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How Safe Is Your Food? Investigating the effectiveness of odor-proof bags
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Kyle Meyer

Locale: Portland, OR
Re: How Safe Is Your Food? Investigating the effectiveness of odor-proof bags on 04/10/2013 09:51:42 MDT Print View

One thing missing from this study is proximity. I certainly understand dogs (or any creature) identifying food when it's literally inches from it's nose, but I'm curious what the rate of detection would have been from 3 or even 6 feet away, emulating the distance a bear bag would be from the snout of a bear. Additionally, trapping odor in a locker with the ability to extract the air easily via a vent would seem to enhance the ability for a dog to detect the bags.

In open air, at 6 feet away, my guess is the results would be very different.

That said, thank you so much for doing this study. We need more of this on BPL.

Daniel Pittman
(pitsy) - M

Locale: Central Texas
Somewhere... on 04/10/2013 10:01:52 MDT Print View

Out there waiting for you, is a bear. He can smell what you're thinking. He can smell what the weather is going to do. He can smell your car's tire pressure. He can smell that your bike isn't shifting right. He can smell how Stonehenge was built. He can smell the origin of the Universe. He can smell everything.

Courtenay Ennis
(courtenayennis) - MLife

Locale: Southwestern BC
Opsak seal sucks on 04/10/2013 10:10:02 MDT Print View

It's great to know there is little practical difference in odourproofness between Opsak and Ziplok.

One observation from using both:

Odour permeability aside, the actual seals on Opsak bags are, in my experience, terrible. The bags can open in packs far more readily than Ziploc seals. Even if the odourproofness was perfect, it's a huge problem that the seals are so weak. I was putting everything in my Opsaks in Ziplocs, first, as a backup, but now I'll just forego the Opsaks altogether.

If Opsak could greatly improve their seal, I might still buy them, in spite of this study, for the additional piece of mind, but in lieu of this study, with their current seals... no point whatsoever.

Thanks for the great study! Love the care taken in designing it.

John Coyle

Locale: NorCal
How Safe Is Your Food? Investigating the effectiveness of odor-proof bags on 04/10/2013 10:28:12 MDT Print View

This article has saved me money, because I don't see any reason to buy OP sacks again. Hard as it is to take for light and ultralight backpackers, it appears the the only sure way to prevent food from being destroyed by bears or other animals is to use a hard sided bear canister. Hats off to BPL for publishing this article, because you are going to get some flak from OP sack companies, I can guarantee you that.

Rob E

Locale: Canada
distance+wind on 04/10/2013 10:32:34 MDT Print View

If a bear is a few feet from food in an odour proof bag I have no doubt that it will zero in pretty quickly.

To me the value of the odour proof bags are when the bear is a few hundred meters away, or some wind. The odour proof bags might possibly mute the smell enough to make it uninteresting in comparison to other natural (or unnatural sources), or possibly increase the difficulty in vectoring in on the source. If nothing else my opsak is coming along anyway to mask the smell of esbit, a task it does admirably.

I look forward to scorning odour proof bags becoming BPL dogma. I will add opsaks to my growing list of things I do or use but don't mention here any more, including but not limited to: trekking poles, hydration bladders, light hikers, white gas stoves, fleece, knife instead of scissors, bear spray, venturing out in the rain, drinking gatorade and using toilet paper.

(Edit: full disclosure, as a forum member, I did not read the article)

Edited by eatSleepFish on 04/10/2013 10:44:38 MDT.

Hk Newman
(hknewman) - MLife

Locale: Western US
Re: How Safe Is Your Food? Investigating the effectiveness of odor-proof bags on 04/10/2013 10:33:50 MDT Print View

Interesting, though the cover photo is reminiscent of American high school locker sweeps since the late 1970's (kind of had a surprise opening up BPL this AM). By the time I slather blueberry preserves and peanut on a bagel for Lunch #1, I figure these smells coat the outer part of any baggie regardless. Still a very good study but wonder what the lifespan of a ziploc vs. odor-proof bag is if washed after every trip. Got my odor-proofs with my Ursack, occasionally wash and reuse it, whereas with ziplocs they are tossed (unless green-minded girlfriend insists on washing and reusing them.. yeah, she wasn't even a hippy chick either). Think most toss Ziplocs, so further study is likely pointless.

