Effects of ultrasound-assisted plasma-activated water pretreatment combined with electrohydrodynamics on drying characteristics, active ingredients and volatile components of yam (Dioscorea opposita)

Abstract

This paper explores the effect of ultrasound (US) assisted plasma-activated water (PAW) or deionized water (DW) pretreatment combined with electrohydrodynamics (EHD) on the drying of yam. The activity characteristics of four pretreatments (plasma activated water combined with ultrasound (PAW + US), plasma activated water (PAW), deionized water combined with ultrasound (DW + US), and deionized water (DW) (control)) and their effects on drying characteristics, rehydration rate, color, reducing sugars, total phenols, infrared spectra, and volatile compositions of yam under EHD drying process were investigated. The results showed that the media pretreaded by ultrasound (US) combined with plasma-activated water (PAW) has lower media of pH (53.84 % lower than that of US + DW), higher nitrite ion concentration (311 times over US + DW), higher oxidation reduction potential (50.58 % higher than that of US + DW), and higher electrical conductivity (99.29 times over US + DW). And ultrasonic pretreatment (US) combined with plasma-activated water (PAW) drying resulted in faster drying, better rehydration rate, higher brightness L * (18.35 % higher than that of US + DW) and whiteness (1.1 % higher than that of US + DW), and retention of more reducing sugars (10.96 % higher than that of US + DW) and total phenols (14.04 % higher than that of US + DW), and a higher variety and content of volatile components. This provides an experimental and theoretical basis for the application of ultrasonic (US) combined with plasma-activated water (PAW) pretreatment to electrohydrodynamic drying.

1. Introduction

As a medicine and food homologous crop, yam has been widely noticed in the medical field and food field in recent years [1]. While, yam is a root food with a long history and belongs to the family of Dioscoreaceae, its pulp is hard, delicate and sweet, rich in protein, allantoin, reducing sugar, total phenol and other nutrients, which has the efficacy of treating asthma and autoimmune disorders, relieving diarrhea, promoting digestion and absorption, and facilitating the secretion of bodily fluids [2], [3]. However, due to the high water content, seasonality, and short shelf life of fresh yam, it is easy to cause mechanical damage, microbial contamination, decay, and nutrient loss, which results in serious economic losses. Drying is the most efficient way to extend the shelf life of produce.

Electrohydrodynamic (EHD) drying is a new non-thermal drying technology developed in recent years [4], and is a more promising method of drying yam [5], [6], whose main principle is the uneven electric field and the combined effect of ionized air [7], which has the advantages of low energy consumption, low temperature, fast drying speed, retaining natural color, uniform texture and high retention of bioactive compounds [5], [8], [9]. Martynenko et al. studied EHD drying of grapes and found that grapes dried using EHD had better sensory and quality properties than those dried using hot air drying [10]. Esehaghbeygi et al. found that bananas dried using EHD had higher rehydration capacity, appearance and quality characteristics than those dried using microwave drying [11]. Martynenko et al. found that EHD drying increased the drying rate of apples and mushrooms [12]. Sriariyakul et al. utilized EHD drying for drying aloe vera mud, and the results showed that EHD drying could significantly shorten the drying time and increase the drying rate [13]. However, EHD drying also has some disadvantages and needs to be improved. When the moisture content of the material is low, the drying speed decreases significantly, which greatly limits the application of EHD in drying [14].

In order to make improvements in drying technology, a number of different pretreatment programs can be used prior to drying the material. Jiang et al. found that the combination of freezing and ultrasonic pretreatment maintained the quality of dried strawberries in terms of total phenols, anthocyanins, increased vitamin C content, antioxidant activity and a* value [15]. Zhang et al. found that infrared combined hot air drying (IR-HAD), under the same drying temperature, the time required for IR-HAD is 31.25 ∼ 38.1 % less than HAD, and the drying rate of IR-HAD is more than 1.56 times of that of HAD [16]. Ultrasound (US) is a pretreatment that can be used to accelerate heat and mass transfer and improve the quality of the dried product [17], [18]. The physical changes induced by the US process are mainly attributed to cavitation, microjets and the sponge effect. These mechanisms help prevent temperature-induced nutrient loss. During US pretreatment, microchannels are formed and cavitation effects occur at liquid–liquid or liquid–solid interfaces. The cavitation interface generates bubbles, and the bursting of these bubbles impacts the surrounding liquid, which leads to the formation of microjets [19]. The results of Mirzaei-Baktash et al. showed that US pretreatment is positively affecting drying by promoting water transport with higher retention of bioactive compounds [9]. Zhang et al. demonstrated that US pretreatment enhanced the microstructure of ginger slices and also shortened the drying time, enhanced flavor and enzyme inactivation [5], [20]. Salehi et al. found that US pretreatment reduced the water loss of carrot slices and had higher brightness and redness parameters than untreated samples [21]. Salehi et al. demonstrated that US-pretreated dried fruit and vegetable slices had higher antioxidant activity, higher total carotenoid content and anthocyanin compounds, higher porosity and rehydration capacity, lower shrinkage, and brighter colors [22]. US pretreatment can also be combined with other means of drying materials, achieving better results than traditional pretreatment methods [23], which can significantly improve drying efficiency and retain nutrients [24], [25].

Plasma activated water (PAW) contains a variety of reactive oxygen species (ROS) and reactive nitrogen species (RNS) particles. These substances can effectively inactivate food-related pathogenic microorganisms and enzymes (e.g. polyphenol oxidase, catalase) that cause deterioration of food quality at room temperature, thus extending shelf life [26]. Zhang et al. found that the active substances in PAW could etch the seed epidermis and increase the hydrophilicity of the seeds, which induced the seeds to absorb more water and stimulating the growth of hypocotyls and radicles, thus improving seed germination rates [19]. Wang et al. found that PAW had an effect on bean sprout seed germination rate, sprout growth, nutrient and functional composition, microbial safety and postharvest quality of bean sprouts [27]. PAW has shown great potential for removing pesticide residues and extending the shelf life of fresh food during food processing, and its low-cost, green, non-polluting and sustainable features are in line with the trend of the development of the food industry [27]. Royintarat et al. found that PAW and US reduced E. coli K12 and Staphylococcus aureus counts in chicken breast meat by 1.33 and 0.83 log CFU/mL, respectively, and were higher than the individual treatments [28]. Zuo et al. found that the combined use of PAW and US pretreatment techniques promotes the transformation of starch materials towards functionalization and enhanced properties [29]. Wang et al. found that PAW was particularly effective in disinfecting Salmonella typhimurium in grass carp when used in combination with US [30]. However, so far, no plasma-activated water pretreatment, especially in terms of ultrasonic-assisted water activation, has been reported to be applied to the field of food drying.

Therefore, this paper uses ultrasonic-assisted plasma-activated water pretreatment combined with electrohydrodynamics to carry out drying experiments on fresh yam, to study the drying characteristics of yam, the changes of active and volatile components, and to provide experimental and theoretical bases for the application of ultrasonic-assisted plasma-activated water pretreatment technology in the field of food drying.

