A polylactic acid / coffee grounds nanocellulose composite film and a preparation method and application thereof

Abstract

This invention belongs to the technical field of biodegradable food packaging materials, specifically relating to a polylactic acid / coffee grounds nanocellulose composite film, its preparation method, and its application. The invention aims to achieve high-value utilization of waste coffee grounds and develop biodegradable packaging materials for preserving edible fungi. This invention uses waste coffee grounds as raw material to extract and prepare nanocellulose, and employs a solvent evaporation method to prepare the polylactic acid / coffee grounds nanocellulose composite film. The nanocellulose can be uniformly dispersed in the polylactic acid matrix, improving the composite film's structural density, crystallinity, mechanical properties, and light-blocking properties. When used for storing button mushrooms, this composite film can significantly delay cap browning, weight loss, and decrease in hardness, reduce the loss of soluble solids, soluble proteins, and other nutrients, and exhibit excellent preservation effects. This provides a theoretical basis for the resource utilization of waste coffee grounds and the development of packaging materials for preserving edible fungi.

Description

Technical Field

[0001] This invention belongs to the technical field of biodegradable food packaging materials, specifically relating to a polylactic acid / coffee ground cellulose nanocomposite film, its preparation method, and its application. Background Technology

[0002] With the rapid development of the global food industry, the market demand for food packaging materials continues to expand. Traditional petroleum-based plastic packaging has long dominated the market due to its low cost and good processing performance. However, petroleum-based plastics are difficult to degrade naturally, and their large-scale use has caused serious "white pollution" problems. At the same time, some plastic additives pose food safety risks and are no longer compatible with the industry development trend of increasingly stringent global environmental policies and the continuous improvement of consumers' health and green consumption awareness. Against this backdrop, green, renewable, and fully biodegradable bio-based food packaging materials have become the core direction for research and industrialization in the food packaging field.

[0003] Polylactic acid (PLA) is an aliphatic polyester synthesized from plant biomass resources. It possesses characteristics such as renewable source, good biocompatibility, complete biodegradability, and excellent processability, making it highly promising for food preservation packaging. However, pure PLA material itself has inherent defects: it is brittle, lacks sufficient mechanical resistance to deformation, and has poor barrier properties against oxygen and moisture, as well as poor control over optical properties. This makes it unsuitable for the preservation packaging needs of fresh foods, especially those with high moisture content, prone to browning, and easily perishable, severely limiting its large-scale practical application in the food preservation field.

[0004] To improve the overall performance of PLA materials, existing technologies often employ blending modification with inorganic fillers and natural polymers. Among these, nanocrystalline cellulose (NCC), a nanoscale functional material prepared from natural cellulose, possesses advantages such as large specific surface area, high mechanical strength, good biocompatibility, and complete biodegradability, and has become one of the core reinforcing phases in PLA modification. Existing research indicates that introducing natural fillers such as NCC can effectively improve the mechanical properties and structural stability of PLA, providing a feasible pathway for the application of agricultural waste in polymer modification.

[0005] Coffee is one of the world's most consumed beverages, and its processing generates a large amount of spent coffee grounds (SCG). Statistics show that the global annual coffee production reaches 11.2 million tons, corresponding to an annual production of over 10 million tons of coffee grounds, with a resource utilization rate of less than 10%. Currently, the mainstream disposal methods for spent coffee grounds are still landfill and incineration. These methods not only cause a huge waste of biomass resources but also may lead to soil, water, and air pollution problems due to leachate and harmful gas emissions. There is an urgent need to develop high-value, large-scale resource utilization pathways for spent coffee grounds. Existing research confirms that coffee grounds contain up to 48% cellulose, far exceeding most common agricultural wastes. This makes it a high-quality, low-cost raw material for extracting and preparing nanocellulose. Extracting cellulose from coffee grounds and preparing nanocellulose, which can then be used to modify PLA-based composite membranes, can simultaneously achieve high-value utilization of agricultural waste and performance optimization of PLA-based packaging materials, resulting in significant economic and environmental benefits.

[0006] Although existing technologies have been used to study the preparation of coffee grounds-based nanocellulose and PLA / NCC composite membranes, significant technological gaps and application shortcomings remain. On the one hand, existing research on the resource utilization of coffee grounds mostly focuses on the preparation and performance optimization of coffee grounds-based carbon materials, adsorbent materials, and crude cellulose materials. Only a few studies involve the preparation of coffee grounds nanocellulose, and these studies often stop at the characterization of the material's structure and basic properties. There is little research on its application in food preservation, making it impossible to achieve the high-value utilization of coffee grounds from waste biomass to functional preservation packaging materials. On the other hand, while existing PLA modification technologies have made some progress, they mostly use commercially available wood pulp and cotton pulp-based nanocellulose, resulting in high raw material costs and difficulty in large-scale promotion. This also means that the technology is not well integrated with the industrial demand for the resource utilization of agricultural waste. Overall, existing technologies lack research that systematically integrates "coffee grounds resource utilization—PLA composite material preparation—food preservation application," especially in the area of ​​edible fungi preservation, where related research is relatively limited.

