Understanding Coffee Brewing: Key Concepts and Techniques

Posted on Dec 14, 2025 in Chemistry

1) Sensory Scoring Criteria

2) Mr. Coffee Brewer Parts

Reservoir: Compartment for holding water used to brew the coffee.

Waterspout: Releases hot water to be filtered and extracted through the ground coffee beans.

Filter Basket: Allows water to pass and extract the coffee flavor while filtering out coffee grounds.

Water Tube: Allows water to move down and be heated to move back up through the other tube.

Check Valve: Allows fluid to flow through it in one direction, opening only when water is pushed through one specific way.

Wires: Conduct the flow of electricity and connect to the heater.

Thermal Fuse: Allows electrical currents to pass through and regulates temperature to prevent overheating.

Thermostat: Regulates temperature and cuts it off if it gets too high.

Heating Tube: Allows water to be heated, boiling it and forcing it upward while also warming the heating plate.

3) Brew Ratio

Relationship between M_grounds and M_water –> Formula on sheet.

Avg 15-20: Lower ratio – high TDS and bitter; higher ratio – low TDS and thin and watery.

4) Absorption Rate

How much water is absorbed by used coffee grounds (formula on sheet).

M_spent = M_feed – M_brew

5) Flow Diagrams and Material Balance Equation

Material balance equation –> Input = Output – accumulation.

6) Changes During Roasting

Physical:

1. Moisture evaporation
2. Bean expansion
3. Color change
4. Mass loss

Chemical:

1. Pyrolysis: Thermal breakdown of organic compounds in hot coffee; increases bitterness, dark color, and smoky flavor.
2. Combustion: Oxidation of aromatic molecules during holding; causes aroma loss, staling, and dull flavor.
3. Breakdown of Acids: Decomposition of chlorogenic acids into quinic and caffeic acids; lowers pH, raising sourness and bitterness.
4. Maillard Reactions: Sugar–amino acid browning; deepens color and body, adds roasted and caramel notes but can increase bitterness if prolonged.

Acidity for Different Roasts: Light – high as acids remain intact; medium – moderate; dark – low as acids degrade.

7) Chemical Reactions in a Hot Brew

1. Oxidation: Aromatic compounds react with oxygen, degrading volatile molecules → causes stale or flat taste and aroma loss.
2. Hydrolysis: Complex molecules (like esters and polysaccharides) break down into smaller acids and sugars → increases acidity and bitterness over time.
3. Acid–Base Reactions: Balance between weak acids and basic compounds shifts → gradual pH drop, sharper and more sour flavor.
4. Precipitation/Polymerization of Organics: Tannins and polyphenols combine or settle → darker color, reduced clarity, and harsher mouthfeel.

8) Acid Concentration From pH: Lower pH = more acidic coffee.

9) Reaction Rate pH vs Time: How fast acids form/decompose during holding or reheating.

10) Energy Unit Conversions: To convert J → kWh: divide by 3.6 × 10⁶. To convert kWh → J: multiply by 3.6 × 10⁶.

11) Power and Energy: Power is the rate at which energy is used or transferred over time. In other words, power tells you how quickly energy is being consumed or produced. Power = Energy/Time = Watts = Joules/second.

12) Specific Heat Capacity: Every kind of matter has a specific heat capacity (C_p), defined as the amount of energy required to raise the temperature of 1 gram of a substance by 1 °C.

It measures resistance from a material to temperature change; the higher the C_p, the more energy it takes to heat it.

E = energy input (Joules/kWh) – m = mass (g) – C_p = specific heat capacity (J/g · °C) – ΔT = temperature change (°C)

13) Mass Transfer During Extraction

14. TDS and PE: Total Dissolved Solids (TDS) is the percentage of coffee solids that are dissolved in the water; it measures how concentrated the coffee is. A higher TDS indicates a stronger, more concentrated cup, while a lower TDS indicates a weaker, more diluted brew.

Percent Extraction (PE) is the percentage of the coffee grounds’ mass that has been dissolved into the brew. It tells you how much of the coffee material was actually extracted from the grounds.

Relation → PE = TDS x (m_brew/m_grounds)

15. Conversion: 1% = 10,000 ppm

16. TDS Change with Extraction Time: As extraction time increases, the TDS value increases; more time allows water to dissolve and carry more coffee compounds into the brew.

18) Effects of Pressure on Flow Rate

Pressure (↑) → Faster flow → ↓ contact time → Lower TDS, sour/under-extracted

Pressure (↓) → Slower flow → ↑ contact time → Higher TDS, bitter/over-extracted

More Grounds (↑ thickness) → Slower flow → Higher TDS, stronger/bitter

Fewer Grounds → Faster flow → Lower TDS, weaker/sour

Finer Grind → Lower permeability → Slower flow → Higher TDS, bitter

Coarser Grind → Higher permeability → Faster flow → Lower TDS, sour

19) Filters

20) Arabica vs. Robusta

21) Wet and Dry Processing Methods

22) Conversions

• Joules → kWh

• 1 gram of water = 1 milliliter = 1 cm³

• 1000 grams of water = 1 kilogram = 1 liter = 1000 milliliters

• pH → [H₃O⁺]

• Watt = Joule/Second