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. 2020 Feb 21;57(7):2640–2650. doi: 10.1007/s13197-020-04300-0

Temporal depletion of packaged tea antioxidant quality under commercial storage condition

Anjan Hazra 1,2, Nirjhar Dasgupta 1, Chandan Sengupta 2, Gargi Saha 3, Sauren Das 1,✉
PMCID: PMC7270307  PMID: 32549614

Abstract

Shelf life studies play a significant role in determination of time duration for the retention of product quality after packaging. Assessment of tea shelf life in terms of antioxidant quality, a prime health benefit trait of tea would substantiate its marketing and consumption preference to the trade and end users. In shelf life analysis of tea with respect to its antioxidant potentialities, both antioxidant activity and incidences of secondary metabolites are responsible. A temporal analysis with regular intervals since 1 year of said characteristics has been carried out in four types of processed teas. To be precise, the overall initial antioxidant concentrations and activities were almost maintained up to 90–120 days and thereafter declination appeared. Beyond 180 days, rapid declination occurs and beyond 330 days, depletion recorded up to 60–75% of the initial activity. Black tea showed maximum ferrous ion chelating activity initially and white tea commenced with slight lower value but it maintained a similar trend up to 150 days while a rapid declination occurred in such activity of black and green tea after 30 days only. It is observed that total tannins or proanthocyanidins amount highest in white tea among all other three types. The preservation of metal chelating activity of white tea was observed as comparable to its stability in tannin composition (r2 = 0.869, P ≤ 0.01) during the storage period.

Electronic supplementary material

The online version of this article (10.1007/s13197-020-04300-0) contains supplementary material, which is available to authorized users.

Keywords: Antioxidants, Health benefits of tea, Shelf life, Storage, Tea

Introduction

With the increment of people’s awareness of the health benefit characteristics of tea, presently it is relevant to assess the shelf life tenure of packaged tea. This value provides a numerical support for labelling of the product with the date of expiry or ‘best before use’ which would specify the period of withholding its quality after packaging. Normally, before marketing, tea is packaged with different amounts in a sealed packet (preferably air tight). Properly conducted shelf life assessment on tea would ensure that the product quality maintained for a particular time period and beyond that, reduced quality of tea might be the customers’ grievances. Thus shelf life studies will provide scientific and logical data towards establishing the product quality for the tea consumers and greatly strengthen brand value of the tea as a whole.

In recent times, few researches focused on temporal changes in antioxidant properties in course of storage of the products after processing (Michalczyk and Macura 2010). Processed and packaged tea possess substantially long shelf-life (with respect to colour, flavour, briskness and antioxidant efficiency) for their low moisture content, due to involvement of a series of processing steps during manufacturing. Even several types of tea like oolong and pu-erh are stored for prolonged duration in order to restore appropriate taste and aroma. Though, in case of other common types of teas like green, black and white, during prolonged storage, it may cause quality deterioration through time. The report says the compounds responsible for tea health benefit properties are not retained in stable condition during storage. Especially, tea polyphenols are mostly inclined to degradation during storage. Friedman et al. (2009) reported that after 6 months of storage, epigallocatechin gallate (EGCG) of green tea bags decreased by one-third and epicatechin gallate (ECG) decreased by half at room temperature in the dark. Theaflavins, the potent antioxidant component of black tea, decreased about 22–37% after 12 months storage (Thomas et al. 2008). In accordance to this, several antioxidants activities of white tea were also found to be drastically reduced with increasing aging time (Xu et al. 2019).

Earlier studies suggested that packaging materials and storage conditions might play a vital role in maintaining stability of antioxidant quality during storage of processed tea (Cordero et al. 2009; Kim et al. 2011) that is possibly due to different level of oxygen permeability in the packaging materials. However the conventional mode of packaging and storage, which is being used for commercial conditions, often causes depreciation of quality after certain period which leads to massive revenue losses. Although controlled environment with specific packaging conditions might safeguard such quality to some extent, most of the recommendations are not commercially feasible for large scale storage and transport. Therefore it is necessary to assess the permissible time duration for maintenance of innate health benefit properties of tea in usual condition. In this present scope of work, judgemental scientific approaches have been taken into considered for assessing the withholding value of antioxidant ability of different packaged teas (black, green, white, oolong) in the commercial condition which would be an additional approach to make a baseline research towards succeeding a symbol ‘Best before use’ by the tea producers and the regulatory authorities.

