Abstract: This experiment uses acidic water electrolysis as a sterilizing solution for finished tofu, and studies the sterilization effect and quality of finished tofu by acidic water electrolysis. The results showed that soaking tofu in acidic water electrolysis is beneficial for reducing the microorganisms in tofu. A soaking time of 20 minutes can reduce the total bacterial count of tofu from 3.64 log CFU/g to 2.34 log CFU/g. In addition, the instrument measurement results showed that the acidic water electrolysis soaking had little effect on the hardness of tofu and slightly affected its color; The sensory evaluation results indicate that soaking tofu in acidic water electrolysis does not significantly affect its sensory quality.

The main types of tofu in our country are northern tofu and southern tofu. Beidou is one of the representative traditional soy products in China, widely welcomed by people due to its good flavor, sensory and reprocessing characteristics. However, most tofu production enterprises in China are still in the small-scale production mode of workshop style. These enterprises have low production efficiency, unstable product quality, and inadequate hygiene conditions. Tofu has a high moisture content and is rich in nutrients such as protein and fat. It is highly susceptible to spoilage and deterioration when contaminated by microorganisms. It is precisely because tofu is extremely intolerant to storage that it cannot solve the problem of quality assurance during transportation, and can only be sold locally, which limits the production of the product. Therefore, it is necessary to conduct research on tofu preservation technology to reduce losses.

Electrolyzed water is a general term for acidic and alkaline water electrolysis with special functions, which is produced by treating water with an electric field in a special device to change indicators such as pH value, oxidation-reduction potential (ORP value), and effective chlorine concentration (ACC). Among them, acidic water electrolysis sterilization is an emerging sterilization technology widely researched at home and abroad, with high efficiency, instantaneous, wide range, no residue, safe and reliable use, and convenient production operation.

This study uses commercially available northern tofu as raw material to explore the bactericidal effect and quality impact of acidic water electrolysis on finished tofu, in order to provide some theoretical data for tofu preservation.

1. Materials and Methods

1.1 Materials and reagents

Fresh northern tofu was purchased from a local supermarket.

Acidic water electrolysis (pH: 2.6 ± 0.2, ORP=1016 ± 24mV, ACC=49.2 ± 3.8mg/L); Tap water (pH: 7.6 ± 0.3, ORP=365 ± 10mV, ACC=4.3 ± 0.8mg/L); Acetic acid and K2Cr2O3 (both analytical grade); KI、 Soluble starch and KCl (both analytical grade).

1.2 Method

1.2.1 Preparation of Acidic Water Electrolysis

Use a water electrolysis device to produce acidic water electrolysis, with a NaCl concentration of 1g/L and an electrolysis time of 15 minutes. The pH value and ORP (oxidation-reduction potential) value are directly measured using a pH meter and oxidation-reduction potentiometer. For each water sample, repeat the measurement of the above indicators three times. The effective chlorine concentration is determined using the iodometric method.

1.2.2 Sterilization of Tofu by Acidic Water Electrolysis

Weigh 6 portions of tofu, each weighing 25g, and randomly divide them into two groups. One group is soaked in tap water, while the other group is soaked in acidic water by electrolysis. The soaking time for 3 servings of tofu in each group is 10, 20, and 30 minutes, respectively. The soaking water is 100ml. The control is untreated. After removal, drain for 10 minutes and measure the total number of bacteria.

Use the plate counting method to detect the total number of bacteria. Take 10g of soaked tofu using aseptic techniques, grind it in a sterile mortar, and transfer it to a triangular flask containing 90ml of sterile physiological saline. After sufficient shaking, dilute it using a 10 fold dilution method. According to GB4789.2-94, inoculate the bacterial suspension onto nutrient agar medium and incubate it in a constant temperature incubator at 37.1 ℃ for 24 hours before counting.

1.2.3 Effects of Acidic Water Electrolysis Soaking on Tofu Hardness and Sensory Quality

Take a whole piece of tofu (250 ± 50g) in a 2000ml beaker, add acidic water in a ratio of 1:4 (W: V) for electrolysis, soak for 10, 20, and 30 minutes respectively, drain for 10 minutes, measure the color and hardness of the tofu, and conduct sensory evaluation on the tofu.

1.2.3.1 Color determination

Using a colorimeter to measure the color of tofu, based on the L * a * b * color system established in 1976. L * is called the brightness index, L *=0 represents black, L *=100 represents white, and the larger the L value, the brighter the sample color. +The a * direction increases in red, the - a * direction increases in green, the+b * direction increases in yellow, and the - b * direction increases in blue. VE ab * represents the degree of color difference between samples.

1.2.3.2 Determination of Tofu Hardness

Conduct a secondary compression experiment using a rheometer: use a 2cm diameter sampler to take a sample from the middle of the tofu, with a height of 2cm; select a 2cm diameter probe and a 2kg force arm. The speed of the loading platform is 1mm/s. The hardness of tofu is calculated according to the Bourne method.

