FR2587035A1 - Process for producing ethanol by the fermentation of sugar-containing media - Google Patents

Abstract

The present invention relates to a process for producing ethanol by the fermentation of sugar-containing media using yeasts, characterised in that at the start of the fermentation, the yeasts are immediately brought into contact with the whole of the sugar-containing medium to be fermented, and in that during the fermentation, the medium around the yeasts receives no supply of sugar up to completion of the fermentation when the yeasts are separated from the fermentation medium.

Description

 The present invention relates to the production of dyestuff by fermentation, in particular from molasses.

 The fermentation processes currently used in the molasses distilleries fall into two groups, insofar as the yeast produced by the fermentation is recycled (fermentation by recovery of yeasts or not (fermentation by mother tank) in the medium fermentation.

 In the fermentation by mother tank, the yeast production is carried out in a mother tank, also called pre-fermentor, fed by a must with a low sugar content (70 g / l) and in which the yeast (saccharomyces) develops. cerevisiae) in the presence of air.

 A residence time of 10 hours is required to produce a yeast wort grading about 3.5 to 40 GL and containing 60.106 to 100.106 yeast germs per milliliter.

It is recalled that a solution is n GL when it contains n cm3 of pure alcohol for a total volume of 100 cm3 at the temperature of 15 C.

The temperature is maintained at 330 ° C throughout the fermentation circuit.

 The must leavened by the mother vat constitutes the "vat stock" deposited at the bottom of the fermentation vat and whose volume represents approximately one-third of it.

 It constitutes the supply of yeast biomass on which will be poured, in the form of a must more concentrated (strong must), the amount of sugar necessary to produce the alcoholic degree of the wine.

 The wort is gradually poured into this vat which fills up in 10 to 15 hours as sucrose is assimilated by the yeast. This fermentation rate of the sugar is maintained during pouring at about 80% of the substrate present, so that when the tank is full it will titrate about 6.50 GL for a fermentation of 80 GL.

 This method of progressive casting makes it possible to control the rise in temperature during the fermentative process. This is of the order of 18.9 kJ (2.6 kcal) per alcoholic degree produced. The strong must is maintained at around 150 C so that the calories released by the fermentation maintain the tank at 330 C.

 If this method is not enough, a system of water runoff on the outer walls of the tank also eliminates calories. In the case of large volumes of cuverie (> 100 m3), a heat exchanger must be mounted in a loop on the withdrawal of the tank.

 The yeast biomass is maintained at about 60.106 germs / ml in this tank which is not aerated and therefore can produce only a very weak growth of yeast.

 The total fermentation time, excluding mother vat, is around 25 to 30 hours for 8 to 90 GL.

 In the yeast recovery fermentation, the wine is sent to a centrifuge which will produce, on the one hand, the wine destined to feed the distillation column, and on the other hand a concentrated suspension of yeast (primary cream) which will have to undergo two treatments before to be put back into fermentation.

- 1st treatment: washing with water
A volume of cream is mixed with a volume of water and
kept agitated by bubbling with air for 1 hour at
1 hour 30. This treatment has been recommended since
several years in order to release the yeast from
toxins accumulated during fermentation, and therefore
make it more active for the next fermentation cycle.

Its main purpose is to dilute the supernatant and to
allow, during the acid treatment that follows, a
better lowering of pH with the minimum of acid
(decrease of the buffer effect).

- 2nd treatment: acid wash
At the end of washing, the yeast passes into a second
centrifuge. The supernatant constitutes the "small
waters "or yeast calves" that have an alcoholic degree
around 40 GL and can not be rejected.

They are recycled
either in the must, which is a source of infection
bacterial,
either in the distillation column, which decreases
the degree of the wine and increases the consumption of steam.

The secondary cream from the second centrifuge
is added with water and acid in order to obtain
a pH = 2 with an acidity of 2 to 2.5 g / l of acid
sulfuric.

This acid treatment is very effective for the destruction
bacteria and should last 1 hour.

 In this process, the "vat" is constituted by yeast treated with acid. It constitutes a biomass, much richer than in the previous process. The casting is done in the same way, with a mashed must containing the sugar necessary for the production of the desired alcoholic degree. The first three hours of casting are however aerobic in order to regenerate the yeast before the actual fermentation.

 The yeast population being much larger (200.106 -ger./ml), the fermentation time for 80 GL is of the order of 15 to 20 hours.

