Microwave irradiation: impacts on physicochemical properties of red wine

ABSTRACT

Microwave irradiation is considered as a potential alternative technology for promoting the progress of wine aging. The effects of microwave irradiation on some important physicochemical properties of young Cabernet Sauvignon dry red wine were studied. The results indicated that there were significant changes in total phenolic compounds (TPC), total monomeric anthocyanin (TMA), titratable acidity (TA), and DPPH-free radical scavenging activity (DFRSA), while pH and electrical conductivity (EC) hardly changed. DFRSA was correlated to TPC, TMA, and TA during microwave treatment. The changing trend of chromatic characteristics (CC), which induced by microwave irradiation, was consistent with that of aging red wine. The results of principal component analysis showed that there was a big difference between the untreated red wine and the treated red wine under different microwave conditions. The results suggested that microwave technology could change some physicochemical properties of red wine and promote the aging of red wine.

RESUMEN

Se considera que la irradiación con microondas es una tecnología alternativa potencial para promover el proceso de añejamiento del vino. Con esta premisa se estudiaron los efectos de la irradiación por microondas en algunas propiedades fisicoquímicas importantes del vino tinto seco joven Cabernet Sauvignon. Los resultados obtenidos indicaron que se produjeron cambios significativos en los compuestos fenólicos totales (TPC), la antocianina monomérica total (TMA), la acidez valorable (TA) y la actividad de eliminación de radicales libres del DPPH (DFRSA), mientras que el pH y la conductividad eléctrica (EC) apenas se modificaron. Se constató que durante el tratamiento con microondas la DFRSA se correlacionó con los TPC, la TMA y la TA. La tendencia de cambio de las características cromáticas (CC), inducida por la irradiación con microondas, es coherente con la del añejamiento del vino tinto. Los resultados del análisis de los componentes principales dieron cuenta de que, bajo diferentes condiciones de microondas, existe una gran diferencia entre el vino tinto sin tratar y el vino tinto tratado. En este sentido apuntan a que la tecnología de microondas podría alterar algunas propiedades fisicoquímicas del vino tinto, promoviendo de esta forma su añejamiento.

KEYWORDS:

• Red wine
• microwave irradiation
• physicochemical properties
• principal component analysis (PCA)
• aging

PALABRAS CLAVE:

• vino tinto
• irradiación de microondas
• propiedades fisicoquímicas
• análisis de componentes principales (PCA)
• añejamiento

1. Introduction

Grape is one of the most valued fruits in the world (García-Lomillo & González-SanJosé, 2017), about 75% of the grapes are utilized into winemaking (Zhu et al., 2015), global wine production is about 27 billion liters per year (Amienyo et al., 2014). From then on, wine consumption is increasing over the years and, along with that, the wine production technology has been drawing attention. Generally, most young red wines are then matured in oak barrels for a few months to several years depending on the variety of grapes and style of wine desired during wine aging (Amienyo et al., 2014). Owing to the increasing of wine consumption, oak barrels, and oak cellar increase year by year. The lengthiness of aging process results in high cost, large quantities of oak barrels occupied, potential microbial contamination, and long cycle for the wine production. To accelerate the aging process and shorten the production cycle of wine, some innovative physical aging technologies have been studied, such as ultrasound, pulsed electric fields, gamma radiation, high pressure (Tao et al., 2014), micro-oxygenated (Alamo-Sanza et al., 2019), wood chips aging (Del Alamo Sanza & Nevares Domínguez, 2006; Gortzi et al., 2013), and microwave aging (Zheng et al., 2011), that these methods in laboratory try to short the vinification time and effectively promote taste quality of the fresh wine. However, there is still a long way to go for the application of these new artificial technologies in the actual production of red wine, making the process of wine aging becomes more efficient and more economical.

Microwave is a technology applied in many food processes to reduce processing time (Guo et al., 2017). It is known that the high-frequency electromagnetic waves destabilize weak hydrogen bonds by enhancing the rotation of polar molecules (R. Li et al., 2014; Liu et al., 2017), which would induce the formation of free radicals (Elias et al., 2009; Thostenson & Chou, 1999). Free radicals are thought to be a key factor in the oxidation of wine. The existence of free radicals in winemaking process makes changes even more complicated during aging red wine (Elias et al., 2009), which may shorten the aging duration. Microwave aging wine may benefit to re-combination of insoluble molecules and speeding up the alcoholization process (Elias et al., 2009), improve wine sensory properties (Zheng et al., 2011), advantage to aging process (C. Li et al., 2010), and accelerate certain necessary chemical reactions for the wine aging process (Lin & Lin, 2000). On the other hand, potential microbial contamination frequently occurred during wine aging process, SO₂ was added into the must or vats to controlled microbial contamination. Microwave sterilization can substantially reduce the microorganisms in foods (Guo et al., 2017), so microwave irradiation is able to destroy microorganisms dangerous for the wine quality when microwave aging young wine, as well as minimizing of SO₂ use (Carew, 2013a; Carew et al., 2013b). In this sense, the application of microwave technology reduces human costs and environmental pollution. So microwave irradiation may become an efficient and feasible artificial wine aging solution.

