One of the things I see constantly about honey is that it is an anti-microbial, so let's take a look at the evidence for this concept.
There are three features of honey that have been studied extensively for anti-bacterial properties - high osmolarity, acidity, and hydrogen peroxide production. This study found there were two phenolic acids - caffeic acid and ferulic acid - also had a role.
High osmolarity means that there are many things dissolved in the solute and that things will want to go from a low osmolarity environment to the higher osmolarity environment. Honey has been tested to have a pH level from 3.4 to 6.1, with an average being around 3.9, so it's actually quite acidic. And honey contains an enzyme called glucose oxidase that will produce hydrogen peroxide when honey is exposed to oxygen, diluted, and brought to a more neutral pH, something that can happen when we put honey on a wound, for instance. It is destroyed when honey is exposed to heat and light. Gluconic acid can behave as a chelating ingredient - meaning it binds with metal ions - and we know that chelating ingredients can help prevent microbial contamination in our products.
Click here for a post on osmosis. Click here for a post on pH. Click here for more information on glucose oxidase and gluconic acid.
Caffeic acid is a powerful anti-oxidant, out-performing almost every other anti-oxidant when tested. It is a possible fungicide, good anti-inflammatory, and may protect skin if applied after sun exposure. Ferulic acid can help reduce inflammation on our skin, and it works synergistically with other anti-oxidants to create a super anti-oxidant party that will help retard the rancidity of oils in your products and fight free radicals on your skin.
With all these great features, could we use honey to preserve our products? No. I've found nothing that shows that we could use honey to preserve anything we make. And I think there are a number of reasons...
- Honey doesn't do well at higher temperatures and breaks down. If it breaks down, it's not going to be an effective preservative.
- Honey doesn't do well when diluted, and our water based products have quite a lot of water in them. (See the second abstract below).
- Honey will be in a less acidic product than it would normally be found. Our products tend to be in the 5.5 range. Although the neutralization of the honey's acidic pH is a good thing for releasing hydrogen peroxide, those enzymes are destroyed when exposed to heat and light, two things our products see regularly.
- You would have to use so much that it would be the main ingredient in the product, and it would feel very very sticky.
I've found websites that say you can use honey as a preservative and they are simply wrong. I know this is a bold position to take, but I think I can say with confidence that any blog, website, or book suggesting that a teaspoon or two of honey will preserve your water based product is suggesting something dangerous to you. I have done more searches than I want to admit through Google, Google Scholar, and EBSCOhost at my university, and I can't find a single study that demonstrated that someone has successfully passed a challenge test while using honey as the sole preservative to preserve a bath and body, cosmetic, or toiletry product. I found one person claiming this in the comments in this post at Chemists' Corner, but you'll notice that he points out that he used a secondary preservative, so that's not evidence of honey being the sole preservative. And Anthony Dweck notes in this article that "Honey in its undiluted form is also a natural preservative and, indeed, there are many learnéd papers citing honey as a viscous barrier to bacteria and infection."
Here are two abstracts that might interest you...If not, join me tomorrow for more information on using honey in our bath and body products!
The anti-bacterial property and preservative nature of honey has been studied by evaluating the role of hydrogen peroxide in these properties, against bacterial strains isolated and identified from pasteurized milk samples. The antibacterial property of honey examined by agar incorporation assay and turbidometry, indicated a concentration dependent inhibition of bacterial growth in all catalase negative strains in comparison with catalase positive strains, highlighting a probable role of hydrogen peroxide. Samples of commercial milk stored at 40C in presence of honey were shown to inhibit opportunistic bacterial growth better compared to samples stored without honey. Due to the bactericidal property of hydrogen peroxide and its preservative nature, honey which is chiefly a combination of various sugars and hydrogen peroxide, can be used a preservative of milk samples.
Citation: Krushna, N., Kowsalya, A., Radha, S., & Narayanan, R. (2007). Honey as a natural preservative of milk. Indian Journal Of Experimental Biology, 45(5), 459-464.
To study the antimicrobial activity of honey, 60 samples of various botanical origin were evaluated for their antimicrobial activities against 16 clinical pathogens and their respective reference strains. The microbiological quality of honeys and the antibiotic susceptibility of the various isolates were also examined. The bioassay applied for determining the antimicrobial effect employs the well-agar diffusion method and the estimation of minimum active dilution which produces a 1mm diameter inhibition zone. All honey samples, despite their origin (coniferous, citrus, thyme or polyfloral), showed antibacterial activity against the pathogenic and their respective reference strains at variable levels. Coniferous and thyme honeys showed the highest activity with an average minimum dilution of 17.4 and 19.2% (w/v) followed by citrus and polyfloral honeys with 20.8 and 23.8% respectively. Clinical isolates of Staphylococcus aureus subsp. aureus, Escherichia coli, Salmonella enterica subsp. Enterica, Streptococcus pyogenes, Bacillus cereus and Bacillus subtilis were proven to be up to 60% more resistant than their equal reference strains thus emphasizing the variability in the antibacterial effect of honey and the need for further research.
C. Voidarou, (., A. Alexopoulos, (., S. Plessas, (., A. Karapanou, (., I. Mantzourani, (., E. Stavropoulou, (., & ... E. Bezirtzoglou (b, e. (n.d). Clinical Microbiology: Antibacterial activity of different honeys against pathogenic bacteria. Anaerobe, doi:10.1016/j.anaerobe.2011.03.012