Materials and methods

Processed tea samples

Four common types of processed tea samples i.e. black, green, white and oolong tea were considered in the current study. The samples under study were obtained from certified tea planters and manufacturers by Tea Board of India for research purposes through National Tea Research Foundation (NTRF). All four types of teas were packaged in twelve aluminium foil pouches each and sealed conventionally. An equal amount (100gms) of all types of teas having same manufacturing date and garden origin were taken for experiments. The packaged teas (12 × 4) were stored in dark and moist free chamber at laboratory at 25 °C. Considering the packaging date as day zero, one packet each from black, green, white and oolong were opened at 30 days interval up to 360 days for pursuing the required experiments leading to assess several biochemical parameters.

Extraction of secondary metabolites

At 30 days interval, all four processed teas were unpacked, amount weighed as required and proceeded for extraction. At first, homogenised fine powders were made from the dried samples using a blender. A total of 2 g of dry powder was extracted in 50 ml of HPLC graded methanol (Merck Millipore, Germany) through gentle agitation for 1 h using a magnetic Stirrer (Labman WMS-400). The methanolic mixture was centrifuged at 5000 rpm for 10 min and collect the clear supernatant. The extraction process was repeated twice further with the same sample for extracting maximum possible secondary metabolite components in the tea sample. The resulting supernatant was air dried to remove the solvent and final dry methanolic fraction of the sample was dissolved in 20 ml solvent for obtaining the stock solution of 100 mg/ml concentration. All the extracted solutions were preserved at 4 °C for further chemical analysis. A total 15 of previously reported antioxidant parameters including concentrations of secondary metabolites and various radical scavenging and metal chelating activity have been checked month wise to assess their depreciation trend through time.

Estimation of secondary metabolites

Total phenols

Total phenols were determined following the method of Singleton and Rossi (1965) with slight modifications by a UV-spectrophotometer (Helios γ, Thermo Electric Corpn., USA) using gallic acid as standard.

Total flavonoids

Total flavonoid content was estimated following the method described by Zhishen et al. (1999) using quercetin as standard.

Proanthocyanidin

Total Proanthocyanidin content was estimated following the method described by Sun et al. (1998) using catechin as standard.

Tannin

Total tannin content was estimated following the method described by Dasgupta et al. (2017) using tannic acid as standard.

Determination of radical scavenging ability

Scavenging ability of the test samples for DPPH (1, 1-diphenyl-2-picrylhydrazyl) and ABTS [2, 2-azino-bis (3ethylbenzothiazoline-6-sulphonic acid)] radical cations were determined following the methodology (Brand-Williams et al. 1995; Re et al. 1999). In all samples, per cent inhibition and IC50 values were calculated by tracing decolourization of the free radical source.

Determination ROS/RNS scavenging ability

Superoxide radical

Superoxide radical scavenging activity was estimated by the nitro-blue tetrazolium (NBT) reduction method according to Fontana et al. (2001).

Peroxynitrite

The Evans Blue bleaching assay method (Bailly et al. 2000) was adopted to measure the peroxynitrite (ONOO-) radical scavenging activity. Peroxynitrite was synthesized in the laboratory following the method described by Beckman et al. (1994).

Hydroxyl radical

Efficacy of the samples under study to scavenge hydroxyl radicals was determined by the method of Kunchandy and Rao (1990).

Nitric oxide

The nitric oxide radical, a reactive nitrogen species scavenging ability was quantified by the Griess Illosvoy reaction (Garratt 1964).

Singlet oxygen

Singlet oxygen radical scavenging activity was estimated with the help of a previously reported method (Pedraza-Chaverrí et al. 2004).

Hydrogen peroxide

Hydrogen peroxide scavenging activity was assayed following the method described by Long et al. (1999).

Hypochlorous acid

The Hypochlorous acid (HOCl) radical was prepared in vitro condition and efficiency of the sample to scavenge this radical was determined by the method described by Aruoma and Halliwell (1987).

Estimation of reducing power

The reducing power of the samples was measured following the method described by Oyaizu (1986).

Estimation of ferrous ion chelation activity

The ferrous ion chelating activity of the samples was assayed according to the standard method of Haro-Vicente et al. (2006).