The main research of microwave technology in red wine is to extract phenolics from grape must (Carew et al., 2014) or grape wastes (Casazza et al., 2010), but there is little research on the aging of young red wine by microwave irradiation. Therefore, before evaluating the potential of microwave-assisted wine aging, the effects of microwave on the physical and chemical properties of wine under different microwave conditions should be studied, which is usually used to determine the quality of wine. As the most important operating parameters, the effects of microwave time, power, and temperature on red wine should be evaluated. In this study, a commercial Cabernet Sauvignon red wine was subjected, the effects on wine physicochemical properties, including total phenolic compounds (TPC), total monomeric anthocyanin (TMA), pH, titratable acidity (TA), electrical conductivity (EC), DPPH-free radical scavenging activity (DFRSA), and chromatic characteristics (CC) were evaluated. The main purpose of this study was to explore whether microwave could actively change the physical and chemical parameters of young red wine and the optimal operating conditions for microwave treatment of young red wine.

2. Materials and methods

2.1. Wine sample

Cabernet Sauvignon Red Wine was used in this study. The young red wine was provided by Louis Langyong Winery (Shaanxi Province, China). The alcoholicity of red wine was 12.0% (v/v) provided by the manufacturer.

2.2. Chemicals and reagents

DPPH, rutin, Folin-Ciocalteu reagent, and gallic acid were obtained from Sigma-Aldrich Co. (St. Louis, MO, USA). Sodium carbonate, potassium dihydrogen phosphate, and sodium hydroxide were purchased from Tianli Chemical Reagent Co. Ltd. (Tianjin, China). All other chemicals and reagents used were of analytical grade.

2.3. Microwave irradiation

A microwave synthesis was used as the microwave reactor in this study. The synthesis, XH-MC-1 (Xiang Hu science and Technology Development Co., Ltd, Beijing, China), was basically a rectangular container with a temperature probe and a bottom magnetic stirring device, its maximum power output is 900 W, its frequency is 2450 MHz, its temperature is manually controlled by the start button and the pause key. During the heating process, a hot spot and cold spot were appeared with the difference of electromagnetic field intensity, the volume of the system was decreasing during microwave heating, so closed magnetic stirring system of liquid sample was used for a microwave temperature uniformity and maintaining a constant sample volume. Low-temperature coolant circulating pump (Gongyi Yuhua Instrument Co., Ltd, Henan, China) provides −20°C coolant, a cylindrical cup is divided into two layers, the inner layer is a microwave treatment solution, the outer layer passes through the coolant, ensuring microwave energy was continuously absorbed by the treated sample.

Three sets of experiments were performed. Firstly, the effect of different microwave irradiation time (5, 10, 15, and 20 min) was investigated, being the microwave treatments performed at 60°C with 500 W. In parallel, another set of experiments was carried out with 500 W microwave irradiation at different temperature (40°C, 50°C, 60°C, and 70°C, respectively) for 15 min. Finally, in order to investigate the effects of microwave power on wine, different power was assayed including 100, 300, 500, 700, and 900 W. For this set of experiments, microwave irradiation time and temperature were 15 min and 60°C, respectively. CK was the untreated red wine with microwave irradiation. Experiments were performed in triplicates.

2.4. Analytical determinations

2.4.1. Determination of total phenolic compounds

TPC was determined using the proposed method by Rapisarda et al. (1999) with some minor modifications. Briefly, 200 μL of sample, 3.6 mL of deionized water and 0.2 mL of Folin-Ciocalteu reagent were mixed and let stand for 1 min. Afterwards, 6 mL of 5% sodium carbonate solution was added to the above mixture. The resulting mixture was incubated for 60 min in the dark at room temperature and then the absorbance was measured at 765 nm using an N4 UV/Vis Spectrophotometer (Shanghai INESA Scientific Instruments Co., Ltd., China). The results were expressed as gallic acid equivalents per liter of wine. All analyses were performed in triplicates.

