You are certainly more up-to-date than I am:)

Ruth,

I think AquaLab is/was the name for certain models of Decagon devices, so I should have referred to Decagon. In any event, that info is out of date because, in my recent email exchange with the company, this note was included at the end:

We are excited to let you know that we have changed our name from Decagon Devices/AquaLab to METER Group Inc.  All content will gradually migrate to our new website at METERgroup.com.  The current website www.aqualab.com will be available for the foreseeable future.

Jim

Jim, I think Pawkit is made by Decagon rather than AquaLab, not that it makes any difference in the answer.

There you go.  There are many different makes/models out there - towards the turn of the century is when the technology to differentiate/compensate began to be used, but not everyone chose to include it in every model.  Know thy model and it's limitations 8-)

An update:  

I wrote to AquaLab asking about the issue of volatiles (such as alcohol) in the sample being measured and got this reply:

"The statement [included in my previous message] was quoted from the 4TE manual that used the chilled mirror sensor. I've attached the link to the manual, this statement is on page 51 section 8.4."

I replied to them, asking specifically:  "But does the statement also apply to the Pawkit, meaning that it too may not give accurate readings with high amounts of volatiles in the sample?"

And the reply to that was:  "Not at all James. Pawkit uses the capacitance sensor, there is not a chilled mirror on the Pawkit. Our Series 4TEV instrument uses the same sensor as the Pawkit."

Sebastian,

You said "as long as the Aw meter was made after 1999, that is, after which point they began including compensation to correct for the fact that volatiles such as ethanol impact the humidity and the humidity sensor."

On eGullet I posted on what I learned from your previous statements on alcohol and aW. Were you referring to any specific models of meters? I ask because I received a reply from Kerry Beal (she and I both have the Pawkit from AquaLabs) stating "A standard dew point  Aw meter without special filters doesn't recognize the Aw lowering effect of alcohol. From the Aqua Lab manual - ‟Samples with a high amount of volatiles condense on the mirror during the reading process, but do not evaporate from the mirror as water does. As a result the reading on samples with volatiles may not be accurate with the dew point technique.”

I would like to clear up the apparent discrepancy.

I don't think i'm going to go into the science behind it, but suffice it to say that an Aw meter will give a reading of the overall Aw of the food system being tested, and will be agnostic of individual ingredients. It considers the system as a whole, and if that system contains alcohol, that will be considered as well as part of that system.  As Clay notes, port, while relatively high alcohol for a wine, is still relatively low in overall alcohol content, and as such the contribution of alcohol to the overall formula will be nominal at your likely use level.  While it may be measurable, it's not likely to be meaningful in terms of it's ability to provide extended microbiology shelf life.

Edit: as longa as the Aw meter was made after 1999, that is, after which point they began including compensation to correct for the fact that volatiles such as ethanol impact the humidity and the humidity sensor.

updated by @sebastian: 02/18/17 11:57:29

Sebastian,

Thanks for those ideas. In regard to "The impact of the alcohol you add will be factored into any Aw reading you take," could you say more about that? Does that mean that the alcohol somehow decreases the available water? How does this happen, given the water that is part of whatever liquor is being used?

The impact of the alcohol you add will be factored into any Aw reading you take.

Perhaps consider grating the cheese, putting it in a sous vide bag, and then into a pressure cooker - sort of a DIY HPP process to reduce the micro load of the cheese? I've no data on it, but theoretically it should work pretty well.  Better yet, grate the cheese, add it AND the port to the bag and pressure treat them both - that way you get the impact heat, pressure, and alcohol working on micro load reduction.

Good point about mold in the cheese. I think the upshot of this is that it will not be possible to predict the shelf life of this ganache with any degree of certainty. I'll go ahead and make it to see what I think of the taste and keep it for a while to determine what happens over time. Perhaps it will go in the same category as the famous crème brûlée chocolate at Kee's in NYC--eat it within two day or....

Jim -

Thanks for the clarification.

Intuitively, I think that the Aw would only take into account the water in the spirit you're adding.

Ports are around 20% ABV, so if your recipe calls for 50gr of port in a 1kg recipe, that's 10gr of alcohol in 1000gr or 1% alcohol by weight. You've also added 40ml of water, but given the extended contact with alcohol, it should be free of live microorganisms.

However, given the Stilton (which is infected with mold spores) and cream in the recipe, the comparatively small amount of alcohol from the port is probably not going to affect shelf life significantly.

I would look to other techniques to reduce Aw and look to pasteurize the cream and cheese to kill as many live microorganisms as possible.

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@DiscoverChoc

I have Greweling's book. I am not talking about a huge alcohol content, just a routine adding of dark rum or Grand Marnier or kirschwasser, etc. Perhaps I was not clear, but my basic question is whether the measured Aw takes into account the alcohol or not. I suspect it does not, but I've never seen a definitive statement on that. Wybauw writes about adding alcohol at 15% of the weight of the ganache to affect shelf life--that's a lot of alcohol and most recipes I've seen don't approach that level.

This question is brought on by my intention to try a chocolate filling using Stilton cheese, cream, dark chocolate, and port, and I said on another forum that I would make this ganache and report back on the water activity. The port is the unknown factor.

I strongly recommend that you get a copy of Peter Greweling's book Chocolates and Confections: Formula, Theory, and Technique if you don't already have it.

What kind of alcohol are you using, and at what proof? You can buy Everclear at 190 proof (95% alcohol, 5% water). An 80 proof spirit is 60% water. Then there is the question of how much alcohol is in your recipe.

Many states regulate the maximum alcohol content - in Massachusetts it was 1% last I looked. Some states require that buyers be 21+.

Even with a lot more info, I don't see a simple way to arrive at an answer.

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@DiscoverChoc

I know that the addition of alcohol to a ganache increases shelf life, but I would like to improve my understanding of the relation of this fact to the measured activity of water. In other words, when I take a reading of the Aw of a ganache that contains alcohol, does the resulting figure include the impact of the alcohol or not? It would seem at a quick glance that alcohol, containing water, might actually increase the reading. If the Aw figure does not take into account the effect of alcohol, is one reduced to guessing what difference it makes in shelf life or is there a way to calculate it? Any clarification would be appreciated.