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Yeast Are People Too: Sourdough fermentation from the microbe’s point of view

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Yeast
Are
People
Too:
Sourdough
Fermentation
from
the
Microbe’s
Point
of
View
Jessica
A.
Lee
An
unbaked
loaf
of
sourdough
bread
is
a
garden,
home
to
micro-organisms
of
diverse
species
and
functions.
It
is
an
intimate
working
relationship
between
microbes
and
humans;
it
is
also
a
potent
reminder
of
the
culinary
benefits
of
biodiversity.
Fermentation
is
primarily
the
business
of
yeast
and
bacteria.
We
humans
aren’t
actually
very
good
at
it;
we
cultivate
microbes
to
do
the
biochemical
work
for us.
So
while
fermentation
is
often
seen
as
a
craft,
it
may
in
fact
more
accurately
be
described
a
sort
of
micro-agriculture.
Michael
Pollan
wrote
The
Botany
of
Desire:
a
Plant’s
Eye
View
of
the
World
(2001)
with
the
purpose
of
bringing
attention
to
the
lives
and
needs
of
the
plants
we
eat.
In
his
book,
Pollan
illustrates
that
agriculture
is
not
simply
a
practice
in
which
humans
manipulate
plants
to
express
the
traits
we
find
desirable,
but
one
in
which
plants
evolve
traits
that
coerce
humans
to
aid
the
survival
of
their
species
and
also
in
which
the
traits
of
plants
help
to
shape
human
history.
In
a
similar
fashion,
humans
and
our
food
are
changed
by
the
microbes
we
work
with,
just
as
much
as
those
microbes
are
changed
by
us.
We
have
done
this
bread-making
thing,
harnessing
the
microbes
we
find
around
us,
for as
long
as
history
has
been
recorded.
However,
it
is
only
in
the
last
few
decades
that
we
have
developed
the
techniques
to
be
able
to
understand
in
greater
chemical
and
biological
detail
what
exactly
happens
within
bread
dough,
and
to
use
our
new
knowledge
to
improve
bread
further.
Here,
Iwill
give
an
introduction
to
that
knowledge,
in
hopes
of
encouraging
bakers
to
think
more
as
microbes
might.
Bread-making
is
the
work
of
micro-organisms
Unlike
cookies,
cakes,
and
quickbreads,
yeast-leavened
bread
is
a
baked
good
that
requires
the
participation
of
biology
in
the
kitchen.
And
sourdough
bread
is
defined
by
that
biology.
The
word
‘sourdough’
is
sometimes
used
to
refer
to
any
bread
leavened
with
‘wild-caught’
yeast
rather
than
the
commercially-cultivated
Saccharomyces
cerevisiae;
or
sometimes
to
the
distinctive
profile
of
flavor,
texture,
and
keeping
qualities
associated
with
bread
made
acidic
by any
means.
However,
here
we
use
sourdough
to
refer
specifically
to
the
community
of
micro-organisms
which
together
leaven
and
acidify
the
dough:
a
diverse
consortium
of
yeasts
and
lactic
acid-producing
bacteria.
Before
the
invention
of
the
term
‘sourdough’,
it
was
the
only
method
by
which
bread
was
made,
for
most
of
history,
before
the
advent
of
commercial
baker’s
yeast
production.
Bread
has always
been
leavened
with
yeasts
captured
from
the
environment,
and
bacteria
175
Yeast
Are
People
Too
176
are
often
captured
at
the
same
time.
However,
sourdough
bread
is
nowadays
a
novelty
rather
than
the
default
method
of
production;
the
default
is
now
bread
made
with
commercial
yeast
grown
as
a
monoculture
a
single
species
and
with
no
significant
bacterial
activity.
The
popularity
of
‘conventional’
bread
is
due
partly
to
the
simplicity
and
regularity
of
the
process,
and
partly
to
the
wide
appeal
of
the
mild
flavor
and
very
regular texture
achievable
without
bacterial
activity.
Sourdough
therefore
finds
itself
in
the
small-scale,
artisanal
niche,
and
is
appreciated
for
qualities
such
as
complex
flavor,
rugged
texture,
and good
shelf-life.
All
of
those
special
qualities
of
sourdough
bread
may
be
attributed
to
the
complex
communities
of
yeast
and
bacteria
that
build
it.
And
an
obvious
but
often-overlooked
fact
is
that
sourdough
micro-organisms
are
interested
in
nothing
more
than
their
own
survival
and
reproduction.
It
just
so
happens
that
many
of
the
processes
they
carry
out
for
survival
contribute
to
an
excellent
artisanal
product.
A
baker,
therefore,
has
an
interest
in
understanding
those
microbial
processes,
in
order
to
facilitate
the
success
of
the
micro-organisms
that
will
make
the
best
bread.
The
creation
of
a
good
sourdough
is
a
symbiosis
between
baker
and
microbes.
Colonization:
what
starts
a
starter?
The
assemblage
of
diverse
yeast
and
bacteria
that
leavens
sourdough
bread
first
begins
its
relationship
with
the
baker
when
the
organisms
colonize
the
flour-water
batter
that
will
become
the
starter.