2.4.2. Determination of total monomeric anthocyanin

TMA in the extracts was determined using the method described by Lee et al. (2005) with some minor modifications. Briefly, pH 1.0 (KCL-HCL) buffer solution 3 ml, the sample (0.1 mL of red wine diluted with 0.9 mL of deionized water) placed for 110 min, then measure the absorbance at 520 and 700 nm, respectively. It is the same reaction condition at pH 4.5 (NaAc-HAc buffer). The results were expressed as mg/L of cyanidin-3-glucoside (cyd-3-glu) equivalents, as follows:

$C=\frac{A\times MW\times DF\times 1000}{\epsilon \times 1}$

where A = (A_520 nm-A_700 nm) pH1.0-(A_520 nm-A_700 nm) pH4.5; MW (molecular weight) = 449.2 g/mol for cyd-3-glu; DF = dilution factor established in D; l = pathlength in cm; ε = 26 900 molar extinction coefficient, in L× mol^–1× cm^–1, for cyd-3-glu; and 1000 = factor for conversion from g to mg.

2.4.3. Determination of EC, pH, and TA

The electrical conductivity (EC), pH, and titratable acidity (TA) of samples were measured by a DDS-11A Conductivity Meter (Shanghai INESA Scientific Instruments Co., Ltd., China), a ST 3100 pH Meter (Shanghai Ohaus Instruments Co., Ltd., China), and a ZDJ-4B Automatic Potentiometric Titrator (Shanghai INESA Scientific Instruments Co., Ltd., China), respectively. The TA was expressed in the equivalent of tartaric acid content (g/L) (OIV-MA-AS313-01 method). All analyses were carried out in triplicates.

2.4.4. Determination of DPPH-free radical scavenging activity

The DFRSA of the samples was measured according to the method described by Yuan et al. (2014) with some minor modifications. Inhibition of the DPPH radical by wine samples was expressed as the percentage of DPPH decrease and was calculated according to the following formula:

DPPH-free radical scavenging activity ratio (%) = $\frac{\mathrm{A}0-\mathrm{A}1}{A1}\times 100$

where A₀ is the absorbance of the control (DPPH solution without sample), A₁ is the absorbance of the sample. DFRSA of each sample was determined in triplicates.

2.4.5. Determination of CC

Chromatic characteristics (CC) of wines were determined following the method of OIV-MA-AS2-07A with some minor modifications, by measurement of the absorbance with 10 times dilution wines at 420 nm (intensity of yellow), 520 nm (intensity of red), and 620 nm (intensity of blue) with quartz cuvette. Color characteristics of the samples were determined by measuring the absorbance of samples at 420, 520, and 620 nm, respectively. Color density (CD) was calculated by the sum of the absorbance at 420, 520, and 620 nm. Color hue (CH) was determined as the ratio of the absorbance at 420 to absorbance at 520 nm. CIELab coordinates, including lightness (L*), red/green color coordinates (a*), and yellow/blue color coordinates (b*) were determined by an SC-80 C automatic colorimeter (Beijing Kangguang instrument Co Ltd., China) following the recommendations of OIV-MA-AS2-11: R2006 method. The overall colorimetric differences between the microwave irradiation treated samples and the untreated wine (CK) (ΔE) were calculated as follows:

$\mathrm{\Delta }\mathrm{E}=[{\left(\mathrm{\Delta }\mathrm{L}\ast \right)}^2+{\left(\mathrm{\Delta }\mathrm{a}\ast \right)}^2+{\left(\mathrm{\Delta }\mathrm{b}\ast \right)}^2{]}^{1/2}$

where ΔL*, Δa*, and Δb* are the differences in the values of each color coordinate between the microwave irradiation treated sample and CK.

2.5. Statistical analysis

Principal component analysis (PCA) attempts to identify potential variables or factors that explain the correlation patterns between a set of observed variables. PCA was carried out with SPSS statistical software.

Statistical analysis was performed by one-way analysis of variance (ANOVA) using the SPSS statistics software version 25.0 (SPSS Inc., Chicago, IL, USA). Minimum significant difference test was performed on the data to determine the difference with statistical significance. The different average values of the same superscripts mean that there is no significant difference between them. PCA attempts to identify potential variables or factors that explain the correlation patterns between a set of observed variables. PCA was also carried out with the SPSS statistics software.

3. Results

3.1. Effects of microwave irradiation time on TPC, TMA, EC, pH, TA, CD, CH, and DFRSA

Physicochemical parameters of Cabernet Sauvignon red wines treated with different microwave irradiation time are shown in Table 1. The content of TPC, TMA, TA, and DFRSA in treated wines was lower than that of CK, TPC, TA, and DFRSA obviously decreased with the increasing of time. Regarding red wine color, CD was lower than CK, the CD slightly changed with the increasing of time; CH in treated samples was slightly higher than CK. Regarding EC and pH, few changes have been found, so it is unnecessary to make PCA of EC and pH.

Table 1. Effect of microwave irradiation time on physicochemical properties of wine.

