Effect of Fermentation With and Without the Addition of Carrots on the Total Antioxidant Capacity of White and Red Cabbage

Abstract

Cabbage is one of the most popular vegetables all over the world, with white cabbage generally being more popular than red cabbage. This study aimed at a comparison of the antioxidant properties of fresh and fermented white and red cabbage. Total phenolic content, the content of anthocyanins and carotenoids, and the Total Antioxidant Capacity (TAC) assayed by ABTS^• scavenging, DPPH^• scavenging, FRAP, and ORAC of fresh white and red cabbage, fermented white and red cabbage (sauerkraut), and sauerkraut juice were compared. The TAC of fresh and fermented red cabbage, and of red sauerkraut juice (110.3 ± 8.9, 47.4 ± 4.6 and 48.9 ± 5.7 mmol Trolox equivalents/kg, respectively) was significantly higher than the TAC of fresh and fermented white cabbage and white sauerkraut juice (5.1 ± 0.2, 7.9 ± 0.9 and 6.6 ± 0.9 mmol TE/kg, respectively, when assayed by ORAC). The TAC of white sauerkraut and white sauerkraut juice could be elevated by fermentation with 20% of black carrots (to 16.4 ± 1.2 and 10.5 ± 0.8 mmol TE/kg, respectively) but the TAC of red sauerkraut and red sauerkraut juice was diminished by a mixture of either orange or black carrots, which are of lower anthocyanin content than the red cabbage (41.8 ± 3.0 and 29.2 ± 3.1 mmol TE/kg, respectively). These results may justify the promotion of the broad consumption of red cabbage, both fresh and fermented, and encourage the usage of red cabbage as a promising material for functional foods.

1. Introduction

Anthocyanins—flavonoid pigments occurring in many flowers, fruits, and other plant organs—are excellent antioxidants [1,2] and are claimed to have numerous positive health effects related and unrelated to their antioxidant properties. They were found to ameliorate oxidative stress, dyslipidemia, and arterial stiffness, positively affect endothelium functions and glucose metabolism, prevent cognitive disorders, inhibit cyclooxygenases, have antiatherogenic, antihypertensive, antithrombotic, anti-inflammatory, antiglycative, anticarcinogenic, and antimicrobial activity [3,4,5,6,7] and induce favorable changes in the composition of gut microbiota [8,9].

The outstanding longevity of the Okinawa population has been attributed, at least in part, to the high consumption of food products rich in anthocyanins, such as purple potatoes, purple sweet potatoes, and purple and black carrots [8,10,11].

Globalization has made it possible to buy specific products like purple sweet potato globally and to produce transgenic plants overproducing anthocyanins, including tomato, rice, maize, and other species [12,13]. However, the transport of food products is costly and generates a carbon footprint, so the tendency to eat local products (zero-kilometer food strategy) is gaining popularity [14,15]. Moreover, there are numerous issues related to the acceptance of transgenic plants in societies and governments [16,17,18]. Therefore, locally cultured anthocyanin-rich crops need closer attention and evidence-based recommendations.

Cabbage (Brassica oleracea var. capitata), native to coastal regions of the Mediterranean, spread across northern Europe due to its ability to withstand cold climates, became an important staple in European cuisine [19,20], and is now cultivated and consumed all over the world [21]. This vegetable is consumed in the form of salads, coleslaw, sauerkraut, soup, and is also employed for other cooking purposes. Red (purple) cabbage (Brassica oleracea var. capitata rubra), rich in anthocyanins (≥10 g/kg dry mass), is a variety of cabbage originating from Southern Europe, but is nowadays produced and harvested worldwide [22], although it is generally less popular than white cabbage. This variety is plentiful year-round, but tastes the best when grown in cooler climates [21]. Interestingly, red cabbage is preferentially consumed in some regions (as Silesia in Poland) [23,24].

