Introduction

The increasing knowledge of consumers about the beneficial effects of fermented products on human health contributes to their popularization and growing consumption. Cabbage and cucumbers are the most commonly preserved plant materials in Eastern and Southeastern Europe. However, the search for new flavor solutions is ongoing, leading to a significant expansion of the range of fermented products to include other plant species, including cauliflower. This vegetable is valued for its mild flavor and delicate texture1, but some varieties are characterized by a slightly astringent or bitter taste and a sulfurous odor, which is due to a number of chemical compounds2,3,4. Cauliflower has a short shelf life after harvest at room temperature, and can be extended to just a few days in refrigerated conditions1. Therefore, the fermentation process of this raw material offers an interesting solution for food producers.

So far, the research conducted on cauliflower fermentation has focused on issues related to determining the effect of lactic acid fermentation on the chemical composition, including the occurrence of amino acids and vitamins B2, B9 and B12 in this vegetable5 and biogenic amines in its fermented juice6. In addition, aspects related to the composition of microorganisms participating in this process were studied7 and their probiotic potential8. The possibilities of using waste from cauliflower fermentation for the production of cosmetics9 or as animal feed10 have also been analyzed. However, no research has yet been conducted on the possibility of improving the antioxidant activity of fermented cauliflower as a component of the human diet by using additives with high antioxidant values during the fermentation process. This study on enriching fermented cauliflower with ingredients that increase its antioxidant activity is therefore a novelty in this field.

When preparing fermented vegetables, additional raw materials are often incorporated into the composition, and spices are used to diversify and enrich the product’s sensory characteristics11, rather than to improve its antioxidant activity. To date, only studies have been conducted on the use of green tea in the production of fermented vegetable kimchi. This ingredient was found to prolong the fermentation time of the product, and the final product exhibited antimutagenic effects. The experiments determined changes in the product’s pH and acidity, but did not examine their effect on the antioxidant activity of the final product12,13. Information on the effect of green tea and a spice mixture on the antioxidant activity of the fermented product (kimchi) can be found in patent KR20170025806A14. The combination of these ingredients was found to increase the content of vitamin C and total polyphenols (from 0.37 g/100 g to 0.65 g/100 g) and also increase the DPPH radical scavenging ability (from 16 to 39%).

Tea leaves contain a number of amino acids, sterols, and vitamins B2, B3, C, and E15. However, its most valuable compounds are flavonoids and catechins, including epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate (EGCG)16,17,18. Catechins are easily soluble in water, but as the extraction period is extended, they deteriorate the sensory qualities of the extract, causing an increasing bitter and astringent taste19. However, a significant change in the sensory qualities of tea leaves and infusions during the fermentation process, including a loss of bitterness, has been observed. This results from changes in their chemical composition (biotransformation of compounds). Among others, a reduction in the content of tannins and catechins, which contribute to the bitter taste of tea, has been observed. They are hydrolyzed to simpler compounds (which reduces their bitterness and astringency) or transformed into new compounds, often more complex, which enriches the flavor profile with a distinctly pleasant, slightly sweet aftertaste20,21,22,23,24.

Green tea leaf extracts exhibit significant health-promoting effects: antioxidant, anti-inflammatory, and anticancer15,25,26,27,28, as well as antibacterial29,30. In this latter respect, catechins, especially EGCG, play a special role. They negatively affect, among other things, the functioning of bacterial cell membranes and the growth of these cells29,31,32. Strong antibacterial effects have been observed in the case of Gram-positive bacteria29, which also include Lactobacillus plantarum. However, previous studies show that some strains of lactic acid bacteria (including Lactobacillus bulgaricus, L. plantarum 299 V, L. rhamnosus LOCK900, Lactobacillus spp., and Lactobacillus brevis GTL 79) are less sensitive to the components contained in green tea compared to other bacteria, and their small doses may even promote bacterial activity33,34,35,36,37,38. It was found that increasing the addition of green tea during the production of fermented vegetable mixture (kimchi) may result in an extension of the fermentation time12, however, the action of extracts from this plant and catechins isolated from it does not completely inhibit the fermentation processes. An example is the popular fermented beverage kombucha, made from sweetened tea infusion. Traditionally, the fermentation process is carried out by a symbiotic colony of acetic acid bacteria, lactic acid bacteria, and yeast (SCOBY), but work has also been conducted to develop this product based solely on lactic acid bacteria (including Lactobacillus spp. and Lactobacillus brevis Y52)39,40. Similarly, fermentation of leaves and waste from green tea production for animal feed with lactic acid bacteria did not inhibit silage formation41,42,43,44. The varied resistance of bacterial strains to tea components results from their genetic differences, which are manifested, among others, in different activity of efflux pumps, detoxification enzymes, activity of cytoplasmic membranes, pH tolerance, and the presence and concentration of various substances in the external environment45,46,47. Therefore, in industrial processes, the selection of the appropriate strain of microorganisms and the appropriate dose of green tea is extremely important.