Finally, a new process proposed by the Company
ALFA LAVAL, under the name of Biostil, has just been installed in a distillery. It aims to continuously produce a wine of low alcoholic degree (5 to 60 GL) on a molasses medium highly enriched in dry matter by the recycling of vinasses.

 The productivity in ethanol is kept high thanks to the low alcoholic degree.

 The resistance to non-sugar substances is due to the use of a selected yeast strain of the Schizosaccharomyces pombe variety.

 The unique, aerated fermenter operates continuously and requires yeast recycling without any treatment.

The expected performances are as follows
- production of 60 GL in 7 to 8 hours, in the presence
20% of non-sugar substances.

This is obviously a remarkable improvement over the results that have just been presented on traditional vat rooms.

This one is done in two directions
- increased productivity of ethanol and
dry matter in fermentation.

 Table No. 1 makes it possible to compare the results obtained according to the methods used. It has been established for wines with 80 GL, but it is possible to obtain higher alcoholic degrees by lengthening the fermentation time. The figure of 100 GL seems to be a maximum.

TABLE N 1
PERFORMANCE OF ALCOHOLIC FERMENTATION PROCESSES OF MOLASSES

DEGREE <SEP> ALCOHOLIC <SEP> MATTER <SEP> DRY <SEP> BIOMASS <SEP> FROM <SEP> TIME
<tb> PRODUCTIVITY
<tb> DU <SEP> FROM <SEP><SEP> YEAST <SEP> IN <SEP> FROM
<tb> ROCEDE <SEP> OF ETHANOL
<tb> WINE <SEP> VINASSE <SEP> FERMENTATION <SEP> FERMENTATION
<tb>(<SEP> GL) <SEP> (%) <SEP> (Sprouts / ml) <SEP> (Hours) <SEP> (g / l / h)
<tb> 8 <SEP> GL
<tb> Motherbottom <SEP> on <SEP> molasses <SEP> 9% <SEP> 60.10 @ <SEP> 25 <SEP> 2.5 <SEP>%
<tb> (P <SEP> = <SEP> 60)
<tb> 8 <SEP> GL
<tb> Recovery <SEP> of
<tb> on <SEP> molasses <SEP> 9% <SEP> 200.10 @ <SEP> 17 <SEP> 3.7 <SEP>%
<tb> yeast
<tb> (P <SEP> = <SEP> 60)
<tb> 6 <SEP> GL
<tb> on <SEP> mer <SEP> molasses <SEP> no
<tb> 20 <SEP>% <SEP> 8 <SEP> 5.9
<tb> Biostil <SEP> + <SEP> accurate
<tb> Vinasse
<tb> * Productivity to increase by 20%, in a system running continuously.

(A batch vat uses only 80% of its volume).

 The productivity gain always comes from an enrichment in yeast biomass, and this is what characterizes the processes: Biostil and Yeast Recovery. In the future, fermentation by the mother tank must be abandoned in favor of the recycling of biomass, which remains essential for the fermentation of products with high non-sugar solids content.

 The object of the present invention is to increase the productivity of ethanol on molasses not to increase the intrinsic productivity but in order to be able to use fermentation media with high non-sugar content.

 Indeed, the production of ethanol at lower energy cost requires the design of fermentation workshops that can operate on media with high concentration of non-sugar substances. This makes it possible to obtain a vinasse relatively charged with dry matter and thus a saving of steam on the workshop of concentration where it is in view. This enrichment of the fermentation medium in nonsugar is done naturally by the recycling of a portion of the vinasse in the must to ferment.

 In the case of sugar by-products (molasses and poor sewage), it was shown that at equal concentrations, the non-sugar substances had an identical biological activity on the yeast, regardless of their vinasse or molasses origin. On the other hand, an increase in these products at doses higher than those found in molasses (purity <60%) causes a decrease in the ethanol productivity (g / l / h) in proportions which increase as a function of non-sugar recycled rate.

To this end, the present invention proposes a method for producing ethanol by fermentation of sugar-containing media, using yeasts, characterized in that - at the beginning of fermentation the yeasts are put into
contact instantly with the entire sweet medium
to ferment, and - in that during fermentation, the environment
yeast does not undergo any sugar intake until the end
fermentation where the yeasts are separated from the middle
ferments.

 This process is therefore characterized in that, contrary to what is done in the techniques known as fermentation by mother tank or fermentation techniques by yeast recovery, the sweet medium is not slowly poured in contact with the yeasts but at the same time. otherwise the contact is made instantly, that is to say in a very short space of time compatible with the industrial realization of this step.

