Welcome again to this Food Biotechnology course!
Today I will talk about lactic acid fermentation
and the main product that I will discuss is
the production of yoghurt. We have a series
of possibilities for lactic acid fermentation,
mostly from milk. And from milk we can produce
yoghurt, sour milk, we can produce kefir,
butter milk can be fermented.
And, we can ferment vegetables, also with
lactic acid producing bacteria. The examples
that I will discuss are the fermentation of
cabbage and the fermentation of olives.
So, for yoghurt production we have two specific
bacterial species that are used in traditional
yoghurt, and this is the streptococcus thermophilus
and the lactobacillus delbrueckii subsp. bulgaricus.
A combination of these two bacterial strains
gives the optimal product with acidity and
with taste and aroma and remaining sugar concentration.
So, the process that is carried out for yoghurt
production is shown here. We have in the starting
- this is very important - we have the standardisation
of milk, giving the milk a specified concentration
of fat. After this standardisation we do the
homogenisation to have a homogenious distribution
of the particles in the milk, the milk fat
globules, which is done at 65° C, and after
the homogenisation there is the heat treatment
at 85°C for 30 minutes. This is very important,
I will talk about this a little later.
After cooling down to 42°C this milk is inocculated
with the two species Lactobacillus delbrueckii
and Streptococcus termophilus. And the fermentation
is carried out either directly in the packaged
food or in bigger vessels and then it is transferred
into the food package where it is sold.
After the fermentation the yoghurt has to
be cooled to very low temperatures, let's
say 4°C or even lower than 4°C, because this
stops immediately the fermentation and the
living bacteria are preserved in the yoghurt.
The homogenisation is shown here. So, we have
in the milk, and this is very important, we
have in the milk a very heterogenious particle
size, ranging from 1 micrometer to 15 micrometer
around, with an average of about 8 micrometer.
After homogenisation the size is reduced to
about 1 to 2 micrometer and, what is very
important, that when the particle size is
reduced, they do not flotate anymore and they
do not separate from the liquid.
The heat treatment, which is very important
for the yoghurt production, is mainly carried
out for destroying the pathogens and to have
a safe product, these have to be removed completely.
In addition, the enzymes that could modify
the product later, they are als inactivated.
And what is also important, that it's not
only the pathogens, that are destroyed, but
also bacteria that are called thermodurics,
which resist higher temperatures, for example
the pasteurisation process, and they can grow
during prolonged storage periods, that we
have with yoghurt, because this is a stable
product, which can be stored for a long time,
that these bacteria are eliminated.
And of course the proteins, the Beta-Lactoglobulin,
the Casein is also inactivated.
The denaturation of the remaining proteins in
the yoghurt, the Beta-Lactoglobulin for example,
this can form a complex with the Kappa-Casein.
And after this, it is possible to have the
interaction with the disulphide bonds or the
formation of the disulphide bonds, building
a three-dimensional network forming a stable gel.
And this stable gel is one part of the
quality of the yoghurt, where we have a stable
yoghurt. So we have two possibilities: Either
we have a liquid yoghurt, which can be used
for drinking, or we have a stable gel,
which is eaten with a spoon, for example.
Good, fermentation is carried out at 42°C,
this is the optimum temperature that gets
some kind of equilibrium for the growth of
the two bacterial strains, of the Lactobacillus
delbrueckii, which is the high acid producing
bacteria, and if the acid is too high, the
taste becomes very tart. And also the Streptococcus
termophilus, again, which grows at different
temperatures. We have this balanced growth
of the both strains forming a balanced product,
a balanced taste in the product.
The fermentation of the yoghurt is very short.
It takes only 4 hours and the PH that is reached
is below 4.5. This is a very safe PH, because
- if you remember earlier - the pathogens
stop growing at the PH of 4.8. So if we go
down with the PH,
we get a more stable and safer product.
After the fermentation sugar can be added
to have a sweet product, or fruits can be
added, or fruit preparations can be added,
because at the low temperature, when it is
stored at 4°C, there is no continuing fermentation.
So we have the possibility to add around 10%
of sucrose, giving it a very nice balanced
taste with sweetness and sourness of the product.
What is very important for the yoghurt, is
that we have life bacteria in the product.
And the codex alimentarius, which is some
kind of regulation of the WHO, the World Health
Organisation, and this states, that we need
at least 10 million living bacteria per gram
in the product. If we have different strains,
for example in probiotics, it is allowed to
have less than 10 million, the minimum number,
that is necessary for probiotic yoghurt,
is 1 million colony-forming units per gram.
Probiotic yoghurt, as it is shown here, can
use different strains, is not Lactobacillus,
is not Streptococcus. We can use Lactobacillus
acidophilus - there are patented strains from
different companies. We can use Lactobacillus
lactis, and Bifidobacterium. So many of these
are patented, they are protected, and they
can be used in probiotic yoghurts.
So this presentation should give an overview
of the lactic acid fermentation in milk products,
showing that we get a stable product with
a low PH, no pathogens present and this can
be safe and stored for a prolonged period.
Thank you!
