AU2020101923A4 - Method for assisting short-term frozen storage of livestock and poultry meat by using low-voltage electrostatic field and product thereof - Google Patents

Abstract

The present invention discloses a method for assisting short-term frozen storage of livestock and poultry meat by using a low-voltage electrostatic field, which comprises: placing livestock and poultry meat in a low-voltage electrostatic field for frozen storage at a temperature ranging from -18°C to -6°C.A voltage of the low-voltage electrostatic field is 2000V to 2500 V, and a current is 0.15 mA to 0.2 mA. A time for the frozen storage is no more than 28 days. The present invention further discloses livestock and poultry meat prepared by the method for assisting short-term frozen storage of livestock and poultry meat by using the low-voltage electrostatic field. The method has the beneficial effects of remarkably reducing a total number of colonies, and TVB-N and TBARS values of the livestock and poultry meat, improving a water-holding capacity of the livestock and poultry meat, having a fresher color, forming small and uniform ice crystals in muscles, and having a low damage degree to a muscle micro-structure, and a quality of the livestock and poultry meat prepared by the method is improved. Fresh meat )U -18L -18 -12L -12 -6L -6 7d 28d FIG 9 T22 160 T 120- -6 -6L .........- 12 -12L 28 80 -18 -18L -6J .29 -6L $ r 40- -12 4-12L 7d 0 -18L 0.01 0.1 1 10 100 1000 10000 Relaxation time/ms FIG 10

Description

)U Fresh meat

-18L -18 -12L -12 -6L -6

7d

28d

FIG 9

T22

160 T

120- -6 -6L .........- 12 -12L 28 80 -18 -18L -6J .29 -6L $

r 40- -12 4-12L 7d 0 -18L

0.01 0.1 1 10 100 1000 10000 Relaxation time/ms

FIG 10

METHOD FOR ASSISTING SHORT-TERM FROZEN STORAGE OF LIVESTOCK AND POULTRY MEAT BY USING LOW-VOLTAGE ELECTROSTATIC FIELD AND PRODUCT THEREOF

TECHNICAL FIELD The present invention relates to the technical field of frozen storage of livestock and poultry meat. More specifically, the present invention relates to a method for assisting short-term frozen storage of livestock and poultry meat by using a low-voltage electrostatic field and a product thereof.

BACKGROUND China is a big country in meat production and consumption, among which pork production is the highest, which is the leading factor in China's meat consumption structure. As the most important storage method of meat, frozen storage is a main form of inter-regional circulation and consumer storage, which can effectively inhibit microorganism reproduction and prolong a shelf life. However, meat and meat products are prone to oxidation of proteins and fats during the frozen storage, which leads to a certain degree of quality deterioration, such as juice loss, tenderness decline and color deterioration, etc., resulting in serious economic losses. At present, main frozen storage methods of meat comprise conventional frozen storage (-18°C), deep frozen storage (below -38°C) and superchilled storage (PC to 2°C below a freezing point), etc. Meat stored by the conventional frozen storage and the deep frozen storage has a long shelf life and is convenient for circulation, but has a high energy consumption, which makes it easy for meat consumed in a short time to be abused by freezing. The superchilled storage can ensure the meat quality, prolong the shelf life and reduce a carrying cost, but the superchilled storage needs precise temperature control equipment and technology, and is difficult to realize large-scale industrial application. There are rare reports about influences of a temperature range from -18°C to -6°C between the superchilled storage and the conventional frozen storage on the meat quality during the frozen storage.

Compared with a high-voltage electrostatic field (>2500 V) which has high energy consumption and low safety, the low-voltage electrostatic field (52500 V) is easy to operate,

Page 1 safe and energy-saving, and is convenient for large-scale industrial production. In view of the short-term rapid consumption of meat in the current market, there is still a lack of frozen storage methods that can avoid freezing abuse, reduce freezing energy consumption, improve the quality of frozen meat and can be applied to large-scale production in a short storage period. Therefore, the present invention studies changes of the pork quality during the short-term frozen storage assisted by the low-voltage electrostatic field under different temperature conditions, aiming to provide theoretical and technical support for the development of new frozen storage and fresh-keeping technology of meat and the formulation of freezing technology.

SUMMARY An object of the present invention is to solve at least the above problems and to provide at least advantages that will be described hereinafter.

Another object of the present invention is to provide a method for assisting short-term frozen storage of livestock and poultry meat by using a low-voltage electrostatic field, which can remarkably reduce a total number of colonies, and TVB-N and TBARS values of pork, improve a water-holding capacity of the pork, have a fresher color, form small and uniform ice crystals in muscles, and have a low damage degree to a muscle micro-structure, and a quality of the pork prepared by the method is improved.