 In the course of the fermentation, there is no more sugar supply until the fermentation is complete and the yeasts are separated alcoholic medium, that is to say the fermented medium, it is what is meant by "the yeast environment does not undergo any sugar intake until the end of fermentation".

 In order to ensure the progress of the fermentation, it is advisable to maintain in the fermentation medium, a biomass of the order of 100 to 130.106 yeasts per ml of medium, which generally gives a fresh weight pellet of the order of 10 *.

 Under these conditions, the fermentation remains stable for weeks at 80 GL for a period of 4 to 6 hours, if the yeasts are recycled after centrifugation in the form of a squeezed yeast pellet or a thick yeast cream ( base preferably of the order of 50% yeast in the form of fresh material).

 It is thus possible to use, without modifying the process parameters, a yeast cream containing 50% of the wet yeast pellet.

 It should be noted that in the process according to the invention the yeasts, after separation of the alcohol, are recycled at the beginning of fermentation without further treatment, in particular without washing.

 It should also be noted that the process must be conducted with a bacterial contamination which must be less than 703, preferably 102 seeds per ml.

In fact, the contaminating flora which is most often of lactic origin produces an organic acidity which disturbs the metabolism of the yeast, which very quickly leads to a decrease in the alcohol productivity which can reach 20% as soon as the acid of bacterial origin reaches 1 g / l.

 Under these conditions, if we want to recycle yeasts as proposed, we must solve the problem of protection against bacterial contamination.

 In order for the biomass to remain constant during the fermentation cycle, it is necessary to provide a low aeration of the order of 0.5 to 2 VVH, preferably 0.5 to 1 WH. This aeration being necessary for the growth and the survival of the cells. This aeration does not affect the fermentation balance, which remains close to 60 to 62 liters of pure alcohol per 100 kg of sucrose consumed.

 Of course, the envisaged process which involves the instantaneous casting of the sweet medium in the bacterial biomass can pose cooling problems since under these conditions there is a considerable temperature rise in the beginning of the fermentation, that is why as it has been said above one of the major interests of the process according to the present invention is not so much to be able to prepare in a reduced time, that is to say a fermentation time of the order of 5 hours, a wine 80 GL, but to be able to prepare with a normal fermentation time, that is to say of the order of 20 hours, a wine also containing 80 GL but containing a much higher dry matter content, that is, that is to say a wine obtained from a sweet material in which non-sugar materials such as vinasse have been recycled.

 The addition of vinasses or non-sugar materials to the sweet medium leads to a slowing down of the fermentation process and in the case of the process according to the present invention, this is not a problem since the fermentative process is much faster than with the processes implemented so far.

 Under these conditions, we obtain fermentation residues rich in non-sugar that do not have to be concentrated which leads to a saving of steam and therefore considerable energy.

 In general, it has been found that it is possible in the process according to the present invention to introduce non-sugar dry matter into the fermentative medium in proportions of 10 to 30% (w / v) of the fermentation medium. in particular from 15 to 25% (w / v). In particular, when the sweet medium used is beet molasses, corresponding vinasses may be used to dilute the sweet medium.

 The sweet media can be of various origins, in particular it can be products from grains (cereals) of fruit, sugar beet or sugar cane, for example.

 In the case of grain alcohol, it is possible to envisage even higher levels of non-sugar materials than for beet molasses, of the order of 40%.

 Although the process according to the present invention is essentially an essentially batch process, since the fermentation does not undergo any external supply of fermenting material, it is not excluded to be able to practice the process according to the present invention in a continuous manner, that is to say, to advance the yeasts and their medium in cascade tanks, in which they will always be in equilibrium with the same surrounding medium, it is also possible to provide a system in which a yeast front moves continuously for, starting from a corresponding end b a beginning of fermentation, end at the end of fermentation in a medium essentially satisfying the wine.

 In practical terms, it is possible to envisage two types of fermentation using the process according to the invention.

 In the case of a batch fermentation, the tank will be filled with must at 330 C for the time necessary for the centrifugation of the yeast from another wine tank feeding the distillation. This operation lasts, in general, 2 to 3 hours which will be necessary to establish the initial fermental volume. If the fermented product is molasses of purity 60, the fermentation time should last 5 hours and one imagines in this case, the industrial constraints to evacuate the calories and carbon dioxide over such a short time.

 This process was developed to work in vinasse recycling, so that the fermentation time is extended to 20-24 hours, by a fermenting medium containing 20% non-sugar dry matter (purity 40%).