Cabbage has been traditionally subjected to fermentation, mainly for the sake of preservation and as a source of vitamins and other phytochemicals in winter and spring, when fresh vegetables and fruits were not available, and also to obtain a product of a different taste. The effect of fermentation on the antioxidant properties of cabbage is therefore of considerable interest, but data from the literature concerning this question are somewhat contradictory. Fermentation for up to 14 days was reported to progressively increase the antioxidant activity of white cabbage juice assayed by DPPH^• and ABTS^• reduction [25]. Lactic fermentation increased the antioxidant capacity of aqueous extracts of Chinese cabbage (Brassica pekinensis) assayed by DPPH^• decolorization [26]. An increase in TAC, a decrease in the content of ascorbic acid, and a profound increase in the ascorbigen content were noted upon fermentation of white cabbage cv. Megaton [27]. Fermentation of red cabbage was reported to increase the TAC estimated by DPPH^• and ABTS^• reduction [28]. However, it was also reported that fermentation of white and red cabbage decreased the phenolic content and the TAC estimated by DPPH^• decolorization and CUPRAC assays of both white and red aqueous cabbage extracts [29]. Another study found a decrease in TAC estimated by ABTS^• reduction and ORAC but saw no change in TAC assayed by DPPH^• reduction after fermentation of red cabbage (cultivar Langedijker Polona) [30].

Another interesting question is the effect of an additive on the TAC of fermented cabbage. Various recipes propose the inclusion of additives for cabbage fermentation, chiefly to increase the sensory properties of the sauerkraut [31,32]. Such additives may also affect the antioxidant properties of the sauerkraut and the sauerkraut juice.

This study was aimed at: (i) re-investigation, in view of divergent results, of the effect of fermentation on the antioxidant capacity of white and red cabbage using several different methods of evaluation of TAC, and (ii) examination of the effects of addition of orange and black carrots on the antioxidant properties of sauerkraut and sauerkraut juice made of white and red cabbage.

2. Materials and Methods

2.1. Reagents and Chemicals

2,2′-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS; CAS no. 504-14-6; cat. no. 10102946001; purity ≥ 99%) was bought from Roche (Warsaw, Poland). 2,2-Diphenyl-1-picrylhydrazyl (DPPH^•; CAS no. 1898-66-4; cat. no. HY-112053, purity ≥ 99.13%), and iron(III) chloride (FeCl₃; CAS no. 7705-08-0; cat. no. 451649; purity ≥ 99.99%) were provided by MedChemExpress (Monmouth Junction, NJ, USA). 2,4,6-2,4,6-Tri(2-pyridyl)-s-triazine (TPTZ; CAS no. 3682-35-7; cat. no. T1253), fluorescein sodium salt; CAS no. 518-47-8; cat. no. 103887), 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH; CAS no. 2997-92-4; cat. no. 440914; purity ≥ 97), gallic acid monohydrate (CAS no. 5995-86-8; cat. no. 398225), potassium persulfate (CAS no. 7727-21-1, cat. no. 216224), Trolox (CAS no. 53188-07-1, cat. no. 648471), the Folin–Ciocalteu phenol reagent (cat. no. 4764) and dimethyl sulfoxide (DMSO; CAS no. 67-68-5; cat. no. D2438) were purchased from Merck (Poznan, Poland). Sodium dihydrogen phosphate (CAS no. 10049-21-5; cat. no. PM306.500, purity 98–103%), and sodium hydrogen phosphate (CAS no. 7782-85-6; cat. no. SPD579.1, purity 98–102%) produced by BioShop Canada Inc. (Burlington, ON, Canada) were obtained from Lab Empire (Rzeszow, Poland). Methanol (CAS no. 67-56-1; cat. no. 6219900110, purity ≥ 99.9%), ethanol (CAS no. 64-17-5, cat. no. 396420113, 96%), glacial acetic acid CAS no. 64-19-7; cat. no. JT9522-2) and sodium acetate anhydrous (CAS no. 127-09-3; cat. no. BN60/6191; purity ≥ 99%) were provided by Avantor Performance Materials (Gliwice, Poland).

