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Oils Formed on the Surface of the Bean After Roasting

Roasted #coffee beans are either matte or glossy. If the grains are shiny, that is, they shine, they are covered with oil: sometimes there is very little of these oils - a few drops, and sometimes the grains are completely covered with oil. In this article, we understand what these oils are and what their amount depends on.

Sometimes the surface of the roasted beans is covered with oil droplets, sometimes the beans are completely covered with oil. In this article, we will explain what these oils are and why they appear.

Is the oil on the surface of the grain good or bad?

Oil droplets on the surface of the coffee are essential oils released during roasting. In fact, it is thanks to #essential oils that the smell of coffee is so bright and memorable. However, this does not mean that the released oils are good.

First, the oxidation of oils as a result of their interaction with oxygen is one of the main causes of coffee aging. Secondly, in order for the prepared drink to be as tasty as possible, we first need essential oils, while the oils on the surface quickly lose their volatile aroma compounds. As a result, this type of coffee smells brighter on the pack, but tastes worse in the cup.

Often times, oils on the surface may not mean the most pleasant taste. The truth is that when sugars burn and acids break down, they are released at these temperatures in the roaster. Therefore, such coffees mostly contain bright bitterness and minimal acidity in taste.

Oftentimes, if good coffee oils turn out, they are present in very small amounts or none at all. But their number depends on many reasons, which are discussed below.

Why are oils released on the surface of the bean?

The degree of roasting is the main and simplest reason. In light and medium roast beans, fats are mostly not formed, only in individual grains in some varieties. Dark roasted beans almost always contain fat. This is because with a darker roast, the grain structure becomes more porous and caramelization processes take place actively.

Due to the darker roasting of espresso coffee, oils are sometimes released on it. Their number varies from rare drops in some grains to completely oily grains with a dark surface.

Frying errors. Often times the oils are released not because of the degree of frying but because of the faults of the roaster.

Professionals refer to the appearance of "Migration" on the surface of oils as oil. It occurs when there is a large moisture difference between the surface and inner layers of the grain.

The moisture content of the grains before roasting is evenly distributed throughout the grain and is equal to 9-12%. The water in the grains is divided into free and bound. The free water evaporates almost completely during "drying", the first stage of roasting. The bound water evaporates more slowly until the end of frying as it is with other elements inside the cell walls.

During roasting, the surface temperature of the beans is higher. This results in a higher evaporation rate of bound water in the outer layers than in the inner layers. For example, if you make a mistake while roasting, apply too much transmission at an insufficient level of radiation energy, the moisture difference between the grain layers will cause the oils to "migrate" - the oils will pass into the layers. Grain with lower moisture and cell wall density.

As a result, even lightly roasted beans may contain oil if the coffee is roasted incorrectly. For example, if heating the drum roaster is not sufficient, but if you roast coffee at high burner power values ​​or pour the coffee into a drum that is too hot, it will cause roasting - the appearance of dark spots on the beans.

Too long cooling after roasting. After roasting, the processes inside the grains do not stop, they continue during cooling.

The grains are cooled by a large air flow in the cooling mixer. If the coffee is cooled for too long, the airflow dries the surface of the bean, although chemical processes continue inside the coffee. Also, when cooling for too long, a large air flow takes away some of the flavoring substances with it, making the coffee less bright.

Shelf life of coffee after roasting. Migration of fats occurs not only immediately after roasting, but also during storage.

The oils are released for several months after roasting. So, for example, if a drop of oil appears on grains today, after a few weeks of storage it will no longer be a drop but much more. By the way, this is one of the reasons why even the darkest roasted coffee from Italian brands is not shiny and covered with oil - usually this coffee is already quite old.

Characteristics of the coffee variety. There are varieties of coffee in which the cellular structure of the beans or the high sugar content cause the surface oils to be released.

In an article on processing methods, we wrote that naturally processed coffee usually contains more sugar and has a more porous cellular structure. Therefore, caramelization processes are more active and the risks of burning sugars and destruction of surface cell walls are higher.

The same goes for low gravity coffee, ie coffee grown at low altitude. If it is roasted too quickly, a "migration" effect of the oils will occur. The same effect will be enhanced if you roast stale green coffee whose moisture level has dropped to 7-8%.

These varieties include any naturally processed Ethiopian, low-growing Brazilian or Peruvian coffee, and some non-standardly processed coffee.


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