In order to achieve these objects and other advantages according to the present invention, a method for assisting short-term frozen storage of livestock and poultry meat by using a low-voltage electrostatic field is provided, which comprises: placing livestock and poultry meat in a low-voltage electrostatic field for frozen storage at a temperature ranging from -18°C to -60 C.

Preferably, a voltage of the low-voltage electrostatic field is 2000V to 2500 V, and a current is 0.15 mA to 0.2 mA.

Preferably, a time for the frozen storage is no more than 28 days.

The present invention further provides livestock and poultry meat prepared by using the method above.

The present invention at least comprises the following advantageous effects:

1. The low-voltage electrostatic field makes a cell membrane of bacteria induce electric charges, and a difference of permeability membrane produced causes cell rupture, and the cell dies due to the disorder of membrane structure and the change of permeability. The frozen

Page 2 storage assisted by the low-voltage electrostatic field can inhibit the growth and reproduction of microorganisms.

2. The low-voltage electrostatic field can inhibit the activity of enzymes and bacteria, and delay the decomposition of enzymes and bacteria during storage, thus inhibiting a TVB-N content.

3. The frozen storage assisted by the low-voltage electrostatic field can effectively inhibit color deterioration.

4. The frozen storage assisted by the low-voltage electrostatic field can improve a water-holding capacity of meat samples, which is mainly caused by that water with natural frequency will resonate when the low-voltage electrostatic field is applied, causing structural changes of water around proteins, changing a binding state between the proteins and the water, making a binding force between the proteins and the water stronger, and enhancing the water-holding capacity of muscles.

5. During the frozen storage, an ice crystal volume in the meat samples keeps increasing, which destroys an original muscle fiber structure, thus reducing the tenderness of meat, i.e. reducing a shearing force; while the frozen storage assisted by the low-voltage electrostatic field enables the ice crystal volume not to increase too much, and have uniform sizes, which will reduce the damage to the original muscle fiber structure, reduce the shearing force and keep the tenderness of meat.

6. It is analyzed that an electrostatic induction phenomenon of the low-voltage electrostatic field may make surfaces of the meat samples charged, thus reducing a contact frequency with surrounding oxygen and inhibiting lipid oxidation. The results shows that the low-voltage electrostatic field can effectively inhibit the oxidation of fats of the meat samples in a later stage of the frozen storage, and the inhibiting effects of the low-voltage electrostatic field at -12°C and -6°C on the oxidation of fats of the meat samples are similar to those of the control groups at -18°C and -12°C respectively on the 2 8 day.

7. The low-voltage electrostatic field can delay the oxidation of proteins of the meat samples in the later stage of the frozen storage stage (2 1 " day to 28h day). In addition, there are no significant differences in the oxidation degree of proteins between the meat samples frozen at -18°C and at -12°C under the low-voltage electrostatic field, as well as the oxidation degree of proteins between the meat samples frozen at -12°C and at -6°C under the low-voltage electrostatic field (P>0.05).

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8. The low-voltage electrostatic field can inhibit the growth of the ice crystals.

9. Compared with non-electric field meat samples, the meat samples under the frozen storage assisted by the low-voltage electrostatic field have higher water content and stronger water-holding capacity, which verifies the previous storage loss test results.

Other advantages, objects and features of the present invention will be partially reflected by the following description, and will be partially understood by those skilled in the art through researching and practicing the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of a total number of colonies of pork in different treatment groups during frozen storage;

FIG 2 is a graph of TVB-N contents of pork in different treatment groups during frozen storage;

FIG. 3 is a graph of coloration of pork in different treatment groups during frozen storage;

FIG. 4 is a graph of storage losses of pork in different treatment groups during frozen storage;

FIG 5 is a graph of cooking losses of pork in different treatment groups during frozen storage;

FIG. 6 is a graph of tenderness of pork in different treatment groups during frozen storage;

FIG 7 is a graph of oxidation of fats of pork in different treatment groups during frozen storage;

FIG 8 is a graph of oxidation of proteins of pork in different treatment groups during frozen storage;

FIG 9 is a cross-sectional view of muscle tissues of pork in different treatment groups during frozen storage; and

FIG 10 is a graph of relaxation time T2 of pork in different treatment groups during frozen storage.

DETAILED DESCRIPTION The present invention will be further described in detail hereinafter with reference to the accompanying drawings, so that those skilled in the art can implement the present invention

Page 4 with reference to the specification.

It should be noted that all the experimental methods in the following embodiments are conventional methods without special instructions, and all the reagents and materials can be obtained from commercial channels without special instructions.

1. Materials and reagents

The raw meat referred to longissimus dorsi (Longissimusthoracis et lumborum LTL) of -month-old hybrid pigs (Duroc * Landrace * Yorkshire) with a quarantine qualification and a mass of about 160 kg provided by Shandong Weifang Tonglu Food Co., Ltd. The slaughtered pigs were hung for 24 hours to remove acid by air cooling, and two longissimus dorsi were taken from six boar carcasses with similar conditions. The longissimus dorsi were put into sampling boxes and transported back to a laboratory in an ice bath.

