Welcome back to the Food Biotechnology lecture
series! Today I will talk about acetic acid
fermentation. We have quite many products,
again, which are produced with acetic acid
and the acetic acid, of course, is very important
for the stability and for the safety of the
products.
Like all the other fermented products, we
have a very long tradition and a very long history
of these products, of the fermentation products,
and this dates back until maybe 2000 years b.C.,
so we live with these products for more
than 4000 years.
For the production of vinegar we can use practically
any fermentable carbohydrate, either polymeric
or monomeric carbohydrates, but this is definitely
a two-step-process, in which the first step
is the fermentation of the carbohydrates by
yeast, and in the second step it is the fermentation
with acetic acid bacteria under oxidative conditions,
where we produce the acetic acid.
As a source for carbohydrates we can use practically
any fruit, any cereals, so we can produce
vinegar from apples, from pears, from grapes,
also from honey, from syrups, from cereals,
meaning from beer, or other hydrolysed starches,
from potatoes for example, or from wine as
well vinegar can be produced.
In general, vinegar is a low-cost product,
which can be high quality, but normally the
raw materials are low-cost materials, so it's
substandard fruits, it's seasonal agricultural
surpluses, that are fermented. Or they are
byproducts from food processing, or fruit
wastes. Because of this very common taste
of the acetic acid the aroma of the orginal
raw material is not so much present and for
this reason we can produce low-cost raw materials
still cleaving some high-quality product.
Of course also in this group of foods we can
produce, or we can get very high-standard
foods, meaning for example traditional balsamic
vinegar from Modena, or Sherry vinegar from
Spain or even in Greece, high-quality vinegar
is produced.
The botanical species: We can divide a little
bit these botanical species for raw materials,
so we have a group with low PH, they are easier
to ferment, because they are more stable,
not so many competitive organisms present,
it means that the PH is low, and fructose,
glucose, sucrose is present. And these are
mainly from fruits, from berries, from grapes,
from apples, which can be fermented.
The next group is from, let's say, a little
bit higher PH, so moderate acid content, again
we have easily fermentable carbohydrates present,
and this is for example from figs or from
dates. Low acid becoming close to neutral
PH, so the PH is higher than 4.5, which is
a very critical PH, especially in relation
to pathogens that can be present during the
fermentation, but again after acetic acid
fermentation the pathogens are eliminated
and this is for example from palm syrup.
And the non-fermentable raw materials, as
we have discussed earlier for beer, the starch
has to be hydrolised before the fermentation.
So, any cereals can be used, or seeds can
be used, so mais or potatoes from any of these
starch, acetic acid can be produced.
So, the first step in acetic acid production
is the fermentation of the carbohydrates to
ethanol, and for this we can use different
species. Sometimes this is a standard process,
a very traditional process, so the yeast that
is present in the environment can start the
fermentation or the yeast is added with a
high inocculum, so as a high concentration
of starting yeast, it can be used saccharomyces,
saccharomaces cerevisiae, or Hanseniaspora,
Zygosaccharomyces or Candida.
So this step is for the alcohol production.
The acetic acid formation is under oxidative
conditions and in these conditions the yeasts
are not as active anymore as they are under
the anaerobic step.
So for the oxidative fermentation acetic acid
bacteria are used and we can see that the
list of acetic acid bacteria is very long.
So we have many, many species that can produce
acetic acid and world-wide they are practically
present everywhere. And they also can spontaniously
start the fermentation or they are added as
some kind of inocculum to the acetic acid
fermentation as when we use some industrial
scale high-volume fermentation.
So Acetobacter, for example, is very common,
Acidomonas, Asaia, Gluconacetobacter, Gluconobacter,
and so on. So you see, the list of these species
is very long.
When we talk about a specific product in vinegar,
in this case this is for example the traditional
balsamic vinegar, meaning in a region in Italy,
in Modena, this vinegar is produced, we can
see here that the central fermentation, the
alcohol production and the acetic acid fermentation
is the same as in any other vinegar production,
but what is done preliminary, before the alcohol
fermentation, is the cooking of the must.
And the must is the squeezed juice of the
grapes. And after the fermentation we have
an aging process, meaning that for several
years, 18 years, this product is ripened,
and then after sensory evaluation the product
is sold. So we have two different transformations
in this process, meaning the microbial transformation,
which is standard, and we have a chemical
and physical transformation during the process.
So the must cooking: You can see that there
is a specific type of grapes used for this
product, and during the must cooking, which
is done in an open vessel at high temperatures,
so 100 degrees and then the temperature is
lowered to 80-90 degrees. During this must
cooking the water evaporates and the sugar
concentration increases and the sugar concentration
is quite high, going from 35-60 degrees Brix,
meaning that the sugar concentration is extremely high.
Again, during the cooking or during the boiling,
the solid particles and the proteins that
are present, flotate to the surface, and they
are removed from the surface, so that the
cooked must does not contain any more solid
particles and this is then fermented to alcohol.
The final alcohol concentration is 5-7 % of
alcohol and after this fermentation the acetic
acid bacteria are added and the oxidation
of the alcohol to acetic acid is carried out.
The problem is that this has to be a two step
process, because at PH 3, which means 1 % of acetic
acid present in the liquid, the yeast is completely
inhibited.
So if we want to have further or more alcohol
production, we have to do this under anaerobic
conditions where no acetic acid bacteria are present.
So, high concentrations of sugar as we have
here, they can inhibit the growth of the yeast
and the low PH and the concentration of acetic
acid above 1 % also inhibits the yeast.
The oxidation of the alcohol is normally done
in the presence of indigenous acetic acid
bacteria, but they can also be transferred
from older fermentations to the new fermentation
having a nice high-concentrated inocculum.
The low alcohol concentration is important.
Low means less than 7 %, because otherwise
the acetic acid concentration would be too
high, which is beyond any nice taste. So this
is then becoming too acidic, and this cannot
be produced for food anymore.
In the traditional process of vinegar production
as it is carried out in Modena, the final
product is stored after the fermentation in
oak barrels, giving the product a very specific
aroma. So the storage is done here in these
barrels, starting at 66 liters and then is
always transferred to smaller barrels, and
the volume loss that we have is mainly due
to the evaporation of water and of the acetic
acid. From the last barrel, which is only
15 liters of size, the vinegar, which is sold,
is taken out. And to refill this 15-liter-barrel
the vinegar is taken out from the bigger barrel
and transferred to the smaller barrel, as
it is shown here, and again we refill the
next barrel from the next barrel, so from
bigger size to smaller size, and the first
barrel is always refilled with cooked must
plus acetic acid bacteria, so that we have
a high concentration of acetic acid bacteria present.
Thank you for the attention!