Lactobacillus bacteria change the nutritional value of fermented raw materials (enriching products with polysaccharides while reducing protein and sugar content) and the sensory quality of raw materials48,49,50,51,52,53, and also increase the content of certain vitamins or slow down their decomposition. This effect has been documented, for example, for vitamin C54,55,56, vitamin B25,57,58, vitamin B95,59,60, vitamin B125,61,62,63, and vitamin K64,65. They also increase the safety of fermented products and extend their shelf life through the production of metabolites: organic acids and bacteriocins48,49,66,67. Some substances contained in fermented products are exogenous antioxidants. It has been proven that, when consumed regularly, they have a health-promoting effect on the human body and are an important element in the prevention of lifestyle diseases (including neurodegenerative, cancer, and cardiovascular diseases). They also slow down the aging process48,68,69,70. Currently, we are observing an increase in the incidence of lifestyle diseases, the prevalence of which is increasing in society due to stressful and unhealthy lifestyles. This leads to the ever-increasing importance of ingredients rich in exogenous antioxidants in the diet and the need to enrich traditional products with antioxidant substances. Fermented cauliflower with tea, thanks to its increased antioxidant activity and high sensory qualities, ideally meets the needs of consumers and food producers.

The aim of this article is to present the results of research on enhancing the antioxidant activity of fermented cauliflower by adding green tea leaves during the fermentation process, without losing the sensory values of this vegetable.

Material and methods

Materials

The research material was white cauliflower (Andromeda F1 variety; purchased in a chain store in Poland), which was then subjected to lactic acid fermentation with the addition of dried China Sencha green tea (Oxalis, Poland) in brine solution (Sól Kłodawska Kamienna Niejodowana Do Przetworów; Kopalnia Soli “Kłodawa” S.A, Poland). The starter was a culture of active Lactobacillus plantarum bacteria (strain LP 299v) (Serowar, Poland).

The cauliflowers weighed approximately 1.5 kg and were approximately 20 cm in diameter. They were mature (estimated age was 140–150 days after sowing), harvested two days before the experiment began. The health condition was very good: there was no discoloration, browning or damage on the inflorescences, and the leaves were fresh, green and firm.

Experiment

The fermentation process was carried out in sterilized glass fermenters with a capacity of 500 ml. Fermenters were anaerobic (no free air access due to the tight cap being fitted and tightened). Each cap was fitted with an airlock, which served three important functions: it prevented air and microbiological contamination from entering the fermenter, vented excess CO2, and thus regulated the pressure within the fermenter. Sterilization of the fermenters, caps, and stoppers was always performed with steam for 3 min at 134 °C. During this process, the sterilization process was monitored physically (monitoring and recording sterilization parameters: temperature, pressure, and time) and chemically (using integrated indicator strips (Class 5). Biological monitoring of the sterilizer was performed in the laboratory once every two months and additionally after each autoclave repair.

The same Lactobacillus plantarum (LP 299v) starter was used in all experiments. No starter-free control was run during the studies. The initial microbiological load was 3 × 105 CFU/ml. The experiment was repeated three times.

At the bottom of each container, except for the fermenters containing control samples, dried green tea leaves were placed (respectively in the amount of 0.5, 1, 2, 3, 4, and 5% of the mass of the main raw material, i.e., according to the mass of cauliflower, approximately 0.65 g, 1.3 g, 2.6 g, 3.9 g, 5.2 g, and 6.5 g, respectively), followed by cleaned, rinsed and dried cauliflower divided into small florets (approximately 130 g in each fermenter). Before being added to the fermenter, the tea leaves were sterilized (poured with boiling water for 30 s, then drained). The whole was flooded with 360 ml of hot brine prepared in the proportion of 1 l of water / 30 g of salt / 5 g of sugar. After the brine had cooled down, previously hydrated L. plantarum bacteria (30 ml/fermenter) were added to each fermenter and the whole was screwed with a sterilized lid. The fermentation process took place in a room with a constant temperature (22 °C) and lasted 14 days (Fig. 1). On the 7th and last day of the experiment, samples were taken from cauliflower and brine for laboratory analyses and a sensory evaluation of the cauliflower was performed.

