can extend the cold chain shelf life of RTE dishes by 2–3 times while reducing nisin dosage (to 0.08–0.15 g/kg), balancing safety and economy.

IV. Key Precautions in Application

1. Avoid Direct Contact with Proteases

Nisin is a peptide substance that is easily degraded by natural proteases in RTE dishes (e.g., trypsin in meat, proteases in vegetables), reducing its activity. Therefore, nisin should be added only after dishes are fully cooked (to inactivate proteases at high temperatures), or mixed with a small amount of protease inhibitors (e.g., citric acid, which slightly inhibits protease activity) during addition.

2. Control pH and Water Activity (Aw)

Nisin is more active in an acidic environment (pH 4.0–6.0). For neutral or weakly alkaline RTE dishes (e.g., some braised dishes with pH 7.0–7.5), a small amount of food acid (e.g., citric acid, lactic acid) should be added to adjust the pH to below 6.0 to ensure nisin activity. Meanwhile, dishes with Aw > 0.95 are prone to microbial growth; dehydration (e.g., drying surface moisture) or thickeners (e.g., xanthan gum, which reduces free water content) should be used to control Aw at 0.90–0.95, forming a synergistic antibacterial effect with nisin.

3. Ensure Cold Chain Stability

Nisin’s role is to "inhibit microbial reproduction," not "kill all microorganisms." If the cold chain is interrupted for a long time (temperature > 10℃ for more than 4 hours), microorganisms may quickly exceed nisin’s inhibitory threshold, leading to product spoilage. Therefore, full-process temperature control (e.g., using refrigerated trucks with temperature recorders and intelligent insulation boxes) should be adopted to ensure that RTE dishes are stably maintained at 0–4℃ throughout transportation and storage.

By targeting Gram-positive bacteria and synergizing with low temperatures, nisin effectively solves microbial spoilage problems in RTE dishes during cold chain transportation, extending product shelf life and sales radius. Its synergistic application with technologies such as MAP and natural extracts not only expands the antibacterial spectrum but also aligns with the consumer trend of "natural, safe, and clean labels," providing reliable technical support for the industrialization and cross-regional development of RTE dishes.