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This report focuses the manufacturing route of yeast and the improvements that could be done in the process. A review is presented dealing with engineering analysis of the production of yeast, in which seven steps in manufacturing yeast are discussed. The key limitations of the process are identified such as the formation of ethanol reduces the yield of yeast. This report also provides a detailed discussion of potentials improving the production of yeast. The report also presents information on the feasibility of implementing those suggestions made.
This report starts with the overview of production of yeast, followed by detailed explanation of every step in the process. Other than that, suggestions are made to improve the manufacturing of yeast together with description of how to achieve that.
In the global baker¿½s yeast industry, every year more than 2 million tons of yeast are produced, earning greater than $2 billion. At present, world population and industrialization are growing and in Asia, bread eating is getting more popular. Due to these factors, the yeast production is increasing at roughly 4% per year (Evans, 1990). In food industry, the production of yeast accounts for the largest domestic use of microorganism. Understanding the process route of yeast enables improvement to make, which results in greater yield or quality of yeast.
The manufacturing of yeast is basically about breeding yeast under aerobic condition. During the aerobic growth, sugars as the carbon and energy source are consumed. Two distinct energy-generating pathways, oxidation or fermentation takes place, depending on the sugar concentration presented.
Oxidative metabolism of glucose
C6H12O6 + 6O2 ?6CO2 + 6 H2O + 38 ATP
Fermentative metabolism of glucose
C6H12O6 ? 2C2H5OH + 2CO2 + 2 ATP
Tentatively, in oxidative metabolism of glucose, glucose is completely oxidized and a high level of energy for adenosine triphosphate (ATP) is synthesised, while the minimal level of energy released in fermentative metabolism of glucose is associated to low yeast growth.
When excessive sugars are present, oxidation is suppressed and fermentation occurs. When there is insufficient oxygen supply, the fermentative pathway outweighs the oxidative metabolism, bringing about ethanol formation, even at a low sugar concentration.
Thus, in baker¿½s yeast industry, it is vital to provide optimal air supply and accurate amount of sugars to ensure that only oxidation occurs, resulting in maximum yield of yeast.
Compressed yeast and active dry yeast are the two major types of baker¿½s yeast manufactured. Generally, the manufacturing route of baker¿½s yeast can be divided into seven major steps. These steps are:
(1.0) Raw materials arrangement
(3.0) Concentration and filtration
(5.0) Extrusion and cutting
(7.0) Shipment of packaged yeast
1.0 Raw materials arrangement
In producing baker¿½s yeast, the most important raw materials needed are the pure yeast culture and molasses. A range of vital nutrients and vitamins is also involved in the manufacturing of yeast.
1.1 Principal raw materials
Saccharomyces cerevisiae is the most common yeast strain employed in yeast production. In order to grow Yeast needs sugar as the carbon and energy source. Since yeast does not contain the appropriate enzymes to hydrolyse starch to fermentable sugars, starch is not utilised. Instead, molasses which is the cheapest source of sugar known are usually used. It contains 45 to 55 weight percent fermentable sugars which is a mixture of sucrose, glucose, and fructose.
1.2 Supplementary raw materials
In addition to pure yeast culture and sugar, a mixture of important nutrients, minerals, and salts is necessary in the yeast production, such as nitrogen, potassium, phosphate, magnesium, and calcium. Iron, zinc, copper, manganese, and molybdenum are also needed in tiny amounts. Molasses malt contains sufficient amount of most minerals like potassium and calcium but not nitrogen, phosphates and magnesium. Therefore, they are supplied to the feedstock through addition of aqueous ammonia, phosphoric acid and magnesium salts.
For yeast to grow, a number of vitamins such as biotin, inositol, pantothenic acid, and thiamine are also compulsory. Inositol and pantothenic acid are present in adequate quantity in cane and beet molasses for yeast production, but not thiamine and biotin. Thus, they are added to the feedstock.
