IT’S ALL ABOUT PREINFUSION

BASIC PRINCIPLES – ADVANCED DECENT PREINFUSION – PCT RATIO

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STÉPHANE RIBES – MAY 2019

• Perfect preinfusion consists in instantaneous and even wetting of the coffee grinds. Its outcome is a

totally wet puck, with even temperature and compression.

• From the brewing perspective, a more even preinfusion improves coffee extraction and taste

• In the traditional espresso process where water hits the coffee puck from only 1 direction, with no

stirring possibilities, targeting a perfect preinfusion is obviously a challenging objective!

• We have assessed the influence of the water flow during espresso preinfusion, considering the

following characteristics:

1. Consistency of the puck density after preinfusion

2. Homogeneity of the Puck Contact Time with brew water and of the puck temperature

Smoothness of the puck surface after the espresso shot

• What’s presented here is a mix of basic observation results and hypotheses based on expected

physical phenomena

ESPRESSO PREINFUSION

SUMMARY (1/2)

• With the Decent Espresso machine, espresso preinfusion can easily be adjusted and perfected

ESPRESSO PREINFUSION

SUMMARY (2/2)

Constant preinfusion flow rates

Variable

preinfusion

flow rate

Very low flow

(0.5 – 1.5 mL/s)

Low medium

(1.5 – 3.0 mL/s)

Medium high

(3.0 – 5.0 mL/s)

High flow

(5.0 – 8.0 mL/s)

2. Even puck

compression

++ + – – – +

1. Even Puck

Contact Time

& Temp.

– –

–

(+ with low

extraction flows)

–

(+ with high

extraction flows)

3. Even puck

surface

– + + – +

• Benefits of a given flow rate for preinfusion often seem to be offset by other negative effects

• A universal compromise working best in all situations may not be possible – the most suitable target

may vary depending on coffee, technique and expectations in terms of taste, mouthfeel…

• For example very slow preinfusions (“Slayer shots”) can undoubtedly lead to delicious espresso coffee,

especially with light roasts, despite a huge difference in contact time for the top and bottom grinds:

maybe in this case more time for the coffee to get fully wet and dwell evenly (also due to low puck

compression) brings more benefits than a more even contact time would offer

ESPRESSO PREINFUSION

WHAT’S BEST FOR MY COFFEE?

• During preinfusion with a quick pressure increase the bottom layers of the puck are more compressed

• The puck density is more homogenous when a slow preinfusion is performed

• This explains why, with the same grind size, dose and extraction pressure (e.g. 9 bar), higher flow

rates during preinfusion induce lower flow rates during extraction (with longer preinfusion a finer

grind is needed to get similar flow during extraction)

Air

Dry coffee Wet coffee

Water

0 bar 1 bar 3 bar 5 bar

9 bar

0 bar 0.2 bar 0.5 bar 1 bar

9 bar

Quick preinfusion

(e.g. 8 mL/s)

Slow preinfusion

(e.g. 2 mL/s)

= 0 s 3 s 5 s 6 s

7 s

= 0 s 5 s 12 s 16 s

22 s

ESPRESSO PREINFUSION

EVEN PUCK DENSITY (1/3)

Higher

density

of coffee

grinds

• Higher puck density → less available space for water around the coffee particles

• In the case of a quick preinfusion, the fluid velocity is higher around the coffee grinds at the bottom

of the puck. The resulting shorter contact time suggests a lower extraction of the coffee grinds from

the bottom of the puck. This could also contribute to increase the likelihood of messy sprays that can

sometimes be seen at the exit of the basket, especially when combined with high extraction flow rates.

Quick preinfusion

(early pressure rise)

Slow preinfusion

(later pressure rise)

Top layers of the puck Bottom layers of the puck

ESPRESSO PREINFUSION

EVEN PUCK DENSITY (2/3)

• With the same grind size and

targeted flow rate for the

extraction phase, decrease the

preinfusion flow rate reduces puck

compression with direct impact on

the maximum pressure reached

during extraction (flow profile)

• Channeling can occur when a high

preinfusion flow rate is combined

with a steep pressure increase

ESPRESSO PREINFUSION

EVEN PUCK DENSITY (3/3)

6 mL/s preinfusion 4 mL/s preinfusion

3 mL/s preinfusion 2 mL/s preinfusion

Channeling

Lower pressure due to

less puck compression

Evidence of

channeling?

• With the Decent Espresso machine, a 20 g coffee dose requires ca. 40 mL of water to perform full

preinfusion (DE1 PRO v1.1 machine – 20 g of coffee in a 18 g basket)

• With a 1 mL/s preinfusion, when the coffee grinds at the bottom of the basket get wet, those from the

top of the puck have already been soaked in hot water for 40 seconds!

