1947 read food yeast

7/29/2019 1947 Read Food Yeast

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95FOOD YEAST

~ y F . O. READSince time immemorial, man has been striving todiscover the underlying requisites for robust health.The importance of the diet was almost immediatelyrealized, but just how and to what extent it was involved, has taken centuries to discover, and evento-day, the picture is by no means complete.At first the emphasis was on heat producing foodswhich supplied the calories necessary for an activelife. Later it was discovered that these were notenough to ensure health-the importance of vitaminswas established. Of still more recent date was thediscovery that proteins too, have an essential partto fulfill in the diet.Man has two general sources of protein at hisdisposal, viz. animal and vegetable, and it is nowgenerally accepted that animal protein is superior tovegetable protein.Proteins are mixtures of very complex chemicalsubstances, many of which have not yet been isolated, but they are known to contain at least a scoreof amino acids. Proteins from vegetable sources areoften deficient in some of these essential constituents, which detracts form their nutritive value.To obtain the full value of the proteins, carbohydrates and fats in the diet, certain vitamins mustalso be present, amongst others the vitamin B group.This also is a very complex mixture containing atleast ten constituents and occurs naturally with protein in milk, meat, eggs, etc. Here again animalsources provide a better balanced vitamin B, as thatderived from vegetable sources is often deficient incertain essential constituents.Thus it follows that persons living largely on avegetable diet, are liable to suffer from a deficiencyofgood proteins and vitamin B.It is true that certain animals that live entirelyon a vegetable diet, are capable of producing thesemissing proteins and vitamins within their ownbodies. Human beings are not so fortunate and haveto rely on an extraneous source. "During comparatively recent times, the import

ant discoverywasmade that yeast can supply all theconstituents necessary to complement the vitamin Bgroup as it occurs in vegetable sources. Moreover itso happens that yeast is an excellent source ofprotein, of the same high nutrit ive value as animalprotein.Of course, for centuries past, our ancestors haveunwittingly been making good the deficiencyof pro-

teins and B vitamins in their diets by consuminglarge quantities of freshly prepared mead and beerfor instance. The natives of Africa had their traditional Kaffir Beer pot, etc. These beverages underwent none of the refinements applied to modern wine"and beer and retained in the dregs large numbers ofyeast cells. With, the progress of civilisation, theseold established items of diet gradually disappeared,to be superseded very often by refined wheaten products, mealie meal, polished rice, etc. Asa resultthe general health of the community gradually deteriorated and very often deficiency diseases ensued.I f yeast is again to be used in the modern diet,'presumablya practical method of application wouldbe to admix it with the wheaten products, mealiemeal, etc. For this to be economic, a large supply ofgood quality yeast at low price must be forthcoming.At present brewers' yeast is in very short supply andmoreover this yeast is most unpalatable, due to itsextremely bitter taste and also due to the large percentage of insoluble calcium salts, with which it isusually mixed. The only other yeast commerciallyavailable is bakers' yeast, which has a blander taste.This however is grown for a specialized purpose, on

an expensive batch process, and would be far toocostly for general application.In an endeavour to be selfsufficient, the Germansstarted producing food yeast about 30 years ago.Unfortunately little information is available as towhat measure of success was achieved, from thephysiological aspect. I t is, however, significant thatproduction on a large scalewas continued right up tothe time of capitulation in 1945. I t may further benoted that -the Germans used the expensive woodsaccharification process to'provide the source of carbohydrate for yeast growth.In 1940, the Brit ish Colonial Office decided tosponsor the production of food yeast, and Dr. Thaysen of the Department of Scientific and IndustrialResearch was asked to investigate the problem.As the source of carbohydrate, Dr. Thaysen chosemolasses, which in normal times is available to theBritish Empire in abundant supply, at a very cheapprice. He then set about developing a suitable strainof yeast. For this purpose the yeast chosen shouldhave the following properties: '(i) I t should growwellon the sugars in molassesand should at the same time produce,a high yield offood yeast.(ii) The dried product must.be palatable.


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96(iii) I t should have a high rate of growth whichwould effect an economy on the size of plant necessary.(iv) The cellsizeshould be large enough to enableefficient handling with the machinery commerciallyavailable.

