When you cook in water every surface of the food is exposed to the heat source, the liquid. Food is totally enveloped within the heated media. Water isn’t the only source of liquid – some cook in oil (frying or confit). This results in the most even exposure possible of food to the heat source. The problem with boiling food is in the thermal chemical reaction that occurs. Light fats and fluids within food become even more fluid and match the density of the surrounding media (the water). Then molecular migration occurs.

The lighter fats and fluids migrate out of the food into the water media and the lightest molecule, the water migrates back into the food. The result is soggy tasteless food. Meats also develop a dryness and lose flavor. When the light fats and juices are All fluids, the water and those around and within food are controlled at a precise temperature. Juices, fats, spices and marinades circulate throughout food while the cooking medium (water) is kept out but is in as direct contact with food as possible.cooked (migrated) out, all that remains are fibrous strands. Most importantly the majority of the flavors are gone too. This happens because the light fats and juices that migrate out are where the flavor is.

The other effect of cooking at a higher temperature (near 200 degrees) is that the protein molecules unravel. When the proteins unravel – they tend to become hydrophobic – meaning they don’t like water. So the natural water and juices in a meat tend to go out of the meat.

With Sous Vide the light fats are cooked (or migrated) into meat fibers rather than cooked out. Because the temperature is kept low in Sous Vide cooking – the proteins do not unravel to the extent that they do in traditional cooking. The meats are kept moist, egg whites are not rubbery. This also causes another chemical reaction to occur, the fibers – the elastic fibers that are tough and gristly become soft and supple. As we have stated, heated water is the most perfect media to cook in.

However, it is best to not have water in direct contact with the proteins, or the vegetables. Therefore the food must be protected – while still allowing the heat source to be around them. When food is vacuum sealed in a thin plastic wrap none of the water goes in, and none of the flavorful juices and fat migrates out into the water. There is no molecular migration between the outside water and food. The water performs as the perfect heat media without permeating and removing flavor and texture. Sous Vide can also be done with oil – but that is a different chapter. Isolated within the vacuum bag, the light fats and juices stay in and migrate throughout meat fibers causing them to become deliciously flavorful, soft and tender, but never soggy.

More vacuum equals better results – forcing marinade in while keeping water out

Precise temperature controls thermal reaction and molecular migration. Sous Vide recipes are actually the results of testing control of thermal molecular reaction and subsequent migration of flavorful fats and juices back into food fibers. Every meat and every vegetable reacts differently to different temperatures- as well as the thickness of the food being heated. Applying heat to food causes the liquid in it to outgas (to dry out) and the existing atmosphere to expand. The goal in Sous Vide is to cook at the lowest possible temperature so food, liquid and atmosphere expansion is limited and can’t defeat the vacuum. In addition, we cook the food long enough to minimize problems with bacteria. But any rise in temperature causes expansion and outgassing. The stronger the vacuum is the less likely it will be. All fluids, the water and those around and within food are controlled at a precise temperature. Juices, fats, spices and marinades circulate throughout food while the cooking medium (water) is kept out but is in as direct contact with food as possible. But simply, more is better. If the vacuum is defeated completely, an air bubble will form within the bag. A bubble will isolate areas of the food from the heated water. This causes a loss of control over the thermal molecular reaction resulting in unevenly cooked food and a loss of flavor and moisture.

Vacuum causes greater permeation of marinades to food. The best Sous Vide recipes using marinade require a precisely weighed portion of solid food and a precise amount of liquid. These recipes are time tested to learn the exact ratios of sold food to liquid in combination with exact temperature control. There are some things in traditional marinades that do not do well in Sous Vide- alcohol for example. They use controlled application of the process of thermal molecular migration of liquid that we explained above. But unlike boiling the liquid marinade isolated within the vacuum sealed pouch is infused into the food instead of water. At this point you might be thinking this isn’t cooking it’s more science. Actually Sous Vide is chemistry. Sous Vide does allow you to use less marinade than traditional cooking. Using thermal molecular reaction to infuse marinade into food may be it’s highest manifestation. Flavor can be infused throughout meat at a level not possible in any other form of cooking. This is done with the meat retaining absolutely perfect texture. More vacuum results in more permeation of marinades. Greater vacuum causes tighter contact of the plastic to the food resulting in maximum heat transfer between the outer heated water and the food. All relationships are direct, more precise temperature control and deeper vacuum causes a more controlled thermal reaction and superior results.

