Method of Increasing Yeast Yield

This invention relates to a method of increasing the yield of yeast cells which can be used as single cell protein.

For the production of usable protein for humans and animals, experimenters are showing much interest in microorganisms which produce single cell proteins. Possible suggested foods for the microorganisms have included hydrocarbons such as petroleum and fractions thereof, e.g., natural gas, naphtha, kerosene, and fuel oil, and other carbon-containing materials such as oxygenated hydrocarbons, carbohydrates, and cellulosic substances. It is desirable to be able to feed crude, instead of refined, materials to the microorganisms in order to avoid refining costs.

Crude methanol in particular is a possible food source for certain microorganisms, as has been suggested in the literature. How to increase yeast cell growth where crude methanol or crude ethanol is the carbon source is the specific problem addressed by the present invention.

Although the prior art has suggested various possible lists of carbon sources for various lists of microorganisms, many microorganisms are very selective as to the foods which they can utilize.

The present invention has solved a problem which hitherto had not even been recognized in the prior art: i.e., 2-methyl-1-propanol is detrimental to the growth of certain yeasts which feed on crude methanol or crude ethanol.

It is an object of the present invention to increase the yield of certain yeasts on the carbon sources crude methanol and crude ethanol.

It has been discovered that in a yeast single cell protein process using as the carbon source a substrate comprising at least one alcohol selected from the group consisting of methanol and ethanol, together with a minor amount of 2-methyl-1-propanol as an impurity, and utilizing selected microorganisms which are capable of assimilating methanol or ethanol, the yield of yeast is increased by removing the 2-methyl-1-propanol from said alcohol prior to introduction of said alcohol to the fermenter.

It is believed that the best mode of removing 2-methyl-1-propanol from the carbon source for the yeasts in the process of this invention is by distillation, although other possibilities exist, such as by gas chromatographic methods and solvent extraction.

The carbon source is preferably crude methanol which contains 2-methyl-1-propanol as an impurity and which can also contain some ethanol. A less preferred but still operable feedstock is crude ethanol which contains 2-methyl-1-propanol and which could also contain some methanol.

The distillation can be readily accomplished since the boiling point of methanol is about 65.degree. C. and the boiling point of ethanol is about 78.degree. C., whereas that of 2-methyl-1-propanol is about 108.degree. C. The methanol and/or ethanol distill, leaving behind the 2-methyl-1-propanol.

Distillation is done, preferably at atmospheric pressure, before the carbon source is fed to the fermenter.

Substantially all (i.e., about 98 to about 99.9 percent) of the methanol and/or ethanol can be distilled from the crude methanol and can be fed to the fermenter.

The methanol will distill at about 65.degree. C. and the ethanol at about 78.degree. C., if the distillation is conducted at a pressure of 1 atmosphere.

The yeasts which are most preferred are Hansenula polymorpha, although certain other yeasts would also be within the scope of this invention.

Yeasts which can be used in the process of this invention comprise species from the genera Candida, Hansenula, Torulopsis, Saccharomyces, Pichia, Debaryomyces, Lipomyces, Cryptococcus, Nematospora, and Brettanomyces. The preferred genera include Candida, Hansenula, Torulopsis, Pichia, and Saccharomyces.

Examples of suitable species comprise:

Candida boidinii

Candida lipolytica

Candida mycoderma

Candida utilis

Candida stellatoidea

Candida robusta

Candida claussenii

Candida rugosa

Candida tropicalis

Candida maltosa

Brettanomyces petrophilium

Hansenula minuta

Hansenula saturnus

Hansenula californica

Hansenula mrakii

Hansenula silvicola

Hansenula polymorpha

Hansenula wickerhamii

Hansenula capsulata

Hansenula glucozyma

Hansenula henricii

Hansenula nonfermentans

Hansenula philodendra

Torulopsis sonorensis

Torulopsis candida

Torulopsis bolmii

Torulopsis versatilis

Torulopsis glabrata

Torulopsis molishiana

Torulopsis nemodendra

Torulopsis nitratophila

Torulopsis pinus

Pichia farinosa

Pichia polymorpha

Pichia membranefaciens

Pichia pinus

Pichia pastoris

Pichia trehalophila

Saccharomyces cerevisiae

Saccharomyces fragilis

Saccharomyces rosei

Saccharomyces acidifaciens

Saccharomyces elegans

Saccharomyces rouxii

Saccharomyces lactis

Saccharomyces fractum

If desired, mixtures of two or more species of yeasts can be employed. The particular yeast employed depends in part on the carbon-containing substrate to be used. All of the yeasts listed above will grow either on methanol or ethanol, and some of them will grow on both carbon sources. However, not all will grow on both methanol and ethanol.