• This additional contact time of the top grinds will be partly (or even fully) compensated at the end of

the extraction when the last amount of brewed coffee will pour out of the basket. This amount

corresponds to the volume of residual fluid in the puck in the last stages of the extraction.

• The ideal balance (same contact time for top and bottom grinds) is obtained when the time to

pour the residual brew water equals the preinfusion time

• This can be achieved thanks to appropriate selection of flow rates for preinfusion and for the (last

step of the) extraction phase

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (1/7)

Preinfusion flow

1.0 mL/s 2.0 mL/s 4.0 mL/s 8.0 mL/s

Preinfusion

duration

40 s 20 s 10 s 5 s

Preinfusion water

• The volume of a 20g coffee dose (dry grinds), tamped in a 58 mm basket, is more or less 30 cm

• Around 20 mL of water suffice to saturate this amount of grinds

• Let’s consider a typical 1:2 brew ratio to get 40 mL of coffee in the cup

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (2/7)

Dry coffee puck in

basket

Preinfusion water

(40 mL)

“Extraction” water

(40 mL)

“Extraction” water

Wet coffee puck

“Extraction” water

= 0 s End of preinfusion Half extraction Full extraction

Extracted coffee

(40 mL)

Extracted coffee

(20 mL)

• In this example with the chosen brew ratio of 1:2, “Extraction” water only pushes down the preinfusion

water through the puck; it is not actually used as brewing water (also valid for lower brew ratios)

• Considering a constant flow extraction phase and our 20 g coffee dose, the contact time of the coffee

grinds with water are:

• top of the coffee puck → preinfusion time + extraction time – time to pour the last 20 mL *

• bottom of the coffee puck → pouring time (once preinfusion is complete)

• In this example (DE1 v1.1 & 20 g of coffee in a 18 g basket), with a constant flow rate during

extraction, preinfusion should be roughly twice shorter than the extraction phase to get an even

overall contact time for all coffee grinds

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (3/7)

* Probably slightly less if puck erosion was considered (not taken into account here)

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (4/7)

Best suggested settings to

reach even Puck Contact

Time *

* DE1 PRO v1.1 machine – 20 g in a 18 g basket

• On top of contact time differences along the height of the puck, the brew temperature varies

significantly within the coffee puck. Identified influencing parameters are:

• Higher brew ratios are positive as they smoothen the phenomenon

• Longer preinfusion and extraction increase the thermal losses and decrease the actual brewing

temperature (hence have detrimental effect on temperature evenness)

• Also, as coffee gets extracted, the resulting fluid gets more concentrated in extracted compounds

which makes it less efficient than pure water to perform further extraction. The consequence, again, is

a lower extraction of the coffee grinds from the bottom of the puck.

• As for temperature unevenness, this effect becomes less visible when the brew ratio increases

• Unclear impact (if any) of preinfusion and extraction flow rates on this phenomenon

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (5/7)

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (6/7)

= 0 s

Preinfusion starts

Preinfusion ends

0 g in cup

20 g in cup

(1 : 1)

40°C

90°C

Brew water

T°C

40 g in cup

(1 : 2)

60 g in cup

(1 : 3)

• Increasing the brew ratio from 1:1 to 1:3 halves the temperature gap between top and bottom grinds

• However, as extraction efficiency decreases with time, the actual benefits in the cup are presumably

lower than those suggested by these average gap values

62°C

70°C

76°C

Puck T°

Top grinds

Puck T°

Bottom grinds

Average T° gap: 28°C

Average T° gap: 20°C

Average T° gap: 14°C

ESPRESSO PREINFUSION

EVEN CONTACT TIME & TEMPERATURE (7/7)

Best suggested settings to

reach even Puck Contact

Time and temperature *

* DE1 PRO v1.1 machine – 20 g in a 18 g basket

• Once extraction is complete, craters can sometimes be observed on the surface of the coffee puck

• The exact root cause of these craters is not fully clear but trying to avoid them seems a reasonable

objective, as they reveal that channeling has probably occurred during extraction

• As far as the generation of craters is concerned, the very beginning of the preinfusion phase is

probably one of the most critical moments: it can be assumed that water droplets hitting repeatedly

the same spot of the top of the puck could create this kind of surface unevenness

• To mitigate the risk of generating craters at the beginning of the preinfusion, one can recommend to

avoid too high or too low preinfusion flow rates:

• With high flow rates (> 5 mL/s) water will obviously hit the surface with more energy

