Published by Jeremy. Last Updated at November 9, 2021.
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If you have opted to use freezer bags instead of vacuum bags for sous vide cooking, you have an extra step on your hands- getting the air out of the bag after you put your ingredients in it.
Air, much like most foods we cook in sous vide, makes bags float when they are in the water bath. As sous vide works best when our bags are surrounded by water at our ideal temperature setpoint, it is our job to get as much air out of the bags as possible to get the bag to sink (which is one reason why vacuum bags are so popular for this cooking method).
So, how do we do it? With the water displacement method. In this one, we share a step-by-step guide on how to do this.
Note: Only use freezer rated bags or vacuum bags when cooking with sous vide. Regular zipper bags, like sandwich bags, are not constructed well enough to withstand being submerged in water or at elevated temperatures. You can read more on why we recommend a vacuum sealer for sous vide cooking here.
What is the Water Displacement Method?
The water displacement method is a simple way to force air out of your freezer bags. When a bag is submerged in water, the weight of water is heavy enough to displace the air from inside the bag to above the water level.
Another way to think of this is if a vacuum sealer pulls air out of the bag, the displacement method is water pushing the air out under its own weight. It really is that simple.
All you have to do is slowly insert the bag into the water bath (keeping the zipper and opening above the water line), and you'll quickly see the bag effectively collapse onto itself (and your food). As this occurs, all the air that was once there is being pushed up to the surface and out of the bag. The more the bag is submerged, the more air is pushed out, and the less buoyant the bag becomes.
The trick here is getting as much of the bag submerged as possible while also keeping the opening at the zipper above the water level to avoid getting any water inside the bag- both water and air need to be outside of the bag. It takes a few times to really get this down, but we do have to recognize that it can be somewhat challenging too as air will always collect at a high point- which could be a small bubble in your bag.
For example, with the roast in these images, the moment I stopped pushing down on the meat to use both hands and zip, it started to float and the air shifted to a pocket away from the seal. I sealed the bag, pulled it out of the water, and noticed the bag still had some air in it. Take two. This time I forced the bag down and zipped again and did not have as noticeable of an air pocket.
This presents a few issues worth discussing:
First, obviously, is getting water into your bag on accident. The easiest way to prevent this is to have your freezer bag 80% sealed except for a small part at one side or the middle, and using that as your pinch point for forcing air out (as seen in the photo above). This allows for better control overall.
The second is that your fingers will get dangerously close to the water level. This is a concern if you are using the water displacement method in your sous vide vessel with the water warmed in excess of 130 °F as most people consider this temperature to be uncomfortable. Anything above 150 °F is generally considered the threshold for pain. So for cooks higher than 130-150 °F, you may risk hurting yourself if you are not careful (and is all the more reason to upgrade to a vacuum sealer if you do higher temperature cooks).
Finally, as we discuss many times on this site, freezer bag zippers can be a point of failure. One reason for this is if any food (likely salt or pepper) dirties up the seal when you add it into the bag. Make sure the zipper is clear of any food debris that could prevent sealing issues as this may let in air (or even worse, water) after the fact.
One alternative to avoid this is to add your food into the sous vide before the temperature reaches this setpoint (so add it in as the water is rising but before it hits 140-150 °F). As we showed in our sous vide wattage calculations, you will likely increase water temperature 10 degrees every 5-7 minutes pending the rating of your unit- so the water bath will continue to rise to set temperature fairly quickly (especially if you start with hot water from your tap).
A second alternative is to perform the water displacement method in a separate bowl of water at a more comfortable temperature (say, room temperature) as illustrated in our photos. Just don't go sticking your fingers into the water at 140 °F- you will not find it pleasant.
Finally, it is worth noting that even with the best application of the water displacement method, you may find that your bags are still buoyant for some recipes- partly from the ingredients and partly from the little bit of air that stays in the bag. If your favorite recipes have floating issues, you may have to add an external weight (we've use a wooden spoon or even a stone pestle) or buy a weighted rack designed for sous vide to hold the bags into place (visible in the image above).
At this point, though, you start running the risk of submerging the zip portion of traditional freezer bags which, in our experience, can sometimes be a point of failure. As such, if this happens to you regularly it could be a sign to upgrade to a vacuum sealer system outright. This is exactly what we did.
Still, with a little practice you'll get the hang of the water displacement method in no time!
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