2. Materials and methods

2.1. Experimental material

2.1.1. Source material

Fresh yam was purchased fresh, intact and uniformly shaped from a fresh food supermarket near Inner Mongolia University of Technology in Hohhot, Inner Mongolia. The fresh samples were stored in a refrigerator at −4 °C for no more than 7 days before the experiment.

2.1.2. Chemical reagents and instruments

The chemical reagents used in this study include: distilled water, gallic acid standard solution, FC color developer, sodium carbonate solution, glucose standard solution, 3.5-dinitrosalicylic acid reagent, and ethanol solution.

The instruments used in this experiment included: electrohydrodynamic system (including a high-voltage power supply, a voltage controller (YD(JZ), Wuhan, China), and a multi-needle plate electrode system), ultrasonic cleaner (KQ-300DE, Kunshan, China), thermohygrometer (GJWS-A2, Tianjin, China), 3nh colorimeter (3nh-nr60cp, Shenzhen, China), electronic balance (BS124S, Shanghai, China), constant temperature water bath (DK-600BS, Jiangsu, China).

2.2. Experimental content and methodology

2.2.1. Preparation and characterization of PAW

An electrohydrodynamic system and deionized water were used to prepare activated water (PAW), and the flow is shown in Fig. 1. The electrohydrodynamic system mainly consists of a high-voltage power supply, a voltage controller (YD (JZ), Wuhan, China), and a multi-needle plate electrode system. The high-voltage power supply can output alternating current (AC) voltage and is connected to the voltage controller, and the voltage range is adjustable from 0 to 50 kV. The upper electrode in the multi-needle plate electrode system is a multi-needle electrode connected to the high voltage power supply. The length and diameter of each needle were 20 mm and 1 mm, respectively, and the distance between two consecutive needles was 40 mm. The lower electrode was a metal plate, grounded, with an area of 1000 mm × 450 mm. The distance from the tip of the multi-needle needle to the electrode of the metal plate was 80 mm. 100 mL of ultrapure water was added to a glass petri dish and then placed in an electrohydrodynamic system for discharge plasma activation treatment. The power supply output frequency was 50 Hz, the output voltage was 30 kV, the working gas was air, and the discharge plasma activation time was 1 h. The pH value was measured by a pH meter (S210, Shanghai) with an accuracy of 0.01. A conductivity meter (DDS-307A, Shanghai) was used to determine the conductivity with an accuracy of 0.01 µS/cm. The redox potential was measured using a redox potentiostat (S210, Shanghai) with an accuracy of 0.01 mV. The absorbance was measured by a UV–Vis spectrophotometer (NanoDrop™One, USA) to determine the nitrite content.

Fig. 1.

Schematic diagram of the experimental process.

2.2.2. Pretreatment experiments

In this study, yam was pretreated using four methods: plasma activated water, deionized water, ultrasonic-assisted plasma activated water, and ultrasonic-assisted deionized water. The pretreated yam slices were put into the electrohydrodynamic system for drying and the experimental flow is shown in Fig. 1. The specific parameters and methods are: (1) Combined pretreatment of PAW and US: PAW with yam slices was put into an ultrasonic cleaner (KQ-300DE, Kunshan, China) for pretreatment, with an ultrasonic power of 180 W, a temperature of 30 °C, and a treatment time of 30 min. (2) PAW pretreatment: Yam slices were soaked in PAW at 30 °C for 30 min. (3) Combined pretreatment of DW and US: The DW with yam slices was put into a medium ultrasonic cleaner for pretreatment. The ultrasonic power was 180 W. The temperature was 30 °C. And the processing time was 30 min. (4) DW pretreatment: Yam slices were soaked in DW for 30 min at 30 °C. To ensure the reliability and accuracy of the results, each measurement was repeated three times.

2.2.3. Drying experimental method

Fresh yam was removed from the outer skin and cut into cylindrical slices with a diameter of 12 mm and a height of 3 mm. Pretreatment was carried out utilizing the four methods described above. A rapid moisture detector (SH10A, Shanghai, China) was used to detect the initial wet-base moisture content of the yam, and the result was 82 ± 0.87 %.

The pretreated yam slices were placed into an electrohydrodynamic (EHD) system for drying as shown in Fig. 1. The electrohydrodynamic (EHD) system is the same equipment used to make PAW. The experiments were carried out in a drying environment with a drying temperature of 15 ± 3 °C, a relative humidity of 30 ± 2 %, and a natural wind speed of 0 m/s. The samples were dried at a temperature of 15 ± 3 °C and a relative humidity of 30 ± 2 %. The drying experiment was terminated when the wet basis moisture content of the sample reached 10 %. The drying characteristics, color, rehydration rate, total phenols of reducing sugars, infrared spectra and volatile constituents of yam officinale were analyzed.

2.3. Determination of dry basis moisture content and moisture content ratio

The dry base moisture content and moisture content ratio of yam slices during drying are defined as follows [1]:

$$M_i=\frac{m_i-m_g}{m_g}$$ (1)

$$\mathit{MR}=\frac{M_i-M_e}{M_0-M_e}$$ (2)

where m_g is the dry mass of the yam slices, m_i is the mass of the yam slices when they were dried to moment i, M_i is the dry basis moisture content of the yam slices when they were dried to moment i, MR is the moisture ratio of the yam slices, and M_e is the equilibrium dry basis moisture content of the yam slices. The equilibrium moisture content M_e was not evaluated during the drying process of the yam slices due to the fact that the equilibrium moisture content of the food ingredients was generally low, and therefore the value of M_e was considered to be zero, and M₀ was the moisture content of the yam slices at moment 0 [31]. The formula for calculating the moisture content ratio can be simplified as:

$$\mathit{MR}=\frac{M_i}{M_0}$$ (3)

2.4. Determination of drying rate

The drying rate of yam is defined as follows [32]:

$$\mathit{DR}=\frac{{M_t-M}_{t+\mathrm{\Delta }t}}{\mathrm{\Delta }t}$$ (4)

where DR is the drying rate, M_t is the dry basis moisture content of the yam slices at time t, M_{t + Δt} is the dry basis moisture content of the yam slices at time t + Δt.

2.5. Determination of dry effective moisture diffusion coefficient

The effective diffusion coefficient of moisture during drying of yam slices was calculated using Fick's second law with the following equation [33]:

$$\frac{\mathit{dM}}{\mathit{dt}}=D_{\mathit{eff}}\frac{d^{2}M}{d{r}^2}$$ (5)

For prolonged drying processes MR < 0.6. Thus, Eq. (5) can be expressed as:

$$\mathit{MR}=\frac{8}{{\pi }^2}\mathit{exp}\left(-\frac{{\pi }^2{D}_{\mathit{eff}}t}{4L^2}\right)$$ (6)

where D_eff is the effective water diffusion coefficient of yam. L is half the thickness of the sample. Taking logarithms on both sides of the above equation. The following equation is obtained:

$$\mathit{ln}\left[MR\right]=-\frac{{\pi }^2{D}_{\mathit{eff}}}{4L^2}t+\mathit{ln}\left[\frac{8}{{\pi }^2}\right]$$ (7)

D_eff can be derived from the relationship between ln[MR] and time, The slope in Eq. (7) is as follows:

$$\text{k=}-\frac{{\pi }^2{D}_{\mathit{eff}}}{4L^2}$$ (8)

The value of D_eff can be calculated from Eq. (8) [34].

