New Zealand Kanuka Honey Has High Levels of Methylglyoxal and Antimicrobial Activity

Dear Editor:

The antimicrobial properties of New Zealand manuka (Leptospermum scoparium) honey have been widely reported, and products with varying levels of manuka factor are popular. ¹ Little work has been undertaken on the properties and potential medical applications of New Zealand kanuka (Kunzea ericoides) honey, and the information that is available suggests that it may have similar properties. ^1,2 Therefore, we determined the basic physicochemical and antimicrobial properties of New Zealand kanuka honey.

Samples of both raw honey and medical-grade honey were tested, the latter being prepared in a two-stage process of filtration and gamma irradiation. Honey must be medical-grade for topical use because the many impurities in raw honey can cause inflammatory reactions.

The testing was undertaken by Industrial Research Limited, Wellington, New Zealand. Methylglyoxal (MGO) content was determined using a variation of the high-pressure liquid chromatography method published by Weigel et al. ³ For this work, chromatography was carried out using a Water (USA) UPLC chromatographic system and Waters BEH C18 2×250-mm 1.7 μm C18 column with an oven temperature of 35°C. The antimicrobial activities were tested by following the Kirby-Bauer disk diffusion antibiotic sensitivity testing method described by Jorgensen and Turnidge ⁴ with the modifications used for honey samples, except that wells were punched with sterilized 0.5-cm-diameter Spluggers and then loaded with honey samples. The pH of honey samples was determined by a variation of the method published by Feás et al. ⁵ For this study, 5 g of honey was diluted with 20 mL of distilled water and mixed thoroughly (as per the Association of Official Agricultural Chemists [AOAC] Official Method 962.19) before the pH of the resultant solution was measured using a digital pH meter (Blueline 15 pH probe, Schott, Germany; and SevenEasy pH meter, Mettler Toledo, Switzerland).

For several samples, the pH was approximately 3.6, with no difference between raw and medical-grade honey. In terms of antimicrobial activity, the average diameters of inhibition zones for 3 micro-organisms are shown in Table 1. A range of dilutions from 10% to 50% were also tested, and none of these diluted samples had any antimicrobial activity. MGO has been shown to be responsible for the pronounced antimicrobial activity of manuka honey. ⁶ The mean of three analyses of MGO was 1024 mg/kg for raw kanuka honey and 1154 mg/kg for medical-grade kanuka honey. These are high levels and would be approximately equal to a commercially available manuka honey with a Unique Manuka Factor rating of 40. ⁶

Therefore, New Zealand kanuka honey has antibacterial activity (including against methicillin-resistant Staphylococcus aureus), antifungal activity, and high levels of MGO. It is likely to have therapeutic uses in humans, and clinical trials are warranted.