Preparation of cellulose nanocrystal cinnamon essential oil chitosan composite coating and its effect on fruit preservation of Crataegus pinnatifida Bge. var. major N. E. Br

Abstract:【Objective】The abundant vitamins, mineral elements, organic acids, pectin and flavonoids made hawthorn be conducive to cardiovascular health via promoting blood circulation and removing blood stasis. However, hawthorn might rapidly deteriorate at ambient temperature due to water loss, microbial invasion, and oxidization. Thus, efficient preservation method is crucial to improvement of the nutritional and commercial value of hawthorn. This study aimed at developing high performance conformal coating using chitosan and cinnamon essential oil (CEO) for hawthorn postharvest storage and exploring the effects of the coating on the storage quality of hawthorn.【Methods】Chitosan (Ch) and CEO were used as film- forming agent and antibacterial components for the preparation of conformal coating to reduce the dehydration and nutrition loss of the fruits of C. pinnatifida Bge. var. major N. E. Br. and prolong its shelf life. 3.0 g caffeic acid (CA) was dissolved in 100 g cellulose nanocrystals (CNC) dispersion (3% wt). The pH of the mixture was adjusted to 9.0 and stirred at room temperature for 12 h. After freezing drying, the CA-modified CNC (CA@CNC) was obtained. Then 1 g solid powder of CA@CNC and 0.5 mL CEO were dispersed in water (15 mL) with stirring and ultrasonic cellcrusher to obtain Pickering emulsion (CA@CNC-CEO), which was then added in Ch solution to form conformal coatings (CA@CNC-CEO-Ch). By altering the contents of pickering emulsion, a series of conformal coatings were finally obtained and the mechanical properties, gas barrier ability, and oxidation resistance of conformal coatings were investigated. The pure chitosan coating (Ch) and the coating with essential oil added directly (Ch-CEO) were set as control group. The diameters of Pickering emulsion were observed and calculated. The viscosity of the mixed solution of coating was tested. The mechanical property, transmission rate of the water vapor and oxygen, DPPH free radical scavenging activity, antibacterial properties, and cell viability of the conformal coatings were determined. The fresh hawthorn fruits were washed and immersed in the coating solution for 2 min and naturally dried. The samples were then stored in PE bags at 20 ℃ with 60% relative humidity to evaluate the preservation activities of the conformal coatings by measuring the hardness, weight loss, decay ratio, the contents of soluble solid, vitamin C and titratable acid (TA) of the hawthorn fruits every 10 days.【Results】CA@CNC was used as stabilizer to improve the dispersity and stability of CEO. The diameter of CA@CNC-CEO decreased to 160 nm and could maintain stable for over 10 days. The viscosity of the prepared coating solution dramatically decreased as the shear rate increased, showing obvious shear thinning behavior, which was conducive to spray and mold. The fracture strain and stress obviously increased comparing with the pure Ch and Ch-CEO coating, which could reach to 30.3 MPa and 4.1%, respectively. Owing to the increasing crosslinking interaction between the chitosan and CA@CNC-CEO, the inner structure of the coating became denser and the water vapor and O2 transmission rate decreased to 668 g ·m^-2 · d^-1 and 672 g·m^-2 ·d^-1 , respectively. The imposed CA could also improve the oxidation resistance of the conformal coatings, thus the DPPH radical scavenging activity of the CA@CNC-CEO-Ch increased from 25.9% to 32.2%. In addition, the CA@CNC-CEO-Ch coating showed great antibacterial activity due to the synergistic effect of the chitosan and CEO. The diameter of inhibition zone against Staphylococcus aureus and Escherichia coli reached to 25.4 and 14.3 mm, which increased more than 25%. The natural origin raw material endowed the coating with good biocompatibility. The cell viability of the freezingdried coating samples was over 92% when the solid content was 2 mg · mL^{- 1} . The obtained conformal coatings were used for C. pinnatifida Bge. var. major N. E. Br. preservation. By investigating multiple indicators treated with different coating, CA@CNC-CEO-Ch-3 (chitosan mass fraction=2%, CA@CNCCEO volume fraction=5% ) showed best preservation capacity due to the high relative content of the CA@CNC-CEO, which contributed to ensure the integrality and increased the cross-linking density, oxidation resistance, antibacterial properties, and gas barrier capacity of the coating. The dense structure of the CA@CNC-CEO-Ch-3 could reduce water loss and oxidation. The hardness and soluble solids were up to 5.4 kg · cm- 2 and 13.9%, respectively. The shelf life of the fruits could reach 40 days when treated with the CA@CNC-CEO-Ch-3. The soluble solid, vitamin C and titratable acid (TA) contents of the fruits when treated with CA@CNC-CEO-Ch-3 was test to be 13.9%, 71.2 mg·100 g^-1 and 9.4%, respectively.【Conclusion】The resultant conformal coating could improve the postharvest fruit quality during storage of C. pinnatifida Bge. var. major N. E. Br. The CA@CNC-CEO-Ch-3 showed the best preservation performance. The conformal coating could reduce the decline of fruit hardness, water loss and other nutrient content like soluble solids, TA, and vitamin C through physical barrier, antibacterial and antioxidant properties. This study would provide theoretical basis and technical support for the preservation of the fruits of C. pinnatifida Bge. var. major N. E. Br.