Fig. 1
figure 1

Technological scheme of experiment.

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The samples were designated according to the amount of dried green tea (GT) used in a fermenter: Control—control sample (without dried green tea), 0.5%—sample containing GT at level of 0.5% of cauliflower mass, 1%—sample containing 1% of GT, 2%—sample containing 2% GT, 3%—sample containing 3% GT, 4%—sample containing 4% GT and 5%—sample containing 5% GT. The experiment was carried out in triplicate.

Laboratory analyses

Each physicochemical test was performed three times.

During fermentation (on day 0, 1, 4, 7, 10 and 14), the pH of brine was measured using an automatic pH meter (ChemLand PL-700 PVS, accuracy 0.02 pH). Brine was stirred before each measurement.

Samples for other physicochemical analyses were taken from fresh cauliflower (RC) on the day of experiment establishment (day 0), while samples from fermented cauliflower (Control-5%) were taken on the 7th and 14th day of fermentation. The dry matter content was determined using the thermogravimetric method, fat content using the Soxhlet method, nitrogen content using the Kjeldahl method (the conversion factor of 6.25 for protein content) and ash content using the dry ash method71.

The amount of carbohydrates (g/100 g ww) was calculated using formula:

$${\text{X}} = {\text{a}}{-}({\text{b}} + {\text{c}} + {\text{d}})$$

where a—dry matter content (g/100 g ww), b—fat content (g/100 g ww), c—protein content (g/100 g ww), d—ash content (g/100 g ww).

Total acidity and vitamin C content in fresh and fermented cauliflower were determined. The total acidity was determined by titration with NaOH solution (0.1 M) in the presence of an indicator (phenolphthalein), and the vitamin C content was determined by the Tillmans method72.

Methanolic extracts were prepared, by doubled homogenization (4000 rpm, 30 s) of cauliflower sample with methanol (5 g/50 ml w/v) with 5 min break. After 30 min of incubation at ambient temperature extracts were filtrated through medium paper filter to dark glass bottles. Total polyphenols content (TPC) was determined according to Turkmen et al.73, using 10% Folin–Ciocalteau reagent (5 ml) and 7.5% Na2CO3 (4 ml) added to 1 ml of diluted extract. After 2 h in the dark the absorbance was measured at 750 nm. Results were expressed as gallic acid equivalents per 1 g of wet weight. The standard curve was prepared for gallic acid concentration ranged from 0.02 to 5.0 mg/ml. Dilution of extracts was chosen to obtain absorbance not higher than 0.700 and results obtained were multiplied by dilution factor.

Total flavonoids content (TFC) was determined by combining diluted extract (2.0 ml) with 10% aluminum chloride (w/v, 0.1 ml), 1 M sodium acetate (0.1 ml), and distilled water (2.8 ml), and the absorbance at 415 nm was measured after incubation in dark for 30 min74. Results were expressed as quercetin equivalents per 1 g of wet weight, using standard curve prepared for 0.01 to 10 mg/ml quercetin solutions.

The antioxidant activity of cauliflower was studied in methanol extracts as free radicals DPPH scavenging ability75, Trolox equivalent antioxidant capacity (TEAC) against ABTS cation radicals76, and ferric reducing antioxidant power (FRAP)77. Briefly, DPPH scavenging ability (DPPH) was determined by combining diluted extract with 0.2 mM DPPH solution in methanol, and after 30 min of incubation in dark the absorbance at 517 nm was measured. The inhibition of absorbance ranged from 20 to 80% was compared with DPPH scavenging ability of Trolox (standard concentration ranged from 0.001 to 0.02 mM, corresponding to the absorbance inhibition from 3 to 80%, respectively) and expressed as µM TE/g ww. TEAC activity was determined by application of cation radical ABTS solution (7 mM ABTS activated by 2.45 mM K2S2O8 for 16 h), previously diluted in methanol to obtain absorbance 0.700 ± 0.020 at wavelength of 734 nm. After 30 min of incubation in dark, the absorbance was measured at the same wavelength. Results were calculated as the absorbance inhibition (ranged from 20 to 80%) and compared to the effect of Trolox (standard curve prepared for concentrations ranged from 0.02 to 2.0 mM, corresponding to the absorbance inhibition from 3 to 90%, respectively), and expressed as µM TE/g ww. In turn, FRAP was determined by combining extract with freshly prepared working solution (TPTZ in 0.04 M HCl, 0.02 M FeCl3 and 0.3 M acetic buffer pH 3.6 combined in ratio 1:1:10, and heated for 30 min at 37 ◦C). After 30 min of incubation the absorbance at 593 nm was measured and compared with ferric reducing ability of Trolox (standard concentration ranged from 0.002 to 0.500 mM).

