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Acids in coffee.


When evaluating coffee, an observation of its acid content is of central importance. Acid in coffee covers a variety of classifications--from elegant to fullness all the way to pleasant, fine acid. But also negative taste effects caused by acid may occur, giving us sufficient cause to take a closer look at the acids contained in coffee and their effects.

Definition of Acid/ Effect of PH-Value

We know, not all acids are alike. And not all acids in green coffee or in roasted coffee actually affect the taste of the extracted beverage in the cup. The bitter taste of coffee is generally attributable to the titratable total acid content. The use of titration solution has a linear relationship with the intensity of the bitter taste: The intensity of a sour taste increases with increased titration solution. Most coffee lovers prefer an agreeable, not too pronounced acidity. However, without sufficient acid in the coffee, its taste is rather flat and bitter.

Despite the above-mentioned influential parameters of the sour taste, the pH-values as well as the value of total titratable acid content does not permit a final statement on the taste of coffee, because the total acid content as referred to above is composed of a mixture of various acids. Each of these individual acids has its own characteristics. Their percentage of the mixture indicates the taste preference of the beverage.

Classification, Individual Acids and Their Characterization

The different acids contained in roasted or green coffee can be categorized according to different preferences. They can also be also be separated by organic and inorganic acids. Organic acids (that is: carbonic acids), in particular, contribute greatly to the sour taste of coffee. A large portion of these acids is already present in certain concentrations of the green coffee, including, for example, citric acid, pyruvic acid and malic acid. Other organic acids, which have an influence on the flavor development only develop during roasting. Hydrolysis of the acids initially present in the green coffee and pyrolysis of carbohydrates form many acids that are perceived as agreeable by the consumer. Acetic acid, formic acid, quinic acid and glycolic acids are examples of acids that reach their highest concentration during the roasting process. As a whole, the percentage of organic acids in Arabica coffee as well as in Robusta coffee increases during the roasting process.

Among the acids in coffee, chlorogenic acid is of further significant importance. Chlorogenic acid is quantitative dominant in green coffee. In comparison, Robusta coffee has a higher chlorogenic acid content than Arabica coffee. However, during roasting, particularly during the beginning stages of the roasting process, chlorogenic acid is strongly reduced and thus contributes substantially to the development of taste-relevant acids. Research has shown that the darker roasted coffee has a lower content of chlorogenic acid than a comparatively lighter roasted coffee. A typical break-down product of chlorogenic acid is the quinic acid as mentioned above.


Each of these individual acids has its own characteristics and its own taste. In terms of quantity, the main components in roasted coffee are quinic acid and citric acids (see Figure 1). The more tart and sour flavor in the coffee is derived from the quinic acid. Citric acid can be characterized as agreeably fresh and lively. Volatile acids, acetic acids and formic acids are primarily typical representatives for the sour aroma of the coffee. While malic acid determines the more agreeably perceived fruity acidity, pyruvic acid is responsible for the caramel sour taste of the coffee beverage. The pyruvic acid is deemed to be very delicate and is the sweetest of the acids in coffee. Glycolic acid has an intensive effect on the entire acidic experience of coffee and is generally described as a bland, fine and soft acid.

Acidity in coffee is oftentimes also related to digestibility. Although the impact of organic acids on the digestibility of the stomach is not known, in general it is believed that a reduction of the total acid content is an effective remedy and leads to a well-tolerated, stomach-friendly coffee.

In general, how does acid develop and what are the influencing factors At the Probat R&D Facility, different tests series have been preformed to examine the development of acids in certain coffees during the roasting process. The degrees of acidity in coffee increase after the product reaches a temperature of about 130[degrees]C. The degree of acidity is tantamount to the content of titratable acids ascertained and thus to the degree of strength of the acids in coffee. However, this development takes place only up to a product temperature of about 170[degrees]C. The acidity drops again after the temperature has reached 170[degrees]C and continues to drop during the further roasting process (see Figure 2). As referenced above, the degree of acidity is analytically an indication of the content of all free acids in coffee. In order to make an objective observation of the taste of the coffee, a detailed description of the individual acids is necessary.

