Whatever cooking method is used, an increase in heat alters the cellular makeup of fruit and vegetables. The more the heat is increased, the more pronounced these changes become, so the benefits of cooking at low temperatures (below 100°C) are obvious. Paired with the sous vide bag low temperature cooking provides an ideal solution as the water-soluble elements are not lost in the cooking liquid.
A precise knowledge of the biochemistry of plants when combined with the precision of a cooking temperature and working in a closed environment allows us to obtain unique results. Innovative textures and concentrated flavours can be obtained without altering the intrinsic qualities of the ingredient (colour, antioxidants, vitamins, etc).
In his reference book about sous vide cooking (Under Pressure), Thomas Keller says that “sous vide has dramatically changed the way we cook vegetables. From the standpoint of flavour, preventing oxidation and convenience, it is dramatically effective.” Sous vide is a great technique to use with vegetables and fruits. The only vegetables for which this technique is not recommended are green vegetables. They lose their vivid colour when locked in a sous vide bag.
It is important to understand the science behind the precise and ideal cooking temperature of fruits and vegetables. Both are built with cells wrapped in a membrane and surrounded by a cell wall. This cell wall is made up of polysaccharides (cellulose, hemicellulose) and pectins. Inside the cell, the principal constituent is the vacuole. Not only is its content of great biological value but it also exerts a pressure – termed ‘osmotic pressure’ – on the cell walls. Between 85 and 90°C, a physical process takes place that leads the pectins to become soluble. At those temperatures the pectins leave the cell wall, where they normally act as cement. This solubilisation makes the cell wall less resistant. The more the temperature and the cooking time are increased, the more the food becomes tender.
Bruno Goussault, the renowned scientist specialized in vacuum cooking (and Keller’s advisor), used to estimate that the temperature necessary for the dissolution of the pectins was 85 °C. He now considers that a few more degrees are needed, that is to say 88 ° C.
This means that a standard temperature to use to obtain a purée is 88° C , but the cooking time will vary depending on the type of product. Of course, this is a minimum. Cooking at higher temperatures ( but below 100°C) will impact the cooking time. Which means that if we want to protect the pectins and preserve the vegetable’s appearance and crunchiness, cooking below 85°C (around 83°C) is recommended.
Cooking vegetables and fruits in a confined atmosphere such as a vacuum bag, allows to concentrate the flavours of each ingredient, with unparalleled purity. Flavours become more pronounced and more nuanced. For example, adding a single bay leaf in a big bag of carrots will permeate the carrots with the bay leave’s aroma but without altering their own flavour.
Applying the technique to artichokes, Keller says, gives a good example of how oxidation can be avoided. Artichokes will begin to darken as soon as you cut them, unless you immediately put them into acidic water or heat them (which neutralizes the enzymatic reaction behind the oxidation). Sealing sous vide also reduces the oxidation and its effects.
Sous-vide is a convenient technique. Once ingredients are selected and cooking scales are set according to the desired result (e.g. al dente or crunchy or pureed) one can simply repeat the procedure over and over again. Using vacuum bags allows for longer conservation and therefore a better and more flexible organization during service. And last but not least, this technique gives you the freedom to get creative without too much hassle. Just imagine cooking a whole artichoke and using the juices with a little seasoning to create a complete dish that is as close to the product as possible.