Production of Yeast by Aeration Process.
This method was invented in Sweden about ten years ago, and is in use in many yeast factories to-day. A decided greater percentage of yeast yield is accomplished by the “Aeration Process.”
After the saccharification of the mash is completed, the extract called “wort” is strained to remove the husks and bran of the grains. Large vats containing a double bottom are used for this purpose, the inner or upper bottom being perforated. Spigots are attached to the bottom of the vats to draw off the “wort.” At first the extract appears opaque and is again returned to the mash. This pouring-back process is continued until the “wort” finally flows perfectly clear from the spigots. The extractive matter still adhering to the husks and bran of the grains is washed out or “sparged” with hot water.
Another way employed for recovering the clear “wort” is by means of the filter press. The percolation method, however, is preferable, as the extraction of the essential properties is more complete.
Fermentation is produced in the “wort” by adding small quantities of compressed yeast also, or by the use of pitching yeast. During the fermenting period a continuous stream of atmospheric air is forced through the “wort” by the aid of air pumps. In order to eliminate atmospheric dust and bacteria, the air before entering the “wort” filtered through cotton, and sterilized by passing it through a solution of salicylic acid. It is also necessary to distribute the air to all parts of the “wort” equally, by means of perfected tubes, which are attached to the main air pipe, branching out in various directions at the bottom of the fermenting vats, with the perforations facing downward. At the beginning the air current is very moderate, and is increased in accordance and in proportion of the yeast growth. At the final stages of fermentation the air current is again moderated. This forcing in of air, or rather oxygen, in the “wort” stimulates in an exceedingly large measure the propagation of yeast, but care is exercised in this respect, however, for if the air pressure be too strong a large per cent. of alcohol, a very important by-product, will be lost. The characteristic feature of this method, distinguishing it from the Vienna Process, is the continued aeration during fermentation, hence called “Aeration Process.”
Obtaining the Yeast.—Fermentation of the “mash” or the “worts” proceeds at a lively rate. In observing the “head” or froth, during the “Vienna Process,” which at first is transparent, gradually assumes a milky or of more opaque appearance, caused by enormous increasing growth of yeast cells, filling up the froth bubbles. When the cells are fully developed the fermentation may be considered finished. Practice assumes, although the assumption is not always reliable, that this stage has been reached when the “head” or froth begins to recede. The only sure method to determine proper maturity of the yeast cell is by microscopic observation.
Placing some of the froth under the object glass of the microscope, the yeast cells most appear well developed and isolated from each other. It should be the exception rather than the rule that budding cells still be visible.
Not until assured that the proper time has been reached should the skimming of the upper portion of the froth be begun. This portion of the “head” contains the so-called “pitching yeast,” and is used largely in starting new propagation.
Large galvanized perforated spoons with long handles are used to skim off the froth. Repeated observations of the froth during the skimming are made, to ascertain the condition of the yeast cells.
The yeasty froth is immediately mixed with ice cold water to arrest further fermentation. This also serves to increase the keeping properties of the yeast.
The water containing the skimmed-off matter is now run through strainers of varying sized meshes, the coarser retaining the husks and bran, while the finer meshes prevent the gummy matter adhering to the yeast cells from passing.
The strained yeast cells are caught up in vessels containing water, where they precipitate in a compact layer, and is then ready to be washed.
In order to watch the settling of the yeast, these vessels are constructed with windows so as to give the operator a perfect vision of the settling.
This operation of washing the yeast in new water and allowing it to settle is repeated several times, at which time nearly all of the impurities have been removed and excellent keeping properties have been attained.
A newer method of washing yeast has lately been introduced by the invention of a specially constructed patented centerfuge. If it be intended to mix starch with the yeast, it is usually done just after the washing has been completed.
Potato or rice starch are used. The utmost carefulness must be observed in the examination of the starches, as they frequently are contaminated with bacteria or acids, which tend to injure the keeping qualities of the yeast and very soon become unfit for use.
After the clear water of the last washing has been removed by decantation, the compact settled mass is pressed dry by hydraulic or filter press, and finally formed by specially constructed machines into pound pieces, familiar to all bakers.
The “mash” or the “wort” after the yeast has been removed contain alcohol in paying quantities, and is recovered by distillation.