Vacuum sealers used for Sous Vide cooking – standard versus chambered

Chamberless Vacuum Sealers

Often referred to as ‘standard’ vacuum sealers, most people get one of these to start. Because they cost only a fraction of what a chambered unit does. Bags are typically cut from a roll of bag stock. The user decides how large a bag is going to be as they must be cut to length from the role. A bag is created by the user by sealing one end. Most manufacturers offer precut bags. People really like precut bags because they eliminate that task of sizing and cutting. Food is placed into the bag and the open end is placed between the jaws of the sealer (in the vacuum position). Typically the user must press down on the top lid until the vacuum engages. When the air is pumped out of the bag, the plastic implodes and the machine will seal and signal the user that the process is complete. Some of the more powerful machines have manual controls that allow the user to limit the amount of vacuum applied. Less vacuum is used to protect more delicate foods from being crushed by the compressing bag.

Wet foods including those with marinade are the bane of standard food sealers. Better machines have methods to contain fluids so they won’t go into the pump. Typically there is a larger tray designed to contain vacuumed liquids in conjunction with a foam filter. But it still can be messy and keeping the measured amount of marinade in the bag can be difficult. If marinade enters the pump, most units will cease functioning. Liquid being drawn into the pump will probably invalidate the warranty. To solve this problem, some manufacturers suggest putting a paper towel in the bag above the food and marinade. This may not work well with Sous Vide as cooking a paper towel with your food is not a very desirable concept. One suggestion is freezing marinade and simply placing the frozen ice cube of marinade into the bag prior to freezing.

You can place the food in a bag, fold the bag over and place the unsealed bag inside another bag. But this method might cause there to be air pockets. A better method may be to hang the bag from the side of the counter to let gravity take affect. Some standard food sealers include a vacuum chamber attachment. This is a bowel with a top that seals over it that is connected by a tube to the sealer pump. Using this method food can be pre-marinated in the chamber, then excess liquid is removed and the food safely sealed into a bag without endangering the food sealer. This method may be the best defense to insure against pulling liquid into the pump. The other possibility is using a frozen marinade that is placed in the bag and then as the food is heated in the Sous Vide the frozen marinade thaws and imparts the flavor to the food. The most important point is that marinade liquid pulled into the pump can be a disaster not covered by warranty. So you should look for features that address this problem when making a purchase decision if you want to do a lot of marinating. To start out with, many of you are going to purchase a less expensive unit with less than adequate liquid containment (usually they have no filter) that will require using one of the preventative methods above.

Another real problem is that the heat seal may not be made if the bag becomes too wet at the point the heater bar makes contact. If too much liquid is present at the point of seal, the user should remove the bag and thoroughly clean and dry it at the seal area or start over with another bag. Air being drawn through the bag opening pulls liquid to the point of seal, making this a larger problem with standard sealers. Please check the manufacturer specifications and instructions for more details.

Chambered vacuum food sealers for Sous Vide cooking

Most people are not going to spend $800+ for a vacuum sealer. But there are advantages to chambered vacuum sealers that you should know about. These units are typically used in commercial kitchens. But you never know, you may grow to love this method of cooking. A chambered vacuum sealer may be something worth saving for if you become a true aficionado.

With a chambered vacuum sealer all of the atmosphere is pulled out of the chamber. The air is completely pulled out from both sides of the food bag inside and out. But a standard vacuum sealer only pulls air through the open end of the bag. All the atmosphere must be removed from the top first in order to pull it up from the bottom. If large and/or moist food form a contact seal with the sides of the bag, air in the bottom my be isolated and unable to be pulled out. With a standard unit this would cause you to have to open the bag and reposition the food so there is a channel to the bottom. In any case even with a successful seal, a standard sealer may not completely remove all the air from a bag especially in those areas at the bottom. This is also due to the fact that a seal must be made to the bag material. The seal is made by compressing rubber (elastomeric) seals (gaskets) on either side of the bag. Every bag will experience some leakage at the seal. Bags are imperfect and even a slight crease will cause air to leak back in.

Every surface of the food storage bag, inside and out, is encapsulated in vacuum in a chamber system. If air is trapped at the bottom of the bag it will expand when the air outside it is removed from the chamber. Typically the expanded bag will form a channel for all atmosphere to escape the bag. That’s the bottom line. All atmosphere is removed. There is no reliance on a gasket seal on the bag’s surface; the air is entirely gone inside and out.

Typically chambered sealers have more controls than standard sealers. For example, some chambered sealers allow the user to set an amount of time for the machine to keep pumping after the target vacuum has been achieved. This allows residual air to be pumped out of porous foods. This is an especially good feature for Sous Vide cooking because as we have explained residual air expands and can cause a bubble within the cooking bag when heated. Unlike standard sealers, chambered machines have gauges that display a readout of the vacuum within the chamber. This lets the user know if vacuum is achieved and if the machine is working properly or needs maintenance.

