Hi there,
so I'm a university student, and I'm currently studying cell biology. I'm learning about how temperature affects the fluidity of the cell membranes, and the metabolic responses than ensue to maintain the membrane's physical state.
Is this the reason why we don't want temperatures to fluctuate during fermentation?
If so, how does the metabolic response of these yeast cells to changes in temperature negatively effect (a) yeast health, (b)fermentation, or (c)the chemical composition of the beverage in question?
Thanks for the insight,
Delaney.
Update
well, for those that are curious, I happened upon a professor of cellular biology today, to whom I posed this question. His reply: "absolutely".
For those less familiar with cell membranes, they are composed primarily of molecules called phospholipids. Phospholipids have two fatty acyl chains each, which can either be saturated or unsaturated. unsaturated fatty acyl chains increase the membrane's fluidity, and saturated fatty acyl chains increase the rigidity of the membrane. In order to carry out necessary biological functions, a cell membrane must maintain a relatively constant fluidity. If a given membrane were to remain unchanged, and the temperature were lowered, it would lose it's fluidity and become gel-like, inhibiting it from functioning properly.
When exposed to temperature fluctuations, the cell membrane responds metabolically by interchanging saturated/unsaturated fatty acyl chains in it's membrane. By either increasing/decreasing membrane fluidity, the cell can resist to changes in temperature. In the context of brewing, this metabolic response is what negatively affects life cycles of yeast/bacteria cells, as they are forced to focus on the homeostasis of cell membrane fluidity.
And that is why temperature fluctuations during fermentation is bad. I'm still curious as to the changes in chemical composition which occur as a result of the homeostasis of membrane fluidity in yeast cells.
UPDATE
Today I had the chance to speak with Dr.George van der Merwe, a specialist in the microbiology of saccharomyces, providing valuable insight which I'd like to share with the HBT community.
As previously stated, cell membrane fluidity is vital for the biological functions of yeast/bacteria. Embedded or attached to the cell membrane are proteins which carry out various functions, such as transporting nutrients into the cell. If cell membrane fluidity is compromised due to heat/cold stress, the functionality of these proteins will be negatively impacted.
According to Dr.Merwe, heat stress is to be avoided as this will cause metabolism to reach levels too high, resulting in excessive biomass production. Furthermore, these higher temperatures will encourage the evaporation of volatile compounds, resulting in a "flat" tasting wine/beer, an indicator of an amateur product. Dr.Merwe indicated that a slower fermentation at the cooler end of a yeast's temperature range will produce a higher quality wine. He indicated that if fermentation is sluggish, the temperature could then be raised slightly to kick off fermentation. The implication is that a slower fermentation at cooler temperatures is better than a quick fermentation at slightly higher temperatures.
That being said, temperature fluctuations within the yeast's acceptable temperature range will not produce ill-effects. It is only when temperatures fluctuate outside of the yeast's range that cold/heat stress will occur, negatively impacting yeast metabolism and overall quality of the beverage in question.
I was curious as to where fatty acyl chains could be acquired/sent by phospholipid molecules in order to adapt to changes in temperature. During metabolic response to temperature fluctuations in saccharomyces, fatty acyl chains of phospholipids are not released into the "broth". Rather, the fatty acyl chains are sent/acquired intracellularly (within the yeast cell) in vacuoles, where they are stored.
Another option is that fatty acyl chains can be interchanged between phospholipids of the cell membrane itself. This is effective because a phospholipid with two unsaturated fatty acyl chains is much more (disproportionaly) resistant to cold temperatures than a fatty acyl chain with only one unsaturated chain. Example 2 is therefore considerably better suited to deal with cold temperatures than Example 1:
Example 1:
phospholipid A:
-1 saturated fatty acyl chain
-1 unsaturated fatty acyl chain
phospholipid B:
-1 saturated fatty acyl chain
-1 unsaturated fatty acyl chain
Example 2:
phospholipid A:
-1 unsaturated fatty acyl chain
-1 unsaturated fatty acyl chain
phospholpid B:
-1 saturated fatty acyl chain
-1 saturated fatty acyl chain
The inverse is true as well. Hotter temperatures render the cell membrane more fluid, therefore the phospholipids will acquire more saturated fatty acyl chains to deal with this stress.
