Nisin is compounded with sodium hexametaphosphate

In the preservation of acidic foods (e.g., pickled fruits/vegetables, fermented milk, acidic beverages, canned goods), microbial contamination (e.g., overgrowth of lactic acid bacteria and molds) and quality deterioration (e.g., texture softening, dull color) are core challenges. The combination of nisin (a natural antimicrobial peptide) and sodium hexametaphosphate (a polyphosphate-based quality improver) addresses these issues through dual "antibacterial synergy + quality protection" effects. It not only enhances inhibition of pathogenic and spoilage bacteria but also maintains food texture and color stability. Additionally, the combination reduces the dosage of individual additives, aligning with the "natural, low-additive" trend in the food industry.

I. Core Advantages of the Combination: Logic of "Synergistic Enhancement" in Acidic Environments

Acidic foods typically have a pH of 2.5–4.5 (e.g., pickles: pH 3.0–3.5; yogurt: pH 4.0–4.5). While this environment inhibits some neutral bacteria, it has limited effect on acid-tolerant microorganisms (e.g., Lactobacillus plantarum, Penicillium) and accelerates mineral loss and cell wall degradation in foods. The combination of nisin and sodium hexametaphosphate complements each other to solve these two problems, with synergistic advantages derived from the compatibility of their mechanisms.

(I) Antibacterial Synergy: 1+1>2 Inhibitory Effect

Nisin, a natural antimicrobial peptide, inhibits Gram-positive bacteria (e.g., Listeria monocytogenes, Staphylococcus aureus—common pathogens in acidic foods) by disrupting bacterial cell membranes (e.g., forming pores, causing intracellular substance leakage). However, it has weak inhibitory effects on Gram-negative bacteria and molds. In acidic environments, its stability decreases (prone to degradation at pH < 3.0), requiring high dosages to maintain efficacy. Although sodium hexametaphosphate has no direct antibacterial activity, it enhances nisin’s inhibitory capacity in two ways:

Disrupting bacterial defense barriers: The phosphate groups of sodium hexametaphosphate bind to calcium ions on the bacterial cell membrane surface, weakening membrane stability and enabling nisin to penetrate more easily. Experiments show that in pickled cucumbers (pH 3.5), 0.1 g/kg nisin alone inhibits Lactobacillus plantarum by 65%, while combining it with 0.3 g/kg sodium hexametaphosphate increases inhibition to 92%.

Broadening the antibacterial spectrum: Sodium hexametaphosphate increases the membrane permeability of Gram-negative bacteria (e.g., E. coli—an occasional contaminant in acidic foods), allowing nisin to bypass their outer membrane barrier (nisin alone has almost no effect on E. coli, but the combination achieves >70% inhibition). Its metal ion-chelating ability also inhibits mold spore germination (mold growth requires magnesium and iron ions), filling the gap in nisin’s mold inhibition.

Furthermore, sodium hexametaphosphate improves nisin’s stability in acidic environments: by forming hydrogen bonds with nisin molecules, it reduces degradation at pH < 3.0, extending nisin’s half-life from 2 days to 5 days and ensuring long-term preservation.

(II) Quality Synergy: Maintaining Texture and Color of Acidic Foods

Acidic environments accelerate quality deterioration: acidic substances decompose pectin in fruit/vegetable cell walls (causing texture softening, e.g., pickled radishes turning mushy over time); polyphenol oxidase (PPO) remains active under acidic conditions, triggering browning (e.g., discoloration in canned apples); and metal ions (e.g., iron, copper) accelerate this process. Sodium hexametaphosphate is key for quality protection, while nisin indirectly supports quality stability:

Texture protection: Sodium hexametaphosphate binds to calcium ions in fruit/vegetable cell walls, forming stable "pectin-calcium-phosphate" complexes that strengthen cell wall structure and prevent acid-induced softening. In pickled carrots (pH 3.2), combining with 0.2 g/kg sodium hexametaphosphate increases hardness by 30% compared to the non-added group, maintaining crispness after 1 month of storage.

Color protection: Sodium hexametaphosphate’s chelating ability traps iron and copper ions in foods, inhibiting PPO activity and reducing browning. Meanwhile, nisin’s inhibition of pigment-producing microorganisms (e.g., certain yellow mold) prevents microbial-induced discoloration (e.g., yellow spots on moldy pickled peppers). In canned strawberries (pH 4.0), the combined group shows 45% less browning than the control and retains a bright red color after 3 months of storage.