[0007] Edible fungi, such as button mushrooms, have vigorous post-harvest respiration and a moisture content exceeding 90%, making them highly susceptible to water and weight loss, cap browning, softening, and nutrient loss. During cold chain storage, their quality deteriorates rapidly, resulting in a short shelf life. Therefore, packaging materials require highly compatible properties in terms of water retention, oxygen barrier properties, light blocking, and air permeability. However, existing pure PLA films suffer from poor compatibility in terms of air permeability, light blocking, and water retention, failing to effectively inhibit post-harvest enzymatic browning and quality deterioration. Furthermore, existing modified PLA composite films lack a systematic technical solution for performance regulation using coffee grounds-derived nanocellulose specifically designed for button mushroom preservation, thus failing to meet the practical application requirements for cold chain storage and preservation of button mushrooms and other edible fungi. Summary of the Invention

[0008] The purpose of this invention is to provide a polylactic acid / coffee grounds nanocellulose composite film, its preparation method and application, providing a solid theoretical basis and technical reference for the high-value utilization of waste coffee grounds and the development of edible fungi preservation packaging materials.

[0009] The objective of this invention is achieved through the following technical solution: This invention provides a polylactic acid / coffee ground cellulose nanocomposite membrane, wherein the composite membrane uses polylactic acid as the matrix and cellulose nanoparticles derived from coffee grounds as the reinforcing phase; the cellulose nanoparticles are uniformly dispersed in the polylactic acid matrix to form a dense and interwoven composite structure, and the cellulose nanoparticles retain the cellulose type I crystal structure.

[0010] Furthermore, the transmittance of the composite film is ≤25% at a wavelength of 400nm and 40%~46% at a wavelength of 550nm.

[0011] This invention provides a method for preparing the polylactic acid / coffee ground cellulose nanocomposite membrane, comprising the following steps: S1 Extraction of cellulose from coffee grounds: Using waste coffee grounds as raw material, coffee grounds cellulose is obtained after alkali treatment, decolorization and purification. Preparation of S2 nanocellulose: The coffee grounds cellulose obtained in step S1 was hydrolyzed with sulfuric acid and purified by dialysis to obtain a nanocellulose suspension; Preparation of S3 composite membrane: Polylactic acid is dissolved in an organic solvent, and the nanocellulose suspension obtained in step S2 is added. After mixing evenly, the mixture is cast into a film, cured and dried to obtain the polylactic acid / coffee grounds nanocellulose composite membrane.

[0012] Further, step S1 specifically involves: pre-treating coffee grounds by boiling them in water, adding a 6.66% NaOH solution at a material-to-liquid ratio of 1:16 g / mL, stirring in a water bath at 81°C for 2 hours, and washing until neutral; then adding a sodium hypochlorite solution at a material-to-liquid ratio of 1:10 g / mL, adjusting the pH of the system to 3, decolorizing by shaking in a water bath at 75°C for 30 minutes, washing until neutral, and drying to obtain coffee grounds cellulose.

[0013] Further, step S2 specifically involves: adding coffee grounds cellulose to a 60% sulfuric acid solution at a material-to-liquid ratio of 1:20 g / mL, stirring at a constant temperature of 45°C until the system becomes a yellowish-brown viscous substance, and then adding deionized water to terminate the reaction; after the system has stood, it is centrifuged, washed, and dialyzed until neutral to obtain a nanocellulose suspension.

[0014] Furthermore, in step S3, the organic solvent is dichloromethane, and the ratio of polylactic acid to dichloromethane is 2.5g:50mL; the polylactic acid is dissolved under constant temperature water bath stirring at 45℃; after the nanocellulose suspension and polylactic acid solution are mixed, they are ultrasonically treated for 10min until the system is homogeneous; after casting, it is allowed to stand at room temperature for 2h to cure.

[0015] The present invention also provides an application of the polylactic acid / coffee grounds nanocellulose composite film in the preservation packaging of fresh food.

[0016] Furthermore, the fresh food is edible fungi, and the application is to use polylactic acid / coffee grounds nanocellulose composite membrane for post-harvest storage and preservation of edible fungi.

[0017] Furthermore, the edible fungus is *Tricholoma matsutake*; the storage and preservation temperature is 4±1℃, and the storage time is ≤12 days.

[0018] Furthermore, when the polylactic acid / coffee grounds nanocellulose composite membrane is used for the preservation of button mushrooms, it can delay the weight loss, decrease in hardness, and browning of the cap during the storage process of button mushrooms, while inhibiting the loss of soluble solids and soluble proteins.

[0019] The beneficial effects of this invention are as follows: This invention uses waste coffee grounds from coffee deep processing as raw material to extract and prepare nanocellulose, which is then used to modify polylactic acid composite membranes. This achieves high-value resource utilization of waste coffee grounds and effectively solves the resource waste and environmental pollution problems caused by existing coffee grounds treatment methods, which mainly rely on landfill and incineration. The coffee grounds cellulose extraction process used in this invention exhibits excellent stability, with an average cellulose yield of up to 25.03% and a relative standard deviation of only 0.46%. It can efficiently remove impurities such as lignin and hemicellulose from coffee grounds, achieving efficient separation and purification of cellulose. This provides a stable and feasible technical path for the large-scale, high-value utilization of coffee grounds.