I am currently brewing lambic beers, so I asked whether these concepts apply to lactobacillus/brettanomyces. He told me that these organisms are not well understood on a microbiological level, and it is therefore a guessing game as to their sensitivy to temperature fluctuations or what not. Furthermore, bacteria are known to release vacuoles containing phospholipids/fatty acyl chains outside of the cell. These compounds may then react with other molecules in the "broth", which will likely result in aromatic/flavour compounds being formed. It is therefore more than likely that this occurs with bacteria such as lactobacillus.
Finally, I found it interesting that these phospholipids, fatty acyl chains, other proteins of the cell membrane are released by saccharomyces when autolysis occurs. Normally, this is to be avoided. Interestingly, these compounds aid in bubble formation with respect to sparkling wines, therefore a certain amount of autolysis is desireable in this circumstance, to a limited extent. A greater percentage of lees would result in more of these compounds being released/formed, producing a yeastier tasting wine. Using less lees would result in a fruitier tasting wine.
Because vacuoles containing these compounds are likely released by brettanomyces/lactobacillus, one would presume that this has an impact on flavor/aromatic properties of the beverage. Because of this, I presume it would be hard to manage the amount of these compounds being released by such organisms. This therefore highlights the importance of pitching them at proven proportions both with respect to saccharomyces and other species, as well as in proportion to the amount of wort/must; the best way to control the release of these compounds would seemingly be accomplished by limiting the amount of substrate available to brettanomyces/lactobacillus via competition with other organisms such as sacch.
Other than temperature, factors such as ABV% will affect cell membrane fluidity, and therefore the cells' ability to carry out it's biological functions. This is why ABV ranges must also be respected for strains of yeast.
Again, the sensitivity of brettanomyces/lactobacillus to such factors is not well understood.
Cheers,
Delaney
so I'm a university student, and I'm currently studying cell biology. I'm learning about how temperature affects the fluidity of the cell membranes, and the metabolic responses than ensue to maintain the membrane's physical state.
Is this the reason why we don't want temperatures to fluctuate during fermentation?
If so, how does the metabolic response of these yeast cells to changes in temperature negatively effect (a) yeast health, (b)fermentation, or (c)the chemical composition of the beverage in question?
Thanks for the insight,
Delaney.
Update
well, for those that are curious, I happened upon a professor of cellular biology today, to whom I posed this question. His reply: "absolutely".
For those less familiar with cell membranes, they are composed primarily of molecules called phospholipids. Phospholipids have two fatty acyl chains each, which can either be saturated or unsaturated. unsaturated fatty acyl chains increase the membrane's fluidity, and saturated fatty acyl chains increase the rigidity of the membrane. In order to carry out necessary biological functions, a cell membrane must maintain a relatively constant fluidity. If a given membrane were to remain unchanged, and the temperature were lowered, it would lose it's fluidity and become gel-like, inhibiting it from functioning properly.
When exposed to temperature fluctuations, the cell membrane responds metabolically by interchanging saturated/unsaturated fatty acyl chains in it's membrane. By either increasing/decreasing membrane fluidity, the cell can resist to changes in temperature. In the context of brewing, this metabolic response is what negatively affects life cycles of yeast/bacteria cells, as they are forced to focus on the homeostasis of cell membrane fluidity.
And that is why temperature fluctuations during fermentation is bad. I'm still curious as to the changes in chemical composition which occur as a result of the homeostasis of membrane fluidity in yeast cells.
UPDATE
Today I had the chance to speak with Dr.George van der Merwe, a specialist in the microbiology of saccharomyces, providing valuable insight which I'd like to share with the HBT community.
As previously stated, cell membrane fluidity is vital for the biological functions of yeast/bacteria. Embedded or attached to the cell membrane are proteins which carry out various functions, such as transporting nutrients into the cell. If cell membrane fluidity is compromised due to heat/cold stress, the functionality of these proteins will be negatively impacted.
