April 7, 2017

Storage-induced chemical changes in active components of honey de-regulate its antibacterial activity

In order to explain the reasons for variability in the antibacterial activity of honeys, the causal relationship between both honey floral sources and the activity, and the effect that honey storage has on stability of compounds conferring this activity were analysed. Honey’s fro a wide range of floral sources were screened against Escherichia coli (ATTC 14948) and Bacillus subtilis (ATCC 6633) using the broth microdilution method. Amongst the 37% considered “active” 37% were originated from buckwheat, 18% was from clover, and 12% were from blueberry, which is an indication that these floral sources produce phytochemicals which inhibit bacterial growth. The stability of the putative phytochemical(s) was analysed in ‘‘active’’ honeys (MIC90 6.25% v/v) by measuring the activity once every three to six months over a one to three year period. A significant decrease in activity was exhibited by both bacteria in the first three to six months of storage. This decrease harmonised with significant changes in chemical composition of honeys which included a major change in colour (p <0.0025), extremely consequential change in concentration of UV-absorbing compounds and appearance of melanoidins.

While these changes reduced E. coli responsiveness to honey, it rendered B. subtilis completely insensitive. Therefore, the data indicates that the presence of phytochemical(s) conferring the antibacterial activity is sensitive to storage. The de-regulation of the antibacterial activity with the associated appearance of melanoidins suggests that the active phytochemical components might be secluded into melanoidin aggregates, resulting in the loss of their function.