The
baker
combines
flour
and
water
and
allows
the
mixture
to
sit
for
hours
or
days
until
microbial
activity
is
observed,
in
the
form
of
bubble
formation,
sour
or
alcoholic
smell,
and
thinner
texture
(these
are
all
signs
of
microbial
activity,
the
specifics
of
which
are
described
below).
This
formation
of
a
‘spontaneous
sourdough’
is
actually
one
of
ecological
succession,
as
will
be
described
below.
Where
did
the
bacteria
and
yeast
come
from
to
begin
with?
Everything
is
everywhere,
and
the
environment
selects.
This
is
known
as
the
Baas-Becking
hypothesis,
after
the
Dutch
microbiologist
who
used
the
words
to
describe
his
observation
that
all
micro
organisms
are
ubiquitous
but
that
the
characteristics
of
any
particular
environment
determine
which
ones
succeed
at
being
most
abundant
there.
Bacteria
and
yeast are
on
particles
of
dust
in
the
air;
on
the
bowls
and
spoons
used
to
mix
the
batter;
in
the
flour
itself;
in
ingredients
such
as
fruit
or
yoghurt
sometimes
added
to
help
kick-start
the
process.
Then,
of
all
of
the
microbes
that find
their
way
into
the
burgeoning
starter,
only
a
few
kinds
are
able
to
survive
in
the
world
of
the
flour-water
paste.
The
initial
several
hours
after
inoculation
see
rapid
changes
in
the microbial
community.
But
once
an
active
sourdough
starter
has
been
established,
the
community
of
micro-organisms
remains
quite
stable
in
abundance
and
composition
(Stolz
2003).
It
consists
primarily
of
one
or a
few
species
of
yeast,
and
up
to
several
species
of
bacteria,
almost
exclusively
lactic
acid
bacteria.
Table
1
lists
common
sourdough
micro
organisms;
no
one
sourdough
contains
all
of
these
species,
but
the
many
species are
found
in
sourdoughs
from
around
the
world.
Once
the
population
has
reached
stability,
Yeast
Are
People
Too
microbial
counts
tend
to
range
107
109
live
active
bacterial
cells
per
gram
of
dough,
and
102
107
yeast
cells
(Siragusa
et
al.
2009).
Yeast
Bacteria
Candida
boldinii
Candida
guilliermondii
Candida
holmii
Candida
krusei/
crusei
Candida
milleri
Candida
stellata
Candida
tropicalis
Hansenula
anomala
Hansenula
subpelliculosa
Hansenula
tropicalis
Pichia
polymorpha
Pichia
saitoi
Saccharomyces
cerevisae
Saccharomyces
dairensis
Saccharomyces
ellipsoideus
Saccharomyces
exiguus
Saccharomyces
fructuum
Saccharomyces
inusitatus
Torulopsis
holmii
Saccharomyces
chevalieri
Saccharomyces
curvatus
Saccharomyces
inusitatus
Saccharmoyces
panis
fermentati
Candida
norvegensis
Enteroccocus
mundtii
Lactobacillus
acidophilus
Lactobacillus
amylovorus
Lactobacillus
brevis
Lactobacillus
buchneri
Lactobacillus
casei
Lactobacillus
casei
Lactobacillus
confusus
Lactobacillus
crispatus
Lactobacillus
crustorum
Lactobacillus
curvatus
Lactobacillus
delbrueckii
Lactobacillus
farciminis
Lactobacillus
fermentum
Lactobacillus
fructivorans
Lactobacillus
hammesii
Lactobacillus
helveticus
Lactobacillus
johnsonii
Lactobacillus
namurensis
Lactobacillus
nantensis
Lactobacillus
parabuchneri
Lactobacillus
paracasei
Lactobacillus
paralimentarius
Lactobacillus
plantarum
Lactobacillus
pontis
Lactobacillus
reuteri
Lactobacillus
rossiae
Lactobacillus
sakei
Lactobacillus
sanfranciscensis
Lactobacillus
spicheri
Leuconostoc
mesenteroides
Pediococcus
acidilactici
Pediococcus
pentosaeceus
Weissella
confusa
Weissella
cibaria
Table
1.
Micro-organisms
commonly
found
in
sourdough
cultures.
Data
from
Scheirlinck
et
al.
(2007);
Maloney
and
Foy
(2003);
Stolz
(2003).
177
Yeast
Are
People
Too
178
The
baker
may
exert
some
control
over
the
factors
that
determine
what
survives:
the
kind
of
food
available
(that
is,
variety
of
flour
and/or
added
sweeteners);
the
mechanisms
of
breathing
that
are
possible
(how
well
the
batter
is
aerated);
the
temperature;
how
much
starvation
the
organisms
must
endure
(the
fermentation
time
before
dough
refreshment);
and
the
physical
nature
of
the
substrate
(the
hydration
of
the
dough)
(Stolz
2003).
In
the
end,
by
setting
his
fermentation
parameters
and
then
attracting
all
the
potential
participants
he
can,
the
baker
will
automatically
end
up
with
the
population
that
is
best
adapted
to
survive
in
his
particular
dough.
Importantly,
the
baker’s
choice
of
fermentation
parameters
is
not
the
only
force
that
controls
these
factors.
