Modified Rosin Resin and Its Integration into Natural Preservation Coatings for Fresh Produce

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For centuries, the challenge of preserving the freshness of fruit has been a persistent concern for fruit growers and those working across the post-harvest supply chain. This concern is far from new—over a thousand years ago, Bai Juyi, a poet from China’s Tang Dynasty, vividly captured the ephemeral nature of lychees in his work The Preface to the Painting of Lychees, noting that they “change colour after a day, and lose their fragrance after two.” This poetic observation underscores the inherent perishability of fresh produce.

Despite the evolution of modern supply chains, which utilise methods such as grading, packaging, and temperature-controlled logistics to slow deterioration, these measures often fall short of ensuring long-term freshness. As a result, the use of surface coatings has emerged as a focal point in the ongoing development of fruit preservation technology. This article explores the film-forming mechanisms, material properties, and the application potential of different types of preservation coatings, highlighting their role in extending the shelf life of fruits and vegetables.

The Film-forming Properties of Preservation Coatings

The core mechanism behind preservation coatings lies in their ability to form a semi-permeable film over the surface of fruits and vegetables. This film acts as a protective barrier, serving multiple functional roles. Its dense structure helps inhibit moisture evaporation and prevents the infiltration of external microorganisms, which are key contributors to post-harvest spoilage.

Equally important is the film’s flexibility—it can adapt to the natural expansion or contraction of produce caused by temperature fluctuations or respiration, without cracking or peeling. Furthermore, high transparency ensures that the natural appearance and recognisability of the fruit or vegetable is preserved, a factor increasingly valued in both retail presentation and consumer trust.

Another vital feature is edibility and water solubility, meaning that these coatings are safe for human consumption and can be easily washed off if needed. In summary, once applied post-harvest, a preservation coating adheres closely to the fruit’s surface, forming a thin, multifunctional protective layer that actively contributes to extending freshness and maintaining commercial value throughout storage and distribution.

After fruits and vegetables are harvested and graded, they are typically transferred to processing facilities, where surface treatment is performed using spraying or dipping techniques. Among these steps, the formulation and application of preservation coatings represents not only a critical control point (CCP) but also a prerequisite programme (PRP) within the fresh produce industry. These coatings are more than a finishing touch—they are an essential part of the post-harvest quality assurance system.

To ensure functionality and safety, the emulsions used for these coatings must meet strict performance benchmarks. These include parameters such as particle size, viscosity, pH, emulsion stability, emulsifiability, and surface tension. In most practical applications, the coating is applied in the form of a water-based emulsion, either sprayed or dipped onto the produce surface. As such, the quality of emulsification, along with the physicochemical stability and processability of the formulation, directly impacts the safety, uniformity, and protective performance of the final film.

Importantly, the materials used in these coatings must comply with international food contact safety standards. These include regulatory benchmarks such as the GRAS list in the United States, EU Regulation 10/2011, and China’s GB 9685-2016. Adherence to these standards ensures that the coatings are safe for consumption, legally compliant, and suitable for direct application to fresh produce surfaces.

To ensure both the stability of the coating and its compatibility with fruit skins, the pH of the emulsion should be carefully adjusted to a mildly acidic range—typically between 5.0 and 6.5—which aligns with the natural conditions of most fruit surfaces. This helps the coating form a consistent film without damaging the produce.

Emulsion Design: Balancing Chemistry and Function

For uniform film formation and effective spraying, the particle size of the dispersion is ideally controlled within the nanoscale range of 200–800 nm. While commercial spray systems can handle a broader range, including particles several microns wide, maintaining a finer, near-nanoscale size improves the evenness of the applied layer.

Equally important is the emulsion’s rheological behaviour. A viscosity between 30–100 mPa·s offers a practical balance: it ensures smooth flow through the nozzle, allows proper spreading on the fruit surface, and supports controlled drying. Together, these properties reduce the risk of common coating defects, such as cracking, uneven coverage, or dripping.

Operational reliability also depends on well-defined standard operating procedures (SOPs). These should ensure that spraying equipment is clean, pipelines are residue-free, and detailed batch records are maintained. Such documentation supports traceability of the coating’s source, formulation, and usage conditions, forming a critical part of quality control.

Types of preservation coatings

What kinds of raw materials are best suited for use in preservation coatings? 

1. Polysaccharide-based Preservation Coatings

Among all options, polysaccharide-based coatings are the most widely used and well-researched. Derived from a variety of natural sources, these materials include chitosan, starch, alginate, cellulose derivatives, pectin, glucan, and gellan gum. They are generally non-toxic, biocompatible, and biodegradable, making them ideal for applications in the preservation of not only fruits and vegetables but also poultry and seafood.

From a structural perspective, polysaccharides possess a strong film-forming capability. Molecular interactions—such as hydrogen bonding between chains—enable these materials to create continuous, uniform, and often transparent films with sufficient mechanical strength to protect delicate produce surfaces.

