Study on the stability and digestibility of the cellulose ether/nanocrystalline cellulose compound emulsion

Abstract: In order to reduce the body's absorption of fats and oils in food and reduce the risk of obesity, this study explored the physical properties of oil-in-water composite emulsions prepared by synergistic effects of different types of cellulose ethers (non-ionic and anionic) and cellulose nanocrystals. Storage and simulated gastrointestinal digestive stability properties. Studies have shown that cellulose ether/cellulose nanocrystalline composite emulsions have the best storage stability for at least one month and the lowest lipid digestibility compared to single-component stable emulsions, thus confirming that cellulose-based materials Potential applications in fields such as food and healthcare.

Key words:cellulose nanocrystals; cellulose ether; emulsion physical stability; lipid digestibility

As one of the three basic nutritional components required by the human body, fat is closely related to human health. Excessive intake of fats in food will lead to fat accumulation in the body, increase body weight and even cause a series of cardiovascular, digestive and endocrine system derived diseases. Although common dieting methods can achieve the goal of weight loss to a certain extent or at a specific stage, it can easily lead to other health risks such as malnutrition, stomach problems, and memory loss. solutions are particularly important. In daily life, in addition to direct blending, the most common use of emulsification technology is to improve the taste of food, improve stability and endow products with unique functionality. Inspired by this, this study reduces the release of oil in food and the absorption rate of human body by enhancing the stability of oil-in-water emulsion. This method is expected to be applied to the suppression of obesity and related fields.

Natural biomass colloidal particles have long attracted the attention of industry and academia due to their advantages of environmental friendliness, wide sources and unique physicochemical properties. Different from the traditional amphiphilic surfactant emulsion, the emulsion with stable colloidal particles has higher interfacial deadsorption free energy and is not easy to be replaced by bile salt in the process of gastrointestinal digestion. Therefore, the emulsion system has excellent stability and plays a unique advantage in the field of diet and health care. Singh et al summarized how to regulate the digestion and lipolysis of lipids by gastrointestinal digestive juices through the construction of a food-grade emulsion, and discussed in detail the issues facing this area, including the interaction between digestive enzymes and the composition and structure of the stable layer at the milk droplet interface, the replacement and displacement of bile salts, and the final state of the milk droplet before absorption. On this basis, some literatures have reported the application of nanocellulose colloidal particle stabilization emulsion in improving lipid digestion and absorption. Deloid et al. found that the addition of nanocellulose to high fat content could reduce the hydrolysis of triglycerides (FFA content of free fatty acid was reduced to about 30%) and reduce fat digestion or absorption in an experiment simulating gastrointestinal digestion in vitro. Bai et al. studied the digestive performance of cellulose nanocrystalline (CNC) emulsion through a three-stage static simulation of gastrointestinal peptic experiment. The results showed that compared with acacia emulsion, CNC emulsion could reduce FFA content to about 60%.

Based on the design idea of a single colloidal particle stabilized emulsion, this study investigated the synergistic effect of CNC nanoparticles and different kinds of cellulose ether polymers on improving the physical storage stability of their complex emulsion and simulating gastrointestinal digestion, as well as reducing FFA release.

1. Experiment

1.1 Reagents and instruments

Whatman Dust-free cotton filter Paper (GE Healthcare Canada); Nonionic methyl cellulose (MC, viscosity =4000mpa·s) and anionic carboxymethyl cellulose (CMC, viscosity =2000mpa·s) (Dow Chemical Company, USA); Mass fraction 99.999% sulfuric acid (Caledon Laboratory Chemicals, Canada), Corn Oil, sodium chloride, sodium hydroxide (Sigma-Aldrich, Canada).

Super fast frozen centrifuge (Sorvall RC-5, Dupont, USA), ultrasonic cell crusher (Sonifier 450, Branson Ultrasound, USA), electronic Analytical Balance (ME204T, Mettler Toledo Group, Switzerland).

1.2 Preparation of cellulose nanocrystals

As mentioned above, under the condition of continuous mechanical stirring, Whatman dust-free cotton filter paper (40g) was treated with concentrated sulfuric acid (700ml) with mass fraction of 64% (wt) at 45℃ for 45min, then diluted with a large amount of ice water for quenching reaction, and the residual sulfuric acid was removed as far as possible through repeated washing and centrifugation. This was followed by dialysis in deionized water for about two weeks to remove residual sulfuric acid and degradation of sugars from the system. After dialysis, the CNC dispersion was treated with ultrasonic dispersion for 45min in an ice bath by cell shredder and filtered, then the PH of NaOH neutralization system was added to neutral, and the neutral CNC water-dispersed liquid was stored in cold storage for later use.

