How does coffee fermentation occur?

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Coffee fermentation plays a crucial role in shaping the flavor of coffee beans. This process occurs in every coffee processing method and needs to be carefully controlled to avoid the production of compounds that could negatively impact the quality of the coffee beans. Join 43 Factory Coffee Roaster to explore how the coffee fermentation process takes place!

What is coffee fermentation?

Fermentation is the process that stimulates microorganisms to metabolically exchange, leading to chemical transformations of biochemical compounds through the activity of enzymes. In biochemistry, it is defined as the extraction of energy from carbohydrates in the absence of oxygen. In the realm of microbiology, fermentation can be succinctly interpreted as the metabolic exchange process of microorganisms, where bacteria transform sugars into energy compounds and release other organic substances.

Coffee fermentation is the stage in coffee processing where the pulped coffee beans are soaked in water until the mucilage completely decomposes. The fermentation process is a natural transformation that occurs when sugar and water, two components naturally present in coffee beans, combine. During fermentation, microorganisms such as fungi and bacteria in nature consume and transform the sugar and acids in coffee.

The purpose of coffee fermentation is to allow positive compounds from the pulp to be transformed into the beans to a reasonable extent. Subsequently, it aids in the removal of the mucilage layer, which is rich in polysaccharides, and reduces the water content in the coffee beans.

Types of coffee fermentation

Coffee fermentation can occur in one of two ways: Aerobic fermentation or anaerobic fermentation. Here’s an explanation:

– Aerobic fermentation: This occurs when oxygen is present in the air. Microorganisms utilize oxygen and organic matter (organic compounds in the coffee cherry’s pulp) to grow and release secondary organic compounds.

– Anaerobic fermentation: This takes place when the coffee cherries are completely submerged in water (after pulping). This environment allows various microorganisms to operate without the need for oxygen.

Anaerobic fermentation helps farmers control and adjust various parameters.

These two processes can occur simultaneously in the coffee processing method. However, anaerobic fermentation, facilitated by acid-resistant microorganisms, is more easily controllable and uniform compared to aerobic fermentation, which is more complex and challenging to regulate.

How does coffee fermentation take place?

The coffee fermentation process occurs in all processing methods (natural, washed, semi-washed), but the fermentation time varies for each method.

Natural Processing Coffee Fermentation

After harvesting, coffee cherries undergo a process of removing impurities, thorough washing, and immersion in a large water tank. During the first 24 hours, green, unripe, overripe, and insect-damaged cherries are eliminated. Subsequently, the entire coffee batch is sun-dried on racks for 10 to 25 days without hull removal until the moisture content drops to around 11-12%. The coffee fermentation process takes place during this drying period as enzymes from various microorganisms are released. The pulp and mucilage break down to form pectinases on the parchment, and the pectin degradation process serves as a carbon source for microorganisms after initial sugar absorption. The fermentation process initiates with the isolation of bacteria, yeast, and mold.

Washed Processing Coffee Fermentation

The fermentation process is stimulated and carried out by microorganisms, fungi…

This method utilizes a significant amount of water. After the coffee cherries are thoroughly washed to remove impurities, they are mechanically pulped to separate the skin, pulp, and mucilage. The resulting beans are then placed in a large tank, submerged in water for 36-72 hours, initiating the underwater fermentation process. The purpose is to eliminate the remaining mucilage, which is later sun-dried or machine-dried after fermentation.

Wet processing helps limit the sticky mucilage on the beans. Mucilage, a substance utilized by bacteria and yeast (naturally occurring in the fruit) during fermentation, contains pectin (polysaccharides), the precursor to flavor-enhancing compounds naturally secreted by the microbial fermentation process. Due to its adhesive nature, water is necessary to separate it.

Compared to dry processing, microorganisms generated in wet processing are less diverse due to the shorter fermentation time (approximately 36-72 hours), a quicker decrease in pH, and the absence of filamentous fungi in the fermentation process.

Semi-Wet Processing Coffee

Similar to wet processing, coffee beans and a considerable portion of mucilage are separated during the pulping process. However, instead of completely removing the mucilage through underwater fermentation, the remaining mucilage is dried along with the beans. When comparing the primary and secondary metabolites of coffee processed using semi-wet and natural methods, similar results were found regarding the distribution of chlorogenic acids (CGA) and trigonelline. However, in wet processing, the overall CGA content was higher.

– In the wet method, the average content of CQA, di-CQA, and FQA were 4.94 ± 0.41, 0.94 ± 0.05, and 0.32 ± 0.02 g/100 g (dm), constituting 81.3%, 13.3%, and 5.3% of the total CGA content (6.08 ± 0.44 g/100 g, dm).

– In the semi-wet method, the average CQA content was 4.70 ± 0.37 g/100 g (dm), followed by diCQA at 0.98 ± 0.02 g/100 g (dm) and FQA at 0.30 ± 0.01 g/100 g (dm). These classes constituted 81.0%, 14.0%, and 5.1% of the total CGA content (5.8 ± 0.38 g/100 g, dm).

The lower sucrose and CGA levels in semi-wet processed beans may be associated with the shorter water-soaking time, leading to the loss of water-soluble components during washing and fermentation.

The types of enzymes produced during the coffee fermentation process.

Researchers have identified over 50 types of yeast and bacterial species present during the coffee fermentation process. They contribute to the production of ethanol, lactic acid, butyric acid, acetic acid, and various extracellular enzymes produced by Bacillus species that break down cellulose, contributing to the decomposition of cellulose and pectin in the coffee cherry’s skin, parchment, and mucilage. The three most important enzymes produced by microorganisms to break down pectin in the coffee fermentation process are pectin lyase, polygalacturonase, and pectin methyl esterase:

– Pectin lyase catalyzes the breakdown of pectin through trans-elimination, releasing unsaturated galacturonic acid.

– Polygalacturonase is the primary enzyme involved in the coffee fermentation process. It catalyzes the hydrolysis of α-1,4 glycosidic bonds into pectic acid (polygalacturonic acid).

– Pectin methyl esterase is responsible for the de-esterification process of the methoxyl groups of pectin, forming pectic acid and methanol.

The News provides some insights into the coffee fermentation process – a process that can yield surprising results for coffee flavor. If carefully controlled, the fermentation process can positively impact the quality of coffee beans, as these methods determine the chemical composition of the final product and influence the sensory characteristics of coffee.

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