. (v) I t should be capable of growing at a reasonably high temperature, as this would facilitate coolingin warm climates.. (vi) It should be capable of withstanding storagein the cream form, for sport periods at ordinarytemperatures without autolysing (yeasts have theproperty of being able to feed on themselves in theabsence of' other foodstuffs-a process known asautolysis).After some lengthy research, Dr. Thaysen succeeded in developing an entirely new strain of yeast

w h ~ c h he named Toru:lopsis utilis variety major,which more nearly satisfied all the above requirements than any other strain examined. For instance,it was capable ofgrowing approximately three timesa.s rapidly as bakers ' yeast under optimum conditions. Furthermore, with the exception of phosphorus and ~ i t r o g e n , which can be supplied in inorganicform, this yeast can obtain all the nutrients requiredfor growth, from molasses.Next he set about devising a continuous process.

The batch process,whereby bakers' yeastis normallyproduced, would be far too costly on the scale visualized for food yeast. As the result of experienceand data gained on a pilot plant at Teddington, acommercial factory designed to produce 4,000 tonsper annum was subsequently erected in Jamaica.In.evitably difficulties were encountered calling formmor changes of design and variations in the process, etc. This greatly hampered production but it isbelieved that the plant willsoon be in full operation.Samplesof foodyeast produced by the pilot plant

at Teddington were subjected to various tests whichproved fairly conclusively that the addition of foodyeast greatly improves the nutritive value of a dietwhose protein is otherwise derived mainly fromcereals. The biological value of the mixture of thecereal proteins with those of yeast, is equal to thatof a similar mixture using milk protein instead ofyeast. Trials undertaken showed that at least l oz.could be taken daily without risk of digestive disturbance. This amount may seem negligible, in thatit would i n c r ~ a s e the amount of dietary protein byonly! oz. dally, but by supplying the amino acidsnormally missing in the proteins of cereals, it nevertheless disproportionately improves the quality ofthe protein in a diet composed largely of cereals.A report on nutritive tests conducted along theselines in the United. Kingdom, can be found in an

official. British publication-British Medical Research War MemorandumNo. 16 (1945). In general,the results of these tests were very favourable and inno cases were harmful effects observed.In South Africa at the instigation of the Ministersof Public Health and Economic Development, theFood Yeast Development Co. (Pty.) Ltd., was formedwith two objects in view, viz.:-(a) Investigating the possibility of producingfood yeast in South Africa from raw materials available locally and to determine the costs of manufacture.(b) Should production be feasible, to providesufficient food yeast to enable its nutritive value tobe tested systematically in South Africa.Some preliminary work was initially undertakenusing a laboratory sizegenerator, and a pure cultureof Torulopsis utilis variety major, obtained fromLondon. This strain of yeast has been patented, andthe Industrial Development Corporation hold thepatent rights in the Union. Subsequently a semicommercial plant with 3: nominal capacity of fourto .five tons per month was designed and erected atMerebank. Here again considerable difficulties hadto be overcome primarily as the result of the lack ofsuitable equipment during the wartime. Ofnecessitymuch of the plant installed was of a make-shift character of local fabrication, especially as it was deemedessential to operate this plant on the continuousprocess only. Minor alterations have been effectedfrom time to time, but basically the process has remained unaltered. The various stages of the processwill be described in outline.

Molasses Pretreatment.Molasses is diluted with an equal proportion ofwater, by weight. Sufficient sulphuric acid is thenadded to give a pH of 4.0. The solution is boiled bymeans of live steam, in an open vat for one hour andthen allowed to stand for 24hours. Calcium sulphate

in the form of gypsum gradually settles out andcarries down with it, most of the suspended organicmatter. This step is essential for two reasons (a) toolarge a concentration of calcium salts in solution hasa deleterious effecton the growth ofyeast andfurthermore might cause precipitation of calcium salts inthe generator and so adulterate the final food yeastproduct. (b) Unless the organic matter in suspensionisremoved, it imparts a very dark colourto the yeast.By means of a float, the clear solution is drawn offfrom the top, leaving the sediment behind. As anadditional precaution the solution is next passedthrough a centrifugal to ensure complete clarificationand stored in vats ready for use in the generator.The percentage ofsugars present isdetermined by theFehling's reduction method-it normally averagesabout 25 grams sugar per 100 ml. solution.