1.3 Preparation of raw materials before fermentation
A few nutrients are put in with molasses before entering flash sterilisation while others are supplied separately to the fermentation. For instance, nitrogen and phosphates are fed to fermentors through a separate nutrient feed tank to acquire better pH control of the process. Bacteria and yeast cultivate under the similar environment, thus managing pH is very imperative. Usually, the molasses mixture is adjusted to pH values between 4.5 and 5.0, in which the most favourable yeast yields are obtained.
Following pH adjustment, the molasses mixture is treated to get rid of any mud and purified with high-pressure steam. Next, the sterilized molasses, generally referred to as mash, is diluted with water and held in separate holding tanks until it is called for the fermentation process.
Yeast breeds in a sequence of fermentation vessels which work under aerobic state. It is because anaerobic conditions result in low yield of yeast as ethanol and carbon dioxide are formed when sugars are used up. Controlling of the quantity of sugars is also essential, to ensure that yeasts can absorb the sugar as soon as it is added. To achieve this, incremental feed system in the final fermentation stages is applied. In general, fermentation steps could be divided into three stages:
2.1 Laboratory stage
2.2 Pure culture stage
2.3 Main fermentation stage
Below is the picture adapted from Midwest Research Institute, depicting the typical fermentation process.
2.1 Laboratory Stage
Yeast production starts with a pure culture of appropriate yeast strain. This yeast strain serves as the inoculums growing in medium under stringent germ-free conditions, happening in the laboratory. The pure yeast culture is mixed with molasses in an uncontaminated flask and is set aside to cultivate for 2 to 4 days. Following growth, the entire contents of this flask are transferred and used to inoculate the pure culture fermentor in the second fermentation stage.
2.2 Pure Culture Stage
The pure culture stage consists of two pure culture fermentors which are batch fermentations. Sterility plays an important role in this stage as infected microorganisms can effortlessly outgrow the yeast. Therefore, before inoculating, fermentation medium is meticulously sterilized by heating the medium under pressure or at atmospheric pressure for extended time. In pure culture stage, yeast is left to grow for 13 to 24 hours. These pure culture fermentations are fundamentally an extension of the flask fermentation in the laboratory stage; with the exception of the pure culture fermentors have supply for sterilized aeration and aseptic shift to the main fermentation stage.
2.3 Main Fermentation Stages
Following the pure culture fermentation is the main fermentation stages, in which the greatest yield of yeast is attained. The main fermentation stages are usually carried out at 30o C and are split into four steps. They are:
2.3.1 Intermediate fermentation
2.3.2 Stock fermentation
2.3.3 Pitch fermentation
2.3.4 Trade fermentation
Following the pure culture stage is the intermediate stage of main fermentations of yeast growth without incremental feeding. Subsequently, the whole fermentor contents from the intermediate stage enter the stock fermentation. The vessel in this stock fermentation is furnished for incremental feeding and has fine ventilation. Yeast from this stock fermentation is split from the mass of the fermentor liquid by centrifuging once the fermentation is finished, producing a stock of yeast for the next stage.
The third stage is the pitch fermentation which is usually carried out in large vessel where intense exposure to air and incremental feed of molasses with nutrients take place. Similar to stock fermentation, the product from pitch fermentor is centrifugalized for pitching the final trade fermentations.
The final stage is the trade fermentation which has the maximum level of aeration. Therefore, the vessels in trade fermentation are usually in a staggered style to diminish the magnitude of the air compressors required to handle enormous air supplies. Similar to the three previous steps, molasses and other nutrients are fed incrementally to the trade fermentor.
Every step of the final fermentation stages takes about 11 to 15 hours. In each stage, following the feed of molasses and nutrients, the liquid in the fermentor is aerated further for 0.5 to 1.5 hours, results in more stable yeast for refrigerated storage. The yeast growth in main fermentation stages is rising with each stage with roughly 120 kg in the intermediate fermentor, 420 kg in the stock fermentor, 2,500 kg in the pitch fermentor, and 15,000 to 100,000 kg in the trade fermentor.
