Water Activity for Controlled Foods

The Food Processing Center Author

03/27/2018 Added

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This lecture discusses the concept of water activity and how it is used to preserve foods such as baked goods, jams and jellies, honey and syrups. Water is present in all foods and usually takes the form of free water, or water that is bound to different molecules. This lecture also describes how water activity affects the growth of bacteria, yeast and mold, which can affect the safety and quality of foods.

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[00:00:05.720]Okay, well, we'll move onto our next lecture.
[00:00:08.790]Welcome to the Science of Safe Foods.
[00:00:10.785]This lecture will cover water activity controlled foods.
[00:00:14.150]It's a little bit shorter lecture,
[00:00:16.380]and then we'll move onto low acid and acidified foods.
[00:00:20.439]So, this is our basic outline for our talk.
[00:00:23.190]We'll talk a little bit about food preservation,
[00:00:25.300]go over some definitions, some regulations,
[00:00:27.890]and exemptions.
[00:00:29.190]A little bit on jams and jellies since
[00:00:31.310]that's probably the most common water activity
[00:00:34.130]controlled product that's sold at the farmer's markets,
[00:00:36.520]and then we'll have a few processing considerations as well.
[00:00:40.434]So, to start with a little bit on the history
[00:00:42.730]of low water activity foods.
[00:00:45.770]So, baking has been arounds since 4,000 BC.
[00:00:51.246]Unleavened breads were first produced by the Egyptians,
[00:00:54.650]the Greeks, and the Romans,
[00:00:56.070]and they further refined that process.
[00:00:58.520]Smoking has also been part of food production.
[00:01:02.570]Native Americans in the northern parts of
[00:01:05.030]the United States, they used smoke,
[00:01:07.350]they used dry circulating air from a fire to dry meats,
[00:01:11.790]fish, herbs, and vegetables.
[00:01:14.419]Also, we've just had drying itself,
[00:01:17.200]so maybe drying items out in the sun.
[00:01:19.560]So, the Greeks and Romans, they dried peas and grapes.
[00:01:23.010]In the Near East, they preserved fruit by wrapping
[00:01:25.770]it in dried palm leaves and burying it in hot sand to dry.
[00:01:30.566]And then sugar, sugar has also been used.
[00:01:33.580]In the 19th century US, women preserved fruit in sugar.
[00:01:38.170]So, again, a long history of understanding that water
[00:01:41.510]is an important part of food safety,
[00:01:44.292]if taking it out can help extend that shelf life.
[00:01:49.680]A few more processes that have been developed over
[00:01:53.510]the years, curing was first developed by the Chinese
[00:01:57.080]and they used those on pork products.
[00:01:59.600]Brining, brining meat and fish, this has also been used.
[00:02:04.500]Again, we know that salt's gonna be binding up that water,
[00:02:07.960]so it's unavailable for bacteria.
[00:02:10.430]Freeze-drying, believe it or not,
[00:02:12.760]freeze-drying has been around for a while too.
[00:02:15.340]In the 15th century, the Incas of Machu Picchu,
[00:02:19.361]they would leave their product out in the cold
[00:02:22.650]mountain temperature.
[00:02:23.670]And so, that water would slowly evaporate.
[00:02:26.448]Essentially, you've got a freeze-dried product
[00:02:29.440]in a natural environment.
[00:02:31.974]So, water activity, we went over this a little bit
[00:02:36.400]in the earlier lecture.
[00:02:38.820]It's actually based on a formula.
[00:02:41.739]The top number, this P is the vapor pressure
[00:02:44.910]of water above a food.
[00:02:46.490]The P sub zero is a the vapor pressure of pure water
[00:02:49.380]at the same temperature.
[00:02:51.470]The water activity of pure water is one.
[00:02:54.909]Essentially what the water activity is gonna do
[00:02:57.420]is that's gonna be a measure of the available water
[00:03:00.360]in the food.