[0020] The polylactic acid / coffee grounds nanocellulose composite membrane prepared by this invention allows for the uniform dispersion of coffee grounds-derived nanocellulose within the polylactic acid matrix, effectively improving the structural density and crystallinity of the composite membrane. Simultaneously, the nanocellulose retains the complete type I monoclinic crystal structure of natural cellulose during the composite membrane preparation process, forming stable intermolecular forces with the polylactic acid molecular chains through hydrogen bonding. This significantly improves the inherent defects of pure polylactic acid materials, such as high brittleness and insufficient structural stability, endowing the composite membrane with excellent mechanical properties. Furthermore, the composite membrane of this invention allows for effective control of light transmittance, exhibiting lower transmittance in the visible light range. This effectively blocks photo-oxidation reactions, meeting the light-blocking performance requirements of packaging materials for fresh food preservation and expanding the application scope of polylactic acid-based materials in the field of food preservation packaging.

[0021] The polylactic acid / coffee grounds nanocellulose composite membrane prepared in this invention, when applied to the storage and preservation of button mushrooms, can precisely regulate gas exchange and moisture transfer within the packaging through appropriate air permeability. This effectively prevents rapid moisture evaporation during the storage process, significantly reducing weight loss, and also prevents moisture accumulation and microbial growth within the packaging. Furthermore, by regulating oxygen barrier properties, it reduces contact between oxygen and the button mushrooms, effectively inhibiting polyphenol oxidase-mediated enzymatic browning and significantly delaying the browning process of the mushroom cap. Simultaneously, this composite membrane effectively inhibits postharvest respiratory metabolism in button mushrooms, slowing down cell structure damage and degradation, effectively maintaining the firmness of the mushrooms during storage, and reducing the loss of core nutrients such as soluble solids and soluble proteins. This allows the button mushrooms to maintain good sensory quality during storage, with a preservation effect significantly superior to that of pure polylactic acid membranes and the unpackaged control group. It effectively solves the problems of rapid postharvest quality deterioration and short shelf life of button mushrooms.

[0022] This invention constructs a closed-loop recycling model of "resource utilization of agricultural waste - preparation of functional nanomaterials - application in food preservation packaging". The entire preparation process is simple to operate and the conditions are controllable. The raw materials used are widely available and inexpensive. The prepared composite film has the characteristics of being completely biodegradable and can replace traditional petroleum-based plastic packaging materials. It has both environmental and economic benefits and can provide a solid theoretical basis and technical reference for the high-value utilization of waste coffee grounds and the development of preservation packaging materials for edible fungi. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 Scanning electron microscope images of the morphology of coffee grounds nanocellulose membrane and PLA / NCC composite membrane; among them, Figure 1 Image b is a scanning electron microscope image of a coffee grounds nanofiber membrane. Figure 1 In the middle, c is a scanning electron microscope image of the PLA / NCC composite film; Figure 2 Fourier transform infrared spectra of coffee grounds nanocellulose membrane and PLA / NCC composite membrane; Figure 3 X-ray diffraction patterns of coffee grounds nanocellulose membrane and PLA / NCC composite membrane; Figure 4 Transmittance curves of coffee grounds nanocellulose membrane and PLA / NCC composite membrane in the wavelength range of 200nm~800nm; Figure 5 The graph shows the weight loss rate of mushrooms in different treatment groups during storage at 4℃. Among them, the CK group is the blank control group without any packaging treatment, the K group is the pure PLA film treatment group, and the P group is the PLA / NCC composite film treatment group. Figure 6 The graph shows the change in hardness of button mushrooms in different treatment groups during storage at 4℃; among them, CK group is the blank control group without any packaging treatment, K group is the pure PLA film treatment group, and P group is the PLA / NCC composite film treatment group. Figure 7 The graph shows the changes in sensory scores of different treatment groups of button mushrooms during storage at 4℃; among them, CK group is the blank control group without any packaging treatment, K group is the pure PLA film treatment group, and P group is the PLA / NCC composite film treatment group. Figure 8 The images show the appearance of mushrooms from different treatment groups during storage at 4℃. Group P is the PLA / NCC composite film treatment group, Group K is the pure PLA film treatment group, and Group CK is the blank control group without any packaging treatment. Figure 9 The graph shows the browning index changes of mushrooms in different treatment groups during storage at 4℃; among them, CK group is the blank control group without any packaging treatment, K group is the pure PLA film treatment group, and P group is the PLA / NCC composite film treatment group. Figure 10 The graph shows the changes in soluble solids content of mushrooms in different treatment groups during storage at 4℃; among them, CK group is the blank control group without any packaging treatment, K group is the pure PLA film treatment group, and P group is the PLA / NCC composite film treatment group. Figure 11 The graph shows the changes in soluble protein content of mushrooms in different treatment groups during storage at 4℃. Among them, the CK group is the blank control group without any packaging treatment, the K group is the pure PLA membrane treatment group, and the P group is the PLA / NCC composite membrane treatment group. Detailed Implementation

[0025] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0026] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0028] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0029] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0030] Experimental materials and reagents: Coffee grounds were the residue after pressing Arabica coffee beans (from Lucky Coffee); polylactic acid (PLA) particles, with a weight-average molecular weight Mw = 280 kDa and a molecular weight distribution Mw / Mn = 1.98, were purchased from NexWalk, Inc., USA; a biuret method protein content assay kit was purchased from Beijing Solarbio Science & Technology Co., Ltd.; sodium hydroxide, sodium chlorite, glacial acetic acid, concentrated sulfuric acid, dichloromethane, sodium hypochlorite, hydrochloric acid, and potassium bromide were all analytical grade reagents.