Water was purified by a Milli-Q system (Millipore, Bedford, MA, USA). Transparent flat-bottom 96-well plates (cat. no. 655101) as well as black-flat-bottom 96-well plates (cat. no. 655209) (Greiner, Kremsmünster, Austria) were used. Absorbance and fluorescence were measured in a Spark microplate reader (Tecan Group Ltd., Männedorf, Switzerland).

2.2. Plant Material

White and red cabbage (Brassica oleracea var. capitata f. alba cv. Amazon F1 and f. rubra cv. Lodero F1), grown in the continental agroclimatic zone, were bought in a supermarket in Rzeszów.

2.3. Preparation of Extracts

External leaves of the cabbage head were removed, the cabbage was washed and chopped into about 1 × 1 × 1 cm fragments, frozen and thawed, and homogenized with a 9-fold excess of deionized water, shaken for 30 min, again frozen and thawed, homogenized again, and centrifuged. The supernatant was aliquoted and stored frozen at −80 °C until analysis, for no more than 1 month.

2.4. Cabbage Fermentation

External leaves were removed from medium-sized cabbage heads, which were then washed, chopped, and divided into three sets of portions, each set containing either 400 g (control) or 320 g of cabbage; the latter portions were supplemented with 80 g of orange or black carrots (20% by weight); all were added with 8 g of table salt. The components were thoroughly mixed, kneaded with a wooden spoon, and left for 30 min. This treatment was repeated three times. Then, the cabbage was placed in 500 mL jars and pressed to obtain a juice layer covering the cabbage. The jars were covered with a paper towel and left at 20 °C for 3 days. Then, the released gas was expelled with a sterile spoon, and the jars were closed tightly and left at ambient temperature for the next 12 days. Every 3 days, the level of juice was controlled, and the cabbage was pressed to be fully covered by the juice.

In the fermented cabbage, both the juice and cabbage fragments were analyzed. The latter were dried with a paper towel and extracted as described above.

2.5. Estimation of the Polyphenol Content

The polyphenol concentration in the extracts was determined using the Folin–Ciocalteu reagent [33]. In brief, 25 µL of a 10-fold diluted extract was pipetted to 125 µL of 1 M Folin–Ciocalteu reagent in wells of a microplate, mixed, and incubated for 4 min. Then, saturated sodium carbonate solution (100 µL) was added to the wells, the plate was incubated for 1 h at room temperature (21 ± 1 °C), and the absorbance of the samples was measured at 750 nm. The polyphenol concentration in the extracts was calculated based on a standard curve prepared with gallic acid and expressed in gallic acid equivalents (GAE).

2.6. Estimation of the Anthocyanin Content

The content of anthocyanin was estimated according to a slightly modified method of Lee et al. [34]. Briefly, 125 µL aliquots of the extract were added with 875 µL of 0.1 M acetate buffer, pH 4.5 or 1.5 M HCl, and the absorbance of both samples was measured at the absorption maximum of anthocyanins (at about 520 nm) and at 700 nm.

• The anthocyanin concentration, c, was calculated as:

  c [mg/L] = A × MW × dilution × 10³)/(ε × l), where

  A = (A_maximum − A_{700 nm})_{pH 1} − (A_maximum − A_{700 nm})_{pH 4.5},

MW, molecular weight; ε, molar absorption coefficient [M⁻¹ cm⁻¹]; l, length of the optical path. The following values were assumed: MW for cyanidin-3-glucoside of 484.8 g mol⁻¹, ε = 26,900 M⁻¹ cm⁻¹.

2.7. Estimation of the Carotenoid Content

The content of carotenoids was estimated in hexane extracts of the white and red cabbage by measurement of absorbance of the extracts at 446 nm. Total carotenoid content TCC was calculated as:

$$TCC[\mathrm{m}\mathrm{g}/\mathrm{k}\mathrm{g}]={\displaystyle \frac{A\times V\times {10}^4}{2592\times m}}$$

where A, absorbance at 446 nm; V, volume of hexane [mL], m, mass of a sample [g] [35].

