Experimental comparison of Nisin in Juice Preservation

As a natural food preservative, the application effect of nisin in fruit juice preservation needs to be clarified by experimental comparisons with traditional chemical preservatives (e.g., sodium benzoate, potassium sorbate), physical preservation technologies (e.g., pasteurization, high-pressure processing), and a blank control group to identify its advantages and limitations. These comparisons focus on the core indicators of fruit juice preservation—changes in microbial count, stability of physicochemical quality, and maintenance of sensory characteristics—while considering the operational costs and safety of different treatment methods. Ultimately, they demonstrate the application value of nisin for specific preservation needs.

I. Experimental Comparison of Microbial Control Effects

The core advantage of nisin lies in its efficient inhibition of Gram-positive bacteria (one of the main spoilage bacteria in fruit juices, such as lactococci, streptococci, and some Bacillus species), but its scope of action differs significantly from that of traditional preservatives. At the same concentration (e.g., 0.1–0.3 g/L), experiments on freshly squeezed apple juice and orange juice showed the following:

In the blank control group, when stored at 25°C for 3 days, the total bacterial count soared from an initial 10³ CFU/mL to over 10⁶ CFU/mL, accompanied by obvious bottle swelling and off-odors.

In the experimental group with Nisin added, the number of Gram-positive bacteria remained below 10³ CFU/mL after 7 days of storage, with no spoilage caused by spore germination.

In the control group with sodium benzoate (0.2 g/L) added, although it inhibited both Gram-positive and Gram-negative bacteria (e.g., E. coli, Pseudomonas), its inhibitory effect on Bacillus was weaker than that of Nisin—after 10 days of storage, the number of Bacillus in the sodium benzoate group was 3–5 times higher than that in the Nisin group.

However, Nisin has limited ability to inhibit Gram-negative bacteria. If the initial contamination of the fruit juice contains a large number of Gram-negative bacteria (e.g., freshly squeezed grape juice due to the bacterial characteristics of the raw material), the total microbial count in the group using nisin alone will decrease less than that in the sodium benzoate group. In such cases, nisin must be combined with synergists such as EDTA (ethylenediaminetetraacetic acid) to achieve a similar broad-spectrum bacteriostatic effect.

II. Experimental Comparison with Physical Preservation Technologies

Nisin exhibits a balanced advantage in cost and quality when used in a composite preservation system of "low temperature+preservative." Taking orange juice treated with pasteurization (65°C, 30 min) as an example:

The pasteurization group achieved an initial microbial count reduction to below 10² CFU/mL and an extended shelf life of 14 days. However, the sterilization process caused a 15%–20% loss of vitamin C in the orange juice and increased volatilization of heat-sensitive aroma compounds (e.g., limonene).

In the experimental group with "4°C refrigeration+0.2 g/L nisin," although the initial microbial control effect was slightly inferior to pasteurization (initial total bacterial count ≈ 10³ CFU/mL), the vitamin C loss was only 5%–8% after 14 days of storage. The retention rate of aroma components was 12%–15% higher than that in the pasteurization group, and no heating equipment was required, reducing energy consumption costs by approximately 30%.

When compared with high-pressure processing (600 MPa, 5 min), nisin is at a disadvantage in long-term preservation—high-pressure processing enables orange juice to be stored at room temperature for more than 30 days, while the nisin group relies on low temperatures (4–10°C); at room temperature, the microbial count in the nisin group exceeds safety standards after 10 days. However, the equipment investment cost of nisin is only 1/10 that of high-pressure processing, making it more suitable for small and medium-sized fruit juice enterprises with short-term preservation needs.

III. Experimental Comparison of Physicochemical Quality and Sensory Characteristics

Nisin provides significantly better protection for the original quality of fruit juice than traditional chemical preservatives and high-temperature treatments. Taking freshly squeezed kiwifruit juice as an example—it is rich in polyphenol oxidase (PPO), which easily causes juice browning, and vitamin C, which is prone to oxidative loss:

In the blank control group, after 5 days of storage, the browning index (OD420 nm) increased from an initial 0.15 to 0.82, and the vitamin C retention rate was only 45%.

In the experimental group with nisin (0.25 g/L) added, the browning index was 0.31 after 5 days, and the vitamin C retention rate reached 78%. There were no significant changes in the juice’s pH value or soluble solids content (pH fluctuation ≤ 0.2, difference in soluble solids ≤ 0.5 Brix).

In the control group with potassium sorbate (0.2 g/L) added, although the browning index (0.35) was close to that of the nisin group, the vitamin C retention rate (62%) was lower. Additionally, a slight chemical taste was detectable in some sensitive populations.

In sensory evaluation, apple juice treated with nisin showed minimal differences from fresh juice after 7 days of storage: its color (brightness L value decreased by 3.2) and flavor (sweet-sour balance score 8.2/10) were comparable to fresh juice (L* value decreased by 1.5, score 9.0/10). In contrast, the pasteurization group had darker color due to thermal processing (L* value decreased by 6.8) and a "cooked taste," with the score dropping to 6.5/10. The sodium benzoate group had a flavor score of only 7.0/10 due to the slight off-odor of the chemical preservative.

IV. Experimental Comparison of Safety and Applicability

The natural nature of nisin gives it an irreplaceable advantage for "clean label" demands. Although traditional chemical preservatives (e.g., sodium benzoate) are safe within compliant dosages, their application scope (e.g., in infant fruit juices) is restricted in some countries and regions, and concerns remain about potential risks of long-term intake. In contrast, nisin is a polypeptide produced by the fermentation of Streptococcus lactis; it can be decomposed into amino acids by proteases in the human digestive tract, with no residual risk. Experimental tests showed that after 30 days of storage in fruit juice, Nisin did not produce any toxic metabolites and caused no damage to the juice’s nutrients (e.g., minerals, dietary fiber).

However, nisin has certain application limitations:

In highly acidic juices (e.g., lemon juice, pH < 3.0), its stability decreases, with approximately 20% loss of bacteriostatic activity after 10 days of storage, requiring an appropriate increase in the added concentration.

In juices containing high levels of fat or protein (e.g., mango juice, walnut juice), nisin may bind to macromolecular substances, reducing its bacteriostatic activity. Emulsifiers such as Tween-80 need to be added to improve its dispersibility.

This stands in contrast to the stability of chemical preservatives (e.g., sodium benzoate, which is stable in the pH range of 2.5–7.0) and determines that Nisin is more suitable for clear juices with a pH of 3.0–5.0 and relatively simple composition (e.g., apple juice, orange juice).

Summary of Experimental Comparisons of Nisin in Fruit Juice Preservation

Compared with traditional chemical preservatives, nisin has advantages such as stronger inhibition of Gram-positive bacteria (especially spore-forming bacteria), better protection of juice nutrients and sensory quality, and higher safety; however, it has the disadvantage of a narrow antibacterial spectrum (weak effect on Gram-negative bacteria). Compared with physical preservation technologies, Nisin is advantageous in terms of low cost, simple operation, and no thermal damage; yet, it requires low temperatures for long-term preservation and has limited applicability to juices with complex compositions.

Therefore, the application of nisin in fruit juice preservation is most suitable for composite systems of "low temperature + synergists" (e.g., EDTA, citric acid), targeting medium-to-short-term preservation scenarios with high quality requirements and a focus on natural labels. For preservation needs such as broad-spectrum or long-term bacteriostasis, or for highly acidic or complex-composition juices, Nisin must be combined with other technologies to form a complementary solution.