LU501049B1 - Method for Improving Gel Properties of Soy Protein Isolate through Pretreatment with Low-frequency and High-intensity Ultrasonic Combined with Curdlan - Google Patents

Abstract

A method of improving the gel properties of soy protein isolate by pretreating with low-frequency and high-intensity ultrasonic combined with curdlan is provided. The invention relates to an SPI gel, which is prepared by adding two kinds of curdlan with different characteristics to improve the gel properties of soy protein isolate (SPI) based on ultrasonic-assisted treatment, and may be used as a new functional gel in the food industry. According to the invention, the method comprises the following steps: pretreating SPI by ultrasonic, adding a certain concentration of thermoreversible curdlan (TRC) and thermoirreversible curdlan (TIRC), uniformly mixing, and then preparing SPI gel through thermal induction. Low-frequency high-intensity ultrasonic waves may produce mechanical, chemical and thermal effects through "cavitation effect", further improving protein function. TRC and protein form an interpenetrating polymer gel network, TIRC particles exist in the protein gel matrix in the form of simple "filler".

Description

DESCRIPTION HUS01049 Method for Improving Gel Properties of Soy Protein Isolate through Pretreatment with Low-frequency and High-intensity Ultrasonic Combined with Curdlan

TECHNICAL FIELD The invention relates to the field of regulation and control of processing characteristics for soybean protein, particularly to a method for improving SPI gel properties under the interaction between protein and polysaccharide by adding curdlan after pretreatment with low-frequency and high-intensity ultrasound.

BACKGROUND Since SPI is particularly rich in nutritional value and functional characteristics, it has been widely used in processed meat products such as sausage, ham, further improving the nutrition and edible quality of meat products. As the most widely used vegetable protein meat raw material, SPI's essential amino acid composition meets the needs of human body; in addition, except the methionine content is slightly lower, contents of other essential amino acids are high. More importantly, the digestibility of SPI is about 60% and may be increased to 90% after heat treatment, which is close to the digestibility of animal meat. In addition, on the amino acid score scale modified by protein digestibility, the score of processed SPI is equivalent to that of animal protein, so SPI is the best substitute for conventional animal protein.

The functional characteristics of SPI mainly depend on protein composition, structure, denaturation degree and aggregation degree. Gel properties of protein may encapsulate water, fat, flavor and pigment in a three-dimensional network structure, thus playing a very important role in forming good food quality and food texture. Heat, acid, salt and enzyme may all induce the formation of protein gel, but the heat treatment is the most common and basic treatment way in food processing. The method of forming gel by SPI thermal induction is as follows: firstly, the polypeptide chain is heated to expand for exposing more action sites, then the expanded protein molecules interact and aggregate, and finally the aggregates aggregate to form a network structure. In addition, gelation capacity and viscoelasticity of SPI largely depend on intermolecular interactions, such as hydrogen bond and covalent bond HUS01049 (disulfide bond), electrostatic and hydrophobic interactions. At present, commercial SPI is still produced by alkali extraction and acid precipitation and spray drying, which leads to a certain degree of denaturation of protein and decrease of solubility, and SPI is sensitive to extreme external environment such as ions, pH value and temperature. Therefore, in the process of food production as well as storage and transportation, SPI gel is easy to be flocculated and aggregated, and the water-holding capacity decreases, resulting in poor taste and texture, further limiting the application of SPI in the food field.

Many techniques have been used to modify and change the structure and aggregation of soybean protein, further improving its functional characteristics. For example, ultrasonic technology, as a common physical technology, makes the medium undergo physical and chemical changes through the interaction between ultrasonic waves and the medium, resulting in cavitation effect, mechanical effect and thermal effect, and due to simple and environmentally friendly operation features with energy saving as well as time saving, ultrasonic technology is widely used in the field of auxiliary component extraction. The principle of ultrasonic wave is destroying noncovalent interactions, even breaking peptide bonds, changing the structure of peptide bonds, further changing the functional characteristics of proteins. In recent years, researches show that ultrasonic treatment may reduce the average particle size of SPI, increase solubility, enhance surface hydrophobicity and change secondary structure, thus promoting SPI to form a modified gel with uniform and compact structure. Therefore, pretreating SPI solution by ultrasonic is beneficial to improving subsequent gel properties.

