GB2091293A - The production of starch and alcohol from starch-bearing grains - Google Patents

Abstract

A starch and alcohol production process is described wherein output from the steeps is processed to separate relatively fine and coarse fractions the former being treated for the recovery of starch and the latter being fed to an alcohol production line comprising saccharification, fermentation and distillation stages. In the disclosed process aqueous underflow liquor from the distillation stage is recycled and directly or indirectly used as steeping liquor for the starch-bearing grains.

Description

SPECIFICATION Production of starch and alcohol This invention relates to a process for the production of starch and alcohol from starchbearing grains, e.g. maize, in which process the grains are steeped in an aqueous liquid and subsequently mechanically coarsely disintegrated, and after degermination the resulting material is separated to obtain on the one hand a suspension of relatively fine material from which starch is recovered, and on the other hand a slurry of coarse material which incorporates fibre-bound starch and which is subjected in an alcohol production line to saccharification and fermentation to produce alcohol which is then recovered by distillation.

Such a process is described in United States Patent Specification No. 3 236 740. By utilising the coarse fraction of the disintegrated grains as starting material for alcohol production this production can proceed in an economically viable manner without the need to separate the microstarch from the relatively coarse fibrous fraction in the disintegrated grains. That separation is difficult to accomplish. In a conventional starch production plant in which the object is to recover as much as possible of the starch content of the grains, as such, an appreciable proportion of the production costs are attributable to the difficulty of this separation, which in addition to necessitating a long initial steeping period requires repeated grinding and screening operations and consumes a lot of energy.And even then the residual fibre (the so-called bran fraction which is normally used as a cattle feed), contains a lot of starch. The starch content may be as high as 2030% by weight.

It has been found that a combined starch and alcohol production process as above referred to can be performed much more profitably than is suggested by the prior art literature. The manner of achieving this improvement is the subject of the present invention as claimed herein.

The coarse fraction from which the alcohol production proceeds is obtained as a wet screen residue. This residue contains protein in addition to starch and fibrous matter. This protein, in the form of gluten, like the fibre, is a marketable commodity. In the process described in the aforesaid United States Patent No. 3 236 740 the fibre and gluten discharge as part of the stillage or underflow from the alcohol distillation column and it is indicated that this stillage can be sent to evaporators or other equipment for recovery of the solids content.

It has been found that a remarkable further increase in profitability can be achieved by carrying out the process in accordance with the present invention which is defined in claim 1 hereof. The process is characterised in that fibre and gluten in said slurry are recovered by a recovery step or steps following the saccharification, and aqueous underflow solution from the distillation is recycled and serves as steeping liquor for the starch-bearing grains.

This process affords very important benefits, resulting from the fact that substantially starchfree aqueous solution run off from the distillation stage (after separation therefrom of the fibre and gluten fractions if these have not previously been removed), is recycled so as to serve as steeping liquor in the starch plant. It has been found that such recycling and use of the aqueous underflow solution can substantially reduce the water and energy consumption of the plant. In addition byproduct values can be increased because of the nature of the substances dissolved in the recycled water.By reason of its composition, in particular its relative freedom from starch and soluble protein, and the presence of certain organic acids, the aqueous underflow solution from the distillation is a more effective steeping liquor than water and use of the recycled solution for that purpose reduces the steeping period which is required. And nutritious dissolved ingredients of the solution increase the concentration and value of the liquid effluent from the steeping vats. The energy required for concentrating the effluent by evaporation is therefore reduced and the profitability of the concentrate, e.g. as animal feed ingredient, is increased.

Another very important advantage of the process according to the invention is that the combined starch and alcohol plant can be designed as a closed system. In consequence of using stillage fluid as steeping liquor the addition of fresh water can be dispensed with or effected only for washing starch obtained from the starch production section of the plant.

Moreover it is not necessary to dispose of any water from the system other than by drying the dewatered products (germs, fibre, gluten and starch) and by evaporating concentrated steep liquor.

It is desirable for at least the greater part of the total aqueous underflow solution from the distillation to be directly or indirectly recycled to the grain-steeping section of the plant for use as steep liquor. The economic advantages of the process tend to be higher the greater is the proportion of the aqueous underflow solution which is used in that way.

Aqueous underflow solution which is to be recycled for use as steep liquor need not be recycled directly to the steeping vats. All or part of the solution can first be used as process water in another section of the plant, for example as germwashing liquid, from which the process water is recovered and then fed to the steeping section.