Nutrient retention: Sodium hexametaphosphate reduces oxidative loss of vitamin C in acidic environments (by chelating metal ions that catalyze vitamin C decomposition). Nisin inhibits microbial consumption of nutrients (e.g., overgrowth of lactic acid bacteria depletes sugars and amino acids). Together, they increase vitamin C retention in acidic foods by >20%.

II. Preservation Applications of the Combination in Typical Acidic Foods

Microbial contamination types and quality deterioration characteristics vary across acidic foods. The combination ratio of nisin and sodium hexametaphosphate must be adjusted based on specific food properties, with a focus on "optimizing dosage and ratio on demand" to balance antibacterial efficacy and quality protection.

(I) Pickled Fruits/Vegetables (pH 3.0–3.8, e.g., pickles, pickled cucumbers)

Key spoilage microorganisms: acid-tolerant lactic acid bacteria (e.g., Lactobacillus plantarum, Lactobacillus brevis) and molds (e.g., Penicillium). Quality issues: texture softening and browning.

Combination scheme: 0.08–0.12 g/kg nisin + 0.2–0.4 g/kg sodium hexametaphosphate (ratio ≈ 1:3). Add synchronously during pickling, evenly dispersed in the brine.

Effects:

Antibacterial: >90% inhibition of lactic acid bacteria; mold growth latency extended from 7 days to 25 days; shelf life extended from 1 month to 3 months.

Quality: Pickled cucumbers retain 85% crispness (vs. 50% in the non-added group); pickles show 50% less browning; no astringency (sodium hexametaphosphate ≤ 0.4 g/kg to avoid masking original flavors).

(II) Fermented Dairy Products (pH 3.8–4.5, e.g., yogurt, fermented milk beverages)

Preservation challenges: "post-fermentation" (residual lactic acid bacteria continue producing acid, causing excessive sourness), pathogenic contamination (e.g., Listeria—grows at low temperatures), and whey separation (affecting texture).

Combination scheme: 0.05–0.08 g/kg nisin + 0.1–0.2 g/kg sodium hexametaphosphate (ratio ≈ 1:2). Add after fermentation, before filling, and mix well.

Effects:

Antibacterial: Inhibits residual lactic acid bacteria; yogurt pH shows no significant decrease over 15 days of refrigeration (4℃) (control pH drops from 4.2 to 3.9 in 5 days); 99% killing rate of Listeria, preventing low-temperature pathogenic contamination.

Quality: Sodium hexametaphosphate improves milk protein stability, reducing whey separation (from 8% to 2%); yogurt texture remains smooth and uniform; no impact on milky flavor (nisin is a natural dairy-derived ingredient with no off-odors).

(III) Acidic Canned Foods (pH 2.5–4.0, e.g., canned tomatoes, canned strawberries)

Canned foods undergo heat sterilization but may have "incomplete sterilization" (e.g., residual heat-resistant mold spores). Acidic environments cause can wall corrosion (releasing iron ions, accelerating browning). Quality issues: dull color and metallic taste.

Combination scheme: 0.1–0.15 g/kg nisin + 0.3–0.5 g/kg sodium hexametaphosphate (ratio ≈ 1:4). Add to raw materials before canning, and sterilize with the cans (nisin retains 70% activity after 10 minutes at 121℃, compatible with canning processes).

Effects:

Antibacterial: Kills residual heat-resistant mold spores (e.g., penicillin-producing bacteria); prevents mold growth after can opening; shelf life extended from 6 months to 12 months.

Quality: Sodium hexametaphosphate chelates iron ions released from can walls, preventing browning (canned tomatoes retain 90% redness); metallic taste reduced (sensory scores show metallic taste detection rate drops from 35% to 5%).

(IV) Acidic Beverages (pH 2.8–3.5, e.g., citrus beverages, fruit/vegetable juices)

Key spoilage microorganisms: acid-tolerant yeasts (e.g., Saccharomyces cerevisiae) and lactic acid bacteria. Quality issues: texture stratification (pulp sedimentation) and vitamin C loss.