All
the
organisms
in
the
starter
are
simultaneously
struggling
to
survive,
and
at
the
same
time,
they
too
are
shaping
their
environment
in
ways
that
make
it
either
more
or
less
hospitable
to
other
organisms.
There
is
evidence
that
certain
combinations
of
yeast
and
bacteria
species
are
more
likely
to
coexist
than
others;
however,
results
from
different
studies
sometimes
disagree
about
the
associations
they
observe
(Scheirlinck
et
al.
2008).
In
opposition
to
the
Baas-Becking
hypothesis
is
the
argument
that
biogeography
may
play
a
role
perhaps
not
everything
is
everywhere,
and
location
does
matter.
Certainly,
the
importance
of
environmental
selection
in
narrowing
down
a
sourdough’s
inhabitants
is
clear:
only
a
handful
of
genera
of
yeast
and
bacteria
have
ever
been
found
in
any
sourdoughs
anywhere
(evident
in
Table
1).
However,
within
those
genera,
the
role
of
selection
is
less
clear,
and
there
is
growing
evidence
that
in
fact
what
determines
whether
L.
plantarum
or
L.
sanfranciscensis
flourishes
in
a
certain
sourdough
is
simply
who
got
there
first:
Scheirlinck
and
colleagues,
in
their
survey
of
traditional
Belgian
sourdoughs,
found
the
sourdough
microbial
community
to
be
more
dependent
on
bakery
environment
than
on
dough
composition
(2007).
This
indicates
that
regardless
of
dough
makeup
and
fermentation
conditions,
a
baker
may
have even
more
control
over
the
population
of
his
sourdough
by
his
choice
of
inoculum.
In
any
stable
sourdough
starter,
regardless
of
the
species
makeup
of
the
microbial
community,
you
will
find
the
same
basic
processes
always
being
carried
out
by
someone
or
other.
In
the
sections
that
follow,
we
discuss
the
daily
business
that
goes
on
in
a
sourdough
community.
A
day
in
the
life
of
a
sourdough:
how
microbes
eat
Upon
being
born
in
a
sourdough,
a
microbe
finds
itself
in
a
world
of
carbohydrates,
proteins,
fats,
and
water.
The
same
basic
ingredients
are
present
in
a
liquid
starter
or
in
a
solid
bread
dough;
figure
1
presents
an
image
of
the
maze
that
is
the
solid
dough,
a
network
of proteins
with
suspended
starch
grains.
A
microbe’s
first
line
of
business
is
to
eat,
in
order
to
generate
energy.
Like
humans,
yeast
and
lactobacilli
get
their
energy
from
carbohydrates
eating,
in
fact,
a
very
small
portion
of
the
very
bread
dough
they
live
in,
and
simultaneously
excreting
their
waste
into
it,
before
we
consume
it.
As
unappetizing
as
this
sounds,
the
changes
that
microbes
Yeast
Are
People
Too
work
on
sourdough
bread
are for
the
better
they
break
down
molecules
to
change
the
texture
and
digestibility
of
the
bread,
and
create
new
molecules
to
change
its
taste
and
storage
properties.
179
Figure
1.
(top)
Scanning
electron
microscopy
image,
optimally
kneaded
dough.
(bottom)
Scanning
electron
microscopy, highly
overkneaded
dough.
From
Belitz,
Grosch
and
Schieberle (2009).
Yeast
Are
People
Too
180
a
d
c
b
Figure
2.
Chemical
structures
of
a)
glucose;
b)
maltose;
c)
amylose;
d)
amylopectin.
The
main
source
of
carbohydrates
in
bread
dough
is
starch,
which
makes
up
approximately
70
per
cent
of
wheat
flour
by
weight
(McGee
2004).
Starch
is
composed
of
long
chains
of
glucose
molecules
strung
together
into
molecules
called
amylose
and
amylopectin,
protected
inside
starch
granules.
Upon
milling,
the
granules
are
damaged
and
starch
becomes
more
physically
accessible,
but
the
long
chains
remain
chemically
unusable
to
yeast
and
bacteria
until
they
are
broken
down
into smaller
fragments.
This
job
is
done
by
the
enzymes
α-
and
β-amylase
and
maltase,
which
chop
the
bonds
between
the
monomers
to
release
smaller
sugars:
primarily
the
single
sugar
glucose
and
its
disaccharide
counterpart,
maltose.
(Figure
2)
Elegantly
enough,
most
of
the
breakdown
of
starch
is
done
by
enzymes
furnished
by
the
wheat
grain
itself.
A
wheat
grain
carries
amylases
and
maltases
in
preparation
for
the
day
it
will
germinate,
when
it
will
need
to
break
down
the
stored
starch
into
glucose
to
give
the
growing
plant
quick
energy
(Taiz
and
Zeiger
2002,
484).
Of
course,
a
wheat
grain
that
has
been
milled
into
flour
will
never
germinate,
but
the
enzymes
are
still
present
and
active
in
the
bread
dough.
The
fate
of
carbohydrates:
yeast
fermentation
Yeast
have
two
ways
of
eating.
Like
animals,
they
can
use
oxygen
to
turn
glucose
into
carbon
dioxide
gas.
