Retention of Original Flavor Characteristics in Defluorinated Instant Qingzhuan Brick Tea Prepared Using Membrane Separation Technology

Abstract

Brick tea is a type of post-fermented food that involves microorganisms. Long-term consumption of brick tea exposes consumers to high fluoride levels, which can adversely affect their health. This study explored the feasibility of selective defluorination of Qingzhuan brick tea through membrane separation technology, and pilot production was conducted to produce defluorinated instant brick tea. The concentration of tea polyphenols increased by more than 10 times after nanofiltration, demonstrating the high selectivity of nanofiltration membranes toward fluoride. Defluorination trends were studied at different initial material concentrations (0.5–4%) and operating pressures (0.1–0.5 MPa) under cyclic defluorination. Defluorinated instant brick tea products were also industrially prepared using 300- (DF-300) and 1000-Da (DF-1000) membranes, followed by vacuum freeze-drying. The DF-1000 and DF-300 products exhibited a defluorination rate of 51.46% and 67.96%, respectively. The products have excellent characteristics in terms of color, aroma, and flavor quality, as well as solubility. Gas chromatography–mass spectrometry indicated that the volatile components in the defluorinated instant brick tea were slightly different from those in the original tea, but the key aroma and flavor characteristics of the defluorinated brick tea remained unchanged. Membrane separation provides technical support for the large-scale production of low-fluoride post-fermented tea.

1. Introduction

Fluoride is an essential trace element for the human body and has both favorable and unfavorable effects on human health [1]. Adequate intake of fluoride can effectively prevent dental caries and can increase bone density, thereby preventing osteoporosis. However, excessive intake of fluoride can cause fluoride toxicity, resulting in conditions such as dental fluorosis and skeletal fluorosis [1]. The Dietary Reference Intakes for Chinese Residents guide recommends a daily fluoride intake of 1.5 mg for adults. Additionally, the “Health Standards for Total Fluoride Intake in Populations” stipulates that the maximum daily fluoride intake for children and adults is 2.4 and 3.5 mg, respectively [1,2]. The World Health Organization states that a daily fluoride intake exceeding 6 mg increases the risk of skeletal toxicity, and an intake exceeding 14 mg increases the risk of fractures [1]. Fluoride toxicity includes water-type fluorosis, coal-burning-type fluorosis, and tea-type fluorosis. Tea-type fluorosis is primarily due to long-term consumption of tea containing excessive amounts of fluoride [1,3].

Border tea, also known as brick tea or compressed tea because of its brick-like shape, is a general term for post-fermented food that involves microorganisms sold to ethnic minorities residing in northwest China. Several types of brick tea are available, including black brick tea, Fuzhuan brick tea, Hua brick tea, Kang brick tea, tight brick tea, and rice brick tea. Studies have demonstrated numerous benefits of brick tea; for example, it facilitates digestion, exerts antioxidant effects, regulates gastrointestinal function, and promotes weight loss and fat reduction [3], rendering it an essential component of the daily lives of ethnic minorities residing in border regions.

The tea plant accumulates excess amounts of fluoride, primarily in its leaves. Older leaves have higher levels of fluoride than do younger leaves [1,4]. Because the raw materials for brick tea are often coarse and old, long-term consumption of brick tea can lead to chronic fluoride toxicity. Epidemiological studies have confirmed that prolonged consumption of brick tea with excessive fluoride content can result in fluorosis [1,4,5]. Therefore, effectively controlling the fluoride content in brick tea has been a pressing challenge for the brick tea industry. Studies have explored several defluorination strategies for brick tea; such strategies include using low-fluoride raw materials as a means of source control [4,6], improving agricultural management practices to reduce fluoride accumulation [7,8], optimizing production processes to reduce fluoride content [1], and changing consumption habits to decrease the bioavailability of fluoride [1,9,10]. Although these strategies signify progress, their practical implementation is marred by challenges.

Nanofiltration (NF) is a pressure-driven membrane separation technique with nanometer-level retention capabilities [11]. The separation precision of nanofiltration lies between that of ultrafiltration and that of reverse osmosis. NF can effectively retain most ions and organic molecules in water and is characterized by high selectivity, low energy consumption, and high rejection rates [12,13]. Therefore, NF technology is increasingly applications in various applications, including water desalination [11], wastewater treatment [11,13], and biopharmaceutical production [14]; it is also used in various stages of food processing [15,16], including recovery of functional compounds [12,17], deacidification [14], and demineralization [18].