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Generator.This consists of a stainless steel vessel fitted withwater spraying pipes, for external cooling and anoverflow pipe which can be adjusted to any desired

working volume. Air is introduced at the basethrough porous ceramic candles having a pore sizeof about twelve microns. Suitable instruments arefitted for metering the volume ofmolasses, ammonia,phosphoric acid and water to be added, and for recording the pH and temperature of the contents ofthe generator.Before proceeding with an outline of the procedure adopted, there are several general considerationswhich should be borne in mind.(a) It is a general property of yeasts that theproduction of alcohol is favoured by a high carbo

hydrate concentration' and anaerobic conditions.Conversely the propagation of the yeast itself is favoured by extremely low concentrations of carbohydrate and an abundant supply of oxygen in solution. In practice the latter is achieved by passing airinto the liquor, through porous candles previouslymentioned. By thus breaking up the air into minutebubbles a large surface area is provided which facilitates solution of oxygen in the liquor. By thismethod a considerably higher percentage of theoxygen passes into solution, than if perforated pipesare employed, as is established practice, in the manufacture of bakers' yeast. By calculating the theoretical amount of sugar that the yeast could consumehourly if growing under optimum conditions andadding it accordingly, the carbohydrate concentration is kept as low as possible.

(b)' The optimum hydrogen ion concentration forthis particular strain of yeast is between pH 4and pH 5.(c) The optimum temperature is 36 to 37C.,and as approximately 1,600 B.T.D. are liberated foreach pound of sugar oxidised, it is necessary to coolthe generator in order to maintain the temperature

between these limits.(d) I t should be constantly borne in mind further that should the conditions of growth departmuch from the optimum even for a short period onlye.g., as regards hydrogen ion concentration or temperature the yeast may become permanentlyweakened, and although optimum conditions aresubsequently re-stored, it may never again regain itsformer activity. Obviously therefore extremely careful and regular control is essential.

(e) Great care is required at all times to guardagainst possible infection. All plant has to be thoroughly sterilized at regular intervals, all liquids fedinto the generator should be absolutely sterile and theair used should be efficiently filtered.

To proceed: to start the generator a certainamount of sterile water, dependent on the weight ofseed yeast available, is run in. Sufficient seed yeastis added to give a wet yeast concentration of aboutone per cent. This is determined by centrifugingfifteen mls. of the liquor, in a laboratory centrifuge,using a calibrated tube. The total weight of yeast inthe generator can then be calculated from the volumeof liquid and the yeast concentration. From this,the total amount of nutrients to be added iscalculated as follows;- .'Assume the weight of,yeast present is ten kg.It is known that the generation time of this strain ofyeast is less than 100 minutes, and consequenty ifthis weight of yeast were grown under optimumconditions, it should be capable of increasing inweight by about five kgs. during the following hour.Therefore sufficient nutrients should be providedduring the hour, to grow five kgs. of new yeast. I tis known that one kg. of yeast requires 0.4386 kgs.sugar, 0.00526 kg.' phosphorous and 0.02113 kg.nitrogen, Consequently by simple multiplication it

follows that the weight of nutrients required is2.193 kg. sugar, 0.0263 kg. phosphorous and 0.1056kg. nitrogen.The pH is watched carefully and should it exceedthe optimum limits, it is adjusted by the addition ofeither sodium carbonate or sulphuric acid, as thecase may be. . .The nitrogen content is determined every fifteenminutes by the rapid formol titration method, whichaffords a ready check on the rate at which nitrogenis being consumed, and hence the rate at which theyeast is growing. At the end of the hour, the yeastconcentration is determined again and as before thetotal weight of yeast present is calculated. I f it hasgrown according to schedule, then there willbe 15kg.present, and during the following hour sufficient nutrients should be added to grow 7.5 kg. of new yeastand so on.This process, whereby a larger quantity of yeast

is grown each hour, is termed the differential growthof yeast and is continued, until the yeast concentration reaches 9 per cent. Experience shows that yeastdoes not grow well at concentrations in excess of 9per cent., probably due to overcrowding. On a batchprocess, once .this stage is reached, the yeast is harvested, the generator is emptied and a new differential is started. Obviously the time taken to reach aconcentration of 9 per cent. is dependent on severalfactors, but it is normal on a batch process, for thethe differential to last about twelve hours.Assuming that 10 kg. of yeast were used as in

oculant and that an hourly growth rate of 50 percent. was maintained throughout the twelve hours,then the final weight would be about 1,300 kg.


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98But if the process could be continued for anotherhour, presumably it should be possible to produceanother 650kg. of yeast. And if the process could bebe continued indefinitely it should be able to produce650kg. of yeast every hour. This in fact isthe essenceof the continuous process whereby it should bepossible to produce 1,300 kg, of yeast in 2 hours asagainst twelve hours by the batch process, using thesame size generator. The Food Yeast DevelopmentCo. has devoted its energies exclusively to continu-ous production, production by a batch process nothaving been attempted. Continuous production isachieved by calculating the nutrients required forthe following hour by the same method as before.