2.6. Determination of rehydration rate

The dried yam slices were placed in a beaker and 100 mL of ultrapure water was added to the beaker, which was immersed in a constant temperature water bath at 37 °C for rehydration experiments, and rehydration of the yam was considered to be completed when the weight of the yam no longer changed. The samples were removed from the beaker and the excess water on the surface of the yam slices was blotted with absorbent paper and weighed. The formula for calculating the rehydration rate was [35]:

$$\mathit{RR}=\frac{m_a}{m_b}$$ (9)

RR is the rehydration rate of yam; m_a is the mass after rehydration; m_b is the mass before rehydration.

2.7. Measurement of color difference

Direct measurement of the color change of the surface of yam slices before and after drying using an automatic colorimeter (3nh-NR60CP, Shenzhen, China), includes brightness (L*), redness (a*), and yellowness (b*) values of the surface of the yam. The colorimeter was corrected for blackboard and whiteboard before testing to ensure the normal use of the colorimeter. In the process of testing, measurements were taken at 5 non-repeating locations on the surface of the same sample, the results are averaged. In order to determine the extent of the overall color change of the dried slices of yam, the physical quantity of color difference (ΔE) was introduced for the analysis, and its expression is as follows [36]:

$$\mathrm{\Delta }E=\sqrt{{\left(L_i^{\ast }-L_0^{\ast }\right)}^2+{\left(a_i^{\ast }-a_0^{\ast }\right)}^2+{\left(b_i^{\ast }-b_0^{\ast }\right)}^2}$$ (10)

where L₀*^, a₀* and b₀* are the brightness, redness and yellowness values of fresh yam slices respectively, L_i*^, a_i* and b_i* are the values of brightness, redness and yellowness of the dried yam slices respectively. ΔE is the overall degree of color change of the sample after drying.

The whiteness of the dried yam is also one of the most important indicators for evaluating the color of yam slices, and the whiteness is calculated as [2]:

$$\mathit{Whiteness}=100-\sqrt{{\left(100-L^{\ast }\right)}^2+{\left(a^{\ast }\right)}^2+{\left(b^{\ast }\right)}^2}$$ (11)

where L*, a* and b* are the brightness, redness and yellowness values of the dried yam slices, respectively.

2.8. Determination of reducing sugar content (RSC)

The determination of reducing sugar content of yam was based on the method of Chen et al. with improvement [20]. The specific methods were as follows:

• (1)Make glucose (reducing sugar) standard curve: Take the glucose (reducing sugar) standard solution with the concentration of 1 mg/mL and 3,5-dinitrosalicylic acid reagent and add it to the test tube, shake it well, heat it up in a pot of boiling water for 5 min. Remove it into a beaker containing cold water and cool it down to room temperature, and then fix the volume to the scale of 25 mL with distilled water, and adjust the absorbance with the tube No. 0 at the wavelength of 540 nm. At 540 nm wavelength, adjust the zero with tube 0 and read the absorbance. Take the absorbance as the vertical coordinate and the mass of glucose (reducing sugar) as the horizontal coordinate, plot the standard curve and obtain the regression equation.
• (2)Determination of reducing sugar: Weigh 3 g of edible flour, put it in 150 mL beaker. Mix it into paste with a little distilled water, put it in 50 °C constant temperature water bath for 20 min, so that the reducing sugar can be leached out. Filtration, wash the residue with 20 mL distilled water, collect all the centrifuged supernatant or filtrate in a 100 mL volumetric flask, and then mix it well, as the reducing sugar solution. Color development and colorimetry: Take three 25 mL graduated test tubes, numbered, add 2 mL of reducing sugar test solution and 1.5 mL of 3,5-dinitrosalicylic acid reagent, the rest of the operation was the same as that of making the standard curve, and then the absorbance of each tube was measured. At last, check the mass of the corresponding reducing sugar on the standard curve, and then substitute it into the formula.

$$\mathrm{R}\mathrm{S}\mathrm{C}=\frac{\mathrm{a}\times {\mathrm{V}}_1/{\mathrm{V}}_2}{\mathrm{m}\times 1000}\times 100\%$$ (12)

where a is reducing sugar content from regression equation, unit: g/kg; v₁ is the volume of reducing sugar solution to be measured, unit: mL; v₂ is the volume taken for color development, unit: mL; m is the mass of sliced yam, unit: g.

2.9. Determination of total phenol content (TPC)

The determination of total phenolic content of yam with the colorimetric method of forintol colorimetric by Zhang et al. and its improvement [19]. The specific method was as follows: the yam samples dried by different pretreatment methods were crushed and sieved through a 60-mesh sieve for spare parts. Then 0.20 g of sample was weighed and 50 mL of distilled water was added. The temperature was maintained at 100 °C in a boiling water bath for 30 min, removed, allowed to cool and then the volume was fixed and the filtrate was filtered for later use. 2.5 mL of extract was taken in a 50 mL centrifuge tube and 30 mL of 60 % ethanol solution was added. After ultrasonication for 10 min, the volume was adjusted to 40 mL with 60 % ethanol solution. 1.0 mL of the filtrate (if the total phenol content is too high, it can be appropriately diluted) or gallic acid monohydrate standard solution, respectively, with a graded pipette to add 2.5 mL of FC colorant (diluted by adding 1 times distilled water). 2.5 mL of 15 % sodium carbonate solution was added and fixed to 10 mL with distilled water, mixed well and 40 °C water bath for 60 min, and left to cool for 20 min. The absorbance at 778 nm was measured, and the total phenol content of the sample was calculated according to the dilution factor and the concentration on the standard curve. The formula for total phenols in yam was as follows:

$$\mathrm{T}\mathrm{P}\mathrm{C}=\mathrm{C}\mathrm{N}\frac{0.001{\mathrm{V}}_{\mathrm{t}}}{\mathrm{m}{\mathrm{V}}_{\mathrm{s}}}$$ (13)

where C is the total phenol content from regression equation, unit: μg/g; N is the dilution factor; V_t is the total volume of the extraction solution, unit: mL; V_s is the amount of liquid used in the measurement, unit: mL; m is the mass of sliced yam.

2.10. Fourier transform infrared spectroscopy and determination of protein secondary structure

Infrared spectroscopy was carried out by taking 130 mg of potassium bromide and 1.3 mg of dried yam slices (the mass ratio of dried iron stick slices to potassium bromide was 1/100), grinding them in an onyx mortar to less than 2 μm and mixing them well, the powder is then pressed into clear flakes using a tablet press. The pressed transparencies were placed in a FTIR spectrometer (Nicolet IS 10, USA) to scan the samples, scanning range of 400–4000 cm⁻¹, 32 scans at 4 cm⁻¹ resolution, removal of water and carbon dioxide interference, acquisition of scanning spectral images.