Ultrasonic treatment, as a means of physical modification, improves the gel properties of SPI; however, when only using ultrasonic wave as a means, the effect of improving the gel properties of protein is limited, and SPI gel has not yet broken through the range of weak gel. In order to further improve the gel properties of SPI, this method combines ultrasonic technology with other technologies (that is, the interaction between protein and polysaccharide by adding polysaccharide) to modify HUS01049 SPI, so as to obtain a positive effect of improving its functional characteristics.

Curdlan is a neutral bacterial extracellular polysaccharide, and its linear molecules are completely connected by D-glucose units through B-(1—3) glycosidic bonds. In addition, curdlan may form two types of gels (TRC and TIRC), which have different physical stability, water and oil retention characteristics, thermodynamic characteristics and rheological characteristics. When the aqueous solution of curdlan is heated to 55-60°C and then cooled to below 40°C, the obtained curdlan is thermally reversible; when the aqueous solution is heated above 80°C, TIRC may be formed. For the two gel types and their unique thermodynamic and rheological characteristics, curdlan has been widely used in food industry to improve the quality of products. It has been studied that the strength of fish gel increases when mixing hairtail minced with curdlan is 1-3%, so two types of curdlan may effectively improve the gelation properties of myofibrillar proteins through different modes of action. At present, curdlan is mostly used to modify animal protein, but there are little researches on the modification of plant protein; protein with good gel properties may be obtained by ultrasonic pretreatment, however, the technical method of compound modification of protein by adding curdlan after low-frequency and high-intensity ultrasonic pretreatment is rarely reported.

Based on this, the invention discloses a method to improve SPI gel properties by low-frequency and high-intensity ultrasonic pretreatment combined with curdlan, and provides a new idea for improving SPI gel properties by adding curdlan.

SUMMARY The objective of the invention is to firstly improve the solubility of SPI by ultrasonic pretreatment, then prepare TRC and TIRC, and finally make two types of curdlan fully react with SPI, so as to improve the ability of SPI to form gel and furthermore improve the quality of SPI gel. According to the invention, firstly, the SPI solution is ultrasonically treated with different output powers (200 W-600 W) of 20 kHz for 10-30 min respectively, and output powers all significantly improve the dissolution characteristics of SPI, when the output power is 400 W and the processing duration 1s 10 min, the improving effect of the SPI dissolution characteristics is best. HUS01049 After ultrasonic pretreatment (400 W, 10 min), SPI is fully and evenly mixed with two kinds of curdlan, and then hydrated at 4°C for at least 12 hours to fully interact with each other, wherein the final concentration of SPI in the reaction system is 100 mg/mL, and the final concentration of two kinds of curdlan is 0.1%-0.5% (w/w, based on the total weight of SPI solution); finally, after the reaction, the sample is weighed and heated from 25°C to 75°C by water bath, then kept for 30 min to prepare SPI gel.

Advantages of Invention:

1. Ultrasonic technology is a green food physical processing technology and may improve the functional characteristics of SPI under low frequency and high intensity (frequency 20-100 kHz, intensity>1 W/cm?). In addition, the high intensity ultrasonic wave may produce the mechanical, chemical and thermal effects through "cavitation effect", which may reduce the average particle size of SPI, increase the solubility, enhance the surface hydrophobicity, change the secondary structure, and further promote SPI to form a modified gel with uniform and compact structure.

2. Two types of curdlan (TRC and TIRC) have different physical stability, water and oil retention capacity and texture characteristics. Through the interaction between protein and polysaccharide, the functional characteristics of SPI gel may be improved. Two types of curdlan promote the expansion of SPI structure, change the tertiary structure of SPI, and significantly enhance the water-holding capacity and gel strength of SPI composite gel.

3. Compared with commonly used polysaccharides (cellulose, starch, xanthan gum and carrageenan), curdlan has unique physical, chemical and functional characteristics, but in hot water (usually about 55°C) the formed TRC by curdlan is reversible to temperature and has lower strength; while curdlan is heated in high temperature (80-100°C) water suspension, elastic TIRC is formed, which not melts again after being heated or cooled, and the gel polysaccharide is colorless and tasteless at high temperature. The gel properties of SPI may be significantly improved by pretreating with low frequency and high intensity ultrasound and adding curdlan. The invention provides a new idea for improving SPI gel properties.

