Abstract: The effects of three kinds of lipids, namely, stearic acid, beeswax and sucrose fatty acid esters, on the properties of rice albumen membranes were investigated using rice albumen as the base material, and the changes of the mechanical properties and water permeability of rice albumen membranes were examined with the additions of different amounts of lipids. The experiments showed that the addition of stearic acid, sucrose fatty acid esters and beeswax improved the water resistance of the membrane, while the addition of stearic acid and sucrose fatty acid esters had little effect on the mechanical properties of the membrane.
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Since protein-based edible membranes have good nutritional, mechanical and gas barrier properties, protein-based edible membranes have become a hot research topic in recent years. In 1996, FriDeric F. Shih produced edible membranes from rice protein concentrate (53% protein mass fraction) and polysaccharides obtained by enzyme treatment of rice flour[1] ; in 1998, seung-yong-Cho et al. produced edible membranes with certain strength and barrier properties from rice protein extracted from sake lees by alkaline method[2] . However, it has not been reported to improve the performance of rice protein film by combining it with other materials. The authors added lipids to the rice protein film in order to reduce its water permeability, which is a characteristic of the poor moisture barrier of rice protein film.
1 Materials and Methods
1.1 Materials and main instruments
Rice protein concentrate: homemade, protein mass fraction 88.48% (N×5.95, dry basis); glycerol: Shanghai Chemical Reagent Purchasing and Supply Station; glutamine transaminase (TG): Jiangsu Yiming Fine Chemical Co.
Micrometer: Harbin Gauge and Sharpening Tools Factory; Physical properties meter: TA × T2i stable Micro systems (UK); Cs501 high-speed disperser, constant temperature bath, magnetic stirrer, PHS-3C PH meter. 1.2 Test methods
1. 2. 1 Membrane Preparation
Prepare rice protein solution (protein quality concentration 5 g/DL), add 0.2% TG as cross-linking agent, glycerol (1g/DL) as plasticizer, and adjust the PH to 11.5 with 2mol/L NaoH and 2mol/LHCl. Add lipids (0.2g/DL), melt at 80 ℃, and dispersed at 20,000 r/min. After degassing, 25 mL of the film solution was poured into a 10 cm diameter mold and dried at 50 ℃ for 5 h. The film was removed. Place the film in a constant temperature and humidity chamber at 23 ℃ and 50% relative humidity to equilibrate the moisture 2D and prepare for use.
1.2.2 Membrane Performance Testing
1) Film Thickness Measurement.
Randomly take 4~5 points, use micrometer (accuracy 0.001mm) to measure its thickness. Take the average value.
2) Determination of tensile strength (T) and elongation (E).
Cut the film into 100mm x 10mm strips and use the tensile probe on the physical tester with a tensile speed of 1 mm/s. The tensile speed is set to the maximum tensile force and tensile distance. The tensile speed is 1 mm/s. The physical properties meter is set to the maximum tensile force and tensile distance. The tensile strength (T) was calculated as follows.
Where :T is the tensile strength, N/m2 ; F is the maximum tensile strength, N; L is the thickness of the film sample, m; W is the width of the film sample, m. The tensile strength of the film sample was calculated as follows.
The formula for elongation (E) is as follows.
Where: E is the elongation, %; L0 is the length of the specimen before stretching, m; L1 is the length of the specimen after stretching, m. The elongation of the specimen after stretching, m is the length of the specimen after stretching, m.
3) Determination of water permeability
According to the principle and steps of the water vapor permeability test method for plastic films and sheets, the method of GB1037-87 was adopted[3] with slight modification. After selecting a flat, dry and homogeneous film and determining its thickness, the film was sealed in the mouth of a permeable cup filled with anhydrous calcium chloride, weighed and put into a desiccator with barium chloride saturated solution at the bottom and 90% relative humidity, and then weighed after 24 h. The film was then put into a desiccator with barium chloride saturated solution at the bottom and 90% relative humidity.
Water vapor transmission rate (WvP) (g. mm/kPa● D ● m2 )
Where:M1 is the original weight of the membrane, the moisture permeable cup and the desiccant, g; M2 is the weight of the membrane, the moisture permeable cup and the desiccant after absorbing water, g; L is the thickness of the membrane, mm; A is the effective measurement area, m2; t is the measurement interval time, D; ΔP is the difference in water vapor pressure between two sides of the membrane, kPa.
2 Results and Discussion
2.1 Effect of the type of lipid-like substance on membrane properties
Nathalie GontarD et al. studied the effects of 11 different lipids on the whiteness of wheat gluten[4] , in this experiment, we selected 4 kinds of lipids from these 11 kinds of lipids, and examined the disperser, thermostatic tank, magnetic stirrer and PHS-3C PH meter. 1.2 Experimental method
1. 2. 1 Membrane Preparation
Prepare rice protein solution (protein quality concentration 5 g/DL), add 0.2% TG as cross-linking agent, glycerol (1g/DL) as plasticizer, and adjust the PH to 11.5 with 2mol/L NaoH and 2mol/LHCl. Add lipoid (0.2g/DL) and melt at 80 ℃, dispersed at 20,000r/min. After degassing, 25 mL of the film solution was poured into a 10 cm diameter mold and dried at 50 ℃ for 5 h. The film was removed. Place the film in a constant temperature and humidity box at 23 ℃ and 50% relative humidity to equilibrate the moisture 2D and prepare for use.
