Heat Transfer

• Thermal Energy
• Heat ≠ Temperature
• Conduction, Convection, Radiation

Conductive Transfer

• Physical contact + temperature gradient
• Common cause of roast defects
• Temperature gradient inside bean
• Water content is critical*

Convective Transfer

• Air as a medium or vehicle
• Most common transfer method in drum and air roasters
• Greater the temp difference between air and bean greater the transfer (potential)

Radiated Transfer

• Electromagnetic radiation (same as the sun)
• Some visible, some not
• Example: light from heat source, heat felt near a heat source
• Metal in roaster acts as a battery, storing energy as heat*

Heat Transfer, Supplemental

1. Drum, Air, Bean
2. Air, Drum, Bean
3. Bean, Drum, Air

Temperature Midway Point ≈ 75 Agtron

• New concept
• Generates easy color to work/train with
• Not meant to be a standard or a sweet spot regarding end product

Quick Review

1. What are the three types of heat transfer?
2. Why is water content important?
3. What is the difference between heat and temperature?

The Roasting Cycle

• Evaporation
• Color VS roast curves
• Defects
• Cooling Time

Drying

• Primary concern of first-third of roast (first phase)
• Evaporative cooling

Roasting

• Maillard reaction (complex)
• Streker degradation (complex)
• Melanoidins
• Organic acids

Much more on this tomorrow

Rate of Change

• Often called Rate of Rise (but rate may not always rise)
• Speed (momentum) of the roast
• Typically measured in 30 or 60-second intervals
• Not useful until after TP
• 30s is more accurate than 60s calculations

Rate of Change

Example:
At 4:00 temp is 148°C
At 4:30 temp is 153°C

The temperature increased 5° in thirty seconds

153° - 148° = 5°

Therefore, RoC is 5°/30s

Or if we want a more accurate/higher resolution RoC we could calculate the rate of change per second:

153° - 148° = 5°
Time = 30s

5/30 = .16°/s (not very useful, in my opinion)

Cooling

• Preferably within 4 minutes
• Ideally as fast as possible
• Quenching with water can be bad (but can be done)

Quick Review

• How much mass is typically lost during the roast and what is most of the ‘material’ that is lost?
• Which occurs first (and thus longest); Strecker degradation or the Maillard reactions?
• Why are darker roasts less acidic?

Basic properties and changes

• Coffee is organic!
• Endothermic VS exothermic reactions

Important changes to observe and measure:

• Temperature over time
• Volume
• Weight
• Color
• Density

Temperature over time

• Temperature evolution
• Typically includes bean and air or environment temp 
(BT & ET)
• Momentum (Rate of change or rate of rise)

Volume Changes

• Cell walls become brittle
• Water vapor inside bean creates a vapor “wall”*
• CO2 build-up
• Other organic gasses

Weight Changes

• Weight difference is a critical measurement
• Weigh coffee before and after roasting and calculate % weight loss
• Density change is also a good measurement to track consistency

Color changes

• Sugar browning
• Melanoidins
• Industry standards for color measurement

Sugar Changes

• Simple sugars already present in green bean
• Broken down and used later to form polysaccharides
• Caramelization
• Maillard VS Caramelization

Acidity Changes

• Acids created early in roast
• Get reduced later in roast
• Therefore acidity provides information on roast level

Workspace Management

Green coffee storage, ideal conditions:

• 60% relative humidity
• Green beans at 10-12% moisture content 
(12% = longest shelf life with 60% relative humidity)
• Keep bags ~15cm off floor, away from pests, water, etc
• Consider GoPro bags (can be expensive)
• Routinely check and monitor condition of stored greens

Workspace Management

Ideal safety conditions

• Production flow
  • Efficiency, organization in layout & movement
  • Physical safety/health (heavy lifting)
• Air quality while roasting
  • Big roasting machine (20+Kg) consider a mask
  • When handling greens consider a mask
  • Not just gases, but micro-fibers & micro-particles from greens
• Fire safety
  • Fire suppression
  • Plan for a roaster fire
  • Share the plan with everyone
  • Revisit the plan regularly (at least yearly)

Roaster Machine Construction

• Drum roaster
• Fluid bed
• Batch VS continuous roaster

Ventilation

• Within the drum, ventilation creates convection heat transfer
• Keep the exhaust stack clean to avoid fires!
• Ensure dangerous gases are evacuated from facility

Electricity

• Electric roasters are fine for sample roasting or lab roasting.
• Electrical heating elements are slow and don’t scale well

Question: how would you compensate for the lag caused by an electrical heating element?

Tools for roasting

• Buckets + shovel/scoop
• Logging sheets or software
• Timer and temperature probes
• Infrared thermometer can be helpful
• Moisture, density, and roast color meters are handy 
(and getting cheaper and smaller)
• Quality Control System (what does this look like?) *A roaster (obviously)

Cleaning, Maintenance, & Troubleshooting

• Ventilation ducts
• Chaff collector
• Exhaust air cleaning
• Cooling tray
• Area surrounding roaster

Tomorrow

We’ll meet in the cupping lab

We’re cupping immediately after a quick meeting

• No perfumes, colognes or heavy fragrances
• Be done smoking in time for your clothes to air-out

Roasting Activity: 5 Roasts

Roast 1: Perform roast with any profile to Temperature Midway Point (sample color creation).

Roast 2: Perform roast with similar profile to Midway Point of Roast 1 but continue to the start of 2nd Crack (and thereby resulting in a longer development time).

Roast 3: Roast coffee to the same profile to First Crack, then end at the same color with a longer development time (over 4 mins).

Roast 4: Roast coffee to the same profile to First Crack, then end at the same color with a shorter development time (under 1:30).

Roast 5: Roast coffee to the same profile to First Crack, then end at the same color with a 2-3 minute development time.