This
process,
called
respiration,
is
the
most
efficient
way
of
obtain
Yeast
Are
People
Too
ing
energy
from
glucose,
so
in
the
presence
of
oxygen
yeast
will
always
respire.
For
every
molecule
of
glucose,
six
molecules
of
oxygen
are
consumed
and
six
molecules
each
of
carbon
dioxide
and
water
are
produced:
C6H12O6
+
6
O2
6
CO2
+
6
H2O
1
glucose
+6
oxygen
6
carbon
dioxide
+
6
water
However,
in
the
absence
of
oxygen,
they
can
still
obtain
energy
from
sugar
by
a
less
effi
cient
method:
fermentation.
Fermentation
is
a
general
term
for
several
ways
of
metabo
lizing
sugar
without
oxygen.
Yeast carry
out
ethanol
fermentation:
they
turn
one
mol
ecule
of
glucose
into
two
molecules
of
ethanol
and two
molecules
of
carbon
dioxide:
C6H12O6
2
C2H5OH
+
2
CO2
1
glucose
2
ethanol
+
2
carbon
dioxide
Without
oxygen,
yeast
are
unable
to
break
apart
all
of
the
C
atoms,
leaving
quite
a
bit
of
chemical
energy
in
ethanol.
A
bread
dough
is
a
heterogeneous
environment,
with
both
air
pockets
and
regions
of
dough
that
oxygen
cannot
penetrate;
therefore,
both
respiration
and
fermentation
take
place in
bread.
In
aconventional
bread
dough,
at
its
fastest,
a
gram
of
commercial
baker’s
yeast
can
ferment
0.3–0.7
g
carbohydrates
per
hour.
In
the
entire
course
of
fermentation,
the
sugar
consumed
by
the
yeast
is
equivalent
to
about
3
per
cent
of
the
total
flour
weight.
That
means
that
in
a
one-pound
loaf,
approximately
5
g
of
carbon
dioxide
is
produced,
equivalent
to
1500
cm3
(more
than
half
a
gallon)
of
gas
at
atmospheric
pressure.
Most
of
this
remains
dissolved
in
the
dough
or
in
small
bubbles
dependent
on
the
structure
of
the
dough
until
baking.
An
equal
amount
of
sugar
is
converted
to
ethanol
at
the
same
time
(Maloney
and Foy
2003).
Sourdough
yeast
are
almost
certainly
slower
at
these
processes
than
standard
bakery
yeast,
as
they
have
not
been
bred
for
optimum
efficiency
in
conversion
of
sugar
to
CO2
or
ethanol.
Instead,
they
compromise
efficiency
for
other
fitness
advantages,
such
as
acid-tolerance
or
the
ability
to
consume
a
diverse
range
of
energy
sources.
The
fate
of
carbohydrates:
bacterial
fermentation
All
of
the
above
processes
occur
in
every
yeast-raised
bread,
both
conventional
and
sourdough.
However,
as
mentioned
above, the
distinguishing
characteristic
of
sourdough
is
the
population
of
lactic
acid
bacteria
it
harbors
alongside the
yeast.
Unlike
yeast,
lactic
acid
bacteria
cannot
use
oxygen
to
break
glucose
all
the
way
down
to
carbon
dioxide;
they
make
their
living
only
by
fermentation.
But
instead
of
producing
ethanol,
they
produce,
as
their
name
suggests,
lactic
acid:
C6H12O6
2
CH3CHOHCOOH
1
glucose
2
lactic
acid
181
Yeast
Are
People
Too
182
Bacteria that
produce
only
lactic
acid
are
classified
as
homofermentative.
Hetero
fermentative
bacteria
produce
both
lactic
acid
and
either
acetic
acid
or
ethanol:
C6H12O6
CH3CHOHCOOH
+
C2H5OH
+
CO2
1
glucose
1
lactic
acid
+
1
ethanol
+
1
carbon
dioxide
or
C6H12O6
+
O2
CH3CHOHCOOH
+
CH3COOH
+
CO2
+
H2O
1
glucose
+
1
oxygen
1
lactic
acid
+
1
acetic
acid
+
1
carbon
dioxide
+
1
water
The
concentration
of
oxygen
present
determines
whether
more
ethanol
or
more
acetic
acid
is
produced
(Kandler
1983).
Both
homo-fermentative
and
hetero-fermentative
bacteria
may
be
found
in
sourdough
bread.
The
fact
that
both
yeast
and
bacteria
like
to
eat
glucose
would
seem
to
be
a
perfect
set-up
for
a
competitive
relationship,
rather
than
the
successful
coexistence
that
we
observe
in
sourdough.
The
answer
lies
in
the
fact
that,
as
mentioned
before,
the
glucose
in
the
above
equations
is
actually
not
very
abundant
in
bread
dough,
but
rather
it
is
the
product
of
substantial
work
that
the
microbes
must
put
into
breaking
down
larger
molecules
and
subsequently
importing
them
into
the
cell
interior.
Carbohydrate
degradation
and
transport
is
one
of
the
main
differentiating
features
among
microbial
species.
Each
species
of
micro-organism
has
the
capability
to process
a
different
suite
of
carbohydrates
to
obtain
the
glucose
it
ultimately
eats,
so
different
species
often
do
not
compete
for
the
same
substrate.
