Time for coffee and tea
Black tea infusion time
The consumption of tea has been said to lower the risk of developing cardiovascular disease and several types of cancer. The ingredients associated with these beneficial effects are the antioxidant polyphenolic molecules abundant in tea, mainly catechins, flavonols, theaflavins and thearubigins.
There have been many research studies involving in vitro and animal models along with human intervention trials that indicate the power of tea to reduce the risks of atherosclerosis and cancer. However, examination of population studies that investigate the relationship of tea consumption and incidence of diseases have not provided the conclusive information that would support the idea.
A study that investigated the effect of tea infusion time suspected that the disparity in the data may be due to the imprecise assessment of tea consumption habits in the conduct of epidemiological studies. The measures of consumption are at best semi-quantitative – such as “number of cups” consumed per day.
The study thinks that more precision could have been achieved by considering the various brands and different tea varieties that are consumed, or the method of preparing the tea (such as brewing time). In countries such as the United Kingdom, black tea is principally consumed mixed with milk, a method which may result in the formation of polyphenol-milk protein complexes. If such a complex protein were to happen, it would reduce the antioxidant potential of the polyphenolic compounds thus removing the capability of black tea to fight atherosclerosis.
It was thus necessary to investigate these possible causes for the equivocal data that seemed to come out of epidemiological studies. First, they wanted to determine the influence of infusion time on the concentration of phenols of six brands of commercial black tea. Then, they wanted to test how the addition of milk would affect the antioxidant potential and total phenol concentration and individual flavonoids.
The polyphenol compounds transferred from the tea bags to the water rapidly, and concentration reached the peak after 7 minutes of infusion, leveling off after that. Catechin concentration reached 87% after 3 minutes, but total phenols achieved only 60% during that period, suggesting a slower transfer. Increases in antioxidants were directly correlated with increase in total phenol and catechin content, but the correlation with catechin was greater (coefficient being 0.80 versus 0.67). The infusion patterns in releasing phenolic and catechin compounds into water were the same for all six brands. Additions of milk did not significantly affect antioxidant concentration.
In the human intervention trial, the tea brand (among the six) that exhibited the highest total phenolic and catechin contents was used.
The test tea leaves were infused for 7 minutes, and each subject was give 400mL of black tea, which is about 2 cups and represents the average daily consumption of tea in the UK. Blood samples were taken 10 minutes before drinking, then 50, 80, and 180 minutes after.
The results showed that the two cups of tea caused significant transient increases of total phenolic compounds, total catechins, and the flavonols quercetin and kaempferol, as measured in blood plasma. During the 3-hour time period of the trial, concentrations of total phenol, quercetin and kaempferol peaked 50 minutes after ingestion, while total catechin concentration was highest after 80 minutes. Again, the addition of milk did not significantly affect the uptake of these nutrients into the blood.
This study should help refine future research methods, and maybe reconcile the equivocal results so far obtained. For us ordinary drinkers, the significance is that if we wish to maximize the release of the beneficial compounds in tea leaves, it will be best to extend infusion time to 7 minutes.