Is it easy for PVP to form a protective film, which can prolong the freshness period of fruits and vegetables?

PVP (Polyvinylpyrrolidone) Easily Forms a Protective Film on the Surface of Fruits and Vegetables, and This Property Has Been Applied in the Field of Fruit and Vegetable Preservation to Effectively Extend Their Freshness Period. The Core Reasons Are Closely Related to Its Molecular Structure, Film-Forming Properties, and Preservation Mechanism, Which can be elaborated from the following three aspects:

I. Why does PVP "easily form a protective film"?

As a water-soluble high molecular polymer, PVP has inherent advantages in film formation, which are specifically reflected in two aspects:

1. Convenience of film formation

PVP can dissolve in common solvents such as water or ethanol. Through simple processes like soaking, spraying, or shower coating, it can evenly cover the surface of fruits and vegetables. After the solvent (e.g., water) evaporates naturally or dries slightly, the long-chain molecules of PVP cross-link with each other to quickly form a continuous, transparent, and flexible film. No complex equipment or high-temperature conditions are required, resulting in a low operational threshold.

2. Adhesion and stability of the film

The polar groups contained in PVP molecules (such as the amide group on the pyrrolidone ring) can form hydrogen bonds with the hydroxyl groups on the surface of fruits and vegetables (e.g., polysaccharide components in the epidermal cell wall). This allows the film to adhere tightly to the surface of fruits and vegetables, making it difficult to fall off due to handling or mild rinsing, thus exhibiting strong stability.

II. How does the PVP protective film extend the freshness period of fruits and vegetables?

The preservation effect essentially relies on "physical barrier + mild chemical regulation" to delay the senescence and spoilage process of fruits and vegetables. The core mechanisms include:

1. Inhibiting water loss to maintain the plumpness of fruits and vegetables

Fresh fruits and vegetables tend to lose water due to transpiration after harvesting, leading to wilting and epidermal shrinkage (e.g., cucumbers, lettuce). The dense structure of the PVP film can effectively block the outward diffusion of water molecules, reduce the rate of water evaporation, and maintain the water content and crisp texture of fruits and vegetables.

2. Isolating oxygen to delay oxidation and respiration

The respiration of fruits and vegetables consumes sugars and produces carbon dioxide, accelerating senescence. Meanwhile, oxygen can trigger oxidative browning of epidermal cells (e.g., browning of cut apples and pears) and nutrient loss (e.g., oxidation of vitamin C). The PVP film can form an "oxygen barrier" on the surface of fruits and vegetables, reducing the oxygen concentration around the epidermis, thereby slowing down the intensity of respiration and oxidation reactions and extending the fresh state of fruits and vegetables.

3. Inhibiting microbial growth to reduce spoilage

PVP itself has a certain antibacterial property (especially against Gram-positive bacteria such as Staphylococcus, and also inhibitory against some molds like Penicillium). The mechanism lies in its ability to bind to microbial proteins, destroy bacterial cell walls or enzyme activity, and prevent microorganisms from colonizing and reproducing on the surface of fruits and vegetables. In addition, the physical barrier of the film can also reduce the contact between external microorganisms (e.g., mold spores in the air) and the epidermis of fruits and vegetables, further reducing the risk of spoilage.

III. Precautions in practical application

1. Safety compliance

PVP has been approved as a "food additive" (code: E1201) by authoritative institutions such as the Food and Agriculture Organization (FAO) of the United Nations and the U.S. Food and Drug Administration (FDA) for use in food coatings, stabilizers, etc. When used within the specified dosage, it is harmless to the human body, and the residual amount on the surface of fruits and vegetables is extremely low and easy to clean.

2. Better effect with compound use

When PVP is used alone, the air permeability and antibacterial property of the film may be limited. In practical preservation, it is often compounded with other substances (such as chitosan, plant essential oils, calcium chloride, etc.). This not only enhances the flexibility and air permeability of the film (to avoid anaerobic respiration of fruits and vegetables leading to alcohol poisoning) but also improves the antibacterial and preservation effects.

3. Suitable types of fruits and vegetables

It is more suitable for fruits and vegetables with relatively smooth surfaces and low spoilage tendency (e.g., apples, pears, citrus fruits, tomatoes, cucumbers, carrots, etc.). For fruits and vegetables with downy surfaces (e.g., peaches) or fragile surfaces (e.g., strawberries), the film-forming process needs to be adjusted (e.g., reducing concentration, using spraying instead of soaking) to prevent the film from blocking the epidermal stomata or damaging the epidermis.

In conclusion, due to its easy film formation, strong film stability, and ability to delay the senescence of fruits and vegetables through multiple mechanisms, PVP is an efficient and safe coating material for fruit and vegetable preservation, and has clear application value in extending the freshness period.