Uyttebroek et al. [19] determined that the commercial nanofiltration membrane NFX exhibited its good retention capability for the recovery of phenolic compounds and quinic acid from apple pomace with good flux and concentration factor at pilot scale. PVDF–12%PDMS NF membrane was found to show the best permselectivity performance to separate free fatty acid from soybean oil, achieving 58% removal rate [14]. Three commercial membranes (NF270, Fortilife XC-N, and PRO-XS2) were evaluated for desalination brine valorization, and the NF membrane was proven to be an efficient strategy for selective eliminating target ions from seawater desalination brine [18]. Moreover, the DK nanofiltration membrane demonstrated superior permeability and selectivity to the other four commercial NF membranes [20].

The present study investigated the use of nanofiltration to defluorinate Qingzhuan brick tea to produce instant brick tea with original flavor. This study involved three stages: feasibility testing, nanofiltration pattern research, and pilot-scale production. The findings of this study can inform the production of original brick tea with low fluoride content.

2. Materials and Methods

2.1. Reagents and Materials

Instant brick tea and Qingzhuan brick tea were entrusted to Zhejiang Minghuang Natural Products Development (Hangzhou, China) and Hubei Zhaoliqiao Tea Factory Co., Ltd. (Xianning, China) for processing, respectively. High-performance liquid chromatography (HPLC)-grade formic acid, methanol, acetonitrile, acetic acid, and anhydrous ethanol were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). High-purity sulfuric acid, nitric acid, and hydrochloric acid, and analytical-grade ascorbic acid, sodium carbonate, sodium hydroxide, potassium hydroxide, glucose, sodium chloride, magnesium chloride, potassium chloride, and ethylenediaminetetraacetic acid disodium (≥99%) were obtained from China National Pharmaceutical Group Corporation Chemical Reagent Co., Ltd. (Shanghai, China). Gallic acid (≥99%), ethyl decanoate (≥99%), and Folin–Ciocalteu phenol reagent (≥99%) were obtained from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). HPLC-grade catechin, catechin gallate, epicatechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, gallocatechin, gallocatechin gallate, tea polyphenols, and theanine were purchased from Shanghai Yuanye Biotechnology (Shanghai, China). Deionized water (18.2 MΩ cm) was prepared using a Milli-Q Gradient system (Billerica, MA, USA).

2.2. Feasibility Analysis of Highly Selective Fluoride Removal

Nanofiltration membranes with different pore sizes, namely NF1 (100–150 Da), NF4 (500–1000 Da), and NF7 (1000 Da), were purchased from Hefei Keruite Environmental Protection Engineering Co., Ltd. (Hefei, China). The feasibility of these membranes for highly selective fluoride removal was evaluated, and then different working pressures (0.1–0.5 MPa) with 0.2% initial concentrations and initial concentrations (0.1–0.5%) with 0.2 MPa were analyzed using NF-7; this feasibility was assessed using two metrics: defluorination rate and ratio of fluoride to tea polyphenols after nanofiltration.

2.3. Factors Influencing Defluorination Using Nanofiltration

The instrument used for nanofiltration was equipped with a crossflow pump, a feed tank, a nanofiltration system (300 Da and 1000 Da), and a pressure control valve. To mimic the parameters used during industrial production, the initial brick tea concentration and working pressure were set to 0.5–4% with 0.5 MPa working pressure and 0.1–0.7 MPa with 2% initial concentration, respectively. The nanofiltration permeates were collected, and the concentrations of fluoride, caffeine, catechins, and tea polyphenols were analyzed. Furthermore, more than 40 fluoride removal cycles were executed using 200 mL of deionized water at an initial brick tea concentration of 2%, and a working pressure of 0.5 MPa.

2.4. Analysis of Defluorinated Instant Brick Tea Products Prepared Through Nanofiltration

2.4.1. Preparation of Defluorinated Brick Tea

The pilot production of defluorinated instant brick tea was implemented at Huangshan Greenxtract Co., Ltd. (Huangshan, China; Figure 1). Specifically, 10 kg of Qingzhuan brick tea was crushed for pretreatment and extracted at 90 °C using a material-to-liquid ratio of 1:14 under constant stirring at 3018 g for 30 min. After coarse filtration using a 120 mesh sieve, the material was passed through a ceramic membrane (0.2 μm). Subsequently, a nanofiltration membrane with a pore size of 300 Da was used for cyclic defluorination, and changes in the conductivity and fluoride content of the permeate were monitored throughout the process. After defluorination, the obtained solution was concentrated to the dead volume through reverse osmosis, followed by instantaneous sterilization at an ultrahigh temperature and freeze-drying to obtain defluorinated instant brick tea (denoted herein as DF-300). Similarly, defluorinated instant brick tea was prepared using a nanofiltration membrane with a pore size of 1000 Da (denoted herein as DF-1000). The products were subsequently evaluated to determine the corresponding defluorination rate; in addition, a sensory evaluation and quality assessment were conducted for the products.