2.8. Estimation of Total Antioxidant Capacity

2.8.1. ABTS^• Scavenging Assay

The ABTS^• decolorization assay [36] was used in a modification described previously [37]. In brief, a stock solution of ABTS^•, prepared by the oxidation of ABTS with 2.45 mM potassium persulfate for 16 h, was diluted with phosphate-buffered saline (145 mM NaCl in 10 mM sodium phosphate, pH 7,4; PBS) to obtain a solution providing absorbance 1.0 at a wavelength of 734 nm when 200 µL of such solution was measured in a well of a 96-well microplate. Wells containing such an ABTS^• solution were added with various volumes (1–10 µL) of solutions of a cabbage extract or juice. The decrease in absorbance after 30 min incubation at ambient temperature (21 ± 1 °C) in the dark, was measured and corrected for the drop of absorbance of a blank sample, containing ABTS^• solution and no additive.

The antioxidant capacity was calculated as a ratio of the slope of the dependence of absorbance change on the volume of a studied extract to the slope of the dependence of absorbance change in standard samples containing Trolox on the concentration of Trolox, as explained in [38]. Total Antioxidant Capacity (TAC) is expressed in mmoles of Trolox equivalents (TE)/kg of cabbage or cabbage juice. The same procedure was applied to the results of DPPH^• scavenging and FRAP assays. TAC is expressed in mmoles of Trolox equivalents (TE)/kg of cabbage or cabbage juice.

2.8.2. DPPH^• Scavenging Assay

A procedure described previously [38] was employed. In brief, increasing volumes of the extract/juice supplemented with water to a final volume of 20 μL were added to Eppendorf tubes containing 300 µL of 0.3 mM DPPH^• solution in methanol and incubated for 30 min at ambient temperature, in the dark. The samples were centrifuged, and 200-μL aliquots of the supernatants were pipetted into wells of a 96-well plate. Their absorbance at 517 nm was then read. The absorbance decrease, with respect to samples containing DPPH^• solution added with water only, was a measure of the antioxidant activity.

2.8.3. Ferric Reducing Antioxidant Power (FRAP) Assay

A slight modification of the method described by Benzie and Strain [39] was used. Briefly, increasing volumes of diluted extract/juice were pipetted to wells of a 96-well microplate containing 200 μL of the working solution (0.3 M acetate buffer, pH 3.6/10 mM TPTZ in 40 mM HCl/20 mM FeCl₃; 10/1/1, v/v/v). The samples were incubated for 30 min at ambient temperature, and their absorbance was measured at 593 nm.

2.8.4. Oxygen Radical Absorbing Capacity (ORAC) Assay

The method of Ou et al. [40] was employed in a slight modification. In brief, increasing volumes (1–10 μL) of the extract/juice were added to wells of a 96-well microplate containing fluorescein (0.2 μM), AAPH (50 mM), and sodium phosphate buffer (50 mM; final concentrations) in a total volume of 200 μL (including the extract volume). The fluorescence was measured at the excitation/emission wavelengths of 478 nm and 520 nm, respectively, every 90 s for 180 min at a temperature of 37 °C. The sum of fluorescence intensities for all measurements was calculated for each sample. The per cent protection of the fluorescein fluorescence was calculated according to the formula:

% protection = 100% [(S_sample − S_AAPH)/(S_blank − S_AAPH)]

where S_sample is the sum of fluorescence intensities for a studied sample; S_AAPH is the sum of fluorescence intensities of a sample containing fluorescein, AAPH and no antioxidant additives; S_blank is the sum of fluorescence intensities of a sample containing fluorescein and no AAPH or antioxidant additives.

The antioxidant capacity was calculated as the ratio of the slope of the dependence of the percentage protection of fluorescence on the volume of the studied extract to the slope of the dependence of the protection of fluorescence of standard samples containing Trolox on the concentration of Trolox. Thus, the determined TAC is expressed in mmoles of Trolox equivalents (TE)/kg of cabbage or cabbage juice.

