Introduction

Lotus root (Nelumbo nucifera), which is cultivated in various countries such as China, Japan, and Korea, can be utilized as an anti-tussive and anti-inflammatory drug in Chinese medicine1,2. The vegetable is also rich in starch, protein, vitamins, and minerals3. Fresh-cut lotus root is one of the fresh-cut products that can meet increasing consumer demand as a novel minimally processed vegetable. However, the slicing of fresh-cut lotus root can cause mechanical damage to tissue, which results in quality deterioration, such as browning and nutrient loss. As a result, the edible and commodity values of fresh-cut lotus root are seriously affected. It is important to prevent quality losses which have a negative impact on the development of the industry.

The methods commonly used in the preservation of fresh-cut fruits and vegetables are mainly soaking and daubing with various preservatives that inhibit the activity of antioxidant enzymes, such as exogenous salicylic acid4, and chitosan coating5. Other methods (e.g., heat treatment and N2) are also available6,7. However, soaking and coating can easily cause a high residue of the reagents in fresh-cut fruits and vegetables, and the air conditioning treatment and heat treatment are less effective than preservatives, which require specialized equipment8. Thus, the development of safe, effective, convenient, and low-cost preservatives applied in fresh-cut fruits and vegetables becomes a research topic.

Acetic acid, an active component of vinegar, has the advantages of easy volatilization, little residue, and low cost; thus, acetic acid is regarded as an environmentally friendly liquid9,10. Anhydrous acetic acid, which is also known as glacial acetic acid, is a safe food additive approved for use in China (GB 2760-2014). Apart from being a sour and flavoring agent, acetic acid is also an antimicrobial component in vinegar and an effective preservative against food spoilage11. The decreased respiratory intensity of the product and the inhibition of microbial growth on the surface caused by acetic acid might prevent fruits and vegetables from spoilage12,13,14. Acetic acid had been applied to maintain the postharvest quality of tomato15. Ozone could reduce the respiration rate and weight loss of strawberry16 and delay the decrease of soluble solids and tomato firmness17. In the food preservation, ozone has been proven to be an effective method to extend the storage time of fresh-cut fruits and vegetables18. In short, ozone, as a strong oxidant, is an alternative agent for food applications due to the advantage of rapid decomposition into oxygen without any residue. Acetic acid and ozone may be able to keep fresh-cut lotus root fresh, but there is little information about the application of acetic acid or ozone in the preservation of fresh-cut lotus root.

In providing a scientific basis for the application of acetic acid or ozone in keeping fresh-cut lotus root fresh, the indicators related to the quality of fresh-cut lotus root must be measured. The mass data obtained by determination may cause confusion in understanding the relationship among indicators or their contribution to the evaluation of the fresh-keeping effect of acetic acid or ozone on fresh-cut lotus root. The grey–correlation method is a multivariate statistical analysis method of calculating the absolute value of the data difference among sequences19. The multi-objective optimization of the Q355C steel gas metal arc-welding process20 was performed by using the grey–correlation method. The correlation degrees between the frost-penetration depths and some parameters, such as thermal characteristics and average annual temperatures, were also described by grey–correlation analysis21. The present study focused on determining the weight loss rate, browning degree, and some nutritional components of fresh-cut lotus root exposed to different concentrations of acetic acid and ozone. The experimental data were analyzed and compared. Then, the importance of different concentrations of acetic acid and ozone affecting the quality indicators of fresh-cut lotus root was classified by using the grey–correlation method based on the variation–coefficient weight. Our findings may provide information and a comprehensive evaluation approach to the preservation of fresh-cut lotus root and other fresh-cut fruits and vegetables.

Materials and methods

Ozonated water preparation

The ozonated water was prepared with a KFT-S series computerized ozone generator (Shanghai Kangfu Environmental Protection Product Technology Co., Ltd, Shanghai, China) using atmospheric air as the oxygen source22.

Plant materials

Lotus root was purchased from a supermarket in Xuchang. Any bruised or diseased vegetable was discarded. The vegetable was washed, peeled, and cut into 0.5 cm-thick slices using a stainless-steel knife. The prepared slices were randomly divided into six groups. Three groups were respectively soaked in glacial acetic acid solutions of different concentrations (0, 1.8% and 5%) for 5 min, and the treatment without glacial acetic acid served as the control of acetic acid treatment. The other three groups were respectively soaked in ozonated water of different concentrations (0, 0.3 mg/L and 0.6 mg/L) for 12 min, and the treatment without ozone served as the control of ozone treatment. After dripping water for 10 min, each of the six groups was initially divided into 15 parts, then respectively weighed and packaged through heat sealing using PE preservation bags, and finally stored at 4 °C in a BCD-290W Haier refrigerator (Qingdao Haier Co., Ltd, Qingdao, Shandong, China). Sampling and measuring of relevant indicators were performed at 0, 4, 8, 12, and 16 days. Each treatment was repeated three times.

