In recent years, the environmental pollution caused by the traditional plastic packaging materials has been more and more attention, the safety of the traditional plastic packaging materials has also been questioned, the use of new materials to replace the traditional food packaging materials has aroused domestic and foreign attention to the research of edible film. The authors explored the preparation of edible film using rice protein concentrate as the main raw material, determined the optimal preparation process, and measured and analyzed the performance of the film.
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1 Materials and Methods
1.1 Materials and equipment
Glycerol: Shanghai Chemical Reagent Purchasing and Supply Station; Glutamine transaminase (TG): Jiangsu and Suzhou Yiming Precision Chemical Industry Co. Ltd.; Micrometer: Harbin Measuring Instruments & Sharpening Tools Factory; 94-2 Magnetic Stirrer: Shanghai Zhiwei Electric Apparatus Company Limited; Physical Properties Instrument: TA× T2i stable Micro systems (UK) products.
1.2 Test methods
1. 2. 1 Preparation of rice protein concentrate Rice protein extraction using alkaline acid precipitation process: NaoH concentration of 0. 1 mol / L, water to material ratio of 9 : 1 (mL : g), extraction time for 4 h, extraction temperature for room temperature; acid precipitation PH value of 5. 5 (because the isoelectric point of rice protein is at a PH value of 4. 6, but the lower the PH value, the more prone to the formation of alcohol-soluble protein salts, resulting in the loss of protein dissolution). Loss of protein solubilization. The difference in solubility between pH 4.6 and pH 5.5 is not significant[1] ).
1. 2. 2 Membrane preparation Add TG into the prepared rice separating egg white solution, add glycerol and mix well. The PH value was adjusted to 11.5 with 1mol/L NaoH and 1 mol/L Hcl. After heating in a water bath, 25mL of the membrane solution was poured into a mold with a diameter of 10cm, and then dried at 50 ℃, the membrane was uncovered, and then placed in a constant-temperature and constant-humidity box at 23 ℃ and 50% relative humidity for two days to equilibrate the moisture before being prepared for use.
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1. 2. 3 Membrane Performance Testing
1) Determination of film thickness: Select flat, dry, uniform film, randomly take 4~5 points, with a micrometer (accuracy 0.001mm) to measure its thickness, take the average value.
2) Determination of tensile strength (TS) and elongation (E): The film was cut into 100mm×10mm strips, and the tensile probe was used on the physical meter with a tensile speed of 1 mm/s, and the physical meter was set to the maximum tensile force and tensile distance.
T is the tensile strength (Mpa); F is the maximum tensile force (N); L is the thickness of the film sample (m); w is the width of the film sample (m).
E is the elongation (%); L0 is the length of the specimen before stretching (m); L1 is the length of the specimen after stretching (m).
3) Determination of water permeability: According to the principle and procedure of water vapor permeability test method for plastic film and sheet, the method of GB1037-87 is adopted with slight changes. The method is as follows: Choose a flat, dry and uniform film, measure its thickness, and then seal it in the mouth of a moisture permeable cup filled with anhydrous calcium chloride. Calcium chloride should be crushed evenly before use, with a particle size of about 2 mm, and dried in an oven for 2 h. After adding calcium chloride, the distance from the mouth of the cup is about 5 mm. The moisture permeable cup is made of lightweight, corrosion-resistant, impermeable and watertight polyethylene plastic cup, and the membrane is sealed with melted paraffin wax over the mouth of the cup and weighed. Then it was put into a desiccator with a saturated solution of barium chloride at the bottom and a relative humidity of 90%, and weighed after 24h.
Wvp(Water vaPor permeability): water vapor permeability M1 is the original mass of the membrane, the moisture permeable cup and the desiccant (g); M2 is the mass of the membrane, the moisture permeable cup and the desiccant after water absorption (g); L is the thickness of the membrane (mm); A is the effective area of measurement (m2 ); t is the measurement interval time (d); Δp is the difference in water vapor pressure between two sides of the membrane (kpa). The following is a summary of the results of this study.