[00:03:03.700]So, if we look at that on a curve,
[00:03:08.810]our water activity of pure water is one.
[00:03:12.400]When we get down to the lower sections,
[00:03:14.730]or the lower values of water activity,
[00:03:17.653]such as below point two,
[00:03:19.610]we have a very strongly bound monolayer of water.
[00:03:23.360]As we move our way up, that water becomes freer and freer
[00:03:26.930]and more available for microorganisms.
[00:03:29.030]Until finally we get to a point where we have free water
[00:03:33.550]and that's where we see the microorganisms start to grow.
[00:03:38.111]In this slide we show, schematically, how salt and
[00:03:42.770]sugar might affect water.
[00:03:44.680]So, in this first diagram, we see that the water
[00:03:48.790]is tightly, or the sugar and water are tightly bound.
[00:03:53.520]As the sugar and water components become less and less,
[00:03:57.930]then we have more unbound water that's available
[00:04:00.760]for microorganisms to use.
[00:04:04.180]We hear this term water activity a lot.
[00:04:07.480]And you get your sample, you send it off to a lab
[00:04:10.090]so you can have that result back but how is that measured?
[00:04:13.540]It's measured by a device that's called
[00:04:16.120]a Water Activity Meter.
[00:04:17.920]And this is one of the more recent ones produced by Decagon.
[00:04:23.010]Any ideas on how much that one, that instrument costs?
[00:04:26.935]100 G.
[00:04:28.140]100 G?
[00:04:30.527]Eh, not quite that bad.
[00:04:33.007](laughing)
[00:04:35.270]Any other thoughts?
[00:04:36.220]25.
[00:04:37.580]25, boy, you guys are really, really up there.
[00:04:41.481]Five?
[00:04:42.870]A little bit higher than that,
[00:04:44.520]so this runs about $10,000.
[00:04:46.860]So, that's why people usually send it out
[00:04:50.189]if they're not doing regular water activity measures,
[00:04:51.560]they send it out to a lab for testing.
[00:04:55.407]So, this table gives an approximate water activity value
[00:04:59.580]of some common foods.
[00:05:01.200]So, we can see a meat product could be up around 96,
[00:05:04.572]as we work our way down to dried powder milk,
[00:05:07.630]we could be at .7.
[00:05:10.323]Our fudge sauce, which is kind of in the middle of
[00:05:14.058]this table, is at .83.
[00:05:16.350]And remember, we see that those regulations are,
[00:05:19.600]if you wanna be exempt from acidified food regulations,
[00:05:23.180]the water activity of that product would be below .85.
[00:05:27.370]So, a lot of your products like syrups, or honey,
[00:05:30.480]fudge sauce, those are gonna be exempt from the regulations
[00:05:34.670]because of their low water activity.
[00:05:39.070]Again, another table, I just wanted to point out
[00:05:41.750]on this particular table, jams and jellies typically
[00:05:45.640]run anywhere from about .8 to .85,
[00:05:49.640]somewhere in that range.
[00:05:54.310]So, on this slide I'm showing a little bit on
[00:05:57.267]the relationship of water activity and moisture,
[00:05:58.410]just to give you an idea that moisture is different
[00:06:02.000]than water activity.
[00:06:03.700]So, water activity, that's the amount of water
[00:06:06.869]that's available for a microorganism to use.
[00:06:09.980]Moisture level, or moisture content,
[00:06:12.930]that's the entire water that's in that product.
[00:06:18.190]So, if we pick out one of the products on both
[00:06:21.300]of these tables, this is the food and the water activity.
[00:06:24.910]This has the food and the percent water.
[00:06:27.330]So, if we look at jams, jellies, and marmalades,
[00:06:29.812]water activity is .75 to .8.
[00:06:33.670]Jams and preserves, the percent water could be 30%.
[00:06:37.660]So, do those numbers relate?
[00:06:39.800]They don't, there's really no relationship
[00:06:41.940]between those two.