Sensory assessment

The fermented cauliflowers were subjected to a point assessment of overall sensory acceptability and assessed using the sensory profiling method. A 5-point scale was used for the assessment, where: 1—meant a very poor quality product, 2—poor, 3—moderate, 4—good, and 5—very good. In the profiling method, the assessment of previously selected descriptors was carried out according to a 10-point scale (0–9), where 0—undetectable, 1—extremely weakly detectable, 2—very weakly detectable, 3—weakly detectable, 4—slightly detectable, 5—moderately detectable, 6—detectable, 7—strongly detectable, 8—very strongly detectable, 9—extremely strongly detectable. The descriptors were: sweet, salty, sour, bitter, cauliflower, pickled cucumber, tea and other; salty, vinegary, sweet, sour, tea, pickled cucumber, bland, and other; texture crunchy, compact, rubbery, soggy, sticky, and unctuous.

The sensory panel consisted of 12 trained individuals, 6 men and 6 women, aged 22–53. Each panelist underwent a training process characterized by fixed stages: recruitment and selection of candidates (selection of candidates based on completed questionnaires and preliminary tests determining the ability to recognize sensory values), theoretical training (initial familiarization with the principles of the conducted research), methodological training and practical training (learning how to conduct sensory evaluations in practice), validation (determining the repeatability of results and consistency with the results of other panelists), qualification for the research panel, and then maintaining the high quality and suitability of the team members for research through repeated exercises (constant calibration), (according standards78,79,80,81). All participants gave their voluntary, informed consent to participate in the study and signed the required document. The researchers informed the participants about the purpose of the study and the anonymization of the results. Each participant had the opportunity to withdraw from the study at any stage of the study.

The sensory evaluation of the products was carried out under conditions consistent with the recommendations specified in the PN-EN ISO 8589:2010 standard82. The testing rooms were lit with neutral white light and isolated from noise, vibration, and undesirable odors. They were maintained at constant humidity, a controlled temperature (22 °C), and ventilation. Product samples for evaluation were prepared in separate rooms from the evaluation rooms. Samples were kept at room temperature during the evaluation. They were coded separately for each panelist with three-digit codes (each panelist received a sample with a different code to avoid the risk of misinterpreting the results of other participants). The code numbers were randomly selected and recorded before the test in a document accessible only to the person responsible for coding83.

Each sample weighed approximately 30 g, allowing for repeated assessment of the sensory profile in terms of the selected descriptor. They were uniform in appearance and relatively similar in shape. They were prepared simultaneously for all panelists and served in sets (each set contained one sample from each trial), with the order of the samples within each set randomized (block randomization). The samples were served on identical glass trays (odorless), without any markings, labels, or descriptions. The assessment was conducted on day 7 of fermentation and then repeated on day 14 of fermentation.

Sensory analyses were carried out in accordance with the principles specified in the PN-ISO 11035:1999, PN-EN ISO 13299:2010, PN-ISO 4121:1998, PN-EN ISO 11036:1999 standards84,85,86,87.

Statistical analysis

The values presented in the tables are arithmetic means of three replicates. The results were statistically analyzed: arithmetic means were calculated for the studied variants of features, the significance of differences between them was demonstrated by the Tukey test (P < 0.05; two-way ANOVA analysis; the variable was the tested parameter, and the predictor of quality was fermentation time and tea dose). The relationship between the main components (antioxidant activity and bioactive compounds) was demonstrated by cluster analysis and the PCA method (Statistica 13.3, Statsoft, Tulsa, USA). Results of sensory analysis were examined using one-way ANOVA analysis (significant differences with tea dose as predictor), as well as nonparametric tests, including median, minimal and maximal values, and variance. Differences between medians for samples of the same tea concentration and different fermentation time were determined using Wilcoxon matched-pairs test (P < 0.05).