 The aeration will be maintained at 0.5 - 1 WH as soon as the tank is filled and during the entire fermentation.

 In the case of a continuous fermentation, the wort and yeast mixture is fed into a set of compartments connected in series. During this journey, the medium is gradually enriched in alcohol and is depleted in sugar.

Each compartment is therefore at an increasing alcoholic degree from the first to the last stage of the fermenter. Sugar, of course, follows the opposite curve.

 Such a process is encountered on industrial site in the form of several tanks mounted in series.

The implementation of the process requires in this case
- a ventilation device to be mounted in each tank
- Pouring the must only in the first tank at the same time as the recycled yeast cream.

 The number of vats should be large enough so that the variation of the alcohol content between each vat does not exceed 0.50 GL.

 The following examples are intended to highlight other features and advantages of the present invention.

EXAMPLE 1 Recycling of yeast ~ on vials ~ ~ 30 ml a) Experimental conditions
The wort is thus constituted - Sucrose: 140 g / l by a purity molasses of 60% - Acidity: 2.5 g / l of sulfuric acid by use
of hydrochloric acid - pH: 5.0 - Phosphate
di-ammonium-
that: 0.5 g / l - Antifoam
The yeast used is a strain of Saccharomyces cerevisiae (UNGDA No. 1) having good performance in alcoholic fermentation of molasses.

 After a pre-culture on Malt Wickerham medium and taken up by physiological water (NaCl: 9g / l) the yeast is ready for inoculation of the must.

A 50 ml flask containing 30 ml of must is inoculated with yeast at a dose of 106 seeds / ml. It is equipped with a micro-leakage device for measuring ethanol production by weighing the carbon dioxide released.

The fermentation takes place in an oven at 330 C, with rotary shaking of the flasks. The yeast biomass is measured by spreading on a Petri dish containing Malt Wickerham agar medium. The recovery of yeast is done by centrifugation. The pellet obtained is resuspended in 30 ml of must without any prior treatment of the yeast (no washing, neither with water nor with acid).

b) Results
After a first fermentation under the conditions of the test, the yeast is recycled many times over the wort for a long period of two to three months. The biomass and the productivity increase up to the 4th fermentation and then remain stable during all the other fermentative cycles, without deenergeneration of the yeast.

 The results of the first four fermentations are shown in figures in Table 2.

TABLE N 2
PRODUCTIVITY GAIN BY YEAST RECYCLING

BIOMASS <SEP> FROM <SEP> YEAST
<tb> TIME <SEP> DEGRE
<tb> FERMEMTATIONS <SEP> germs / ml <SEP> PRODUCTIVITY
<tb> DE <SEP> ALCOHOLIC
<tb> SUCCESSIVE <SEP> FERMENTATION <SEP> FROM <SEP> WINE <SEP> FROM ETHANOL
<tb> Start <SEP> of <SEP> End <SEP> of
<tb> Fermentation <SEP> Fermentation <SEP> (Hours) <SEP>(<SEP> GL) <SEP> (g / 1 / h)
<tb> 1st <SEP> Fermentation
<tb> 106 <SEP> 60.106 <SEP> 24 <SEP> 8.3 <SEP> 2.7
<tb> (standard SEP test)
<tb> 2nd <SEP> Fermentation <SEP> 60.106 <SEP> 100.106 <SEP> 16 <SEP> 8.3 <SEP> 4
<tb> 3rd <SEP> Fermentation <SEP> 100.106 <SEP> 130.106 <SEP> 8 <SEP> 8.3 <SEP> 8.2
<tb> 4s <SEP> Fermentation
<tb> 130.106 <SEP> 130.106 <SEP> 5 <SEP> 8.3 <SEP> 13
<tb> and <SEP> next
<tb> 130.106 seeds / ml represents fresh weight of 10% of the wine.

c) Conclusions
The interpretation of these results will be made according to four essential points which highlight the technological parameters of the method according to the invention.

1. Growth of biomass on concentrated must
The first three fermentations show that it is possible to integrate, on the same concentrated must of sugar, a growth of the yeast without attenuating the production of ethanol. The air contained in the 50 ml flasks is a controlled aerobiosis that allows a good growth of the yeast.

2. Stabilization of the biomass at the 4th fermentation
It should be remembered that a growing yeast cell gives by successive buds, a number of daughter cells, then dies. Its life depends to a large extent on the conditions of the nutrient medium in relation to its metabolism. A mortality rate therefore appears in any population of recycled yeast and, together with the growth rate, constitutes the two essential factors for the maintenance of biomass.