Measurement of weight loss rate

The weight loss rate was calculated using Eq. (1).

$${\text{Weight loss rate }}\left( \% \right) = \frac{{M_{0} - M_{i} }}{{M_{0} }} \times 100,$$
(1)

where \(M_{0}\) is the weight of fresh-cut lotus root before packaging; \(M_{i}\) is the weight of fresh-cut lotus root stored for i day; i refers to 0, 4, 8, 12, or 16 days.

Measurement of browning degree

The browning degree of fresh-cut lotus root was determined using a visible photometer (722N, Youke Instrument and Meter Co., Ltd., Shanghai, China) at 410 nm. Put 2 g lotus root and 20 mL pure water into a mortar, fully grind and filter, keep the filtrate at 25 °C for 5 min, and measure the absorbance value at 410 nm (A410). The result is represented by A410 for browning degree23.

Measurement of reducing sugar

The reducing sugar was detected with a visible photometer (722N, Youke Instrument and Meter Co., Ltd., Shanghai, China) at 540 nm using the 3, 5-dinitrosalicylic acid method24.

Measurement of ascorbic acid

The total ascorbic acid was measured by 2, 4-dinitrophenylhydrazine method using a visible photometer (722N, Youke Instrument and Meter Co., Ltd., Shanghai, China) at 500 nm described by GB/T 5009.86-2003 (China). The analysis steps include sample preparation, oxidation treatment, color reaction, 85% sulfuric acid treatment, colorimetric determination, and standard curve drawing. With slight variations in sample preparation: Weigh 5 g of lotus root into a mortar, add 5 mL of 2% oxalic acid solution, grind thoroughly, transfer to a 100 mL volumetric flask, and make up to the mark with 1% oxalic acid solution.

Measurement of free amino acid

Free amino acid was detected with a visible photometer (722N, Youke Instrument and Meter Co., Ltd., Shanghai, China) at 570 nm using the ninhydrin colorimetric method24.

Calculating the coefficient of variation and the objective weight

The coefficient of variation (\(V_{i}\)) can avoid the equal division of weight and make the result more reasonable25.

$$V_{i} = \frac{{\delta_{i} }}{{{\text{X}}_{i} }},$$
(2)

where \(V_{i}\) is the coefficient of variation of the indicator Ci; \(\delta_{i}\) is the standard deviation of the indicator Ci; \({\text{X}}_{i}\) is the mean value of the indicator Ci; i = 1, 2, …, n, and n is the number of indicators (n = 1, 2, …, 5).

The objective weight was calculated using Eq. (3).

$$W_{i} = \frac{{V_{i} }}{{\mathop \sum \nolimits_{i = 1}^{n} V_{i} }},$$
(3)

where Wi is the object weight of the indicator Ci; Vi is the variation coefficient of the indicator Ci, and i is the number of indicators (n = 1, 2, …, 5).

Normalizing raw data

The ideal processing was selected as the reference sequence (X0), and the other treatments (acetic acid and ozone) were taken as the comparison sequence (Xj). Except for the weight loss rate, the minimum value of browning degree and the maximum value of other quality indicators in the comparison sequence were the corresponding values for ideal processing. The weight loss rate of the ideal processing was the smallest value among those values of weight loss rate in the comparison sequence except for 0. The raw data were preprocessed and converted into dimensionless data using Eq. (4)26.

$$C_{j}^{\prime } \left( i \right) = \frac{{C_{j} \left( i \right)}}{{C_{0} \left( i \right) }}, \left( 4 \right)$$
(4)

where j is the number of samples (j = 0, 1, 2, …, 30), and i is the number of indicators (i = 1, 2, …, 5).