2.4.2. Sensory Evaluation

Anhui Agricultural University does not require ethics committee approval for human sensory ethical, we ensured that the sensory evaluation followed stringent protocols to protect the rights and privacy of all participants. These measures included securing voluntary participation, providing comprehensive information about the study’s requirements and potential risks, obtaining explicit written or verbal consent from participants, maintaining the confidentiality of participant data, and allowing participants the freedom to withdraw from the study at any time.

This study established a sensory evaluation panel comprising 10 trained students majoring in tea science. The evaluation criteria were based on the following proportions proposed by Liu et al. [21]: instant brick tea color (10%), liquid color (25%), aroma (25%), taste (30%), and solubility (10%). The comprehensive score was calculated as follows: (Liquid Color Score × 0.25) + (Dry Leaf Color Score × 0.10) + (Aroma Score × 0.25) + (Taste Score × 0.30) + (Solubility Score × 0.10). Each of the aforementioned five components was scored along with comments. The evaluation was repeated three times, and the final score was calculated as the average of the scores from all 20 participants.

2.4.3. Quality Analytical Methods

The fluoride content in brick tea and nanofiltration permeates was measured using a fluoride ion-selective electrode (9609BNWP, Orion, Waltham, MA, USA) [22]. The contents of catechins and caffeine were determined using HPLC (Waters e2695, Waters, Shanghai, China), and the total polyphenol content was measured using the Folin–Ciocalteu method (BeckMan DU730, BeckMan, Brea, CA, USA) in accordance with the national standard [23]. The conductivity of nanofiltration permeates was monitored using a handheld conductivity meter (PT-11). The flavor quality of the defluorinated brick tea products was assessed using Portable Electronic Nose 3 (AIRSENSE Analytics, Schwerin, Germany), and the key volatiles were analyzed and identified using gas chromatography–mass spectrometry (Agilent 8890A+5977B, Agilent, Santa Clara, CA, USA) combined with headspace solid-phase microextraction (HS-SPME-GC-MS) [24].

2.5. Calculations

The defluorination rate (%) and loss of catechins and total polyphenols (%) at a specific time t were defined as follows:

Defluorination rate, loss rate (%) = V(C₂ − C₁)/C₂ × 100%

where V denotes volume of the brick tea infusion (L) and C₂ and C₁ denote concentrations of fluoride, catechins, or total polyphenols in the liquid phase before and after nanofiltration (mg·L⁻¹).

2.6. Statistical Analysis

All experiments were performed at least in triplicate, and the data were statistically analyzed using IBM SPSS software (version 24.0; SPSS, Chicago, IL, USA). The results are presented as means ± standard deviations and were plotted using GraphPad Prism 5.0 (GraphPad Software, San Diego, CA, USA). A single-factor analysis of variance was performed using IBM SPSS Statistics 25 software, and significance was assessed using least significant difference test (p < 0.05) after an analysis of the results of Bartlett’s test for equal variances.

3. Results and Discussion

3.1. Feasibility Analysis of Highly Selective Fluoride Removal Using Nanofiltration Technology

Nanofiltration of brick tea extracts was performed using membranes of different pore sizes at an operating pressure of 0.5 MPa and an initial brick tea concentration of 0.1%. The results demonstrated that the number of molecules passing through the membrane per unit time was higher for membranes with larger pores, resulting in a higher defluorination rate for such membranes (Figure 2). The NF membranes with smaller pore sizes exhibited the highest retention efficiency, while those with larger pore sizes had the highest permeate flow rate [25,26]. Consequently, as the membrane pore size increases, the rejection rate of the membrane decreases.

The ratio of fluoride content to tea polyphenol content was used as an indicator for evaluating the selective removal of fluoride from the tea liquid using nanofiltration performed at different operating pressures (0.1–0.5 MPa) and initial material concentrations (0.1–0.5%; Figure 3,

Table 1. Ratio of fluoride content to tea polyphenol content for evaluating the feasibility of highly selective fluoride removal using NF-7.