 Beyond the 4th fermentation, the same amount of must and air being always available, one can imagine that the number of cell divisions remains constant.

If the biomass does not increase any more, it must be admitted that a balance is now established between the number of cells that die and those that are born by cell division.

 In fact, although the biomass and the productivity remain constant from the 4th fermentation, one nevertheless observes by microscopic observation, numerous cells of lewresmortes. These do not interfere with the production of alcohol and are probably a source of nutritive protein for growing yeasts.

3 - No washing of the yeast is necessary
In view of the industrial processes currently proposed, it has been seen that it seemed indispensable for the yeast to undergo two treatments before being recycled to the water and to the acid.

 However, the tests just described clearly show that the yeast can pass directly from the alcoholic medium (wine) in the sweet medium (must) without undergoing any pretreatment (washing or acid treatment).

 Successive fermentations are not affected and keep a biomass and productivity quite stable. This calls into question the merits of washing yeasts with water, in order to "eliminate toxins" from fermentation. The washing processes currently proposed for the recovery of yeasts have only one purpose: bacterial decontamination.

It therefore appears that by maintaining a very low level of bacterial contamination in the medium (<10 or 10 seeds / ml), it is possible to eliminate the process of washing and acidic treatment of the yeasts. This represents a major technological advance, because we know the constraints imposed by such a process and the nuisance brought by the recycling of yeast water from the second centrifugation.

4. Very high ethanol productivity: 13 g / 1 / h
The yeast biomass here is 130.106 germs / ml.

It is of the same order, and even lower than that found on the industrial site in the yeast recovery process. In the tests, the fermentation lasts 5 hours (productivity: 13 g / l / h) and in the industrial sector, it goes from 20 to 24 hours. (productivity: 4g / l / g).

This difference can be explained as follows
Like any biochemical reaction, the conversion of sucrose to ethanol is at a rate that is proportional to the concentration of the substrate (sucrose)
In addition, the ethanol produced is inhibitory and accumulates in the yeast cells.

 This has been explained previously the conduct of pouring must in industrial environment.

 In order to eliminate calories more easily, the tank is gradually fed with sugar by a cold must (14 C).

This has two consequences
- the yeast is very quickly in a rich environment
alcohol (80% of the final GL)
- the sugar concentration in the tank does not exceed
not 20% of that of must during casting.

 In contrast to this progressive pouring observed in the factories, the laboratory tests were conducted in instant casting, that is to say that, from the beginning of the fermentation, the yeast (130.106 seeds / ml) is placed in a medium containing 14% sucrose, ie the maximum concentration of substrate.

 The initial ethanol does not exceed 10 GL, due to the alcohol provided by the yeasts. The enrichment in alcohol is then progressively following the normal curve of fermentation, but there is not from the beginning a high alcoholic degree.

 It is therefore believed that the differences in sugar and ethanol concentrations that have just been demonstrated are largely responsible for the differences in fermentation rates observed.

The method of the invention therefore proposes, in order to obtain a high productivity, an instantaneous casting where all of the biomass is put as soon as possible in contact with the totality of the must - ferment
COMPARATIVE EXAMPLE
Continuous Fermentation Application on Ungerm Fermenter
This process is often proposed as an industrial application, because its advantages are numerous (simplicity of implementation, facilitated automation ... etc).

It may therefore seem interesting to propose its use in the production of molasses alcohol. It should be noted, however, that the operation of such a continuously fed fermentor will impose on the yeasts minimal concentrations of substrate (sugar: 2 to 5 g / l) and maximum concentrations of ethanol (8 g / l). This goes against the process according to the invention in which the productivity of ethanol increases with the concentration of the substrate.

Two types of editing are carried out - the first, without yeast recovery, in which the
biomass will depend on the growth rate of the yeast, - the second, with total recycling of the yeasts.

 The yeast population then, as in the previous trials, decreased yeast growth and mortality rates.

 The conditions remain identical to those of the former with regard to - the constitution of the must, - the yeast strain, - the preculture and the inoculation of the must, - the biomass measurements.

The fermentation circuits operated under the following conditions - the must is stored in the fridge (40 C) during a
period not exceeding 3 days. No contamination
bacterial does develop under these conditions, so
that all the fermentation media will remain,
during all the tests, at a very high rate of bacteria
low (<102 seeds / ml), - the fermenter is a glass column comprising
its base a sintered glass plate by where will be sent
the air. It will also maintain
the homogeneous medium graciated with the gas transfer that
maintain along this column.

