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Old 11-17-2009, 04:23 PM   #11
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Excellent! Thanks everyone for the info.

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Old 11-17-2009, 07:39 PM   #12
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I would say the right answer is a combination of all the responses thus far. Osmotic pressure plays a huge factor as does water activity. I agree with GN that the reason a lot of high sugar food products don't require refrigeration in the store is a combination of pH factors, preservatives (though these probably paly more of a role after the product is opened), osmotic pressure, water activity, and the big reason is that these products are pasteurized and stored under vacuum pressure...you wouldn't open a can of grape jelly, use some and leave it on your counter at 78 degrees for a week and a half and expect to find no spoilage growth. Also the cheese example someone mentioned is partially correct I think. Again I think a combination of water activity and osmotic pressure are at play here. Typically the harder the cheese is the saltier it is as well meaning high osmotic stress as well as low water activity.

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Old 11-17-2009, 08:14 PM   #13
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Everything, as far as the OP is concerned comes back to water activity level... osmotic pressure, even salt content (although this doesn't apply to the OP's question). Low water activity level means osmotic imbalance. High salt content means low water activity level.

pH can also be a concern, but I don't believe it is in regards to the OPs question.

This isn't to say that 0.85 aw is the cut-off for any microbial growth. It just means that there won't be rapid and progressive growth. Mold can grow on the surface of foods with aw's of over 0.8.

The bottom line is that for the OPs question, there is only one measurable parameter that needs to change to allow the yeast to thrive... water activity level.

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Old 11-18-2009, 12:57 AM   #14
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Even better sugar and salt mixed together. That's what makes bacon delicious and long-lasting!

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Old 11-18-2009, 01:28 AM   #15
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wait are we saying the same thing here menschmaschine? I agree there is a whole slew of microbes that do well sub .85 aw (Zygosaccharomyces bailii and penicillium for example) and I realize water activity and osmotic pressure are inversly proportional to each other, but I am confused on which your are saying is the driving factor...in other words would one change the water activity to shift the osmotic pressure or would one change the osmotic pressure to shift the water activity? I was under the impression that while water activity is a "measurable parameter" it is dependent on osmotic factors (as well as other factors) so while water activity would answer the OP question wouldn't osmotic forces be more specific? I know I am just splitting hairs, but I enjoy stimulating exchanges.....now this is just getting weird.

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Old 11-18-2009, 02:15 AM   #16
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Quote:
Originally Posted by sw341034 View Post
wait are we saying the same thing here menschmaschine? I agree there is a whole slew of microbes that do well sub .85 aw (Zygosaccharomyces bailii and penicillium for example) and I realize water activity and osmotic pressure are inversly proportional to each other, but I am confused on which your are saying is the driving factor...in other words would one change the water activity to shift the osmotic pressure or would one change the osmotic pressure to shift the water activity? I was under the impression that while water activity is a "measurable parameter" it is dependent on osmotic factors (as well as other factors) so while water activity would answer the OP question wouldn't osmotic forces be more specific? I know I am just splitting hairs, but I enjoy stimulating exchanges.....now this is just getting weird.
Sure there are microbes that thrive below 0.85 aw, but it's partially a matter of how quickly. The 0.85 number is used primarily for pathogenic microbes and food-spoilage microbes (I believe it's tested with Staph aureus), but S. cerevisiae is probably pretty close to the same number.

We very well may be saying the same thing, but my head is already spinning because I've been infected with a rhinovirus (my aw must have gone above 0.85) and I just don't have the energy to put much more thought into it right now.
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Old 08-18-2011, 08:41 AM   #17
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bacteria cannot grow in high salt or high sugar concentrations...dis is mainly bcoz of osmotic pressure..
when bacteria r suspended in highly concentrated sugar solution, then the bacterial cell tries to attain equilibrium between its cell and the surrounding environment...as we all know..water always move from low concentration to high concentration [osmosis], water from the bacterial cell moves into itz surrounding medium to attain equilibrium...as a result it [bacterium] gets deprived of water and bacterial cell finally becomes flaccid

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Old 09-05-2011, 11:43 PM   #18
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Originally Posted by menschmaschine View Post
Everything, as far as the OP is concerned comes back to water activity level... osmotic pressure, even salt content (although this doesn't apply to the OP's question). Low water activity level means osmotic imbalance. High salt content means low water activity level.
Ehhh... Osmotic imbalance means low water activity. I can derive equations governing osmosis from first principles. In theory, water activity is just a different way of looking at chemical potential, and differences of chemical potential across membranes is what drives osmosis.

Once you start talking about food processing or anything like that, water activity is a measured quantity rather than a predicted quantity. That's mainly because we don't fully understand the interaction of water with complex systems, especially ones that are primarily solid-phase.

To put it in terms of the original question, water activity is the what, but osmotic pressure (or more accurately chemical potential) is the why. Nature doesn't like pure or unmixed substances being in proximity. The honey will always pull the water out of the cells to dilute the honey.
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Old 09-06-2011, 06:13 PM   #19
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It's actually a balancing act involving chemical potential (which is a function of pressure) and osmotic pressure. As the activity of water outside the cell (in something like honey) is lower than the activity of water inside a cell there is a chemical potential across the cell membrane. Matter moves from high chemical potential to lower so water moves out of the cell. This dilutes the honey thus increasing the chemical potential of water in the honey but also decreases the pressure in the cell thus lowering the chemical potential of water in the cell. This process continues until the water in the honey is dilute enough and the osmotic pressure difference across the membrane is high enough that the chemical potentials are the same. No more water migrates.

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Old 09-11-2011, 12:46 PM   #20
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Quote:
Originally Posted by susmithanukala View Post
bacteria cannot grow in high salt or high sugar concentrations...dis is mainly bcoz of osmotic pressure..
when bacteria r suspended in highly concentrated sugar solution, then the bacterial cell tries to attain equilibrium between its cell and the surrounding environment...as we all know..water always move from low concentration to high concentration [osmosis], water from the bacterial cell moves into itz surrounding medium to attain equilibrium...as a result it [bacterium] gets deprived of water and bacterial cell finally becomes flaccid
I'm having a difficult time getting through this one. Water moves from low to high? Obtaining equilibrium would be moving from high concentration to low concentration.

Tehehehehe... he said "flaccid"

EDIT. Dammit I just realized I responded to a month old post.
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