There is also a sealer bar in chambered machines that functions the same as in standard machines. When the desired vacuum is met, the bag is automatically sealed by compressing the opening with a hot bar. Some chambered sealers also allow adjustment for bag sealing time. This would enable the use of bags from different manufacturers because the sealing heat can be adjusted for each manufacturer’s bags melting characteristics. Better yet, heater adjustment allows the use of generic commercial bags. These are off the shelf bags not made by a sealer manufacturer. They cost a small fraction of what sealer manufacturer’s bags do. One manufacturer claims that the savings in bags very quickly pays the difference in cost between a chambered and standard machine. So bag cost is an issue to be considered . If you use your chambered machine a lot, the bag savings may pay for it. Those purchasing standard sealers should consider bag costs when choosing between manufacturers.

A big advantage of chambered systems is that replacement parts are available for most critical parts. The heater bar used to seal bags will wear out. A chamberless sealer must be thrown out when the heater fails. With a chambered unit the sealer bar is simply replaced. Pump parts and even the whole pump can be replaced, allowing these machines to last for decades or longer.

So what about marinade? A chambered system can apply vacuum to the lightest liquids causing absolutely no pull on them. Because air is removed from both sides of the bag, nothing in the chamber moves until the air is let back in after the seal is made. Standard vacuum sealer users become accustomed to the bag imploding down to nothing around food as vacuum is applied. In a chambered system, everything remains expanded (looks the same) even as the vacuum is applied. Implosion occurs only as atmosphere is let back into the chamber so it can be opened. This is a strange effect to the standard sealer user who has become accustomed to the bag slowly collapsing in their hands as vacuum is applied. Total stillness in the chamber allows vacuum to be applied to any level of marinade or even pure liquid without pulling any into the machine or the seal area on the bag…ever.

Pump power

More money typically equals a larger pump. A larger pump means faster pump down time and deeper vacuum. In either system–chambered or chamberless–as distinct models go up so does the size of the pump. Two pump designs are typically used: piston pump and rotary vane pumps. Piston pumps work like the pistons in your car. They have rings or seals around the pistons that wear out. When this happens, the pump will take much longer to pump down and will not be able to achieve a deep vacuum. All the less expensive standard chamberless systems will have dry piston pumps. When they wear out completely your only option is to buy a new system. Dry pumps can be made with very expensive elastomeric seals that will wear for a very long time. Therefore, some of the less expensive chambered vacuum sealers use dry pumps and some of those may be designed so the seals can be replaced. At the upper end of chambered sealers would be those that are oil filled. They actually  have oil sumps like your auto. Like your car, part of their maintenance is to periodically change the oil. Not only does the oil protect the seals, it serves to cool the pump. Heat is a very large byproduct of vacuum pumps as they run. Oil filled pumps should have at least ten times the lifetime as dry ones.

Piston pumps reach higher vacuums, while rotary pumps are faster at pumping down. Rotary pumps being less efficient are going to need much larger motors to get down as low as a much smaller piston pump sealer. But it will be a much, much quicker pump-down, almost immediate. Using the same motor size, a rotary pump won’t be able to reach as high of a vacuum as a piston pump. There will be a point it won’t be able to pump below. So for really porous or large foods such as large cuts of meat the piston pump is going to be better for Sous Vide. But, some manufacturers using rotary pumps are claiming to be able to reach the highest vacuums. Again be aware that by design the piston pump should always be able to reach higher vacuums.

Most people are not going to buy an expensive vacuum sealer for Sous Vide cooking to start

Before spending too much, you should know that you enjoy the process and results of Sous Vide. A $50 dollar standard vacuum sealer will be do fine to begin with. After a little experimentation with thermometer and temperature controller heat control we recommend getting a water oven (available for $300). Then you have the advantages of accurate automated temperature control which means you can simply vacuum seal and place food into the water oven and walk away. Later you come back to the delicious results cooked with relatively none of your time being consumed. If your use of Sous Vide grows a better vacuum sealer may be desirable to save money on bags and to cook larger and/or more porous food.

Sous Vide cooking – safety considerations

Some very dangerous bacteria grow in oxygen free environments. When you vacuum seal foods for cooking there are some things you need to remember to avoid botulism.

  • Uncontaminated food that is heated and served within 4 hours is safe. Eating undercooked food or raw food is unsafe.
  • Any meat cooked for longer than 4 hours must reach a temperature of 131° F (55° C) within 4 hours and maintain a temperature of 131° F (or higher) over the subsequent span of cooking which must be at least 4 hours.
  • Foods to be stored should be chilled immediately. Research each individual  food to determine the correct storage temperatures and timeframes.
  • Pregnant woman and people with compromised immunity should take extra precautions relative to cooking temperatures, cooking time and food storage.

Cooking in sheet plastic has not been proven to be either dangerous or safe. Only cook in bags that are temperature rated.

NOTIFICATION: Dr. Terry Simpson and all of his affiliated organizations assume no responsibility for personal injury or property damage sustained by or through use of this information.