1.2.2 Membrane Performance Testing
1) Film Thickness Measurement.
Randomly take 4~5 points, use micrometer (accuracy 0.001mm) to measure its thickness. Take the average value.
2) Determination of tensile strength (T) and elongation (E).
Cut the film into 100mm x 10mm strips and use the tensile probe on the physical tester with a tensile speed of 1 mm/s. The tensile speed is set to the maximum tensile force and tensile distance. The tensile speed is 1 mm/s. The physical properties meter is set to the maximum tensile force and tensile distance. The tensile strength (T) was calculated as follows.
Where :T is the tensile strength, N/m2 ; F is the maximum tensile strength, N; L is the thickness of the film sample, m; W is the width of the film sample, m. The tensile strength of the film sample was calculated as follows.
The formula for elongation (E) is as follows.
Where: E is the elongation, %; L0 is the length of the specimen before stretching, m; L1 is the length of the specimen after stretching, m. The elongation of the specimen after stretching, m is the length of the specimen after stretching, m.
3) Determination of water permeability
According to the principle and steps of the water vapor permeability test method for plastic films and sheets, the method of GB1037-87 was adopted[3] with slight modification. After selecting a flat, dry and homogeneous film and determining its thickness, the film was sealed in the mouth of a permeable cup filled with anhydrous calcium chloride, weighed and put into a desiccator with barium chloride saturated solution at the bottom and 90% relative humidity, and then weighed after 24 h. The film was then put into a desiccator with barium chloride saturated solution at the bottom and 90% relative humidity.
Water vapor transmission rate (WvP) (g. mm/kPa● D ● m2 )
Where:M1 is the original weight of the membrane, the moisture permeable cup and the desiccant, g; M2 is the weight of the membrane, the moisture permeable cup and the desiccant after absorbing the water, g; L is the thickness of the membrane, mm; A is the effective measurement area, m2; t is the measurement interval time, D; ΔP is the difference in the water vapor pressure between the two sides of the membrane, kPa.
2 Results and Discussion
2.1 Effect of the type of lipid-like substance on membrane properties
Nathalie GontarD et al. investigated 11 different species of
The effect of lipids on the protein properties of wheat gluten[4] , in this experiment, we selected four kinds of lipids from these 11 kinds of lipids, and examined their effects on the edible membrane properties of rice protein.
2.1.1 Membrane thickness
Table 1 lists the thicknesses of several types of films. The thickness of the film with sucrose fatty acid ester was about 0.084 mm, the surface was smooth, the color was light, but the film was heavily adhered to the mold and was difficult to remove. The film with beeswax was yellowish in color, slightly less smooth, and thicker, about 0.102 mm. The film formed by stearic acid was homogeneous and flat, with a thickness of about 0.077 mm, but the transparency of the film was not as good as that of the former two. Acetyl tartaric acid molecule contains a free carboxyl group, has strong acidity, and is a dispersion of PH 2~3 in water[5] . Since 66% to 78% of rice protein is alkali-soluble protein[6] . After the addition of acetyl tartaric acid ester, the PH value of the membrane solution decreased, and the rice protein precipitated and could not cross-link to form a network, so the membrane could not be formed.
Note: RPI: Rice Protein Concentrate; SA: Stearic Acid; WB: Beeswax; SESA: Sucrose Fatty Acid Ester; DTEM: Acetyl Tartaric Acid Ester.
2.1.2 Mechanical properties of membranes
Edible membranes provide a barrier to substances such as water vapor and oxygen, and for ease of application, require a certain degree of strength and flexibility.
As can be seen in Figures 1 and 2, the tensile strength and elongation of the membranes with stearic acid, beeswax, and fatty acid esters of sucrose were reduced compared to the monolithic (blank) membranes without the addition of lipids. The addition of lipids to the membrane fluid changes the mechanical properties of the membranes due to their intrinsic mechanical properties (e.g., lipid membranes are generally less strong and flexible) and the introduction of hydrophobic groups that reduce the water content of the membrane.
Among them, the tensile strength and elongation of beeswax decreased more than the other two, which were 11.34% and 42.13%, respectively. This is due to the fact that the dispersion of relatively large molecular weight of wax in the membrane fluid reduces the probability of protein-to-protein cross-linking and lowers the mechanical properties of the membrane[6] . The addition of stearic acid and sucrose fatty acid ester decreased the tensile strength by 0.89% and 2.98%, and the elongation by 9.97% and 10.68%, respectively. Although stearic acid and sucrose fatty acid esters reduce protein-protein cross-linking, stearic acid and sucrose fatty acid esters are amphoteric substances containing both polar and nonpolar groups, which interact with the polar and nonpolar side-chain groups of proteins through van der Waals' forces, hydrophobic bonding, electrostatic forces, and hydrogen bonding[4, 5] to form lipid-protein complexes.