Herbert Sitz

Locale: Pacific NW
Re: distance+wind on 04/10/2013 10:46:15 MDT Print View

I tend to agree with Rob E. I no longer have premium membership to read the BPL article, but it seems to me there is a big difference between testing at close range and at more of a distance.

I know I can reliably and quickly distinguish between a faint odor and absence of odor at very close range. As distance increases that ability disappears. I see no reason why this would be different with dogs or bears. And I'm pretty sure that as distance is added the ability to discern odor from an OPSAK would fail more quickly than ability to discern odor from a ziploc.

I csn personally confirm that there is a big difference between the amount of odor that escapes a regular ziploc and what escapes an OPSAK. I can't smell odor from OPSAK even at close range, although of course I'm not surprised that a dog or bear could. But increase distance of dog or bear from ziploc and OPSAK to 10 yards, 100 yards, a quarter mile, or longer, and I suspect at some distance the animal will be able to smell contents of the ziploc but not be able to smell contents of OPSAK. Of course that's much harder to reliably and easily test.

I say all of this with belief that bears have powers of smell greater than any dog. I believe there are studies that show that, although for now I'm satisfied with this from Wikipedia: ". . . the Silvertip Grizzly found in parts of North America, ha[s] a sense of smell seven times stronger than that of the bloodhound. . .". Wikipedia entry on Olfaction

Edited by hes on 04/10/2013 10:53:47 MDT.

Will Tatman
Opsaks on 04/10/2013 11:02:37 MDT Print View

Well done article, I appreciate the thoroughness.

As someone who uses OpSaks exclusively for food protection, I will now have to do some further thinking about food selection and packaging. As a vegan, I don't interact much with Cheese or Salami, which I assume might hold the titles for 'stinkiest trail foods' But I do hike with Peanut Butter, a top contender. I tend to just throw a jar straight into an Opsak along with my day-to-day meals (themselves in a gallon bag per day), and one extra ziplock full of trash. At the end of the day, I seal up the opsak, wrap it in a wind jacket, and use it as a pillow. Same method I used on the GET last year with no issues. So, not terribly comforting to read this article, but until I know more about how proximity, layering, and food selection factor in, not enough to deter me from continuing to use OpSaks instead of hanging or canisters (at least in the desert/mountain SW).

John Nausieda
(Meander) - MLife

Locale: PNW
Re: Opsak seal sucks on 04/10/2013 11:02:39 MDT Print View

There are ways to beef up the sealing of an Opsak by using this product at Amazon.

Also wondering if Nylofume bags were mentioned or could be studied in the future.

Stephen Barber
(grampa) - MLife

Locale: SoCal
"odor-proof" bags vs ziplocks on 04/10/2013 11:04:46 MDT Print View

@ Rob and Herbert:

Since you didn't read the article, let me summarize it: There was no real or statistical difference in the dog's ability to locate drugs enclosed in either a ziplock or OP bag.

If there is no difference at close range, enclosed in a locker, indicating that about the same quantity of scented molecules are escaping each type of bag, I don't see how you can expect a difference at a distance when the food is enclosed within a bear bag or pack?

Billy Ray
(rosyfinch) - M

Locale: the mountains
odor proof bags may still do some good on 04/10/2013 11:07:02 MDT Print View

If an 'odor-proof' bag just slows down the emissions of smells, then it may do you some good. Reduced emissions my reduce the distance at which bears can pick up a trace of things inside. In other words, a bear may need to be, say, closer than 100 yards to pick up the smell of something in any plastic bag. Whereas a bear a half a mile away may pick up the scent of food outside of a bag. So it could make a difference as to whether a bear comes to your camp or not.

In addition, it seems to me the above study did not determine if the Op Sack could reduce odor emissions to a lower level than that of a regular zip lock. It may be that it does, but both were over the threshold of what was required for the dogs to pick up the scent in such a small area as was used.

That's my 2 cents.

Using the above logic it may be a good idea to line your bear canister with a plastic bag.


Andrew Zajac

Locale: South West
great study on 04/10/2013 11:07:14 MDT Print View

I think this study was great and shows that the most important aspect of avoiding negative interactions with wildlife is your behavior. I also really appreciated the inclusion of some statistics. Speaking from the perspective of finishing a masters thesis in ecology, statistics can get nit picky to a ridiculous level. This is usually (from my perspective) to no appreciable gain in understanding or interpretation of the data, so I won't nitpick methods. However, there are accepted ways of reporting these statistics that allow the reader to know what tests were performed and what the results are. These can be found with a quick google search and their inclusion would be much appreciated.

