Roasting of Coffee Beans and Adulteration

After natural / wet processing of coffee cherries, de-hulled coffee beans are roasted to give typical flavor and aroma. Roasting is a time temperature dependent process where chemical and physical changes takes place in the green coffee. During roasting, there is also loss of dry matter, primarily as CO2 gas and other volatile products of the pyrolysis. About half of total CO2 generated are retained in the roasted coffee together with a major proportion of the important volatile flavor substances. Roasting is normally carried out under atmospheric pressure with hot air and combustion gases as the primary heating agent. Though heat may also be provided by contact with hot metal surface.

Fig: Different stages of coffee roasting (×704.jpeg )

After the initial removal of moisture, the actual roasting begins when the temperature inside the bean reaches approximately 200°C. Different variety of beans differ in moisture and density. Therefore they are roasted at different temperature. Degree of roasting plays a crucial part in determining the flavor characteristic of extract, subsequently brewed from roasted coffee. During roasting, intense heat breaks down starch in the beans into simple sugar.  Heat causes caramelization of the sugar that gives brown color to the coffee beans. Sugar is rapidly lost during roasting process and may disappear entirely in darker roast. During roasting (at 205°C) change in flavor occurs due to development of aromatic oils, acids and increases in caffeine content. One of the important oil in coffee is “caffeol” which is developed at about 200°C and is largely responsible for coffee’s aroma and flavor. The prominent changes in coffee beans after roasting are summarized below,

Physical changes:
1. During roasting, external color changes to yellowish brown (light roast), light brown (medium roast) and very dark brown or almost dark (dark roasting).
2. Beans becomes shiny due to the exudation of oil onto the surface.
3. Increase in bitterness and typical aroma of coffee.
4. Moisture loss from 12 % (in green beans) to about 2 – 3 % (in roasted beans) which also causes the reduction in           weight of the beans.
5. Beans swells in volume by 40 – 60 % due to swelling
6. The breaking strength is decreased after roasting

Chemical changes:
1. After roasting, pH of coffee decreases. The pH is 6 in green beans which is reduced to 4.4 in Robusta and 5.1 in           Arabica variety.
2. Carbohydrates are decomposed to give aldehyde and organic acids.
3. Lipids are broken down to free fatty acids, esters and formic acids.
4. Water extracts and total sugar increases.
5. Chlorogenic acid decompose to give simple phenol, caffeic acid and pyrogallol
6. Decomposed sugar and amino acid react to form maillard reaction
7. Phenolic acids are broken down to catechol and ethyl alcohol
8. Trigonelline is broken down to nicotinic acid, ester and pyridine.
9. Combustion of carbohydrates and other product release CO2 which is entrapped by bean.
10. Dry matter is lost to some extent
11. Caffeine is almost stable during roasting process. Only small portion during dark roasting is lost during decomposition.

The color of roasted beans as perceived by human eyes are labelled as light, medium light, medium, medium dark and dark. A more accurate method includes the use of spectroscopy. Roasted color can be co-related with percentage loss of moisture of coffee. A light roast shows moisture loss of about 3 – 5 %, medium roast shows 5 – 8 % loss and dark roast shows 8 – 14 % moisture loss. Practically, roasting time ranges from few minutes to about 30 minutes. If roasted for longer time than 30 minutes, undesirable burnt flavor is developed.

Not only chemical changes, physical changes that takes place during are also equally important. Reduction of moisture increases shelf life of the coffee beans. Popping of coffee beans during roasting causes considerable decrease in density of the beans. Extent of popping is the function of degree of roast and speed of roasting.  Uniformity of the roast color is generally desired.

Size grading:

Most of the coffee whether prepared by wet / dry process is marketed according to bean size. Bean size can be accessed by conventional laboratory test of “screen analysis”. Perforated plate screen are used with aperture of specified dimensions. Beans of commercial Robusta variety are generally smaller than Arabica variety. Average 100 beans of Robusta variety weighs 12 – 15 grams while Arabica variety weighs 18 – 22 grams.

Not only on the basis of bean size, grading can be performed on the basis of percentage of defected beans, level of contamination and cup quality of coffee. For determining cup quality of brewed coffee by organoleptic means, samples are submitted to professional panelist.

Adulteration in coffee:

It may be defined as the addition of foreign matter of any kind to reduce the strength and quality. The substitution of cheaper substance impart low quality to the product. The use of cereal in so called blending should be considered and adulteration. The cereal act as a diluent, increasing the weight and bulk without corresponding benefit to the purchaser. It is not a common practice to treat inferior or damaged coffee by some process for the improvement of their appearance and imitation of superior grades.

To detect the color, suspected coffee is mixed with cold water to obtain sediment. The sediment is examined chemically and microscopically. Lead, tin, copper and arsenic are the objectionable matter to be present in coffee.

One of the common adulteration in coffee granules is mixing powder of root or chicory plant. There are several chemical method for detection of chicory root depending on positive or negative test. Grounded chicory root when mixed with cold water, sink quickly coloring the water and itself being soon softened. Grounded roasted coffee powder on other hand floats imparting no color.

Fig: Chicory root ( )

Chicory root is easily bleached with chlorinated soda but coffee is slowly affected by this bleaching agent. Also, the coloring pigment of chicory is not precipitated by iron salt while pigment of coffee bean is precipitated with iron salt.

Adulteration with chicory root can also be detected by coiling the suspected coffee infusion in test tube with two drops of concentrated Hydrochloric acid (HCl). Then coffee infusion ( 15 drops) is added to Potassium Ferrocyanide solution (1  part salt, 8 part water) and boiled until liquid becomes dark green. Upon adding of 6 drops of Potassium hydroxide solution and boiled, if chicory root is present, the liquid becomes brown and murky. Otherwise, precipitate will be separated and settle to bottom of tube leaving the supernatant solution of light yellow color.



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.