Results and discussion

Acidity and pH

During fermentation, the pH of brine decreased significantly in each experimental variant. The lowest value was observed on the last day of the experiment in sample with the addition of 4% tea. In all fermenters, the final pH remained at a similar level (Table 1), which suggests that the presence of dried green tea, even in the highest dose used, did not negatively affect the fermentation process.

Table 1 Changes in pH of brine during fermentation.
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The acidity of cauliflower and brine during the fermentation process in all tested experimental variants was higher than the acidity of raw cauliflower and fresh brine. After 7 and 14 days of fermentation, the acidity in the control sample was significantly lower than the acidity of cauliflower samples fermented with the addition of tea. In cauliflower samples fermented for 7 days, there are no significant differences in the acidity of samples with different doses of green tea. However, with the extension of fermentation time, the acidity of cauliflower increased; on day 14, it was the highest in samples with doses of 0.5–1% of tea (Table 2).

Table 2 Acidity of raw and fermented cauliflower and brine.
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The increase in acidity and decrease in pH of the brine during the fermentation process is caused by the formation of organic fermentation products, including lactic acid, acetic acid, and alcohol88,89,90,91.

Proximate composition

The dry matter content as well as ash and carbohydrates content in fermented cauliflower were higher compared to the raw material, while the protein content was statistically significantly lower (p < 0.05). There was no effect of the green tea dose on the protein content in fermented cauliflowers (Table 3).

Table 3 The proximate composition of raw and fermented cauliflower.
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According to literature data, changes occurring in raw materials and brine depend on the type of raw material and its composition, as well as on the biology of microorganisms involved in the fermentation process and on the conditions in which this process is carried out. The most frequently observed changes in the proximate composition occurring in raw materials during fermentation were a decrease in dry mass and a decrease in the amount of carbohydrates, including water-soluble sugars: glucose, fructose, and sucrose. The above-mentioned carbohydrate components were transformed by microorganisms into lactic acid, acetic acid, and propionic acids. Sometimes there were also changes in the fat and protein content and the composition of amino acids, but they were multidirectional—in some raw materials their content decreased, while in others it increased5,90,91,92,93,94.

Bioactive compounds and antioxidant activity

In contrast to the basic components, the content of bioactive components changed significantly depending on the addition of green tea leaves.

The addition of tea leaves to the brine caused a significant increase in the content of total polyphenols already at a concentration of 0.5%. Tea leaves, unlike cauliflower, are very rich in polyphenols16,18, therefore increasing the amount of tea caused a further increase in the total polyphenols content in fermented cauliflowers (Table 4). After 14 days of fermentation, the TPC was lower than after 7 days. Lactic acid bacteria are relatively resistant to polyphenol content (unlike other bacteria) because they have developed mechanisms to counteract their impact, including dissociation of polyphenol-substrate complexes, inactivation of polyphenols through high-affinity binders, alteration, or degradation of phenolics95. These effects occur through the production of enzymes such as tannases, esterases, phenolic acid decarboxylases, and glycosidases, which modify the polyphenol profile96. The amount of huge polyphenols molecules like tannins decreased, however small molecular mass compounds, like catechins or gallic acid increase, therefore antioxidant activity of the fermented products increase. However, hydrolysis of glycosidic bonds occurs, resulting in increased polyphenol solubility and extractability of antioxidant compounds, therefore, polyphenol content decreases with prolonged fermentation96. Kim et al.97 demonstrated that L. plantarum strains resulted in significant bitterness changes and increased antioxidant, antiglycation, and anti-aggregation activities. The solubility of polyphenols increases, thus potentially increasing their solubility and recovering more antioxidant compounds96. Although LAB have been found to cause changes in polyphenol content during fermentation, a control sample The total content of flavonoids (TFC) in cauliflower samples fermented for 7 days was higher than in raw cauliflower, but after 14 days it decreased by approx. 50–75%, while the samples without tea or with tea added up to 2% did not differ statistically significantly. In turn, the content of ascorbic acid (AA) in samples fermented with tea added at a concentration of 1–5% did not differ statistically significantly (p < 0.05). Yang et al.98, who subjected fruit and vegetable juice to 14-day fermentation with L. plantarum, also found an increase in TFC content and antioxidant activity of DPPH, TEAC and FRAP until day 6–8, and then a decrease in these values.