Sodium chloride, light magnesium oxide, SDS (sodium dodecyl sulfate), thibabituric acid, trichloroacetic acid, hydrochloric acid, chloroform, absolute ethyl alcohol, and xylene produced by Sinopharm Chemical Reagent Co., Ltd. Plate count agar produced by Beijing Solarbio Science and Technology Ltd.; tris(hydroxymethyl)methyl aminomethane produced by AMRESCO; DTNB (5,5'-dithiobis(2-nitrobenzoic acid)) produced by Sigma-aldrich; and HE staining kits produced by Beijing Zhongke Wanbang Biotechnology Science and Technology Ltd.

All the reagents were analytically pure.

2. Instruments and devices

Electronic balance produced by Sartorius (Beijing); SW-CJ-1FD clean bench produced by Suzhou Puaide Purification Equipment Technology Co., Ltd.; KDN-4C Kieldahl apparatus produced by Syance Science and Technology Ltd.; CR-400 colorimeter produced by Konica Minolta Holdings, Inc.; C-LM3B muscle tenderness meter produced by Beijing TENOVO International Co., Limited.; UV-1800 ultraviolet and visible spectrophotometer produced by Shimadzu Instrument (Suzhou) Inc.; MesoMR23-060H-I low-field nuclear magnetic resonance spectrometer produced by Shanghai Niumag Electronic Technology Co., Ltd.; Nikon CI-S inverted microscope produced by Nikon Corporation.; and DENBA+ freshness-holding electric field device produced by AGUA Corporation, etc.

3. Sample treatment

Fats, fascias and connective tissues visible on 12 longissimus dorsi were removed, and then the longissimus dorsi were divided into meat pieces of about 5cm*4cm*3cm, packed in

Page 5 transparent polyethylene bags and sealed. After the meat samples were fully mixed, the meat samples were randomly divided into 6 groups, which were labeled as -18L, -18, -12L, -12, -6L and -6 respectively, among which -18L, -12L and -6 groups were frozen at -18°C, -12°C and -6°C with a low-voltage electrostatic field, while -18, -12 and -6 groups were frozen at -18°C, -12°C and -6°C without an electrostatic field, as shown in Table 1. The samples were taken on the 7h, 14, 2 1st and 28 h days respectively, and all indexes were analyzed after thawing the samples at 4°C for 10 hours. A voltage of a low-voltage electrostatic field generating device was 2500 V, and a current was 0.2 mA.

Table 1 Experimental Grouping Design

Group Temperature Condition

-18L -18 0 C Low-voltage electrostatic field

-18 -18 0 C Control

-12L -12 0 C Low-voltage electrostatic field

-12 -12 0 C Control

-6L -60 C Low-voltage electrostatic field

-6 -60 C Control

4. Test on indexes of frozen meat samples:

4.1 Determination of a total number of colonies

The total number of colonies was determined according to a national standard method. 25 g of meat samples was weighed and diluted in 10 times; then two suitable gradients were selected, and 1 mL of diluted sample homogenate was evenly mixed with plate count agar; after the mixture was cooled and solidified, the plate was turned over, and the mixture was cultured at 37 0C for 48 hours.

4.2 Determination of TVB-N content

The TVB-N content was determined according to a national standard method. Meat samples were chopped and then 10 g of the chopped meat samples were taken and added with 100 mL of distilled water, shaken for 30 minutes, and then filtered. 5 mL of filtrate was taken and put into a digestive tract, added with 10 mL of distilled water and 5 mL of 20 g/L magnesium oxide suspension, and then the TVB-N content was determined by using an automatic Kieldahl apparatus.

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4.3 Determination of color

A luminance value L*, a redness value a* and a yellowness value b* of the sample surface were directly determined by using a portable colorimeter. The colorimeter was calibrated with a white board before use. Each sample was determined in parallel for five times. A total color difference value AE was calculated. A calculation formula is as follows:

AE=V(AL*)2+(Aa*) 2+(Ab*) 2

wherein, AL* is a difference of L* values between the treated group and a fresh meat sample, Aa* is a difference of a* values between the treated group and the fresh meat sample, and Ab* is a difference of b* values between the treated group and the fresh meat sample.

4.5 Determination of water-holding capacity

Storage loss

Masses of the meat sample before and after frozen storage were accurately weighed. A calculation formula of the storage loss is as follows:

Storageloss% M X100% M,

wherein, M 1 is the mass of the meat sample before frozen storage, and M 2 is the mass of the meat sample after frozen storage.