Maltose
is
one
of
the
most
common
sugars
found
in
bread
dough,
but
it
must
be
broken
down
into
its
constituent
two
molecules
of
glucose
before
it
can
enter
the
fermentation
or
respiration
pathways.
Because
consuming
pure
glucose
saves
that
step,
when
it
does
happen
to
be
present
many
organisms
will
shut
down
their
ability
to
use
other
substrates
so
that
they
can
concentrate
on
consuming
as
much
glucose
as
possible
a behavior
known
as
‘glucose
repression’
(Maloney
and
Foy
2003).
Typical
San
Francisco
sourdough
provides
just
one
example
of
the
interlocking
metabolic
relationships
in
yeast
and
bacterial
consortia.
Many
yeasts,
including the
S.
cerevisiae
sold
commercially,
are
able
to
recognize
the
disaccharide
maltose
and
transport
it
into
the
cell,
where
it
is
then
broken
down
into
two
molecules
of
glucose
and
used
for
energy.
However,
S.
exiguus,
a
species
of
yeast
commonly
found
in
San
Francisco
sourdough,
cannot.
At
the
same
time,
L.
sanfranciscensis,
one
of
the
lactic
acid
bacteria
most
commonly
found
in
sourdough,
prefers
maltose
to
glucose.
It
takes
one
molecule
of
maltose
into
the
cell,
breaks
it
down
into
two
molecules
of
glucose,
and
then
commonly
uses
one
of
those
glucose
molecules
and
excretes
the
other.
Consequently,
glucose-eating
yeast
nearby,
such
as
S.
exiguus,
thrive
(Neubauer
et
al.
1994).
It
may
well
be
that
S.
exiguus
simply
lost
the
ability
to
consume
maltose because
it
had
no
need
to,
and
in
fact
is
better
off
not
competing
with
the
bacteria
with
whom
it
coexists.
At
the
Yeast
Are
People
Too
same
time,
the
high
concentration
of
glucose
in
the
immediate
vicinity
triggers
glucose
repression
in
most
other
organisms,
so
that
they
lose
their
ability
to
consume
maltose,
leaving
L.
sanfranciscensis
(which
is
not
subject
to
glucose
repression)
alone
to
consume
maltose
without
competition
(Stolz
et
al.
1993,
Wink
2009).
An
acid
environment
Every
time
a
sourdough
bacterium
eats,
it
produces
acid.
Although
acid
appears
to
be
simply
a
byproduct
of
the
way
bacteria
obtain
energy,
it
also
constitutes
one
of
the
most
important
chemical
characteristics
of
the
sourdough
environment.
In
a
sourdough
microbial
community,
acidity
acts
as
a
powerful
weapon
to
keep
other
organisms
at
bay.
The
pH
in
a
sourdough
starter
can
fall
as
low
as
4
or
lower,
and
sourdough
bacteria
are
tolerant
of
acidic
environments
but
many
other
bacteria
are
not.
Once
bacteria
start
producing
acid,
they
quickly
clear
the
field
of
competitors
so
that
they
can
continue
to
reproduce
and
to
generate
yet
more
acid.
So
while
the
single
cell
sees
acid
production
as
having
mainly
to
do
with
energy
production,
from
the
perspective
of
the
population
it
also
serves
the
purpose
of
self
defense,
and
confers
such
an
evolutionary
advantage
that
it
would
be
unfair
to
call
it
just
an
accidental
byproduct
of
metabolism.
The
fate
of
proteins
Like
humans,
bacteria
and
yeast
not
only
need
to
eat
carbohydrates
to
get
energy;
they
also
need
to
consume
proteins
in
order
to
build
new
cells.
And
as
with
sugars,
microbes
generally
do
not
consume
proteins
whole,
but
need
them
broken
down
into
their
building
blocks
(amino
acids).
About
10
per
cent
of
wheat
flour
is
composed
of
the
long
protein
chains
glutenins
and
gliadins.
To
the
wheat
grain,
they
are
a
way
of
storing
protein
for
the
future
baby
plant;
in
the
bread
dough,
they
interact
to
form
the
elastic
gluten
matrix
that
traps
gas
and
enables
bread
to
rise.
Lactic
acid
bacteria
possess
proteases
enzymes
to
break
down
proteins
into
their
constituent
amino
acids
(Gerez
et
al.
2006).
This
means
that
as
microbes
do
their
job
mining
amino
acids
from
the
dough
they
are
in
fact
gradually
tearing
down
the
walls
around
them
bit
by
bit.
And
their
activity
is
strikingly
evident
in
the
changes
in
the
texture
(rheological
properties)
of
the
dough
after
fermentation:
sourdoughs
are
measurably
softer
than
doughs
fermented
only
with
yeast
(Martinez
Anaya
2003).
To
the
baker
or
bread
consumer,
these
changes
can
be
construed
as
either
positive
or
negative,
depending
on
the
desired
texture;
to
the
micro-organism,
the
main
benefit
is
nutritional.
Studies
investigating
the
role
of
sourdough
in
bread
dough
rheology
have
revealed
that,
in
fact,
bacterial
enzymes
are
not
responsible
for
most
of
the
protein
breakdown
that
occurs
during
sourdough
fermentation
(Theile
et
al.