Controlling fermentation system
Fermentors are usually equipped with an incremental feed system. This incremental feed system may be a pipe or a series of pipes that distributes the molasses over the entire surface of the fermentor liquid. The rate at which the molasses is fed is critical and may be controlled by a speed controller connected to a pump or by a valve on a rotameter, which delivers a certain volume of molasses at regulated time intervals. Nutrient solutions of vitamins are kept in small, separate tanks and are added through rotameters into the fermentor. The rate of this feed is not as critical as the molasses feed rate. However, if ammonia is used as a nitrogen source, additions must be made in a manner that avoids sudden pH changes. Nitrogen salts and phosphates may be charged in a shorter period of time than the molasses.
Fermentors used in the final stages must also be equipped with heat exchangers to remove the heat produced from the production process and to cool the fermentor. The type of heat exchanger system depends on the size of the fermentation vessel. Because large volumes of air are supplied to the fermentation vessels during this stage of production, the fermentor size and the type of aeration system selected are interdependent. The different types of aeration systems include horizontal, perforated pipes; compressed air and mechanical agitation; and a self-priming aerator. In the horizontal, perforated pipe system, air is blown through a large number of horizontal pipes that are placed near the bottom of the fermentor. With this aeration system, the only agitation of the fermentor liquid is carried out by the action of the air bubbles as they rise to the surface. Typically, this type of aeration system requires from 25 to 30 cubic meters (m3) (880 to 1,060 cubic feet [ft3]) of air to produce 0.45 kilograms (kg) (1 pound [lb]) of yeast. The self-priming aerator operates with a turbine that draws air through a hollow, vertical shaft into the fermentor liquid. Because air is drawn through the shaft of the turbine without a compressor, the pressure of the air at the outlets is not very high and the depth to which the turbine can be submerged is limited.
3.0 Concentration and Filtration
After fermentation, the yeast is concentrated and filtered. In concentrating, a centrifugal yeast separator, for example, the continuous dewatering centrifuge is used to retrieve yeast cells from the last trade fermentor. To achieve yeast solids concentration between 18 and 21 percent, two or three passes through the separators are usually needed. The centrifuged yeast solids are then preceded to filtration. Filter presses and rotary vacuum filters are the two types of filtering systems utilised. Generally, filter press comprising frame of 58 to 115 cm is utilised with operated pressure between 860 and 1030 kPa, producing filter cake with 27 to 32 precent solids. In rotary vacuum filter, by rotating the filter drum in a channel of yeast cream, the drum is covered with yeast. The yeast is then removed by the blades at the bottom when the drum rotates, forming cake with roughly 33 percent solids.
Diagram of filter press
Diagram of rotary vacuum filter (wikipedia, 2010)
Subsequent to filtration, the filter cake is mixed with water, emulsifiers, and cutting oils to produce the final product. Emulsifiers texturize the yeast and prevent water spotting of the yeast cakes. Cutting oils, for example, soybean or cottonseed oil, help in extruding the yeast.
5.0, 6.0, 7.0 Extrusion and cutting, Packaging, Shipment of packaged yeast
The yeast cake is then extruded through nozzles to create continuous ribbons of yeast cakes. The ribbons are then cut. For producing compressed yeast, the yeast cakes are covered with wax paper and cooled to below 8o C. For producing dry yeast, the yeast cakes are dried in continuous drying system, vacuum packed, and heat-sealed. Then, they are ready for delivery in refrigerated wagons.
Improvements in manufacturing of bakers¿½ yeast
The following key components are identified in improving production of baker¿½s yeast:
(1) Greater yield of yeast
(2) Better quality of yeast
(3) Shorter time of production
(4) Reduced damage to environment
Some methods are suggested in order to achieve these goals. They are:
(1) Incorporating enzyme in manufacturing of yeast
(2) Removing volatile organic compounds in yeast production
(3) Utilising genetically engineering improvements in yeast strains
(4) Improving fermentor design
(5) Implementing computer-based feed rate manipulation
(6) Changing feed entrance point during fermentation
Incorporating enzyme in manufacturing of yeast
Enzyme could be utilised to shorten the time of yeast production. Enzymes function as the catalysts as they can increase the rate of fermentation of yeast without being consumed.