Determination of the secondary structure of proteins: Scanning spectral images were obtained to analyze the amide I spectra in the range of 1600–1700 cm⁻¹ by baseline adjustment, gaussian inverse plethysmography, second-order derivatives, and curve fitting using PeakFit software (San Rafael, USA), obtaining the protein secondary structure of yam [37].

2.11. Determination of volatile components

Headspace solid-phase microextraction coupled with gas chromatography (HS-SPME-GC–MS) was used for the determination of volatile components of dried products of yam [16]. Weighing 5 g of the dried sample (the mass difference between samples was not more than 3 %) was placed in a 20 mL headspace vial, and after incubation at a temperature of 50 °C for 3 min. The sample was inserted into the headspace vial for solid-phase microextraction for 40 min, and then the separation and identification were carried out by gas chromatography-mass spectrometry (GC–MS) after de-attachment at 250 °C for 5 min.

For gas chromatography-mass spectrometry (GC–MS), the column was DB-wax (30 m × 0.25 mm × 0.25 μm), the temperature of the injection port was 250 °C, the split ratio was 10:1, and the carrier gas was high-purity helium at a flow rate of 1.00 mL/min. The temperature-rise procedure was as follows: hold at 40 °C for 5 min, rise to 120 °C at 5 °C/min, and rise to 24 °C at 10 °C/min. The temperature of the ion source was 220 °C, the interface temperature was 250 °C and the scanning range was 30–500 m/z.

2.12. Statistical analysis

All experiments were conducted three times and final data were obtained by averaging the results of each group and expressed as mean ± standard deviation. Differences between groups were determined by one-way analysis of variance (ANOVA) using SPSS software. Differences between means were determined at a significance level of 0.05. The data obtained were normalized and after processing, heat maps were done using origin software to analyze the correlation between the different pretreatments and the various indicators. In addition the different indicators were subjected to Pearson correlation analysis to see the correlation between the different indicators.

3. Results and discussion

3.1. Characterization of different pretreatment media

Fig. 2 shows the parameters of pH, nitrite ion concentration, oxidation reduction potential (ORP), and electrical conductivity (EC) in the DW, PAW, DW + US, and PAW + US groups. pH was used to reflect the degree of acidification of PAW. As can be seen in Fig. 2, the pH values of DW and DW + US groups were at 6.83 ± 0.09 and 7.04 ± 0.07, respectively. The pH values of PAW and PAW + US groups were lower at 3.44 ± 0.07 and 3.25 ± 0.07, respectively. The plasma activated water lowers the pH, ultrasound has a small effect on the pH value within the medium. Nitrogen oxides (NO_x) generated from nitrogen and oxygen in the air during plasma discharge were dissolved in the water and reacted with water molecules to form HNO₂ and HNO₃, which causes the pH of the plasma activated water to decrease. Ikawa et al. found that hydrogen ions generated by hydrolysis of water molecules during plasma-activated water preparation can also cause a decrease in pH value [38].

Fig. 2.

Pretreatment media index parameter. Different letters indicate significant differences between sample means (p < 0.05).

Nitrite concentration refers to the level of nitrite ion (NO₂⁻) in a particular system, and is a common nitrogen substance [39]. From Fig. 2, it can be seen that the nitrite concentration from high to low is PAW + US > PAW > DW > DW + US, which is 3.11 ± 0.08 mg/kg, 3.05 ± 0.08 mg/kg, 0.02 ± 0.01 mg/kg, 0.01 ± 0.01 mg/kg, respectively. Ultrasound has a small effect on the concentration of nitrite within the medium. Chou et al. found that during plasma discharge, nitrogen in the air is ionized, producing a variety of reactive nitrogen substances such as NO, NO₂, NO₂⁻, NO₃⁻, and so on [40]. The combination of these reactive particles with water increases the nitrite concentration within the PAW, which in turn is higher than the DW.

ORP is an important indicator to characterize the redox capacity of aqueous solution, reflects the oxidizing power of an aqueous solution and the concentration of oxidizing agent in it [40]. As can be seen in Fig. 2, the ORP of DW group and DW + US group was lower at 51.67 ± 1.25 mV and 51.40 ± 0.10 mV, respectively. The ultrasonic waves had less effect on the ORP within the medium. The ORP of PAW + US group and PAW group was higher at 77.40 ± 0.89 mV and 477.77 ± 0.72 mV, respectively. Aaliya et al. found that the discharge of reactive oxygen species (ROS) particles during PAW preparation caused an increase in ORP [41].

EC reflects the nature and concentration of reactive ions in solutions treated with electrohydrodynamic systems, the higher the concentration of charged ions in solution, the higher the conductivity of the solution [42]. As can be seen in Fig. 2, the EC in the DW, DW + US, PAW, and PAW + US groups were 4.41 ± 0.11 μS/cm, 3.76 ± 0.14 μS/cm, 324.3 ± 1.28 μS/cm, and 373.33 ± 8.62 μS/cm, respectively. Xu et al. found that a large number of active particles were dissolved in water during the preparation of plasma activated water, leads to an increase in conductivity [43]. Tabtimmuang et al. found that ultrasound was able to promote normally difficult or slow chemical reaction processes within crude glycerol [44]. These may have contributed to the highest EC values within the PAW + US group.

3.2. Drying characteristics

Fig. 3 shows the drying characteristics as well as the rehydration rate of the yam under different pretreatments. Fig. 3(a) reflects the effect of different pretreatment methods, changes of water content with drying time in yam. It can be seen that PAW + US pretreatment had the lowest water content and the best drying efficiency. It was followed by DW + US, in the next was PAW and lastly DW (control). This indicates that PAW combined with US can be more effective in improving drying efficiency, this is because PAW pretreatment improves drying efficiency and US pretreatment promotes the interaction of PAW with the yam chips, which leads to an increase in the rate of decrease in moisture content and improves drying efficiency.

Fig. 3.

Drying characteristics and rehydration capacity. (a): Variation curve of moisture content with drying time; (b): Plot of average drying time vs. average drying rate used for different pretreatment methods; (c): Effective water diffusion coefficients of yam under the effect of different pretreatment methods; (d): Rehydration capacity. Different letters indicate significant differences between sample means (p < 0.05).

Fig. 3(b) shows the plot of average drying time versus average drying rate for different pretreatment methods. The drying rate mainly depends on the electric field strength and the nature of the dried material, especially the free moisture initially available on the surface of the dried material [17]. It can be seen that the average drying time of PAW is shorter than that of the DW group by 2.5 h, and the average drying rate is increased by 57.45 %, which indicates that the PAW pretreatment can effectively shorten the drying time and increase the drying rate. The average drying time of PAW + US is shorter than that of the DW + US group by 2.5 h, and the average drying rate is increased by 74.67 %. This shows that ultrasound assisted plasma activated water pretreatment can effectively shorten the drying time and increase the drying rate.