1.2.3.3 Sensory evaluation test of tofu

Refer to GB/T14195. Ten graduate students from the School of Food Science at China Agricultural University were selected, who have a good understanding of the processing technology and characteristics of northern tofu. Before sensory evaluation, the samples were numbered with three digit random numbers and divided into 2cm × 2cm × 2cm cubes. The samples were placed in numbered paper trays and provided to evaluators for sensory evaluation in a well lit and partitioned laboratory. Rate according to Table 2. Hardness is perceived by the compression of the sample in the mouth through the teeth, tongue, and upper jaw; Taste refers to the granular texture of tofu in the mouth; Flavor refers to the unique bean aroma of tofu products. Including odor and taste, perceived through smell and taste; Tofu has a good sensory color, which can be perceived through observation; The overall acceptability takes into account the appearance, color, flavor, taste, and hardness of tofu, which the evaluator considers the best.

2. Results and Analysis

2.1 Antibacterial effect of acidic water electrolysis on tofu

It can be seen that the total bacterial count of untreated tofu is 3.64 log CFU/g, and the total bacterial count of tofu treated with acidic water electrolysis is significantly lower than that of untreated tofu (p=0.05), a decrease of 1.0 logarithmic value compared to the total bacterial count of untreated tofu. However, the difference between tofu soaked in tap water and untreated tofu is not significant. This indicates that ordinary tap water has little bactericidal effect on tofu, while the bactericidal effect of acidic water electrolysis is significantly better than tap water. From Figure 1, it can also be seen that the total number of bacteria in tofu varies slightly after soaking in acidic water for different times. The total number of bacteria in tofu soaked in acidic water electrolysis for 20 minutes was the smallest, reaching 2.34 log CFU/g. The sterilization effect of soaking for 10 minutes and 30 minutes was not as good as soaking for 20 minutes. The above results indicate that a soaking time of 10 minutes is not sufficient to maximize the bactericidal effect of acidic water electrolysis, while a soaking time of 30 minutes is too long, which will cause the bactericidal effect of acidic water electrolysis to decrease with prolonged contact time with tofu.

2.2 Effect of Acidic Water Electrolysis Soaking on Tofu Hardness

The texture of tofu plays a crucial role in determining its quality and consumer acceptability. Tofu is a protein gel material, and its hardness is one of its important texture characteristics. This experiment used a rheometer to determine the effect of acidic water electrolysis soaking on the hardness of tofu. The experimental results show that the effect of acidic water electrolysis soaking on the hardness of tofu is not very significant. After soaking for 30 minutes, the decrease in tofu hardness is not very obvious. This indicates that soaking in acidic water electrolysis for 30 minutes can maintain the hardness of tofu and will not have adverse effects on its hardness.

2.3 Effect of Acidic Water Electrolysis Soaking on Tofu Color

The effect of acidic water electrolysis soaking on the color of tofu was measured using a color colorimeter. The results showed that the L, a, and b values of tofu treated with acidic water electrolysis were not significantly different from those of the control. However, the L value of tofu treated with acidic water electrolysis for 30 minutes was lower than that of the control. The larger the L value, the brighter the sample color, indicating a slight change in tofu color after 30 minutes of acidic water electrolysis treatment. From the VEab * values, it can also be seen that the tofu treated with acidic water electrolysis for 10, 20, and 30 minutes had values of 0.40, 0.54, and 1.41, respectively. This means that there is a very small difference in color between the tofu treated with acidic water electrolysis for 10 minutes and the control, while the other two treatments have slight differences from the control.

Sensory evaluation of tofu soaked in acidic water electrolysis

The sensory evaluation analysis of tofu soaked in acidic water electrolysis is shown in Table 4. From Table 4, it can be seen that there is no significant difference in sensory evaluation indicators between soaked tofu and the control group. In terms of hardness, the hardness score of soaked tofu was lower than the control score. After data analysis, the difference was not significant (p>0.05), which is consistent with the instrument measurement results. In terms of flavor, due to the similar odor of acidic water electrolysis to sodium hypochlorite, it is examined whether soaked tofu has this odor and affects its inherent flavor. From Table 4, it can be seen that the flavor scores of soaked tofu are lower than those of the control group, but the difference is not significant (p>0.05). The evaluators did not find any odor in tofu soaked in acidic water electrolysis. In terms of taste, soaking in acidic water for 30 minutes resulted in the lowest score, but there was no significant difference compared to the control group. In terms of color, there was no significant difference between all treatments soaked in acidic water electrolysis and the control, which is consistent with the instrument test results. The results indicate that soaking in acidic water electrolysis has little effect on the color of tofu, and there is no visible difference visually. In terms of overall acceptability, the tofu soaked in acidic water electrolysis scored similar to the control group, with no significant difference (p>0.05). From this, it can be concluded that the acidic water electrolysis soaking treatment has little effect on the overall acceptability of tofu.

3 Conclusion

Soaking tofu in acidic water electrolysis is beneficial for reducing the total number of bacteria in tofu. The experimental results of soaking time show that soaking tofu in acidic water electrolysis for 20 minutes can achieve good sterilization effect. Acidic water electrolysis soaking has little effect on the hardness of tofu and slightly affects the surface color of tofu. The sensory evaluation results indicate that soaking tofu in acidic water electrolysis does not significantly affect its sensory quality.