According to Dr.Merwe, heat stress is to be avoided as this will cause metabolism to reach levels too high, resulting in excessive biomass production. Furthermore, these higher temperatures will encourage the evaporation of volatile compounds, resulting in a "flat" tasting wine/beer, an indicator of an amateur product. Dr.Merwe indicated that a slower fermentation at the cooler end of a yeast's temperature range will produce a higher quality wine. He indicated that if fermentation is sluggish, the temperature could then be raised slightly to kick off fermentation. The implication is that a slower fermentation at cooler temperatures is better than a quick fermentation at slightly higher temperatures.
That being said, temperature fluctuations within the yeast's acceptable temperature range will not produce ill-effects. It is only when temperatures fluctuate outside of the yeast's range that cold/heat stress will occur, negatively impacting yeast metabolism and overall quality of the beverage in question.
I was curious as to where fatty acyl chains could be acquired/sent by phospholipid molecules in order to adapt to changes in temperature. During metabolic response to temperature fluctuations in saccharomyces, fatty acyl chains of phospholipids are not released into the "broth". Rather, the fatty acyl chains are sent/acquired intracellularly (within the yeast cell) in vacuoles, where they are stored.
Another option is that fatty acyl chains can be interchanged between phospholipids of the cell membrane itself. This is effective because a phospholipid with two unsaturated fatty acyl chains is much more (disproportionaly) resistant to cold temperatures than a fatty acyl chain with only one unsaturated chain. Example 2 is therefore considerably better suited to deal with cold temperatures than Example 1:
Example 1:
phospholipid A:
-1 saturated fatty acyl chain
-1 unsaturated fatty acyl chain
phospholipid B:
-1 saturated fatty acyl chain
-1 unsaturated fatty acyl chain
Example 2:
phospholipid A:
-1 unsaturated fatty acyl chain
-1 unsaturated fatty acyl chain
phospholpid B:
-1 saturated fatty acyl chain
-1 saturated fatty acyl chain
The inverse is true as well. Hotter temperatures render the cell membrane more fluid, therefore the phospholipids will acquire more saturated fatty acyl chains to deal with this stress.
I am currently brewing lambic beers, so I asked whether these concepts apply to lactobacillus/brettanomyces. He told me that these organisms are not well understood on a microbiological level, and it is therefore a guessing game as to their sensitivy to temperature fluctuations or what not. Furthermore, bacteria are known to release vacuoles containing phospholipids/fatty acyl chains outside of the cell. These compounds may then react with other molecules in the "broth", which will likely result in aromatic/flavour compounds being formed. It is therefore more than likely that this occurs with bacteria such as lactobacillus.
Finally, I found it interesting that these phospholipids, fatty acyl chains, other proteins of the cell membrane are released by saccharomyces when autolysis occurs. Normally, this is to be avoided. Interestingly, these compounds aid in bubble formation with respect to sparkling wines, therefore a certain amount of autolysis is desireable in this circumstance, to a limited extent. A greater percentage of lees would result in more of these compounds being released/formed, producing a yeastier tasting wine. Using less lees would result in a fruitier tasting wine.
Because vacuoles containing these compounds are likely released by brettanomyces/lactobacillus, one would presume that this has an impact on flavor/aromatic properties of the beverage. Because of this, I presume it would be hard to manage the amount of these compounds being released by such organisms. This therefore highlights the importance of pitching them at proven proportions both with respect to saccharomyces and other species, as well as in proportion to the amount of wort/must; the best way to control the release of these compounds would seemingly be accomplished by limiting the amount of substrate available to brettanomyces/lactobacillus via competition with other organisms such as sacch.
Other than temperature, factors such as ABV% will affect cell membrane fluidity, and therefore the cells' ability to carry out it's biological functions. This is why ABV ranges must also be respected for strains of yeast.
Again, the sensitivity of brettanomyces/lactobacillus to such factors is not well understood.
Cheers,
Delaney