Fig. 3(c) shows the effect of different pretreatment methods on the effective water diffusion coefficient within the yam. As seen in Fig. 3(c), the effective water diffusion coefficients of the other three pretreatment groups with different conditions were higher than those of the control group (DW), which were PAW + US > PAW > DW + US > DW in descending order. Among them, PAW + US was 1.25 times higher than that of the DW + US group, which indicated that the combination of the pretreatment of PAW and US could significantly increase the effective water diffusion coefficient. And PAW was higher than DW group, indicating that PAW pretreatment alone also led to a significant increase in the effective water diffusion coefficient, which also favored the volatilization of water inside the yam.

The ultrasound combined with plasma activated water group had the fastest average drying rate, highest effective water diffusion coefficient. Tripathy et al. found that PAW pretreatment weakened the cell wall network of Centella asiatica leaves and produced more intracellular interstitial spaces and pores on the surface structure of the leaves, which contributed to water diffusion during its drying process [45]. During PAW pre-processing, the interaction of reactive particles (RONS) with materials increase its intracellular interstitial space, number of cavities and micropores, which led to changes in physical and chemical properties, altered the microstructure of the yam slices, transfered of water from the sliced yam, improved drying effect. When ultrasonic waves penetrate materials containing liquid media, it can cause a series of rapid alternating compression and expansion (sponge effect), which created micro-channels within the material, accelerated the flow of water molecules within the material, increased drying rate [19].

3.3. Rehydration capacity

Rehydration capacity is an important index used to evaluate dried products, and the rehydration capacity reflects the degree of damage to the internal structure of the material [5]. For some dry foods, agricultural or other products, rehydration capacity reflects their ability to return to close to their original state (e.g., taste, texture, appearance, etc.) after reabsorbing water. Dry products with good rehydration capacity can better maintain their original characteristics after the rehydration process, and provide consumers with a better experience. Fig. 3(d) shows the effect of different pretreatment methods on the rehydration capacity of yam slices whose rehydration capacity is closely related to the changes in microstructure, which determines the macroscopic properties of yam. From the Fig. 3(d), there was a significant difference in the rehydration capacity of the different pretreatment methods (p < 0.05). The rehydration rates of PAW + US, PAW, DW + US, and DW were, in order, 2.16 ± 0.03, 1.92 ± 0.09, 1.61 ± 0.04, and 0.96 ± 0.03, respectively. The rehydration rates of PAW + US were higher than those of PAW, DW + US and DW, indicating that both ultrasound and plasma activated water pretreatment can improve the rehydration ability of yam slices Ultrasonic assisted plasma activated water pretreatment can significantly improve the rehydration ability of yam, with the best effect.

3.4. Color difference

The color of food products can largely influence consumers' first impression and purchase intention [46]. Table 1 shows the changes of surface color parameters before and after drying of yam, which was pretreated with different methods and then dried. High-quality dried yam slices should have higher brightness and whiteness, lower redness, yellowness, and color difference. Corresponding to the data and pictures in Table 1. It can be clearly seen that the brightness and whiteness were higher in the PAW + US group compared to the DW + US group at 62.1 ± 0.107 and 59.70 ± 0.023. And the redness, yellowness and color difference were lower at 0.16 ± 0.01, 12.84 ± 0.005, and 9.94 ± 0.008, respectively. Cavitation of the US pretreatment facilitated the penetration of PAW into the yam cell membranes, resulting in superior color parameters for the yam. PAW also had higher brightness and whiteness (62.04 ± 0.001 and 58.97 ± 0.076), and lower redness and yellowness (0.28 ± 0.01 and 13.42 ± 0.009), when comparing PAW with the DW group. This suggests that the reactive particles in the PAW may have played a role as a color protector.

Table 1.

Color changes of yam slices before and after drying under different pretreatment methods.

Pretreatment Methods	L*	a*	b*	ΔE	Whiteness
PAW + US	62.1 ± 0.107a	0.16 ± 0.01bc	12.84 ± 0.005ab	9.94 ± 0.008b	59.70 ± 0.023a
PAW	62.04 ± 0.001ab	0.28 ± 0.01a	13.42 ± 0.009a	11.76 ± 0.006a	58.97 ± 0.076ab
DW + US	52.47 ± 0.006bc	0.27 ± 0.007c	13.68 ± 0.011c	12.03 ± 0.048a	59.05 ± 0.007a
DW	52.7 ± 0.03b	0.36 ± 0.005b	15.47 ± 0.06ab	7.06 ± 0.014c	55.45 ± 0.012b

Note: Different letters indicate significant differences between sample means (p < 0.05). L*: luminance value; a*: redness value; b*: yellowness value; ΔE: total color difference; Whiteness: whiteness value; PAW + US: Plasma activated water combined with ultrasonic pretreatment group; PAW: Plasma activated water immersion group; DW + US: Deionized water combined with ultrasonic pretreatment group; DW: Deionized water immersion group.

3.5. Infrared spectroscopy and protein secondary structure

Fig. 4(a) shows the FTIR spectra of the yam under different pretreatment conditions, which were used to determine the effect of different pretreatment methods on the absorption bands of the yam slices. The broad and strong peaks appearing near 4000–3000 cm⁻¹ are N–H, O–H and C–H stretching vibrations; the peaks appearing at 3000–2000 cm⁻¹ correspond to the stretching vibrations including methyl, methylene, etc. which are C C, C N; the typical protein bands are concentrated at the vicinity of 2000–1300 cm⁻¹, and there are amide I, amide II, and amide III groups which are C C, C O, and C N stretching vibrations; in the range of 900–500 cm⁻¹, polysaccharides are mainly present [28]. Yam contains a large proportion of protein, and protein is a macromolecule whose secondary structure directly determines its properties, so it is particularly important to explore the protein secondary structure of yam slices.

Fig. 4.

Infrared spectra and protein secondary structure of yam slices under different pretreatment conditions. (a): Infrared spectra of yam under different pretreatment methods; (b): Proportion of the secondary structure constituting the protein of yam.

As can be seen in Fig. 4(a), under different pretreatment conditions, the peak positions of the infrared spectra are the same, but the peak heights are different, indicating that pretreatment does not change the type of material composition during the drying process, but can change the content of the ingredients. Broader peaks near 3274 cm⁻¹, the energy band intensities (% transmittance, T) of PAW + US, DW, and PAW pretreated dried yam were significantly lower than those of DW + US pretreated dried yam. In studying the effect of physicochemical modification of yam starch on FTIR spectral lines, a broad band of –OH stretching on starch due to hydrogen bonding is also observed near this spectral line by Falade et al. [47]. Near 2930 cm⁻¹, the transmittance of yam was found to be DW + US > PAW + US > DW > PAW in descending order. The transmittance of yam slices under different pretreatments was found to be PAW > DW + US > PAW + US > DW in descending order near 1726 cm⁻¹ and 1625 cm⁻¹. The wave intensities of the dried yam slices under different pretreatment were found to be PAW > DW + US > PAW + US > DW in the order of range of wave numbers around 1200 cm⁻¹ was significantly lower. The intensity of absorption peaks around 1000 cm⁻¹ was in the order of strongest to weakest as DW > PAW + US > DW + US > PAW. Candoğan et al. also observed fingerprinting characteristic peaks below 1000 cm⁻¹ and concluded that this region mainly contains characteristic bands of proteins, lipids, phospholipids, and nucleic acids produced by a wide range of biomolecules [48].

