Issue 105 / May - June 2015
Antimicrobial Activity of Honey
Honey has been used as food for millennia. Not only is it delicious, though, but it's also a "super" food, renowned for its medicinal and antibacterial qualities
Honey has been used by humans for millennia. It's beloved by kids and adults alike. Honey is one of those versatile food items which can also be used as a cosmetic ingredient or antibacterial agent. In stores, you may find that there are honey brands which are sold as natural ointments, because honey's antibacterial properties have been known for centuries. By the same token, even though it is a food item and sweet, it does not spoil over time. You may keep honey at room temperature without any spoilage as long as you want; the only change you will observe is its crystallization, which is a natural process and can be easily reversed by heating honey in hot water. To prove this point, you may check out several honey samples which were found in ancient tombs. These samples are thousands of years old. In one particular case, a 3000-year-old batch of honey which was found in an ancient Egyptian tomb ÔÇô and it was still edible today.
Honey's historical importance and usage, as well as all its features, have attracted many scientists to research its chemical properties to understand the biochemical mechanism behind its medicinal properties. Earlier research shows that there might be several mechanisms behind honey's antimicrobial qualities.
These factors can be listed as: its high acidity (pH 3.5-4); its low water content (less than 17% with high sugar content); the presence of hydrogen peroxide; the presence of several organic compounds such as phenolic and methylglyoxal, which may vary according to the floral source.
Recent research by Susan M. Meschwitz of Salve Regina University shows that honey may prevent bacteria from communicating with other bacteria via quorum sensing, a chemical way that bacterial cells coordinate with their neighbors to coordinate certain behaviors, such as biofilm formation, virulence (infecting cells), and antibiotic resistance.
In order to understand how each of these factors is formed, we need to understand how honey is made by bees and what its content is.
Bees collect plant nectar, which is a mix of various different sugars, proteins, and other organic compounds, into a water solution. Of the more than 100 compounds found in honey, many are volatile organic compounds, such as phenolics, that contribute to flavor. Most honey contains the same phenolics, but in different concentrations. Since the composition of nectar that bees collect highly depends on the type of plant, we usually end up having many varieties of honey: There are more than 300 distinct honeys in the U.S. and more than 3,000 worldwide. However, the common and dominant ingredients of all honey are fructose and glucose, which exist mostly as sucrose, also known as table sugar, in plant nectar.
The process between plant nectar and the final product and its properties is highly dependent on the activities of the bees. The first process starts in the honey stomach of worker bees as soon as they collect the nectar from plants. The worker bees break down the sucrose, table sugar, into its component sugars, which are fructose and glucose, using digestive enzymes. This increases the viscosity of the nectar as the break down process continues. Worker bees spit up the processed nectar solution when they arrive at the hive. In the hive, house bees take the processed nectar and continue the process; by using several different enzymes, they further break down molecules and sucrose. At the end of the process, most of the sucrose is turned into fructose and glucose, which makes most of the final honey. Later, glucose is further processed, which results in some of the honey's medicinal properties.
When the breakdown of the sucrose is achieved at the desired level, the processed nectar is placed in honeycomb to start the second phase of the process: reducing the water content of the nectar, which is another of the contributing factors in its medicinal properties. Nectar as gathered by the worker bees from the plant contains about 70% water and is reduced down to around 17% water as it becomes honey. This gives honey its thick consistency. Bees remove the excess moisture from the nectar by rapidly fanning their wings (their wings beat 200 times per second or 12,000 beats per minute) over the open cells of the honeycomb in the hive. This results in rapid evaporation of the water in the nectar and may take up to three days.
Reducing the water concentration is a critical step in achieving most of honey's properties. The most important one is that the low water content prevents bacterial and mold growth in honey and this makes it a very good antibacterial agent. In addition, honey has a very low amount of water activity, which also prevents the growth of bacteria, fungi, or mold. Water activity is measured on a scale of 0 to 1, and honey's water activity is around 0.6 on this scale. However, mold and bacteria require a water activity of 0.75 in order to be able to grow. Honey's low water activity (it has few free water molecules) and its low water content dehydrate the microbes (resulting their death) and prevent spoiling. The same properties are also effective when honey is applied on an open wound and prevent bacterial growth on and around the wound.
As mentioned before, one of the factors contributing to honey's antibacterial properties is its high acidity. Honey's acidity ranges between 3.5 and 4 on the pH scale, which makes it more acidic than apple cider vinegar (pH around 4.5). The acidity of honey is the result of several acidic compounds and therefore highly depends on the plant source. These compounds include formic acid, citric acid, butyric acid, phenolics, and gluconic acid. The major contributor is gluconic acid, which is produced by the action of bee enzymes on some of the glucose molecules in the honey. Bacteria and mold prefer neutral mediums to grow. Therefore, honey's high acidity, in addition to its low water activity and water content, further enhances its antibacterial properties and provides a harsh condition for microbes.
Hydrogen peroxide is one of the well-known wound cleaning solutions used to kill bacteria. Most of us may not know that honey also includes hydrogen peroxide. Honey's hydrogen peroxide is produced as a side product during the enzymatic production of gluconic acid from glucose by glucose oxidase.
In addition to all these chemical and medicinal properties mentioned so far, these compounds further impact bacterial and mold growth by disrupting the communication between them, according a recent research. As scientists further research honey and its properties, they may even find new features and molecules, such as peptides or proteins, which could contribute to its antibacterial effectiveness. All these components may vary according to the floral variety of the bee's environment and the plant nectar which is collected from that environment. They impact the flavor, appearance, and aroma of the honey as well as its medicinal properties. Since honey has multiple features, which all contribute into its antibacterial properties at the same time, one can easily say that honey is one of those miracle foods, just as ancient tribes knew, and it deserves more attention. However, an adulterated honey may not show the same effectiveness as natural honey, as it may lack one or more of the chemicals or properties, even though it may have the same content and flavor.
Rachel Petkewich. Honey. Chemical and Engineering News. 85, 2007, p. 29 (accessed online on 2/21/15, http://cen.acs.org/articles/85/i6/Honey.html)