Tabla 1. Efecto del tiempo de irradiación con microondas en las propiedades fisicoquímicas del vino

Microwave irradiation time (min)	CK	5	10	15	20
TPC (mg/L)	2098.7 ± 13.4^a	2085.26 ± 8.7^ab	2078.3 ± 10.6^ab	2060.2 ± 12.5^bc	2055.1 ± 9.7^c
TMA (mg/L)	44.9 ± 0.5^a	43.25 ± 0.7^b	43.9 ± 0.5^bc	43.3 ± 0.3^bc	42.7 ± 0.2^c
EC (ms/cm)	2.15 ± 0.02^a	2.14 ± 0.01^a	2.12 ± 0.02^a	2.12 ± 0.02^a	2.16 ± 0.02^a
pH	3.71 ± 0.01^a	3.72 ± 0.01^a	3.72 ± 0.01^a	3.72 ± 0.01^a	3.71 ± 0.01^a
TA (mg/L)	5.36 ± 0.03^a	5.32 ± 0.02^b	5.28 ± 0.01^c	5.22 ± 0.02^d	5.22 ± 0.01^d
CD	0.595 ± 0.003^ab	0.592 ± 0.001^ab	0.587 ± 0.005^b	0.578 ± 0.002^c	0.576 ± 0.001^c
CH	0.815 ± 0.002^a	0.836 ± 0.001^c	0.830 ± 0.001^b	0.831 ± 0.001^b	0.834 ± 0.001^c
DFRSA (%)	80.53 ± 1.37^a	78.28 ± 0.7^b	78.10 ± 0.33^b	77.47 ± 0.39^bc	76.59 ± 0.43^c

Note: Means ± SD (n = 3); different letters in the same row indicate significant differences (P ≤ 0.05).

Nota: Medias ± DE (n = 3); las distintas letras en la misma fila indican diferencias significativas (P ≤ 0.05).

PCA is used for exploratory data analysis of multivariate data to identify associations among elements that can be used to explain the physicochemical properties of Cabernet Sauvignon red wine and microwave irradiation. The method aims to find the maximum variation within the data by reducing its dimensionality and results in a linear combination of elements defined as principal components (PCs). The outcomes of PCA are commonly presented as a biplot of scores (score of every sample on PC1 and PC2) and loadings (loadings of every element on PC1 and PC2), whereas the positioning of loadings reflects microwave irradiation influence on physicochemical properties of red wine. In this study, PCA was performed by varimax rotation. The principal components (PC1 and PC2), which explained 89.6% of the variation of the exploratory data, were selected. The scores of Figure 1 were able to clearly differentiate the untreated wine (CK) from treated samples using different microwave irradiation time. In the loading of Figure 1, PC1 explains 48.3% of the variance and is positively related to the DFRSA, TPC, TA, and CD, while PC2 explaining 41.3% of the variance is defined by TMA and DFRSA in the positive axis and with CH in the negative axis.

Figure 1. Scatterplot of scores (a) and loadings (b) for PC1 and PC2 of samples treated at different microwave irradiation time and CK.

Figura 1. Diagrama de dispersión de las puntuaciones (a) y las cargas (b) para PC1 y PC2 de las muestras tratadas con diferentes tiempos de irradiación con microondas y CK

3.2. Effects of microwave irradiation power on TPC, TMA, EC, pH, TA, CD, CH, and DFRSA

Physicochemical parameters of Cabernet Sauvignon red wines treated with different microwave irradiation power are shown in Table 2. Microwave irradiation did not change the pH of wine, while the content of TPC, TMA, and TA in treated wines was lower than that of CK, no pattern was found about EC. The capacity of DFRSA obviously decreased with the increase of microwave irradiation power. Regarding wine color, CD of some treated samples was slightly lower than CK, CH slightly increased after microwave irradiation. Few changes in EC and pH have been found, so it is unnecessary to make PCA.

Table 2. Effect of microwave power on physicochemical properties of wine.

Tabla 2. Efecto de la potencia de las microondas en las propiedades fisicoquímicas del vino