Essentially, the fermentation process of yeast is about breaking apart the sugar molecules which are made up of carbon, hydrogen, and oxygen atoms, into ethyl alcohol and carbon dioxide. This reaction provides the yeast cells with the energy required for their growth and reproduction. Enzymes could be employed in making this reaction faster, thus shorter time is needed to produce yeast. Incorporating enzyme in yeast production might be economical since yeast can be manufactured faster and enzyme is highly selective and reusable.
However, there are drawbacks in using enzyme as it is sensitive to denaturation, and expensive. Besides that, enzymes have limited operating conditions.
Removing volatile organic compounds in yeast production
The fermentation process in yeast production produces volatile organic compounds (VOC) as the by-products such as ethanol, butanol, isopropyl alcohol, 2,3 butanediol, and acetate. These by-products form as a result of excess sugar present in the fermentor or an insufficient oxygen supply to the fermentor. Under these conditions, anaerobic fermentation occurs and results in the excess sugar being broken down to form alcohols and carbon dioxide.
Removing volatile organic compound from fermentation could reduce the negative impact to the environment. Wet scrubbers, thermal incinerators and catalytic incinerators can be employed to control volatile organic compounds emissions from yeast fermentation vessels.
In wet scrubbers, volatile organic compounds like ethanol and acetaldehyde are taken up into the absorbing liquid such as water. Packed towers are one of the wet scrubber¿½s types. In packed towers of the scrubber, the upcoming gas stream containing volatile organic compound comes into the bottom of the scrubber and passes up through the packing. Water runs down through the packing counter current with the incoming gas stream. In the packing part of the scrubbers where the volatile organic gas compounds and water contact, water absorbs the pollutants in the gas stream. The uncontaminated gas stream then goes up and out the top of the packed tower while water flows down through the packing and out the bottom of the scrubber.
Incineration is the oxidation of organic compounds by exposing the gas stream to high temperatures in the presence of oxygen and sometimes a catalyst. Carbon dioxide and water are the oxidation products.
Thermal incinerator and catalytic incinerator are chambers which could be utilised to burn waste such as volatile organic compounds. Typical thermal incinerators are furnaces oxidation with a burner positioned at one end. In thermal incinerator, the air loaded with solvent, runs by the burner together with an auxiliary fuel. A system of valved ductwork directs the volatile organic compound gases upwardly into the combustion chamber. The gases combined with the solvent-laden air leaving the burner increase the temperature of the mixture to the extent where the volatile organic compounds are oxidized. With most solvents, oxidization occurs in less than 0.75 second at a temperature of 870¿½C. Applying thermal incinerators in fermentation process should result in volatile organic compounds destruction efficiencies of about 98 percent.
Catalytic incinerators make use of a catalyst to promote the burning of volatile organic compounds. The solvent-laden air is preheated by the combustor and then brought into contact with the catalyst bed, where oxidation takes place. Platinum is the common catalyst used.
Catalytic incinerators can achieve destruction efficiencies comparable to thermal incinerators while operating at lower temperatures, 315¿½ to 430¿½C. Thus, catalytic incinerators can operate with considerably lower energy costs than can thermal incinerators. The materials of construction may also be less expensive because of the lower operating temperatures. However, since catalyst is sensitive to temperature, the operating temperature range for the catalyst sets the upper VOC concentration that can be incinerated.
Utilising genetically engineering improvements in yeast strains
To produce better yeast production, application of recombinant DNA technology to improve yeast strains is suggested. By genetically manipulating yeast strain, the desired traits of yeast strain could be chosen. Yeast strain could be modified to have the good characteristics such as better resistance to volatile organic acid, increased sugar utilization, and improved ability to encounter ambient temperature or pH. Baker¿½s yeast strains can be altered to be able to stay alive and become accustomed to adverse environmental conditions that change steadily or rapidly.
