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The technological process involved in the production of yeast in very large commercial plants is much well established and a large body exists with a lot of literature that covers the whole process of yeast production. This production is done and propagated in some well-organized stages. Indeed, there are four stages involved in this process. These are the fermentation process where the growth of the yeast takes place, separation or concentration of the yeast, dehydration by simple filtration or drying, and finally the process of packaging the yeast. It is basically used in the bakery and the brewery sector in most settings of the world.

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Fermentation is the main process that is important to the whole manufacturing process for the yeast. This, therefore, exposes this whole process to a lot of microbial contamination due to the nature of the whole process. The large vessels used in the whole process prove it to be logistically impossible to keep them completely sterile and thus some sort of contamination must adhere to the whole process no matter how safe and clean the system may look like. Certain microbial are contained in the final finished yeast products as much as the contamination is kept at minimal levels during the process of production. The subsequent processes after fermentation may also still provide a risk of contaminating the whole sample. These include the washing, packaging, transportation, storage, and dehydration processes. The main microbial associated with yeast production include lactic acid bacteria (LAB), Enterococcus spp, Bacillus spores, and the coliforms and strains of E. coli. These contaminations could be from raw materials, water, chemicals, defoamers, inadequate fermentation tanks, and other storage factors.


This study involved surveys that were independent of some commercial production plants over a period of six months. In the surveys, 2 pre-treated raw materials both molasses and synthetic sugar were used. 9 raw materials and one processing air, seven seed yeast, one cream yeast, eight compressed yeast, and eight dry yeast production were collected for this particular survey.

For each of the surveys, raw material and seed, cream, and the compressed plus dry yeast lines of production were corresponding to the same fermentation process where the batch was aseptically collected. The incoming air was sampled after proper filtering by the use of a sterile air sampler that is connected to the aseptic fermentation tank. This air sampler holds about six plates which are the standard 90mm Petri dishes. These had three duplicate copies of selective media types that were placed in the air sampler and then exposed to the incoming air for about twenty minutes.

This was followed by the liquid samples that were assayed aseptically and directly onto the sterile Whirl Pak bags through the tank or pipeline sampling valves. These valves were steam-strayed with 70 percent alcohol just before the sampling process. For purposes of the solid samples, aliquots were collected aseptically through the use of sterile gloves that were disposable and then placed in the sterile Whirl Pak bags. All these samples were stored in ice bags and analysis have done within twenty-four hours. The duplicate plates containing the process of air analysis were incubated just immediately after the whole process of transportation. The samples were analyzed twice with the first done immediately on the very day of sampling and then again after preliminary incubation at 37 degrees Celsius and for 24 hours. This was meant to boost the bacterial counts before the analysis process. Both the preliminary and non-preliminary incubated samples were analyzed and processed using the same criteria to avoid biases in the results obtained from the samples.


The results obtained during these three replicate surveys of a commercial yeast process are tabulated using the means, differences, and standard deviation. The counts were made for pH, aerobic, Enterococcus, coliforms, and E.coli that counts for seed, cream, and compressed yeast production. These are shown before and after the preliminary incubation of the samples. The standard deviations show that the bacterial counts were very different from the three replicate surveys conducted over the six months period.

Most of the samples where the aerobic plate counts were present were very elevated as compared to the Enterococcus, E Coli, and coliforms respectively. Bacterial counts for Pi samples were higher than that of the non-PI samples. The aerobic plates and the Enterococcus count that increased by about two to four log CFU/ml after the incubation at the primary level at 37 degrees Celsius for 24 hours.

Based on the factors related to the raw materials used in the study or experiment, aerobic plates, Enterococcus, coliforms and E. coli had very low counts even after the primary incubation with an exception of the ones containing soda ash which had over 2 logs CFU/ml Enterococcus plus the corresponding aerobic bacteria. This was due to the presence of the sterilized raw molasses before the entry of fermentations. The average pH values for the raw materials were 4 for the mineral mix and 5 for the inoculums; compressed and dry yeast at 6.5 with defoamers at 8; soda ash at 11.5. The measurements were not very different before and after preliminary incubation of the raw materials because they had a difference of (P> 0.05).


The earlier studies on the possible causes of contamination by bacteria on the yeast production from sources like coliforms, Enterococcus, and E. coli through the whole process have been reconfirmed by this study. The contamination was detected in the production of the cream, dry and compressed yeast as well as at the raw materials level of the initial stages in the production of the yeast in one way or the other. It is very normal to find bacteria contaminating the fermented products like in the case of yeast production in most of the plants across the country. This is because the conditions used in the experiments are very favorable for the growth and development of aerobic, facultative, anaerobes, and mesophilic bacteria that grow at pH values between 4 and 5. These conditions are very similar to the conditions utilized in the commercial fermentation processes of cream, compressed or dry yeast.

In most food and beverage production units, the isolation and detection of the microbes in such samples is considered as an avenue to assess the quality of the products and the processing conditions. They are used as indicator organisms to the hygiene observation in the fermentation process in these plants like in the production of yogurt. The presence of such organisms will therefore clearly show a failed system of sanitation in these processes on a wide scale than ever imagined.

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