Shelf life analysis

Results from total studied 15 antioxidant parameters in four types of teas at 30 days intervals to estimate the shelf life of processed teas in terms of their antioxidant activities i.e. the major health benefit attribute of tea. The periodical depreciation of individual antioxidant parameters has been plotted with standard error of the mean. The interrelationships among the studied parameters have been tested for four different types of tea by Pearson’s bivariate correlation analysis with R package and visual representations have been made with the assistance of MVApp program (Julkowska et al. 2018). The declination percentage of each individual parameter (i.e. flavonoid content, proanthocyanidin content, ABTS and superoxide radical scavenging activity etc.) in each observation (30 days interval) has been calculated by dividing the changes (difference between initial and momentary result) into each data by its initial appearances. The overall mean percentages of declination for 15 studied parameters have been used as predictor value of shelf life. A four parameter logistic regression model was plotted with the percent residual of overall antioxidant quality against storage duration using Quest Graph™ program. From the 4-PL sigmoid curve, probable days for 25% and 50% quality preservation have been calculated according to standard methodology for determination of shelf life for food and food products (Sharma et al. 2018). Similar analyses have been performed separately for studied four types of processed tea.

Result and discussion

This study considered determination of secondary metabolites which contribute to the overall antioxidant properties. From month wise antioxidant quality data (at 30 days intervals), it revealed that total phenols occur much higher at initial time but after 120 days, there is a sharp decline in green tea, whereas remarkable depletion takes place after 210 days in black and 180 days in white teas. Oolong tea, though initiate with a lower amount of total phenols than the other three types, retains its total phenol content level till 330 days. Initial flavonoid content is higher in green and white teas and degradation trends also almost identical, after 90 days, flavonoid contains drop down much and maintain its gradual depletion. Black and oolong teas, though initially have lesser amount than the other two, notable degradation starts after 90 days of preservation in case of oolong tea and 120 days in black tea. In all four types of teas, Proanthocyanidin depletion is at a faster rate (after 90 days). Oolong tea shows highest occurrence of Proanthocyanidin at initial stage but sharp decline occurs after 60 days, green starts from more or less same amount and falls after 90 days, thereafter both maintain similar lower trend. Black and white teas initially show much lower amount and minimum amount of depreciation occur along with the time duration. Tannin content is higher in white tea, rest of the tea types having almost similar amounts at the initial time, but in all the cases the depletion pattern is more or less same, after 90–150 days, the rate of degradation is remarkable (Fig. 1, Table 1).

Fig. 1.

Fig. 1

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Changes in different secondary metabolites during storage of four types of packaged tea. Initial concentrations have been considered as 100% and their remaining fractions has been calculated in each month

Table 1.

Incidences of secondary metabolites and reducing power of the four types of tea samples during 12 months storage