In the fermentation process, an aqueous mineral medium such as those known in the art to be useful for fermentations employing yeasts is supplied to the fermenter. This aqueous mineral medium provides nutrients such as nitrogen, phosphorus, magnesium, calcium, potassium, sulfur, and sodium as well as trace quantities of elements such as copper, manganese, molybdenum, zinc, iron, boron, and iodine. As is known in the art of fermentation, the concentrations and relative amounts of the nutrients can vary, depending on the yeast selected and other factors. In addition, the aqueous mineral medium preferably contains vitamins such as biotin and thiamine, as is known in the art.

Although a minor amount of the nitrogen utilized by the yeast can be provided by any ammonium salt used in preparing the aqueous mineral medium, the major amount of assimilable nitrogen is provided by the addition of anhydrous ammonia or aqueous ammonia, i.e., ammonium hydroxide, to the fermentation vessel. Although the amount of anhydrous or aqueous ammonia added will depend upon the pH desired for the fermentation mixture, generally it will be such as to maintain the fermentation mixture at a pH within the range of about 3 to about 5.

The fermentation reaction is an aerobic process wherein the oxygen needed is supplied from a free oxygen-containing gas such as air or oxygen-enriched air which is provided to maintain the contents of the fermentation vessel at a pressure which is not critical but which is generally within the range of about 0 to about 150 psig, preferably being within the range of about 0 to about 60 psig.

Although the aeration rate can vary over a considerable range, the aeration generally is conducted at a rate which is within the range of about 0.5 to about 6, preferably about 0.7 to about 4, volumes (at the pressure employed and 25.degree. C.) of oxygen-containing gas per liquid volume in the fermenter per minute.

The fermentation temperature can vary somewhat but generally will be within the range of about 25.degree. C. to about 65.degree. C., preferably being within the range of about 28.degree. C. to about 50.degree. C.

Although the retention time of the fermentation mixture in the fermenter can vary considerably, depending in part on the fermentation temperature and yeast culture, it generally will be within the range of about 2 hours to about 20 hours, preferably being within the range of about 4 hours to about 14 hours. Preferably, the fermentation is conducted in such a manner that the alcohol substrate acts as the limiting growth factor, thereby providing good conversion of the alcohol to yeast cells.

Although the fermentation can be conducted as a batch or semi-continuous process, continuous operation is preferred. In a continuous process the alcohol substrate, the aqueous mineral medium, the ammonia, and the free oxygen-containing gas are added continuously to the fermenter. Although the volume ratio of added alcohol to added aqueous mineral medium can vary over a wide range, generally it will be within the range of about 0.5:9.5 to about 5:5, preferably being within the range of about 0.8:9.2 to about 4.5:5.5. If desired, the alcohol substrate, at least in part, and/or the ammonia, in part, can be added to the aqueous mineral medium prior to passing the aqueous mineral medium to the fermenter. Each of the streams can be introduced into the fermenter at a predetermined rate or in response to a need which can be determined by monitoring such things as concentration of alcohol substrate, pH, dissolved oxygen, oxygen or carbon dioxide in the gaseous effluent from the fermenter, and cell density. The feed rate of the various materials can be varied so as to obtain as rapid a cell growth as possible consistent with efficient utilization of the alcohol substrate, thus obtaining a high yield of yeast cells based on the alcohol charged.

The fermentation process can be conducted in any of a variety of fermentations well known in the art. For example, the fermentation can be carried out in a foam-filled fermenter or in a fermenter in which foaming is maintained at a minimum, e.g., through use of an antifoam agent.

