Fermentative yeasts

Biochemistry resulted from the early elucidation of the pathway of enzymatic conversion of glucose to ethanol by yeasts and its relation to carbohydrate metaboHsm in animals. The word enzyme means "in yeast," and the earfler word ferment has an obvious connection. Partly because of the importance of wine and related products and partly because yeasts are relatively easily studied, yeasts and fermentation were important in early scientific development and stiU figure widely in studies of biochemical mechanisms, genetic control, cell characteristics, etc. Fermentation yeast was the first eukaryote to have its genome elucidated. 

In this thiamine pyrophosphate-mediated process, ben2aldehyde (29), added to fermenting yeast, reacts with acetaldehyde (qv) (30), generated from glucose by the biocatalyst, to yield (R)-l-phen5l-l-hydroxy-2-propanone (31). The en2ymatically induced chiral center of (31) helps in the asymmetric reductive (chemical) condensation with methylamine to yield (lR,23)-ephedrine [299-42-3] (32). Substituted ben2aldehyde derivatives react in the same manner (80). 

The production of soda crackers is also based on a mixed fermentation. Doughs for cracker production are inoculated with very smaH amounts of bakers yeast. During the first 3—5 h of the 18-h fermentation, yeast activity predominates thereafter bacterial fermentation causes a rapid decrease in pH through formation of lactic acid. 

Cheese whey soHds contain 70—75% lactose, which can serve as the carbon source for lactose fermenting yeasts such as Klujveromjcesfragilis. The total volume produced is considerably smaller than for the other yeasts described. 

For preparative purposes fermenting baker s yeast (Saccharomyces cerevisiae) is commonly used instead of a purified enzyme preparation. However, isolated pyruvate decarboxylates can also be used30. In this context, the most important substrate is benzaldehyde31 which is converted by n-glucosc fermenting yeast to (7 )-l-hydroxy-l-phenyl-2-propanone. This conversion has gained considerable industrial importance because ( )-l-hydroxy-1-phenyl-2-propanonc is an important precursor for the synthesis of (-)-cphedrin. 

In the reduction of racemic /i-ketosulphoxides (e.g. 464a) with actively fermenting yeast (Saccharomyces cerevisiae) the enantiomers are reduced at sufficiently different rates to allow isolation of optically active /1-hydroxy sulphoxide 524 and unreacted optically active /1-ketosulphoxide with at least 95% optical purity617,618 (equation 323). 

In current industrial practice, benzaldehyde is added to fermenting baker s yeast Saccharomyces cerevisiae) with resultant PAC production occurring from the yeast-derived pyruvate. Typically PAC concentrations of 12-15 g F are produced at yields of 65-70% theoretical in a 10-12 h biotransformation process. [2], Appreciable concentrations of benzyl alcohol are produced as by-product due to oxidoreductase activity in the fermentative yeast. 

A kinetic resolution was also observed in the reduction of racemic a-ketosulphoxides 277 by fermenting yeast (equation 153). Both the starting ketones 277 and the corresponding )S-hydroxysulphoxides 278 formed have been recovered in almost enan-tiomerically pure form. 

Hexose diphosphate was found by Harden and Young69 in cell-free alcoholic-fermentation liquors. In 1930, it was observed that addition of fluoride to fermenting-yeast extracts leads to an accumulation of 0-phospho-D-glyceronic acid,60 which is also a metabolite of muscle extracts.61 Attention was turned, therefore, to the pathway from hexose diphosphate to 0-phos-pho-D-glyceronic acid. In 1932, Fischer and Baer62 described the synthesis of D-glycerose 3-phosphate, and, in 1933, Smythe and Gerischer63 noted... 

Bottom-fermenting yeasts, 26 465 Bottom-pressure casting method, 23 269 Bottom-spray fluidized-bed units, 16 448-449... 

Top-fermenting yeasts, 26 465 Topical Discussion Group on Rubber Recycling, 27 471-472 Topicals... 

Although dilute aqueous acid hydrolyzed the methyl and benzyl a-D-fructofuranosides approximately 8 and 16 times as rapidly as sucrose, their method of preparation showed them to be unaffected by any of the enzymes active in a fermenting yeast suspension. Purified yeast invertase, proven free of a-D-glucosidases (maltases), could therefore contain no enzyme capable of hydrolyzing the above two a-D-fructo-furanosides, but did contain constituents that readily cleaved the beta isomers and also sucrose. The latter is accordingly a /S-D-fructofuranos-ide. When the evidence is put in this way, the present uncertainties as to whether purified invertase preparations include one or a number of /3-D-fructofuranosidases, and whether or not sucrose, methyl and... 