Cooking loss

The mass of the meat sample was weighed before frozen storage. The meat sample was put into a polyethylene bag for water bath at 80°C for 30 minutes after freezing storage, then cooled with running water, and weighed after absorbing the water on the surface of the meat sample with absorbent paper. A calculation formula of the cooking loss is as follows:

Cookinglosso = X100% M1

wherein, Mi is the mass of the meat sample before frozen storage, and M 3 is the mass of the meat sample after cooling.

4.6 Determination of shearing force

The meat sample was put into a retort pouch and sealed, and then heated in a boiling water bath until a center temperature of the meat sample reached 72°C, and then the meat sample was taken out and cooled to a room temperature. The meat sample was trimmed to a size of

Page 7

3cm*lcm*lcm and then stored at 40 C for 12 hours, and then determined by using a tenderness meter, wherein a shearing velocity was 5mm/s.

4.7 Determination of TBARS content

2 g of meat sample was minced, mixed with 3 mL of thiobarbituric acid (a volume fraction of 1%), and then 17 mL of trichloroacetic acid (a volume fraction of 2.5%) was added. After boiling the mixture for 30 minutes, the mixture was cooled with cold water. A sample solution was mixed with chloroform at a volume ratio of 1:1 and swirled, centrifuged at a room temperature (3000*g, 10 minutes), and an absorbance at 532 nm was determined. A calculation formula of the TBARS content is as follows:

TBARS content'(mgMDA/100 kg of meat sample) A 5 3 2 X9.48

In the formula: A5 3 2 - absorbance of the sample solution at 532 nm

Ms - mass of the meat sample (g)

9.48 - a constant obtained from a dilution factor and a molar extinction coefficient (152000 M-cm 1 ) of a red thiobarbituric acid reaction product

4.8 Determination of sulfydryl content

1 g of meat sample was mixed with 25 mL of 0.1 M Tris-HCl (pH 8.0) containing SDS (a volume fraction of 5%) and homogenized (13500 rpm, 30 seconds). The mixture was soaked in a water bath at 80°C for 30 minutes, cooled with cold water and filtered. A protein concentration of the filtrate was calculated by measuring an absorbance at 280 nm and according to a standard curve prepared by bovine serum albumin (0 mg/mL to 2 mg/mL). 2 mL of 0.1 M Tris-HCl (pH 8.0) and 0.5 mL of 0.1m Tris-HCl (pH 8.0) containing 10 mM of DTNB were added into 0.5 mL of filtrate, reacted in the dark at a room temperature for 30 minutes, and then an absorbance at 412 nm was determined. A calculation formula of the sulfydryl content is as follows:

Sulfydriyl conen/(nml/mg) =(A 4 1 2 -AD)X 10X6 13600xlxc

In the formula: A4 12 - absorbance of the sample solution at 412 nm

Ao - absorbance of a blank solution at 412 nm

13600 - molar extinction coefficient (L/(mol cm))

c - protein concentration of the sample solution (mg/mL)

4.9 Observation of ice crystal morphology in muscular tissue Page 8

A cuboid with a length of 15 mm and a cross section of about 5mm*5mm was taken from a center of the meat sample along a fiber direction, and put into a Carnoy's fluid at 4°C for 24 hours. The mixture was dehydrated by ethanol with different concentration gradients, then soaked in absolute ethanol+xylene (40 minutes) and xylene (40 minutes) respectively for transparent treatment, and then embedded after wax immersion at 65°C, then cut into 4 pm slices and baked at 65°C for 1 hours. The slices were stained by using a hematoxylin and eosin staining kit, and a tissue structure was observed by an optical microscope.

4.10 dimension of relaxation time T2

The transverse relaxation time T2 in the sample was determined by CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence in nuclear magnetic resonance analysis software. The sample was put into a center of a RF coil at a central position of a permanent magnetic field to collect T2, and then an obtained signal value was inverted by nuclear magnetic resonance T2 inversion software to obtain a T2inversion spectrum. Parameters of the CPMG pulse sequence are as follows: a main frequency of 23 MHz, an offset frequency of 286.7813 kHz, a 90°pulse time of 17ps, a 1800 pulse time of 35 ps, a sampling point number of 54996, a repetition time of 3000 ms, an accumulative frequency of 4 times, and an echo number of 2000.

5. Statistical analysis of data

According to the present invention, Microsoft Excel 2019 software was used to process data, GraphPadPrism 8 was used to draw an analysis chart, index determination referred to four parallel measurement results, which were expressed as mean plus or minus standard error, and IBM SPSS Statistics 25 statistical analysis software was used for Duncan method multiple comparisons and significance analysis (P<0.05).

6. Results and analysis

As shown in FIGS. 1 to 10 and Table 2, -18L represents -18°C+ow-voltage electrostatic field treatment group; -18 represents -18°C treatment group; -12L represents -12°C+ow-voltage electrostatic field treatment group; -12 represents -12°C treatment group; -6L represents -6°C+ow-voltage electrostatic field treatment group; and -6 represents -6°C treatment group. Different letters in the same storage time and different treatment groups represent significant differences (P<0.05).

