dinsdag 15 december 2015

Part 8 - Aerobic Biosynthesis

Part 8 - Aerobic Biosynthesis
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v  Anaerobic ande aerobic biosynthesis
Yeasts do not need oxygen to convert glucose to ethanol or to synthesize glycerols, saturated fatty acids and proteins from amino acids. However, oxygen is needed for :
                - biosynthesis of ATP in Electron Transport Chain (see Part 3),
                - biosynthesis of unsaturated fatty acids (UFA), and
                - biosynthesis of ergosterol.


v Phospholipids bilayer
A yeast cell membrane is constructed of phospholipids. A phospholipid consists of a phospho-head and two fatty acid-tails (lipid = fat ). The phospho-head is hydrophilic (love water); the fatty acid-tails are hydrophobic (fear of water).
For this reason, the yeast cell membrane is formed by a "tail-to-tail” bilayer of phospholipids. The phospholipids float against each other. They are not fixed to each other. They allow proteins and enzymes to move freely between them. At low temperature they become tightly close and at high temperature they drift loosely. The yeast cell membrane is a fluid mosaic model.




v Proteins, Carbohydrate & Sterol
The cell membrane consist of not only phospholipids, but also many other substances, which can be divided into 3 groups. (1) Different proteins, for different functions, synthesized from different amino acids. 
(2) Carbohydrate is a carbon where water is included (H-C-OH). Carbohydrate attached to protein or phospholipid is called glycoprotein resp. glycolipid. 
(3) Sterol, which is called cholesterol in animal and ergosterol in yeast.




v Unsaturated fatty acids (UFA) & medium chain fatty acids (MCFA)
The yeast cell membrane is a fluid mosaic model.The fluidity of the yeast cell membrane is considerably reduced by low temperature and high ethanol concentration. The phospholipids go tightly against each other. This can prevent cellular transport systems from functioning correctly. Therefore, during alcoholic fermentation yeasts must adapt the membrane fluidity to the changing environmental conditions. They can do that by synthesizing unsaturated fatty acids (UFA) or medium chain fatty acids (MCFA). They both have a lower melting point and more flexibility, and therefore they could modulate the membrane fluidity. Only for UFA, oxygen is required to dehydrogenate at a defined position in fatty acids 
(-CH2-CH2- + O à -CH=CH- + H2O). The enzyme, desaturase OLE1, catalyses this  dehydrogenation, and is activated by low temperatures and the presence of oxygen.





v Ergosterol
The yeast can also modulate the membrane fluidity by increasing its proportion of ergosterol. Ergosterol is a fatty substance that is located between the fatty acid tails in the  membrane. It ensures that the phospholipids are not too close together at low temperature and not too far apart at high temperature. Ergosterols (like fatty acids) are synthesized from acetyl-CoA by the mevalonate pathway. It is a very complicated pathway of about 30 steps. The key step is, without any doubt, the reaction catalysed by squalene monooxygenase which uses oxygen as a substrate to transform squalene into squalene 2.3-expoide. Without oxygen, the ergosterol synthesis will stop there.




                                                                                     

v  Membrane fluidity adaptation during fermentation
l Red wines are fermented at relatively high temperatures (28-30oC) and are aerated in order to enhance colourextraction. High temperatures cause excessive fluidity which can alter the organization and the dynamic properties of the membrane. The increasing ethanol concentration creates a new aggressive environment. Under these conditions, the yeast must increase their proportion of UFA and ergosterols to compensate for this effect and consequently enhance their tolerance to ethanol. These changes can be done without problems because oxygen is introduced during the racking process.

l White wines are made at low temperatures (14-18oC) and without aeration to conserve aromas. The low temperature and the increasing ethanol concentration prompt the yeasts to adapt their membrane fluidity by increasing the proportion of UFA and ergosterols. However, these can not go on when the oxygen is running out. The yeasts need to use another strategy to fluidize their membranes and the only possibility is incorporating medium chain fatty acids (MCFA).

l Long-chain fatty acids (LCFA) and medium-chain fatty acids (MCFA)  can form esters with alcohols. The volatility of the esters (boiling point) is dependant on the length of the compound: generally the longer the chain, the less volatile. As we already know esters contribute aromas to wines, and these aromas will completely be gone within 1 or 2 years by hydrolysis. That explains why esters are of more significance to the young white wines than to the reds.

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P.S.
Next post we'll take a look at the acetic acid, the main volatile acid in wine.