2.9. Statistical Analysis

The results are presented as means ± SD of at least three replicates. The statistical significance of differences between antioxidants within individual assay methods was estimated using one-way ANOVA with the post hoc Tukey’s test, for p = 0.05, using XLSTAT, version 27.1.3.0 (Lumivero, Denver, CO, USA). PCA analysis was performed using STATISTICA, version 13.3 (TIBCO, Palo Alto, CA, USA).

3. Results

Comparison of the antioxidant capacity of fresh white and red cabbage estimated by various methods is shown in

Table 1. Antioxidant properties of fresh white and red cabbage.

Parameter	White Cabbage	Red Cabbage
Polyphenol content [mg GAE/kg]	821 ± 10	6915 ± 91
Anthocyanin content [mg/kg]	6.3 ± 2.5	3610 ± 93
Carotenoid content [mg/kg]	1.5 ± 0.1	10.5 ± 0.1
TAC, ABTS• decolorization [mmol TE/kg]	5.8 ± 0.2	87.9 ± 3.3
TAC, DPPH• decolorization [mmol TE/kg]	2.4 ± 0.1	63.9 ± 2.0
TAC, FRAP [mmol TE/kg]	1.2 ± 0.1	36.2 ± 0.9
TAC, ORAC [mmol TE/kg]	5.1 ± 0.2	110.3 ± 17.2

. Significant differences were seen between white and red cabbage. The latter, apart from the presence of anthocyanins, had higher polyphenol content (due, i.a., to anthocyanins), and higher TAC values estimated by all assays employed.

Fermentation of cabbage caused a decrease in pH to values in the range of 3.2–3.5, with no significant differences between white and red sauerkraut. The presence of black carrots during the fermentation led to a slight elevation in the pH of both white and red sauerkraut (from 3.31 ± 0.03 to 3.45 ± 0.02 and from 3.24 ± 0.04 to 3.40 ± 0.04; mean ± SD, respectively); in the red sauerkraut, such an effect was also caused by the addition of orange carrots (Figure 1).

Comparison of the antioxidant properties of fresh and fermented cabbage demonstrated that the fermented cabbage retained most of the polyphenols (Figure 2) and, in the case of red cabbage, anthocyanins (Figure 3). In white cabbage, the total polyphenol content did not differ significantly between fresh cabbage and sauerkraut (821 ± 10 vs. 798 ± 9 mg/kg). In contrast, red sauerkraut had significantly less polyphenols than fresh red cabbage (3990 ± 100 vs. 6915 ± 99 mg/kg). The juice contained comparable amounts of polyphenols and anthocyanins as the solid sauerkraut. The addition of black carrots increased the polyphenol content in the white sauerkraut (up to 1291 ± 57 mg/kg) and white sauerkraut juice (from 689 ± 10 to 1001 ± 19 mg/kg). In contrast, the addition of orange or black carrots decreased the content of anthocyanins and polyphenols in the red sauerkraut (down to 3245 ± 113 and 3387 ± 127 mg/kg, respectively) and red sauerkraut juice (from 3467 ± 204 to 2484 ± 78 and 2777 ± 31 mg/kg, respectively).

The addition of carrots enriched the sauerkraut in carotenoids—the effect being more significant for orange carrots. The carotenoid concentration did not increase significantly in the sauerkraut juice (Figure 4), apparently due to the hydrophobicity of these compounds and practical insolubility in aqueous solutions.

The pattern of Total Antioxidant Capacity of fresh white and red cabbage, white and red sauerkraut, estimated by ABTS^• scavenging (Figure 5), was similar to that of polyphenol content. It was much higher for the red cabbage and its products as compared with the white cabbage (87.9 ± 3.3 vs. 5.8 ± 0.2 mmol TE/kg, respectively). TAC of white sauerkraut and white sauerkraut juice (9.2 ± 0.1 and 9.3 ± 0.6 mmol TE/kg, respectively) was higher than that of fresh white cabbage, while TAC of red sauerkraut and red sauerkraut juice (47.3 ± 0.1 and 29.4 ± 0.8 mmol TE/kg, respectively) was considerably lower than that of fresh red cabbage.