The volume in fermentation is 300 ml. food
must is done from the bottom using a pump peris
talique. The evacuation of wine and yeasts is
from the top, with the throttle, thanks to a
flush tube at the level of the fermenting medium.

In the case of a fermentation without yeast recovery,
only the fermenter is used, - the yeast decanter, refrigerated, allows
decantation from wine and their recycling in
the fermenter by the must supply circuit.

A double wall system allows the yeasts to
deposit avoiding the turbulence of the arrival of the wine
and gases. 50 to 80% of the biomass is recycled.

The rest is stored in the fridge, and centrifuged
manually twice a day, so as to complete
transfer of yeast to 100% in the fermentor, - the must is fed in such a way as to maintain
in the fermenter sugar concentrations and in
alcohol close to a wine at the end of fermentation.
decisive parameter was the remaining sugar whose con
centers should be maintained between 3 and 5 g / l,
expressed in sucrose, - the alcoholic degree was of course close to
80 GL, under these conditions, the performance of the process is
- the fermentation time (hours) i
Volume in fermentation (ml)
Flow must (ml / h)
- the yeast population in the fermenter: germs / ml
- the ethanol productivity obtained: g / l / h - the air supply: was made from a
bottle of compressed air and could be varied from O to
6 WH (volume of air per volume fermented and by
hour).

- the fermentation balance: Alcohol produced
Sugar consumed
was measured by the aeration rate, taking into account
ethanol driven by the gases.

RESULTS
The two fermentation processes: with or without yeast recovery, worked for several weeks to stabilize the must supply according to the biomass present.

The performances are identical in both cases - alcoholic degree: 8 to 8,30 GL - sugar remaining: 3 to 5 gil - fermentation time: 30 hours - biomass of yeast: 20,106 germs / ml - Productivity of ethanol
80 x 0.8 = 2.13 g / l / h
30
The results are much lower than those obtained in Example 1. They highlight several important points.

The yeast biomass remains very low (20.106) compared with that observed in Example 1 (130.106).

This difference is considerable and highlights the inhibitory action of the millet in which the yeast has remained.

 In the case of non-recovery of yeast, only the growth rate is involved, and it is obviously very low.

 For the circuit with yeast recovery, there is practically no increase in biomass.

Assuming that the growth rate remains unchanged, it would seem that the survival time of the yeasts in the medium maintained at 80 GL can not exceed 30 hours.

 This low concentration biomass is therefore responsible for a much longer fermentation time (30 hours instead of 5 hours), which gives a relatively low ethanol productivity (2 g / l / h) and close to the figures obtained. in the factory in the mother-tank process.

Claims (14)

 1. Process for producing ethanol by fermentation of sugar-rich media by yeasts characterized  in that, at the beginning of the fermentation, the yeasts are brought into instantaneous contact with all the sweet medium to be fermented, and  - In that during fermentation the yeast environment undergoes no sugar intake until the end of fermentation where the yeasts are separated from the fermented medium.  2. Method according to claim 1, characterized in that the biomass is between 100 and 130.106 yeast germs / ml of medium.  3. Method according to claims 1 and 2, characterized in that the yeasts after separation fermented medium are recycled at the beginning of fermentation without further treatment.  4. Method according to claim 3, characterized in that the bacterial contamination is less than 103 germs / ml, in particular 102 germs / ml.  5. Method according to one of claims 1 to 4, characterized in that the aeration of the fermentation medium is maintained between 0.5 and 2 VVH of air.  6. Process according to claim 5, characterized in that aeration is maintained between 0.5 and 1 VVH of air.  7. Method according to claim 6, characterized in that the sweet medium is added non-sugar material before the start of fermentation.  8. Process according to claim 7, characterized in that the non-sugar material is a vinasse.  9. Method according to one of claims 1 to 8, characterized in that the sweet medium is a molasses.  10. Method according to one of claims 1 to 9, characterized in that the non-sugar solids represent 10 to 30 t (w / v) of the fermenting medium.  11. Method according to one of claims 1 to 10, characterized in that the non-sugar solids represent 15 to 25% (w / v) of the fermenting medium.  12. Method according to one of claims 1 to 11, characterized in that the sweet medium is a molasses debetteraves.  13. Method according to one of claims 1 to 11, characterized in that the fermentation is conducted in a single vessel.  14. Method according to one of claims 1 to 11, characterized in that the fermentation is conducted in cascaded tanks.