[00:06:42.773]So, if somebody's talking about moisture level,
[00:06:45.210]they're not talking about water activity.
[00:06:47.520]Two separate animals, essentially.
[00:06:53.284]Navigating regulations, the FDA has specific rules
[00:06:57.578]on water activity and pH regarding a food product.
[00:07:02.179]So, in this first one, if your product is below .85,
[00:07:07.520]like your baked goods, jams, jellies,
[00:07:11.090]like we talked about, syrup, fudge sauce, honeys,
[00:07:13.960]those types of things, those are gonna be exempt
[00:07:17.330]from certain regulations.
[00:07:19.510]However, you still have to use good manufacturing practices.
[00:07:23.470]All of your utensils, everything you use,
[00:07:25.610]have to be cleaned, sanitizing, good employee hygiene,
[00:07:29.350]those kinds of things.
[00:07:31.770]If your water activity is above .85 but
[00:07:34.410]your pH is below 4.6,
[00:07:36.928]you still have to have good manufacturing practices
[00:07:40.180]but you may be also following those
[00:07:42.450]acidified food regulations.
[00:07:45.370]If your water activity is above .85,
[00:07:48.540]your pH is above 4.6, now you're getting into the area
[00:07:52.860]of low acid canned foods.
[00:07:54.984]So, that'd be like your Campbell's soup or
[00:07:58.680]your canned vegetables, those kinds of products.
[00:08:03.370]In acidified food, we'll see this definition next
[00:08:05.985]talk as well, this is right out of the regulations.
[00:08:10.000]And acidified food is a low acid food to which acid
[00:08:12.870]or acid foods are added to produce a product
[00:08:15.570]which has a finished equilibrium pH of 4.6 or less
[00:08:19.250]and a water activity of greater than 0.85.
[00:08:23.600]So, this is our key for our low moisture products.
[00:08:29.730]Exempt products, jams, jellies, syrups,
[00:08:32.300]as I mentioned before.
[00:08:35.562]So, if you're looking at the home-based processor
[00:08:38.170]who might be selling at the farmer's markets,
[00:08:40.085]you could be doing whole fruits and vegetables,
[00:08:42.730]mixed greens.
[00:08:44.400]These are what I maybe call maybe semi-processed or
[00:08:50.973]processed for shelf stability
[00:08:54.480]based on the way you make the product.
[00:08:56.800]These are all okay to be selling at the farmer's markets.
[00:09:01.870]And again, this is based on their intrinsic factors
[00:09:04.800]of pH and the low water activity that they have.
[00:09:10.670]So, a little bit on fruit jellies.
[00:09:14.510]Why are these particular products so stable?
[00:09:18.494]It's because of their solids content,
[00:09:20.770]which could be around 65%, which is required
[00:09:24.800]due to the sugar, fruit, pectin,
[00:09:28.103]and because of the acid content,
[00:09:29.020]primarily citric acid that's either being added
[00:09:31.670]or coming in from the fruit.
[00:09:33.840]So, they have a very low pH around 3.1, 3.2.
[00:09:38.966]Four major ingredients, we've got sugar, acid,
[00:09:42.940]pectin, and water that are all contributing
[00:09:46.060]to the composition of that particular product.
[00:09:52.920]I found this online, I thought this was really cool.
[00:09:56.987]I think it's, if you're interested in maybe learning
[00:09:58.210]more about the science, you can go and download this
[00:10:01.230]PDF file, it's from CompoundChem.com.
[00:10:05.420]And it's the chemistry of jam making.
[00:10:08.120]So, I went ahead and copied it and
[00:10:09.630]put it into this presentation because it was really nice
[00:10:12.500]how they put all of this chemistry together.
[00:10:15.700]We make, I don't make jams and jellies,
[00:10:18.610]I know some of you folks do and some of your family.
[00:10:22.020]I had a grandmother that made lots of jams and jellies
[00:10:24.740]that was really good.
[00:10:27.703]But this was really nicely put on how everything
[00:10:30.740]kind of comes together for making this type of a product.