In carrying out the invention it is preferred to recover the fibre and gluten separately, i.e. as separate fractions. Because of the conversion of micro-starch to sugar in the alcohol production line the separation of gluten results in a substantially starch-free gluten fraction. The separation can be achieved in a simple and inexpensive way, e.g. in a centrifuge. Provided that unacceptable degradation of the protein by adverse processing conditions as hereafter referred to are avoided, the starch-free gluten fraction will command a very good price which increases with the protein concentration. In some embodiments of the invention the protein is recovered in a high concentration and the overall protein recovery is also high.

The said conversion of the micro-starch also has the result that the separated fibre-fraction is itself substantilly starch-free. The absence of any significant quantity of starch from the fibre fraction enables the fibre dewatering and drying appliance to operate with a favourably high throughput capacity and with lower energy consumption.

The invention has been made primarily for use in the production of starch and alcohol from maize. However other starch-bearing grains can be used as feedstock.

For further increasing the benefits of using the invention it is recommended to adopt one or more of the following features.

One of these optional features is the separation of the gluten fraction from its mother stream at a stage following the fermentation stage, and preferably following the distillation stage of the alcohol production line. An important advantage of observing this condition is its contribution to the attainment of a higher protein concentration and a higher protein recovery in the separated gluten fraction. The fermentation has the effect of solidifying and aggiomerating the protein present in the slurry feeding the alcohol production line and consequently of making more protein mechanically separable from the mother stream with the gluten fraction, e.g. by means of a centrifuge. Consequently not only does the separated gluten fraction contain a high concentration of protein but also the glutenprotein recovery per unit weight of the starchbearing feedstock grains is greater.Another advantage is that there is no need for separating sugar from the gluten fraction. Separation of the gluten fraction from the bottom run-off from the distillation stage is the most preferred procedure because no special measures are then necessary for avoiding significant losses of either sugar or alcohol. It is in all cases, for practical reasons, desirable for the fibre fraction to be separated from the mother stream before removal of the gluten fraction.

Another optional feature which affords important advantages is the separation of the gluten fraction at a stage following the distillation stage as above referred to, in combination with the performance of the distillation under reduced pressure conditioris. With low pressure or vacuum-distillation of the alcohol the temperature in the still can easily be kept below the level at which gluten-protein is liabie to suffer degradation. That procedure therefore promotes the financial benefits which can be derived from the invention.

Certain embodiments of the invention, selected by way of example, will now be described with reference to the accompanying drawings comprising Figs. 1 to 3 which are flow-sheets of three different processes.

In the process represented by Fig. 1, maize is steeped in vats 1 in process water delivered via line 2, acidified by SO2. The steep water is drawn off as maize steep liquor and concentrated in an evaporator 3. The so-called concentrated corn steep liquor (CCSL) is dried to make a cattlefeed or used for other purposes. When used as a cattlefeed it can be mixed with the fibre or bran fraction recovered from another point in the plant as hereafter described.

The steeped maize is coarsely milled in mills 4 in order to free the germs which are then separated from the other parts of the feedstock by means of hydrocyclones or gravity separators 5.

The separated germs are washed free from adhering starch in a washer 6 by means of process water delivered via line 7.

The underflow from the germ separators 5 is supplied to screen units 8 for separating the solids into fine and coarse fractions. The fine fraction contains the high quality starch, protein and very fine fibres which flow from the screen units as a suspension in process liquid containing dissolved substances. The coarse fraction leaving the screen units 8 contains all except the finest fibre together with starch bound to such fibre, some free starch, protein and solubles. The screening units may for example be sieve-bends having a screen slot width of 50 microns. However the grain size according to which the degerminated solid material is classified is open to choice in accordance with the required performance of the plant.

The suspension which contains the aforesaid fine fraction and leaves the screen units via line 9 is subjected to further processing for the production of starch as in conventional starch production techniques. The starch is normally separated from a protein-rich fraction, i.e. the gluten, in hydrocyclones and/or other separators 10, 11. In the illustrated process the starch is washed with water in hydrocyclones 1 The wash water leaving the top of the hydrocyclones is recycled as process water. The gluten is concentrated in a centrifuge 12. The concentrated gluten is then dewatered and dried. The process water from the centrifuge is mainly recycled to the steep vats 1 via line 13.