Initial Concentration (%)	Fluoride (mg)/Tea Polyphenols (mg)			Working Pressure (MPa)	Fluoride (mg)/Tea Polyphenols (mg)
	Original Tea Infusion	Permeate	Permeate/Original Tea Infusion		Original Tea Infusion	Permeate	Permeate/Original Tea Infusion
0.10	0.33	3.67	11.03	0.1	0.24	10.48	44.41
0.20	0.33	6.38	19.19	0.2	0.28	3.17	11.33
0.30	0.32	11.82	37.04	0.3	0.30	19.18	63.89
0.40	0.33	4.02	12.10	0.4	0.30	36.50	123.68
0.50	0.30	7.95	26.71	0.5	0.33	3.67	11.04

). As presented in Table 1, compared with the ratio observed before nanofiltration, the ratio observed for the permeate was greater than 10 at all initial material concentrations tested, indicating that the nanofiltration technology eliminated fluoride with high selectivity. Therefore, the feasibility of using nanofiltration for preparing low-fluoride instant tea was validated. NFX was employed to concentrate from 59.5 mg/L to 1256.1 mg/L in the final retentate of 10 selected phenolic compounds and quinic acid in apple pomace extracts [19], indicating that membrane technology can be employed to concentrate valuable phytochemicals extracted from agricultural products at a pilot scale.

3.2. Factors Influencing Defluorination Using Nanofiltration

3.2.1. Characteristics of Cyclic Defluorination

The constructed nanofiltration system was operated at a pressure of 0.5 MPa and an initial material concentration of 2% (industrial standard). Water was continuously added to the system to investigate the patterns of cyclic defluorination (Figure 4). The defluorination rate gradually decreased with increasing cycle number, and the total defluorination rate was 82.40%. Notably, the loss rate of tea polyphenols in the tea liquid remained relatively low, with a total loss rate of 0.84%, suggesting that the system demonstrated high selectivity for fluoride.

3.2.2. Regulation of Defluorination Under Different Initial Concentrations and Working Pressures

Initial material concentration is a crucial factor affecting the efficiency of defluorination through nanofiltration [15,17]. To examine the effect of initial material concentration on defluorination rate, the nanofiltration system was operated at a pressure of 0.5 MPa with 20 cycles of water addition. The defluorination rates were found to be 39.78%, 41.07%, 51.39%, 74.74%, and 34.97% at initial material concentrations of 0.5%, 1%, 2%, 3%, and 4%, respectively (Figure 5a). As the initial material concentration increased, the defluorination rate first increased and then decreased. The highest defluorination rate was obtained when the initial material concentration was 3%. The decline in defluorination rate when the initial material concentration increased from 3% to 4% can be explained as follows: An increase in initial material concentration raised not only the fluoride ion content but also the content of other tea components, such as polyphenols, catechins, and caffeine, which may have led to membrane clogging. This, in turn, resulted in the slower circulation of the tea liquid through the nanofiltration system, a decrease in permeate volume, and a decline in defluorination rate. The loss rates of tea polyphenols remained low at the various initial material concentrations tested (Figure 5b).

Ramdani et al. [25] reported the same trends in defluorination rate with increasing initial material concentrations. In their study, the rejection rate reached its maximum value when the fluoride concentration was 4 mg/L; subsequently, the rejection rate decreased with increasing fluoride concentration. This finding is primarily because an increase in fluoride concentration implies an increase in the ionic strength of fluoride ions, which leads to more fluoride ions being adsorbed on the membrane surface, reducing the equivalent charge density and eventually, the rejection rate [26]. Additionally, a rapid increase in fluoride concentration leads to a weakening of the Donnan effect, resulting in a smaller concentration difference between the membrane pores and attenuated electrostatic repulsion, thereby lowering the rejection rate [27,28].

Nanofiltration is a pressure-driven process; excessively high pressure levels can cause membrane rupture, affecting the lifespan of the nanofiltration process, whereas extremely low pressure levels can severely compromise the efficiency of nanofiltration. To examine the effects of different operating pressure levels on defluorination rates, the nanofiltration system was operated with an initial material concentration of 2% and with 20 cycles of water addition. The defluorination rates were found to be 75.94%, 52.45%, 51.39%, and 52.31% at operating pressure levels of 0.1, 0.3, 0.5, and 0.7 MPa, respectively (Figure 5c). A previous study revealed that the transmembrane pressure difference increased continually with operating pressure, allowing a large volume of material to pass through the nanofiltration membrane, thus reducing the fluoride ion concentration in the material [20,25]. As displayed in Figure 5d, prolonged nanofiltration increased the loss rate of tea polyphenols (7.38%) when the system was operated at a low pressure of 0.1 MPa. However, the loss rate of tea polyphenols remained low as the operating pressure increased. Higher applied pressure was shown to promote membrane fouling and osmotic pressure, thus exacerbating the decay in flux [29].