The TAC estimated by DPPH^• scavenging (Figure 6), FRAP (Figure 7), and ORAC (Figure 8) showed similar relationships. However, TAC values obtained by DPPH^• scavenging (2.3 ± 0.1 and 63.9 ± 2.0 mmol TE/kg for fresh white and red cabbage, respectively) and FRAP (1.2 ± 0.1 and 36.2 ± 0.9 mmol TE/kg for fresh white and red cabbage, respectively) were lower than those found by ABTS^• scavenging. The TAC values obtained by ORAC for fresh white cabbage (5.1 ± 0.2 mmol TE/kg) were somewhat lower than the values obtained by ABTS^• scavenging, but those found for fresh red cabbage (110.3 ± 0.9 mmol TE/kg) were higher than those found by ABTS^• scavenging. All methods demonstrated a decreased TAC of red sauerkraut as compared with fresh red cabbage (29.4 ± 0.8, 20.2 ± 1.2, 16.4 ± 0.7, and 48.9 ± 4.7 mmol/kg for ABTS^• scavenging, DPPH^• scavenging, FRAP and ORAC assays, respectively). In contrast, the TAC of white sauerkraut was higher than that of fresh white cabbage (9.3 ± 0.1, 3.5 ± 0.4 and 7.9 ± 0.9 mmol/kg by ABTS^• scavenging, DPPH^• scavenging, and ORAC, respectively) or did not differ from the TAC of fresh white cabbage (2.2 ± 0.2 mmol/kg by FRAP). The TAC of white sauerkraut juice did not differ from that of white sauerkraut in all assays; the TAC of red sauerkraut juice did not differ from that of red sauerkraut in the ORAC assay (48.9 ± 5.7 mmol/kg) but was lower than the TAC of the sauerkraut in all other assays (29.4 ± 0.8, 20.2 ± 1.3 and 16.4 ± 0.7 mmol/kg in the ABTS^• scavenging, DPPH^• scavenging and FRAP assays, respectively).

Supplementation of cabbage to be fermented with black carrots resulted in an increase in the TAC of the white sauerkraut as estimated by ABTS^• scavenging, DPPH^• scavenging and ORAC (13.2 ± 0.9, 4.2 ± 0.3 and 10.5 ± 0.8 mmol/kg, respectively), with no significant change observed in the FRAP assay (13.7 ± 0.7 mmol/kg) but a reduction in the TAC of red sauerkraut fermented with carrots, both orange and black (to 36.2 ± 0.1, 18.8 ± 1.0, 13.8 ± 1.3 and 36.6 ± 4.0 mmol/kg for orange carrot and 39.7 ± 0.1, 20.3 ± 1.2, 13.7 ± 0.7 and 41.7 ± 3.0 mmol/kg for black carrot in the ABTS^• scavenging, DPPH^• scavenging, FRAP and ORAC assays, respectively).

Pearson’s correlation coefficients between the means of the polyphenol content, anthocyanin content, and results of TAC assays were highly significant while those between the carotenoid content and any other parameter were devoid of statistical significance (

Table 2. Correlation coefficients between the parameters describing white and red cabbage and their products.

r	Anthocyanin Content	Carotenoid Content	TAC: ABTS• Scavenging	TAC: DPPH• Scavenging	TAC: FRAP	TAC: ORAC
Polyphenol content	0.99 *	0.37	0.98 *	0.98 *	0.99 *	0.98 *
Anthocyanin content		0.32	0.98 *	0.99 *	0.99 *	0.99 *
Carotenoid content			0.42	0.35	0.35	0.36
TAC: ABTS• scavenging				0.99 *	0.98 *	0.98 *
TAC: DPPH• scavenging					0.99 *	0.99 *
TAC: FRAP						0.99 *

* p < 0.001.