Microwave power (W)	CK	100	300	500	700	900
TPC (mg/L)	2098.7 ± 13.4^a	2060.9 ± 7.4 ^c	2076.6 ± 10.3^bc	2065.2 ± 12.5^bc	2088.5 ± 9.7^ab	2054.8 ± 8.5^c
TMA (mg/L)	44.9 ± 0.5^a	44.2 ± 0.4^ab	43.9 ± 0.8^ab	43.3 ± 0.7^b	43.6 ± 0.7^b	44.1 ± 0.7^ab
EC (ms/cm)	2.15 ± 0.01^a	2.16 ± 0.02^a	2.15 ± 0.01^a	2.12 ± 0.02^a	2.14 ± 0.01^a	2.14 ± 0.01^a
pH	3.71 ± 0.01^a	3.71 ± 0.01^a	3.71 ± 0.01^a	3.72 ± 0.01^a	3.71 ± 0.01^a	3.71 ± 0.01^a
TA (mg/L)	5.36 ± 0.03^a	5.05 ± 0.01^d	5.13 ± 0.02^c	5.22 ± 0.02^b	5.05 ± 0.02^d	5.09 ± 0.03 ^cd
CD	0.595 ± 0.003^a	0.589 ± 0.001^b	0.584 ± 0.002^c	0.578 ± 0.001^d	0.557 ± 0.002^e	0.561 ± 0.001 ^f
CH	0.815 ± 0.002^a	0.826 ± 0.001^b	0.830 ± 0.001^c	0.831 ± 0.001^c	0.817 ± 0.001^a	0.824 ± 0.001^b
DFRSA (%)	80.53 ± 1.37^a	78.92 ± 1.05^ab	78.27 ± 0.76^bc	77.47 ± 0.39^bc	77.28 ± 0.68^bc	76.74 ± 0.99^c

Note: Means ± SD (n = 3); different letters in the same row indicate significant differences (P ≤ 0.05).

Nota: Medias ± DE (n = 3); las distintas letras en la misma fila indican diferencias significativas (P ≤ 0.05).

In this study, PCA was performed by varimax rotation. PC1 and PC2 were selected, which explained 74.9% of the variation of the exploratory data. The scores of Figure 2 were able to clearly differentiate CK from treated samples at different microwave irradiation power. In the loading of Figure 2, PC1 explains 43.1% of the variance and is positively related to the DFRSA, TMA, TA, and CD, while PC2 explaining 31.8% of the variance is defined by TPC in the positive axis and with CH in the negative axis.

Figure 2. Scatterplot of scores (a) and loadings (b) for PC1 and PC2 of samples treated at different microwave power and CK.

Figura 2. Diagrama de dispersión de las puntuaciones (a) y cargas (b) para PC1 y PC2 de las muestras tratadas con diferente potencia de microondas y CK

3.3. Effects of microwave irradiation temperature on TPC, TMA, EC, pH, TA, CD, CH, and DFRSA

Physicochemical parameters of Cabernet Sauvignon red wines treated with different microwave irradiation temperatures are shown in Table 3. Few variances were observed in pH and EC, while TPC, TMA, and TA in treated wines were lower than CK. Particularly about TA, there was no significant difference between different microwave temperature treatment groups. DFRSA decreased with the increasing of microwave irradiation temperature. Regarding wine color, CD of treated samples was obviously lower than CK, CH increased after microwave irradiation.

Table 3. Effect of microwave irradiation temperature on physicochemical properties of wine.

Tabla 3. Efecto de la temperatura de irradiación con microondas sobre las propiedades fisicoquímicas del vino

Microwave treatment temperature(°C)	CK	40	50	60	70
TPC (mg/L)	2098.7 ± 13.4^a	2075.9 ± 8.0^ab	2079.4 ± 14.8^ab	2065.2 ± 12.5^b	2053.3 ± 19.2^b
TMA (mg/L)	44.9 ± 0.5^a	44.2 ± 0.4^ab	43.8 ± 0.7^ab	43.3 ± 0.7 ^b	43.6 ± 0.7^b
EC (ms/cm)	2.15 ± 0.01^a	2.17 ± 0.01^a	2.14 ± 0.01^a	2.12 ± 0.02^a	2.15 ± 0.01^a
pH	3.71 ± 0.01^a	3.74 ± 0.01^b	3.71 ± 0.01^a	3.72 ± 0.01^a	3.71 ± 0.01^a
TA (mg/L)	5.36 ± 0.03^a	5.21 ± 0.01^b	5.24 ± 0.02^b	5.22 ± 0.02^b	5.21 ± 0.01^b
CD	0.595 ± 0.003^a	0.567 ± 0.003^cd	0.566 ± 0.001^d	0.578 ± 0.001^b	0.571 ± 0.003^c
CH	0.815 ± 0.002^a	0.825 ± 0.001^c	0.818 ± 0.001^b	0.831 ± 0.002^e	0.828 ± 0.001^d
DFRSA (%)	80.53 ± 1.37^a	78.05 ± 0.44^b	77.82 ± 0.35^b	77.47 ± 0.39^b	76.85 ± 0.84^b

Note: Means ± SD (n = 3); different letters in the same row indicate significant differences (P ≤ 0.05).

Nota: Medias ± DE (n = 3); las distintas letras en la misma fila indican diferencias significativas (P ≤ 0.05).