Many of these coatings also offer selective gas permeability, especially for oxygen and carbon dioxide, which can help moderate the respiration rate of fresh produce. This function slows down ripening and senescence, while preventing undesirable anaerobic conditions and the development of off-odours. In addition, by acting as a physical barrier to moisture, polysaccharide-based coatings reduce water loss during storage and transportation—helping to preserve both freshness and firmness, and ultimately lowering weight loss.

2. Protein-Based Preservation Coatings

Similar to polysaccharides, protein-based materials also exhibit notable film-forming properties, making them well-suited for food preservation applications. Common examples include gelatin, whey protein, casein, collagen, soy protein, zein, and egg white protein. These are all sourced from natural food-grade materials and can be applied directly to the surface of perishable foods.

One of the advantages of proteins lies in their rich array of functional groups, which allows for a wide range of chemical modifications. These coatings can be tailored and enhanced using various physical or chemical methods, such as heating, pH adjustment, chemical cross-linking, or enzymatic treatment. As a result, their performance can be adjusted to suit different storage environments or product types.

Thanks to their biocompatibility, protein-based coatings can be effectively combined with antimicrobial agents, antioxidants, and even other polymers, extending their functional range. They are also frequently used in composite coatings, where they are blended with polysaccharides or lipids to balance moisture resistance, flexibility, and mechanical strength.

Studies have shown that coatings made from materials like chitosan or zein can offer notable preservation and antimicrobial effects, particularly in applications involving ready-to-eat fish products stored under vacuum packaging—demonstrating the practicality of protein-based coatings in real-world scenarios.

3. Lipid-Based Preservation Coatings

Lipid-based preservation coatings represent another important category of natural film-forming materials. Commonly used substances include waxes—such as shellac, carnauba wax, and beeswax—as well as fatty acids, vegetable oils, glycerides, natural resins, and animal fats. These materials are widely employed in the preservation of fruits, vegetables, and processed food products.

What distinguishes lipid-based coatings is their inherent hydrophobicity. They serve as some of the most effective barriers to moisture, forming films that significantly reduce water loss and thereby help slow down drying and spoilage. This makes them particularly valuable in applications where moisture control is a priority.

In addition to their functional benefits, lipid coatings can also enhance surface appearance, giving food products a glossier, more appealing finish. This aesthetic effect is especially advantageous in the commercial presentation of fresh produce and confectionery.

However, their limited permeability to gases—while beneficial for water resistance—can pose challenges in certain applications, as we will explore in the following comparative sections.

4. Naturally Derived Polymers: Modified Rosin Resin as a Promising Alternative

Beyond the well-established categories of polysaccharide, protein, and lipid-based coatings, researchers have begun to explore other naturally derived polymers for food preservation. One such material showing strong potential is modified rosin resin.

At its core, modified rosin resin is an ester-based modified polymer. It is typically produced by reacting rosin with dicarboxylic acids, which introduces unsaturated double bonds or carboxyl groups into the structure. This is followed by further esterification or ring-opening copolymerisation, processes that help form a more densely cross-linked polyester network. The resulting structure greatly enhances the film-forming ability and mechanical strength of the resin.

According to existing patent, when applied as a preservation coating, modified rosin resin has demonstrated promising results. For example, in citrus fruit, it was shown to maintain weight loss within 9% to 10%. Compared with the control group, its water-blocking effect is improved by 37% to 50%, showing a significant effect.

Comparison with Polysaccharide-Based Coatings

When compared to polysaccharide-based coatings, modified rosin resin demonstrates several distinct advantages—particularly in moisture resistance and film durability.

Polysaccharide coatings, by nature, are hydrophilic and readily soluble in water. While this makes them safe and biodegradable, it also means that their films tend to have high water vapour permeability and are prone to moisture absorption. Under humid conditions, this can lead to poor moisture retention and even increase the risk of microbial growth on the produce surface.

In contrast, modified rosin resin is ammonia-soluble, which enhances its compatibility with aqueous coating systems while maintaining superior water resistance. Its films are more effective at minimising water loss, making it especially suitable for produce sensitive to dehydration.

Functionally, polysaccharide coatings often prioritise gas exchange regulation and active ingredient delivery, serving as carriers for antimicrobial or antioxidant agents. Modified rosin resin, on the other hand, excels at forming a robust physical barrier and improving surface aesthetics, such as by adding gloss or creating a smoother finish. These characteristics make it a strong candidate for pairing with or even substituting polysaccharide components in certain preservation contexts.

Comparison of Protein-Based Coatings

In comparison with protein-based coatings, modified rosin resin offers a different balance of strengths—particularly in terms of moisture control and mechanical resilience.

Protein-based coatings are well-regarded for their excellent oxygen barrier properties. Materials like zein and whey protein are especially effective at limiting oxygen transmission, which is crucial for slowing down oxidation and respiration processes in certain foods.

