Last updated 05:00 23/07/2013
Why is tea made with microwave-heated water so lousy compared to tea made with water boiled in a kettle? Because a proper cup of black tea must be made with water that's come to a rolling boil.
A kettle is designedto heat water evenly to 100°C. Heat at the bottom of the kettle - whether from a heating element embedded in an electric device or from a burner on the stove - creates a natural convection current: The hot water rises and the cool water falls in a cyclical fashion, which uniformly heats the contents of the kettle to a boil (at which point an electric kettle clicks off or a stovetop kettle whistles).
But microwaves don't heat water evenly, so the boiling process is difficult to control. Microwave ovens shoot tiny waves into the liquid at random locations, causing the water molecules at those points to vibrate rapidly. If the water isn't heated long enough, the result is isolated pockets of very hot or boiling water amid a larger body of water that's cooler. Such water may misleadingly exhibit signs of boiling despite not being a uniform 100°C.
For instance, what appears to be steam rising from a mug of microwaved water is only moist vapour evaporating off the water's surface and condensing into mist on contact with cooler air. It's the same principle that makes our breath visible on frigid days. Why is water temperature so important togood-tasting tea?
When tea leaves meet hot water, hundreds of different compounds that contribute flavour and aroma dissolve and become suspended in the water. Black tea contains two kinds of complex phenolic molecules, also known as tannins: orange-coloured theaflavins and red-brown thearubigins. These are responsible for the colour and the astringent, brisk taste of brewed black tea, andthey are extracted only at near-boiling temperatures.
Water also cooks certain volatile compounds, chemically altering them to produce more nuanced flavours and aromas, such as the earthy, malty, and tobacco notes in black tea. When the water isn't hot enough to instigate these reactions and produce these bold flavours, tea tastes insipid. Overheated water results in bad tea, too - and this is also easier to do in a microwave than in a kettle since there's no mechanism to indicate when the water has reached a boil. The longer water boils, the more dissolved oxygen it loses - and tea experts say that dissolved oxygen is crucial for a bright and refreshing brew.
Microwaved water can also be taken to several degrees above boiling if heated for too long (which is impossible in a kettle because the metallic surface prevents overheating). Such ultra-hot water destroys desired aromatic compounds and elicits an excessof astringent, bitter notes by overcooking the leaves. Overheated water can also accentuate naturally occurring impurities in the water, contributing negatively to the flavours of the final brew.
It's possible that the material of the heating vessel also affects tea's flavour. Modern day kettles are invariably made from stainless steel. While stainless steel is considered a nonreactive material, research has shown that minuscule amounts of chromium, iron and nickel can migrate from a container or a utensil into the food. These don't pose a safety threat, but they may subtly affect the taste of water boiled in a kettle.
In contrast, only glazed ceramics, glass and plasticsare safe to use in microwaves. It's not inconceivable that the lack of trace metal ions is partly responsible for a lousy cup of microwave tea.Microwaved water isn't totally useless for all tea. In fact, water that's microwaved to below boiling is ideal for green tea. The mellow, brothy flavours prized in green tea are mostly derived from specific savoury-tasting amino acids that start to dissolve at 60°C.
While mouth-puckering tannins are desirable in black tea, with green tea, boiling water extracts too many astringent notesand too much bitter caffeine which can overwhelm the delicate amino acids. Caffeine is extremely soluble at 100°C, but significantly less so at 63°C to 80°C, the ideal temperature range for brewing green tea.
- Sunday Magazine