Determining the absolute difference between comparison and reference sequence

The absolute difference was calculated using Eq. (5)27.

$$\Delta_{j} \left( i \right) = \left| {C_{0}^{\prime } (i) - C_{j}^{\prime } (i)} \right| ,$$
(5)

The minimum absolute difference was measured as follows:

$$\Delta min = \mathop {min}\limits_{j} \mathop {min}\limits_{i} \left| {C_{0}^{\prime } (i) - C_{j}^{\prime } (i)} \right| ,$$
(6)

The maximum absolute difference was calculated as follows:

$$\Delta max = \mathop {max}\limits_{j} \mathop {max}\limits_{i} \left| {C_{0}^{\prime } (i) - C_{j}^{\prime } (i)} \right|,$$
(7)

where j is the number of samples (j = 0, 1, 2, …, 30), and i is the number of indicators (k = 1, 2, …, 5).

Calculating the grey–correlation coefficient

The grey–correlation coefficient is considered as the correlation between the ideal and actual standardized value. This coefficient can be calculated using Eq. (8) 27.

$$\zeta_{j} \left( i \right) = \frac{\Delta min + \rho \Delta max}{{\Delta_{j} \left( i \right) + \rho \Delta max}},$$
(8)

where j is the number of samples (j = 0, 1, 2,…, 30); i is the number of indicators (i = 1, 2,…,5); \(\rho\) is the resolution ratio, which enlarges the difference among various coefficients. \(\rho\) = 0.5 is generally used.

Calculating the grey–correlation degree

In obtaining the grey–relational degree, the following equation was used:

$$X_{j} = \mathop \sum \limits_{j = 1}^{i} \zeta_{j} \left( i \right) \times W_{i} ,$$
(9)

where \(\zeta_{j} \left( i \right)\) is the grey–correlation coefficient of the indicator Ci; \(W_{i}\) is the object weight of the indicator Ci; j is the number of samples, and j = 0, 1, 2, …, 30; i is the number of indicators, and i = 1, 2, …, 5.

Statistical analyses

Each sample was made in three parallel. The experimental results were analyzed by variance analysis and compared by using the Student–Newman–Keuls test. p ≤ 0.05 was considered significant.

Results and discussion

Effect of acetic acid and ozone on the storage quality of fresh-cut lotus root

The demand for fresh-cut lotus root, a convenient and nutritive vegetable, is increasing in market. However, given the mechanical damage during processing, the quality deterioration in fresh-cut lotus root is aggravated, resulting in short storage time and huge economic losses. In extending the storage time of fresh-cut lotus root and ensuring product quality, the utilization of acetic acid or ozone could be a desirable method because of its cost-effectiveness, safety, and simplicity. It was found that the storage quality of fresh-cut lotus root exposed to different storage time and different concentrations of acetic acid and ozone varied (Fig. 1).

Fig. 1
figure 1

Effect of acetic acid and ozone on the storage quality of fresh-cut lotus roots. Each value represents the mean of three replicates ± standard deviation (SD). Columns with different letters at the same day are significantly different at the same time (p < 0.05).

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Weight loss of fruits and vegetables is due to the decrease in water content in tissues caused by transpiration during storage and transportation. The weight loss rate is an important indicator for the preservation effect of fruits and vegetables. As shown in Fig. 1A, the weight loss rate of fresh-cut lotus root increased with the increase of storage time. Acetic acid caused a significant increase in moisture loss of fresh-cut lotus root during storage. And the higher weight loss rate also indicated significant loss of water and nutrients, as well as stronger respiration and transpiration. Compared with acetaldehyde, acetic acid treatment may promote the activity of enzymes related to the depolymerization of plant cell wall (e.g., pectin methyl esterase, polygalacturonase, and β-galactosidase) to destroy the integrity of tissue cells of fresh-cut lotus root, which is unfavorable to the maintenance of the water-holding capacity of tissue cells28. Contrary to acetic acid, ozone could reduce the weight loss rate of fresh-cut lotus roots. And the water loss rate of 0.3 mg/L ozone was significantly lower than other treatments when the storage time was more than 4 days. Maybe ozone reduced the respiration and transpiration of fresh-cut lotus roots.

As an important factor affecting the appearance quality of fruits and vegetables, color is an indicator to evaluate the quality of fresh-cut fruits and vegetables29. As shown in Fig. 1B, the effect of acetic acid on reducing browning degree of fresh-cut lotus root was significant during storage. It was indicated that acetic acid could inhibit the yellowing and maintain the eating quality and market acceptability of the product, as well as ClO211. And the inhibition effect of 1.8% acetic acid was better than 5%. When the storage time was more than 4 days, the effect of ozone on browning inhibition was little or no, but that of 0.3 mg/L ozone was better than 0.6 mg/L.