The transmission spectra in Fig. 4(a) were analyzed by baseline adjustment, Gaussian inverse convolution, second-order derivatives, and curve fitting for amide I spectra in the range of 1600–1700 cm⁻¹ using the appropriate software. The peak positions were used to quantitatively analyze the changes in secondary structure ratios. The α-Helix is located at 1651–1664 cm⁻¹, the β-Sheet at 1609–1637 cm⁻¹ and 1680–1689 cm⁻¹, the β-Turn at 1665–1680 cm⁻¹, random coil at 1637–1648 cm⁻¹. The content of different structures in the secondary structure of proteins was statistically derived, and the results obtained are shown in Fig. 4(b). From Fig. 4(b), it can be seen that random coil and β-Sheet are the main forms of secondary structure of the protein of yam slices. Higher levels of β-Turn in PAW + US vs. PAW groups, the DW + US and DW groups had lower levels of β-Turn. There was no significant difference in the amount of random coil and α-Helix in any of the four pretreatment groups. Ultrasound assisted plasma activated water pretreatment can cause changes in the secondary structure of proteins within the material.

3.6. Reducing sugar and total phenol content

The experimental results of total phenol and reducing sugar content determination are plotted from Fig. 5. Reducing sugars are a class of carbohydrates with reducing properties, mainly includes all monosaccharides with aldehyde or ketone groups in their molecular structure and some disaccharides with aldehyde groups in their molecular structure [41]. As can be seen in Fig. 5, the reducing sugar content of PAW + US and DW + US groups were 113.3 ± 2.259 g/kg and 102.73 ± 2.356 g/kg, respectively, and PAW + US was 1.1 times higher than DW + US group. This indicates that PAW combined with US pretreatment can promote the production of reducing sugars. This is because US pretreatment can lead to an increase in the content of reducing sugars, and with the basis of US pretreatment, coupled with the active substances in PAW may also have a promotional effect on the content of reducing sugars, which will ultimately lead to an increase in the content of reducing sugars. The content of reducing sugars in the PAW and DW groups were 129.5 ± 1.672 g/kg and 99. 98 ± 2.373 g/kg, respectively, and were higher in the PAW group than the DW group was higher. This indicates that PAW pretreatment also has the effect of promoting reducing sugar production.

Fig. 5.

Total phenols and reduced sugar content. Different letters indicate significant differences between sample means (p < 0.05).

Phenolic compounds in food exhibit significant antioxidant capacity. The main reason for the antioxidant, antibacterial, antitumor, and immune-enhancing properties of yam is its richness in total phenolic [30]. As seen in Fig. 5, the total phenol content of PAW + US and DW + US groups was 0.65 ± 0.01 g/kg and 0.57 ± 0.01 g/kg, respectively, which was 14.04 % higher in PAW + US than in DW + US group. This indicates that PAW promotes the synthesis or release of phenolic compounds. US also causes phenolic compounds to be released due to the disruption of cellular structure. Ordóñez-Santos et al. in their study on the effect of ultrasonication on the juice of currants from Cape Town mentioned that the increase in phenolic compounds in the samples treated with ultrasonication could be due to a greater disruption of the cell wall, which promotes the release of bound phenolics into the cell wall of the pectin, cellulose, hemicellulose and lignin traces [49]. Therefore, ultrasonic-assisted plasma-activated water pretreatment can improve phenolic compounds. It can also be seen that the PAW group had higher levels than the DW group, 0.78 ± 0.012 g/kg and 0.54 ± 0.01 g/kg, respectively, which suggests that PAW pretreatment also has the effect of increasing phenolic compounds.

3.7. Volatile components of yam under different pretreatment conditions

Drying after different pretreatments, the volatile components of this process are mainly derived from lipid oxidation and degradation, meladic reactions, interaction of oxidized lipids with amino acids or proteins, and vitamin degradation [19]. According to the NIST database, combined with retention time, a total of 43 volatile components were identified, including 10 alkanes, 8 aldehydes, 4 alcohols, 7 ketones, 3 esters, 2 olefins, 3 benzenes, and 4 other substances. From Fig. 6(a), it can be seen that the distribution of the number of volatile components is as follows: alkanes > aldehydes > ketones > alcohols > olefins > benzenes > lipids.

Fig. 6.

Volatile components. (a): Amounts of different volatile compounds in the slices of yam. Different letters indicate significant differences between sample means (p < 0.05). (b): Content of volatile compounds in yam under different pretreatments.

The types of volatile components in yam under different pretreatment conditions were similar, but the amount of each volatile component was different, as shown in Fig. 6(b). Nonanal, decanal, pentanal-, 4-methylhexanal, benzoin aldehyde, 2-nonenal, (E)-, 2,3,7-trimethyldecane, 3,7-dimethyldecane, (Z)-3-nonen-1-ol, 1-octen-3-ol, 2-methyl-3-heptanone, butylated hydroxytoluene, 1,2,4,5-tetramethylbenzene, and 1,2,3,4-tetramethylbenzene were the major yam tablets' volatile compounds. It can be seen that aldehydes are the most abundant volatile compounds, followed by benzenes, alcohols and ketones.

Fig. 7(a) shows the fingerprints of volatile substances in yam under different pretreatment methods, which were established based on the parameters of peak retention time and peak area of volatile substances. There were significant differences in the fingerprints of volatile substances in yam under different pretreatment methods. And the total content of each volatile substance was different. The contents from high to low were: aldehydes > benzenes > alcohols > ketones > alkanes > other > esters > olefins. From Fig. 7(a), it can be seen that the peaks of PAW + US are higher than those of DW + US group, and the peaks of PAW are higher than those of DW group. This indicates that not only ultrasound-assisted plasma-activated water pretreatment can promote the generation of volatiles, but also activation water pretreatment alone has the effect of increasing the generation of volatiles.

Fig. 7.

Fingerprint mapping, OPLS-DA plot. (a): Fingerprints of volatile components of yam under different pretreatment conditions; (b): OPLS-DA in slices of yam under different pretreatment; (c): Model cross-validation results.

Supervised discriminant modeling (OPLS-DA) was used to perform multivariate statistical analysis of volatile components in yam slices under different pretreatments. The relationship between volatile components and different pretreatment was modeled [50]. In Fig. 7(b) of the OPLS-DA model, the independent variable fit indicator R_x² = 0.983, the dependent variable fit indicator R_y² = 0.998, and the model prediction indicator Q² = 0.996. R² indicates the degree of fit of the model to the data, and Q² denotes the predictive effect of the model to the new data, which are all more than 0.5, indicating that the model's fitting results are acceptable. From the Fig. 7(b), it can be seen that the yam under different pretreatments were all within the confidence interval with good repeated aggregation. In order to further validate the model fitting results, the cross-validation model was validated using the 200 substitution test. As shown in Fig. 7(c), the intersection of the Q² regression line with the vertical axis is less than 0, indicating that the model is not overfitted and the validation is valid, and the results are considered to be useful for analyzing the discriminant analysis of yam under different pretreatments, which is similar to that of Wang et al. [20].

