Smells are caused by volatile compounds that stimulate receptors in our nose. There is a large variety of such compounds with a great diversity of molecular structures. But many of the smells encountered in the fridge are due to volatile fatty acids. For example, when butter goes rancid, it releases butyric acid, a particularly foul smelling compound that is also a component of foot odour, vomit, and curiously, parmesan cheese. It is also the notorious ingredient in stink bombs. French chemist Michel Eugene Chevreul first described isolating butyric acid from butter in 1817 and coined its name from the ancient Greek word for butter.
As everyone knows, acids can be neutralized by bases. Baking soda, or sodium bicarbonate, is a base. It reacts with butyric acid to form sodium butyrate which has no smell because it is not volatile. That is why baking soda can help eliminate fridge odour. But there has to be a large available surface area for contact, so that opening just one little corner of the box does no good. Spreading the baking soda in a plate is the best way to go. If this doesn鈥檛 solve the smell problem, washing with diluted bleach followed by hydrogen peroxide to get rid of the chlorine smell is the next step. Then follow with a bicarbonate solution rinse to change the residual smelly volatile fatty acids to sodium salts. Activated carbon deodorizers are also available for fridges. You just plop these in and they adsorb all kinds of volatile compounds.
Not all smells in a refrigerator originate from food. Nasty volatile compounds can be produced by a complex mix of bacteria and household mildew which can set up shop in the fridge. These produce a variety of acids with disturbing smells. Pseudomonas bacteria can produce a foul smell and are the likely culprits in a freezer, or on smelly dishcloths and old towels. In the fridge they coexist with microbes that inhibit the bacteria or break down the smell, but these microbes cannot live in the freezer. So, in the freezer they have less competition and can multiply and produce a smell.
Interestingly, while butyric acid is undesirable in the fridge, it is welcome in the human colon where it is released by butyrate-producing bacteria as they digest 鈥渇ermentable fiber鈥� as found in oat bran, pectin, guar gum and resistant starch. Butyrate is used as an energy source by cells that line the colon, keeping them healthy and allowing for their multiplication while, paradoxingly, inhibiting the multiplication of colonic tumour cells. This explains why inadequate fiber in the diet has been linked with an increased risk of colon cancer. The proper maintenance of the colonic epithelial layer is also critical in not allowing bacteria to enter the circulatory system. Butyrate, however, can pass through this layer and once it enters the circulatory system it curbs inflammation and helps the immune system to function properly, reducing the risk of auto-immune disease. Children who lack butyrate-producing bacteria have been shown to be more susceptible to allergic disease and diabetes.
Resistant starch is a form of fiber particularly favoured by butyrate-producing bacteria. Like all starch, it is composed of amylose and amylopectin, polymers of glucose. But in this case, the molecules are compressed into tiny granules making them less accessible to attack by digestive enzymes in the small intestine. As a result, the starch passes into the colon where it can be metabolized by bacteria. Oats, lentils, chickpeas and beans are good sources, and interestingly, potatoes, pasta and rice become richer in resistant starch when cooked and then cooled. Green bananas are also an excellent source but their resistant starch changes to regular starch as they ripen.
The take-away message here is that chemicals cannot be labeled as 鈥済ood鈥� or 鈥渂ad,鈥� context matters. While we don鈥檛 want butyric acid in our fridge, we do want it produced in the colon. That means increasing fiber intake to feed the bacteria that produce it, and placing a dish of baking soda in the fridge to eliminate it. And what can you do with the used baking soda? I wouldn鈥檛 suggest using it in baking because that can lead to the liberation of butyric acid, and you do not want your cake to smell of stinky feet. Give it to the kids to build a volcano by reacting it with vinegar. After all, you expect volcanoes to be smelly.