The respiration and transpiration intensity of fresh-cut water chestnut changed continuously during storage and caused tissue internal carbohydrate synthesis and the catabolism unceasing transformation, which directly affected the content of soluble solids in fresh-cut water chestnut28. Severe dehydration can cause metabolic disorders, for example, the accelerated hydrolysis caused by the increased hydrolase activity. On the one hand, enhanced hydrolysis increases the respiratory matrix, promotes cellular respiration, and accelerates nutrient consumption. On the other hand, the increase in nutrients provides convenience for microbial activity and accelerates decay. Acetic acid could increase moisture loss of fresh-cut lotus root and inhibit the browning root (Fig. 1A,B). Therefore, acetic acid may decrease the content of water-soluble nutrients and increase the respiratory matrix in fresh-cut lotus root. According to the different influence on the weight loss rate and browning degree (Fig. 1A,B), it could be concluded that the effect of ozone on the nutrient of fresh-cut lotus root might be different from acetic acid.

As shown in Fig. 1C, the changing trend of reducing sugar in fresh-cut lotus root treated with acetic acid initially decreased and then increased. This was basically consistent with the change in the respiration intensity of fruit and vegetable after harvest. Post ripening resulted in the decrease and increase. However, the content of reducing sugar in fresh-cut lotus root exposed to 1.8% or 5% acetic acid was lower than the control (acetic acid). It was confirmed that, for fresh-cut lotus root, acetic acid could enhance respiratory intensity and reduce the content of reducing sugar. And the effect of 1.8% acetic acid was better than 5% to maintain reducing sugar content. 0.6 mg/L ozone caused reducing sugar to initially increase and then decrease, and the trend caused by other ozone treatments initially increased, then decreased, and increased. Ozone may inhibit respiratory intensity of fresh-cut lotus root. When fresh-cut lotus root was stored for 4, 8, or 12 days, ozone caused increase in the content of reducing sugar, and the effect of 0.6 mg/L ozone was better than 0.3 mg/L to maintain reducing sugar content.

As shown in Fig. 1D, acetic acid and 0.3 mg/L ozone significantly increased the content of ascorbic acid. This was consistent with the fresh-cut water chestnuts treated with salicylic acid, boiling water, or N24,5,7,30. When the storage time was 0 day, the ascorbic acid of 0.6 mg/L ozone treatment was the highest, and it had no significant difference with 5% acetic acid. When the storage time was 4 days, the ascorbic acid of 5% acetic acid treatment was the highest. When the storage time was more than 4 days, the ascorbic acid of 1.8% acetic acid was the highest. It was reported that, after 2.5 min of blanching, the preservation rate of ascorbic acid in peas reached 100%. The oxidation rate of ascorbic acid increased with increasing temperature. When the temperature reached the boiling point, the oxidation rate dropped to zero. This might be due to the inhibition of oxidative and inactivating enzyme activity by blanching31. The content of ascorbic acid in fresh-cut lotus roots treated with acetic acid and ozone was higher than that in CK, which might be due to their influence on enzyme activity related to ascorbic acid synthesis and decomposition, thereby reducing the oxidation rate of ascorbic acid and promoting its synthesis.

As shown in Fig. 1E, the content of free amino acid in fresh-cut lotus root exposed to acetic acid or ozone appeared a trend of first rising and then falling. When the storage time was 0, 4, or 8 days, acetic acid and 0.6 mg/L ozone significantly increased the content of free amino acid, and the free amino acid of 0.6 mg/L ozone was the highest. When the storage time was more than 8 days, the effect of 0.3 mg/L ozone was better than 0.6 mg/L to maintain free amino acid content, and that of 1.8% acetic acid was also better than 5%.

The synthesis and catabolism of substances caused by the respiration and transpiration of fresh-cut lotus root might lead to the increase and decrease of reducing sugar, ascorbic acid, and free amino acid content. It was indicated that the respiration of fresh-cut lotus root was affected by acetic acid and ozone. However, determining which of acetic acid and ozone works well on extending the storage time of fresh-cut lotus root is difficult.

Weight value of impact factors

Using Eqs. (2) and (3), Vi and Wi were calculated, and the results are listed in Table 1. Based on the different contributions of these indicators, the weights of weight loss rate, browning degree, reducing sugar, ascorbic acid, and free amino acid accounted for 39.3%, 9.0%, 9.8%, 11.4%, and 30.5%, respectively.