D,L-Lactaldehyde, CHs-CHOH-CHO, is reduced by fermenting yeast to 1,2-propylene glycol. In this reaction 1,2-propylene glycol containing excess of the dextrorotatory component was obtained by Neuberg and Vercellone by means of top yeast. On the other hand, acetol (see p. 84) and methylglyoxal (see p. 85) yield a levorotatory 1,2-propylene... 

The first suitable representative of this class of substances, 6-methyl-5-heptene-2-one, was investigated by Neuberg and Lewite. This ketone, which is a natural constituent of ethereal oils, can also be obtained by degradation of various aliphatic and cyclic terpenes. By means of fermenting yeast it is converted to 6-methyl-5-heptene-2-ol,... 

As already mentioned, the secondary alcohols that are obtained are optically active. It should be stressed that the reduction of ketones to carbinols by means of fermenting yeast is completely different from the method of resolution of racemic alcohols by treatment with living microorganisms (Pasteur). In the latter case one of the enantiomorphs is removed by oxidation during metabolism in the former it is produced by true asymmetric hydrogenation, without the intermediate formation of the inactive form, (Cf. Mayer and Levene and Walti. )... 

Levene and Walti also reduced phytochemically l-hydroxy-3-buta-none to the levorotatory 1,3-butanediol and l-hydroxy-2-heptanone to the dextrorotatory 1,2-heptanediol. It seems that the optically active glycols that are obtained by bioreduction of hydroxy ketones with fermenting yeast are configurationally related. But the 1,3-butanediol that is obtained by the reduction of the l-hydroxy-3-butanone has the opposite configuration from the product of bioreduction of the isomeric d,Z-acetaldol (see p. 81). 

The first experiments made by Neuberg and Nord with the simplest diketone, diacetyl, showed at once that this substance can be hydrogenated phytochemically with comparitive ease. Acetylmethylcarbinol appears as an intermediate (see below), and the end product of reduction is asymmetric and levorotatory. Reduction was effected by the action of fermenting yeast on diacetyl. The 2,3-butanediol that is formed can be isolated by alcohol-ether extraction of the fermentation mixture after concentration in the Faust-Heim apparatus or by steam distillation in an atmosphere of carbon dioxide under ordinary pressure it is then carefully concentrated with the birectifier and obtained in the pure state by final fractionation. 

The phytochemical reducibility of quinones was first demonstrated in the case of p-xyloquinone. This compound is worthy of interest since it is very easily formed from diacetyl by purely chemical means through a type of aldol condensation followed by ring closure. It is reduced to p-xylohydroquinone by fermenting yeast. Benzoquinone, thymoquinone and a-naphthoquinone similarly yield the corresponding hydroquinones. Tetrabromo-o-quinone and anthraquinone proved resistant to attack, while phenanthraquinone could be reduced phyto-chemically to phenanthrahydroquinone in poor yield (9%). Phytochemical reduction can also be accomplished in the dicyclic terpene series. According to unpublished experiments by Neuberg and Peiser, 2,3-dihy-... 

A tetraketone, 1,4,9,10-anthradiquinone, is reduced by fermenting yeast to quinizarin ... 

Neuberg, Lustig and Cagan were able to reduce furoin to hydro-furoin by the action of fermenting yeast ... 

Thiophene-carbinol is synthesized from 2-thiophene aldehyde by fermenting yeast. The behavior of 2,2 thenoin and 2,2 thenil on bioreduction is exactly analogous to that of furoin and furil. "... 

In such experiments, a preliminary preparation of the often difficulty obtainable pure thioaldehydes is not actually necessary. It suffices to bring the fermenting yeast in contact with the ordinary (non-thio) aldehydes and with ammonia and hydrogen sulfide (ammonium hydrogen sulfide). Such conditions frequently apply for animals and plants, for example, in bacteriological processes and probably also in the metabolism of higher organisms. 

It appeared probable that the reduction of disulfides to mercaptans by means of yeast would be particularly easy, since the two substances can be interconverted quite smoothly by ordinary chemical means. This prediction did not prove to be quite correct. True, Neuberg and Schwenk could reduce a disulfide by means of yeast, but they could not do so with the expected ease. Ethyl disulfide was chosen for the experiment because its boiling point (151°) differs very much from that of ethyl mercaptan (36°). Yeast which has been killed by boiling does not convert the disulfide into the mercaptan, but fermenting yeast does in view of the physiological importance of the disulfide group in cystine and glutathione, this observation is worthy of note. 

---