Question

what are mechanism involved in preparing yeast from slant to fermenter? please make the answer as detailed as you can, perhaps you could add reference so I can study this further

Answer 1

Solution

• Yeast culture requires a sugar source, typically glucose, is added as it provides energy to the growing yeast cells.
• After dissolving yeast extract and peptone in water, sterilize the mixture using an autoclave.
• To make yeast media slants add agar to the liquid media, and pour the mixture into slants, allowing it to solidify at room temperature.

Note- Autoclave for 15 minutes at 121°C.

Pour around 25-30mL of the agar medium on to sterile test tube and let it set in a laminar flow chamber.

From yeast broth culture cells on the agar slants and incubate at 30°C.

Conditions necessary for fermentation in large fermenter-

• Most yeasts require an abundance of oxygen for growth, therefore by controlling the supply of oxygen, their growth can be checked.

• In addition to oxygen, they require a basic substrate such as sugar. Some yeasts can ferment sugars to alcohol and carbon dioxide in the absence of air but require oxygen for growth.

• They produce ethyl alcohol and carbon dioxide from simple sugars such as glucose and fructose.

                                                   C₆H₁₂O_{6              Þ}              2C₂H₅OH               +              2CO₂

                                                   Glucose          yeast         ethyl alcohol              +              carbon dioxide

  In conditions of excess oxygen (and in the presence of acetobacter) the alcohol can be oxidised to form acetic acid. This is undesirable if the end product is a fruit alcohol, but is a technique employed for the production of fruit vinegars (see later section on mixed fermentations).

  Yeasts are active in a very broad temperature range - from 0 to 50° C, with an optimum temperature range of 20° to 30° C.

  The optimum pH for most micro-organisms is near the neutral point (pH 7.0). Moulds and yeasts are usually acid tolerant and are therefore associated with the spoilage of acidic foods. Yeasts can grow in a pH range of 4 to 4.5 and moulds can grow from pH 2 to 8.5, but favour an acid pH (Mountney and Gould, 1988).

  In terms of water requirements, yeasts are intermediate between bacteria and moulds. Bacteria have the highest demands for water, while moulds have the least need. Normal yeasts require a minimum water activity of 0.85 or a relative humidity of 88%.

  Yeasts are fairly tolerant of high concentrations of sugar and grow well in solutions containing 40% sugar. At concentrations higher than this, only a certain group of yeasts – the osmophilic type – can survive. There are only a few yeasts that can tolerate sugar concentrations of 65-70% and these grow very slowly in these conditions (Board, 1983). Some yeasts – for example the Debaromyces - can tolerate high salt concentrations. Another group which can tolerate high salt concentrations and low water activity is Zygosaccharomyces rouxii, which is associated with fermentations in which salting is an integral part of the process.