BRIEF DESCRIPTION OF THE FIGURES HUS01049 FIG. 1 shows solubility of SPI solution pretreated by ultrasonic; note: Control: SPI solution without ultrasonic treatment; 200 w-10 m, 20 m and 30 m: SPI solution pretreated by ultrasonic with output power of 200 W for 10, 20 and 30 minutes respectively; 400 w-10 m, 20 m and 30 m: SPI solution pretreated by ultrasonic with output power of 400 w for 10, 20 and 30 minutes respectively; 600 w-10 m, 20 m and m: SPI solution pretreated by ultrasonic with output power of 600 w for 10, 20 and 30 minutes respectively; FIG. 2 shows particle size distribution of SPI solution pretreated by ultrasound, note: Control: SPI solution without ultrasonic treatment; 200 w-10 m, 20 m and 30 m: SPI solution pretreated by ultrasonic with output power of 200 W for 10, 20 and 30 minutes respectively; 400 w-10 m, 20 m and 30 m: SPI solution pretreated by ultrasonic with output power of 400 w for 10, 20 and 30 minutes respectively; 600 w-10 m, 20 m and 30 m: SPI solution pretreated by ultrasonic with output power of 600 w for 10, 20 and 30 minutes respectively; FIG. 3 shows fluorescence spectrum of SPI-TRC sol; note: control: SPI sol treated by ultrasonic; TRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI sol with 0.1%,

0.2%, 0.3%, 0.4% and 0.5% TRC added after ultrasonic treatment; FIG. 4 shows fluorescence spectrum of SPI-TIRC sol; note: control: SPI sol treated by ultrasonic, TIRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI sol with 0.1%,

0.2%, 0.3%, 0.4% and 0.5% TIRC added after ultrasonic treatment; FIG. 5 shows SPI gel strength; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment; N-U-TRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TRC added; N-U-TIRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TIRC added; U-TRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TRC added respectively after ultrasonic treatment; U-TIRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI gel with 0.1%, 0.2%,

0.3%, 0.4% and 0.5% TIRC added respectively after ultrasonic treatment; FIG. 6 shows SPI gel cooking loss; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment; N-U-TRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TRC added; N-U-TIRC-0.2%: SPI gel without ultrasonic HUS01049 treatment but with 0.2% TIRC added; U-TRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TRC added respectively after ultrasonic treatment; U-TIRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI gel with 0.1%, 0.2%,

0.3%, 0.4% and 0.5% TIRC added respectively after ultrasonic treatment; FIG. 7 shows storage modulus of SPI-TRC gel; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment, N-U-TRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TRC added; U-TRC-0.1%, 0.2%, 0.3%, 0.4% and

0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TRC added respectively after ultrasonic treatment; FIG. 8 shows storage modulus of SPI-TIRC gel; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment; N-U-TIRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TIRC added; U-TIRC-0.1%, 0.2%, 0.3%,

0.4% and 0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TIRC added respectively after ultrasonic treatment; FIG. 9 shows loss modulus of SPI-TRC gel; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment, N-U-TRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TRC added; U-TRC-0.1%, 0.2%, 0.3%, 0.4% and

0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TRC added respectively after ultrasonic treatment; FIG. 10 shows loss modulus of SPI-TIRC gel; note: N-U: gel without ultrasonic treatment; U: SPI gel after ultrasonic treatment; N-U-TIRC-0.2%: SPI gel without ultrasonic treatment but with 0.2% TIRC added; U-TIRC-0.1%, 0.2%, 0.3%, 0.4% and 0.5%: SPI gel with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% TIRC added respectively after ultrasonic treatment;

DESCRIPTION OF THE INVENTION The present invention will be further explained in detail with reference to the drawings and embodiments below.