• With very low flow rates (< 1 mL/s), water distribution above the puck may be suboptimal

• A moderate speed of pressure increase after PI has also proven to reduce the occurrence of craters

• Other actions not linked with the preinfusion flow rate (e.g. puck preparation, headspace, shower

screen cleanliness & type) to avoid puck craters & channeling in general, have note been analyzed

ESPRESSO PREINFUSION

EVEN PUCK SURFACE AFTER PREINFUSION

• With the Decent Espresso machine, the preinfusion process can easily be modified to benefit from

the best of each flow rate family and improve the overall preinfusion performance

ESPRESSO PREINFUSION

HOW TO MAKE IT MORE DECENT (1/2)

Constant preinfusion flow rates

Variable

preinfusion

flow rate

Very low flow

(0.5 – 1.5 mL/s)

Low medium

(1.5 – 3.0 mL/s)

Medium high

(3.0 – 5.0 mL/s)

High flow

(5.0 – 8.0 mL/s)

2. Even puck

compression

++ + – – – +

1. Even Puck

Contact Time

& Temp.

– –

–

(+ with low

extraction flows)

–

(+ with high

extraction flows)

3. Even puck

surface

– + + – +

• With the Decent Espresso machine, on top of the possibility to select any flow rate value to

perform preinfusion, a 2 step approach can further improve the consistency of the process:

medium high flow rate first, followed by a lower flow rate phase

• A moderate high flow rate at the very beginning of the preinfusion reduces the overall

preinfusion time and hence improves the Puck Contact Time homogeneity (avoids too high

PCT ratio)

• Low flow rate in the second preinfusion step limits puck compression and a too steep

pressure increase once preinfusion is complete

• The switch between the 2 preinfusion steps can easily be triggered by a pressure threshold

(typically around 1 bar)

• If the most even preinfusion is the target (and hence the most even overall extraction) it is also

possible to adjust all preinfusion and extraction parameters (flow rates, thresholds) taking into account

the proposed chart (page 14)

• Impacts of other parameters (e.g. dose, puck preparation, headspace) will be quantified and

discussed in a future study

ESPRESSO PREINFUSION

HOW TO MAKE IT MORE DECENT (2/2)

ESPRESSO PREINFUSION

ADVANCED DECENT PREINFUSION (1/3)

• Advanced preinfusion with a standard Flow profile (1/2)

• Anticipated switch to the “extraction” phase with lower flow rate (exit pressure: 4 bar → 1 bar)

• A higher initial preinfusion flow rate can be selected to compensate for longer actual preinfusion

and avoid too high PCT ratio

ESPRESSO PREINFUSION

ADVANCED DECENT PREINFUSION (2/3)

• Standard Flow profile (2/2)

• Finer grind is required to

maintain maximum extraction

pressure (less puck

compression, lower average

flow rate before the pressure

peak)

• The smoother pressure rise

(1.5 bar vs 3.7 bar / second)

is also beneficial to lower the

risk of channeling during

extraction

Lower pressure

Standard preinfusion “Advanced” preinfusion

Same grind size (EK2.8)

“Advanced” preinfusion

Finer grind (EK2.6)

ESPRESSO PREINFUSION

ADVANCED DECENT PREINFUSION (3/3)

• For a Pressure priority shot an

advanced profile is needed

• Exit pressures of steps 1 and 2

can be adjusted to reach

optimum preinfusion time

• An additional step between

stages 2 and 3 could be

imagined (e.g. pressure ramp

to ensure smoother transition)

• Trials to replace step 2 by a

pressure priority phase

(e.g. 1 bar) have not been

very successful (hard to

get smooth transitions

between flow and

pressure priority

modes)

ESPRESSO PREINFUSION

HOW GOOD IS YOUR PUCK CONTACT TIME RATIO?

Excess Contact Time of Top grinds

ECTT = preinfusion time

(6 s in this example)

Excess Contact Time of Bottom grinds

ECTB = time to pour the

“residual brew water”

Actual average

final extraction

flow rate in cup =

2.2mL/s

18 g dose → 18 mL

residual brew water

ECTB = 18 / 2.2 = 8.2 s

PCT ratio = 6 / 8.2 = 0.7

* Probably slightly less if puck erosion was considered (not taken into account here)

• PCT ratio = ECTT / ECTB

• ECTT = preinfusion time

• ECTB = time to pour the

“residual brew water”

• Residual brew water volume

= more or less 1 mL / g of dry

coffee (dose) *

• Target for PCT ratio = 1

• PCT ratio < 1: too short preinfusion