PCA goal is to analyze variance and reduce the observed variables, so there is no need to analyze pH and EC in PCA. In this study, PCA was performed by varimax rotation. PCA is analyzed using exploratory data of TPC, TMA, DFRSA, TA, CD, and CH to explain physicochemical properties of Cabernet Sauvignon red wine and microwave irradiation temperature. As shown in Figure 3, the total variance explained was 85.3%, the biplot of PC1 and PC2 captures 47.2% and 38.1% of the total variation of the data, respectively. PC1 and PC2 scores' diagram could discriminate between treated groups and CK well at different microwave irradiation temperatures. In the loading diagram, PC1 is defined by TPC, TMA, DFRSA in the positive axis and with CH in the negative axis, while PC2 is positively related to the DFRSA, TA, and CD.

Figure 3. Scatterplot of scores (a) and loadings (b) for PC1 and PC2 of samples treated at different microwave irradiation temperature and CK.

Figura 3. Diagrama de dispersión de las puntuaciones (a) y las cargas (b) para PC1 y PC2 de las muestras tratadas con diferentes temperaturas de irradiación con microondas y CK

3.4. Effects of microwave treatment on CIELab

As shown in Table 4, the L*, a*, and b* of microwave irradiation red wine changed significantly, except a* changed slightly under different microwave irradiation temperatures. In terms of microwave power, wine treated with 900 W had the highest L*, a*, and b* values and the highest ∆E* values as well. In terms of microwave irradiation time, the highest values of L*, a* and b* and ∆E* were exposure time of 20 min. For the treatment condition of microwave temperature, there was no obvious difference for each result on a* of the young red wine samples. The obvious color changes were not perceived because of the lower ∆E* values under different microwave irradiation temperatures, but the influence on TPC, TMA, and DFRSA was significant with the increasing of microwave temperature, the lower temperature was selected during red wine aging process, so microwave processing selected 40°C as a reasonable experimental parameter.

Table 4. Effects of microwave treatment on the chromatic characteristics of red wine.

Tabla 4. Efectos del tratamiento de irradiación con microondas en las características cromáticas del vino tinto

	L*	a*	b*	∆E
Microwave irradiation time
CK	30.88 ± 0.12^a	57.72 ± 0.15^a	22.66 ± 0.15^a	0.00
5 min	31.27 ± 0.14^b	57.55 ± 0.16^ab	23.76 ± 0.16^b	1.18
10 min	31.72 ± 0.1^c	57.41 ± 0.16^b	23.77 ± 0.07^b	1.43
15 min	31.57 ± 0.05^c	57.56 ± 0.12^ab	23.89 ± 0.11^b	1.42
20 min	32.03 ± 0.05^d	57.58 ± 0.09^ab	24.14 ± 0.16^c	1.88
Microwave power
CK	30.88 ± 0.12^a	57.72 ± 0.15^a	22.66 ± 0.15^a	0.00
100 W	33.14 ± 0.06^e	57.03 ± 0.27^b	24.34 ± 0.15^b	2.90
300 W	31.18 ± 0.09^b	57.01 ± 0.09^b	23.48 ± 0.09^b	1.13
500 W	31.57 ± 0.05^c	57.56 ± 0.12^a	23.89 ± 0.11^c	1.42
700 W	32.52 ± 0.07^d	56.53 ± 0.17 ^c	24.01 ± 0.07^d	2.43
900 W	33.76 ± 0.16^f	57.43 ± 0.37^a	24.65 ± 0.19^f	3.51
Microwave irradiation temperature
CK	30.88 ± 0.13^a	57.72 ± 0.15^a	22.66 ± 0.15^a	0.00
40°C	31.76 ± 0.07^b	57.79 ± 0.1^a	22.95 ± 0.11^b	0.93
50°C	31.76 ± 0.16^b	57.78 ± 0.21^a	23.02 ± 0.07^bc	0.95
60°C	31.57 ± 0.05^b	57.56 ± 0.12^a	23.89 ± 0.11^c	1.42
70°C	31.74 ± 0.08^b	57.59 ± 0.23^a	22.65 ± 0.15^a	0.71

Note: Means ± SD (n = 3); different letters in the same row indicate significant differences (P ≤ 0.05).

Nota: Media ± DE (n = 3); las distintas letras en la misma fila indican diferencias significativas (P ≤ 0.05).