However, like polysaccharides, protein films are also hydrophilic and therefore sensitive to moisture. Without the inclusion of plasticisers—such as glycerol or sorbitol—these coatings are prone to absorbing moisture, which can lead to swelling, softening, or even partial dissolution in high-humidity environments. Such changes weaken both their barrier properties and mechanical strength.

Moreover, unmodified protein films tend to be brittle, making them more susceptible to cracking during handling or storage. These drawbacks often necessitate blending protein materials with polysaccharides or lipids to enhance their performance.

In contrast, modified rosin resin naturally forms a denser, more hydrophobic film, which offers greater stability under humid conditions. While it does not inherently match the oxygen barrier strength of certain proteins, it compensates with structural integrity, water resistance, and visual enhancement, making it a strong complementary material in composite coating systems used for fresh produce preservation.

Comparison of Lipid-Based Coating

When evaluated alongside lipid-based coatings, modified rosin resin presents a more balanced profile, particularly in applications requiring both moisture control and respiration regulation.

Lipids are among the most effective moisture barriers known in food preservation. Their strong hydrophobic properties create coatings that provide outstanding resistance to both water vapour and gas transmission. However, this high level of impermeability comes at a cost: limited gas selectivity. In particular, lipid coatings can overly restrict the exchange of oxygen and carbon dioxide—processes that are essential for the natural respiration of fresh produce.

As a result, applying lipid-based coatings to certain fruits can suppress respiration to an undesirable extent, causing internal moisture accumulation, which may lead to fungal growth or trigger anaerobic fermentation. These outcomes pose risks to both product safety and shelf life.

In contrast, modified rosin resin offers strong moisture resistance while still maintaining semi-permeable characteristics, allowing for a more controlled gas exchange. This makes it particularly well-suited for respiring fruits that require moisture protection without complete sealing.

Given these differences, lipid-based coatings are more commonly used for preserving low-respiration or processed foods, such as nuts, dried fruits, confectionery, chocolates, and ready-to-eat cereals. Modified rosin resin, meanwhile, is better suited to fresh produce where both barrier and breathability are needed.

Limitations of Modified Rosin Resin

Despite its many strengths, modified rosin resin also comes with several limitations that must be addressed for optimal use in food preservation coatings.

One of the primary challenges lies in the formulation of stable emulsions. During preparation, issues such as poor dispersion, large particle size, phase separation, and low redispersibility may occur. These problems not only affect the consistency of the coating but also its performance on fruit surfaces. For instance, on hydrophilic fruits like pears or strawberries, inadequate adhesion can result in poor spreading, edge shrinkage, or even film peeling.

Therefore, achieving a stable and uniform emulsion system is essential to unlocking the full potential of modified rosin resin. Several strategies have proven effective in this regard: introducing hydrophilic functional groups, incorporating emulsifiers, or blending with other hydrophilic polymers such as water-soluble polysaccharides or protein-based materials.

Another challenge is related to its glass transition temperature (Tg), which typically exceeds 70 °C. While this high Tg contributes to structural strength at ambient temperatures, it also makes the coating prone to brittleness under low-temperature storage conditions, particularly below 5 °C. In such environments, the film may develop fine cracks as a result of glassification or internal stress shrinkage, compromising its protective function.

These drawbacks highlight the importance of material optimisation and composite formulation when using modified rosin resin—especially in temperature-sensitive supply chains.

• When combined with polysaccharide-based coatings (such as chitosan, alginate, or pectin),
  • modified rosin resin helps improve water resistance and enhances mechanical properties. The resulting films show better flexibility, strength, and even surface gloss, making them ideal for delicate fruits like strawberries and tomatoes.
• When blended with protein-based materials (such as gelatin, soy protein, or whey protein),
  • the resin improves the coating’s stability in high-humidity environments, while also addressing the brittleness often seen in pure protein films. The composite coatings show markedly improved flexibility and extensibility, making them well-suited for medium-sized fruits like apples, pears, and mangoes.
• In lipid-based systems, modified rosin resin plays multiple functional roles.
  • It can act as an emulsification base, interfacial stabiliser, or tackifier, improving the uniformity and adhesion of lipid films on fruit surfaces. Moreover, it helps balance gas exchange—mitigating the overly airtight nature of lipid coatings and thus reducing the risk of anaerobic respiration. Both lipids and modified rosin resin also serve as carriers for bioactive agents such as antimicrobials and antioxidants, enabling controlled release and encapsulation. This approach is especially suitable for tropical fruits like bananas, passionfruit, and lychees, where wax-plus-film composite coatings are often applied.

Summary

In summary, modified rosin resin offers a compelling combination of barrier performance, structural versatility, and natural origin. Its compliance with food contact safety standards makes it a safe and sustainable choice for modern preservation systems. Through intelligent formulation and strategic blending with other natural materials, it can give rise to coatings that are not only functionally robust, but also adaptable across a wide range of fresh produce, helping to address the challenges of moisture loss, respiration control, and shelf-life stability in today’s global food supply chain.