Yeast Cell a Wonderful Microorganism (Lecture Version)

What's Yeast?
- Yeast are unicellar microorganisms with can obtain the energy in the aerobic or anaerobic
- Fungi species Saccharomyces ceriviseae are used
- Structure & composition of the yeast cell
- Yeast cell is oval to round with length of 8-10 micron & breadth of 5-7 micron
- Yeast cell can only be seen under a microscope consist

Yeast Cell Structure
Bud scar
Cell wall ; give the yeast cell its shape & stability (manan+glucan)
Plasma ; containing cytoplasm
Plasma membrane ; semipermeable membrane
Vacuole ; store metabolic products temporarily (phosphate)
Polymetaphosphate granule
Mitochondria ; provide energy for the cell during respiration
Lipid granule
Endoplasmic reticulum
Nuclear membrane
Nucleus;control the metabolism & contain the genetic information
Nucleolus

Yeast metabolism
- The purposes of metabolism are;
- To take in usable substance as food
- To produce the energy
- Yeast able to utilize these sugars;
- In the presence of oxygen (aerobic)
- Is called respiration
- Produces more energy
- When oxygen is excluded (anaerobic)
- Is called fermentation
- Produces less energy

- Energy production by respiration for fermentation result from numerous reaction by specific enzyme
- The enzymes of the respiration are located in the mitochondria
- The fermentation enzymes are mainly in the cytoplasm
- The yeast cell can only take up substance depend on enzyme spectrum

Carbohydrate metabolism
- Louis Pasteur discoverer : Fermentation that is life without oxygen
- Yeast is only organism which can change from respiration to fermentation
- Fermentable carbohydrates include;
- Monosaccharides ; glucose, fructose, manose, galactose
- Disaccharides ; maltose, sucrose
- Trisaccharides ; raffinose, maltose (not by all yeasts)
- Glycogen & trehalose are a small fraction of sugar as a chemical energy for use when external sugar is not available
- The most important chemical energy reserves are;
- ADP (adenosine diphosphate)
- ATP (adenosine triphosphate)
- Is important as an energy reserve & it’s essential for all life process

Nitrogen metabolism
- Yeast requires nitrogen to form its own cellular protein
- The main nitrogen sources are amino acids & lower peptides
- It can’t use inorganic nitrogen compounds other than ammonium salts
- Yeast can’t simply use the amino acids in wort directly but are absorbed in a particular sequence;
- Degradation, transformation & synthesis reaction
- These processes are also formation of fermentation byproducts such as;
- Higher alcohol(fusel oil), diketone, esters & organic acid
- Formation of metabolic products are depend on a temperature , pressure & pH

Metabolism of inorganic substances & growth factors
- The following cations & anions affect enzymic reactions;
1. Potassium
- Stimulates all enzymic reactions
- Important for energy metabolism
2. Sodium
- Activates enzymes
- Important in the transportation of substances through the cell membrane
3. Calcium
- Assists break formation
- Can be replaced by manganese
4. Magnesium
- Very important for reactions especially during fermentation
5. Copper
- Even a low concentration inhibits some enzymes
6. Iron
- Important for the enzymes involved respiration
- Increase cell budding
7. Manganese
- Stimulate cell reproduction & cell growth
- Can be replaced by iron
8. Zinc
- Increase protein synthesis
- Very important for fermentation
- Requirement is about 0.2 ppm
- Zinc deficiency lead to defective fermentation
9. Sulphate
- Necessary for yeast cell material synthesis
10. Phosphate
- Important for the formation of high energy substances
- Fermentation is not possible without phosphate
- Deficiency result in inadequate enzymic activity
11. Nitrate
- Is reduced by bacteria to nitrite which poisons yeast cell & is determental to fermentation

Energy metabolism of yeast
- Respiration;
C6H12O6 + 6O2 >>> 6H2O + 6CO2

- Fermentation;
C6H12O6 >>> C2H5OH + 2CO2
- Only part of energy is available to the organism, the remainder is converted into heat

During respiration of glucose the heat of reaction is;
deltaH = 2,824 kJ/mol = 15,570 kJ/kg

- Yeast respires at most 2% of the sugar in the initial fermentation phase since after that no more oxygen is available
- As a result reaction heat is produced by the yeast
- Heat production during fermentation assumed;
105.5 kJ/mol = 586.6 kJ/kg
- This means that during fermentation only about 3.7% of the amount of energy which would be produced by respiration is released for the yeast