2.1.3 Water permeability of membranes
The hydrophilic nature of proteins makes them less hygroscopic, so their application is very limited. Adding lipids to protein membranes and introducing hydrophobic groups can improve the moisture barrier property of protein membranes. Figure 3 shows the water permeability of several types of lipid-protein membranes. It was found that the water permeability of the lipid-added protein membranes decreased considerably compared to that of the monolithic protein membranes. The water permeability of stearic acid, beeswax, and sucrose fatty acid ester membranes decreased by 42.23%, 35.04%, and 41.42%, respectively. kester, Fennema, and KamPer et al. showed that waxes, due to their strong hydrophobicity and high melting point, generally have better water-blocking properties than the other lipids[7-9] , but the permeability of the beeswax membranes in the present experiments was not as good as that of the other lipids. However, in this experiment, the water permeability of the beeswax membrane was higher than the other two membranes, which may be due to the following reasons: (1) the membrane formed by beeswax is thicker, and with the increase of the thickness, the partial pressure of water vapor on both sides of the membrane increases, which leads to the increase of the water permeability of the membrane [10]; (2) beeswax is easy to form fine particles in drying, and the temperature decreases in equilibrium after drying, the particles become smaller, and some small pores are formed, which facilitate the water molecules to pass through. These small pores are favorable for the passage of water molecules. When stearic acid and sucrose fatty acid esters are added into the membrane solution, these lipids can form lipid-protein complexes with rice protein through van der Waals, hydrophobic bonding, electrostatic force and hydrogen bonding, and small molecular weight lipids are filled in the network structure of the membrane, forming a dense membrane with relatively low water permeability.
2.1.3 Water permeability of membranes
The hydrophilic nature of proteins makes them less hygroscopic, so their application is very limited. Adding lipids to protein membranes and introducing hydrophobic groups can improve the moisture barrier property of protein membranes. Figure 3 shows the water permeability of several types of lipid-protein membranes. It was found that the water permeability of the lipid-added protein membranes decreased considerably compared to that of the monolithic protein membranes. The water permeability of stearic acid, beeswax, and sucrose fatty acid ester membranes decreased by 42.23%, 35.04%, and 41.42%, respectively. kester, Fennema, and KamPer et al. showed that waxes, due to their strong hydrophobicity and high melting point, generally have better water-blocking properties than other lipids[7-9] , but the permeability of the beeswax membranes in the present experiments was not as good as that of the other lipids. However, in this experiment, the water permeability of the beeswax membrane is higher than the other two membranes, the reasons may be: (1) the membrane formed by beeswax is thicker, with the increase of thickness, the partial pressure of water vapor on both sides of the membrane increases, which leads to an increase in the water permeability of the membrane[10] ; (2) beeswax is easy to form fine particles during the drying process, after drying, when equilibrating the moisture, the temperature decreases, the particles become smaller, and form some small pores, which facilitate the water molecules to pass through the membrane. These small pores are favorable for the passage of water molecules. When stearic acid and sucrose fatty acid esters are added into the membrane solution, these lipids can form lipid-protein complexes with rice protein through van der Waals, hydrophobic bonding, electrostatic force and hydrogen bonding, and small molecular weight lipids are filled in the network structure of the membrane, forming a dense membrane with relatively low water permeability.
2.2.2 Water permeability of membranes
The water permeability of the protein-membrane with different levels of stearic acid, beeswax, and sucrose fatty acid ester was investigated, as shown in Fig. 6. The water resistance of the membrane increased significantly with the addition of lipids, and the water permeability of the membranes decreased almost linearly with the addition of lipids from 0% to 0.3% by mass, and then ceased to decrease with the addition of lipids above 0.3% by mass. The hydrophobic groups in the membrane increased with the increase in the amount of lipids, but after the addition of 0.3% by mass, the addition of lipids did not limit the appearance of hydrophilic groups of rice protein and glycerol in the membrane network structure, and the water-blocking property of the membrane was not improved. Nathalie GontarD et al. reported that the water resistance of beeswax membranes was more significant than others[4] , but this was not found in the present experiments, probably because the small pores of beeswax membranes are favorable for water molecules to migrate inside the membranes.
3 CONCLUSIONS
The hydrophilicity of rice protein determines that the water barrier property of rice protein-based edible film is poor, so the high water permeability of rice protein-based edible film can only be applied to food products with low water content, such as candied fruits, candied fruits and nuts. In this experiment, the addition of lipids to the rice protein membrane was investigated to improve the water molecule barrier property of the membrane through the water-repellent groups of the lipids. The experimental results showed that the moisture barrier property of the edible rice protein film was greatly improved with the addition of stearic acid, sucrose fatty acid esters and beeswax, and the addition of stearic acid and sucrose fatty acid esters had less effect on the mechanical properties of the film, and the addition level of 0.3% by mass was more appropriate.
References .
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