Chitosan based coating extend the lifespan of fresh fruit -

Chitosan based coating extend the lifespan of fresh fruit

Multifunctional biomass-derived conformal coating from quaternary ammonium based water-soluble chitosan (WSC) and tannic acid (TA) developed to solve the fruit spoilage problems. The coating material has excellent transparency, toughness, UV resistance, antibacterial ability, oxidation resistance and washability. The coated passion fruit exhibited an apparent retardation effect in weight loss, shrinkage index, and firmness, prolonging the shelf life 7 days longer. This technique effectively prevent oxygen oxidation and water loss, and delay fruit ripening, showing good potential in fruit preservation, especially in the preservation quality and economic benefits of passion fruit.

Materials and methods

Purple passion fruits (Passiflora edulis Sims) were transported to the laboratory in cold storage immediately after the crop. In this study fruits used were weighed between 80 and 110 g and have no apparent damage. Escherichia coli ATCC 11229 (E. coli) and Staphylococcus aureus ATCC 6538 (S. aureus) were also used.

Synthesis of water-soluble chitosan (WSC)

The WSC polymer was synthesized by grafting quaternary ammonium groups to the chitosan via reaction of the amino groups on chitosan with glycidyltrimethylammonium chloride (GTMAC). Typically, 5 g of chitosan was dissolved in 200 mL deionized water containing 1 mL of AcOH. After stirring for 30 min, 8.3 g of GTMAC was added dropwise in the above solution with continuous stirring. The resulting mixture was stirred at 400 rpm at 50 ◦C for 18 h. After the reaction, the undissolved polymer was removed by centrifugation of the mixture at 4000 rpm for 20 min at room temperature. The supernatant was collected and the WSC polymer was precipitated out by adding a mixed solvent of methanol and acetone (1/1 in volume). The purification process was repeated three times, and the purified WSC was dried in vacuum overnight as white powder (6.6 g, yield: 49.6%) and stored at 4 ◦C for further use.

Preparation of WSC/TA

A homogeneous coating solution was first prepared by mixing WSC solution (1 wt%) and TA solution (1 wt%) in a weight ratio of 2:1. CS (the degree of deacetylation of 90%) and WSC/TA films were prepared by solution casting method in polytetrafluoroethylene mold with the dosage of 30 mL solution (1 wt%). The films are dried in the air, then removed and sealed.

Antibacterial performance test

The antibacterial performance of the WSC and the coating solution were determined according to a standard test method. In brief, Escherichia coli and Staphylococcus aureus were first taken out from − 20 ◦C and slowly thawed at room temperature. Then, the E. coli and S. aureus were activated on the plates of Nutrition Agar (NA) medium by plate crossing method at 37 ◦C overnight under shaking. After that, a small amount of E. coli and S. aureus was inoculated into a conical flask containing the Nutrition Broth (NB) medium at 37 ◦C for 12 h to expand culture. Finally, under aseptic conditions, the cultured bacterial solution was diluted with NB to obtain a stock bacterial solution with an absorbance value of 0.01 at 600 nm. Next, the WSC and WSC/TA coating solutions with different concentrations were inoculated with an activated bacterial solution and cultured using the same procedure in 37 ◦C incubator for 3 h. The same treatment was performed on the activated bacterial solution using only NB as a negative control.

Antioxidant capacity tests

1. DPPH free radical scavenging activity: Briefly, the DPPH test solution was first diluted with methanol to 0.6 mM to obtain an absorbance of 0.7–0.9 at 517 nm. The stock solutions of WSC (100 μL) and WSC/TA (100 μL) were continuously diluted in half with distilled water on a 96-well plate separately, and reacted with 100 μL of DPPH radical solution for 30 min in dark. The absorbance of the reaction solution and the initial DPPH radical solution diluted with distilled water were measured with a UV–vis reader at 517 nm.

2. ABTS free radical scavenging activity: The ABTS radicalcation was generated by reacting 10 mL of 7 mM ABTS stock solution with 10 mL of 2.45 mM potassium persulfate solution in the dark at room temperature overnight. The resulting mixture was diluted to obtain a stock solution with an absorbance of 0.7 ± 0.005 units at 734 nm

Evaluation of storage quality of passion fruit

The dip-coating method was used to apply functional coating on purple passion fruit. The collected passion fruit were washed with tap water and then randomly divided into two groups with 15 fruits in each replication. One group was soaked in the coating solution for 10 s and then air-dried (Coated group). The other one did not receive any treatment as control group. After drying, both groups were stored at room temperature (18 ± 2 ◦C) and evaluated daily for 21 days. Each measurement was carried out at least three times.

