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Thermodynamics of Sous-vide; the Science of Sous-vide

<b>Thermodynamics</b> of Sous-vide; the Science of Sous-vide

Sous-vide Science

Sous-vide cooking uses a hot water bath set to a precise and consistent temperature to cook food in a bag which has had the air removed from it. You know all that by now, but do you understand how the food actually cooks? The scientific details that explain how warm water cooks a steak to the perfect temperature? Well hopefully you want to, so we’re going to give an outline of the thermodynamics (the study of the relationships between heat and energy) and Sous-vide science here!

The resulting uniform done-ness of a sous-vide cooked food item is due to the immersion cooker/circulators ability to maintain the temperature of the water bath to a tenth of a degree or better. Cooking is about moving temperature from a higher heat source to a lower heat receiving product. Sous-vide allows for this to happen without raising the lower heat product beyond the temperature of the source.

The Science

Think of temperature as energy (the average energy of a molecule of substance), and of energy as something chasing equilibrium (as most things of the natural world do). In sous-vide we use an immersion cooker/circulator to supply heat to the water bath, and through convection the energy from the heated water is transferred to the food we are cooking. The food temperature rises as it cooks and when the energy levels between the water and the food reaches equilibrium, the food stops rising in temperature.

The food only cooks as much as the temperature of the molecules in it rises. When the energy gradient no longer exists and all molecules between the water, bag, and meat are at equilibrium, the temperature is maintained on target by the precision cooker and will not rise (or lower, until the machine is turned off).

Simply put, the cooker heats the water to a determined target temperature and the energy in the heated water is slowly transferred throughout the food until everything is at the same temperature.

In meats, cooking distorts protein structures. Actin and myosin which make up the bulk of the proteins (which themselves make up only 20% of the actual meat- as 80% is actually water, fats and connective tissues) and when cooked actually increase their ability to hold moisture. Because most water baths are between 128° F and 140° F for meats, the shrinkage of the tissues occur vertically across the proteins, which keeps the meats juicy. When the shrinkage occurs longitudinally over protein bundles (which happens above 140° F) the ability of the proteins to retain water diminishes and meats ‘dry out’ causing them to be tough, less juicy, and less flavorful. It’s no surprise that most prefer meats cooked to internal temperatures of 128-140° F.

Molecules Under Pressure

By cooking under vacuum *remember, that what sous-vide means!* we can contain energy transfer better than we could in a free environment. As the heated water essentially attacks the bagged food on every front (total surface area contact of a submerged bag in the water bath) energy transfer occurs quickly and efficiently. Any juices extruded from the bagged food into the bag not only help cook the food evenly (as molecular energy transfer occurs between those juices as it does the water, and the water and food) but it retains the flavors dissolved into it as well as keeps the foods from drying out.

Evaporation and condensation occur rapidly under pressure, or systems like a bagged foods. In a container such as a bag, molecules favor condensation and thus the extruded liquid in the bag.

Sous-vide over other methods

BBQ, oven baking, stovetop frying, smokers- why do we prefer to cook sous-vide over other methods. I think we all know that if you have the time, patience, understanding and experience you can make some amazing foods using the other methods. Don’t get me wrong I’m not saying stop using these other means of cooking, but it’s obvious why sous-vide is something everyone and anyone can and should use.

Sous-vide and the capability to cook with consistent precision and work with a wide window of food done-ness is why. Not needing to remove food from the heat source at a specific minute interval of the cooking period is why. Obtaining perfectly cooked food thats absorbed the aromatics and spices we’ve added is why.

Movement of Energy

Our sous-vide cooker heats the water bath until the water bath reaches the target temperature. The food is bagged and placed in the water bath which has a higher temperature (more heat energy) than the food. The differing temperatures between the food and the water bath causes the higher heat energy from the water to move into the lower heat energy of the bagged food. The lack of air in the bag speeds this process up (heating water takes less than 1/24th the time as heating air – thermal conductivity) as air will insulate.

As the heat energy moves into the food, the food temperature rises. In meats, the protein structures become distorted relative to their raw form. Once the temperature of the food is the same as the temperature of the bath (remember, temperature can be described as the average energy of a molecule of substance) the energy of the water bath system is at equilibrium with the food and it will no longer cook any further. The temperature of the bagged food stuffs will not continue to rise.

Energy is conserved. It is not created nor destroyed. When the sous-vide is on and circulating, the energy from the electrical outlet is transferred into heat via the cooker and into kinetic energy via the circulator inside the cooker. The electrical energy is transferred to the water (and the container) in the form of heat. Heat can be lost from the system via evaporation of water or radiative heat loss from the water surface or through the container. It’s also moved into the food through convection, which is how we cook the food. The food can only take on as much energy as the average (temperature) of the water- which is the temperature we’ve set our food to cook too. Cooking is done when the water temp and the food temp have equilibrated.

The energy is transferred from high temp to low temp, food cooks to a set temperature, temperature reaches equilibrium and food stops cooking (at a specific doneness).

Note- Leaving the food in the bath while maintaining cooking temperature for say, 4 extra hours on a 1 hour cooking period, will not ‘cook’ the food any more but it may tenderize the food to a potentially undesirable level. Some cuts of meat do better with very long cook times, while others do not.

 

Featured image from www.byjus.com

 



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