Yeast Production - General Discussion
Many scientific publications emphasize varied aspects of the commercial production of bakers yeast. Companies producing bakers yeast offer different guidelines relative to the appropriate application of their products. What follows is an attempt to synthesize and clarify some of the available information.
An enormous number of strains of Saccharomyces cereviae exist, many of which have already been selected for baking. Among the characteristics individual strains share are the substances they use for growth, especially how and which sugars they utilize, as well as the manner in which they reproduce, and their appearance through the lens of a microscope. Characteristics differentiating individual strains include how much sugar they tolerate, and how quickly they grow. The potential characteristics of a particular baker's yeast are determined by the strain of yeast that is selected. The actual characteristics of bakers yeast from a particular strain are determined by its composition.
The following is a simplified description of the production of bakers yeast: Yeast needs moisture, food, and warmth to grow. As a living organism, yeast is cultivated, rather than manufactured. A carbohydrate food source such as molasses is required for reproductive yeast growth. Molasses is a by-product of sugar beet and sugar cane refining, and supplies the least expensive source of sucrose, glucose and fructose, in addition to some minerals and assimilable nitrogen. Either beet or cane molasses, or a combination of both, can be used. Prior to its use, the molasses is diluted with water, clarified, and heat sterilized. The molasses is supplemented with nitrogen, phosphate, sulphuric acid and sodium carbonate, and small amounts of minerals and trace elements. Nitrogen is supplemented using ammonia or various ammonium salts. Phosphate is supplied in the form of phosphoric acid or ammonium or di-ammonium phosphate. Sulphuric acid and sodium carbonate are included as processing aids for pH control. Minerals such as calcium and magnesium are added as are trace amounts of iron and zinc. Oxygen is provided in the form of filtered air. All of these substances are contained in an aqueous (water) solution or broth referred to as the "wort."
Appropriate strains of yeast cultures are multiplied in phases. Starter cultures are grown under strictly sterile conditions in a laboratory. Single healthy and vigorous cells are selected from desired yeast strains. They are isolated for growth on pure culture slants, or in test tubes containing the required nutrients. This enables them to begin the process of reproduction. These pure cultures are subsequently transferred to flasks, then larger vessels until they have grown to the extent required for a commercial starter.
Yeast propagation proceeds in a series of increasingly larger sanitized tank fermenters. This large-scale fermentation continues under controlled conditions of continuous nutrient addition, temperature control, and optimum aeration. Conditions of growth affect how fast yeast multiplies and how much protein and carbohydrate it accumulates. For instance, rapid growth usually results in yeast with higher protein and enzymes, lower carbohydrates, higher initial activity and lower stability. Alternatively, slow growth usually results in yeast with lower protein and enzymes, higher carbohydrates, lower initial activity, and higher stability.
When all of the nutrients have been consumed, the yeast cells are separated from the remaining nutrient matter by centrifugation (separator). They are then washed and recentrifuged to yield a creamy suspension of pure, active yeast known as "yeast cream" which has a solids (yeast cells) content of approximately 15-18%. From this stage, a series of manufacturing processes take place that prepares the yeast for its final form. These are described briefly below:Compressed Yeast (also called cake, wet, and fresh yeast)
The water content of the yeast cream is further reduced by passing the yeast cream through a filter press or rotary vacuum filtration unit. Once pressed, the pressed cake is extruded through a rectangular nozzle to form a strand that is cut into the proper length and weight. The strain chosen for compressed yeast and the growth conditions tend to favor high activity or gassing power in both lean and sweet dough.
Active Dry Yeast
Active dry yeast begins as compressed yeast but the press cake is extruded through perforated plates or screens in the form of thin spaghetti-like strands. These strands are cut into elongated pellets as they enter a tunnel dryer and pass through a series of drying chambers maintained at different temperature levels. (Drying may also be carried out in rotating drums or in the fluid bed system.) The pellets are then ground into small granules or beads. The combination of strain chosen for active dry yeast, the growth conditions, and the drying method tend to favor stability over activity. This means that active dry yeast has lower activity or gassing power than compressed yeast in lean dough. Active dry yeast has lower activity or gassing power than compressed yeast and higher activity or gassing power than instant active dry yeast in sweet dough.
