Inactivated yeast or semi-purified fractions of it have been used as additives for animal and fish feeds. These preparations have been proposed to encourage animal growth and health by various mechanisms, including immunomodulation, oxidative status, binding of toxins and pathogens, and interactions with gut ingredients.
“Inactivated yeast” or “yeast autolysate” contains the water-soluble and insoluble components of the yeast cell, the composition of which is primarily amino acids, peptides and carbohydrates. Yeast autolysate is manufactured by breaking down the cells using endogenous or exogenous enzymes. There have been many reports on the manufacturing processes.
At the end of yeast fermentation, yeast cells were separated by centrifugation from the spend medium and washed twice with process water and stored in large storage tanks at 4 oC. First step for the manufacturing of “Inactivated Yeast” is the cell inactivation, disruption and autolysis. For that purpose several methods may be used.
Autolysis by endogenous enzymes occurs naturally in yeasts when they complete the cell growth cycle and enter the death phase. Autolysis is a traditional disruption method in yeast autolysate production but it has some disadvantages such as low yield, difficulty in solid–liquid separation due to the high content of residue in the autolysate.
Plasmolysis is a modified autolysis process in the presence of a so-called accelerator, such as inorganic salts. Despite its simplicity, yeast autolysate manufactured by plasmolysis may have limited use.
Mechanical disruption is process that includes homogenization, sonication, bead milling or high pressure treatment.
Hydrolysis which is performed by acid or exogenous enzymes is the most efficient method of solubilizing yeast. Despite a high production yield, acid hydrolysis is less attractive to manufacturers because of the relatively high capital investment cost. The enzymatic degradation of the yeast is carried out by means of suitable enzyme preparations of bacterial, vegetable, yeast, or animal origin. If part of the enzymatic degradation is done before the yeast is inactivated, the own internal enzyme activity of the yeast may also contribute to the process.
After the cell inactivation and disruption, soluble part may be separated by centrifugation and insoluble faction is collected and washed twice with process water. Next step is drying the final product. For that purpose spray dryer or drum dryer may be used. Drying temperature should not be more than 120 oC. Dry final product is packed and kept cool and dry until delivery.