1.3 Preparation of cellulose ether solution

Due to the unique thermal gel properties of methyl cellulose, in order to ensure the full dissolution of samples, this study followed the heating and cooling method recommended in the product manual to prepare cellulose ether aqueous solution. Firstly, powder samples were dispersed in hot water (1/3 total volume) at 90℃, and continuously stirred mechanically for 30min. Remove the heater, add the remaining 2/3 volume of ice water, and stir continuously until the solution changes to a clear and transparent state. Carboxymethyl cellulose is directly dissolved in deionized water under the action of mechanical agitation and configured to the required concentration. All samples were refrigerated for future use.

1.4 Preparation of cellulose based emulsion

After the cellulose ether aqueous solution and CNC dispersion were uniformly mixed by the vortex oscillator, NaCl (the final salt concentration of 50mM) was added to shield the electrostatic repulsion in the system. Then, cellulose ether /CNC emulsion was prepared by mixing corn oil with 1/4 oil/water volume ratio in a pre-oscillatory way and undergoing ultrasonic treatment with ultrasonic cell crusher (6-level intensity and 50% pulse) in an ice water bath for 3min. The emulsion prepared by cellulose ether alone or CNC was used as the control group.

1.5 Size determination of cellulose nanocrystals

The shape and size of CNC was tested by Asylum MFP-3D atomic force microscope (Oxford Instruments, USA). The test conditions were FMR giant spiral arm (Nano World), the legal elastic constant was 1.2~5.5N/m, and the resonance frequency was 60~90kHz in room temperature atmosphere.

1.6 Determination of particle size of cellulose based emulsion

The Malvem Mastersizer 2000G laser particle size analyzer (equipped with 633nm laser, Malvem Instruments, USA) was used to determine the particle size of the cellulose ether /CNC emulsion.

1.7 Morphology observation of cellulose based emulsion

The Axiovert 100M optical microscope (Zeiss, Germany) is used to visualize the emulsion topography. After the emulsion was diluted 100 times, it was dropped in the center of the slide and covered with a cover slide. It should be pointed out that the irregular or oval gray areas in some optical microscope images may be leaking oil droplets.

1.8 In vitro analysis of simulated digestion of cellulose based emulsion

According to the standardized static method reported by Brodkorb et al., the in vitro simulation of human gastrointestinal digestion experiment consisted of the following three stages: 1) simulation of oral digestion. Fresh emulsion samples were mixed with simulated saliva (SSF) electrolyte solution in the final ratio of 1:1, then 75 U/mL saliva A-amylase IX-A was added, and finally 0.75 mM CaCl2 was added. SSF electrolyte solution consists of 15.1 mM KCI, 3.7 mM KH2PO+, 13.6 mM NaHCO3, 0.15 mM MgCl2(H2O), 0.06 mM (NH4)2CO3. The simulated digestion temperature was 37C,pH=7.0, and time was 2 min. 2) Simulated gastric phase digestion. The orally digested emulsion sample was first mixed with the simulated gastric fluid (SGF) electrolyte solution at the final ratio of 1:1, then 2000 U/mL pepsin (P7125) was added, and finally 0.075 mM CaCl2 was added. SGF electrolyte solution is composed of 6.9 mM KC1, 0.9 mM KH2P04, 25 mM NaHCO3, 47.2mM NaCI, 0.1 mM MgCl(H2O)6, 0.5 mM (NH4)2CO3. The simulated digestion temperature was 37C,pH=3.0, and time was 2h. 3) Simulate intestinal digestion. The above-mentioned gastric chyme was first combined with the simulated intestinal fluid (SIF) electrolyte solution in the final ratio 1: 1 Mix, then add 10 mM fresh bile solution, 200 U/mL pancreatic enzyme (P3292) and 2 000 U/mL lipase (L3126, Type II), and finally add 0.3 mM CaC12e SIF electrolyte solution composed of the following components: 6.8 mM KCI, 0.8 mM KH2PO4, 85 mM NaHCO3, 38.4 mM NaCl, 0.33 mM MgCI2(H2O)6. The simulated digestion temperature was 37℃,pH= 7.0 and time was 2 h. The released free fatty acid (FFA) content was measured using pH-stat automatic titration technique (Metrohm 916 Ti-Touch, Switzerland) to quantify the degree of lipolysis of corn oil in the emulsion and thus characterize the emulsion stability.

2. Results and discussion

2.1 Morphology and size of cellulose nanocrystals

According to the morphology and size of CNC film, the length of a single CNC nanoparticle is about 200 nm and the diameter is about 10 nm. The size distribution of the nanoparticles is uniform, and the rod-like morphology of the nanoparticles is "thick in the middle", similar to the rice granular structure.