2004).
But
sourdough
bacteria
are
still
ultimately
responsible
for
protein
breakdown
in another,
indirect
way:
once
again,
it is
a
function
of
their
acid-producing
behavior.
183
Yeast
Are
People
Too
184
Just as
wheat
grains
contain
enzymes
to
break
down
starch
and
release
sugar
upon
germination,
they
also
contain
enzymes
to
break
down
gluten
and
release
smaller
peptides
for
the
growing
baby
plant
to
use
(Bottari
et
al.
1996).
Importantly,
they
are
at
their
most
active
at
low
pH
(around
4.5)
(Belozersky
et
al.
1989).
Though
rarely
seen
in
conventional
bread
dough,
this
is
just
the
pH
that
lactic
acid
bacteria
create
in
sourdough.
So
here
is
yet
another
way
in
which
bacterially
produced
acids
ostensibly
just
a waste
product
from
their
method
of
getting
energy
are
in
fact
essential
to
their
survival:
without
those
acids,
they
would
not
be
nearly
so
successful
at
getting
the
protein
they need.
And,
of
course,
this
particular
evolutionary
advantage
exceptional
protein
procurement
through
acid
production
only
plays
out
in
the
environment
of
the
bread
dough.
In
dairy
or
meat
products,
such
acid-loving
plant
proteases
are
not
present,
so,
unable
to
manipulate
the
enzymes
of
others,
bacteria
must
make
their
own
(Christensen
et
al.
1999).
As
mentioned
before,
proteolysis
in
sourdough
bread
affects
dough
rheology;
but
human
interest
in
this
proteolysis
goes
beyond
that of
bread
dough
quality,
to
that of
human
health.
The
disease
of
greatest
concern
to
potential
bread-eaters
is
probably
celiac
disease,
an
autoimmune
disorder
in
which
the
products
of
the
digestion
of
gluten
in
the
stomach
cause
an
allergic
reaction
at
the
stomach
lining
and
unpleasant
symptoms,
including
abdominal
discomfort
and
malabsorption
(Sabatino
and
Corazza
2009).
Today,
there
is
still
only
one
effective
treatment:
lifelong
avoidance
of
all
gluten
containing
foods.
However,
interest
has
arisen
in
the
proteolytic
properties
of
sourdough
bread.
Preliminary
investigations,
such
as
those
by
Di
Cagno
and
colleagues
(2004),
have
found
that
some
sourdough
breads
can
be
tolerated
by
celiac
patients;
others
have
found
that
culturing
wheat
flour
with
a
combination
of
extracted
fungal
proteases
and
certain
live
sourdough
bacteria
can
make
the
toxic
peptides
all
but
disappear,
and
that
both
bread
and
pasta
of
decent
culinary
quality
can
be
made
from
treated
flour (Rizzello
et
al.
2007,
De
Angelis
et
al.
2010).
Further
investigations
are
necessary,
however,
to
bring
the
process
to
commercial
viability.
Making
flavor
There
is
no
end
to
the
list
of
important
functions
that
dough
acidification
plays
in
sourdough
bread.
Not
the
least,
of
course,
is
the
sour
flavor
from
which
sourdough
gets
its
name.
This
flavor
is
also
dependent
on
the
environment
the
bacteria
live
in:
for
instance,
a
higher
concentration
of
acetic
relative
to
lactic
acid
provides
a
sharper
flavor,
and,
as
explained
above,
bacteria
produce
more
acetic
acid
when
provided
with
more
oxygen.
Stepping
away
from
the
acid
question
temporarily,
another
remarkable
talent
of
sourdough
micro-organisms
is
the
production
of
many
other
complex
flavor
compounds;
see table
2
for
a
brief
list
of
some
of
the
compounds
and
their
associated
flavors.
Most
of
these
molecules
are
simply
‘secondary
metabolites’
byproducts
of
Yeast
Are
People
Too
Compound
Flavor
Producedbybacteriaoryeast
alcohols
ethanol
alcoholic
either
n-propranol
fusel-like,
burning
either
n-pentanol
(amyl
alcohol)
fusel-like,
burning
yeast
n-hexanol
alcoholic
either
carbonyls
acetaldehyde
pungent
yeast
propionic
acid
rancid
either
n-butyric
acid
rancid
butter
either
i-butyric
acid
sweaty
either
n-valeric
acid
rancid
butter
either
hexanoic
acid
unpleasant,
copra-like
yeast
acetone
aromatic,
sweet
either
methylpropanal
malty
yeast
2-methyl-1-butanal
malty
yeast
3-methylbutanal malty
yeast
2,3-butanedione
(diacetyl)
butter
yeast
3-hydroxy-2-butanone
(ace-
toin)
butter
only
both
n-hexanal
fruity
either
trans-2-heptanal
green,
fatty
either
methional
malty
yeast
esters
ethyl
acetate
ether,
pineapple
either
2-acetyl-1-pyrroline
roasty
yeast
i-amyl
acetate
fruity
yeast
phenethyl
acetate
fruity
yeast
2,3-methylbutyl
acetate
apple
peel,
banana
yeast
n-hexyl
acetate
pear,
bittersweet
only
both
ethyl
n-propanoate
rum,
pineapple
only
both
ethyl
n-hexanoate
pineapple,
banana
either
Table
2.