The screen residue or slurry from the screen units 8 is used as starting material for the production of alcohol. For this purpose the slurry is delivered via line 14 to an alcohol production line comprising a liquefaction stage 15, a saccharification stage 1 6 in which saccharification is effected by heating the inflow together with acid, and/or by enzymes, a fermentation stage 17, a yeast separator 1 8 and distillation units 1 9 from which the alcohol is derived as the overhead fraction.

The stillage from the distillation units is cooled by heat exchange with the feed stream to such units in a heat exchangers 20 and the fibrous material and the gluten are then separately removed from this liquor. The fibre fraction is removed by fine screening units 21. The fibre fraction constituting the screen residue is fed onto dewatering screens 22 and subsequently passed to a drying station (not shown) for drying preparatory to use as cattlefeed, e.g. in combination with the material obtained by drying the concentrated maize starch liquor derived from the concentrator 3.

The underflow from the screening units 21, containing gluten and solubles, is concentrated by means of a centrifuge 23 and the concentrate comprising high-protein gluten is dewatered and dried at further stations (not shown). The liquid stream from the centrifuge 23 is recycled to the steep vats 1 via line 24 and the line 2, for use as steep liquor.

In addition to the aqueous solution which is recycled to the steep vats 1 via lines 13 and 24, further quantities of the solution leaving the distillation units 1 9 feed back into the steep vats via other processing stages in the plant. For example, washing liquid recovered from the germ washer 6, and water recovered from the fibre dewatering units 22, is recycled to the said vats.

The lines via which such further recirculations occur have been omitted in order to simplify the flow sheet. The total amount of recycled solution is sufficient to fulfil the entire steep liquor requirements. No extra water has to be added.

The process described with reference to Fig. 1 has very important advantages.

The liquid stream leaving the centrifuge via line 24 has very advantageous properties for use as process water, particularly steep liquor. This fact is attributable to various causes, including the substantially complete prior removal of microstarch and the presence in the water of lower content of soluble protein. In a normal starch plant the micro-starch tends to clog the spaces between the grain in the steeps and to increase the viscosity in the following evaporator. In the process according to the invention the soluble protein content of the fluid in line 24 is lower than in normal process water because part of the soluble protein originally present has been brought to a solidified form and removed from the liquid by the centrifuge 23.The lower is the protein content of the steeping liquor the more effective is the steeping because one of the main objectives of the steeping process is the extraction of soluble protein from the grain. The required steeping time is much less than in a normal starch plant, partly because of the composition of the process water used for the steeping and partly because the operation of the alcohol line makes it unnecessary to dissolve the protein matrix of the most firmly bound starch particles in the feedstock grain. The presence of organic acids, mainly lactic acid in the underflow of the distillation units is a considerable help in the steeping process, lactic acid being a very good dissolving agent for protein. The starchfree property of the liquid stream from the gluten centrifuge also makes the liquid very suitable for the washing of the germ in washer 6.

The illustrated plant operates as a closed system as hereinbefore referred to. The only fresh water introduced is that which is introduced for starch washing in hydrocarbons 11. No water is disposed of except in the product drying operations and in the evaporator 3. The latter need be of no higher capacity than in a starch plant (without alcohol production) with the same grind (grain feedstock throughput) so that the evaporation requires no higher energy consumption.

Further advantages can be very well appreciated from the following data compiled from laboratory tests.

By the coarse milling, degerminating and screening operations a coarse fraction for feeding the alcohol production line can easily be separated, containing, say, 3040% by weight of the total starch in the maize feedstock and insoluble and soluble protein in amounts of 810% and 35% by weight respectively based on the weight of such starch, and also containing other soluble material, mainly organic acids and minerals. By operation of the alcohol production line substantially the entire starch content of this coarse fraction is converted to recovered alcohol.

In a conventional starch production plant in which the grain is processed entirely for starch production, 23% by weight of the total starch is inevitably lost with the fibre fraction used for cattlefeed. The reduction of the starch loss even to that value is only achieved at the cost of high energy consumption because of the need for repeated millings of the coarse fraction in order to loosen fibre-bound starch and for counter-current washing of the re-milled material with process water for recovering the starch particles adherent to the fibrous matter.