2.2 Appearance stability of cellulose-based emulsion

According to the apparent stability of different cellulose-based emulsions (oil-out or delamination problems), in addition to CMC, CNC or MC alone can be used to prepare oil-in-water emulsion, and the emulsion has no oil-out phenomenon. However, due to the difference in the density of the two phases of water and oil, the low viscosity of the system and the larger particle size of the emulsion droplets, the stratification phenomenon of the single CNC nanoparticle emulsion was serious after 30 days of storage at room temperature, and its appearance was slightly inferior to that of MC emulsion. Due to the synergistic effect between components, the cellulose ether /CNC composite emulsion generally has high stability. However, unlike the cellulose derivatives such as MC, CMC does not have emulsifying property, the interaction between CMC and CNC will weaken the stability of the emulsion system, and the emulsion stratification is still obvious.

2.3 Particle size of cellulose based emulsion

According to the particle size of 30-day storage stability of cellulose-based emulsion, the stability of cellulosether /CNC composite emulsion is excellent, although some stratification phenomenon appears, but the particle size of emulsion basically remains unchanged with the extension of storage time. Similarly, the particle size of single CNC emulsion did not change significantly with the extension of storage time, but the stratification phenomenon was serious. This is related to the irreversible adsorption of CNC nanoparticles on the water-oil interface, that is to say, under the action of external high-power ultrasonic dispersion, once CNC nanoparticles used to stabilize the water-oil interface are adsorbed on the interface, it will be difficult to disengage from the interface. MC can stabilize oil-in-water emulsion alone. However, due to the low interfacial dissociation energy of polymer stabilized emulsion, the polymer adsorbed on the interface will gradually break off from the interface and return to the aqueous phase with the extension of time, and finally reach a dynamic reversible adsorption equilibrium. Therefore, the particle size of amphibious polymer MC stabilized emulsion will increase and its stability will decrease during long-term storage.

2.4 Emulsion simulated digestion of free fatty acid content in vitro

The effect of in vitro simulated digestion experiment on the stability of the emulsion was evaluated by detecting the change of the content of free fatty acids with digestion time. As described by DeLoid et al., triglycerides (MAGs), the main component of edible oils, are lipopolysis by pancreatic lipase during digestion in the small intestine to produce free small molecular acids. According to the in vitro simulated digestion curve of cellulose based emulsion, cellulose ether /CNC emulsion maintained good overall stability after triple simulated oral, gastric and intestinal digestion experiments, consumed the least NaOH volume, released the least FFA content, namely the lowest degree of triglyceride lipolysis. The results showed that the cellulose ether (MC or CMC) /CNC composite shell could stabilize the oil-water interface well, and play an isolated role in the digestion and lipolysis of pancreatin and lipase in the simulated intestinal fluid. However, the emulsion stabilized by MC or CNC alone showed a higher FFA release, indicating that the stability of these emulsions was poor after simulated physiological digestion experiment. Digestive enzymes could penetrate through the stabilizer shell into the oil phase of the emulsion and have lipolysis reaction with corn oil. Since CMC alone cannot stabilize oil-in-water emulsion, the single CMC sample will not be listed in this digestion experiment.

2.5 Optical microscope image of emulsion simulation digestion in vitro

As can be seen from in vitro simulated digestion optical microscope images of cellulose based emulsion, cellulose ether VCNC, MC alone or CNC stabilized emulsion can maintain good milk drop integrity (morphology and size) after simulated oral and gastric digestive tract experiments. Anionic cellulose ether CMC cannot stabilize oil-in-water emulsion alone because it cannot reduce the surface tension of the system. It will not be discussed here. The rupture or degradation of the emulsion mainly occurred in the digestive tract of the simulated small intestine. After the treatment of the whole simulated digestive system (simulated saliva & simulated gastric fluid & simulated intestinal fluid, SSF&SGF&SIF), the cellulose ether /CNC composite emulsion showed the best emulsion stability, that is, the lowest content of free fatty acids and the smallest change in the size of milk drops. However, after SSF&SGF&SIP simulation digestion, the content of free fatty acids was the highest, the size of milk droplets increased significantly, and irregular or oval gray areas appeared in the optical microscope images, which may be leaked oil droplets. After SSF&SGF&SIF simulation digestion, the content of free fatty acids of the single CNC stabilized emulsion was slightly lower than that of cellulose ether emulsion, but irregular gray areas still appeared in the optical microscope pictures, indicating that the emulsion was also damaged to a large extent.

3. Conclusion

1) Compared with the O/W emulsion stabilized by cellulose ether alone or CNC, the synergistic effect between cellulose ether and CNC improved the physical stability of the composite emulsion. After 30 days of storage at room temperature, the cellulose ether /CNC emulsion had no obvious particle size change or oil extraction problem, and the stratification was improved.

2) Cellulose ether modified CNC emulsion showed better tolerance to artificial saliva, gastric fluid and intestinal fluid in the simulated gastrointestinal digestion experiment, and had the lowest content of free fatty acids in liolysis, which revealed the potential application value of this type of complex emulsion in healthy diet and reduced the risk of obesity.

3) The stability enhancement effect of nonionic cellulose ether MC on composite emulsion was greater than that of anionic cellulose ether CMC.