Instant Active Dry Yeast
The manufacturing process for instant active dry yeast is similar to that for active dry yeast with a few exceptions. Ascorbic acid may be added as a dough conditioner to help strengthen the dough. Prior to being extruded, the press cake may be plasticized with sorbitan monostearate (an emulsifier) as an aid to yeast rehydration in the dough. The yeast mass is extruded through smaller perforated plates or screens than those used for active dry yeast, cut into small oblong, thread like particles and dried in a fluid bed dryer. The combination of strain chosen for instant yeast, the growth conditions, the drying method, and the addition of emulsifiers tend to place instant yeast intermediate between compressed and active dry yeast relative to activity or gassing power in lean dough. Instant active dry yeast has lower gassing power than compressed or active dry yeast in sweet dough.Yeasts Available to Bakers
The following information is typical for each type of yeast, but may vary somewhat according to product and company:Compressed Yeast (also called cake, wet, and fresh yeast)
Fleischmann's compressed yeast is available in supermarkets in 0.6 oz cakes, and Red Star compressed yeast is available in some supermarkets in 2 oz. cakes. It is found in the dairy or deli case. Compressed yeast is available to commercial bakers from a variety of companies in 1 and 2 pound packets. Compressed yeast has approximately 30% solids and 70% moisture content. It is highly perishable and must be stored at a uniformly low temperature (about 40 F) to prevent excessive loss of activity or gassing. Compressed yeast generally has a shelf life of approximately two weeks from its make or packaging date when kept at 73.3 degrees F. (23 degrees C)
At 32-42 F. (0 - 5.5 C) compressed yeast loses approximately 10% of its gassing power over a 4 week period. At 45 F (7.2 C) yeast will lose 3-4% of its activity per week. At 95 F (35 C), one half of the gassing power is lost in 3-4 days. Once yeast starts to deteriorate or lose its fermentative activity, it does so quickly, losing almost all of its activity (autolysis) by the third week. It has, however, been shown that compressed yeast can be successfully stored for two months at 30 degrees F. (-1 degree C). When this is done, good bread can be made from yeast stored for two, but not three, months.
To use compressed yeast, crumble it into the dry ingredients or soften it in tepid water.
Active Dry Yeast
Fleischmann, Red Star, and SAF active dry yeast are available in supermarkets in 1/4 oz (7 g) packets and/or 4 oz (113.4 g) jars. Active dry yeast is available to commercial bakers from a variety of companies in 1 and 2 pound, and 500 g packets. It also is available in these sizes to home bakers at warehouse or club stores, and via mail order. Active dry yeast has approximately 92.0% solids and 8.0% moisture content. It is advisable to store active dry yeast in a cool, dry place that does not exceed 80F. The shelf life of active dry yeast stored at room temperature is approximately 2 years from its make date. Once opened, active dry yeast is best stored in an airtight container in the back of the refrigerator, where it will retain its activity for approximately 4 months. To rehydrate active dry yeast, blend one part yeast with four parts lukewarm water, wait 10 minutes, and stir. Depending upon the particular product and company, lukewarm water ranges from 90-115 F. Temperatures lower than 90 F and higher than 115 F should be strictly avoided. (Pyler) Active dry yeast may also be blended with the dry ingredients.
Instant Active Dry Yeast
Fleischmann, Red Star, and SAF instant active dry yeast is available in supermarkets in 1/4 oz (7 g) packets and/or 4 oz (113.4 g) jars. The Fleischmann product is marketed as RapidRise, the Red Star product is marketed as QUICK.RISE, and the SAF product is marketed as Gourmet Perfect Rise. Fleischmann also markets an instant active dry yeast named Bread Machine Yeast. Instant active dry yeast is available to commercial bakers in 1 and 2 pound, and 500 g packets. It also is available in these sizes to home bakers at warehouse or club stores, and via mail order. Instant active dry yeast has 96.0% solids and 4.0% moisture content. It is advisable to store instant active dry yeast in a cool, dry place that does not exceed 80 F. The shelf life of instant yeast stored at room temperature is approximately 2 years from its make date. Once opened, instant active dry yeast can be stored in an airtight container in the back of the refrigerator, where it will retain its activity for approximately 4 months. Three methods are recommended when using instant active dry yeast: The first is to blend it thoroughly with the flour before adding water. The second is to mix all the ingredients except the instant yeast for one to two minutes, sprinkle the instant yeast on top of the partially mixed ingredients, and continue mixing. The third is to blend one part yeast with five parts lukewarm water, wait 10 minutes, and stir.