Nevertheless, recombinant DNA technology is limited by the not significant matter of public perception and acceptance of its potential benefits. It is not reasonable for the baking industry to refuse recombinant yeast unless there is an obvious benefit to the industry and its customers. However, for the most part classical genetic breeding programs have already produced yeasts that are rapid with regards to fermentation and quite capable of simultaneous uptake of maltose and glucose. Why a baker would move to a recombinant strain with such properties if yeasts are available through more publicly acceptable technology. It is far more likely that recombinant strains will be accepted in those circumstances where yeast is required to perform against its natural ¿½biological design¿½.
Improving fermentor design
In order to achieve better yield of yeast, the key step is to provide sufficient oxygen such a way that only oxidation occurs and the respiratory capacity of the yeast is utilized to the maximum, minimising the formation of ethanol (Blanco & Rayo 2008).
The air sparger system of fermentor could be improved in order to accomplish this goal. It is recommended to change the equipment design of the aeration from horizontal, perforated pipe system to mechanical agitation systems to reduce ethanol formation.
The efficiency of aeration with a given volume of air is greatly increased by mechanical agitation. In a mechanical agitation aeration system, air under pressure is supplied to a circular diffuser pipe. Directly above the air outlets, a horizontal turbine disk provides mechanical agitation, which distributes the air bubbles uniformly. Agitation systems have baffles to keep the fermentor liquid from rotating in the direction of the motion of the disk. This uniform and proper distribution of air bubbles throughout the malt mixture reduces the volume of air needed to grow the yeast. In an agitated system, only 10 to 15 m3 of air are required to produce 0.45 kg of yeast, while 25 to 30 m3 of air is needed in the horizontal, perforated pipe system (Baker, Williamson 1992).
Implementing computer-based feed rate manipulation
To improve the yield of yeast, the key issue is to make sure that the actual sugar concentration in the fermentor is continuously kept at a low but optimal amount to suppress the ethanol emission from yeast fermentation. By employing the computer-based feed rate controls, the fermentation process is constantly monitored with the aid of a computer to predict the exact requirement for sugar.
The amount of molasses needed relies on the yeast cell concentration, specific growth rate, and yeast cell yield. Since no sensors are available that can quickly and reliably give a direct measurement of cell mass, computer-aided material balance techniques can be used to calculate continuously the cell concentration, specific growth rate, sugar consumption rate, and other growth-related parameters (Wang 1979).
A computer can process information taken from direct measurement of airflow, carbon dioxide production, oxygen consumption, and ethanol production to anticipate the demand for sugar by the system. The result is an indirect method for monitoring yeast production that is regulated by a computer.
By continuously adding only the exact amount of molasses required by the fermentation, conditions of excess sugar are eliminated, thus minimizing ethanol formation.
However, this type of process control system is extremely difficult to refine and implement because of the time delays between ethanol formation in the fermentor, its detection in the stack, and the computer adjustments to the feed rates.
Changing feed entrance point during fermentation
The quality of yeast is enhanced by having better dough-leavening ability and increased shelf life enhance. In baker¿½s yeast production without considerable loss of biomass at a commercial scale, these two goals could be achieved by changing the feed entrance point during the fermentation time.
This method is invented by Zamani et al from Research and Development Department of Iran Mellas Co. It is proposed to switch the molasses feeding point to the circulation loop for the last seven hours of the fed-batch process while all other operating conditions remained unchanged.
The molasses mixture is circulated at a constant rate through the circulation loop and a plate heat exchanger in order to control the temperature
In this method, the assimilation of nitrogen from the medium to yeast is improved, resulting in higher protein content in yeast. Furthermore, protein synthesis in heterogeneous bioreactor is better done in the presence of ethanol. It is speculated that the higher level of protein contributes to higher level of enzymes level in yeast. Thus, high leavening ability may be dependent on high level of proteins and in turn high level of enzymes content.
The drawback of this method is that the high sugar zone in the circulation loop throughout the fermentation process reduces the biomass yield. Although having high biomass yield is very important economically, losing roughly 3% yield may be tolerable when this new feeding method results in reasonably higher leavening ability.