[0031] Experimental instruments and equipment: KQ5200DE CNC ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd., Jiangsu Province); CP224C electronic balance (Ohaus Instruments (Shanghai) Co., Ltd.); HWS-26 electric thermostatic water bath (Shanghai Yiheng Scientific Instrument Co., Ltd.); DHG-9070A electric thermostatic drying oven (Shanghai Qixin Scientific Instrument Co., Ltd.); SHZ-D(Ⅲ) circulating water multi-purpose vacuum pump (Shanghai Lichen Bangxi Instrument Technology Co., Ltd.); TU-1901 / 1900 double-beam UV-Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.); Quanta250 scanning electron microscope (FEIG, USA); X Pert Pro MPD X-ray diffractometer (Malvin Panaco); RE52A Fourier transform infrared spectrometer (Henan Fujing Instrument Equipment Co., Ltd.); RW20digital digital display mechanical stirrer (IKA, Germany); texture analyzer; handheld digital refractometer.

[0032] Data processing: All experimental data were replicated three times. Data summaries were completed using Microsoft Excel, graphs were generated using Origin 2019, and significance analysis was performed using SPSS 26.0. P < 0.05 was considered significant.

[0033] Example 1: Preparation of coffee grounds nanocellulose 1.1 Extraction of cellulose from coffee grounds Dissolve 5g of coffee grounds in 100mL of distilled water, boil for 15 minutes, cool, and filter. The filter residue should be prepared at a ratio of 1:16 (g) of grounds to liquid. mL -1 Add 6.66% (v / v) NaOH solution, stir for 2 hours in a water bath at 81°C, then wash with distilled water until neutral. The pretreated product, after washing until neutral, is then processed at a material-to-liquid ratio of 1:10 (g / v). mL -1 Add sodium hypochlorite solution, adjust the pH of the system to 3 with 10% hydrochloric acid (by volume), place in a 75°C constant temperature water bath and shake to decolorize for 30 minutes, remove and wash with distilled water until neutral, place in a 105°C electric thermostatic drying oven and dry to constant weight to obtain purified coffee grounds cellulose.

[0034] 1.2 Determination of cellulose yield from coffee grounds Three parallel experiments were set up. The mass of coffee grounds raw material dried to constant weight was accurately weighed. After cellulose extraction was completed according to the method in 1.1 above, the product was dried to constant weight and the mass of purified cellulose was weighed. The cellulose yield was calculated according to formula (1):

[0035] Cellulose yield (1) Where: m0 is the constant weight of coffee grounds raw material, g; m1 is the constant weight of purified cellulose after extraction, g.

[0036] The cellulose yields of the three parallel experiments were 24.92%, 25.15%, and 25.03%, respectively, with a statistically average yield of 25.03% and a relative standard deviation (RSD) of only 0.46%. This indicates that the extraction process has excellent stability and can efficiently remove impurities such as lignin and hemicellulose from coffee grounds, thus achieving the separation and purification of cellulose.

[0037] 1.3 Preparation of coffee grounds nanocellulose (NCC) Nanocellulose was prepared by sulfuric acid hydrolysis. 10g of the coffee grounds cellulose obtained in step 1.1 was used, and the mixture was added at a material-to-liquid ratio of 1:20 (g / g). mL -1 Add 60% sulfuric acid solution and stir at 45℃ until the solution becomes a yellowish-brown viscous consistency. Add 100mL of deionized water to terminate the reaction. After the system stands for 12 hours, discard the supernatant. The remaining suspension is washed by centrifugation 5-6 times at 4000r / min. Pour the resulting suspension into a dialysis bag and dialyze it in deionized water until the system is neutral to obtain the NCC suspension, which is then stored at 4℃ for later use.

[0038] Example 2: Preparation of coffee grounds nanocellulose membrane The coffee grounds NCC suspension refrigerated at 4°C in Example 1 was brought to room temperature and ultrasonically dispersed for 10 minutes until the system was homogeneous, thus obtaining a pretreated coffee grounds NCC dispersion. 5 mL of the pretreated NCC dispersion was uniformly coated onto a tetrachloroethylene plate that had been wiped and dried with anhydrous ethanol using a casting method. The plate was allowed to cure at room temperature for 2 hours. After the film was fully formed, it was carefully peeled off, and the resulting coffee grounds nanocellulose membrane was stored in a desiccator for later use.