FIG. 1 shows influences of different treatments on a total number of colonies of pork during frozen storage:

The total number of colonies is one of the important indexes to measure the spoilage of

Page 9 meat. If the total number of colonies of the meat reaches 6 ig (CFU/g), the meat will be regarded as spoilage. As shown in FIG. 1, the total number of colonies of the meat samples in each treatment group increases gradually during frozen storage, which are all less than 5 lg (CFU/g), indicating that the freshness of the meat samples decreases within 28 days but do not deteriorate. On the 7th day, a growth trend of the -18L group is the slowest, and the total number of colonies is 4.08 Ig (CFU/g), which is significantly lower than that in the -18 group 4.25ig (CFU/g), and the total number of colonies in the -6L group is 4.30ig (CFU/g), which is significantly lower than that in the -6 group (4.43lg (CFU/g), P<0.05)). There are significant differences in the total number of colonies among the -18, -12L, -12 and -6L groups (P>0.05). On the 2 8th day, the total number of colonies in the -12L group reaches 4.50 lg (CFU/g), which is significantly lower than that in the -12 group (4.79lg (CFU/g), P<0.05)). There are no significant differences in the total number of colonies between the -18 group (4.48 lg (CFU/g)) and the -12L group, as well as between the -12 group and the -6L group (4.71lg (CFU/g), P>0.05)). Compared with the frozen storage of the control groups -18°C, -12°C and -6°C, the low-voltage electrostatic field can inhibit the growth and reproduction of microorganisms, and the inhibiting effects of the low-voltage electrostatic field frozen storage at -12°C and -6°C on the microorganisms are respectively similar to those of the control groups -18°C and -12°C. Through analysis, the results may be possibly caused by that the low-voltage electrostatic field environment causes cell membranes of bacteria to induce charges, and a potential difference of permeable membranes generated results in cell rupture, so that the cells are died due to the disorder of membrane structures and the change of permeability.

FIG 2 shows influences of different treatments on a TVB-N content of pork during frozen storage:

Volatile basic nitrogen (TVB-N) refers to basic nitrogen-containing substances such as ammonia and amine produced by protein decomposition, and these substances are volatile. The higher the TVB-N content is, the lower the freshness of meat is. As shown in FIG 2, the TVB-N content in each group increases with the extension of storage time. At the early stage of storage (0 to 14 days), there are no significant differences in the -18L, -12L and -6L groups, and no significant differences in the -18, -12 and -6 groups (P>0.05). The results show that there are no significant differences in the TVB-N contents of meat samples at -18°C, -12°C and -6°C within 14 days of storage, regardless of whether a low-voltage electrostatic field is added. On the 28 day, the TVB-N content in the -18L group is 8.40 mg/100 g, which is significantly lower than that in the -18 group (8.91 mg/100 g, P<0.05). There are no significant differences in the -18

Page 10 group and the -12L group (8.73 mg/100 g). There are no significant differences in the -12 group (9.03 mg/100 g) and the -6L group (9.29 mg/100 g). The results show that the accumulation of the TVB-N content can be inhibited by the frozen storage at -18°C assisted by the low-voltage electrostatic field after the meat sample is stored for 28 days, and the inhibiting effects of the frozen storage at -12°C and -6°C under the low-voltage electrostatic field on the TVB-N content are respectively similar to that of ordinary frozen storage at -18°C and -12°C. Through analysis, the results may be possibly caused by that the low-voltage electrostatic field can inhibit the activity of enzymes and bacteria, and delay the decomposition of enzymes and bacteria during storage, thus inhibiting the TVB-N content.

Table 2 and FIG. 3 show influences of different treatments on color of pork during frozen storage:

Table 2 Influences of different treatments on color of pork during frozen storage

St Group ge -18L -18 -12L -12 -6L -6 timed

L* 54.11±0.81 54.11+0.81 54.11+0.81 54.11+0.81 54.11+0.81 54.11+0.81

Od a* 7.55±0.21 7.55±0.21 7.55+0.21 7.55+0.21 7.55±0.21 7.55±0.21

b* 4.96±0.38 4.96±0.38 4.96+0.38 4.96+0.38 4.96±0.38 4.96±0.38 L* 53.25±0.61a 52.7+0.44a 52.65±0.67a 52.53±0.54a 52.4840.78a 52.4540.63a

7d a* 7.66±0.39a 7.51±0.54a 7.63±0.39a 7.33±0.60a 7.46±0.62a 7.10±0.72a

b* 5.48±1.03a 5.56±0.83a 5.62±1.05a 5 .2 1 ±0. 87 ab 5.85±0.65a 4 . 2 5±0.3 6b

L* 52.46±0.69a 51.71±0.72a 52.01±0.76a 51.66±0.80a 51.40±0.71a 51.14±0.59a

14d a* 7.67±0.69a 7.20±0.64a 7.55±0.64a 7.07±0.45a 6.94±0.64a 6.85±0.77a

b* 5. 0 8 ±0.4 6 ab 5.98±0.56a 4 . 8 0±0. 8 3 ab 5. 6 3±0.7 6 ab 4 .6 5± .01b 5. 7 2±0.9 4 ab