1. 2. 4 Preparation process of egg white film
Rice egg white concentrate → add water and Wan Gan Fang oil to disperse at high speed → adjust PH value → add TG reaction → adjust PH value to 11.5 → heating and stirring → pouring the film → drying → uncovering the film
2 One-way test
2. 1 Effect of protein volume fraction
The membranes were prepared at 3, 5, 7 and 9 g/dL of protein concentration, and the experiments showed that the mass concentration of 7 and 9 g/dL was too high, the solution was viscous and poorly flowing, and it was difficult to spread evenly on the mold, which led to the uneven thickness of the membrane, and the solvent evaporation time was too long for 3 g/dL, and the protein was easily denatured, which resulted in the bad smell. 5% egg white is more suitable.
2.2 Effect of Glycerol Addition on the Membrane
Edible membranes made from proteins are generally brittle due to the large number of hydrogen bonds, disulfide bonds, electrostatic forces and hydrophobic interactions between or within protein molecules. Polyalcohols and lipids with small relative molecular mass can be used as plasticizers. They can enter the protein molecules and compete with the hydrogen bonding and electrostatic forces in the protein chains to increase the fluidity of the polypeptide chains, reduce the degree of cross-linking between protein molecules, and make the membranes soft and elastic[2, 3] . Glycerol, malonyl alcohol, sorbitol, polyethylene glycol, fatty acids and monoglycerol lipids can be used as plasticizers for protein membranes, with glycerol being the most commonly used.
The effects of different glycerol additions on the tensile strength, elongation and water permeability of the films were investigated (see Table 1). The elongation and water permeability increased with the increase of glycerol, while the tensile strength decreased. At GLY/RpI of 0:10 and 2:10, after drying, the film formed was very brittle with cracks on the surface and could not be removed completely. When GLY/RpI increased from 4:10 to 8:10, the elongation of the membrane increased from 45.2% to 130%, up to 2.9 times, and the tensile strength decreased from 4.61 Mpa to 2.7 Mpa. The water permeability of the membrane increased with the increase of the proportion of glycerol because, on the one hand, glycerol is hydrophilic, and, on the other hand, glycerol made the network structure of the membrane looser, which was conducive to the passage of water vapor. This is because glycerol is hydrophilic on the one hand, and on the other hand, glycerol makes the membrane network structure looser, which facilitates the passage of water vapor.
Table 1 Effect of Glycerol Additions on Membranes
GLY/RpI | E/(%) | T/(Mpa) | Wvp/(g .mm/ kpa .d . m )2 |
0 : 10 | | | |
2 : 10 | __ | __ | __ |
4 : 10 | 45. 2 | 4. 61 | 24. 5 |
6 : 10 | 89.3 | 3. 28 | 33. 6 |
8 : 10 | 130 | 2. 7 | 36. 3 |
Note: * RpI is rice protein isolate; GLY is glycerol; 6_ indicates that the membrane is too brittle and was not measured.
2.3 Effect of heat treatment on membranes
The denaturation of rice protein starts at 65 ℃, and when the temperature is higher than 65 ℃, the original highly regular spatial arrangement of the protein changes, the protein polypeptide chain unfolds, and the disulfide bond embedded in the inner part of the protein breaks, and the sulfhydryl group is reduced to sulfhydryl group under alkaline conditions[2] . The sulfhydryl groups are oxidized to disulfide bonds when the solvent in the membrane solution evaporates, forming the network structure of the membrane. As shown in Table 2, with the increase of temperature, the tensile strength of the membrane increases, but the elongation and water permeability decrease. However, compared with 80 ℃, T decreases and E increases at 90 ℃ and 100 ℃, which is consistent with the report of M. B. perez-gago et al[4] .
Table 2 Effect of Heating Temperature on Membrane
Heating Temperature/°C | E/(%) | T/(Mpa) | Wvp(g .mm/ kpa . d . m )2 |
70 | 60. 4 | 3. 25 | 33. 6 |
80 | 45. 2 | 4. 61 | 24. 5 |
90 | 54 | 3. 91 | 21. 15 |
100 | 64. 8 | 3. 74 | 25.5 |
Table 3 shows the effects of different time at 80 ℃ on the membrane properties. The results show that E and Wvp decreased with the increase of time and T, but the changes leveled off after 60 min, indicating that the protein chains were fully unfolded after 60 min at 80 ℃.