[0039] Example 3: Preparation of polylactic acid / coffee ground cellulose nanofiber (PLA / NCC) composite membrane Accurately weigh 2.5g of PLA particles and dissolve them in 50mL of dichloromethane solvent. Stir continuously in a 45℃ constant temperature water bath until the PLA is completely dissolved. Add 5mL of the pretreated coffee grounds NCC dispersion from Example 2 to the PLA dissolution system, and sonicate for 10min to mix the system evenly to obtain a casting solution. Use a casting method to evenly coat the casting solution onto a tetrachloroethylene plate that has been wiped with anhydrous ethanol and dried. Allow it to cure at room temperature for 2h. Peel off the formed composite film and store it in a desiccator for later use to obtain the PLA / NCC composite film.

[0040] Example 4: Structure and performance characterization of PLA / NCC composite membrane The coffee grounds nanocellulose membrane prepared in Example 2 and the PLA / NCC composite membrane prepared in Example 3 were used as test samples for structural and performance characterization. The specific test methods and results are as follows: 4.1 Scanning Electron Microscopy (SEM) Morphology Analysis The surface morphology of the samples was observed using field emission scanning electron microscopy (FET), with an accelerating voltage of 5 kV. Before testing, the samples were placed on conductive adhesive on a copper platform, and the sample surface was sputtered with gold before being placed in the sample chamber for observation. Figure 1 As shown, where Figure 1 b is a SEM image of the surface morphology of the coffee grounds nanocellulose membrane. Figure 1 c is a SEM image of the surface morphology of the PLA / NCC composite film. Figure 1 It is evident that the coffee grounds nanocellulose membrane has a smooth and dense surface, a uniform structure, and good film-forming properties. The PLA / NCC composite membrane, on the other hand, has a slightly rough surface, exhibiting a granular and layered interwoven structure with a certain degree of microporous distribution, indicating that the nanocellulose was successfully introduced into the PLA matrix and formed an interfacial interaction, with only slight local aggregation. Coffee grounds NCC can form a certain degree of network structure within the PLA matrix and interact with PLA molecular chains through hydrogen bonding, thereby improving the overall structural stability and mechanical properties of the material. The rough yet relatively dense hierarchical structure of the PLA / NCC composite membrane helps to extend the diffusion path of gas within the membrane, thus enhancing the material's barrier properties.

[0041] 4.2 Fourier Transform Infrared Spectroscopy (FT-IR) Analysis The infrared spectrum of the sample was measured using transmission mode, with a scanning range of 4000–400 cm⁻¹. -1 .like Figure 2 The image shows the infrared spectra of coffee grounds nanocellulose membranes and PLA / NCC composite membranes. Figure 2 It can be seen that the characteristic absorption peak positions of the two membrane materials are basically the same, and no new characteristic absorption peaks appeared in the spectrum of the PLA / NCC composite membrane, indicating that no new functional groups were generated during the preparation of the composite membrane. Infrared spectral analysis of the coffee grounds nanocellulose membrane shows that it has a peak at 2955 cm⁻¹. - A significant O–H stretching vibration absorption peak appears at ¹, indicating a high hydroxyl content in the membrane material. This phenomenon stems from the selective degradation of the amorphous regions of cellulose by sulfuric acid hydrolysis, which fully exposes the hydroxyl groups in the crystalline regions that were originally embedded within the structure. Furthermore, the coffee grounds nanocellulose membrane exhibits a peak at 2909 cm⁻¹. -1 The C–H stretching vibration peak is retained at 1469 cm⁻¹. -1The presence of –CH2– bending vibration absorption peaks, typical characteristic peaks of cellulose structure, indicates that hydrolysis only refined the structural scale of cellulose and fully exposed the hydroxyl functional groups, without destroying its basic carbon skeleton structure. FT-IR analysis confirmed that coffee grounds NCC completely preserved the basic chemical structure of cellulose during preparation, while exposing more active hydroxyl groups. These hydroxyl groups can form stable intermolecular forces with PLA molecular chains through hydrogen bonding, providing a chemical basis for constructing a homogeneous and stable composite system.

[0042] 4.3 X-ray diffraction (XRD) spectrum analysis XRD tests were performed at 40 kV and 40 mA using CuKα radiation (λ = 0.15406 nm). The sample scanning angle ranged from 4° to 90°, with a resolution of 0.02° and a scanning rate of 4° / min. Figure 3 The image shows the X-ray diffraction patterns of coffee grounds nanocellulose membranes and PLA / NCC composite membranes. Figure 3 It was found that the PLA / NCC composite membrane exhibited two characteristic diffraction peaks at 14.8° and 22.6°, the positions of which are related to the crystal structure of PLA and the dispersion state of NCC. The coffee grounds nanocellulose membrane showed three strong diffraction peaks at 15.1°, 16.7°, and 22.9°, which corresponded perfectly to the diffraction of the characteristic crystal plane of cellulose type I. Among them, the strong diffraction peak at 22.9° is the core characterization peak of the crystallinity of cellulose type I. The test results show that sulfuric acid hydrolysis selectively degrades only the amorphous region of cellulose without destroying the molecular arrangement and lattice structure of the crystalline region. The NCC obtained after film formation still retains the complete monoclinic crystal structure of natural cellulose type I. During the preparation of the PLA / NCC composite membrane, the coffee grounds NCC did not undergo crystal transformation or structural damage due to blending and film formation processes, and its original crystal properties were well preserved. This provides a key crystallographic basis for NCC to be used as a reinforcing phase to improve the structural performance of the PLA matrix.

