Cooking is science and tradition

Cooking is all about science. In this blog our chef Peter Coucquyt will give you insights in the chemistry going on in the kitchen...

zondag 20 februari 2011

Brining or pickling

One of the new trends in kitchens is brining or pickling. What is brining and what happens during this process?
Originally this method was used as a preserving method and not so much for altering or enhancing the flavor and texture. This was just an unavoidable side effect. By putting food in a jar with salt, sour, alcoholic beverages or sugar you could preserve it. (For example: pickles, pickled onions, anchovies, herring, cherries in alcohol, ...).
A high content of salt, a high acidity or low humidity inhibits the growth of microorganisms, which extends the shelf life of products. Brining or pickling will result in microbiological stability, but not in chemical stability. In all cases there will be chemical changes in the food products, resulting in new flavors, textures and colors. Osmosis and diffusion are displacement phenomena that take place in a brined or pickled food product. These processes are mainly responsible for the microbiological stability, while the high concentration of salts, acids and/or alcohol will cause the  chemical changes. Yet these substances are also responsible for the displacement phenomena.
Osmosis is the movement of water molecules through a selectively-permeable membrane. The membrane is usually an intact cell membrane, present in many food products, but it can also be artificially created, e.g. when making alginate caviar. Water will move from an area of high solute concentration to an area of low solute concentration. This process does not require any energy so it occurs naturally and goes on until an equilibrium is achieved. This is the essence of osmosis and is life critical in the cells of all living organisms. Yet osmosis can still occur after the death of the organism, if the cells remain intact.
Osmosis can be and is being utilized in the kitchen.  All plants consist of cells, surrounded by a cell wall that gives strength to the plant. The fluid in the cell is under pressure, providing a crisp and fresh texture. When bitten, the cells snap. Older vegetables lose their moisture (carrots, beans, ...). The pressure in the cells decreases and the vegetable loses its crispness. However, this process is reversible with the aid of osmosis. Soak old vegetables (peeled carrot, limp lettuce, ...) in cold, clean water. They will soak up the water like sponge. After an hour or so, they are crisp again. Since the concentration of solutes in the washing water is lower than in the cells, there will be water movement from the washing liquid to the cells, resulting in a recovered pressure in the cells.
When cleaning lettuce in salted water, there will be water movement from any organism, like bugs, to the salted water. This will essentially dehydrate the bugs causing them to die and release the lettuce. But there will also be water movement from the lettuce to the salted water, causing a drop of cell pressure which we will experience as a reduction in crispiness. In most cases this in not desirable. The effect can again be reversed by washing the lettuce afterwards in clean ice water. Now the lettuce will take up water since the concentration of solutes is higher in the cells than in the ice water.
Diffusion is the movement of solutes, like salts or dissolved sugars, from an area of high solute concentration to an area of low solute concentration. This movement does not require an intact membrane, which is critical for osmosis. Yet diffusion can occur through a membrane if the solutes can pass through it. Diffusion is greatly involved in the marination of a food product. The substances, like salts and flavors in the marinade will move in the food product by diffusion.
When brining or pickling. Water will be extracted from the food product by osmosis, resulting in a concentration of solutes in the food product. Meanwhile, diffusion will cause the food product to enrich itself with solutes in the brining solution. It is also possible that substances in the food product leach out to the brining solution. Consider the curing of a piece of meat, say bacon. Over time, the salt concentration of the bacon will be equal to the salt concentration in the bath.  Water is drawn out of the meat by osmosis and diffusion makes the meat saltier, all because the salt concentration of the bath is higher than in the meat. The meat will also lose acidity, acids migrate by diffusion from the meat to the brine, this is because the latter contains no acid.
Brining or pickling will preserve food microbiologically. It will actually make water unavailable for any living organism in the brine or brined food product. Since the concentration of solutes, in this case the concentration of sodium and chloride is far greater outside the cell of any bacteria, or other harmful organisms, than inside. The result is that bacteria will not be able to uptake water, which is essential to remain alive, on the contrary, water will be forced out of the cell, causing the bacteria to dehydrate and eventually  die. The same happens in a pickling solution, with added pH effects.
Brining or pickling will not preserve food chemically. In fact brining or pickling will promote chemical changes in the food product. Today this is one of the main reasons why foodstuffs are pickled in gastronomy. By brining or pickling, new flavours will arise, the texture and color will also be modified. Acid, alcohol and high salt concentrations have a similar effect on proteins as heat does. HervĂ© This established the term for the increase in strength without heat, in 2003. He called it “cocteren”. Acids will strengthen cell walls of plant cells while salts will weaken them. Brining vegetable will lead to a loss of crispiness, because the weakened cell walls can not support the pressure, adding to that, the pressure is also lost due to osmosis. 
By brining and pickling, we can alter following properties of the food.
- The water balance: The water balance of the food product will be altered by osmosis.
- The taste and flavor: Diffusion will cause of movement of odorants and tastants from and to the food    product. The substances in the brining or pickling solution will cause chemical reactions, creating or destroying flavors.
- Texture and color: The substances in the brining or pickling solution will cause chemical reactions, altering color and texture.
- Shelf life: Brining and or pickling creates an environment that is inhospitable for food pathogens. 

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