Osmotolerant Instant Active Dry Yeast
A yeast product available commercially but not readily available to home bakers is osmotolerant instant active dry yeast. Osmosis is the means by which yeast cells absorb oxygen and nutrients and give off enzymes and other substances. Osmotolerant instant active dry yeast is recommended for use in dough characterized as sweet, salty or low absorption. A yeast with good osmotic tolerance is called for because the amount of available water in these doughs is limited. Fermipan markets osmotolerant instant active dry yeast as Fermipan Brown and SAF markets osmotolerant instant active dry yeast as SAF Gold. In the absence of osmotolerant instant active dry yeast, it is sometimes recommended that the amount of active dry or instant active dry yeast in a sweet, as opposed to a lean, dough be increased.It is worth noting that there is disagreement among the yeast companies as to whether or not active dry and instant active dry yeast should be frozen, and if in doing so the shelf life of the yeast is prolonged. The most convincing argument against freezing is that under normal conditions, there are temperature fluctuations in freezer units caused both by repeated opening and closing of the freezer door and, in contemporary freezer models, by the self-defrosting (freeze and thaw) cycle. These temperature fluctuations can cause damage to the yeast cell structure.
One topic upon which there is agreement is that if active dry or instant active dry yeast has been refrigerated, and is going to be rehydrated in lukewarm water, it is best to allow the portion of yeast to be used to come to room temperature prior to blending it with the lukewarm water. Otherwise, temperature shock might damage the yeast cells.
Unlike compressed yeast, which disperses in cold water without any problems, the temperature of the water during rehydration is important when working with dry yeast. When yeast is dried, the cell membrane becomes more porous. During rehydration, the membrane recovers. However, in the process of rehydration, some cell constituents are dissolved in the water used. The optimum water temperature for cell membrane restoration is 104 F. Warm water is effective in this process, because it leads to more rapid cell membrane recovery. Cold water impedes this process, because it slows membrane recovery and allows more cell constituents to leach out during the reconstitution process. The effect is not that great between 70 and 100 F, but at lower temperatures, approximately one-quarter to one-half of soluble yeast cell constituents can be lost. This leaching action effects yeast activity in the following manner: Most yeast enzymes remain, but the soluble chemicals are depleted, and it is these chemicals that promote enzyme activity. Furthermore, even under optimal conditions, glutathione is released or leached out of the cell and can effect dough consistency. Glutathoine contributes to dough slackening and can cause dough to soften and become sticky.
Guidelines for working with compressed yeast and active dry yeast appear relatively standardized for both commercial and home settings. This is not necessarily the case for instant active dry yeast as marketed to the home baker. The fineness and porosity of instant active dry yeast particles allow it to be dissolved quickly and homogenously into the dough for most applications. Lallemand is clear in its description of how to work with instant active dry yeast and the methods are easily adapted to home baking. The following is taken from the Lallemand Baking Update entitled Instant Yeast:"Rehydration
The recommended methods for rehydrating instant dry yeast are aimed at avoiding direct contact with excessive amounts of cold water. Using either warm water or a slow rehydration helps to optimize baking performance.
Blend with flour. The simplest method for using instant yeast is by blending the instant yeast thoroughly with the flour before adding (cold) water. The flour absorbs much of the water so that it doesn't come into direct contact with the yeast. Using this technique, the mixer can be started immediately after adding the water, and the dough can be checked at the end of mixing for undissolved yeast particles by stretching it into a thin film. When using ice-cold water, it is best to leave the yeast in the flour for about thirty minutes before adding the cold water and starting the mixer. This improves activity by giving the yeast time to absorb some of the moisture from the flour, as long as the temperature of the flour itself is not extremely cold.
Sprinkle on dough. Another method is to mix all the ingredients except the yeast for one to two minutes, then sprinkle the instant yeast on top of the partially mixed dough and continue mixing. Because the instant yeast is added at a time when most of the (cold) water has been absorbed by the flour, a cold shock is prevented. In this method, the exact timing for sprinkling the instant yeast is critical, and it is important to check if all the yeast particles have disappeared at the end of mixing. When using ice-cold water, it is best to wait three to five minutes after the yeast addition before restarting the mixer. This improves activity by giving the yeast time to slowly absorb moisture from the partially mixed dough.