[0043] 4.4 Measurement of light-blocking performance The optical properties of the samples were determined using a UV-Vis spectrophotometer. The thin film samples were cut into 10mm × 10mm square specimens (approximately 0.1mm thick), cleaned with anhydrous ethanol, and fixed onto a test holder. The transmittance curves were measured within the wavelength range of 200–800 nm, with the transmittance value at 550 nm serving as the key indicator for evaluating the material's transparency. Figure 4 The image shows the transmittance curves of coffee grounds nanocellulose membrane and PLA / NCC composite membrane in the wavelength range of 200–800 nm. Figure 4It can be seen that the transmittance of coffee grounds nanocellulose membrane and PLA / NCC composite membrane differs significantly at various wavelengths (P<0.05): when λ=200nm, there is no significant difference in transmittance between the two groups of samples, both being 1.3%; when λ=400nm, the transmittance of nanocellulose membrane rises to 79.67%, while that of PLA / NCC composite membrane is only 24.65%, significantly lower than that of nanocellulose membrane; after wavelength 400nm, the transmittance of nanocellulose membrane remains high, at 81.7%, 82.95%, 83.65%, and 84.15% respectively at wavelengths of 500~800nm, while the transmittance of PLA / NCC composite membrane, although slowly increasing, remains at a low level, at 39.1%, 45.7%, 42.15%, and 42.7% respectively at the corresponding wavelengths, significantly lower than that of nanocellulose membrane at all wavelengths. Test results show that the light transmittance decreases after NCC and PLA are combined. This is because NCC is uniformly dispersed in the PLA matrix to form crystal particles with a size similar to the wavelength of visible light. When light passes through, it is scattered and blocked, thus reducing the light transmittance. This characteristic is suitable for the preservation requirements of button mushrooms. Button mushrooms are prone to browning due to photo-oxidation after harvesting. The low light transmittance of the composite film can block the photo-induced browning process, providing key support for the preservation of button mushrooms.

[0044] Example 5: Application of PLA / NCC composite film in the preservation of button mushrooms 5.1 Test Samples and Grouping Fresh button mushrooms were purchased and pre-cooled at 4℃ for 24 hours. Samples of uniform size and maturity, without open caps, pests, diseases, or mechanical damage were selected. Each sample weighed (480±1) g and was placed in a PE preservation box. Three treatment groups were set up: Blank control group (CK group): No packaging treatment was performed; Pure PLA membrane treatment group (Group K): Mushrooms were wrapped with pure PLA membrane; PLA / NCC composite film treatment group (Group P): The mushrooms were wrapped with the PLA / NCC composite film prepared in Example 3.

[0045] All groups of PE food storage boxes were stored at (4±1)℃ for 12 days, and samples were taken every 2 days to determine various quality indicators.

[0046] 5.2 Test Indicators and Methods 5.2.1 Determination of weight loss rate The weight loss rate of the sample was determined by weighing, and the mass loss rate was calculated according to formula (2):

[0047] The mass loss rate (2) is given by formula: m1 is the initial mass of the sample, g; m2 is the mass of the sample at a specific storage time point, g.

[0048] like Figure 5 The figure shows the curve of weight loss rate during the storage of button mushrooms. Figure 5 It was found that the weight loss rates of groups CK, K, and P all increased continuously with prolonged storage time, with significant differences between groups (P < 0.05). On the second day of storage, the weight loss rate of group CK reached 2.6730%, group K was 0.552%, and group P was only 0.485%, indicating that group P's water retention advantage was initially apparent. On the sixth day of storage, the weight loss rate of group CK rose to 5.1860%, group K was 3.356%, and group P was only 2.188%. By the twelfth day of storage, the weight loss rate of group CK reached a peak of 8.6650%, group K was 5.0295%, and group P was only 3.635%, significantly lower than the other two groups throughout the entire process. The results indicate that the PLA / NCC composite film, with its suitable air permeability, can precisely regulate gas exchange and moisture transfer within the packaging, preventing rapid moisture evaporation and preventing moisture accumulation and microbial growth, significantly inhibiting weight loss during mushroom storage, and exhibiting excellent water retention and freshness preservation performance.

[0049] 5.2.2 Hardness Measurement The hardness of button mushrooms was determined using a texture analyzer. A uniform area in the center of the button mushroom cap that was undamaged and free of brown spots was selected as the test site. The test speed was set to 2 mm / s before the test, 1 mm / s during the test, and 2 mm / s after the test. The trigger force was 0.05 N and the compression was 50%. Three caps were randomly selected from each sample, and the test was repeated twice for each cap. The maximum pressure value during the puncture process was recorded to characterize the hardness.

