Author: Alfred Gitau Mwaura

PART 26: COFFEE ROASTING SCIENCE

Roasting transforms green coffee beans into aromatic roasted coffee through a series of complex physical and chemical reactions.

During roasting, coffee undergoes:

• Moisture loss
• Expansion
• Chemical transformation
• Aromatic compound development

Roasting is therefore both a science and an art.

59. Stages of Coffee Roasting

Coffee roasting occurs in several stages.

Stage 1: Drying Phase

Temperature range: approximately 100°C – 160°C

In this phase:

• Moisture evaporates
• Beans begin to turn yellow
• Grassy aromas appear

This stage prepares beans for chemical reactions.

Stage 2: Maillard Reaction Phase

Temperature range: 160°C – 200°C

This phase is responsible for:

• browning of beans
• development of complex aromas

Sugars react with amino acids to create hundreds of flavor compounds.

These compounds contribute to:

• caramel notes
• nutty flavors
• toasted aromas

Stage 3: First Crack

At around 196°C, coffee beans produce a cracking sound.

This occurs because internal pressure causes the bean structure to expand and fracture.

First crack marks the beginning of drinkable coffee.

Most specialty coffees are roasted between:

First crack and second crack.

Stage 4: Development Phase

After first crack, the roaster controls flavor development.

Short development produces:

• brighter acidity
• lighter body

Longer development produces:

• heavier body
• deeper sweetness

Stage 5: Second Crack

At approximately 220°C, beans begin a second cracking stage.

At this point:

• oils migrate to the surface
• roast flavors dominate origin flavors

Very dark roasting can mask coffee origin characteristics.

PART 27: ROASTING DEFECTS

Even high-quality green coffee can be ruined by poor roasting.

60. Underdeveloped Coffee

Occurs when roasting time is too short.

Flavor characteristics:

• grassy
• sour
• vegetal

This defect occurs when the Maillard reaction is incomplete.

61. Baked Coffee

Occurs when roasting temperature is too low for too long.

Flavor characteristics:

• flat
• dull
• lifeless

Baked coffee lacks sweetness and complexity.

62. Scorched Coffee

Occurs when beans are exposed to excessive heat early in roasting.

Characteristics:

• burnt surface
• uneven roasting

Flavor characteristics:

• bitter
• burnt sugar notes

PART 28: COFFEE EXTRACTION SCIENCE

Brewing coffee extracts soluble compounds from roasted coffee grounds.

These compounds include:

Acids
Sugars
Lipids
Aromatic oils

Extraction determines the final taste of coffee.

63. Ideal Extraction Range

Optimal extraction typically occurs between:

18% – 22% extraction yield

Under-extraction produces:

• sour coffee
• weak body

Over-extraction produces:

• bitterness
• harsh flavors

64. Factors Affecting Extraction

Extraction depends on several factors.

Grind Size

Fine grind increases extraction speed.

Coarse grind slows extraction.

Water Temperature

Ideal brewing temperature:

90°C – 96°C

Low temperature produces weak extraction.

High temperature produces bitterness.

Brew Time

Longer brew time increases extraction.

Short brew time reduces extraction.

Coffee-to-Water Ratio

Common brewing ratio:

1:15 to 1:18

Example:

20 grams coffee → 300 ml water.

PART 29: WATER CHEMISTRY FOR COFFEE

Water makes up approximately 98% of brewed coffee.

Therefore water chemistry strongly affects flavor.

65. Important Minerals in Water

Certain minerals improve extraction.

Key minerals include:

Calcium
Magnesium
Bicarbonates

These minerals help dissolve coffee compounds.

66. Ideal Water Composition

Specialty coffee standards recommend:

Total hardness: 50–150 ppm

pH: 6.5–7.5

Water that is too soft produces flat coffee.

Water that is too hard produces dull flavors.

PART 30: SENSORY CALIBRATION TRAINING

Professional cuppers must train their senses regularly.

Calibration ensures that cuppers evaluate coffee consistently and accurately.

67. Aroma Training

Cuppers train their sense of smell using aroma kits.

These kits help identify aromas such as:

Chocolate
Citrus
Floral
Spice

68. Flavor Recognition

Cuppers practice identifying flavor categories including:

Fruity
Floral
Sweet
Nutty
Chocolate
Spicy

They must also detect defects such as:

Musty
Phenolic
Sour
Fermented

69. Sensory Memory Development

Sensory training involves building a library of flavor memories.

This allows cuppers to recognize flavors quickly during evaluation.

PART 31: BUILDING A NATIONAL COFFEE QUALITY STANDARD

For Kenya to maintain its global reputation, it must maintain strict quality standards.

70. Quality Standards Should Include

Transparent grading systems
Independent cupping laboratories
Traceability systems
Farmer training programs

71. Importance of National Coffee Education

A strong coffee sector requires education at multiple levels.

Training should include:

Farmers
Factory managers
Mill operators
Roasters
Baristas

72. The Role of Kenya Coffee School

Kenya Coffee School aims to become a national hub for coffee education and innovation.

Through its programs, it trains:

Farmers
Youth baristas
Coffee entrepreneurs
Coffee technicians

FINAL MESSAGE

The future of Kenyan coffee will depend on knowledge, transparency, and collaboration across the value chain.

When farmers, processors, roasters, and baristas all understand coffee science and quality, the industry becomes stronger.

Through education, innovation, and empowerment, Kenya Coffee School and Barista Mtaani are working to build a coffee industry that is fair, transparent, and globally respected.

• Coffee farm agronomy for maximum cup quality
• Coffee processing factory engineering
• Coffee roasting profile design
• How to build a specialty coffee brand
• The future of Kenyan coffee in global markets