Question

Each year in the US, millions of people become ill and thousands die from eating food contaminated with bacteria. Controlling growth of microorganisms on food is a particular challenge, and over the course of many centuries various methods have been developed for controlling microorganisms in food. Some of these include preservation by irradiation, drying, freezing, canning, and salting. Preservation by salting is based on the principles of osmosis. One example of this is the preservation of ham by applying large amounts of salt (“salt-cured”) or sugar (“sugar-cured”) to the meat within 48 hours after slaughter. Using information you learned in this lab, speculate about how this process works to preserve ham.

Answer 1

Curing is one of the process of food preservation routinely employed to preserve a variety of foods including meat, fish and vegetables. It is usually done by adding a combination of salt, nitrates, nitrites, or sugar to the food product.

Salt (sodium chloride) and sugar (sucrose) are both electrolytes. These compounds increase the shelf life of the food product by killing the microbes that attack the food. The destruction of microbes is based on the simple concept of osmosis. When the food product containing the microbes is immersed in solution containing the above electrolytes, the salt/sugar in the aqueous solution tries to reach equilibrium with the salt or sugar content of the food product with which it is in contact. The result is that the available or free water molecules that are present within the food are drawn out to the outside, and in its place, the salt/sugar molecules are inserted. This results in a decrease of available water (water activity –a_w) in the food product.

Water is an important molecule for the growth and survival of all microbes. Without free and available water, it is difficult for the microbe to survive in the food product, and hence it eventually dies. Additionally, high levels of salt and sugar exert anti-microbial activity up to some extent by interfering the enzymatic activities of the microbes and weakening the molecular structure of microbial DNA.