Roasting of coffee beans

ROASTING OF COFFEE BEANS
V.SUNDARALAKSHMI

Roasting is the key unit operation in converting green coffee beans into flavorful roast coffee. It is the
heart and soul of any coffee manufacturing operation because it is the roasting process during which
flavor is created and physical bean properties are determined.
Sound is a good indicator of temperature during roasting
1. At approximately 196 °C (385 °F), the coffee will emit a cracking sound. This point is referred
to as "first crack," marking the beginnings of a "light roast”, large amount of the coffee's
moisture has been evaporated and the beans will increase in size
2. When the coffee reaches approximately 224 °C (435 °F), it emits a "second crack", this sound
represents the structure of the coffee starting to collapse.

Chemical Components of Green Coffee Beans and
Roasted Coffee Beans

MAJOR CHEMICAL REACTIONS DURING
• Maillard reaction
• Strecker degradation
• Caramelization

CHEMICAL AND PHYSICAL CHANGES IN THE
COMPONENT OF COFFEE BEANS DURING ROASTING
CHANGES OF CARBOHYDRATES:
 Sucrose, which is the most abundant green coffee beans, acts as an aroma precursor during roasting.
 Sucrose is the major source of the aliphatic acids (formic, acetic, glycolic, and lactic) produced during
coffee roasting.
Formation of 5-Hydroxymethyl-2-furfural (HMF) and 5-hydroxymethyl-2-furoic acid (HMFA) occurs through
cleavage of sucrose

 On a dry-weight basis, almost half of green coffee beans are reported to be made
of polysaccharides, which include cellulose, mannan, and arabinogalactan.
 In the green coffee beans, polysaccharides are retained in the coffee bean cell wall
as part of the insoluble polysaccharide complex
 Roasting process increases the solubility by loosening the cell wall structure as it
swells.

During roasting level of chlorogenic acids decrease whereas the levels of quinic acid and of γ-
quinide( internal ester of quinic acid) and syllo-quinic acid( isomeric product of quinic acid)
increased during the roasting process
 Some chlorogenic acids are converted to lactones of chlorogenic acids which includes
feruloylquinic acid lactones, caffeoylquinic acid lactones, and p-coumaroylquinic acid
lactones.
 Chlorogenic lactones are formed during roasting by a loss of water molecules from the quinic
acid moiety and the formation of an intramolecular ester bond.
 Formation of lactones was highly dependent on the degree of roasting.
 Optimum degree of roasting to achieve a maximum of lactones is light medium roast.

CHANGES OF TRIGONELLINE
Trigonelline is a pyridine derivative known to contribute indirectly to the formation of
desirable flavor products, including furans, pyrazine, alkyl-pyridines, and pyrroles, during
coffee roasting.
It is a percursor of flavour and aroma compounds.
 Thermal degradation or pyrolysis of trigonelline yields N-methylpyridinium and nicotinic
acid are the major nonvolatile products.
 Trigonelline level during roasting decreases while the thermal degraded non volatile
products increases.

CHANGES OF PROTEIN AND FREE AMINO ACIDS
 Roasting leads to protein denaturation with degradation.
 The Maillard reaction is a chemical reaction between reducing carbohydrates and
various amino acids, peptides, and proteins, which contain free amino groups.
 The green coffee bean protein subunits are integrated into the polymeric structure
of melanoidins formed during roasting. The melanoidins are defined as brown,
highmolecular- weight products containing nitrogen and are end products of the
Maillard reaction.

FORMATION OF AROMA COMPOUNDS
 Green coffee beans lack the color and characteristic aroma of roasted coffee, both of
which are formed during the roasting process.
 Coffee oil, which comprises about 10% of the roasted beans, carries most of the coffee
aroma. The aroma is made up of a complex mixture of volatile compounds.
 The aroma of coffee brew is mainly caused by some alkylpyrazines, furanones, and
phenols, and by 2-furfurylthiol, methional, and 3-mercapto-3-methylbutyl formate.
 Aroma of coffee is formed due to wide rande range of interaction betweeninteractions
between all the routes involved in the Maillard reaction, caramelization, Strecker
degradation, and the breakdown of sulfur amino acids, hydroxy-amino acids, proline and
hydroxyproline, trigonelline, quinic acid moiety, carotenoids, and minor lipids

KEY REACTION OF COFFEE ROASTING THAT
IMPACT FLAVOR QUALITY

The major compositional changes and chemical processes that affect the development of flavor
compounds in coffee upon roasting:
 Loss of water ⇨ drying of the bean, low moisture reaction system
 Release of carbon dioxide ⇨ expansion of the bean
 Migration of lipids to the bean surface ⇨ retaining aroma components generated
 Loss of sugars (including sucrose) ⇨ flavor and color formation (Maillard chemistry and
caramelization)
 Decrease of free amino acids ⇨ flavor and color formation (Maillard and Strecker chemistry)
 Partial decomposition of polysaccharides (e.g., arabinogalactan) ⇨ release of arabinose which in turn
reacts leading to flavor formation (e.g., Maillard reaction)
 Partial decomposition of proteins ⇨ release of amino acids which in turn reacts leading to flavor
formation (e.g., Maillard reaction)
 Loss of CGA ⇨ formation of bitter taste and color
 Decrease of trigonelline ⇨ formation of N-containing products (aroma, taste, color)
 Formation of melanoidins ⇨ color formation (polymerization of polysaccharides, proteins, and
polyphenols)
 Partial lipid degradation ⇨ aroma active aldehydes interaction between intermediate decomposition
products

Schematic Presentation of the most Important Flavor
Precursors in Green Coffee and the Transformation into key
Aroma Compounds

Colour development:
 Coffee beans during roasting changes from green to yellow, orange, brown, dark
brown, and finally to almost black. The color development is very much
interlinked with flavor development.
 Therefore, the bean color is the best indicator of the degree of roast and a most
important quality criteria.
Volume increase and structural changes:
 Coffee beans swell during roasting and increase in volume.
 The microstructure changes from a dense to a very porous structure.

Roasting of coffee beans

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