David Wood
(RedYeti) - MLife

Locale: South Eastern UK
OP Sacks help but they are not really OP on 04/10/2013 11:08:12 MDT Print View

I've carried a small ziplock bag of curry spices for livening up meals in the past - trouble is everything in the pack smells of curry by the end of a weekend (it should be obvious I'm not talking about bear country here).

However, placing the spices in an small OP sack completely removed the problem.

Interestingly though - I could just smell the spices through the OP sack if I sniffed at it up close.

I've also used them when carrying out empty cans of fish we'd eaten on really long hikes (multi-week) where we could resupply at huts regularly but only dump rubbish occasionally and hadn't good access to detergent based cleaning. The smell of three day old cans of fish cannot be disguised easily in a pack. The OP sacks solved that.

Just pointing out that (as Ari mentioned above) it's not that they have no effect at all, just that it's not 100% as they claim. There are uses for them, and maybe they even reduce the chances of a bear catching the scent - but that's likely to remain conjecture.

Heather Branch
thanks for the study! on 04/10/2013 11:17:29 MDT Print View

I debated whether to renew my BPL membership. This article alone makes me glad I did! I will continue to use my Backpacker's Cache bear barrel [with liner bags and twist locks], despite its weight. I am glad to see I am not the only one who found the OPSak closures terrible. It would be smart if they improved the closures. What I do is put essential food in my bear barrel, put the barrel well away from my site at night, and start out with overflow I could survive without [e.g. coffee and my extra night freeze dried food package] in an OPSack hung in a very light cloth bag at bear bag height etc dimensions or as close as possible. Coffee is pretty essential for many of us, I know, but I always put caffeine pills in the barrel as backup:)

Mary D
(hikinggranny) - MLife
My own experiment with OP sacks and dog on 04/10/2013 11:23:19 MDT Print View

I did a similar experiment a few years ago, using dry dog food with zippered freezer bags (Ziploc brand) and with OP sacks (Loksak brand), and my Labrador/Golden Retriever cross dog (service dog reject). I was concerned because after I packed my dog's kibble into single-serving freezer bags, I could smell the food through the bag, and my nose isn't very sensitive. My results contradict this study.

I tried two layers of freezer bag. That worked for me, but when I left it on the floor, my dog immediately pounced on it and tried to chew into the bags.

I then put a single freezer bag with dog food inside an OP Sack and left it on the floor. My dog walked right by it several times without noticing the smell.

I ran this trial several times, with pretty much the same results. The only difference was that my dog became conditioned to associate a plastic bag on the floor with food, so after a couple of replications he started sniffing the OP sack before moving on. He still did not detect the food inside.

Admittedly, my dog, although of a hunting breed, has never been specifically trained to follow scents, but his food, for him, was an extremely powerful motivator. To motivate him more, I ran the tests just before dinnertime (which caused a lot of barking). Since he didn't smell his food inside the OP Sacks, I had to assume that the OP sacks are at least somewhat effective. The single freezer bag plus OP sack was definitely far more effective than doubled freezer bags.

It may be that those "controlled substances" have a more powerful smell than dry dog food. Or it may be that my dog has a lousy sense of smell. But at least with the OP sacks, I didn't have to worry about my dog getting into his or my food!

Since bears reportedly have a much stronger sense of smell than do dogs, both this study and my one dog study may be moot. However, I have read several anecdotal reports from reliable persons of a bear walking right by a hanging OP sack without investigating it (in most cases the OP sack was inside an Ursack). Maybe the bear wasn't hungry? Or had a head cold?

I will continue to use odor-proof sacks, although I would never rely on them alone, only as a supplement to my Ursack or canister. Nor would I use them to store food inside my tent. But I have demonstrated to my own satisfaction that OP sacks keep my dog out of the food!

PS: I fully agree with others above about the OP sack seals; they're horrible! I plan to try the Nylafume bags, although my dog won't be with me this summer.

I'd love to see the results of a similar trial using mice or other rodents. I had one get in my car one time while I was on a 3-day backpack trip; it ate most of an apple I left on the front seat. It also evidently investigated an OP sack in the back of the car that contained food for a subsequent trip. There were mouse droppings nearby, but no evidence the sack had been chewed. I never figured out how the mouse got in my car, but another backpacker reported the same thing happening at the same trailhead a week before.

Edited by hikinggranny on 04/10/2013 12:14:26 MDT.