Table 4 Bioactive compounds and antioxidant activity of fermented cauliflower.
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Fermented cauliflower was characterized by several times lower antioxidant activity of DPPH and TEAC than raw cauliflower, only FRAP did not differ statistically significantly (p < 0.05). Water-soluble compounds which show the antioxidant activity, like ascorbic acid or some phenolic compounds have leached into the brine. On the other hand, lactic acid fermentation causes increase of low-molecular mass phenolic compounds and amino acids which shows higher antioxidant activities than macromolecules96. The addition of green tea leaves to the brine caused a multiple increase in antioxidant activity of fermented cauliflower, regardless of the method used (Table 4). Changes in antioxidant activity resulted from changes in the content of bioactive components (TPC, TFC, AA) and metabolites formed during fermentation. Green tea is a very rich source of polyphenols, the main ones being catechins18. Catechins are easily soluble in water, so they are extracted from tea leaves into the brine and can diffuse into cauliflower19. During fermentation, tea epigallocatechin gallate, epigallocatechin and epicatechin were hydrolysed to gallocatechin gallate and gallocatechin22,99. Many authors have confirmed that during fermentation their amount decreased22,23,100. Enzymes responsible for the hydrolysis of polyphenols lead to the formation of simpler phenolic compounds and their aglycones or conjugated glycoside forms, which proves the bioavailability and functionality of the obtained products101,102.

Sensory evaluation and acceptability

In this study, fermenting cauliflower for 14 days with higher doses of dried tea (4% and 5%) resulted in a slight color change to light yellow (Fig. 2). This was likely due to the effect of the tea leaf extract, which lightens and clarifies during fermentation100.

Fig. 2
figure 2

Fermented cauliflowers on 7th (A) and 14th (B) day of fermentation. Symbols: C—control sample (without dried green tea), 0.5%—sample containing GT at level of 0.5% of cauliflower mass, 1%—sample containing 1% of GT, 2%—sample containing 2% GT, 3%—sample containing 3% GT, 4%—sample containing 4% GT and 5%—sample containing 5% GT.

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On the 14th day of fermentation, the best sensory scores were obtained for cauliflower samples fermented with 0.5% and 1% dried tea (Table 5). The cauliflower had a salty-sour taste, typical of cauliflower and pickled cucumbers, and an aroma typical of pickled cucumbers (it was the most noticeable aroma), additionally sour and slightly salty (Fig. 3A–D).

Table 5 Overall sensory acceptability of fermented cauliflower determined by point method.
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Fig. 3
figure 3

Profiles of taste, odor, and texture of fermented cauliflower. (A) Taste, fermentation for 7 days, (B) Taste, fermentation for 14, (C) Odor, fermentation for 7 days, (D) Odor, fermentation for 14 days, (E) Texture, fermentation for 7 days, (F) Texture, fermentation for 14 days. Symbols: CC—control sample (without dried green tea), C0.5—sample containing GT at level of 0.5% of cauliflower mass, C1—sample containing 1% of GT, C2—sample containing 2% GT, C3—sample containing 3% GT, C4—sample containing 4% GT and C5—sample containing 5% GT.

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Green tea water extracts prepared with long brewing times are characterized by a very intense, bitter taste. In this study, the bitter taste was clearly noticeable only in samples fermented for 14 days with more than 1% tea (Fig. 3A,B). Products fermented with green tea leaves may be characterized by a bitter taste, but usually this impression is much weaker than in the case of a long-brewed tea infusion. The reason for this can be chemical changes occurring both in green tea and in the infusion itself. During fermentation, the content of ingredients that affect the taste of tea is reduced. This causes the taste to become stronger, but less bitter, sour–sweet20,21,22,23,100,103 with a noticeable umami aftertaste104. During fermentation, the content of tannins105, catechins22,23,100 and sucrose103 decreases. In addition, polyphenols are oxidized22,100, proteins are hydrolyzed, which are converted into amino acids62,70. In infusions, the content of simple sugars increases100 and new phenolic acids and catechin derivatives are produced23. The acids produced, including lactic acid103, cause a decrease in pH22,100,103.