[00:10:34.960]If we look at this chart, we have sugar and
[00:10:38.870]we have the fruit acids.
[00:10:42.910]These contribute to the firmness and the rigidity.
[00:10:45.910]It's gonna help form that gel structure.
[00:10:49.113]It's gonna draw, the sugar itself is gonna draw
[00:10:52.340]the water away, it's gonna bind the water,
[00:10:55.170]and as long as you have your final sugar content
[00:10:59.470]between 65 and 69%,
[00:11:01.930]then you're gonna have a good gel structure.
[00:11:05.300]For the fruit acids, you want your pH somewhere
[00:11:08.820]between 2.8 and 3.3.
[00:11:12.520]And again, that acid is gonna be critical to help
[00:11:15.150]form that gel structure.
[00:11:18.200]Regarding the pectin, pectin is also an interesting molecule
[00:11:23.913]in that the chain is gonna bind to itself
[00:11:27.800]and it's gonna form a gel network and trap the liquid.
[00:11:31.490]So, it's gonna trap water,
[00:11:32.770]so it's gonna be unavailable to the bacteria.
[00:11:36.530]With all of these different chemicals that are
[00:11:39.030]found in jams and jellies,
[00:11:40.900]we have an extremely stable product.
[00:11:44.400]One comment on standards of identity,
[00:11:47.540]in 1940, the FDA created Standards of Identity
[00:11:50.980]for jams and jellies, and these are all listed here.
[00:11:54.720]And if you're making jams or jellies,
[00:11:57.620]it's highly recommended you try to make it
[00:11:59.380]according to a standard identity because
[00:12:01.690]that's based on how it's made, it's inherently safe.
[00:12:06.636]You can't really see the numbers in this chart too well
[00:12:10.030]but I wanna point out that this is available
[00:12:11.970]through the University of Nebraska extension website.
[00:12:15.750]This is a neb guide and it was written by Dr. Durward Smith,
[00:12:21.850]who's a retired faculty member from our department,
[00:12:24.400]and the title is
[00:12:26.286]Fruit Jellies Food for Entrepreneurs Series.
[00:12:29.150]And in this, it has this table along with instructions
[00:12:32.650]on how to make standard of identity jellies.
[00:12:37.330]If you're doing that type of thing,
[00:12:38.630]you might wanna put this up because
[00:12:41.618]it will be a really nice reference for you.
[00:12:44.360]And here I just have underlined peach.
[00:12:47.189]So, you just pick out which type of fruit you're using
[00:12:48.687]and it will give you all the parameters you need
[00:12:51.470]to make the standard of identity.
[00:12:54.650]The other thing that the Neb Guide does is it gives
[00:12:57.500]you some processing considerations as well,
[00:12:59.630]such as how to hot pack, what temperatures you need
[00:13:03.300]to be at, and checking the pH to make sure
[00:13:06.750]you're at sufficient pH, not only for safety,
[00:13:10.170]but to make that gel structure.
[00:13:12.289]It also talks a little bit about the processing part
[00:13:15.630]and the use of a refractometer to make sure
[00:13:18.650]you're at the correct solids and the correct Brix.
[00:13:25.350]And this is in your notebook,
[00:13:27.210]so you can actually go directly to IANR Pubs
[00:13:31.510]and download that document.
[00:13:34.695]To summarize, food preservation has been around
[00:13:39.950]for thousands of years, and the primary way that
[00:13:43.220]we reserved it is to remove water,
[00:13:45.380]as we've seen in a lot of different categories.
[00:13:49.600]Jams and jellies, as long as you're making them
[00:13:51.970]according to standard of identity,
[00:13:54.080]they're gonna be inherently safe.
[00:13:56.240]I'd highly recommend if you're doing those types
[00:13:58.220]of processes to check, make sure that you're making
[00:14:01.480]according to a standard of identity.

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Water Activity for Controlled Foods

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