Table 3 Effect of Heating Time on Membrane
heater Time/min | E/(%) | T/(Mpa) | Wvp(g .mm/ kpa . d . m )2 |
20 | 45. 2 | 4. 61 | 45.5 |
40 | 34. 1 | 4. 75 | 32. 7 |
60 | 26.5 | 4. 86 | 33. 65 |
80 | 22. 6 | 4. 88 | 27. 34 |
2. 4 Effect of glutamine transaminase on membranes
TG is a transferase enzyme that catalyzes acyl transfer reactions, using the Y-carboxamide group of glutamine residues as the acyl donor and the ε-amino group of lysine residues as the acceptor, forming intramolecular and intermolecular isopeptide bonds of ε-(Y-Glu) LYs and cross-linking smaller protein matrices into a macromolecular network[5, 6] .
As can be seen from Table 4, the tensile strength of the membranes increased significantly with the addition of the enzyme, and at 0.20% by mass, the tensile strength of the membranes increased 1.5-fold compared with that of the membranes without enzyme cross-linking, but the E was reduced to 50% of the original one. The water permeability of the membrane decreases rapidly with the addition of TG, which is attributed to the cross-linking effect of the TG data, resulting in a denser structure of the membrane. However, the addition of 0.40% by mass of TG showed little change compared to 0.20% by mass of Wvp, indicating that the addition of more than 0.20% by mass did not have a significant effect on the permeability of the membranes, probably due to the fact that the substrate was already in contact with the enzyme and cross-linking was taking place, and the addition of more enzyme did not increase the extent of cross-linking.
The results of adding 0.20% TG at different times are shown in Table 5. With the increase of enzyme reaction time, the tensile strength of the membrane increased, the elongation decreased, and the water permeability of the membrane also decreased. Thus, the degree of protein cross-linking increased.
3 Orthogonal tests
Referring to the results of the one-way test, five factors, namely glycerol, heat treatment temperature, time, enzyme addition and action time, were examined, and orthogonal tests were conducted at different levels, and the results are shown in Table 6.
According to the above polar analysis, it can be seen that in terms of tensile strength: B>A>E>C>D; in terms of elongation: A>B>C>D>E.
In terms of permeability: A>C>B>D>E . The tensile strength and elongation are negatively correlated, with high tensile strength and low elongation. In Table 6, the tensile strength of experiment No. 4 reaches 3.91Mpa, but its elongation is only 66.39%; and in experiment No. 14, the elongation is 154.62%, but its tensile strength is only 1.48Mpa. In the practical application of the membrane, the elongation or tensile strength of the membrane is too small to be of any value. Considering these three indexes, the better process condition is A2B3C4D3E2.
The total extraction rate of alginate reaches 85.6%, which is much higher than that of traditional extraction methods. At present, the advantage of membrane separation technology in the production of alginate is that the product can be concentrated to a certain concentration and purity by nanofiltration, and then combined with the traditional extraction process for appropriate treatment, which can reduce the intermediate loss of the product, increase the yield and purity, and also increase the utilization rate.
3 CONCLUSIONS
1) The ultrafiltration of yeast extracts can be regarded as a pretreatment process for the nanofiltration process, and the operating pressure and time as well as the flow rate of the feed solution have a great influence on the ultrafiltration flux.
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2) The operation of nanofiltration plays the role of concentration and purification, the operating pressure has a great influence on the nanofiltration flux, the concentration factor and the operation mode also affect the separation performance of nanofiltration membrane, and the use of recirculation is more time-saving and energy-saving than that of the intermittent type.
3) After ultrafiltration, nanofiltration, crystallization and drying, the alginate product was obtained with an extraction rate of 85.6%, which was higher than that of the traditional extraction method.
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