But now it is necessary to add water as well in orderto accommodate the new yeast grown at a concentra-tion of 9 per cent. i.e. if 650 kg. of yeast are to begrown, and the concentration is to remain at 9 percent., the volume should be increased by 1 ~ X 650= 7,222 litres. From this figure is deducted thecombined volume of the sugar solution, the ammonia.and the phosphoric acid to be added, to obtain thevolume ofwater required. The rate offlowof all theseliquids is then adjusted to the calculated figure.using the flowmeters previously mentioned. In thisexample the generator has been assumed to contain1,300 kg. of yeast at a concentration of 9 per cent.i.e. the volume of liquid is 1 ~ X 1300 = 14,444litres.

Thus the overflow pipe would be adjusted to aworking volume of 14,444 litres and any liquid inexcess of this would automatically run out.The yeast concentration is determined each houras previously, and should it vary the amount of nu-trients added are altered accordingly.Throughout the above, it has been assumed thatthe yeast will increase in weight at a steady hourlyrate of 50 per cent. In practice, of course, this is verydifficult to attain, though rates of growth approach-

ing 50 per cent. have been achieved.In spite of all the precautions taken, infectiontroubles have been experienced, which sometimeslimit the duration of the continuous phase to lessthan a week.Concentration andWashing.

The yeast liquor runs out of the generator con-tinuously and is passed through a yeast separator,which concentrates it to about 50 per cent. yeast.This cream is then diluted again with water to ap-proximately the original volume and again passedthrough the separator.. This washing procedure isrepeated two or three times until the waste water isperfectly clean.

Drying, Milling and Bagging.The yeast cream is dried on a double drum drierto about 5 per cent. moisture. I t emerges from thedrier in a flaky form and is put through a hammermill to break it up into a fine powder, to facilitatebagging. I t is stored in 50 lb. paper lined bags. Thedried product is slightly hygroscopic, but if suitablystored will keep almost indefinitely.Using as a standard, the figures published by theColonial Food Yeast Development Co., the nominalcapacity of the Merebank plant is four to five tonsmonthly. I t is now producing regularly at seven tonsmonthly and some further increase is likely.Seven tons monthly corresponds to about 610 lbs.of food yeast per working day, and since the totalcapacity of the single generator is 83 cubic feet, this

is equivalent to 7.3 lbs. food yeast per cubic foot ofgenerator capacity per day.. The Jamaica plant wasdesigned for an output of only 5.6lbs. per cubic footof generator capacity while the full production cap-acity of German plants was less than 0.6 lbs. percubic foot of generator capacity.I t is therefore evident that the technique of foodyeast production evolved at Merebank is not onlyequal to, but probably superior to methods in useelsewhere.I t can safely be said that the first object of theFood Yeast Development Co., has been achieved, inthat the technical difficulties in the production havebeen mastered.I t is of interest to note that large quantities offood stuff of excellent nutrit ive value, can be pro-duced largely from a product of which there is nor-mally a 'surplus over and above Union requirements.Furthermore by growing, say sugar, on a givenacreage of land, and using this sugar plus a smallamount of phosphate and ammonia to.grow yeast, ayield of protein can be obtained, which is severaltimes as high as could be obtained directly by grow-ing the best protein producing agricultural crops on

the same acreage, and many times higher than theprotein which could be obtained from animals fedentirely off a similar acreage.In furtherance Of the second object, practicallythe entire output to date has been placed at the dis-posal of the National Nutrition Council, who areconducting a series of tests to evaluate its nutritiveproperties. Although the value of foodyeast has beenclearly established by nutritive tests conducted inthe United Kingdom and elsewhere, it is consideredadvisable to carry out further tests under SouthAfrican conditions, as a check. For obvious reasons

these tests to be conclusive, should be of fairlylengthy duration. The outcome of these tests isstill awaited.


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the analyses done here. Arrangements had now beenmade, however, to get it analysed overseas.Mr. HAYES said that he could not see how anorganism such as a yeast could be patented, as wasclaimed in this paper. He also mentioned the fact

that the production of food yeast was not confinedto the Merebank pilot plant . Messrs. DehydratedProducts, of Johannesburg, had been in productionfor some time. Their output was about 8 to 10 tonsper month and the average yield, he believed, wasabout 40to 45 per cent. on sugar. The yeast had beenassayed regularly at the Institute of Medical Research and the vitamin content was found to beslightly higher than that claimed for the overseasproduct.Mr. READ stated that the yeast strain used atMerebank was not a naturally occurring one, but wasthe result of laboratory research in England.He was aware that food yeast was being producedin Johannesburg, but the process used was the simple

batch process which, unlike the continuous process,did not call for any special research or technique inhandling. For large-scale production the batch process could not, however, be compared with the continuous process.Dr. HEDLEY congratulated Mr. Read on the resultsachieved, which he appreciated because he startedthe proGess at Merebank and knew of all the difficulties that had to be overcome. The pilot plant was

pu t down in an alcohol distillery, which was probablythe worst place for such a plant. The walls of theroom and the atmosphere were very badly infestedwith all kinds of infection, and the process oftenbroke down after a short run. The use of theaeration candles was made difficult by the excessiveamounts of foam formed.