One hundred kilograms of mash yields an average of 11 per cent. yeast and 28 per cent. of alcohol, if fermented according to the “Vienna Process.” The “Aeration Process” yields 25 per cent. of yeast and 18 per cent. of alcohol. The remaining grains in the liquids are much sought after for their value as desirable fodder for cattle.
Yeast Adulteration.—High class compressed yeast should be free from all adulterants. Most manufacturers, on account of the slimy matter of yeast, causing many difficulties in pressing, add from 5 to 10 per cent. of potato starch, claiming that it increases the keeping qualities by absorbing part of the moisture. The writer, however, does not agree with them.
Starch is undoubtedly at times added to yeast in large excess; it then becomes an adulteration; this fraud is, however, readily detected by treating the sample of yeast with iodine. For this purpose break up a little of the yeast in a test tube with some water, shake it up well and add a few drops of tincture of iodine; after standing a little while the starch will settle at the bottom of the tube in a dark blue layer.
Plaster of paris has also been found in yeast; this, besides being fraudulent, is decidedly criminal, and verily, is giving a “stone for bread.”
Nature of and Examination of Compressed Yeast.—A good sample of compressed yeast should have a creamy white color. A brownish discoloration would indicate that fermentation had been too far prolonged before skimming. It should have an odor of apples, not cheesy; neither should it have an acid odor or taste. A piece of blue litmus paper pressed against the cut of the yeast should remain neutral or at the most show but a faint sign of red; a marked change in the paper from blue to red would indicate acidity.
A microscopic view of good yeast dissolved in water should have the appearance as seen in [Fig. 2], shown above. When broken it should show a fine fracture, irregularly rounded. Should it be crumbly, deterioration has set in. In lukewarm water it should melt readily, and not be sloppy to the touch. The dissolved yeast placed in a glass tube should settle slowly and evenly with the water above it perfectly clear. During this test adulteration with plaster of paris is readily detected, as it would be the first to settle out, and by carefully decanting the fluid, examination of the sediment would disclose plaster of paris.
If the solution of yeast and water does not clear itself the yeast is spoiled, and is of no use for the fermenting of doughs. It is contaminated with wild yeast and harmful bacteria, and would be instrumental in starting putrefactive fermentation, ruining the flavor of baked goods. Should we desire to ascertain the amount of starch present in an adulterated yeast, the following method is applicable:
Weigh a small beaker and a small glass rod on a very accurate scale; or, better still, on an analytical balance, and assume the weight to be 17.5 g. In the beaker place 10 g. of the compressed yeast under examination; break it up fine with the glass rod, and place the beaker in a hot water bath for several hours, weighing occasionally until two consecutive weighings are exactly equal; for instance, 21.2 g. We deduct from this the weight of the beaker and glass rod, giving us the following figures: 21.2 g. - 17.5 = 3.7 g.
The quantity of moisture evaporated out of the yeast would therefore be 10 g. - 3.7 = 6.3 g. According to the findings of Hayduck, pressed yeast contains originally 73.5 per cent. and dry starch 36 per cent. of moisture.
We now proceed to make deductions to determine the quantity of starch contained as an adulterant in the mixture. We set the example: “What per cent. of starch is contained in a mixture of yeast and starch if 10 g. of the mixture gives off by evaporation 6.3 g. of moisture, yeast containing 75.3 per cent. and starch 36 per cent. of moisture?”
Solution.—One hundred g. of pure press yeast, heated to dryness, gives off 73.5 g.; therefore, 10 g. heated should give off 7.35 per cent. moisture.
In our test the loss is but 6.3 g., consequently a deficiency of 1.04 g. This in itself indicates starch adulteration.
Starch gives off 36 per cent. of moisture; therefore, 1 g. gives off 0.36 g., and 1 g. of yeast 0.735 g. of moisture. With each 1 g. of starch addition the moisture loss is found to be 0.735 - 0.36 = 0.375 g. deficient.
In the 10 g. mixture under examination there is contained as many times 1 g. of starch as 0.375 g. is contained in 1.05 g., which is equal to 2.8 g. In a 100 g. mixture the result would be 28 g., or 28 per cent., which is the per cent. of starch adulteration in our mixture examined.