Storage time (days)
Initial 30 60 90 120 150 180 210 240 270 300 330
Phenolics (mg g−1 gallic acid eq.)
 Black 407 ± 32.6 362.5 ± 21.8 347.6 ± 31.3 329 ± 36.2 327.9 ± 22.9 306.4 ± 24.5 267.6 ± 13.4 178.6 ± 21.4 150.1 ± 6 112.3 ± 9 110.3 ± 7.7 89.5 ± 9.9
 Green 605.6 ± 48.5 531.9 ± 31.9 501.3 ± 45.1 470.7 ± 41.8 467.1 ± 32.7 231.1 ± 18.5 212.2 ± 10.6 208.4 ± 25 191.4 ± 7.7 182.7 ± 14.6 140.4 ± 9.8 127.3 ± 14.7
 White 502.8 ± 40.2 494.1 ± 29.6 447.5 ± 40.3 387.6 ± 42.6 376.3 ± 26.3 364.4 ± 29.2 285.6 ± 14.3 279.6 ± 33.6 274 ± 11 143 ± 11.4 139.3 ± 9.8 136 ± 16.6
 Oolong 242 ± 19.4 234.2 ± 14.1 218.8 ± 19.7 218.1 ± 24 213.4 ± 14.9 207.5 ± 16.6 205.7 ± 10.3 187.6 ± 22.5 179.1 ± 7.2 143.7 ± 15.5 133.6 ± 9.4 128.4 ± 12.8
Flavonoids (mg g−1 quercetin eq.)
 Black 217.4 ± 17.4 208 ± 12.5 200.6 ± 18.1 184.9 ± 20.3 167.8 ± 11.7 141 ± 11.3 136.8 ± 6.8 136.3 ± 16.4 124.4 ± 5 115.9 ± 9.3 50.5 ± 3.5 37.4 ± 3.7
 Green 344 ± 27.5 317.7 ± 19.1 304.6 ± 37.4 221.1 ± 24.3 205.7 ± 14.4 194.4 ± 15.5 188.8 ± 19.4 187.5 ± 22.5 184.4 ± 7.4 173.6 ± 13.9 94.2 ± 6.6 77.6 ± 8.8
 White 342.2 ± 27.4 295.6 ± 17.7 230.8 ± 20.8 204 ± 22.4 199.6 ± 14 192.7 ± 15.4 168 ± 8.4 154.1 ± 18.5 146.4 ± 5.9 102.5 ± 8.2 93.3 ± 6.5 84.4 ± 9.4
 Oolong 258 ± 20.6 250.1 ± 15 200.8 ± 18.1 129 ± 14.2 99.2 ± 6.9 94.4 ± 7.6 90 ± 4.5 85.6 ± 10.3 82 ± 3.3 79.6 ± 6.4 70 ± 4.9 66.4 ± 7.6
Proanthocyanidins (mg g−1 catechin eq.)
 Black 148.7 ± 11.9 122.5 ± 7.4 91.6 ± 8.2 88.4 ± 9.7 66.3 ± 8.6 55.1 ± 4.4 51.4 ± 2.6 46.7 ± 5.6 45.6 ± 1.8 41.4 ± 13.3 40.4 ± 2.8 34.6 ± 3.5
 Green 274.4 ± 21.9 226 ± 13.6 147 ± 13.2 140.4 ± 15.4 125.2 ± 8.8 116.2 ± 9.3 109.2 ± 5.5 96.8 ± 11.6 92.3 ± 3.7 87 ± 7 82.2 ± 5.8 75.8 ± 7.6
 White 86.3 ± 6.9 80.7 ± 4.8 66.9 ± 6 64.4 ± 7.1 63.5 ± 4.4 61.6 ± 4.9 61.2 ± 3.1 57.3 ± 6.9 56.4 ± 2.3 55 ± 4.4 52.4 ± 3.7 52 ± 5.2
 Oolong 300.7 ± 24.1 266 ± 16 102.5 ± 9.2 100.4 ± 13.8 99.6 ± 7 89.6 ± 7.2 86 ± 4.3 84 ± 10.1 80.8 ± 3.2 78 ± 6.2 72.4 ± 5.1 70 ± 7
Tannins (mg g−1 tannic Acid eq.)
 Black 262.5 ± 21 126.8 ± 7.6 121.2 ± 10.9 118.9 ± 13.1 103.9 ± 7.3 98.5 ± 7.9 96.5 ± 4.8 95.8 ± 11.5 95.6 ± 3.8 84.4 ± 6.8 75.7 ± 5.3 41.5 ± 5.1
 Green 296.5 ± 23.7 226.1 ± 13.6 199.7 ± 18 149.9 ± 16.5 133.3 ± 9.3 126 ± 10.1 119.6 ± 6 106.3 ± 12.8 101.2 ± 4 86.1 ± 6.9 58.1 ± 6.1 10.6 ± 1.8
 White 408.3 ± 32.7 328.3 ± 19.7 257.1 ± 23.1 246 ± 27.1 218 ± 15.3 215.7 ± 17.3 191.4 ± 9.6 132.2 ± 15.9 124 ± 5 83.3 ± 7.7 82.7 ± 5.8 78 ± 7.8
 Oolong 328.3 ± 26.3 162 ± 9.7 153.3 ± 13.8 121.8 ± 13.4 120.4 ± 8.4 119.6 ± 9.6 112.4 ± 5.6 110 ± 13.2 108.4 ± 4.3 104.8 ± 8.4 96.8 ± 6.8 88.4 ± 8.8
Reducing Power (mg g−1 ascorbic acid eq.)
 Black 131.6 ± 10.5 109.5 ± 6.6 105 ± 9.5 97.3 ± 10.7 95 ± 6.7 93.9 ± 7.5 93.1 ± 4.7 92.7 ± 11.1 88.4 ± 3.5 87.6 ± 7 27.6 ± 1.9 26.1 ± 2.6
 Green 325.7 ± 26.1 246.9 ± 14.8 192.6 ± 17.3 190.4 ± 20.9 189.2 ± 13.2 188.1 ± 15 186.7 ± 9.3 178.1 ± 21.4 166 ± 6.6 162 ± 13 145.4 ± 10.2 34.8 ± 3.5
 White 212.2 ± 17.7 195.5 ± 11.7 193.2 ± 17.4 192.1 ± 21.1 190.9 ± 13.4 189.2 ± 15.1 182 ± 9.1 181.6 ± 21.8 164.8 ± 6.6 140.9 ± 11.3 134 ± 9.4 124.4 ± 12.4
 Oolong 199.6 ± 14.5 181.1 ± 10.9 168.4 ± 15.2 164.4 ± 18.1 167.6 ± 11.7 161.2 ± 12.9 159.6 ± 8 152.4 ± 18.3 150 ± 6 148.4 ± 11.9 51.6 ± 3.6 41.2 ± 4.1
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Reduced phenol content in white tea than the green teas and similar to the black has been observed. White tea is generally deliberated as the least processed and unfermented tea. However, this statement is not firmly correct because, inactivation of enzymes like polyphenol oxidases and peroxidases are not activated before withering and these remain essentially active for colour development in white teas too (Jiang 2008). Consequently, white tea polyphenols are oxidised slightly and the antioxidant activity of this remains slightly lower than green tea as observed in present study. Initially the occurrence of highest proanthocyanidin content expressed as catechin equivalent in oolong tea is supported by the previous report of Zhang et al. (2011) who conducted HPLC–DAD-ESI–MS based analysis of ripened and aged pu-erh, green, oolong, black, white and yellow teas and found incidence of total catechin is highest in oolong tea.