L* 52.22±0.49a 5 1 .6 1 ±0. 7 6 ab 51.83±0.47a 5 1 .4 9 ±1. 6 6 ab 5 1 .0 9 ±0. 5 4 ab 50. 2 6±0.8 5b

21d a* 7.42±0.57a 6 .56±0.3lbc 6 .7 4 ±0. 6 7 ab 6 .71±0.4 8ab 6 .12±0. 4 6cd 5. 7 3±0.3 8d

b* 4 . 8 9 ±0. 6 6 b 6.12±0.49a 2.94±0.36c 5.96±0.99a 4 .3 9±0. 6 5b 4 . 6 5±0.55b

L* 51.32±0.45a 5 0 .8 7 ±0. 8 3 ab 50. 7 6 ±0.7 7 ab 50. 2 7 ±0.7 2 abc 5 0 .0 5 ±0.7 8 bc 49.40±0.60c

28d a* 6.71±0.33a 6.36±0.24a 6.46±0.37a 6.42±0.90a 5.82±0.59a 5.65±0.49a

b* 4 . 9 4 ±0. 5 8 b 5.15+0. 4 2b 3.49±0.57c 4 . 6 2 ±0.4 5b 4 .55+0. 5 7b 6.22±0.76a

Color is one of the important indexes for the sensory quality of meat, which directly affects purchasing preferences of customers. L*, a* and b* values represent brightness, redness and

Page 11 yellowness values respectively. In a certain range, the greater the L* value is, the better the glossiness of the meat is, the greater the a* value is, the fresher the meat is, while the greater the b* value is, the less fresher the meat is. Table 2 shows the L*a*b* values of the meat samples in each treatment group during storage. It can be seen that with the extension of storage time, the L* value and the a* value both decrease slowly, while the b* value has no significant change, indicating that the color of the meat sample gradually deteriorates during frozen storage. During storage, the L* value and the a* value of the low-voltage electrostatic field group are slightly higher than those of non-electrostatic field groups under the same temperature. A total color difference value AE indicates a difference between the color of the meat sample and fresh meat. As shown in FIG 3, the value of AE increases with the extension of storage time, indicating that the difference between the color of the meat sample and the fresh meat in each group is increasing and the color deterioration gradually appears. On the 28h day, the AE value of the -18L group is 2.91, which is lower than that of the -18 group (3.46), and the AE value of the -12L group is 3.82, which is lower than that of the -12 group (4.02), and the AE value of the -6L group is 4.43, which is lower than that of the -6 group (5.24). These results show that the frozen storage assisted by the low-voltage electrostatic field can effectively inhibit color deterioration.

FIG. 4 and FIG 5 show influences of different treatments on a water-holding capacity of pork during frozen storage:

During frozen storage, the decreasing water-holding capacity of the meat sample causes severe juice loss, nutrient loss and quality decline after thawing, resulting in certain economic losses. Storage losses of pork in different treatment groups during frozen storage are as shown in FIG. 4. It can be seen from FIG 4 that with the extension of storage time, the storage loss shows an increasing trend. During the whole storage period, the storage loss of the -18L group is significantly lower than that of the -18 group, and the storage loss of the -6L group is significantly lower than that of the -6 group (P<0.05), and there are no significant differences between the -18 group and the -12L group (P>0.05). From the 14 h to 28h days, there are no significant differences between the -12 group and the -6L group (P>0.05). On the 2 8 hday, the storage losses of the -18L group to the -6 group are respectively 7.12%, 8.34%, 8.21%, 10.05%, 10.10% and 11.15, and the storage losses of the low-voltage electrostatic field groups are significantly lower than those of the non-electrostatic field groups at the same temperature (P<0.05). Cooking losses of pork in different treatment groups during frozen storage are shown in FIG 5. With the extension of storage time, the cooking loss shows a gradually increasing trend. From the 14th to 28h days, the cooking loss of the -18L group is significantly lower than

Page 12 that of the -18 group (P<0.05), but there are no significant differences between the -18 group and the -12L group as well as between the -12 group and the -6L group (P>0.05). On the 28f day, the cooking losses of the -18L group to the -6 group are respectively 28.32% and 30.12. The results of the storage losses and the cooking losses show that the frozen storage assisted by the low-voltage electrostatic field can improve the water-holding capacity of the meat sample. There are no significant differences between the frozen storage assisted by the low-voltage electrostatic field at -12°C and the control group at -18°C, and there are no significant differences between the frozen storage assisted by the low-voltage electrostatic field at -6°C and the control group at -12°C (P>0.05). Through analysis, the results are possibly caused by that water with natural frequency will resonate when the low-voltage electrostatic field is applied, causing structural changes of water around proteins, changing a binding state between the proteins and the water, making a binding force between the proteins and the water stronger, and enhancing the water-holding capacity of muscles.

