Aspects of the Nitrogen Metabolism of Yeasts

When a yeast cell starts to bud the parent cell contains high levels of amino acids and nucleotides which migrate to the young cell and facilitate its rapid growth. These metabolites are held within the cell by the various membrane systems. Most yeast cells have high membrane integrity under normal conditions.
A theoretical consideration of the essential amino acids found in yeast pools showed that they would provide a valuable food adjunct when protein content was low.
This project was carried out to establish if membrane integrity could be reduced sufficiently to permit the efflux (and ‘case’ harvesting) of these pool metabolites in strains of yeast suitable for use in continuous culture Tubular reactors. Viability had not to be reduced significantly by the method of membrane disruption.
Two methods of disruption offered themselves
i) Metabolic disruption. A short term breakdown due. to the addition of fermentable carbohydrate to 4 starving yeast culture.
ii) Chemical lesion. A permanent or semi-permanent breakdown due to treatment of membrane with surfactant chemicals.
Both these two methods were studied. Early success with i) and disappointing results with ii) resulted in the work reported here concentrating on the metabolic disruption method. Preliminary work established the principles of a technique by which large amounts of pool amino acids were produced in shake-flasks and in continuous 6L and 22L Tubular reactors. Viability was not greatly reduced.
A process of continuous fermentative production of amino acids was achieved by cyclically feeding, starving and shock excreting yeast cultures.
New stable hybrid strains of yeast with higher yields than their parents were obtained after mass mating and selection experiments. The best of these was used to provide supplies of amino acids which were used in some small scale feeding trials. In all cases the food value of the adjunct was demonstrated.
The work has indicated that this process has provided a method for the relatively inexpensive production of a wide range of metabolites, including nutritionally important quantities of amino acids.