The key technology of brewing coffee the method of "steaming" coffee

Steaming, often referred to as Bloom in English instructions, refers to the preparation of a small amount of hot water to evenly wet the surface of coffee powder before formal injection in the process of hand flushing.

Steaming has two purposes:

The first is to release the gas in the coffee particles, mainly carbon dioxide. In general, the closer it is to baking time, the more bubbles there will usually be during pre-soaking. Deeper roasted coffee beans, because they contain less water, release more gas during steaming than lighter roasted coffee beans.

Second, if the coffee particles after the exhaust gas can be evenly saturated with water, the subsequent extraction can be uniform. In short, the air between the coffee particles is heated and expands during steaming, releasing a large amount of carbon dioxide, and the temporary void layer between the coffee particles will provide the space required for hot water to penetrate the powder layer during formal water injection.

If the initial step of hand washing is less steamy, a large amount of hot water will follow a local and few channels through the powder layer, which will lengthen the drip time, which may lead to excessive extraction, because the extraction time will be longer, more substances will be dissolved, and the taste may be more sour and irritating, or the mouth may be full of impurities. However, once you can pay attention to the steaming action and then start the formal water injection, the hot water can pass through the filter layer composed of coffee powder uniformly, and then achieve the purpose of uniform extraction.

Here are a few things that will help to achieve a smooth steaming:

Before steaming, make sure the coffee powder is as level as possible in the strainer or filter paper.

Steaming when the force of water injection is not large, gentle.

The amount of water used in steaming is preferably 10% of the total amount of water. For brewing 240 ml of coffee, the amount of water required for steaming is about 24 - 30 ml.

During stewing, the amount of extract flowing down should be as small as possible; when there is too much extract in the next pot, reduce the amount of water injected, or pay attention to the degree of grinding.

Steaming time is about 30 seconds to 60 seconds. When the expansion of the coffee powder surface is over and the surface is about to collapse, it means that the steaming is completed and the first water injection can be started. Ideally stewed, the coffee grounds expand slowly in contact with hot water, as if the coffee grounds were constantly pushing hot water upward. The sum of the surface tension of the water and the pressure of the carbon dioxide released from the coffee powder at this time will slightly increase the gravitational acceleration of the water and the downward pull caused by capillary phenomenon. The best time to steam will be when the expansion reaches its limit, the sum of the surface tension of water and the pressure generated by the release of carbon dioxide from coffee powder is slightly equal to the downward pull caused by the acceleration of gravity and capillary phenomenon of water. Stable water injection speed, at this time, the void layer before coffee powder particles has not collapsed, which is a good time to start the first water injection. Although longer stewing time may give a thicker taste, it also affects the brewing result due to the larger surface area in contact with air and the faster the temperature drops.

Whether a cup of hand-brewed coffee tastes pleasant, in addition to the nature and freshness of the coffee beans, mostly depends on whether a good filter layer is formed, and steaming will determine whether the hot water penetrates the coffee powder sufficiently and well. In Bloom is a prelude to coffee extraction and one of the key steps that coffee extraction cannot ignore.