Flavoring
compounds
detected
in
sourdoughs.
Compounds
produced
by
yeast;
either
yeast
or
bacteria;
or
only
in
the
presence
of
both.
Data
from
Maloney
and
Foy
(2003).
sundry
metabolic
activities,
especially
the
processing
of
amino
acids
(Maloney
and
Foy
2003).
Many
of
these
flavoring
compounds
remain
in
the
final
bread; in
addition,
the
baking
process
contributes
greater
flavor
to
the
crust
through
Maillard
reactions
the
heat-induced
combinations
of
sugars
and
the
amino
acids
released
by
the
bacterially
mediated
proteolysis
described
above
(McGee
2004).
185
Yeast
Are
People
Too
186
In
general,
the
greater
the microbial
diversity
in
a
sourdough,
the
more
different
processes
that
can
occur,
and
therefore
the
more
complex
the
flavor
profile
(De
Vuyst
et
al.
2002).
As
table
2
shows,
some
flavor
compounds
are
detected
only
in
doughs
inhabited
by
both
yeast
and
bacteria.
In
addition,
longer
fermentation
times
allow
for
microbes
to
produce
more
secondary
metabolites.
These
are
both
reasons
that
conventional
bread,
made
with
only
S.
cerevisae
and
short
fermentation
times,
has a
simpler
flavor
profile
than
traditional
sourdough
breads.
Fighting
spoilage
That
sourdough
bread
spoils
more
slowly
than
conventional
bread
has
long
been
an
accepted
fact.
Of
course,
there
is
significant
interest
in
the
mechanisms
behind
sourdough
keeping-properties,
and
in
the
possibility
of
harnessing
those
mechanisms
to
improve
any
bread
even without
the
full
sourdough
process.
The
main
culprits
of
bread
spoilage
targeted
by
scientific
study
are
the
Bacillus
species,
several
of
which
are
blamed
with
causing
‘ropiness’
in
baked
bread:
‘unpleasant
fruity
odor,
followed
by
enzymatic
degradation
of
the
crumb
that
becomes
soft
and
sticky
because
of
the
production
of
extracellular
slimy
polysaccharides’
(Pepe
et
al.
2003).
We
have
already
discussed
the
self-defense
strategies
of
sourdough
bacteria
that
allow
them
to
dominate
the
living
bread
dough
community;
the
inhibition
of
Bacillus
invasion
in
baked
bread,
after
the
native
lactic
acid
bacteria
have
been cooked
to
death,
is
yet
another
issue.
However,
studies
indicate that
the
two
main
weapons
of
sourdough
bacteria
self-defense
do
live
on
through
the
baking
process
and
are
likely
the
cause
for
the
bread’s
keeping
properties.
These
weapons
are
acidity
and
antibiotic
compounds.
The
mechanism
of
acidity
production
is
the
same
that
has
been
discussed
previously.
In
addition,
many
lactic
acid
bacteria,
especially
L.
plantarum,
L.
bavaricus,and
L.
curvatus,
have
been
found
to
be
capable
of
inhibiting
Bacillus
growth
by
the
production
of
antibiotic
compounds
that
they
excrete
small,
protein-based
molecules
called
bacteriocins.
Many
of
these
bacteriocins
are
heat-resistant
and
thus
probably
survive
baking
(Corsetti
et
al.
2004,
De
Vuyst
and
Leroy 2007,
Lavermicocca
et
al.
2000).
The
choice
of
conventional
yeast-bread
over
sourdough
for
flavor
or
ease
of
production
requires
a
compromise
in
keeping
qualities.
Chemical
preservatives
are
one
solution;
however,
growing
consumer
demand
for
‘all-natural’
foods
has
prompted
greater
interest
in
the use
of
compounds
derived
from
sourdough
bacteria,
and
the
possibility
of
isolating
the
compounds
for
use
even
in
bread
fermented
without
lactic
acid
bacteria.
Although
many
potential
anti-rope
bacteriocins
have
been
identified
in
the
laboratory,
only
a
few
have
been
proven
to
work
in
bread.
Among
these
is
nisin,
produced
by
Lactococcus
lactis
(common
in dairy products),
which
kills
other
bacteria
by
poking
holes in
their
cell
membranes
(Lubelski
2008).
The
commercial
success
of
nisin
raises
hopes
for
future
developments with
other
bacteriocins.
The
search
for
novel
biological
anti-spoilage
agents
among
sourdough
microbes
is
not
unlike
the
search
for
biological
pesticides
among
soil-associated
bacteria
that
Yeast
Are
People
Too
produced
Bacillus
thuringiensis
as
an
extremely
popular
insecticide,
or
even
the
search
among
rainforest
plants
for
the
next
million-dollar
pharmaceutical
drug.
Sourdough
microbiota
are
an
equally valuable
repository
of
genetic
and
biochemical
diversity,
and
the
remarkable
properties
of
the
bread
they
produce
may
be
interpreted
as
a
sensual
reminder
of
the
value
of
biodiversity
in
the
food
we
eat.