The types of volatile compounds detected in yam were few, but their contents were concentrated, and their odors, contents, and forms were carefully analyzed. Aldehydes were the main volatile components in the pretreatment process of yam, and the content of aldehydes was higher in each pretreatment method. Aldehydes are volatiles derived from lipids and have strong fatty, fruity and spice odors [51]. Aldehydes volatiles play a very important role in the overall volatile composition of dried yam [16]. A total of eight aldehydes were identified under different pretreatment conditions, and the most important volatile components were nonanal (greasy and sweet orange flavor) and decanal (rotten orange flavor); both components are colorless and transparent liquids because aldehydes mainly originate from oxidative degradation of lipids or degradation of amino acids induced by the Merad reaction, and they are an important aromatic component [52]. Among the eight aldehydes detected, the highest aldehyde content was found in the PAW + US treatment group, which indicates that ultrasound-assisted plasma-activated water pretreatment has the effect of increasing aldehyde content and is more conducive to the synthesis of aldehyde volatiles.

Under different pretreatment conditions, benzenes were second only to aldehydes in the yam. A total of three benzene volatiles were detected. Butylated hydroxytoluene, which is an unscented, odorless and tasteless volatile compound and is a white powder at room temperature. The content of PAW + US was 70.97 % higher than that of the PAW group. This suggests that the combination of PAW and US pretreatment can promote the generation of butylated hydroxytoluene, which is a kind of substance. 1,2,3,4-tetramethylbenzene, which has a camphor-like odor, is a colorless or white crystal, and the content in the DW + US group was significantly higher than that in the other groups and was 5.77 times higher than that in the DW group. 1,2,4,5-tetramethylbenzene, which has a relatively distinctive, light, aromatic odor, was colorless and liquid in the DW + US group, and the content of 1,2,4,5-tetramethylbenzene in the DW + US group was 5.77 times higher than that of the other groups. Liquid, was significantly higher in the DW + US group than in the other groups and was 3.64 times higher than in the DW group. This suggests that this combined pretreatment of DW + US served to promote the production of these aldehyde volatiles. Therefore, appropriate pretreatment methods can be more favorable to the formation of volatile benzene compounds in the dried products of yam.

Under different pretreatment conditions, identification of four alcoholic volatiles from yam. Alcohols generally have more specific floral and fruity aromas [50]. Mainly by reduction of the corresponding aldehydes [53]. (Z)-3-nonen-1-ol gives the herbaceous or greasy odor to yam. 1-Octen-3-ol, which has the aroma of mushrooms, lavender, roses, and hay, is a colorless to yellowish liquid in its pure state and is an indicator of lipid oxidation in a variety of food products [54]. 1-Decanol has a candelilla, sweet, floral and fruity aroma, as well as a strong, pungent odor, and is a colorless, viscous liquid It is a colorless viscous liquid, and when solidified, it is a leaf-shaped or rectangular plate crystal. 1-Octanol has a special aromatic odor, and can be used in the spice industry for the preparation of spices, and it is a colorless transparent oily liquid. In summary, the alcohols in the yam are colorless oily liquids. The compound has a subtle fresh herbal aroma, a weak pungent oily flavor, and also a somewhat clean and soft lavender floral and fruity aroma. All four compounds were significantly higher in the DW + US group than in the other groups, so for these alcohols, US combined with DW pretreatment played a big role.

Ketones are formed through autoxidation, β-oxidation and decarboxylation of unsaturated fatty acids [55]. Because ketones have a low sensory threshold, the overall aroma produces some effect. Despite the low content of ketone volatiles in yams under different pretreatment methods, they can still give them butter and blue cheese flavors. A total of seven ketone volatile compounds were identified in the yam. 2-Methyl-3-heptanone has a fruity-like odor and may have some slightly sweet and fruity notes as a colorless liquid. 5–10-Methoxy-2,7,7,10-tetramethylundeca-2,5-diene-4-one has a slightly fruity or floral aroma. 3-(hydroxyphenylmethyl)-2,2,3-trimethyl-octan-4-one has a slightly fruity or floral odor. 2,2-dimethyl-4-hexen-3-one has a crisp melon or fresh grassy aroma and also has a somewhat pungent and distinctive chemical odor, and is usually a colorless liquid. 1-(2-Methylphenyl)ethyl ketone has a mixture of floral and chemical notes, and is usually a colorless to pale yellow liquid. In summary, the ketone volatile compounds may have a slight fruity and floral aroma or may be difficult to distinguish due to the mixture of flavors. However, the PAW + US group contained very few ketones, probably because the combination of the active substances in PAW and US prevented the production of ketones.

Alkanes are classified as saturated hydrocarbons and can be produced by thermal decarboxylation of saturated fatty acids, oxidative decomposition of highly unsaturated fatty acids and free radical reactions in fatty acyl chains [19]. A total of 10 alkanes were detected. Propylcyclopropane content of PAW + US is 1.4 times as large as that of the DW + US. 2,2-dimethylbutane content of PAW + US is 2.9 times more than that of DW + US. 1-iodononane content of PAW + US was 2.01 times higher than that of DW + US. Among these alkanes, propyl cyclopropane, 2,2-dimethylbutane, 2-cyclopropylbutane, 2,3,7-trimethyldecane, and 1-iodonononane, all of which are the most abundant in the PAW + US group. This indicates a significant increase in the amount caused by ultrasound assisted plasma activated water pretreatment.

Other volatile compounds, among which 2-pentylfuran has a variety of beneficial effects, such as antimicrobial, analgesic, antimutagenic, antiplatelet, antiallergic, antitussive, anti-inflammatory and antioxidant properties [56]. 3-Ethoxy-3-methyl-1-butyne may have a certain irritating and chemical reagent specific odor. 2,2-Dimethylpropyl-2,2-dimethylpropylsulfinylsulfoxide may be white to light yellow solid crystals or colorless to light yellow liquid at room temperature and atmospheric pressure, the specific form may be affected by temperature, pressure and purity of factors such as the effect. The final n-hexyl disulfide is a colorless to light yellow liquid with a peculiar odor. The content of these compounds in the four groups is not significant at high or low levels.

Under different pretreatment conditions, there were three types of ester volatile compounds in the yam. Esters have a citrusy, fruity, almond-like aroma [57]. Ethyl hexadecanoate has a soy sauce aroma and a weak fat-like odor, in the form of a colorless liquid or white solid [5]. 2-Ethylhexyl salicylate has a faint aromatic odor, possibly with some resemblance to a more refreshing herbaceous scent, and is a colorless to pale yellow liquid. Methylnonylphosphonofluoric acid phosphonate is a more complex organic compound, and its odor has not been reported. However, all three esters were reduced in the PAW + US group. This decrease is mainly due to the acidity of PAW, which hydrolyzes the ester compounds, as found by Wu et al. [5].