Table 1 Weights of various indicators used in the comprehensive evaluation of fresh-cut lotus roots.
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Grey–correlation analyses

Given these indicators (Fig. 1), determining the optimum treatment was difficult. Thus, a comprehensive evaluation of the fresh-keep effect of fresh-cut lotus root treated with acetic acid and ozone was necessary. Grey–correlation analysis is a multivariate statistical analysis used to evaluate the correlation among different sequences within a design21. In removing the dimensional differences of each indicator, the influence of treatments with different concentrations of acetic acid and ozone on the quality of fresh-cut lotus root was evaluated by grey–correlation analysis based on the detection of weight loss rate, browning degree, reducing sugar, ascorbic acid, and free amino acid.

Given the non-uniform dimension of the raw data shown in Fig. 1, these data should be made dimensionless before comprehensive evaluation26. Selecting the preservative agent with excellent quality properties for fresh-cut lotus root was desirable to meet quality targets. Therefore, in the ideal process selected as the reference sequence (X0), the reducing sugar, ascorbic acid, and free amino acid were the highest values of the corresponding indicators in the comparison sequence, and the browning degree was the lowest value of the corresponding indicators in the comparison sequence. With regard to the weight loss rate, the lowest value in the comparison sequence was 0, and the following calculation cannot be performed using the value of 0. Thus, the weight loss rate in the ideal process was the minimum value of the corresponding indicators in the comparison sequence, excluding 0. All raw data were normalized using Eq. (4). The standardized data of evaluation indices are listed in Table 2.

Table 2 Standardized data of evaluation indices.
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The absolute difference between the comparison and reference sequence was calculated using Eq. (5), and the results are shown in Table 3. In accordance with Eqs. (6) and (7), the minimum absolute difference was 0.000, and the maximum absolute difference was 16.375.

Table 3 Absolute value between the reference sequence and comparative sequence.
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Equation (8) was used to count the correlation coefficient between the treatments (acetic acid and ozone) and the quality indicators of fresh-cut lotus root. Then, the calculated correlation coefficients are given in Table 4.

Table 4 Grey–relational coefficient and grey–correlation degree between the reference sequence and comparative sequence.
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The grey–correlation degree calculated using Eq. (9) is shown in Table 4 and Fig. 2. Basically, the grey–correlation degree in acetic acid or ozone treatment decreased with the increase of storage time. During storage for 4, 8, 12, or 16 days, the comprehensive scores of ozone treatments were higher than CK (ozone), and those of acetic acid treatments were lower than CK (acetic acid). And the quality of fresh-cut lotus root exposed to 0.3 mg/L ozone had minimal change during storage. Thus, 0.3 mg/L ozone could be used to preserve fresh-cut lotus root stored for more than 4 days.

Fig. 2
figure 2

Effect of acetic acid and ozone on the comprehensive score of fresh-cut lotus roots.

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Grey–correlation analysis is an evaluation method of determining the primary and secondary factors as well as the degree of their correlation in accordance with the gray system21. Given the simple calculation and reasonable evaluation, grey–correlation analysis had been used to comprehensively evaluate the effects of processing methods on turnip chips, Hypsizygus marmoreus, and black sesame seeds26,32,33. In providing a scientific basis for the application of acetic acid and ozone in keeping fresh-cut lotus root fresh, the weight loss rate, browning degree, and content of reducing sugar, ascorbic acid, and free amino acid in fresh-cut lotus root were measured. In addition, the experimental data were analyzed and compared (Fig. 1). However, the information was so cumbersome and complex, making it difficult to determine which of the acetic acid and ozone treatments works well. Consequently, the importance of different concentrations of acetic acid and ozone affecting the quality of fresh-cut lotus root was classified by grey–correlation analysis based on the variation–coefficient method.

Conclusions

Both biochemical and statistical analyses were carried out to determine the effects of different concentrations of acetic acid and ozone applications on the storage time of fresh-cut lotus root. The influences of various treatments on the quality indicators of fresh-cut lotus root could be compared and evaluated comprehensively using grey–correlation analysis by analyzing the weight loss rate, browning degree, and content of reducing sugar, ascorbic acid, and free amino acid. Basically, the grey-correlation degree of acetic acid or ozone treatment decreased with increasing storage time. Compared with the control, ozone treatment during storage increased the comprehensive score of fresh-cut lotus roots, while acetic acid treatment decreased the comprehensive score. It could be concluded that fresh-cut lotus root stored for 4 days need not to be exposed to acetic acid or ozone, but 0.3 mg/L ozone could be used to preserve fresh-cut lotus root stored for more than 4 days to improve storage quality.