According to the literature, tea fermentation also changes its odor to a milder, floral-fruity one, which is caused by the ketones, aldehydes and esters formed21,24,100,106,107. However, in this research, odor was most clearly noticeable with the notes of the fermented main raw material (cauliflower), similar to the odor of a pickled cucumber, while tea or fruity-floral notes were completely imperceptible. The odor components of fermented brassicas (including cauliflower) are mainly products resulting from the degradation of glucosinolates108,109, and the aroma profile of fermented cabbage consists of as many as 61 volatile compounds, including, among others, esters, terpenes, acids, alcohols, sulfur compounds, carbonyl compounds, and nitriles110,111,112,113. Fermented vegetable products also contain diactetyl, a component responsible for the characteristic buttery flavor and odor of the products102.

Changes in the texture of fermented cauliflower in this study were observed only on day 14 of fermentation in the 4% and 5% tea samples (Fig. 3E,F). Vegetables subjected to the fermentation process (including yellow beetroot and kimchi mixture) changed their texture: a decrease in their hardness, elasticity, and chewiness was observed in comparison with raw materials. The softening of plant tissues resulting from the influence of salt was the reason. This occurred as a result of cell swelling, disruption of membranes (plasmolysis) and cell walls in plant tissues by water accumulating in them, and then leakage of cell contents into the brine114,115.

PCA analysis

Principal components analysis showed that the two initial components explained 74.31% of the variance, while PC1 explained 43.22%. The antioxidant activity determined by all methods correlated significantly with total phenolic compounds, and to a lesser extent with flavonoid content (Fig. 4). A weak correlation was found between them and ascorbic acid content, as well as the content of basic components, especially fat content.

Fig. 4
figure 4

PCA biplot of the first two principal components for bioactive compounds, antioxidant activities, and proximate components of fermented cauliflowers. TPC, total phenolic compounds; TFC, total flavonoids content; AA, ascorbic acid; TEAC, Trolox equivalent antioxidant capacity; FRAP, ferric reducing antioxidant power; DPPH, DPPH radical scavenging ability; RC, fresh cauliflower; n = 3; Symbols: CC—control sample (without dried green tea), C0.5—sample containing GT at level of 0.5% of cauliflower mass, C1—sample containing 1% of GT, C2—sample containing 2% GT, C3—sample containing 3% GT, C4—sample containing 4% GT and C5—sample containing 5% GT.

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The distribution of samples in the quadrants of a two-factor case coordinate chart showed samples mutual correlations. Samples found in quadrant I are strongly correlated with high levels of bioactive compounds and antioxidant activities. There are cauliflowers with tea addition fermented for 7 days. Fresh cauliflower constituted a separate cluster, furthest from the others, located in the second quadrant, indicating a negative correlation with antioxidants content and properties. Cauliflower samples fermented without the addition of tea for 7 and 14 days formed the cluster in the third quadrant, while cauliflower samples fermented with the addition of green tea for 14 days formed another cluster in III and IV quadrants, indicating an increase in the strength of correlation with antioxidant properties with increasing tea concentration in the samples.

Summary

Fermenting cauliflower in brine with the addition of tea leaves enriches this traditional product with ingredients that increase its bioactivity. This is an alternative to traditional fermented products: it supplements the diet with minerals and antioxidants. It enhances the positive effects of fermented vegetables on human health and may contribute to improved dietary antioxidant intake.

Green tea increases polyphenols and antioxidant activity. The addition of tea did not significantly change the nutritional value of fermented cauliflower, its color, or crispness. The optimal concentration of 0.5–1% is to maintain sensory quality. The most recommended dose of tea leaves is 0.5–1% of cauliflower weight and fermentation lasting 14 days. The research results may have potential application in the development of functional fermented products.

One limitation of this study was the small number of panelists. Research conducted on a larger sample, taking into account differences by gender, age, education, and place of residence, most accurately reflects the perception of the product among general consumers. A second limitation was that only chemical and sensory analysis was conducted (no biological testing). Biological tests would demonstrate the product’s usefulness in the diet of healthy people and people with various diseases. Therefore, further research primarily involves microbiological testing of the product, which will determine whether it affects the effectiveness of reference strains of microorganisms (e.g., Bacillus cereus, B. subtilis, Enterococcus faecalis, Escherichia coli, Listeria monocytogenes or Salmonella enteritidis). Our next planned study is a bioavailability test and cellular cytotoxicity testing on selected cancer cell lines, such as the LoVo line of human colon cancer. This would allow us to assess whether the product can support therapeutic processes. Conducting a comprehensive consumer study will also be crucial to determine whether the product, with our chosen green tea dose and fermentation time, will appeal to a wider consumer audience.