3.3. Analysis of Defluorinated Instant Brick Tea Products Prepared Through Nanofiltration

3.3.1. Preparation of Defluorinated Instant Brick Tea Products

To further analyze the quality characteristics of defluorinated instant brick tea, pilot production experiments were conducted at Huangshan Greenxtract Co., Ltd. Defluorination was performed using nanofiltration membranes with pore sizes of 300 and 1000 Da and with cyclic water addition, resulting in the production of DF-300 (0.62 g/kg, defluorination rate of 67.96%) and DF-1000 (0.94 g/kg, defluorination rate of 51.46%) products. A blank experiment was also conducted. The results indicated that the conductivity of the permeate was linearly correlated with the defluorination rate (R² ≥ 0.9483, p < 0.0001, Figure S1). This finding provides a theoretical basis for research on real-time online monitoring of fluoride content during industrial production. Long-term consumption of these defluorinated instant brick tea products poses little health risk. Salgado et al. [30] successfully reduced the sugar found in red grape must by using nanofiltration at the pilot plant scale. The feasibility of nanofiltration for producing high-quality whey powder was also demonstrated at pilot scale, resulting in a powder with an ash and moisture content of 4% and 2.5%, respectively [31].

3.3.2. Analysis of Defluorinated Instant Brick Tea Products

Table 2. Sensory evaluation of defluorinated instant Qingzhuan brick tea products.

Products	Color of Instant Brick Tea (10%)	Color of Tea Liquid (25%)	Aroma (25%)	Taste (30%)	Solubility (10%)	Total Score
CK	82.50	84.60	84.60	78.90	84.10	82.63
DF-300	76.90	87.70	85.60	75.80	88.00	82.56
DF-1000	78.70	86.80	85.60	78.80	89.80	83.59

presents the sensory evaluation results obtained for the defluorinated instant brick tea produced in the pilot tests. According to the five evaluation criteria, namely powder color, liquid color, aroma, taste, and solubility, the results obtained for the two defluorinated instant brick tea products were as follows: both DF-300 and DF-1000 presented as a fine, brown powder and a brownish-red liquid, had a pure aged aroma and a mellow yet slightly astringent taste, and exhibited excellent solubility, comparable in quality to the blank control group (Table S1, Figure 6a–c and Figure S2). The contents of tea polyphenols, caffeine, and catechins, including epigallocatechin, epigallocatechin gallate, epicatechin, and epicatechin gallate, remained generally stable during industrial production (Figure 6d). Therefore, the flavor quality of the defluorinated instant brick tea prepared using nanofiltration was comparable to, or even superior to, that of the blank control, preserving the original flavor characteristics of brick tea. Salgado et al. [29] reported a 1–2% reduction in alcohol by volume in red and white wines whose production process involved nanofiltration of the grape musts before fermentation. Moreover, their study indicated no significant differences in sensory evaluation results between control and filtered wines.

3.3.3. Analysis of Key Volatiles in Defluorinated Instant Brick Tea Products

Qualitative and relative quantitative analyses of the key volatile compounds in defluorinated instant brick tea were performed using headspace solid-phase microextraction combined with gas chromatography–mass spectrometry. A total of 65 volatile compounds were identified in the original instant brick tea, comprising 13 alcohols, 15 aldehydes, 14 ketones, 5 esters, 6 acids, 2 alkanes, and 10 other compounds (Figure S3). The average relative concentration of alcohols was 23.64%, and these alcohols primarily comprised linalool, 1-octanol, and 1-heptanol. The average relative concentration of ketones was 22.62%, and these primarily comprised α-ionone, β-ionone, 4-ketoisophorone, and methyl heptenone. Moreover, the average relative concentration of aldehydes was 17.23%, and these primarily comprised low-molecular-weight fatty aldehydes (such as hexanal, heptanal, and nonanal), aromatic aldehydes (such as benzaldehyde), and alkenals from fatty aldehydes (such as trans-2-hexenal, trans,trans-4-heptadienal, and 2-octenal). A total of 10 key components significantly influence the aroma quality of Qingzhuan brick tea and were identified in Qingzhuan brick tea, including (E,E)-2,4-heptadienal, linalool, (E)-2-nonanal, decanal, safranal, hexanal, 1-octen-3-one, α-ionone, (E)-β-ionone, 6-methyl-3,5-heptadien-2-one [21,32].