Sourdough
microbes
and
humans
have
led
a
symbiotic
existence
for
millenia,
with
humans
have
creating
an
environment
in
which
the
micro-organisms
may
thrive,
and
micro-organisms
working
to
transform
their
living
environment
into
bread
rich
in
flavor,
toothsome
in
texture,
free
of
pathogens,
and
slow
to
spoil.
It
is
only
natural
that
interest
in
traditional
sourdough
baking
is
re-emerging
in
conjunction
with
interest
in
traditional
forms
of
sustainable
agriculture
and
the
resurrection of
heirloom
crops
and
animals;
sourdough
micro-organisms
are
equally
important
teammates
in
food
production.
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... The strongest rancid flavors were detected in the 2B and 4A samples, while the 3A and 3B samples, both produced with L. alimentarius PFC91, had the weakest rancid flavor, which may be important in terms of desirability. These results are supported by previous VAC composition findings that the level of pentanoic acid (X105), defined as the flavor of rancid butter (Lee, 2011), was the strongest in the 4A sample (2.3%) and weakest in the 3A sample (1.24%) (Fig. 6). ...
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Previous workers from this laboratory observed considerable variation in the proportions of acetic and lactic acids produced in pure broth culture as compared to consistently high proportions of acetic acid produced in the sourdough and flour suspension systems. In the latter the proportion of acetic acid was always in the range of 20 to 35% of the total, whereas in pure broth culture frequently less than 5% acetic acid was produced. In the natural environment, the sourdough bacteria, tentatively identified as lactobacilli, coexist with a yeast, Saccharomyces exiguus, and this study was undertaken to determine whether this yeast or flour ingredients including glucose or other factors were involved in this variable production of acetic acid. The proportion of acetic acid produced in broth culture on maltose, the preferred carbohydrate source, was found to depend almost entirely on the degree of aeration. Essentially anaerobic conditions, as obtained by thorough evacuation and flushing with CO2 or N2, resulted in very low (5% or less) proportions of acetic acid. Aerobic conditions, achieved by continuous shaking in cotton-plugged flasks, yielded high levels (23 to 39% of the total) of acetic acid. Similar effects of aeration were observed with glucose as the substrate, although growth was considerably slower, or in nonsterile flour suspension systems. It is theorized that, under aerobic conditions, the reduced pyridine nucleotides generated in the dissimilation of carbohydrate are oxidized directly by molecular oxygen, thereby becoming unavailable for the reduction of the acetyl phosphate intermediate to ethyl alcohol, the usual product of anaerobic dissimilation of glucose by heterofermentative lactic acid bacteria. Comparative studies with known strains of homo- and heterofermentative lactobacilli showed similar effects of aeration only on the heterofermentative strains, lending additional support to the tentative grouping by previous workers from this laboratory of the sourdough bacteria with the heterofermentative lactobacilli.
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A proteinase, representing the bulk of the enzyme activity for the hydrolysis of gliadin, was extracted from endosperms isolated from germinated seeds (four days) and was purified by ion-exchange chromatography and preparative isoelectric focusing. The optimal pH for gliadin hydrolysis was 4.25. The Mr, determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, was 30 000; the isoelectric point was 4.5. The enzyme activity was totally inhibited by E-64 and cystatin, while inhibitors of other classes of proteinases were barely effective or ineffective. The activity was stimulated by sulphhydryl compounds. The proteinase hydrolysed to small peptides the gliadins from durum and soft wheat seeds. Other protein substrates were weakly degraded or not degraded. The proteinase appears to belong to the cysteine class and to play a key role in the initial mobilization of the main reserve protein in the starchy endosperm.
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The utilisation of glucose and maltose was investigated with Lactobacillus strains isolated from sourdough starters. These preparations have been in continuous use for a long period to produce sourdough from rye, wheat and sorghum. The major metabolic products formed by resting cells from glucose or maltose were lactate, ethanol and acetate. Upon fermentation of maltose, resting cells of Lactobacillus sanfrancisco, L. reuteri, L. fermentum and Lactobacillus ep. released up to 13.8 mM glucose after 8 h. The ratio of released glucose per mol of utilised maltose was up to 1:1. Glucose formation was high when starved cells of L. sanfrancisco and Lactobacillus sp. were used. This is consistent with maltose utilisation via maltose phosphorylase which phosphorylates maltose without the expenditure of ATP and thus allows the cell to waste glucose in the presence of abundant maltose. The glucose formed may be utilised by the lactobacilli or other microorganisms, e.g. yeasts. However, the release of glucose into the medium by sourdough lactobacilli prevents competitors from utilising the abundant maltose by glucose repression. In strains of L. sanfrancisco, maltose utilisation was very effective and not subject to glucose repression. Therefore, they overgrow other microorganisms sharing this habitat. Wild isolates of L. sanfrancisco were initially unable to grow on glucose. Upon growth on maltose such strains required adaptation times of up to 150 h to grow on glucose. After subsequent transfer of glucose-grown cells to fresh medium the strains resumed growth both on glucose or maltose. They readily lost their ability to grow on glucose upon exposure to maltose. L. sanfrancisco exhibited biphasic growth characteristics on media containing glucose, maltose or both carbon sources. Evidence is provided that biphasic growth and metabolite formation are dependent on the redox potential.