1. The steps are as follows: treating SPI solution by ultrasonic with different output power of 20 kHz (200 W-600 W) for 10-30 min (pulse duration 5 s and off duration 2 s) respectively; then, analyzing the solubility (FIG. 1) and particle size HUS01049 (FIG. 2); finally, selecting output power of 400 W and treating duration of 10 min as the best conditions for ultrasonic pretreatment;

2. Continuously stirring the curdlan solution in a constant temperature water bath at 55°C for 15 min to prepare TRC; in addition, continuously stirring the curdlan solution in a constant temperature water bath at 90°C for 15 min to prepare TIRC;

3. Adding 0.1%-0.5% (w/w, based on the total weight of SPI solution) of two kinds of gel polysaccharides into SPI solution with protein concentration of 100 mg/mL pretreated by ultrasonic for 400 W and 10 min, mixing evenly, and hydrating at 4°C for at least 12 hours to make them fully interact; then, determining the fluorescence intensity of the sol (FIG. 3 and FIG. 4); from FIG. 3 and FIG. 4, it can be seen that the adding TRC or TIRC significantly reduces the tryptophan fluorescence intensity of SPI composite sol in a concentration-dependent manner, and TRC or TIRC may interact with Trp residues and enhance the polar environment during the expansion of the tertiary structure of SPI, which effectively reduces the overall tryptophan fluorescence intensity of SPI-TRC and SPI-TIRC sol.

4. The SPI composite gel is prepared as follows: weighing 5 g of the sample after reaction, putting it in a 10 mL beaker and then putting it in a water bath pot, heating it from 25°C to 75°C and keeping it for 30 min, then taking it out, and immediately putting the taken sample in an ice bath to stop the reaction, and then placing it overnight at 4°C to determine the gel strength (FIG. 5) and cooking loss (FIG. 6); it can be seen from FIG. 5 that after ultrasonic treatment, the gel strength is improved, and adding TRC and TIRC also significantly increases the gel strength, and there is a synergistic effect between ultrasound and polysaccharide. The gel strength of the ultrasonic treatment group with polysaccharide is higher than that of the ultrasonic treatment group and the polysaccharide treatment group. In addition, adding TRC and TIRC enhances the hydrogen bond interaction and hydrophobic interaction among SPI, TRC and TIRC, and improves the texture characteristics of gel. As can be seen from FIG. 6, ultrasonic treatment and adding TRC and TIRC significantly reduce the cooking loss of gel, and there is a synergistic effect between them, further improving HUS01049 the water retention of gel.

5. 1.5 mL of the mixed solution of curdlan and SPI is heated from 25°C to 75°C at a speed of 2°C/min, and the storage modulus (G', Pa) (FIG. 7 and FIG. 8) and loss modulus (G", Pa) (FIG. 9 and FIG. 10) are tested with a constant frequency of 1 Hz and a strain of 1%. After ultrasonic treatment, G' and G" both increase, and adding TRC and TIRC significantly increases the G' value of composite gel, especially the composite gel with high concentration of gel polysaccharide, and the synergistic effect between ultrasonic and polysaccharide is obvious, which shows that ultrasonic treatment and adding curdlan may form a stronger gel structure. G" describes the viscous properties of gel, and high viscosity is related to good water retention. In the current research, the value of G" gradually increases with the increase of the concentration of two kinds of curdlan, and the water retention is significantly enhanced.

Claims (5)

CLAIMS LU501049

1. A method of improving gel properties of soy protein isolate by pretreating with low-frequency and high-intensity ultrasonic combined with curdlan comprises the following steps: (1) pretreating SPI solution by low-frequency high-intensity ultrasound with different power and different duration; (2) preparing TRC and TIRC at different temperatures; (3) respectively adding two kinds of curdlan into SPI solution after ultrasonic pretreatment; (4) evenly mixing the solution prepared in step (3) and then hydrating at 4°C for at least 12 hours, so as to make curdlan fully react with SPI; (5) weighing 5 g of the reacted sample, putting it in a 10 mL beaker, placing the beaker in a water bath, and preparing SPI gel by thermal induction.

2. The method according to claim 1, characterized in that 400 W of output power and 10 min of treating duration are finally selected for ultrasonic pretreatment of SPI solution.

3. The method according to claim 1, characterized by continuously stirring the curdlan solution in a constant temperature water bath at 55°C for 15 min to prepare TRC, and continuously stirring the curdlan solution in a constant temperature water bath at 90°C for 15 min to prepare TIRC.

4. The method according to claim 1, characterized in that the final concentration of SPI in the reaction system is 100 mg/mL, and the final concentration of two kinds of curdlan is 0.1%-0.5% (w/w, based on the total weight of SPI solution).

5. The method according to claim 1, characterized in that the gel polysaccharide may significantly improves gel properties of SPI through physical interactions such as hydrogen bonding and hydrophobic interaction between the gel polysaccharide and

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