Compared with the original instant brick tea, DF-300 and DF-1000 exhibited a slight lower variety of volatile compounds and an overall decrease in their relative concentrations. However, chromatographic analysis along with the results of sensory evaluation revealed that the defluorinated instant brick tea samples retained the characteristic aroma of brick tea, containing relatively high concentrations of key compounds (

Table 3. Analysis of key components in defluorinated instant brick tea products.

Key Components (μg/mL)	CK	DF-1000	DF-300	Odorthresholds	Odor Quality
Linalool	0.0884 ± 0.0101 b	0.1385 ± 0.1051 a	0.0993 ± 0.0782 ab	0.0006	Citrus-like, flowery
(E)-2-nonanal	0.0239 ± 0.0031 a	ND b	0.014 ± 0.0078 a	0.0028	Citrus-like, soapy
Decanal	0.0080 ± 0.0028 a	ND a	0.0032 ± 0.0021 a	/	/
Safranal	0.0470 ± 0.0057 a	0.0432 ± 0.0327 a	0.0298 ± 0.0235 a	/	Saffron-like
Hexanal	0.2284 ± 0.0300 a	0.0331 ± 0.0221 b	0.0154 ± 0.0114 b	0.0024	Green, grassy
(E,E)-2,4-heptadienal	0.0041 ± 0.0027 a	ND b	ND b	0.000032	Fatty, flowery
1-octen-3-one	0.0025 ± 0.0002 a	ND b	0.0002 ± 0.0001 a	0.000016	Mushroom-like
α-ionone	0.0364 ± 0.0041 a	0.0319 ± 0.0219 a	0.0209 ± 0.0151 a	0.0004	Flowery, violet-like
(E)-β-ionone	0.0284 ± 0.0079 a	0.0205 ± 0.0116 a	0.0111 ± 0.0083 a	0.000021	Flowery, violet-like
6-methyl-3,5-heptadien-2-one	0.0914 ± 0.0102 a	0.0198 ± 0.0215 b	NDc	/	/

Note: ND indicates not detected, / indicates odor quality or odorthresholds not found, CK indicates control group. A p value of <0.05 was considered to indicate statistical significance.

). The aroma quality of DF-300 was superior and distinct, with a pure aged scent. In summary, defluorination through nanofiltration has minimal impact on the flavor characteristics of brick tea, with the defluorinated instant brick tea retaining the original flavor characteristics.

The effort to develop methods for controlling fluoride levels in border tea processed from coarse leaves has consistently posed a challenge for the tea industry and is, therefore, warranted. Presently, several strategies have been explored to inhibit fluoride accumulation in tea leaves or reduce bioavailability. It was found that the fluoride content in the bud to fifth leaves of Xiangbo Lǜ variety ranged from 96.0 to 114.8 mg/kg, and was considered a low-fluoride tea variety [33]. Zhang et al. have indicated that controlling the closure of the anion channel can effectively reduce the fluoride absorption in tea roots [34]. Highly selective adsorbents can significantly lower the fluoride content in brick tea infusion while maintaining its flavor quality [1]. Furthermore, dietary factors can decrease the bioavailability of fluoride by reducing the absorption of fluoride and promoting its excretion [9]. The technology provided in this study further improves the comprehensive fluoride reduction system for tea products.

4. Conclusions

This study investigated the defluorination of brick tea by using defluorination rate and product flavor as the main evaluation criteria. Highly selective defluorination of fluoride ions in the tea extract was performed using membrane separation technology. The nanofiltration membrane selectively removed fluoride ions from the tea extract. The defluorination efficiency was influenced by membrane pore size, initial material concentration, and operating pressure. The defluorinated instant brick tea produced through nanofiltration using a membrane of pore size 1000 Da and 300 Da in industrial pilot tests received a satisfactory sensory evaluation. The products, presented as a brown powder with relatively fine particles and a bright brownish-red liquid, had a pure aged aroma and a mellow taste with lingering sweetness, and exhibited excellent solubility. Although the types and quantities of volatile components in the defluorinated instant brick tea were different from those in the original tea, the aroma and flavor characteristics of the brick tea remained unaffected.