Weight loss

The passion fruit of the blank control group and the coated group were weighed and recorded daily by electronic balance and the weight loss was calculated.

Shrinkage index

The method was taken as lessons and the shrinkage index was evaluated by scores 0: no shrinkage; 0–1: 0%–10% shrinkage; 1–2: 10%– 20% shrinkage; 2–3: 20%–30% shrinkage; 3–4: 30%–40% shrinkage; 4–5: 40%–50% shrinkage; 5–6: 50%–60% shrinkage; 6–7: 60%–70% shrinkage; 7–8: 70%–80% shrinkage; 8–9: over 80% shrinkage, respectively.

Firmness:

The firmness of passion fruit was determined by a texture analyzer. The P2/N probe was selected with the following parameters: probe 20 mm, puncture pro-test speed 10.0 mm/s, puncture test speed 10.0 mm/s, puncture post-test speed 10.0 mm/s and trigger force 5.0 g. Each group of samples was pricked three points on the equator of passion fruit surface, and the average value was calculated as the firmness.

Results

Synthesis and characterization of WSC

WSC was prepared from grafting quaternary ammonium groups on the molecular chain of chitosan according to a reported procedure, which not only improves its water-solubility in a wide pH range, but also effectively enhances its antibacterial properties of chitosan. The solubility difference of chitosan and modified chitosan at different pH values further proved the successful synthesis of WSC. With the increase of pH from 2 to 9, chitosan gradually precipitated out and made the solution turbid due to its decreased solubility at higher pH. On the contrary, the modified chitosan, WSC, exhibited excellent solubility in a wide pH range from 2 to 9. At all these conditions, transparent solutions were formed and remained stable without precipitate for 24 h.

Coating properties of WSC/TA solution

The viscosity of the coating solution is one of the key factors for the success of the conformal coating on fruit surface. So we measured the viscosity of TA, WSC, WSC/TA solution (1 wt%) and H₂O at room temperature as a function of shear rate. The viscosity of WSC measured at low shear rate is about 160 mPa s, which is considered as a promising food packaging material. The coating solution showed shear thinning behavior, indicating that spraying is also a viable coating method, because the high shear rate at the nozzle can reduce the viscosity of the solution, resulting in a more uniform coating on the fruit surface. The formation of uniform surface coating should not only have high viscosity adhesion on the fruit surface, but also have a certain wettability to make it spread on the fruit surface. Therefore, we measured the affinity of WSC/TA solution to fruits by measuring the contact angle. The contact angles of WSC/TA solution on the surface of orange, apple and banana were about 71.43◦, 77.31◦ and 94.85◦ respectively, which decreased to 52.61◦, 41.23◦ and 34.70◦ after 540 s. It can be seen that WSC/TA solution not only has a lower contact angle, but also decreases faster with time, which proves that WSC/TA solution can better attach to fruit surface and spread quickly. It is imperative for passion fruit to have good antibacterial ability as the fruit would go rot very rapidly and aggressively once invaded by microorganisms. Therefore, chitosan based materials are widely used in the field of preservation due to their excellent antibacterial ability. The WSC exhibited strong antibacterial activity against E. coli and S. aureus; while TA only had weak antibacterial ability.