The yeast cells can be recovered from the fermentation mixture by conventional means, e.g., by centrifugation or filtration.

The applicant conducted experiments to investigate the effect of minor amounts of selected oxygenated substances when present in methanol used in a fermentation process.

Methanol feeds of varying purity were employed in a series of continuous fermentation runs conducted at 39.degree.-40.degree. C. and substantially atmospheric pressure in a 4-liter fermenter with a 2-liter working volume. The yeast culture Hansenula polymorpha NRRL Y-11,170 was used in each run. The retention time of the fermentation mixture in the fermenter was approximately 7.3 to 8.3 hours. Oxygen was supplied by enriching air with oxygen so as to keep the dissolved oxygen in the fermentation mixture at a level above 15 percent of that which would be dissolved in the fermentation mixture saturated with air at atmospheric pressure at the fermentation temperature employed. This oxygen-enriched air was fed to the fermenter at a rate of about 1 volume (at about atmospheric pressure and about 25.degree. C.) per volume (liquid volume in fermenter) per minute. Each carbon source in Table I was mixed with the same particular aqueous mineral medium in an amount such that the mixture of aqueous mineral medium and carbon source fed to the fermenter in each run contained about 10 volume percent methanol, except as noted in Table I (runs 7 and 8). The mixture of aqueous mineral medium and carbon source was prepared by mixing, for each liter of solution, 2 ml 85 percent H.sub.3 PO.sub.4, 1 g KCl, 1.5 g MgSO.sub.4.7H.sub.2 O, 0.2 g CaCl.sub.2.2H.sub.2 O, 0.1 g NaCl, 5 ml of a trace mineral solution, 1 ml of a biotinthiamine hydrochloride solution, 100 ml of carbon source, about 2 drops of Mazu DF-37C antifoam agent, and sufficient deionized water to make 1 liter of solution. The trace mineral solution was prepared by mixing, for each liter of solution, 0.06 g CuSO.sub.4.5H.sub.2 O, 0.08 g KI, 0.30 g MnSO.sub.4.H.sub.2 O, 0.20 g Na.sub.2 MoO.sub.4.2H.sub.2 O, 0.02 g H.sub.3 BO.sub.3, 2.00 g ZnSO.sub.4.7H.sub.2 O, 4.80 g FeCl.sub.3.6H.sub.2 O, 3 ml H.sub.2 SO.sub.4, and sufficient deionized water to make 1 liter of solution. The biotin-thiamine hydrochloride solution was prepared by mixing, for each 100 ml of solution, 4 mg biotin, 400 mg thiamine hydrochloride, and sufficient deionized water to make 100 ml of solution. Aqueous ammonium hydroxide (from 2 parts concentrated ammonium hydroxide and 1 part deionized water, by volume) was added to the fermenter at such a rate as to maintain the pH of the fermentation mixture within the range of 3.4 to 3.7.

In these experiments, prior operating time associated with a particular feedstock measured the time from which that particular feedstock was added to the fermenter. In each run, the rate of introduction of each feedstock was approximately constant. In each run, the retention time was about 8 hours, which was long enough for the culture to reach an essentially steady state. Because the cells consumed essentially all of the methanol and therefore grew to their maximum density by the end of their retention time of about 8 hours, it is believed that the minor differences in the input per hour of feedstock for the various runs and the differences in the number of yeast cells at the beginning of each run did not significantly influence cell yield.

The various carbon sources used, together with the corresponding prior operating time in hours, are given in Table I. The resulting cell densities in the fermenter effluent are also recorded, as well as calculated cell yields based on methanol added. To determine these cell densities and yields, the yeast cells were separated from the fermentation effluent by centrifugation, washed by suspension in water and recentrifuged, dried overnight at 100.degree. C., and weighed.

The data in Table I clearly show that 2-methyl-1-propanol lowered the yield of the yeast Hansenula polymorpha, and conversely in the absence of the 2-methyl-1-propanol, the yield of yeast cells significantly increased.

While this invention has been described in detail for purposes of illustration, it is not to be construed as limited thereby. Rather, it is intended to cover reasonable changes and modifications which would be apparent to one skilled in the art.