The tests have established that the preparation of the fibre fraction for the market as cattlefeed is itself simplified and cheapened. To be more specific; the dewatering and drying of a given weight of fibrous residue from the screening units 21 has been found to require less energy than is required for dewatering and drying a similar weight of coarse fibres recovered from a conventional starch plant. Moreover dewatering and drying appliances of a given size can operate with a higher throughput capacity. These advantages are directly attributable to the virtual absence of starch from the said screen residue.

Because of this starch-free property of this residue it has 1 5--2 5% less dry weight than a fibrous screen residue containing the same weight of fibres obtained as by-product from a conventional starch plant.

Likewise the concentration of the gluten fraction in the centrifuge 23 and the subsequent dewatering and drying of the gluten benefit in simplicity and in terms of the cost of these operations. This is because in comparison with the processing of a gluten fraction separated in a conventional starch plant, the gluten-containing liquid stream is substantially free of starch. The concentrate can be relatively easily and efficiently dewatered on vacuum filters, which can operate with a higher effective throughput capacity, and less water has to be evaporated in the gluten drying station. The gluten also has a higher quality, not only because of its freedom from starch but also because it has a particularly high protein content as a result of the development of difficultly separable protein to a solidified more easily separable form in the fermentation stage 17.The protein content can exceed 80% and even be as high as 90%, whereas normal gluten contains about 70% protein.

For ensuring the high quality of the gluten it is preferable to distil off the alcohol under reduced pressure conditions as hereinbefore referred to. If the temperature in the still units is sufficiently low, there is no risk that the colour or the digestibility of the protein will be adversely affected by passage through such units. Of course the maximum still temperature which the protein can tolerate depends upon its residence time in the still units.

For a plant as represented in Fig. 1 and operating at a throughput capacity of 200,000 tons of maize per year it can be calculated that the recovery of separate fibre and gluten fractions from the liquid effluent from the distillation units 1 9 in the manner described and the recycling of the remaining aqueous solution as process water, preferably wholly or mainly as steep liquor, will make possible an increase in profitability amounting to many hundreds of thousands of pounds sterling.

A plant as illustrated in Fig. 1 can be used for producing starch and alcohol from other starch bearing grains. However, as already indicated, maize is commercially the most important raw material.

By way of modification of the plant above described, a part of the liquid effluent from the distillation units 1 9 can if desired be recycled to the fermentation stage 1 7 in order to reduce the concentration of sugar in that stage. All that is necessary is the installation of a diluent recycling line 25 as shown in broken line in Fig. 1 together with flow control means for controlling the proportion of the effluent from the distillation units which is thus recirculated through the fermentation stage. Instead of deriving diluent liquor from the said still effluent, it can be derived from the process water stream from the centrifuge 23, e.g. by installing a branch line 26 from process water line 24 as is also shown in broken line in Fig. 1. That alternative may in some cases be preferable in order to avoid increasing the concentration of fibre and/or protein in the fermentation stage.

It will be apparent that internal recirculation of process liquid, as for example is indicated by lines 25, 26, or for washing separated products as is done in other embodiments of the invention described hereinafter, does not increase the water input or output of the combined plant. The advantages of the closed system operation as above described remain, no matter how much or how often the stillage fluid is thus internally recirculated.

The processes represented by Figs. 2 and 3 are in many respects similar to the Fig. 1 process. In order to avoid unnecessarily repetitive description, corresponding parts of the plants are designated by the same reference numerals in the three figures. The following description explains the respects in which the processes shown by Figs. 2 and 3 differ from the process according to Fig. 1.

In the process according to Fig. 2 the fibre and gluten fractions are recovered from the slurry passing along the alcohol production line, at separating points located between the saccharification and fermentation stages and consequently less protein passes through the fermentation stage. In order to avoid unacceptable sugar losses with the separated fibre fraction the screen residue on fine screening units 21 is washed with part of the cooled still effluent which for that purpose is fed to over-screen spray heads 27 via lines 28, 29, 30. The underflow from the screen units 21 is fed to a centrifuge 23 as in Fig. 1 and the liquid stream leaving this centrifuge is conducted via line 31 to the fermentation stage 17 whereafter the alcohol production and recovery proceeds as in the Fig. 1 process.The concentrate from the centrifuge 23 is further dewatered in a vacuum filter unit 32 from which the gluten fraction is discharged preparatory to being dried. In order to keep down loss of sugar with the separated gluten fraction the filter cake on the vacuum filter 32 is washed with part of the cooled still effluent which is fed to spray heads 34 via lines 28 and 35. The liquid removed by the vacuum filter is conducted away along line 33 and combines with the liquid stream leaving centrifuge 23.