99The PRESIDENT said he welcomed this paperbecause, as technologists, we were most interested in

the by-products of sugarcane. Higher efficiencieswere often claimed in other countries than wereobtained here, but in this paper it was stated that inthe conversion ofmolasses into yeast some of the bestefficiencies in the world had been obtained in thelocal pilot plant.Mr. DU TOIT said that the au thor claimed thatI' ton of yeas t could be obta ined from 0.4386 tonssugar, bu t that was of course not dry yeast, but wetyeast containing about 50 per cent. moisture and 25per cent. protein. Even at this figure, however,assuming that we could recover 3.5 tons of sucroseper acre every second year, the annual yield ofprotein would be about one ton per acre.According to C. A, Brown in Indust rial and En-gineering Chemistry (New Edition, 1943), the Germans obtained about 34 kgs, yeast from 200 kgs.molasses in the first world war, using the Delbriickprocess. This yeast contained about 47-56 per cent,protein, bu t on account of the loss of energy someGerman dieticians considered the feeding of themolasses to animals directly more economical, and

the process was found to be uneconomical after thefirst world-war, as it could not compete with theproduction of other protein concentrates. He noticedthat the loss in energy in the process now being usedat Merebank was also very considerable.

Apart from the protein value, food yeast was alsovery valuable as a rich source of vitamin B. He wasunder the impression that it contained about 20 partsper million aneurin, 60 parts per million riboflavine,and about 500 parts per million nicotinic acid; bu the had not seen any vitamin analyses of SouthAfrican food yeast, and would be interes ted in anysuch figures which the author might have. He hadseen a reference to the fact that lack of iron stimu- Mr. READ agreed that the pilot plant was erectedlated vitamin B production in toralopsis util is , and in a very bad place, but he though t if the processwould like to know what other effect iron had on the worked here, it would probably work anywhere else,production of food yeast. Infection was, of course, the big trouble, bu t eventhat had been cut down considerably, and it was nowMr. READ, in reply, stated that he considered the possible to work continuously for about eight daysyield of 34 kgs. of yeast from 200 kgs. molasses, or, in exceptional cases, up to eleven or twelve days.as obtained in Germany during the first world-war, The foaming problem was largely tied] up with thewas very low. At Merebank a yield of one ton of infection problem. I f infection were eliminated, notfood yeast was obtained from 4.5 to 5 tons of molasses 0 much trouble would be experienced with foaming.

and theoretically even less should be used; but there D D id th t th t ld .f th 1 f f d t b M r. ODDS sal a e presen wor reqinre-was, 0 course, e oss 0 energy re erre 0 y r. t f . th . hb h d f 35d T it Th ti 11 60 t . f men s 0 sugar were in e neig our 00 0U 01 . eore ica y, a per cen . conversion a '11' t b t ld b . d t 100 'II't t h ld b btai d b t ' ,m l IOn ons, u wou e increase 0 mr IOnsugar: 0 yeas s au e a amen, u in practice tons if h b ' ld t all th honly 45 per cent. was realized. ons.l every uman emg cou ge a e s u g ~ r . erequired. The latter figure was, however, the ceilingLarge quantities of iron and even calcium had a at present for direct consumption of sugar, but ifvery bad effect on the growth of yeast, but he could sugar could be economically utilised for food yeastnot say what the effect on vitamin B formation was. production or in other important ways it would openHe had, in fact, no data on the vitamin B content up new possibilities for the sugar industries of theof South African food yeast, as it was difficult to get world.


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100Mr. READ stated, in reply to Mr. Duchenne, that

food yeast was ofmuch the same foodvalue as bakers'yeast or any other yeast; but whereas it was esti-mated that a pound of dry food yeast produced frommolasses in a 40,OOO-ton per annum plant cost about6d. a pound, wet bakers' yeast costs about 4s. or

a

16s. a pound of dry yeast.The yeast strain used in the production of food

yeast formed invert sugar from sucrose very readily,but the alcohol was only formed to the extent of oneto two per cent.

1947 read food yeast

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