Kettle or Microwave to Heat Water for Tea

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Is it demonstrably better to use a kettle or microwave to heat water for tea? Using a tea kettle is better since the water is heated evenly, and you know when the water starts to boil. Both ways will remain popular, but you might find that one has some advantages over the other.

Many people like to start their day with a hot cup of tea. If you're a fan of tea, then you probably make it a certain way. Some people heat water for their tea using a microwave. Others make tea more traditionally by heating water on a stovetop using a tea kettle.  

How Water is Heated in a Kettle

When a kettle is used to boil water, the liquid is evenly heated. This is because the water is heated by the process of convection.

Convection occurs when there is a difference in temperature between two parts of a liquid or gas. The heat energy is always transferred from an area with a higher temperature to an area with a lower temperature.  

Diagram illustrating how heat is transferred in a boiling pot

Here’s how convection works:

  • When a kettle of water is heated on a stove, the heat comes from the bottom.
  • The molecules in the warmer liquid move fast and spread far apart. They occupy a large volume which results in a decreased density. The warmer liquid rises from the bottom and transfers heat energy to the other molecules.
  • As the liquid cools off, the molecules slow down and come closer together. They occupy less volume resulting in an increased density. The cooler molecules sink back down through the fluid.
  • When they reach the bottom of the liquid, the cycle starts over again. This results in a uniform water temperature throughout the container.

Related Article: Best Induction Tea Kettles 

These can also be used on electric and gas stoves

How Water is Heated in a Microwave Oven

A microwave oven heats foods and liquids with microwaves, one type of wave on the electromagnetic spectrum. These waves produce constantly changing magnetic and electric fields. For this discussion, our focus is on the changing electric fields.

The three main parts of the microwave are:

  • Magnetron – a vacuum tube that generates energy that heats food
  • Waveguide – inside the wall; directs energy into the food
  • Chamber – holds food and safely contains microwave radiation

A microwave oven works like this:

  • When the start button is pressed on the microwave, the magnetron converts the electricity from the electrical outlet into microwaves.
  • These waves go through a hollow metal tube called a waveguide into the cooking chamber.
  • The microwaves bounce off the metal walls of the oven’s food chamber.
  • When the waves reach the food, they are absorbed by the food’s water content (in this case, the water is heated). This absorption makes the molecules move back and forth, creating the heat that cooks the food.
  • When these microwaves come into the chamber, they go all over the inside compartment. This results in the liquid at the top of the mug being hotter than the water at the bottom. Also, the water at the top does not drop to the bottom.

A study by Zhao et al. found when using a microwave to heat a liquid; the electric fields were unevenly distributed. This means there can be hot spots anywhere in the liquid. Zhao referred to this as “unusual convection.” This occurrence keeps the liquid always heated at the top, and the water at the top does not drop to the bottom. This resulted in uneven water temperature.

While you can stir the water, it may not be at the optimal temperature for brewing tea. As you’ll see in the next section, the water temperate is important when brewing tea.

Ideal Water Temperature for Brewing Tea

When the correct water temperature is used for brewing tea, the best flavors of the tea are brought out. For example, if the water is too cold for green and black tea, the tannins will be under-extracted. The tea won’t have the full flavor it was designed to create.

If the water is too hot, the tea will be astringent (dry, bitter flavor), and the aromatic compounds will be decreased because the tannins were over-extracted. This is especially problematic in brewing white and green teas. The bitter flavor can easily overwhelm the other flavors in the tea.

There are different temperature ranges for brewing each type of tea. The water temperatures for many teas fall below the boiling point of water (212°F).

Suggestions for starting points for water temperature for various teas are given in the table below. The optimal brewing temperature depends upon personal taste. A kitchen thermometer is helpful to gauge the tea’s temperature.

Type of Tea

Water Temperature

Time to Steep**

Black 

190°F

1.5-4 min

Green 

165°F (others say 185-195°F)

1.5-2 min

White

160°F

2- 3 min

Herbal

200°F

3-4 min

If a tea kettle is used, you’ll know when the water has reached the boiling point. After the water has just begun to boil, let it cool to the desired temperature.

If you prefer, you can add bottled or filtered water to cool it down. Another alternative is to cool the water down by pouring it from one bowl or mug to another.

Best Water to Brew Tea

Frank et al., researchers at Cornell University, tested different types of water used to brew tea to determine if it affected the flavor and the amount of Epigallocatechin Gallate (EGCG) in the tea.  Three types of water were used: bottled, tap, and deionized water. The same temperatures, vessel, time, and water-to-leaf ratios were used.

Green and black tea contain Epigallocatechin Gallate (green tea has much more), an antioxidant which prevents free radical damage to cells. (Free radicals are known to contribute to chronic health problems.) EGCG is also known to inhibit cellular oxidation, which can lead to tissue and cell damage. 

The results showed the type of water used to make black tea did not affect EGCG levels. However, green tea made with either bottled or deionized water contained twice the amount of EGCG than tea made with tap water.

Type of Water for the Highest Concentration of EGCG

Type of Tea

Type of Water

Green

Bottled or deionized

Black

No difference

In addition to EGCG testing, a panel of testers was asked to evaluate black and green tea made with bottled, deionized, and tap water. After tasting black tea, they were asked to assess various flavor attributes of black tea.

They thought black tea tasted the same, regardless of the type of water used, except for one attribute. They remarked black tea brewed with tap water was earthier. However, this did not affect whether they liked the tea or not. The researchers concluded water might not be an important factor in whether or not someone likes black tea.

The panelists preferred green tea brewed with tap water over both bottled and deionized water. One reason given was the tea brewed with tap water was much less bitter than with the other two types of water. 

If you use hard water to make your tea, be sure to clean your tea kettle frequently to avoid limescale build-up.

Panelist Results – Type of Water for Best Flavor

Type of Tea

Preferred Type of Water

Green

Tap

Black

Tap was earthier; overal - no difference

Making Water for Tea in a Microwave Oven

Using a microwave to heat water for your tea has one advantage that makes it appealing. It is faster to heat water in the microwave for your tea than to use a kettle to boil the water.,

Not everyone has a tea kettle or, if they do, want to use it every time they make tea. If this is the case, perhaps experimenting with various times will enhance the flavors.

As we discussed, an important issue for brewing tea is the water temperature. 

Determine the optimal time to heat the water for your tea.

  • Use the same mug, brand of teabags or amount of loose tea, and type and amount of water.
  • Microwave the water for different lengths of time – 90 seconds, 110 seconds, etc.
  • When the water is ready, stir the water and take the temperature with a kitchen thermometer.
  • Start with the temperature in the table above for the kind of tea you are brewing, and then adjust if necessary.

Once you get the temperature sorted out, then you can make tea using bottled, deionized, and tap water and decide which one you prefer for brewing your tea.

Final Thoughts

There is a surprising amount of debate about whether it is better to use a kettle or microwave to heat water for tea.

A kettle heats the water evenly, and you will be alerted when the water starts to boil. Using a kettle is going to take slightly more time preparing the water.

The microwave oven produces water that is heated unevenly. However, with a bit of experimentation, you can determine the amount of time to microwave water to attain a specific temperature.

The other advantage to a microwave is the obvious one – it takes less time to heat water in a microwave. If every minute in the morning counts, then you might gravitate towards using a microwave as opposed to heating water in a kettle for your tea.

If you don’t have a kettle and don’t want to use the microwave, boiling water in a pot or saucepan on the stove is an option.