3.5. Pearson correlation analysis

A bivariate correlation analysis was performed using all of the scale values, including TPC, TMA, EC, pH, TA, DFRSA, CD, CH, L*, a*, b*. Pearson’s correlation coefficients of data reaching statistical significance (p < .05) are shown in Table 5. When comparing the TPC with the TMA, TA, and DFRSA, the correlation coefficients were positive (range: 0.342 to 0.629), while TPC with the EC and CH, the correlation coefficients were negative (range: −0.370 to −0.367). Bivariate correlations between TMA and DFRSA were positive and highly statistically significant (p < .01; correlation coefficient = 0.643), while TMA and CH were negative and highly statistically significant (p < .01; correlation coefficient = −0.472). Bivariate correlations between pH and a* were positive (0.386). When comparing the TA with the DFRSA, CD, and a*, the correlation coefficients were positive (range: 0.332 to 0.696), while TA with the L* and b*, the correlation coefficients were negative and highly statistically significant (−0.664 and −0.509). The correlation between DFRSA and CD was positive and highly statistically significant (0.637). The correlation between CD and CH was positive (0.341), while the correlation CD and L* was negative (-0.398). The correlation between CH and b* was negative (0.345). The correlation between L* and b* was positive and highly statistically significant (0.720). The correlation between a* and b* was negative (-0.359). In particular, the L* and b* showed the strongest positive linear correlation (correlation coefficient = 0.720), followed by the TA and a* (0.696), DFRSA and TMA, CD, TPC (0.643, 0.637, 0.629), TA and CD (0.527). While the TA and L* showed the strongest negative linear correlation (−0.667), followed by the TA and b* (−0.509).

Table 5. Pearson correlation coefficients of the physicochemical properties of red wine.

Tabla 5. Coeficientes de correlación de Pearson respecto de las propiedades fisicoquímicas del vino tinto

	Pearson correlation coefficient (p-value)
Variable	TPC	TMA	EC	pH	TA	DFRSA	CD	CH	L*	a*	b*
TPC	1	0.430**	−0.367*	NS	0.342*	0.629**	NS	−0.370*	NS	NS	NS
TMA		1	NS	NS	NS	0.643**	NS	−.472**	NS	NS	NS
EC			1	NS	NS	NS	NS	NS	NS	NS	NS
pH				1	NS	NS	NS	NS	NS	0.386*	NS
TA					1	0.332*	0.527**	NS	−0.664**	0.696**	−0.509**
DFRSA						1	0.637**	NS	NS	NS	NS
CD							1	0.341*	−0.398*	NS	NS
CH								1	NS	NS	0.345*
L*									1	NS	0.720**
a*										1	−0.359*
b*											1.000

*Significant difference, P < 0.05; **Highly significant difference, P < 0.01; NS: Not significant (p > 0.05).

*Diferencia significativa, P < 0.05; **Diferencia altamente significativa, P < 0.01; NS: No significativa (p > 0.05).

4. Discussion

In a word, PCA results showed that the untreated wine was different from microwave-treated wines in the scores of Figures (1–3) under all analytical conditions. The effects of microwave in the analyzed physicochemical parameters are discussed in the following.

4.1. Effect of microwave on EC, pH, and TA of red wine

As mentioned earlier, different combinations of microwave power, time and treatment temperature have no significant effect on the EC of wine. Zhang et al. (2016b) found that the continuously monitored curve of EC was significantly different from the values recorded at the beginning and end of red wine with artificial aging technology; the main reason was the changing temperature of online monitoring EC, which was temperature-dependent electrical conductivity. No significant differences of EC were observed in microwave aging red wine might be attributed to a slight effect on nutrients. Regarding pH and TA, pH increased slightly, the change of TA is inversely related to the variation in pH. Microwave aging red wine could cause the decrease of TA and the increase of pH, the same results occur in other artificial aging wine and conventional aging wine (Zhang et al., 2017；Zhang et al., 2016a).

4.2. Effect of microwave on TPC and TMA of red wine

Phenolic compounds are useful to health as they play an important role in reducing the risk of many diseases which occurring to the oxidation in the human body (Duthie et al., 2006; Sun et al., 2017). Generally, conventional aging red wine is a natural and gradual oxidizing process involving oxygen reacting with the most readily oxidizable wine constituents, resulting in physicochemical changes involving antioxidants, astringency, bitterness, browning reactions, color, protein constituents, etc. (Zhang et al., 2016a). For the microwave aging red wine, complicated changes take place in wines because of the instantaneous high temperatures and polar molecules rotate at high speed. It is known that the high-frequency electromagnetic waves destabilize weak hydrogen bonds by enhancing the rotation of polar molecules (R. Li et al., 2014; Liu et al., 2017), which would induce the formation of free radicals (Elias et al., 2009; Thostenson & Chou, 1999). The existence of free radicals in aging red wine, which induced by microwave, makes changes even more complicated. In our research, different combinations of microwave power, time and treatment temperature have significant decreasing trends on the TPC of aging red wine, this phenomenon on decreasing of TPC was observed by other authors (García-Falcón et al., 2007) in bottle storage.

