Iced Tea: Does Cold Brewing Enhance Antioxidant Content?

Iced tea is a must-have when the heat is on. Whether it's flavored or plain, we all love it more than ever. However, hot water is reputed to be an indispensable criterion for the release of tea's beneficial molecules. Does iced tea release just as many beneficial molecules? Let's take it a step further: does iced tea have any added health benefits over hot tea?

But what exactly is iced tea?

Iced tea refers to tea that's cold, but there are many ways to make iced tea:

- Tea can be brewed at room temperature for a few minutes to several hours before putting in the fridge.

- Iced tea can be infused overnight with water that has already been chilled in the fridge.

- Tea can be infused by placing ice cubes on the leaves and leaving them to melt

I have deliberately ruled out the method where the tea is infused hot and then put in the fridge. This method requires an initial heating step, which is not the purpose of this article, which focuses solely on cold-brewing methods.

What's going on in my iced tea carafe?

The literature on iced tea and the molecules released is still relatively sparse. Among the studies that have looked at the molecules obtained after cold-brewing tea are Lantano et al. and Castiglioni et al. in 2015.

Another story about antioxidant molecules!

Lantano and colleagues infused black, oolong and green teas either cold (4°C for 12h, enough time to drink morning tea and get to work) or hot (90°C 3min, 85°C 4min and 75°C 4min respectively).

Molecular analysis of each infusion showed that cold infusion increased the overall concentration of polyphenols compared with hot infusion (between 6 and 8 mg/g leaf). In addition, the concentration of EGCG, the polyphenol with one of the most powerful antioxidant potentials in tea, increases with cold infusion (between 2 and 3 mg/g leaf for green tea and oolong). By contrast, the concentration of EC, another powerful tea polyphenol, was not detected in iced tea (although the amount of EC is zero in black tea and low in hot-brewed oolong).

To understand whether or not cold brewing improved the antioxidant power of tea, the authors measured the FRAP value, an experimental value obtained when antioxidant molecules are brought into contact with oxidized molecules. The higher the FRAP value, the greater the antioxidant molecules' ability to counteract oxidation. The authors found that the FRAP value was slightly higher for iced green tea (1653.5 vs. 1463.1) and iced black tea (1218.8 vs. 1111.8). On the other hand, this value is multiplied by 4 for iced oolong (1278.5 vs. 325.4).

It's true that iced tea is potentially more antioxidant, but what does it taste like?

Like Lantano et al. Castiglioni et al. were interested not only in the antioxidant power of cold-brewed green and white teas, but also in the organoleptic sensations of iced tea, i.e. the perception of tea taste.

To carry out their study, the authors purchased white (Bai Mu Dan, White Lung Ching, White Salima Peony) and green (Green Lung Ching, Green Salima Peony) teas and infused them either hot (70°C for 7 min) or at room temperature (15min to 2h). The authors found that the total quantity of polyphenols was highest in the tea cold-brewed for 2h for Bai Mu Dan, White and Green Salima Peony. The amount of polyphenols was no different for White Lung Ching and lower for cold-brewed Green Lung Ching. Furthermore, similar to Lantano et al. they demonstrated that cold infusion for 2h increased the antioxidant activity of Bai Mu Dan, White and Green Salima Peony, but not Green or White Lung Ching.

These results are in line with those obtained by Lantano et al. but they also suggest that the type of tea may also condition the cold diffusion of antioxidant molecules.

What about practical health benefits?

Unfortunately, very few data on the health effects of iced tea are reported in the scientific literature.

The high temperature of tea was at one time the source of a debate on the risks of chronic inflammation of the esophagus (“Barrett's Esophagus”) and the consequences for the occurrence of esophageal carcinoma. A prospective study conducted from 2004 to 2008 on 50045 people and published by Islami et al. in 2019 looked at the effects of daily consumption of black and green tea at temperatures above or below 60°C on the occurrence of esophageal carcinomas. The authors found that hot tea increased the risk of esophageal carcinoma (particularly for those who drank more than 700mL per day), but this was not the case for tea at temperatures no higher than 60°C.

This dichotomy between hot and cold was also observed by Maufrais et al. in 2018 for mate. Indeed, 33 people consumed either cold (3°C) or hot (55°C) mate infusions. 90 minutes after ingesting cold mate, heart rate decreased slightly, but catabolism (in this case, lipid oxidation), baroreflex (the body's reaction to control blood pressure) and energy utilization increased slightly. In addition, a stronger hypoglycemic action of iced green tea was also reported by Chen et al. in 2019 (abstract issued by one of the collaborators, Jiayi Jiang, in the same year).

These various data could suggest that iced tea modulates cell metabolism, which could be of interest for metabolic disorders such as diabetes or those associated with cardiovascular disorders.