What is the best ratio when PVP and chitosan are used together for preservation ?

The optimal ratio of PVP (polyvinyl pyrrolidone) and chitosan requires dynamic adjustment based on the type of fruit or vegetable, chitosan molecular weight, and application process . The core principle is to maximize preservation effectiveness through " physical film-forming complementarity + antimicrobial synergy ." The following analysis focuses on the basic ratio range , influencing factors , and practical application recommendations , along with specific examples and adjustment strategies.

1. Basic ratio range: synergistic effect range of PVP and chitosan

According to research and experimental verification, the effective compounding range of PVP and chitosan is usually:

• PVP concentration : 0.05%~0.2% (mass-to-volume ratio, the same below)
• Chitosan concentration : 0.5%~2%
  This range can simultaneously meet the "film density" and "air permeability balance" requirements and meet food grade safety requirements. For example:
• Peach preservation : 0.1% PVP + 1.5% chitosan compound, after 15 days of storage, the decay rate dropped to 5% (12% for the PVP alone group and 25% for the untreated group), and the firmness retention rate increased by 15%;
• Strawberry preservation : 0.1% PVP + 1% chitosan compound, the mold rate after storage for 7 days was only 8% (18% for the PVP alone group), and the strawberry surface did not feel sticky.

2. Key influencing factors: the core basis for dynamic adjustment of the ratio

1. Fruit and vegetable skin characteristics and storage stability

• Thick-skinned, long-lasting fruits and vegetables (apples, citrus) :
  A higher chitosan concentration (1.5%-2%) combined with a lower PVP concentration (0.05%-0.1%) can be used to enhance disease protection by leveraging chitosan's strong antimicrobial properties. For example, treatment of citrus with 0.1% PVP + 2% chitosan reduced the incidence of penicillium mold by 60%.
• Thin-skinned, fragile fruits and vegetables (strawberries, peaches) :
  Reduce the chitosan concentration (0.5%-1%) and increase the PVP concentration (0.1%-0.2%) to prevent excessive film thickness from blocking pores. For example, strawberries treated with 0.1% PVP + 0.8% chitosan experienced only a 6% weight loss after 10 days of storage (compared to 10% for chitosan alone), with no anaerobic respiration odor.

2. Chitosan molecular weight and solubility characteristics

• Low molecular weight chitosan (<100,000 Da) :
  This material has good water solubility but weak film-forming properties. Therefore, the concentration should be increased to 1.5%–2% , and combined with 0.1%–0.2% PVP to enhance film toughness. For example, a combination of low molecular weight chitosan (50,000 Da) and 0.15% PVP can form a uniform, transparent composite film suitable for preserving leafy vegetables.
• High-molecular-weight chitosan (>100,000 Da) :
  It has excellent film-forming properties but requires acidic dissolution (such as 1% acetic acid). When compounding, the concentration can be reduced to 0.5%-1% , and 0.05%-0.1% PVP can stabilize the film structure. For example, a high-molecular-weight chitosan (200,000 Da) compounded with 0.1% PVP can achieve a film thickness of 2-3 μm, suitable for tomato preservation.

3. Application process and environmental conditions

• Soaking method :
  Chitosan concentration should be controlled between 0.5% and 1.5% (to avoid prolonged soaking that may result in a thick film), PVP concentration between 0.05% and 0.1%, and soaking time should be 5 to 8 minutes. For example, cucumbers soaked in 0.1% PVP + 1% chitosan for 5 minutes retain 85% of their crispness after 10 days of storage.
• Spraying :
  A higher chitosan concentration (1.5%-2%) and a lower PVP concentration (0.05%) can be used . By precisely controlling the spray pressure (0.2-0.3 MPa), an ultra-thin composite film of 1-2 μm can be formed. For example, after spraying grapes with 0.05% PVP + 1.5% chitosan, the grain shedding rate was reduced by 40% after 20 days of storage.
• High temperature and high humidity environments :
  Reduce the chitosan concentration to 0.5%-1% and increase the PVP concentration to 0.1%-0.2% to enhance the membrane's air permeability. For example, lychees stored at room temperature in summer can reduce their peel browning index by 30% after being treated with 0.15% PVP + 1% chitosan.

3. Practical application suggestions: four steps to determine the optimal ratio

1. Preliminary experimental screening of basic ratios

• Gradient test : For target fruits and vegetables, three groups of ratios were designed (such as 0.1% PVP + 1% chitosan, 0.1% PVP + 1.5% chitosan, 0.15% PVP + 1% chitosan). The test indicators include:
  • Physical indicators : film thickness (laser thickness gauge), film transparency (transmittance > 80% is excellent);
  • Preservation indicators : weight loss rate (<5% is excellent), decay rate (<10% is excellent), respiration rate (reduction of more than 30% is excellent);
  • Sensory indicators : sticky feeling on the skin (no feeling is excellent), taste (no odor is excellent).

2. Optimize dissolution process and pH control

• Dissolution order :
  first dissolve chitosan in 1% acetic acid solution (stir for 30 minutes until clear), then add PVP (needs to be dissolved in deionized water in advance), and finally use NaOH solution to adjust the pH to 5.5~6.5 (close to neutral to avoid PVP degradation).
• Compound stability :
  If flocculation occurs, add 0.1%~0.2% glycerol (plasticizer) or 0.05% citric acid (pH buffer) to enhance solution stability.

3. Adapt to storage environment and shelf life requirements

• Short-term storage (<7 days) :
  High chitosan concentration (1.5%~2%) + low PVP concentration (0.05%) can be used to quickly form an antibacterial film. For example, after blueberries are treated with 0.05% PVP + 1.5% chitosan, the decay rate is only 5% after being stored at room temperature for 3 days.
• Long-term storage (>7 days) :
  It is necessary to balance breathability and antibacterial properties . Use 0.1% PVP + 1% chitosan and refrigerate (0~5℃). For example, after apples are treated with this ratio, the hardness retention rate reaches 80% after refrigeration for 30 days.

4. Security Verification and Compliance Confirmation

• Residue detection :
  High performance liquid chromatography (HPLC) was used to detect PVP residues (needed to be less than 0.01 mg/kg), and the antibacterial activity of chitosan was verified by inhibition zone experiment (the inhibition zone against Escherichia coli was greater than 15mm, which was excellent).
• Regulatory compliance :
  Ensure that chitosan is food grade (degree of deacetylation > 85%) and that the compound concentration complies with the maximum usage of chitosan in the "Food Additives Usage Standard" (GB 2760) (usually ≤2%).

4. Typical Case: Reference for the Optimal Ratio of Different Fruits and Vegetables

Summarize

The optimal ratio of PVP and chitosan is not a fixed value , but needs to be dynamically adjusted around the balance of " antibacterial properties, film-forming properties, and breathability ". The core strategy is:

• Fruits and vegetables with thick skin : focus on the antibacterial properties of chitosan (1.5%~2%), combined with low concentration of PVP (0.05%~0.1%);
• Fruits and vegetables with thin skin : focus on PVP film-forming properties (0.1%-0.2%), combined with medium-to-low concentration chitosan (0.5%-1%);
• Complex environment : Optimize membrane performance through compounding processes (such as pH adjustment and plasticizer addition), and verify safety and effectiveness through preliminary experiments.

Through the above methods, the shelf life of fruits and vegetables can be extended while ensuring that food safety and quality are not affected.