).

PCA analysis of variables used to characterize the antioxidant properties of the material studied led to the isolation of two principal components (

Table 3. Eigenvalues, variance contributions, and cumulative variance contributions of two principal components generated by the PCA of parameters used for the characterization of antioxidant properties of cabbage products.

	Variable 1	Variable 2
Eigen value	3.82	1.29
Variance contribution	0.5454	0.1839
Cum. variance contribution	0.5454	0.7293
Iterations	8	32

).

The sequence of the importance of variables for the characterization of cabbage and its products was as follows: TAC-ABTS^• (0.973) > TAC-ORAC (0.967) > TAC-DPPH^• (0.965) > carotenoid content (0.832) > polyphenol content (0.740) > TAC-FRAP (0.624) > anthocyanin content (0.003).

A charge plot of the antioxidant parameters is shown in Figure 9.

The t-values for individual cabbage preparations with respect to new variables 1 and 2 are shown in

Table 4. The t-values for individual cabbage preparations with respect to new variables 1 and 2.

	Principal Component 1	Principal Component 2
FWC	−1.599	−0.019
WS	−2.261	2.079
WS+OC	−1.240	1.683
WS+RC	−1.047	0.924
WSJ	−1.539	−0.606
WSJ+OC	−1.515	−0.186
WSJ+BC	−0.968	−1.390
FRC	5.616	1.156
RS	1.203	−0.235
RS+OC	0.812	0.119
RS+BC	1.307	0.475
RSJ	0.765	−1.707
RSJ+OC	0.071	−1.202
RSJ+BC	0.394	−1.092

.

4. Discussion

The present results point to the much higher antioxidant capacity of red cabbage, red sauerkraut, and red sauerkraut juice as compared with white cabbage. Similar conclusions were reached in other studies, usually employing a smaller range of assays [41,42,43,44], although ethanolic extracts of white cabbage were reported to have a higher TAC and higher H₂O₂-scavenging activity than extracts of red cabbage [45].

As expected, various assays provided different values of TACs of cabbage and its products. This may be partly due to different reaction mechanisms on which these assays are based. FRAP is based on single electron transfer (SET) reactions, utilizing antioxidants, as well as DPPH^• and ABTS^• scavenging assays, which include both SET and hydrogen atom transfer (HAT) reactions. In contrast, ORAC focuses on HAT reactions [46,47,48]. Lower TAC values of cabbage were observed for the FRAP and DPPH scavenging assays, while the ORAC and ABTS^• scavenging assays yielded higher values. It suggests a significant contribution of antioxidants reacting through the HAT mechanism to the TAC of cabbage. In the FRAP assay, addition of black carrots did not elevate the TAC of sauerkraut, while such elevation was observed with the three other methods. It suggests that black carrots and white cabbage have a similar content of Fe³⁺-reducing antioxidants. It is known that glutathione and other thiols show poor reactivity in this assay [49,50]. While ABTS^• and DPPH^• are radicals of synthetic molecules, not found in nature, and reactions of antioxidants with these radicals depend on their stereoselectivity, the peroxyl radicals acting as oxidants in the ORAC assay, generated at a relatively low rate, resemble to a higher extent the antioxidant activity in vivo and in food products. The much higher TAC of red cabbage, red sauerkraut, and red sauerkraut juice in comparison with white cabbage and products of its fermentation evidences the higher ability of antioxidants present in red cabbage to react with the physiologically relevant peroxyl radicals.

The increase in TAC of white cabbage after fermentation, also observed by others [25], is intriguing and contrasts with the decrease in the TAC after fermentation of red cabbage. This may be due to (i) the better extractability of antioxidants present in white cabbage after fermentation, (ii) the generation of antioxidants during fermentation, combined with the resistance of antioxidants to lactic fermentation. The TAC of red cabbage is apparently conditioned to a considerable extent by anthocyanins. From 9 to 36 different anthocyanins, predominantly cyanidin derivatives have been detected in various red cabbages. Among them, a large number occurs in acylated forms [51,52]. Anthocyanins are well extractable to aqueous solutions and relatively unstable. A decrease in the anthocyanin content found in this study confirms previous observations [28,52]. The composition of anthocyanins was also demonstrated to be altered by the fermentation of red cabbage [30]. Fermentation was reported to cause a small (about 10%) decrease in the anthocyanin bioavailability as well [52].

