Water Displacement Method for Sous Vide Bags

The water displacement method removes the air from a zipper-lock bag without a vacuum sealer. You close the zipper except for one corner, lower the bag into water so the water pushes the air out through the gap, then seal the corner at the surface.

It needs a zipper-lock freezer bag and a pot of water. The bag then goes into the sous vide bath like a vacuum-sealed pouch. The rest of the cook, from setting the bath to searing, is in how to sous vide.

How to seal a bag with the water displacement method

These steps combine the instructions in Anova's guide to the method and Jason Logsdon's guide to zipper-lock bags at Amazing Food Made Easy.

1. Fill the bag. Put the food in a zipper-lock freezer bag in a single layer. Seasonings, liquids and marinades can go in with it.
2. Close the zipper except for one corner. The open corner is where the air leaves.
3. Lower the bag into the water slowly. The water presses the plastic against the food and pushes the air up toward the gap.
4. Work out trapped bubbles. Massage the bag around the food as it goes down so air caught between pieces can escape.
5. Stop when the water reaches the zipper. Everything below the zipper line should be under water, with the open corner just above the surface.
6. Seal the corner. Press the last part of the zipper closed while the bag is still in the water.
7. Check for trapped air. Anova's guide says air bubbles left around the food cause uneven spots. If you see an air pocket, open the corner and repeat.
8. Clip the bag in place. Anova's guide says to clip the bag to the side of the pot so it does not block the circulator's pump.

Logsdon uses gallon-size bags and says the extra room at the top makes them easier to seal. Anova's guide also names gallon-size freezer bags.

Why air in the bag matters

Air causes two separate problems: it slows heating and it makes the bag float.

Heat transfer. Water moves heat into food far better than air does. Douglas Baldwin's guide to sous vide cooking puts the thermal conductivity of water at 23 times that of air. A pocket of air between the plastic and the food insulates that spot, so it heats more slowly than the rest. Anova's guide describes the result as uneven spots in the finished food.

Floating. A bag with air in it rises. Any food above the surface is not in contact with the water, so it heats unevenly and can stay below the bath temperature. Baldwin's guide states that pouches must be completely submerged, and not tightly arranged or overlapping, to prevent undercooking.

Undercooking here is a safety problem as well as a texture problem. Every time in the calculator assumes the whole piece is under water for the full cook, and the pasteurization times explained in is sous vide safe depend on the same thing.

Air is not the only gas involved. Baldwin's guide notes that at higher cooking temperatures pouches often balloon with water vapor and have to be held under water. Logsdon notes that food can release gas during longer cooks, and that you can open the bag, let the gas out and reseal it.

Which bags are suitable

Both step-by-step guides above specify zipper-lock freezer bags. The maker of one brand of those bags says not to use them for sous vide.

The sources disagree on heat limits, so here is what each one says.

The Ziploc FAQ describes sous vide as boiling in water, and gives as its reason the 230°F (110°C) softening point of the bags against the 212°F (100°C) boiling point of water. Sous vide baths run below boiling, as what is sous vide explains. The maker's instruction still applies to its own product.

America's Test Kitchen reaches a different conclusion based on the plastic. It reports that many name-brand bags are made of polyethylene and polypropylene, and it points to makers that publish what their bags are made of.

In practice:

• Read the box. Follow the maker's stated uses and heat limits for the bag you have.
• If you want a bag whose maker says it is suitable for sous vide, look for that statement on the box. SC Johnson makes it for its silicone bags and not for its polyethylene bags.

Vegetables need much higher temperatures than meat, according to Baldwin's guide, so the margin between the bath and the bag's limit is smallest for them. The vegetables chart uses 183°F (83.9°C) for every vegetable in the calculator. Check that against the limit on the bag before you start.

Some foods need no bag. Egg bites, cheesecake and creme brulee cook in glass jars, and the egg bites guide covers how tight to close the lids.

When a vacuum sealer is the better choice

Anova's packaging guide describes vacuum sealer bags as very sturdy and suggests them for solid foods and foods with protruding bones.

A vacuum sealer is the better choice for:

• Long cooks. Anova's packaging guide recommends double bagging zipper bags for longer cooking times. Tough cuts such as short ribs stay in the bath far longer than a steak.
• Higher temperatures. The same guide recommends double bagging at higher temperatures, and Baldwin notes that pouches balloon with vapor as the temperature rises.
• Bones. The guide lists rigid items such as exposed bones as a reason to double bag a zipper bag. Ribs are an example.
• Storage. Anova's packaging guide says vacuum bags extend the shelf life of cooked and uncooked food. If you cook ahead and chill, as the meal prep guide describes, or freeze portions to cook from frozen, a vacuum seal suits that better than a zipper.

The displacement method does work with liquid in the bag. Logsdon's steps include marinades and other liquids.

How to keep bags submerged

Start by removing the air again. If a bag that sank at the start is floating later, open a corner, lower it until the air or gas is out, and reseal it.

If it still rises:

• Use a rack. Baldwin's guide says to hold pouches under water with a wire rack or some other restraint.
• Clip the bag to the pot. Anova's guide clips the bag to the side of the pot, which also keeps it out of the circulator's pump.
• Add weight. Anova's packaging guide mentions weighting a bag with butter knives and recommends double bagging when you do.
• Give each bag space. Baldwin's guide warns against pouches that are tightly arranged or overlapping.

Long cooks need one more check. Two Instant Pot manuals, covered in the Instant Pot sous vide guide, address it. The Duo Evo Plus manual says to check the water level on cooks over 4 hours so the pouch contents stay submerged. The Duo Plus manual says to keep the seal of a resealable bag above the water.