Herbert Sitz

Locale: Pacific NW
Re: "odor-proof" bags vs ziplocks on 04/10/2013 11:37:19 MDT Print View

Stephen -- The test is what I assumed it was. That test does not indicate that the "same quantity of scented molecules are escaping each type of bag". It indicates, or is at least consistent with, there being enough molecules escaping each type of bag for both to be easily detectable by the animal, even for them to seem roughly equally strong at small distances. As distance increases, however, if one bag is allowing fewer molecules to escape then it could become undetectable sooner than the other.

I'm not saying I know this is the correct explanation for the test's results. I'm saying that the above explanation is consistent with (1) common sense, (2) my own experience that OPSAKs let out less odor than ziplocs, and (3) the BPL test.

To claim that there is no difference between OPSAKS and ziplocs is inconsistent with my own experience (there's a clear difference for me and other people), so it's hard for me to believe that there is "no difference" between them for dogs and bears. I'm not saying the test was wrong; I'm just suggesting that the conclusions people are drawing from it are stronger than the results of the test warrant. That is, there is another explanation for why there seems to be no statistical difference b/w OPSAKs and ziplocs for dogs at close range, and that explanation squares better with my own experience (and with common sense that as you make a barrier stronger (e.g, double bag vs. single bag ziplocs), fewer items will cross it).

Edited by hes on 04/10/2013 11:42:28 MDT.

Rob E

Locale: Canada
Re: "odor-proof" bags vs ziplocks on 04/10/2013 12:22:20 MDT Print View

Stephen, the difference between the ziplocks and the odour proof bag has to do with absolute detection levels and dilution. Close enough the scent density coming from both bags is higher than the absolute detection level of the dog, but further away when you have dilution and spreading, it might become significant enough to drop below the absolute detection level.

I will use a rough analogy, so bear with me (pun intended): If I can hear both a whisper and an airhorn from 1 foot away, should I expect to be able to hear both 500 feet away? Up close sound volumes from both sources are above the absolute detection level of my ears, but further away the drop in energy manifests itself as an inability to hear the whisper.

Edited by eatSleepFish on 04/10/2013 13:00:14 MDT.

Willie Evenstop
(redmonk) - F

Locale: Greater Yellowstone Ecosystem
How Safe Is Your Food? Investigating the effectiveness of odor-proof bags on 04/10/2013 12:29:58 MDT Print View

Once the dogs knew there was something to find, they worked hard for their reward.

At first the dogs were playful, but once conditioned to realize they were in the room until they found the drugs....

Dogs are highly conditioned to please their worker.

How many false positives were there in a locker room with no drugs given the same conditioned dogs ?

Edited by redmonk on 04/10/2013 12:32:33 MDT.

Bruce Warren
(Aimee) - F - MLife

Locale: South Texas
Raccoon test for Investigating the effectiveness of odor-proof bags on 04/10/2013 12:36:34 MDT Print View

I expected that result. I bought a pack of those bags 6 years ago. After working at Dow Chemical for a while, I was under the opinion that no thin plastic polymer will block the passage of aromatic hydrocarbons for long. Only metal does. I did a raccoon test on my porch using the aluminum coated anti-static bags sold for storing circuit boards. See them at, they are much cheaper and come in lots of sizes. They also totally block moisture and do a good job on blocking oxygen. I have stored crackers in an antistatic bag for two years and they were not stale.

I carefully put cat food inside the anti-static bag and made sure nothing touched the outside. I zipped it up good and laid the bag on the porch. Oscar the Raccoon had been eating the cat food from the bowl for a few weeks. With no food in the bowl that night, I checked the next morning. I saw muddy paw prints on the anti-static bag, but he did not chew on it at all. That told me that no bear will smell it either. But one caveat, after using an aluminum coated bag a few times the aluminum coating will get scrapes or maybe tiny holes. I wrinkled a bag all over for a few minutes and dragged it over my gravel driveway a few feet. Repeating the same Oscar test, he ripped open the bag and ate all the food. There were no holes when I did a water test. So put your anti-static bag inside another bigger one when you use it; making two layers, treat it gently and it will last a long time with no odor leaks

Jay Wilkerson
(Creachen) - MLife

Locale: East Bay
How safe is your food? Investigating the effectiveness of odor-proof bags. on 04/10/2013 12:40:12 MDT Print View

Excellent read and very informative! Wonder if freezer bags make a difference?