The phenolic content, anthocyanin content and results of TAC assays showed high correlation (r ≥ 0.98), while correlating poorly with the carotenoid content. PCA analysis showed a possibility of compression of antioxidant parameters to two principal components. The t-values for principal component 1 were negative for white cabbage and its products, and positive for red cabbage and its products.

The red cabbage contains more polyphenols and anthocyanins than black carrots, since the replacement of 20% cabbage with black carrots decreased the anthocyanin content in the sauerkraut. Indeed, data from aqueous extracts of red carrots pointed to the polyphenol content of black carrots of 1189 [53] and 78.9–2915 mg/kg [54], significantly lower than that found here for red cabbage (6914 ± 99 mg/kg). Our previous study estimated the anthocyanin content of black carrot to be 256 ± 11 mg/kg [55] as compared to 3610 ± 93 found for the red cabbage in this study. In contrast, it is advisable to add vegetables rich in antioxidants, like carrots, to the white cabbage subject to fermentation, to elevate its antioxidant capacity. The considerable antioxidant capacity of red sauerkraut juice, comparable to that of solid red sauerkraut, found in this study, is worth emphasizing since, in contrast to white sauerkraut juice, its consumption is far from popular.

The high anthocyanin content of red cabbage is not its only benefit as, compared with white cabbage, it was reported to contain also more vitamin C and DL-α-tocopherol [56]. It has been emphasized that the assets of red cabbage include high contents of antioxidants, phytochemicals, vitamins (especially C, E, A, K) and mineral components such as calcium, manganese, magnesium, iron, and potassium. We have not studied the ascorbate content since its decrease during cabbage fermentation and concomitant formation of ascorbigen is well documented [27,57,58,59]. It is also low in saturated fats and cholesterol [60]. However, it should be kept in mind that the cabbage contents of antioxidants and other components depend on the growth conditions and fertilization of cabbage [61,62,63,64].

Cabbage (Brassica oleracea var. capitata) belongs to the most widely grown and consumed vegetables in the world, especially in areas where climatic conditions restrict the list of cultivated species [65,66]. Numerous health effects of red cabbage were reported, including anti-inflammatory [67], antimicrobial [68,69] and anticancer action (inhibition of proliferation of cancer cells) [70,71,72]. Red cabbage administered to rats with diabetes induced by high-fat/high-fructose diet ameliorated oxidative stress and improved metabolic profile and hepatic function [73]. Red cabbage reduced diabetic nephropathy in rats with diabetes induced by streptozotocin [74]. In rats fed high-cholesterol diet red cabbage diminished hepatic injury and dyslipidemia [75]. Aqueous extract of red cabbage reduced oxidative stress induced by Triton WR-1339 in rats [76]. Red cabbage extract rich in anthocyanins decreased cognitive impairment in aging mice and had beneficial effects on gut microbiota [77]. Red cabbage juice, but not white cabbage juice, increased the stress resistance and lifespan of Caenorhabditis elegans [78]. These findings speak in favor of the broader consumption of red cabbage and products of its fermentation.

5. Conclusions

This study confirmed the much higher TAC of red cabbage and red sauerkraut as compared with white cabbage and white sauerkraut. The TAC of white sauerkraut was increased by the addition of black carrots to the cabbage while the TAC of red sauerkraut and red sauerkraut juice was decreased by the addition of orange and red carrots. Enrichment of fermented cabbage with carrots increased the carotenoid content of both white and red sauerkraut. The high TAC of red sauerkraut juice is noteworthy and suggests its broader use.