Anthocyanins are an important secondary metabolite in grape berries and wines, have important biological and biochemical activities (He et al., 2008). They provide color and antioxidant activity to grape berries and wines; the composition and content of anthocyanins play an important role in wine color stability (Han et al., 2019). In the research of microwave irradiation under different conditions on TMA, which has significant decreasing trends. Monomeric anthocyanins are able to interact with acetaldehyde, pyruvic acid, condensed tannins, and hydrolyzable tannins during winemaking process (Buchweitz et al., 2012); the decrease of monomeric anthocyanins suggests that new more stable compounds formed (Agazzi et al., 2018). The microwave irradiation could accelerate the winemaking process by monomeric anthocyanins react with other compounds to the progressive formation of new pigments allowing color stabilization. A similar trend was obtained by other authors with other artificial aging technologies (Burin et al., 2011; Gómez Gallego et al., 2013; McRae et al., 2012). The anthocyanins were decreasing with the increasing power, the microwave under 500 W not only accelerates the aging of red wine faster but also keeps more nutritional ingredient and antioxidant activity (such as TPC, TMA, and DFRSA), so microwave processing selected 500 W as a reasonable experimental parameter.

4.3. Effect of microwave on CC of red wine

The CC of red wine are judged the intrinsic quality of red wine and ultimately affect consumer satisfaction. The change of color during aging from red-purple to brick-red hues is attributed to the progressive formation of new pigments (Atanasova et al., 2002). The CD of red wine decreasing and CH of red wine increasing were induced with microwave irradiation, similar chromatic changes were previously reported in conventional and artificial wine aging process (Aadil et al., 2013; García Martín & Sun, 2013). Regarding CIELab parameters, L*, a quantitative component of the color clearly increases was caused by different microwave conditions. García Martín and Sun (2013) and Heras-Roger et al. (2014) have reported increase of L* during red wines aging process. The coordinate b* also increased during young red wine treated with microwave irradiation, García Martín and Sun (2013) reported similar results with conventional aging technology on red wine. The coordinate a* slight decreased under different microwave power and microwave time, while there was no significant change under different microwave temperatures. The results showed that microwave temperature had little effect on the color of red wine, which is regarded as a promising result. The phenolic composition of red wine affects its color, so the aforementioned changes of phenolic compound could explain the tiny changes of CC. Red wine producers are currently looking for technologies that enhance the redness of wine, avoiding the browning that occurs during aging. Finally, ΔE was calculated in order to assess whether the color changes by microwave irradiation. Increasing of microwave treatment time makes the deeper degree of red wine, so microwave processing selected 20 min as a reasonable experimental parameter of the artificial aging red wine.

4.4. Effect of microwave on DFRSA of red wine

Red wine polyphenols are mainly flavonoid and non-flavonoid compounds, the flavonoid including anthocyanins, flavonols, flavanols, and proanthocyanidins (Monagas et al., 2005). The study described the high correlation between phenolic composition and the antioxidant capacity in red wine; anthocyanins are the main component and have a significant contribution to the capacity of antioxidant (Lingua et al., 2016). The Pearson’s correlation coefficient results of TPC and TMA are positive linear correlation to DFRSA (correlation coefficients are 0.629 and 0.643), our results are in accordance with other findings (Fernández-Pachón et al., 2004; Lucena et al., 2010). The DFRSA decreased with the increase of microwave time, power, and temperature, the decreasing of DFRSA was same as the natural wine aging. So, microwave-treated young red wine is the process of accelerating the aging red wine.

5. Conclusions

Microwave treatment performed at different microwave powers, times, and microwave temperatures significantly changed the results of TPC, TMA, TA, and DFRSA of the red wine, while no significant impact on pH and EC was detected. Small differences in CC were also observed, the colorimetric difference in relation to the TA, TPC, and TMA. The effect of PCA on the physicochemical properties of wines confirmed that microwave irradiation modified the properties of red wine and wines were effectively divided into two groups: the microwave-treated wines and the untreated wine. PCA also can identify associations among elements that can be used to explain the physicochemical properties of Cabernet Sauvignon red wine and microwave irradiation. The current study proved that microwave irradiation can change some physicochemical properties of red wine. Microwave power and irradiation time are two such factors, which influence each other to a great extent. Lower microwave temperature and longer time may be wise approaches. Based on the results obtained in this article, the conditions suggested for the application of microwave in red wine processing are 500 W power, temperature equal to 40°C and 20 min exposure time. In the processing of aging red wine, microwave irradiation technology can be efficiently used to reduce aging time and changed the physicochemical properties of aging wine. The change of individual compounds by HPLC has not yet been studied, the physicochemical properties of aging red wine using the microwave irradiation are still worth studying and the mechanism of aging red wine is still further studied to better employ this novel technique.

Acknowledgments

We thank Dr. AG Xie, Dr. ZJ Qiu, and Dr. B Zhang at Institute of College of Food and Bioengineering, Henan University of Science and Technology for the constructive suggestion.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The project was supported by the Education Department of Henan Province (No. 172102310543) and the National Natural Science Foundation of China (Projects 31701665, 31870093).