Add to warm water. The traditional method for using conventional active dry yeast (ADY) can also be used with instant yeast. Blend one part instant yeast with three to four parts lukewarm (95-105F/35-40 C) water. Wait ten minutes, then stir and add the fully rehydrated instant yeast to the mixer. This method is useful with high-speed mixers where the very short mix time of five minutes or less does not allow for complete rehydration. It is also useful …to avoid direct contact with ice-cold water. Although the traditional method requires more time and attention, it gives the highest level of yeast activity…"
Fleischmann suggests that it takes slightly longer to rehydrate instant active dry yeast at the same dough temperature as compressed yeast or active dry yeast, since instant active dry yeast is the driest of bakers yeasts. In this case, a factor of an extra 4 degrees is suggested. For example, if a particular dough calls for a dough temperature after mixing of 78-80 F, then 82-84 F is recommended, i.e. (78 + 4 =82 and 80 + 4 = 84). Alternatively, the same dough temperature may be maintained after mixing, but the dough should be allowed to ferment for a slightly longer period of time.
Although warm rehydration maximizes the performance of instant active dry yeast, companies such as Fleischmann and Red Star suggest that home bakers use water ranging in temperature from 120 to 130, which is excessive. Since, leaching of cell constituents is minimized during rehydration when water is between 70-100 F, using lukewarm to warm water temperature in the dough is advised.We have communicated with Fleischmann and have been informed that the vast majority of home baking complaints that Fleischmann receives about yeast failures stem from the dough being either too cold, or held at cold proofing temperatures. While 120° F. is certainly excessive for the experienced baker who has control of ingredients, weights, time and temperature, using this temperature does help the inexperienced baker to achieve a faster proof and and to obtain something tangible at the end of the baking process. It is important to note that Fleischmann's recommendations for their experienced retail and commercial customers are dramatically different, and comport with The Artisan's findings.
Yeast Conversion
The yeast conversion ratio is 100 percent compressed yeast to 40 percent active dry yeast to 33% instant active dry yeast. When converting compressed yeast to active dry yeast or instant active dry yeast in a commercial setting, it is important to take the difference in dry matters into account by making up the difference in weight with water. Table 1 illustrates the conversion from compressed yeast to active dry yeast. (1 oz is rounded to 30 g in the table)Table 1
Compressed Yeast Active Dry Yeast Additional Water 3 oz (90g) 1 .20 oz (36 g) 1.80 oz (54 g) 6 oz (180 g) 2.40 oz (72g) 3.60 oz (108 g) 9 oz (270 g) 3.60 oz (108 g) 5.40 oz (162 g) 12 oz (360 g) 4.80 oz (144 g) 7.20 oz (216 g) 1 lb. (16 oz) (480 g) 6.40 oz (192 g) 9.60 oz (288 g) 1 lb. 8 oz (720 g) 9.60 oz (288 g) 14.4 oz (432 g) Table 2 illustrates the conversion from compressed yeast to instant active dry yeast. (1 oz is rounded to 30 g in the table .)
Table 2
Compressed Yeast Instant Active Dry Yeast Additional Water 3 oz (90g) 1 oz (30 g) 2 oz (60 g) 6 oz (180 g) 2 oz (60 g) 4 oz (120 g) 9 oz (270 g) 3 oz (90 g) 3 6 oz (180 g) 12 oz (360 g) 4 oz (120 g) 8 oz (240 g) 1 lb. (16 oz) (480 g) 5.28 oz (158 g) 10.72 oz (322 g) 1 lb. 8 oz (720 g) 7.92 oz (238 g) 16.08 oz (482 g) The companies specializing in yeast packaged for home baking recommend substituting 1 cube compressed yeast (0.6 oz) for 2 1/4 teaspoons active dry yeast for 2 1/4 teaspoons instant active dry yeast. As stated above, a more precise ratio is 100 percent compressed yeast to 40 percent active dry yeast to 33% instant active dry yeast. Table 3 provides a guide to converting compressed yeast to active dry yeast to instant active dry yeast. (We refer those interested in exact conversion measurements in ounces and grams to the Yeast Conversion Chart. The chart allows home bakers to choose whether or not to include additional water in the conversion.) The ratio of active dry yeast to instant active dry yeast is 1.25:1.
Table 3
Compressed Yeast Active Dry Yeast Instant Active Dry Yeast 1 cube 2 1/2 tsp 2 tsp 3/4 cube 1 7/8 tsp 1 1/2 tsp 1/2 cube 1 1/4 tsp 1 tsp 1/4 cube 5/8 tsp 1/2 tsp
Last Edited on: 02/03/2002 08:20:25 PM