MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.6. MICROBIOLOGY OF BAKERY PRODUCTS

15.6.1. Yeast as a Dough Leavening Agent

In the baking industry, Yeast drives Alcoholic Fermentation in dough and causes it to rise through the generation of carbon dioxide. Alongside standard compressed yeast, bread production also utilizes dried yeast, instant yeast, yeast cream, liquid yeast, and yeast strains resistant to various adverse environmental factors.

Compressed yeast consists of Saccharomyces cerevisiae biomass with a moisture content of 67–85%, formed into blocks. Compressed baker's yeast is classified as a perishable product; therefore, it must be stored and transported at temperatures of 0–4 °C. At this Temperature, yeast Cells enter a state of anabiosis. However, even under these conditions, yeast requires energy to maintain vital Functions, which it obtains by breaking down nutrient reserves, primarily Glycogen and trehalose. Once 80–90% of glycogen and 30–40% of trehalose are depleted, the yeast begins breaking down Proteins, leading to Cell autolysis. To ensure guaranteed shelf life during storage, the trehalose content must be at least 8% calculated on a dry matter (DM) basis.

The dosage of compressed yeast depends on the dough-making method: 0.5–1.0% is added for the sponge-and-dough method, 2.0–2.5% for the straight dough method, and 3.0 to 6.0% relative to flour weight for single-phase accelerated Methods.

Dried yeast. Since compressed yeast is a perishable product, it is preserved by drying. The advantages of dried yeast include excellent transportability and storage stability. The guaranteed shelf life of domestic dried yeast, accounting for specified activity losses, ranges from 5 to 12 months. Imported dried yeast has a shelf life of up to 2 years in unbroken packaging.

Dried yeast is categorized into two main types: high-activity dry yeast and high-activity "instant" dry yeast. They differ in their preparation method prior to dough mixing. High-activity dry yeast is activated in warm Water (35–38 °C) for 10–15 minutes before being added to the dough. Typically, one part of yeast is activated in 4–5 parts of water. "Instant" yeast, conversely, must not come into contact with water; it is mixed with flour prior to kneading or added During the first minutes of mixing. Both types of yeast are added to the dough at 0.5% of the flour weight for the sponge method and 0.8–1.0% for the straight dough method. In rich doughs with high sugar and fat content, The amount of dried yeast can be increased to 1.4–1.6%. "Instant" yeast is recommended for straight and accelerated dough-making methods with a sugar and fat content of 7% or more relative to flour weight.

Yeast cream is an aqueous suspension of Saccharomyces cerevisiae cells obtained by propagating them in a nutrient medium, followed by concentration using separators or Separation on filters. Yeast cream must meet the following parameters: moisture content no higher than 75%; leavening power no more than 75 minutes; color ranging from white-gray with a yellowish tint; flavor and odor characteristic of yeast, free of moldy, putrid, or other foreign smells. In consistency, yeast cream is an aqueous suspension with a layer of yeast cells settling to the bottom.

Liquid yeast is a semi-finished product obtained by fermenting a water-flour mash with thermophilic lactic acid Bacteria of the species Lactobacillus delbrueckii, followed by cultivating Saccharomyces cerevisiae yeast on the fermented mash. Liquid yeast is used when Processing flour with inferior baking qualities, as well as in hot-climate regions where There is a genuine risk of ropy bread spoilage.

The preparation process for liquid yeast comprises two cycles: propagation and production. The propagation cycle involves the gradual multiplication of pure cultures of thermophilic lactic acid bacteria and yeast in a liquid medium (malt wort) and saccharified flour mash until reaching the volumes required for bread production. During the propagation cycle, microorganisms are cultivated without harvesting the culture, whereas regular harvesting of liquid yeast takes place during the production cycle. The propagation cycle is carried out 1–2 times a year.

In the production cycle, liquid yeast is harvested every 2–3 hours around the clock. Half or one-third of the finished yeast is taken for dough mixing, and the harvested portion is replenished with an equal amount of fermented mash. The readiness of liquid yeast is determined by its leavening power using the rising-ball method, which should be between 15 and 25 minutes. The acidity of liquid yeast should be 8–12°, leavening power no more than 30 minutes, and mass fraction of moisture 86–90%.