82 beanscenemag.com.auR&D LAB
Dr Monika Fekete is the Founder of Coffee Science Lab.82 beanscenemag.com.auR&D LAB
Dr Monika Fekete is the Founder of Coffee Science Lab.Dr Monika Fekete looks at temperature-dependent variables and
their consequences on grinder and espresso machine performance.
O
n a chilly winter
morning, hot cup of
coffee in hand, I’m
once again thinking
about the many
ways temperature has influenced the
liquid I’m drinking.
In the June edition of BeanScene,
we investigated how changes in grind
temperature affect the viscosity of
espresso shots, shedding some light
on why shots tend to speed up over
time. This time, let’s zoom out and
look at a complete map of temperature-
dependent variables, and the complex
ways they influence the end result (see
image 1). Rather than discussing the
details of each effect, I’m going to focus
on how they are all interconnected.
On a cold day, coffee beans will sit at
a significantly lower temperature in the
bag and in the hopper than they would
on a summer day. This, on one level,
will affect how the beans age while they
wait to be used. The cold weather is
perfect for keeping your beans fresher
for longer: every 10°C increase in
storage temperature speeds up the
release of volatiles by 50 per cent,
according to a research article published
in 1993. This alone is a powerful reason
to keep the coffee bags and hopper
away from heat sources, such as direct
sunlight or the espresso machine itself,
as much as practically possible.
After grinding, that aroma
evaporates even faster. Grinds release
volatiles three times faster for every
10°C increase. So grinds at 50°C lose
aroma 27 times faster than grinds at
20°C – luckily we don’t usually let them
sit around for too long.
Warm grinds affect more than just
the aroma. As we discussed in the last
edition, warmer grinds do increaseoverall extraction temperature. One
effect of this was that the viscosity of
the overall hotter espresso decreased,
resulting in a faster shot. This effect
becomes even more pronounced as the
machine itself – including the basket
and portafilter – heat up too.
Another potential effect of the
heating grinder is a possible shift
in particle size distribution. This
is a much-debated question with
little solid data to back it up. Until
I can report back to you with a set
of my own experiments, let me
refer to investigations by Maxwell
Colonna-Dashwood, former UK
Barista Champion and Owner of
Colonna capsules. He based his
findings on data provided by grinder
manufacturers, which, as stated in
his blogpost ‘The heat is on – more
grinding puzzles’, found that cooler
conditions seem to produce more fines
than hotter conditions, possibly due
to the increased brittleness of colder
grinds. This, in addition to the change
in viscosity, could help explain why
shots speed up as grinder and machine
warm up during the day.
One of the most important
temperature dependent variables isluckily somewhat easier to control:
brew water temperature. This can be
quite accurately set on many espresso
machines. However, as Ona Coffee
Head Trainer Hugh Kelly warns in
one of his blogposts, we first need
to understand what the temperature
reading on the machine actually
refers to.
Hugh says if the reading is taken
from the boiler, the water will have
changed in temperature before it hits
the coffee grinds. Some machines
have different heating mechanisms
in the group heads to maintain brew
temperature, but each is different.
So 93.5°C in the boiler could mean less
than 93.5°C is actually applied to coffee,
and sometimes more.
Calibrating the output temperature
with a Scace device and a good quality
temperature probe – a job best left to
machine technicians – can help get
more precise readings. Keep in mind
though that brew water temperature
will determine extraction temperature
together with the temperature of the
grind and the metal parts in contact
with the coffee.
According to a heat transfer model
first proposed by Matt Perger throughCause and effect
“WHEN OPTIMISING EXTRACTION
TEMPERATURE, MY ONE PIECE OF
ADVICE IS TO REMAIN PATIENT: THE
SHEER BULK OF THE MACHINE MEANS
THAT CHANGES WILL NEED SOME
TIME TO TAKE EFFECT.”
82 beanscenemag.com.au
R&DLAB
Dr MonikaFeketeis theFounderofCoffeeScienceLab.Dr Monika Fekete looks at temperature-dependent variables and
their consequences on grinder and espresso machine performance.
O
n a chilly winter
morning, hot cup of
coffee in hand, I’m
once again thinking
about the many
ways temperature has influenced the
liquid I’m drinking.
In the June edition of BeanScene,
we investigated how changes in grind
temperature affect the viscosity of
espresso shots, shedding some light
on why shots tend to speed up over
time. This time, let’s zoom out and
look at a complete map of temperature-
dependent variables, and the complex
ways they influence the end result (see
image 1). Rather than discussing the
details of each effect, I’m going to focus
on how they are all interconnected.
On a cold day, coffee beans will sit at
a significantly lower temperature in the
bag and in the hopper than they would
on a summer day. This, on one level,
will affect how the beans age while they
wait to be used. The cold weather is
perfect for keeping your beans fresher
for longer: every 10°C increase in
storage temperature speeds up the
release of volatiles by 50 per cent,
according to a research article published
in 1993. This alone is a powerful reason
to keep the coffee bags and hopper
away from heat sources, such as direct
sunlight or the espresso machine itself,
as much as practically possible.
After grinding, that aroma
evaporates even faster. Grinds release
volatiles three times faster for every
10°C increase. So grinds at 50°C lose
aroma 27 times faster than grinds at
20°C – luckily we don’t usually let them
sit around for too long.
Warm grinds affect more than just
the aroma. As we discussed in the last
edition, warmer grinds do increase
overall extraction temperature. One
effect of this was that the viscosity of
the overall hotter espresso decreased,
resulting in a faster shot. This effect
becomes even more pronounced as the
machine itself – including the basket
and portafilter – heat up too.
Another potential effect of the
heating grinder is a possible shift
in particle size distribution. This
is a much-debated question with
little solid data to back it up. Until
I can report back to you with a set
of my own experiments, let me
refer to investigations by Maxwell
Colonna-Dashwood, former UK
Barista Champion and Owner of
Colonna capsules. He based his
findings on data provided by grinder
manufacturers, which, as stated in
his blogpost ‘The heat is on – more
grinding puzzles’, found that cooler
conditions seem to produce more fines
than hotter conditions, possibly due
to the increased brittleness of colder
grinds. This, in addition to the change
in viscosity, could help explain why
shots speed up as grinder and machine
warm up during the day.
One of the most important
temperature dependent variables isluckily somewhat easier to control:
brew water temperature. This can be
quite accurately set on many espresso
machines. However, as Ona Coffee
Head Trainer Hugh Kelly warns in
one of his blogposts, we first need
to understand what the temperature
reading on the machine actually
refers to.
Hugh says if the reading is taken
from the boiler, the water will have
changed in temperature before it hits
the coffee grinds. Some machines
have different heating mechanisms
in the group heads to maintain brew
temperature, but each is different.
So 93.5°C in the boiler could mean less
than 93.5°C is actually applied to coffee,
and sometimes more.
Calibrating the output temperature
with a Scace device and a good quality
temperature probe – a job best left to
machine technicians – can help get
more precise readings. Keep in mind
though that brew water temperature
will determine extraction temperature
together with the temperature of the
grind and the metal parts in contact
with the coffee.
According to a heat transfer model
first proposed by Matt Perger throughCause and effect
“WHEN OPTIMISING EXTRACTION
TEMPERATURE, MY ONE PIECE OF
ADVICE IS TO REMAIN PATIENT: THE
SHEER BULK OF THE MACHINE MEANS
THAT CHANGES WILL NEED SOME
TIME TO TAKE EFFECT.”