As another important class of hydrocarbons, olefins are formed by the reaction of one unsaturated alkane radical with the combination of another unsaturated alkane radical or hydrogen radical [19]. A total of two olefins were identified in this work, namely styrene and 2,4,4,6,6,8,8-heptamethyl-1-nonene. Among them, styrene has a unique aromatic odor, and its content of PAW + US is 3.92 times higher than that of DW + US. And 2,4,4,6,6,8,8-heptamethyl-1-nonene, whose odor is currently uncertain. Moreover, the olefin content of the PAW + US group was higher than that of the other groups. This proves that ultrasound-assisted plasma-activated water pretreatment has a promotional effect on the generation of olefinic compounds.

In summary, ultrasound-assisted plasma-activated water pretreatment can lead to a significant increase in the content of volatile substances (aldehydes, benzenes, alkanes, olefins), and make it possible for the yam to emit a fresh, fat-like, fruity, and floral flavor. There are two main reasons for this phenomenon, one is due to the expansion and collapse of cavitation bubbles by the acoustic-pressure cycle of ultrasound, which leads to the rapid rupture of the bubbles and the generation of very high local pressures and temperatures, and when the cavitation bubbles collapsed near the walls of the plant tissues, the strong-induced microfluidic flow would puncture the cell walls and cause them to be disrupted, which promotes a more efficient extraction of the volatile compounds. Another is that ultrasound can increase the mass transfer rate and speed up the contact between the activated water and the yam slices, thus delivering the solution, such as the highly active factor RONS, to the slices quickly and affecting their quality, which may lead to elevated content of volatile components. In conclusion, the analysis of the volatile components of yam can prove the advantages of ultrasound-assisted plasma-activated water as a method, and this method deserves more in-depth research.

3.8. Statistical analysis

All the data (average drying time, average drying rate, rehydration rate, color difference value, effective diffusion coefficient of moisture, reducing sugar, total phenol) of yam under different pretreatment conditions were normalized and correlation analysis was carried out. Fig. 8(a) shows the correlation thermograms of the effects of different pretreatment methods on the drying indexes of the yam, where dark blue is positively correlated and yellow is negatively correlated. As you can see from the graph, most notably, the average drying rate and average drying time, rehydration capacity, brightness (L*), whiteness, effective moisture diffusion coefficient, and total phenols in the PAW + US group have the largest positive correlation values, which means they are highly correlated, and all of them are significantly higher. The redness value (a*), the yellowness value (b*), and the total color difference (ΔE) have the largest negative correlation values. So, this proves that the combined use of ultrasound-assisted plasma-activated water as drying pretreatment is effective for yam.

Fig. 8.

Statistical analysis. (a): Correlation thermogram between different pretreatment methods and drying indexes. (b): Summary graphs of drying rate, reducing sugars, total phenols, and volatile constituents of yam under different pretreatment conditions. (c): Cyclic thermograms of different volatile components of yam under different pretreatment conditions, A: Propylcyclopropane; B: styrene; C: Butylated hydroxytoluene; D: Phosphonate methyl nonyl fluoride; E: 3-(Hydroxyphenyl)-2,2,3-trimethyloctan-4-one; F: 2-(1,1-dimethylethyl)-3-ethylethylene oxide; G: 2-Cyclopropylbutane; H: 4-Methylhexanal; I: Pentadehyde-; J: 2,3,7-Trimethyldecane; K: decanal; L: 2-Ethylhexyl salicylate; M: 1,1,3,5-Tetramethylcyclohexane; N:2,2,4,4,4,4,5,5,7-Octamethyl-3,6-dioxa-2,4,5,7-tetrasilaneoctane.

Fig. 8(b) shows the normalized summary plots of total drying rate, reducing sugars, total phenols, and volatile constituents of yam under different pretreatment conditions. From this, we can see that, the PAW + US group had the best efficiency and highest content of total drying rate, reducing sugars, total phenols, volatile components. Followed by the PAW group, followed by the DW + US group, and finally the DW group. It can be seen that ultrasonic-assisted plasma-activated water pretreatment in these aspects can make the food drying effect has a significant improvement. And these results provide some theoretical basis for drying after US combined PAW pretreatment. This method improves the traditional drying method and achieves the effect of low energy consumption and high efficiency, which is of renewed significance for improving the quality attributes of dried products in the food industry.

Fig. 8(c) shows the cyclic thermograms of different volatile components of yam under different pretreatment conditions. The data obtained from the experiments were normalized, screening of 14 differentially volatile compounds (VIP > 1, P < 0.05). This graph uses shades of color to indicate correlation, with dark purple being a positive correlation and light pink being a negative correlation. Decanal, 2,3,7-trimethyldecane, 4-methylhexanal, 2-cyclopropylbutane, butylhydroxytoluene, styrene, and propylcyclopropane positively correlate maximally in the PAW + US group. 2,2,4,4,4,4,4,5,5,7-Octamethyl-3,6-dioxa-2,4,5,7-tetrasilanyloctane, 1,1,3,5-tetramethylcyclohexane, 2-ethylhexyl salicylate, 2-(1,1-dimethylethyl)-3-oxirane, 3-(hydroxyphenyl)-2,2,3-trimethylethyloctan-4-one, and n-ethylnonanoylfluoride, a methylphosphonate positively correlate maximally in the PAW group. 2-Ethylhexyl salicylate, pentanal-, butylated hydroxytoluene, and propyl cyclopropane positively correlated with maximum in the DW + US group. The positive correlation between 4-methylhexanal and 2-cyclopropylbutane was greatest in the DW group. And it is clear that the PAW + US and PAW groups have more number of positive correlations than the DW + US and DW groups.

4. Conclusions

In this study, a novel method of ultrasound-assisted plasma-activated water pretreatment combined with electrohydrodynamic drying was proposed, and compared it with the unpretreated control group. The results show that, plasma-activated water lowers the pH inside the medium and increases the concentration of nitrite ion concentration, oxidation reduction potential, electrical conductivity. Compared to the control group, ultrasonic-assisted plasma-activated water pretreatment resulted in significant enhancement of drying characteristics, rehydration rate, color, total phenols, reducing sugars, and volatile compositions of yam. In summary, compared to conventional drying technology, ultrasonic-assisted plasma-activated water pretreatment combined with electrohydrodynamic drying technology embodies a number of advantages, to achieve better drying results, for the drying field opens up a new pathway.

CRediT authorship contribution statement

Wurile Bai: Writing – original draft, Visualization, Validation, Methodology, Conceptualization. Peng Guan: Data curation, Visualization, Validation. Jiaqi Liu: Data curation, Visualization, Validation. Junjun Lian: Data curation, Visualization, Validation. Zhiqing Song: Conceptualization, Methodology. Hao Chen: Conceptualization, Methodology. Ru Xing: Supervision, Writing – review & editing. Jingli Lu: Conceptualization, Project administration, Resources, Supervision, Writing – review & editing. Changjiang Ding: Conceptualization, Project administration, Resources, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.