According to the final recorded data at 330 days, the degradation order for all four types of tea antioxidant secondary metabolites were as follows—(i) Phenol–Green > Black > White > Oolong, (ii) Flavonoid–Black > Green > White > Oolong, (iii) Proanthocyanidin—Black ≈ Oolong > Green > White, (iv) Tannin—Green > Black > White > Oolong. It is interestingly noted, although black tea contains lower antioxidant value than green tea to some extent, it is less affected by storage period than green tea. In other words, this result someway leads to the statement that black tea possess health promoting properties as much as green tea (Hazra et al. 2017).

As it is hard to quantify the total antioxidant activity based on single active components (Pinelo et al. 2004), therefore total form of antioxidant activity has been estimated along with individual radical scavenging property. Free radical scavenging ability has been studied by ABTS and DPPH radical scavenging activity (% inhibition of free radicals) assay. Both the assays showed per cent scavenging ability initially higher in green and oolong tea, but depletion rate is almost similar to the other two tea types. IC50 calculated data represented reverse complement of per cent scavenging representation (it is the expression of the concentration of the extract which needed to scavenge 50% of the radical present in specific amount of solvent) (Supplementary Table 1). Fe+2 chelating activity is similar in green and black teas, where reduction of per cent chelating ability occur after 60 days. White tea retains its chelating ability all most same up to 120 days and oolong tea shows a gradual decrease at each 30 days intervals. Black and green teas show nearer value of per cent scavenging in Peroxynitrite assay and both decrease after only 30 days of storage and black exhibits minimum value from 210 days onwards. It has been noted that oolong and white teas showing moderate values of per cent scavenging till up to 330 days of storage (Fig. 2).

Fig. 2.