FIG. 6 shows influences of different treatments on tenderness of pork during frozen storage:

The tenderness of meat can be characterized by a shearing force, and a small shearing force indicates that the tenderness of meat is high. As shown in FIG. 6, with the extension of storage time, the shearing force of meat samples in each treatment group increases continuously, indicating that the tenderness decreases continuously. During frozen storage, a volume of ice crystals in the meat sample keeps increasing, which destroys an original muscle fiber structure, thus reducing the tenderness of the meat. On the 14 h day, the shearing force values of the -18 group and the -12 group are 32.16 N and 32.10 N respectively, both of which are significantly lower than that of the -6 group (33.95 N, P<0.05). On the 21' day, the shearing values of the -18 group and the -12 group are 32.42 N and 32.78 N respectively, both of which are significantly lower than that of the -6 group (34.51 N, P<0.05). There are no significant differences in the tenderness of the meat samples frozen at -18°C and -12°C (P>0.05), which are better than that of the meat sample frozen at -6°C.

FIG. 7 shows influences of different treatments on oxidation of fats of pork during frozen storage:

Oxidation of fats affects the sensory quality, functional characteristics and nutritional quality of meat to a certain extent, and a content of malondialdehyde in the meat can reflect the degree of oxidation of fats. It can be seen from FIG. 7 that the TBARS value keeps increasing during storage, which indicates that the degree of oxidation of fats of the meat sample increases gradually. At the early stage of storage (0 to 14 days), there are no significant differences in the Page 13

TBARS values of the treatment groups. On the 21' day, the TBARS value of the -18L group is 0.1217 mg MDA/100 g, which is significantly lower than that of the -18 group (0.1501 mg MDA/100 g, P<0.05). On the 28 h day, the TBARS value of the -18L group reaches 0.1517, which is significantly lower than 0.1754 mg MDA/100 g in the -18 group, the TBARS value of the -12L group is 0.1691, which is significantly lower than 0.1849 mg MDA /100 g in the -12 group, the TBARS value of the -6L group is 0.1896, which is significantly lower than 0.2101 mg MDA/100 g in the -6 group (P<0.05), while there are no significant differences in the TBARS values between the -18 group and the -12L group as well as between the -12 group and the -6L group (P>0.05). Through analysis, the results are possibly caused by that an electrostatic induction phenomenon of the low-voltage electrostatic field may make surfaces of the meat samples charged, thus reducing a contact frequency with surrounding oxygen and inhibiting lipid oxidation. The results shows that the low-voltage electrostatic field can effectively inhibit the oxidation of fats of the meat samples in a later stage of the frozen storage, and the inhibiting effects of the low-voltage electrostatic field at -12°C and -6°C on the oxidation of fats of the meat samples are similar to those of the control groups at -18°C and -12°C respectively on the 28h day.

FIG. 8 shows influences of different treatments on oxidation of proteins of pork during frozen storage:

A sulfhydryl will form disulfide bonds when proteins are oxidized, so the oxidation degree of proteins may be characterized by determining a sulfhydryl content. The lower the sulfhydryl content is, the higher the oxidation degree of proteins is. It can be seen from FIG. 8 that with the extension of storage time, the sulfhydryl content in each treatment group gradually decreases, and the phenomenon of oxidation of proteins gradually intensifies. There are no significant differences in the sulfhydryl content among the treatment groups during at the early stage of storage (0 to 14 days). On the 2l1 day, the sulfhydryl content in the -18L group is 86.27 mg/mL, which is significantly higher than that in the -18 group (82.46 mg/mL), and the sulfhydryl content in the -6L group is 79.55 mg/mL, which is significantly higher than that in the -6 group (75.78mg/ml, P<0.05).On the 28 h day, the sulfhydryl content in the -18L group decreases to 83.38 mg/mL, which is significantly higher than that in the -18 group (80.45 mg/mL), the sulfhydryl content in the -12L group is 78.25 mg/mL, which is significantly higher than that in the -12 group (73.94 mg/mL), and the sulfhydryl content in the -6L group is 73.94 mg/mL, which is significantly higher than that in the -6 group (69.82 mg/ml, P<0.05). There are no significant differences in the sulfhydryl contents between the -18 group and the -12L group as

Page 14 well as between the -12 group and the -6L group (P>0.05). The low-voltage electrostatic field can delay the oxidation of proteins of the meat samples in the later stage of the frozen storage stage (2 1 ' day to 28h day). In addition, there are no significant differences in the oxidation degree of proteins between the meat samples frozen at -18°C and at -12°C under the low-voltage electrostatic field, as well as the oxidation degree of proteins between the meat samples frozen at -12°C and at -6°C under the low-voltage electrostatic field (P>0.05).