[0050] like Figure 6 The figure shows the curve of hardness change of button mushrooms during storage. Figure 6 It was found that the hardness of mushrooms in groups CK, K, and P all decreased continuously with the extension of storage time during the storage period, and the hardness differences among different groups at the same time point were all significant (P<0.05). At the beginning of storage (0d), there was no significant difference in hardness among the three groups, which were 6.21N for group CK, 6.20N for group P, and 6.20N for group K. As storage progressed, the rate of decrease in hardness showed a differentiated pattern of group P < group K < group CK: group CK decreased the fastest, with a hardness of only 1.03N on day 12, a decrease of 83.4% from the initial value; group K was the second fastest, decreasing to 1.8N on day 12, a decrease of 71.0%; the rate of decrease in group P slowed down significantly, remaining at 2.65N on day 12, a decrease of only 57.3%, and the hardness of group P was significantly higher than that of groups K and CK at all time points. The results showed that the PLA / NCC composite film could effectively inhibit the respiratory metabolism of mushrooms and slow down cell structure damage by regulating the transfer of gas and moisture within the packaging, thereby enabling mushrooms to maintain high firmness for a long period of time during storage. Its preservation performance was significantly better than that of pure PLA film and blank control.

[0051] 5.2.3 Sensory quality evaluation Twenty professionally trained evaluators conducted a comprehensive sensory evaluation of the appearance, acceptability, smell, and texture of the mushrooms during storage, according to the scoring criteria in Table 1.

[0052] Table 1 Sensory evaluation criteria for mushrooms during storage

[0053] like Figure 7 The figure shows the change curve of sensory scores during the storage of button mushrooms, as shown in the figure. Figure 8 The images shown depict the appearance of different treatment groups of button mushrooms during storage. Figure 7 , Figure 8 It was found that the sensory scores of the CK, K, and P groups of button mushrooms differed significantly during storage (P < 0.05). In the early stage of storage (0-2 days), the appearance of the button mushrooms in all three groups was good, and there was no significant difference in sensory scores. On the second day, the scores were 8.5 for the P group, 8.4 for the K group, and 8.4 for the CK group. From the fourth day onwards, the preservation effect was significantly differentiated. The caps of the CK group turned yellow and developed sporadic brown spots, and the score dropped sharply to 5. The K group only showed slight browning and scored 6.5. The P group had a stable appearance and maintained a score of 7.5, which was significantly better than the other two groups (P < 0.05). From the 6th to the 8th day, the browning of the CK group intensified and the fruiting bodies shrank, and the scores dropped to 3 and 1, respectively. The K group showed the second most severe browning and scored 4.5 and 3. The P group only showed slight browning and maintained a score of 6 and 5. In the final stage of storage (12 days), the CK group completely browned and shrank, losing its commercial value. The K group still had a decent morphology but obvious browning. The P group had a small amount of browning but its morphology was intact. The results showed that the PLA / NCC composite film could effectively delay the browning and quality deterioration of button mushrooms, maintain their sensory quality during storage, and its preservation effect was significantly better than that of pure PLA film and blank control.

[0054] 5.2.4 Browning Index Determination The degree of browning of button mushrooms was determined by spectrophotometry. The button mushroom samples were accurately weighed and mixed at a material-to-liquid ratio of 1:10 (g). mL -1 Add boiling distilled water, blanch for 30 seconds and then cool rapidly. After homogenization, centrifuge at 10000 r / min for 10 min. Take 10 mL of the supernatant and dilute to 50 mL with distilled water. Measure the absorbance at 420 nm using a UV spectrophotometer. Multiply the result by the dilution factor of 10 and calculate the browning index (BI) according to formula (3).

[0055] In the formula: A 420 This represents the absorbance at 420nm.

[0056] like Figure 9 The figure shows the curve of browning index change during the storage of button mushrooms. Figure 9It was found that the browning index of groups CK, P, and K all increased continuously with prolonged storage time, with significant differences among groups (P < 0.05). On the second day of storage, the browning index of group CK rose to 2.05, group K to 1.72, and group P to only 0.88; on the sixth day of storage, the browning index of group CK reached 5.21, group K to 3.545, and group P to only 2.51; by the twelfth day of storage, the browning index of group CK reached a peak of 6.36, group K to 5.02, and group P to only 4.12, significantly lower than the other two groups throughout the entire process. The results indicate that the PLA / NCC composite membrane can inhibit enzymatic browning by regulating oxygen barrier and permeability, reducing oxygen contact, while avoiding moisture accumulation and reducing the risk of microbial browning, thus exhibiting the best browning inhibition effect on *Pleurotus ostreatus* during storage.

[0057] 5.2.5 Determination of soluble solids content The soluble solids content (SSC) of the samples was determined using a handheld digital refractometer. 5.0 g of fresh mushroom sample was accurately weighed, and after being crushed by tissue grinding, the sample was centrifuged at 10000 r / min for 15 min to obtain the supernatant. An appropriate amount of the clear supernatant was dropped onto the prism surface of the refractometer, and the soluble solids content (%Brix) was directly read at room temperature (25±1)℃.

[0058] like Figure 10 The figure shows the curve of soluble solids content change during the storage of button mushrooms. Figure 10 The results showed that the SSC content of the three groups of mushrooms differed significantly during storage (P < 0.05). Initially, the content in all three groups was 5.1 mg / g, with no significant difference. The CK group showed a trend of first increasing and then decreasing, rising from 5.1 mg / g to a peak of 6.32 mg / g from 0 to 4 days, and then falling sharply to 3.2 mg / g from 4 to 12 days, a decrease of 49.4% from the peak to 12 days. The P and K groups showed a gradual decrease, with the P group decreasing from 5.1 mg / g to 2.1 mg / g at 12 days (a decrease of 58.8%), and the K group decreasing to 1.8 mg / g (a decrease of 64.7%). The decrease in the P group was significantly smaller than that in the K group. These results indicate that the PLA / NCC composite film can regulate the packaging microenvironment, inhibit the respiratory metabolism of mushrooms, slow down the decomposition and consumption of organic matter, maintain the SSC content more gradually, effectively delay the loss of nutrients in mushrooms, and has a significantly better preservation effect than pure PLA film.