Fig. 2

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Changes in various radical (ABTS, DPPH, Ferrous ion and Peroxynitrite, Superoxide, Nitric oxide, HOCl and Hydrogen peroxide, Hydroxyl, singlet oxygen) scavenging ability during storage of four types of packaged tea. In all samples, per cent inhibition was calculated by tracing decolourization of the free radical source in compared to the blank reaction

Superoxide radical scavenging ability initiates more or less same points in black, green and white teas and trends of reduction start after 30 days of preservation and then more decline at 120 days. Beyond that black tea shows rapid decline than the other two types. Oolong tea possessed a moderate scavenging ability and rate of exhaustion is much less and upholds its per cent scavenging ability much better than black one till 330 days. Nitric oxide scavenging power is much higher in black, green and white than oolong tea and noteworthy decrease starts beyond 180 days of preservation. Like superoxide, nitric oxide scavenging ability is being maintained at uniform rate till 330 days of conservation in case of oolong tea. HOCl radical scavenging activity observed to have similar value in all four types of tea and uniformly started to decline after 210 days. Black and green teas show much higher scavenging values of H2O2 radical scavenging assay at initial time and both sharply decline at 150 days. White tea shows from lower initial rate and degradation initiate at 60 days, beyond this, the degradation rate as like black and green teas. But, exceptionally in oolong tea, initial per cent scavenging aptitude is moderate that is preserved till 300 days (Fig. 2).

Hydroxyl radical scavenging ability is higher in white tea and lowest in oolong tea, but after temporal degradation, oolong tea is showing to some extent higher than those of the other three. Singlet oxygen (1O2) radical hunting activity is higher in green tea and after gradual depletion, sharp degradation recorded at 300 days. Only white tea shows degradation more or less up to 50% from its initial value after 60 days. In this assay, oolong tea shows almost similar radical scavenging values all throughout the experimental period. Reducing power occurs higher radical hunting value in green tea at initial point and loses about 50% ability after 60 days. White oolong and black teas performing more or less steady state radical scavenging ability under prolonged preservation time, till 300 days (Figs. 1, 2). However, experimental results of all scavenging assays pointed out that after 4–6 months preservation, the antioxidant quality of all three types of teas gradually reduced, except oolong tea, where upholding radical hunting ability has observed for prolonged time than other three types of teas.

Carloni et al. (2013) reported that the white tea and black tea shows the highest metal chelating activity while the green teas display the lowest. In the present study, black tea had maximum ferrous ion chelation activity at beginning and white tea started with a lower value but it maintained a similar rate up to 150 days while a rapid declination occurred in such activity of black and green tea after a month only. The fact of condensed tannins or proanthocyanidins amount highest in white tea among all types was also observed by Zhao et al. (2011). The maintenance of metal chelating activity of white tea was observed as comparable to its stability in tannin composition during the same period (r2 = 0.869, P ≤ 0.01). According to previous report, these tannins are also called secondary antioxidants along with the function of primary antioxidants by donating hydrogen atom or electrons. Tannins perform the role of secondary antioxidant by chelating metal ions such as ferrous ion and interfering one of the reaction steps of Fenton reaction and finally retarding oxidation (Karamać et al. 2006). Tea with higher antioxidant activity for augmented catechins content and but not possessing higher metal chelating activity also observed by Lin et al. (2008). They opined that the chelating ability probably due to the occurrence of components other than catechins. Adjacent hydroxyl and carbonyl functions bearing compounds of a specific group have been proposed to have major role in metal chelation. This group can be found in exclusive compounds of fermented teas like theaflavins which are present in larger amounts in white and black teas than green tea (Carloni et al. 2013). In present study it is reflected that, after certain time of storage metal chelating activity of black and green tea drastically declined whereas the same is steadily maintained by both semi-fermented oolong tea and slightly fermented white tea. The fact might be interpreted by presence of stable metal chelating compounds like theaflavins along with non-oxidised polyphenols in white and oolong tea whereas green and black tea contains a single reciprocal counterpart.