During test roasting it was noticed that the individual acids reacted differently during the development of the roast. In particular, the high concentration of chlorogenic acids in green coffee significantly degrades at the beginning of roasting process. With increasing roasting time, the intensity of the degrading process slows down (see Figure 2), however, does not stagnate. Also citric acid and malic acid will be partially broken down during the process. In turn, other compounds initially not present or present only in low quantities in green coffee are generated in the roasting process, such as quinic acid, one of the important products generated during the degrading process of chlorogenic acid, lactic acid, acetic acid as well as formic acid.




Moreover, during their trials, Probat researchers ascertained that the build-up or break-down of acids do not necessarily follows linear physical laws. Thus, for example, glycolic acid develops during roasting to an optimal value and remains constant during further processing. Between the break-down of chlorogenic acid and the build-up of quinic acid exists a clear correlation: the content of quinic acid increases linear to the breakdown of the chlorogenic acid. On the other hand, other acids such as acetic acid and formic acids develop only during the roasting process, but will be increasingly broken down again with longer roasting time.

In addition to the roasting time, the degrees of the roast, the color of the coffee beans, also have a significant effect on the acidity. Therefore, roasting time and the degrees of roast should always be considered together. Generally, the acidity of a coffee can be reduced through extending the roasting time (see Figure 3).

However, if the roasting time is identical, the acidity can be influenced by changing the degree of the roast (color). Darker roast degrees have a tendency to produce less acidity. Hence, acidity is higher during identical roasting time, the lighter the degree of roasting. It also had been ascertained in the Probat laboratory that the influence of the roasting time on the acidity of lighter coffee is markedly more pronounced than of darker roasted coffee. As a rule it holds true that the acidity which breaks down with increasing roasting time is reduced with darker degrees of roast. On the other hand, darker roasted batches yield a higher content of acids which develop only during the course of the roasting process. Figure 4 provides an overview of the influence of the roasting time as well as the degree of roast.

Influential Factors In Addition To Roasting Time And Degree Of Roast

In addition to the trials referenced above, determining the influence of various acidity at different roasting times and roast degrees, the Probat Laboratory established in another trial series of trials to determine the influence of acid degrading in coffee.

Roasted coffee was stored for a period of nine months in a freezer. Upon expiration of each three-month period, samples were taken and analyzed. It was found that the individual acids in the samples reacted differently. Thus, the content of titratable acids, the acidity, increased initially during a storage time of three months and then the acidy levels started to decline and finished at the starting level. In the course of the test series, the acidity then declined again to level off at the end of the trial period to almost the starting level.

No significant influence on the content of the total chlorogenic acid was noted during storage. Same applied to the organic acids, citric acid, glycolic, glycolic acid and malic acid. Other organic acids changed during the storage process. While the content of formic acid, acetic acid and quinic acid partially increased marginally, the concentration of pyruvic acid is altogether lower than during the initial measurement. The chemical analysis of malic acid showed that the tested type of green coffee brands (Arabica or Robusta) as well as the type of roaster, has an influence on the change of acidity during storage. Subject to these two parameters, build-up and break-down of malic acid could be noted.

Further trials were performed to determine if the way coffee was stored had an impact on the acid content in coffee. For this reason, coffee samples were stored at room temperature and in a freezer for five weeks. The weekly analyses revealed that the storage methods affected the change in the acidity content for example; the acidity in the sample which was stored in the freezer increased substantially compared to the sample that was stored in atmospheric conditions in a room. Similar results were noticed with reference to the content of quinic acids, which however did not change noticeably in the sample stored in atmospheric conditions. In regard to other organic acids it was noticed that their content during storage first decreased and then increased towards the end of the storage time.

Acids, in particular organic acids, contribute significantly to the taste of the coffee. A well-balanced acid is the prerequisite for a good cup of coffee. The degree of acidity and the development of acidity is significantly influenced by the selection of the coffee type. Once the green coffee type has been selected, acidity can be influenced by the selected roast profile determining roasting time and roast degree (color). The type and the length of storage is important to consider and monitor since it also has an impact on the development and the content of individual acids in the roasted coffee product.

By Probat Werke R&D Group
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Title Annotation:Coffee Chemistry
Publication:Tea & Coffee Trade Journal
Date:Dec 1, 2011
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