Characterizations and properties of WSC/TA film

One of the factors affecting the mechanical properties of composites is the dispersion of filler in the matrix. Therefore, to monitor the dispersion of TA in WSC matrix, the morphologies of WSC and WSC/TA films were characterized by SEM. It can be seen that the surface of WSC film is rough and bumpy suggesting that WSC polymer chains tend to entanglement and aggregation during film formation. The appearance of fruit affects people’s desire to consume, although WSC/TA composite film has color, it is relatively transparent and flexible, which will not hinder people’s observation of fruits. The color is due to the effect of phenolic acid on the optical properties of the film. WSC/TA composite film, which has the advantages of flexibility, transparency and UV resistance, is a potential food packaging material. Fruit preservation coating should have a certain amount of water retention, because the fruit will lose water and shrink in the storage process, so we measure the hydrophilicity of the coating through contact angle measurement. The wetting rate of water droplets on WSC/TA film is slower, indicating that WSC/TA film has lower water permeability and reduces the diffusion of water in the coating. It is imperative for fruit to have good antibacterial ability as the fruit would go rot very rapidly and aggressively once invaded by microorganisms. Therefore, successfully added TA without affecting the antibacterial ability of WSC, which laid a foundation for the practical application of preservation coating. In addition to the advantages mentioned above, a good preservation coating material should also be easy to remove, which is one of the key points related to consumers’ eating experience. WSC/TA coating will absorb water and swell, wrinkling up from the PET surface, and can be easily removed with a glass rod. This shows that WSC/TA coating not only protects fruit, but can be easily removed with water. And the coating made of biomass material will not cause environmental pollution, which can be safely thrown into the trash.

Evaluation of storage quality of passion fruit

Having demonstrated that the WCS/TA material has excellent antibacterial and antioxidant properties, then applied this functional coating to the purple passion fruit by dip-coating methods and evaluated the effectiveness of the coating in preserving the freshness of the fruits. After only 5 days post-harvest, the uncoated fruits have already wrinkled, which quickly deteriorated over time and showed clear enzymatic browning and dehydration on their exteriors, thereby seriously affecting their appearance and color. However, the coated fruits showed slight indentation even on the 10th day and still maintained an integral appearance on the 15th and 20th days. The above results proved that the WSC/TA coating can effectively extend the freshness of passion fruit by maintaining their integral appearance for a longer time, indicating that the antibacterial and antioxidant abilities of the WSC/TA play a certain role in the preservation of passion fruit. These results revealed that WSC/TA coating treatment can effectively reduce the weight loss of passion fruit, which may be due to the fact that the WSC/TA coating affects the permeability to carbon dioxide, oxygen and water vapor of passion fruit, reducing the evaporation of water through pores on the passion fruit skin and thus allowing internal pressure to saturate for a lower weight loss rate. The shrinkage index is another important index to evaluate the quality of passion fruit, which affects the appearance of passion fruit and accelerates its aging process. It can conclude that the shrinkage index of passion fruit in both groups shows an upward tendency and during storage, the shrinkage index of the control group showed a significant rise from the 6th day and reached 7.80 on the 21st day, producing approximately 78% of the crumpled area. In contrast, the value of the coated group was 6.29 on the 21st day of storage, meaning that only 62.90% shrinkage area was produced in coated group and it was significantly lower than that of the control group. The results revealed that the coating of WSC/TA had a good preservation effect and extended the shelf life of passion fruit. Firmness is also an important index to judge fruit maturity and quality evaluation, for the increased activities of the enzyme related to the cell degradation during storage, which is susceptible to bring the loss of firmness, even the microbial invasion. The firmness changes of passion fruit stored at room temperature during 21 days of storage. After 21 days of storage, the firmness of the control group decreased to 419.62 g, dropped 50.91% from the initial, and the firmness of the coated group decreased to 507.26 g on the 21st day, decreased by 41.14% compared to the initial. By contrast, the passion fruit in the coated group had no obvious surface damage. Overall, the treatment of WSC/TA can effectively decrease the loss of firmness of passion fruit, may be achieved by regulating the activities of the related degrading enzymes, and the antibacterial ability of WSC/TA has an obvious effect on microbial disease protection.

Conclusion

In this study, a multifunctional coating material with excellent UV resistance, antibacterial and antioxidant activities was fabricated from two biomass derived materials, WSC and TA, and was used as the conformal coating for the preservation of passion fruit. After applying the coating on the surface of passion fruit by dip-coating, the resulting coating on the fruit could effectively reduce weight loss and delay the softening of passion fruit. Furthermore, compared to the uncoated fruits, the passion fruits coated with this functional material could store 7 days longer at room temperature. Although all the above results is still at laboratory stage, this multifunctional conformal coating has great potential to be applied in the field of fruit preservation for its simple preparation, convenient use and environmental friendliness.

Reference:

Zhou, Y., Zhong, Y., Li, L., Jiang, K., Gao, J., Zhong, K., Pan, M. and Yan, B., 2022. A multifunctional chitosan-derived conformal coating for the preservation of passion fruit. LWT, p.113584.