In this study, the Pearson’s correlation coefficient based matrix depicts almost all secondary metabolites were highly correlated with different antioxidant capacities except the cases where iron chelation and superoxide radical scavenging activity had weak correlation coefficient with phenols and flavonoids in black tea. On the other hand, nitric oxide and hydroxyl radical scavenging activities were not significantly associated with both proanthocyanidin and tannin content. In green tea, almost all secondary metabolites, especially flavonoids and tannins were highly correlated with different antioxidant activities except the cases where DPPH and nitric oxide radical scavenging activity had weak correlation coefficient with proanthocyanidin. On the other hand reducing power and iron chelation activities were not significantly associated with phenol content. In white tea, among all antioxidant activities worked out, superoxide radical, nitric oxide radical and hydrogen peroxide scavenging activity had quite striking responses and weak correlation with the secondary metabolites. Except these cases, phenol, flavonoids, proanthocyanidin and tannins had significant association with other antioxidant activities. In oolong tea, DPPH and hydrogen peroxide scavenging activity has no direct correlation with flavonoid, proanthocyanidin and tannin but show positive link with phenol content. However, the ABTS scavenging activity was not correlated with phenol but was strongly associated with other three studied secondary metabolites (Fig. 3, Supplementary Table 2). Correlations between phenolic compounds and antioxidant activity have already been reported (Sellappan et al. 2002). Here, strong correlation among secondary metabolites and various antioxidant activity has been observed in individual types of tea. As compositional difference due to varietal difference, climate, soil composition, harvesting, manufacturing and storage practices is a complex phenomenon in tea (Carloni et al. 2013; Friedman et al. 2009; Jiang et al. 2019), so consideration of the health-promoting activity changes trend line through time will be better than the comparison among antioxidant activity of different tea.

Fig. 3.

Fig. 3

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Representative pairwise Pearson’s Correlation Coefficients based diagram among selected antioxidant parameters from Green tea. (The colour and size of the pie reflected the strength and significance of the correlation values)

Present study revealed, both antioxidant activity and secondary metabolites responsible for such activity of the investigated four processed teas decreased markedly throughout the storage period. In order to determine the shelf-life of four types of processed (packaged) teas, 15 antioxidant parameters have been assessed and temporal data of each parameter have been generated at 30 days interval for 1 year. The declination percentage of each individual parameter (i.e. flavonoid content, proanthocyanidin content, ABTS, and superoxide radical scavenging activity etc.) at each point of time (30 days interval) has been calculated. Subsequently, estimation of shelf life of processed teas in terms of their antioxidant activities i.e. the major health benefit attribute of tea has been extrapolated. The mean declination percentages from 15 studied parameters have considered as predictor value of shelf life. In regression analysis, percent residual quality after subtracting the decline’s share has been considered as dependant variable against ‘storage duration’. Best fit curves have been extrapolated by 4-parameter logistic regression fit (Fig. 4) and from this, credible days for 50% and 25% initial quality preservation have been calculated. The calculated results have been depicted in Table 2, where oolong tea showing the greater value of preservation time at 50% and 25% of antioxidant quality maintenance and black tea showing shortest preservation time in all the designated declination percentages. The predicted shelf life period of maximum days in oolong tea could be inferred from above order as most of the antioxidant components least degraded in it throughout the storage time. It is supported by an earlier report which revealed that most catechins survive and maintain a substantial amount during the manufacturing process of oolong tea (Meng et al. 2018).

Fig. 4.

Fig. 4

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Shelf life extrapolation curve for four different types of processed tea determined by four parameter logistic regression model. Overall antioxidant properties combining studied 15 parameters in each month was plotted to draw the extrapolation lines

Table 2.

The estimated shelf life of various Indian teas from storage data in terms of overall antioxidant properties

Residual quality (%) Predicted shelf life of antioxidants (days)
Black Green White Oolong
50 183.0 191.0 250.0 307.7
25 331.6 371.7 378.8 579.3
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Conclusion

In the present study, antioxidant qualities of various processed tea and its temporal depletion during storage in market conditions have been taken into account. Incidence of depleted secondary metabolites and antioxidant activities across the time in different four types of tea (black, green, white and oolong) was assessed, which revealed that oolong tea allows retaining the positive qualities of tea for prolong time than those of the other three types. A prediction calculation for shelf-life of tea antioxidants has been derived from experimental results and the anticipated time period for at least 50% and 25% preservation of whole antioxidant properties in four types of Indian processed tea has been estimated. This approach might have a dual beneficial role in the industry as well as to the end users in managing storage and marketing schedule.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary material 1 (XLSX 16 kb) (16.3KB, xlsx)
Supplementary material 2 (XLSX 15 kb) (15.2KB, xlsx)

Acknowledgements

We acknowledge the role of National Tea Research Foundation, India for providing necessary funds and research assistance for conducting this study.

Author contributions

AH carried out all experiments, NDG, CS, GS, and SD analysed the data, AH and SD written the manuscript, all authors read and accepted the current form of the manuscript.

Compliance with ethical standards

Conflict of interest

All authors have declare that they have no conflict of interest.

Footnotes

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