The remainder of the cooled still effluent, i.e.

the part thereof which is not fed to the spray heads 27 and 34 is recycled via lines 28, 29 and 36. Part of this liquid is used for washing the separated germ in washer 6, and part is fed directly into the steep vats 1 for use as steep liquor. The wash liquid recovered from the germ washer and from the fibre dewatering screens 22 is subsequently recycled to the steep vats via recirculation lines (not shown) so that such additional liquid also becomes used in the steeping of the starch bearing grains. An additional yeast separator 37 is installed, in line 36, in order to remove surplus yeast from the recycled liquid before it reaches the washer 6 or the steep vats.

Although the reduction of protein in the fermentor may be of advantage in some circumstances the recovery of the gluten fraction prior to the fermentation stage has the disadvantage as compared with the Fig. 1 process that the yield of protein in the recovered gluten is lower, for reasons previously explained.

In the process represented in Fig. 3 the fibre and gluten fractions are recovered from the alcohol production line after the fermentation stage. The protein yield benefits accordingly. The fermented liquor is supplied to the screen units 21 for the separation of the fibre fraction and the screen residue retained by the screens is washed with part of the cooled still effluent which is conducted along lines 38, 39, 40 to the overscreen spray heads 41. The purpose of this washing is to reduce losses of alcohol in the fibre fraction to a tolerable level. As in the Fig. 2 process the underflow from the screen units is supplied to a centrifuge 23 and concentrate from this unit is further dewatered in vacuum filter 32 from which the gluten fraction is discharged.But in the Fig. 3 process a part of the concentrate from the centrifuge 23 is returned (with yeast) to the fermentation stage via a line 42. The filter cake on vacuum filter 32 is washed with cooled still effluent supplied to spray heads 43 via lines 38 and 44. The remainder of the cooled still effluent is used as steep liquor, part of it being firstly used in the germ washer 6 as in the previously described processes.

In the Fig. 3 process the input to the distillation units is substantially free not only of fibre but also of undissolved protein..

Flow control valves (not shown) will of course in practice be provided for controlling relative volume flow rates of liquid through branched lines, for example for controlling the relative amounts of cooled still effluent supplied as wash water for washing separated fractions and then as steep liquor. In general it will be most profitable to design the plant so that the bulk of the aqueous solution in the still effluent becomes used as steep liquor.

The described processes, in making more effective use of the potential value of the different constituents of the starch-bearing grain, also have the merit of reducing plant waste effluent for disposal.

By way of modification of the processes described with reference to the drawings, fibre and gluten recovered from the carrying stream can be recovered together, as a composite fraction.

This composite fraction can be marketed as such, e.g. as a cattle feed.

Claims (6)

1. A process for the production of starch and alcohol from starch-bearing grains, in which process the grains are steeped in an aqueous liquid and subsequently mechanically coarsely disintegrated, and after degermination the resulting material is separated to obtain on the one hand a suspension of relatively fine material from which starch is recovered, and on the other hand a slurry of coarse material which incorporates fibre-bound starch and which is subjected in an alcohol production line to saccharification and fermentation to produce alcohol which is then recovered by distillation, characterised in that fibre and gluten in said slurry are recovered by a recovery step or steps following the saccharification, and aqueous underflow solution from the distillation is recycled and serves as steeping liquor for the starchbearing grains.

2. A process according to claim 1, wherein said fibre and gluten are recovered as separate fractions.

3. A process according to claim 1 or 2, wherein said fibre and gluten are recovered from the underflow stream from the distillation.

4. A process according to claim 3, wherein the distillation is performed under reduced pressure conditions.

5. A process according to claim 1 or 2, wherein the fibre and gluten fractions are separated from the liquid stream in the alcohol production line at a stage following the fermentation stage.

6. A process according to any preceding claim, wherein the fibre and gluten fractions are recovered from the aqueous underflow solution from the distillation, the gluten being initially separated as a concentrate by means of a centrifuge, and wherein at least part of the liquid stream from this centrifuge is used firstly for washing the separated germ and then as steep liquor.