Combination scheme: 0.06–0.1 g/kg nisin + 0.15–0.3 g/kg sodium hexametaphosphate (ratio ≈ 1:3). Add during beverage blending, mixed well with other additives (e.g., sweeteners, flavors).

Effects:

Antibacterial: >95% inhibition of yeasts; shelf life of beverages at room temperature (25℃) extended from 15 days to 30 days (control becomes turbid within 10 days).

Quality: Sodium hexametaphosphate disperses pulp particles, preventing stratification (sedimentation rate drops from 12% to 3%); vitamin C oxidation reduced (retention rate increases from 60% to 85%), maintaining nutritional value and taste.

III. Key Control Points for Combined Application

The efficacy of the nisin-sodium hexametaphosphate combination depends on three key factors: "dosage control, process compatibility, and safety compliance"—to avoid reduced efficacy or safety risks from improper use.

(I) Dosage Control: Avoid "Excess Affects Flavor, Insufficiency Reduces Efficacy"

Nisin dosage limit: According to GB 2760 National Food Safety Standard for the Use of Food Additives, the maximum allowable dosage of nisin in various foods is 0.2 g/kg. Excess causes a slight bitter taste (especially in mild-flavored acidic beverages), so dosage should be controlled at 0.05–0.15 g/kg.

Sodium hexametaphosphate dosage limit: Maximum allowable dosage is 5.0 g/kg (pickled fruits/vegetables) and 0.5 g/kg (fermented milk). Excess causes astringency and may affect calcium absorption (no risk at recommended dosages), so practical dosage should be 0.1–0.5 g/kg based on food type.

Combination ratio: Recommended ratio is 1:2–1:4 (nisin:sodium hexametaphosphate) for optimal antibacterial and quality effects. For example, 1:2 in fermented milk balances post-fermentation inhibition and whey separation; 1:3 in pickled fruits/vegetables balances crispness and antibacterial efficacy.

(II) Process Compatibility: Align with Acidic Food Processing to Ensure Uniform Dispersion

Addition timing: Add in the "late processing stage" (e.g., brine blending for pickled foods, final mixing for beverages) to avoid damage to the combination by early high temperatures (e.g., for pre-sterilization addition in canning, nisin degrades partially at high temperatures—increase dosage by 10%–20%) or strong acidity (sodium hexametaphosphate hydrolyzes to orthophosphoric acid at pH < 2.5—adjust pH to >3.0 before addition).

Dispersion method: Both additives are powdered; dissolve first in a small amount of water (or food raw material liquid, e.g., whey from fermented milk) to make a 5%–10% solution before adding. This avoids local high concentrations causing flavor abnormalities (e.g., bitter taste from excess nisin) or uneven quality (e.g., pulp softening from excess sodium hexametaphosphate).

(III) Safety Compliance: Meet National Standards to Avoid Residue Risks

Compliance: Both are nationally approved food additives. Nisin is a "natural food additive" (derived from lactic acid bacteria fermentation); the acceptable daily intake (ADI) of sodium hexametaphosphate is 0–70 mg/kg body weight, with no safety risks at recommended dosages.

Residue testing: No residue concerns (nisin is decomposed by digestive enzymes in the human gut; sodium hexametaphosphate is metabolized to phosphate). Ensure raw materials meet purity standards (e.g., nisin purity ≥ 95%, sodium hexametaphosphate heavy metal content ≤ 10 mg/kg) to avoid impurity contamination.

The combination of nisin and sodium hexametaphosphate provides a dual "antibacterial + quality" solution for acidic food preservation: nisin specifically inhibits acid-tolerant pathogens and spoilage bacteria, while sodium hexametaphosphate enhances its antibacterial efficacy and addresses texture softening and color browning in acidic environments. Their synergy not only improves preservation but also reduces individual additive dosages, aligning with the "natural, efficient, low-additive" direction of the food industry. From pickled fruits/vegetables to fermented milk, and from canned goods to acidic beverages, optimizing the combination ratio and process compatibility allows this system to meet preservation needs of different acidic foods. With growing consumer demand for "preservative-free, long-shelf-life" foods, its applications will expand further (e.g., prepared acidic dishes, low-temperature acidic meat products), becoming a core technology for acidic food preservation.