Re: how does borax (sodium tetraborate) kill yeast?

Sean –
You are definitely heading in the right direction with the difference between boric acid and borax. So, let’s dig in a little further to figure out how borax works. As you probably already know, borax is a “soap”, or more correctly, a detergent. We will discuss the mechanism a little later. First, we need to talk a little about yeast.

All living cells contain a membrane made up of proteins, lipids (fats), and other related molecules. For a closer look about cell membranes, go to this link:
http://www.kapili.com/biology4kids/cell/memb.html

The reason why we are focusing on membranes is because they are important to cell survival. What we are going to concentrate on are the hydrophobic (it does not mix with water) and hydrophilic (molecules which “love water”) as well as proteins found in the membrane. Similar to the membrane, these proteins also have hydrophobic and hydrophilic portions. In general, the hydrophobic portion carries a positive charge and the hydrophilic portion carries a negative charge. The important point to remember is that the membrane (including lipids and proteins) is made up of both negatively and positively charged portions.

So, what is the link to borax (sodium tetraborate)? Similar to a membrane, sodium tetraborate is also made up of negative and positive charges; specifically, the sodium (Na2+) carries a positive charge, while the tetraborate (B4O7) carries a negative charge. The detergent properties of borax allow it to interact with the membrane (made of positive and negative parts). The result is that the opposites will attract (that is the negative portion of borax (tetraborate) will be attracted to any positive portions and/or the positive portion of borax (sodium) will be attracted to any negative portions. In essence, the positive and negative portions are “sectioned out” (or solubilized). When you add borax, what you are actually doing is destabilizing the membrane… or causing its components to come apart and then interact with the sodium tetraborate. Without the membrane, the yeast (and other cells) are not able to survive, and they are therefore killed.

I hope that I have clarified this for you a bit! Good luck in the science fair :o)

Glynis K.
Food Science Graduate Student
Rutgers University