[0059] 5.2.6 Determination of soluble protein content The soluble protein content of button mushrooms was determined using a biuret method protein content assay kit.

[0060] like Figure 11 The figure shows the curve of change in soluble protein content during the storage of button mushrooms. Figure 11It was found that the soluble protein content of groups CK, K, and P all decreased with storage time, and the differences between groups were significant (P < 0.05). Initially, the content of all three groups was 5.45 mg / g, with no significant difference. From day 2 onwards, the content of group P was significantly higher than the other two groups, and the advantage continued to widen. By day 12 of storage, the soluble protein content of group P was still 3.555 mg / g (approximately 65.2% of the initial content), significantly higher than group K's 2.6 mg / g (47.7%) and group CK's 1.295 mg / g (23.8%). The results indicate that the PLA / NCC composite film can regulate the packaging microenvironment, reduce oxygen concentration to inhibit respiratory metabolism and reduce protein substrate consumption, while simultaneously reducing endogenous protease activity, thus doubly delaying protein degradation and efficiently preserving the nutrients of mushrooms, demonstrating significant preservation advantages.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A polylactic acid / coffee grounds nanocellulose composite membrane, characterized in that, The composite membrane uses polylactic acid as the matrix and coffee grounds-derived nanocellulose as the reinforcing phase. The nanocellulose is uniformly dispersed in the polylactic acid matrix to form a dense and interwoven composite structure, and the nanocellulose retains the cellulose type I crystal structure.

2. The polylactic acid / coffee grounds nanocellulose composite membrane according to claim 1, characterized in that, The composite film has a transmittance of ≤25% at a wavelength of 400nm and a transmittance of 40%~46% at a wavelength of 550nm.

3. A method for preparing a polylactic acid / coffee ground cellulose nanocomposite membrane as described in any one of claims 1-2, characterized in that, Includes the following steps: S1 Extraction of cellulose from coffee grounds: Using waste coffee grounds as raw material, coffee grounds cellulose is obtained after alkali treatment, decolorization and purification. Preparation of S2 nanocellulose: The coffee grounds cellulose obtained in step S1 was hydrolyzed with sulfuric acid and purified by dialysis to obtain a nanocellulose suspension; Preparation of S3 composite membrane: Polylactic acid is dissolved in an organic solvent, and the nanocellulose suspension obtained in step S2 is added. After mixing evenly, the mixture is cast into a film, cured and dried to obtain the polylactic acid / coffee grounds nanocellulose composite membrane.

4. The method according to claim 3, characterized in that, The specific steps of step S1 are as follows: after pre-treating the coffee grounds by boiling them in water, add a 6.66% NaOH solution at a ratio of 1:16 g / mL, stir in a water bath at 81°C for 2 hours, and wash until neutral. Add sodium hypochlorite solution at a material-to-liquid ratio of 1:10 g / mL, adjust the pH of the system to 3, decolorize by shaking in a 75°C water bath for 30 minutes, wash until neutral, and then dry to obtain coffee grounds cellulose.

5. The method according to claim 3, characterized in that, Step S2 specifically involves adding coffee grounds cellulose to a 60% sulfuric acid solution at a material-to-liquid ratio of 1:20 g / mL, stirring at a constant temperature of 45°C until the system becomes a yellowish-brown viscous substance, and then adding deionized water to terminate the reaction. After the system has stood, it is centrifuged, washed, and dialyzed until neutral to obtain a nanocellulose suspension.

6. The method according to claim 3, characterized in that, In step S3, the organic solvent is dichloromethane, and the ratio of polylactic acid to dichloromethane is 2.5g:50mL. The polylactic acid is dissolved under constant temperature water bath stirring at 45℃. After the nanocellulose suspension and polylactic acid solution are mixed, they are ultrasonically treated for 10min until the system is homogeneous. After casting, it is allowed to stand at room temperature for 2h to cure.

7. The application of the polylactic acid / coffee grounds nanocellulose composite film according to any one of claims 1-2 in the preservation packaging of fresh food.

8. The application according to claim 7, characterized in that, The fresh food is edible fungi, and the application is to use polylactic acid / coffee grounds nanocellulose composite membrane for post-harvest storage and preservation of edible fungi.

9. The application according to claim 8, characterized in that, The edible fungus is *Tricholoma matsutake*; the storage and preservation temperature is 4±1℃, and the storage time is ≤12 days.

10. The application according to claim 9, characterized in that, When the polylactic acid / coffee grounds nanocellulose composite membrane is used for the preservation of button mushrooms, it can delay the weight loss, decrease in hardness, and browning of the cap during the storage process, while inhibiting the loss of soluble solids and soluble proteins.

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