Sous vide cooking has become almost inseparable from vacuum sealing—after all, sous vide is often translated as “under vacuum.” But despite that literal interpretation, food prepared sous vide doesn’t actually exist in a vacuum. The pressure inside a sealed bag is essentially the same as the pressure outside—it’s just that the air has been removed. A more accurate translation might be “without air,” which better reflects the role packaging plays in this technique.
Why Remove Air at All?
Vacuum packing is primarily used to extend shelf life, not necessarily to improve cooking outcomes. You’ll find vacuum-sealed foods all over the grocery store—from sliced meats and cheese to coffee and ready-to-eat meals. Removing oxygen from the packaging slows spoilage in two big ways: it limits oxidation (the same reaction that browns apples and rusts metal), and it halts the growth of aerobic bacteria, which need oxygen to thrive.
In sous vide cooking, vacuum sealing has practical benefits even when safety or shelf life isn’t a concern. For example, sealed bags tend to sink in the water bath, unlike unsealed bags, which float and need to be weighed down. Vacuum sealing also helps prevent “warmed-over” flavors caused by certain chemical reactions in reheated foods.
Still, sealing food tightly in a regular zip-top bag and using the water displacement method can work just fine for short cooking times. The main objective is keeping air away from the food. Air doesn’t conduct heat well, so getting rid of it ensures better temperature control and reduces water loss through evaporation.
Choosing the Right Packaging
We recommend bags that are strong enough to handle freezing temperatures and high-heat cooking, keep juices in, keep water out, and block gases from entering or escaping. There are many options online to choose from, but something as simple as a Ziploc bag can do the trick for simple, short cook times.
Sealing Food with a Zip-Top Bag
If you need to seal food along with a liquid, or if you don’t have a vacuum sealer, ordinary zip-top bags offer a handy alternative to vacuum sealing. This method allows you to quickly remove much of the air from the bag, and it doesn’t require much liquid in the bag to remove nearly all the air.
- Fill a bowl or sink with water, and place the food along with some oil or water in a zip-top bag. The liquid is needed to remove as much air as possible and to prevent the bag from floating. If you’ll be cooking the food immediately, there’s no need for an extra bowl: just use the warm water bath itself to seal the bag.
- Slowly lower the open bag into the water. The water squeezes the air out of the bag and presses the plastic tightly around the food.
- Seal the bag when the water level is 1–2 mm below the zip seal.
Troubleshooting Sous Vide Packaging Issues
Even with good equipment, things can go wrong. Here are a few common issues and how to fix them:
The vacuum ran but didn’t seal the bag.
- The bag wasn’t placed correctly over the sealing bar—try folding the edge back and smoothing it flat.
- The sealing bar wire might be unplugged—double-check connections.
The food wasn’t sealed tightly enough.
- The vacuum level may have been set too low.
- The food could have been too warm—chill it first.
- The bag may have a slow leak or weak seal.
The food shape was distorted.
- The vacuum level was too high—delicate foods need gentler pressure.
- Try wrapping the food in plastic wrap for extra support, or use a rigid container for storage.
The bag burst or the seal failed.
- The cooking temperature may have exceeded the bag’s rating.
- Some bags weaken at temps above 85 °C / 185 °F—switch to a more heat-resistant option.
The food smelled bad after cooking.
- Bacterial growth likely occurred—discard the food.
- This could result from inadequate refrigeration or too-low cooking temperatures.
The food was too dry or had a strange texture.
- The food may have boiled during sealing—cool it before sealing and use a lower vacuum level.
The bag fogged up during cooking.
- This usually signals a bad seal or bag rupture—repack and seal again.
The plastic became opaque.
- The bag material wasn’t heat-resistant enough—try a different kind of bag.
The bag exploded during sealing.
- The bag edge may have been outside the chamber, trapping air inside—make sure the entire bag is properly positioned before sealing.
Is using plastic to cook sous vide harmful?
The safest plastics for use with food are high-density polyethylene, low-density polyethylene, and polypropylene. Virtually all sous vide bags are made from these plastics, as are most brand-name food storage bags and plastic wraps such as Saran wrap. Concerns about the safety of sous vide bags are misplaced.
Less expensive bulk plastic wraps sold to the catering trade are not as safe, however. These products are still commonly made from polyvinyl chloride or polyvinylidene chloride, which can contain harmful plasticizers that have been shown to leach into fatty foods such as cheese, meat, or fish. Legitimate concerns exist about food exposed to these plastics at higher temperatures.
Polyethylene-based plastic wraps are available at only slightly higher cost and do not raise such concerns. Many professional kitchens use clear, rigid plastic storage containers that are made from polycarbonate. These plastics also may be a cause for concern because they contain bisphenol A (BPA), a chemical that can disrupt hormone activity and can leach into food or beverages.
Cracks and crazing due to wear and tear increase the rate at which BPA leaches out of polycarbonates. Polycarbonate containers are currently approved for food use. If you are concerned about BPA contamination, however, replace any polycarbonate containers that have cracks or crazing.
Whether you’re using a chamber sealer and multilayered bags or a zip-top bag and water displacement, understanding the materials and common pitfalls can help you avoid frustration and get the best results.
FURTHER READING
- Getting the Air Out—Modernist Cuisine at Home, p. 52
- Bags, Rigid Containers, and Jars in Sous Vide—Modernist Cuisine at Home, p. 53
- Vacuum Sealers—Modernist Cuisine at Home, p. 54
- How to Use a Handheld Pump—Modernist Cuisine at Home, p. 59