Yeasts can grow in the presence or absence of air. Anaerobic growth, growth in the absence of oxygen, is quite slow and inefficient. For instance, in bread dough, yeast grow very little. Instead, the sugar that can sustain either fermentation or growth is used mainly to produce alcohol and carbon dioxide. Only a small portion of the sugar is used for cell maintenance and growth. In contrast, under aerobic conditions, in the presence of a sufficient quantity of dissolved oxygen, yeast grow by using most of the available sugar for growth and producing only negligible quantities of alcohol.
This means that the baker who is interested in the leavening action of carbon dioxide works under conditions that minimize the presence of dissolved oxygen. On the other hand, a yeast manufacturer that wants to produce more yeast cell mass, works under aerobic conditions by bubbling air through the solution in which the yeast is grown.
The problem posed to the yeast manufacturer, however, is not as simple as just adding air during the fermentation process. If the concentration of sugar in the fermentation growth media is greater than a very small amount, the yeast will produce some alcohol even if the supply of oxygen is adequate or even in abundance. This problem can be solved by adding the sugar solution slowly to the yeast throughout the fermentation process. The rate of addition of the sugar solution must be such that the yeast uses the sugar fast enough so that the sugar concentration at any one time is practically zero. This type of fermentation is referred to as a fed-batch fermentation.
The baker’s yeast production process flow chart attached below can be divided into four basic steps. In order these steps are, molasses and other raw material preparation, culture or seed yeast preparation, fermentation and harvesting and filtration and packaging. The process outlined in the flow chart takes approximately five days from start to finish.
The basic carbon and energy source for yeast growth are sugars. Starch can not be used because yeast does not contain the appropriate enzymes to hydrolyze this substrate to fermentable sugars. Beet and cane molasses are commonly used as raw material because the sugars present in molasses, a mixture of sucrose, fructose and glucose, are readily fermentable. In addition to sugar, yeast also require certain minerals, vitamins and salts for growth. Some of these can be added to the blend of beet and cane molasses prior to flash sterilization while others are fed separately to the fermentation. Alternatively, a separate nutrient feed tank can be used to mix and deliver some of the necessary vitamins and minerals. Required nitrogen is supplied in the form of ammonia and phosphate is supplied in the form of phosphoric acid. Each of these nutrients is fed separately to the fermentation to permit better pH control of the process. The sterilized molasses, commonly referred to as mash or wort, is stored in a separate stainless steel tank. The mash stored in this tank is then used to feed sugar and other nutrients to the appropriate fermentation vessels.
Baker’s yeast production starts with a pure culture tube or frozen vial of the appropriate yeast strain. This yeast serves as the inoculum for the pre-pure culture tank, a small pressure vessel where seed is grown in medium under strict sterile conditions. Following growth, the contents of this vessel are transferred to a larger pure culture fermentor where propagation is carried out with some aeration, again under sterile conditions. These early stages are conducted as set-batch fermentations. In a set-batch fermentation all the growth media and nutrients are introduced to the tank prior to inoculation.
From the pure culture vessel, the grown cells are transferred to a series of progressively larger seed and semi-seed fermentors. These later stages are conducted as fed-batch fermentations. During a fed-batch fermentation, molasses, phosphoric acid, ammonia and minerals are fed to the yeast at a controlled rate. This rate is designed to feed just enough sugar and nutrients to the yeast to maximize multiplication and prevent the production of alcohol. In addition, these fed-batch fermentations are not completely sterile. It is not economical to use pressurized tanks to guarantee sterility of the large volumes of air required in these fermentors or to achieve sterile conditions during all the transfers through the many pipes, pumps and centrifuges. Extensive cleaning of the equipment, steaming of pipes and tanks and filtering of the air is practiced to insure as aseptic conditions as possible.
At the end of the semi-seed fermentation, the contents of the vessel are pumped to a series of separators that separate the yeast from the spent molasses. The yeast is then washed with cold water and pumped to a semi-seed yeast storage tank where the yeast cream is held at 34 degrees Fahrenheit until it is used to inoculate the commercial fermentation tanks. These commercial fermentors are the final step in the fermentation process and are often referred to as the final or trade fermentation.
Commercial fermentations are carried out in large fermentors with working volumes up to 50,000 gallons. To start the commercial fermentation, a volume of water, referred to as set water, is pumped into the fermentor. Next, in a process referred to as pitching, semi-seed yeast from the storage tank is transferred into the fermentor. Following addition of the seed yeast, aeration, cooling and nutrient additions are started to begin the 15-20 hour fermentation. At the start of the fermentation, the liquid seed yeast and additional water may occupy only about one-third to one-half of the fermentor volume. Constant additions of nutrients during the course of fermentation bring the fermentor to its final volume. The rate of nutrient addition increases throughout the fermentation because more nutrients have to be supplied to support growth of the increasing cell population. The number of yeast cells increase about five- to eight-fold during this fermentation.
Air is provided to the fermentor through a series of perforated tubes located at the bottom of the vessel. The rate of airflow is about one volume of air per fermentor volume per minute. A large amount of heat is generated during yeast growth and cooling is accomplished by internal cooling coils or by pumping the fermentation liquid, also known as broth, through an external heat exchanger. The addition of nutrients and regulation of pH, temperature and airflow are carefully monitored and controlled by computer systems during the entire production process. Throughout the fermentation, the temperature is kept at approximately 86 degrees Fahrenheit and the pH in the range of 4.5-5.5.
At the end of fermentation, the fermentor broth is separated by nozzle-type centrifuges, washed with water and re-centrifuged to yield a yeast cream with a solids concentration of approximately 18%. The yeast cream is cooled to about 45 degrees Fahrenheit and stored in a separate, refrigerated stainless steel cream tank. Cream yeast can be loaded directly into tanker trucks and delivered to customers equipped with an appropriate cream yeast handling system. Alternatively, the yeast cream can be pumped to a plate and frame filter press and dewatered to a cake-like consistency with a 30-32% yeast solids content. This press cake yeast is crumbled into pieces and packed into 50-pound bags that are stacked on a pallet. The yeast heats up during the pressing and packaging operations and the bags of crumbled yeast must be cooled in a refrigerator for a period of time with adequate ventilation and placement of pallets to permit free access to the cooling air. Palletized bags of crumbled yeast are then distributed to customers in refrigerated trucks.