FIG. 9 shows influences of different treatments on ice crystal in tissues of pork during frozen storage:

The growth of the ice crystals during frozen storage will destroy muscle cells, cause mechanical damage and quality decline of muscle tissues. Moreover, sizes, shapes and distributions of the ice crystals have an impact on the meat quality. FIG. 9 shows cross-sections of muscle tissues of meat samples of different treatment groups at the 7* and 28* days of frozen storage, in which a red part refers to muscle fibers and a white part refers to pores left by the ice crystals. It can be seen that the muscle fibers of fresh pork are arranged neatly and have a complete structure, and the gaps between the muscle fibers are small. After frozen storage, water in the muscle tissues forms ice crystals; with the extension of storage time, the ice crystals grow continuously, causing mechanical damage to the muscle fibers. On the 7th day, the ice crystals in the -18L, -18 and -12L groups are small and uniform, the ice crystals in the -6L group are smaller than those in the -6 group, and the muscle fibers are arranged more orderly. On the 2 8th

day, an aggregation phenomenon appears to some muscle fibers in the -6 group, which is caused by compression of the ice crystals with a large volume, and this is because that the small ice crystals are migrated to the large ice crystals with tight structure during frozen storage and merged to form the ice crystals with larger volume. In general, a muscle tissue structure of the -18L group is better than those of the -18 group, the -12L group, the -12 group and the -6L group, and muscle tissue states of the -18 group and the -12L group are closer. The test results show that the low-voltage electrostatic field can inhibit the growth of the ice crystals, and the muscle tissue state and the ice crystal morphology assisted by the low-voltage electrostatic field at -12°C are similar to those at -18°C.

FIG 10 shows influences of different treatments on relaxation time T2of pork during frozen storage:

FIG. 10 shows relaxation time T2of meat samples in different treatment groups on the 7th and 2 8 th days of frozen storage. Three peaks represent bound water (T21), immobile water (T22) and free water (T23) respectively, and a peak area ratio represents a relative proportion of the Page 15 three forms of water E4]The water binding capacity of the meat mainly depends on a capacity of the muscle to hold the immobile water. As shown in FIG. 10, with the extension of storage time, the T22 peak area decreases, indicating that a content of the immobile water decreases. On the 7f and 2 8 th days of storage, the T22 peak area in the -18 group is smaller than that in the -18L group, the T22 peak area in the -12 group is smaller than that in the -12L group, and the T22 peak area in the -6 group is smaller than that in the -6L group, indicating that the content of the immobile water and the water-holding capacity of the meat samples in frozen storage assisted by the low-voltage electrostatic field are higher and stronger than those of the meat samples in the non-electric field, verifying the previous storage loss test results.

In conclusion, the pork quality deteriorates in different degrees during frozen storage at -18°C to -6°C, and the higher the temperature is, the more serious the quality deterioration is. The frozen storage assisted by the low-voltage electrostatic field at -18°C, -12°C and -6°C can effectively inhibit the increasing of the total number of colonies and the accumulation of the TVB-N content of the pork, maintain the water-holding capacity, and delay the oxidation of the fat and proteins and the growth of the ice crystals. In addition, during the short-term storage (28 days), the preservation of the pork quality at -12°C assisted by the low-voltage electrostatic field is basically consistent with that of the frozen storage at -18°C, and the preservation of the pork quality at -6°C assisted by the low-voltage electrostatic field is basically consistent with that of the frozen storage at -12°C.

Although the implementation of the present invention has been disclosed above, it is not limited to the applications listed in the specification and the embodiments, and can be fully applied to various fields suitable for the present invention, and additional modifications can be easily implemented by those skilled in the art. Therefore, the present invention is not limited to the specific details and illustrations shown and described herein without departing from the general concept defined by the claims and the equivalent scope.

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Claims (4)

1. A method for assisting short-term frozen storage of livestock and poultry meat by using a low-voltage electrostatic field, comprising: placing livestock and poultry meat in a low-voltage electrostatic field for frozen storage at a temperature ranging from -18°C to -6°C.

2. The method for assisting short-term frozen storage of livestock and poultry meat by using the low-voltage electrostatic field according to claim 1, wherein a voltage of the low-voltage electrostatic field is 2000 V to 2500 V, and a current is 0.15 mA to 0.2 mA.

3. The method for assisting short-term frozen storage of livestock and poultry meat by using the low-voltage electrostatic field according to claim 1, wherein a time for the frozen storage is no more than 28 days.

4. Livestock and poultry meat prepared by using the method according to any one of claims 1 to 3.