**2.2. Lepteridine**

Lepteridine is also uniquely derived from the *Leptospermum* nectar in New Zealand and appears to be present in all manuka honeys. This compound has been quantified by liquid chromatography, and is principally responsible for the fluorescence exhibited by manuka honey at MM2 wavelengths [26]. Lepteridine has also been shown to be chemically stable in elevated storage temperature [26] and has been proposed as an additional chemical and fluo‐ rescence marker for manuka honey.

The concentration of Lepteridine in field collected manuka honey is reported to be in the range between 5 and 50 mg/kg [26] and has not been examined in commercial samples pre‐ viously. Concentrations as low as 1 ppm were detected by using the reported fluorescence method.

Again the field collected manuka honeys (*n* = 27) and the commercial samples (*n* = 17) dis‐ played similar characteristics. The concentration of Lepteridine was correlated linearly (*R*<sup>2</sup> = 0.9433) with the fluorescence signal at MM2 (**Figure 3**). Nonetheless, the commercial samples contained significantly lower concentrations of Lepteridine than the field samples, being a mean value of 28 and 6 mg/kg, respectively (*p* < 0.0001) which is a fourfold difference. Furthermore, seven of the commercial samples contained less than the reported lower con‐ centration of 5 mg/kg; four of which contained 4 mg/kg Lepteridine.

**Figure 3.** Correlation between Lepteridine concentration and MM2 fluorescence in field and commercial manuka honeys.

#### **2.3. Correlation of MM1 and MM2 fluorescence signal**

uted throughout the range of recorded concentrations and signal. These observations rein‐ force the earlier findings that Leptosperin is a reliable fluorescence marker in manuka honey

Lepteridine is also uniquely derived from the *Leptospermum* nectar in New Zealand and appears to be present in all manuka honeys. This compound has been quantified by liquid chromatography, and is principally responsible for the fluorescence exhibited by manuka honey at MM2 wavelengths [26]. Lepteridine has also been shown to be chemically stable in elevated storage temperature [26] and has been proposed as an additional chemical and fluo‐

The concentration of Lepteridine in field collected manuka honey is reported to be in the range between 5 and 50 mg/kg [26] and has not been examined in commercial samples pre‐ viously. Concentrations as low as 1 ppm were detected by using the reported fluorescence

Again the field collected manuka honeys (*n* = 27) and the commercial samples (*n* = 17) dis‐ played similar characteristics. The concentration of Lepteridine was correlated linearly

**Figure 3.** Correlation between Lepteridine concentration and MM2 fluorescence in field and commercial manuka honeys.

centration of 5 mg/kg; four of which contained 4 mg/kg Lepteridine.

 = 0.9433) with the fluorescence signal at MM2 (**Figure 3**). Nonetheless, the commercial samples contained significantly lower concentrations of Lepteridine than the field samples, being a mean value of 28 and 6 mg/kg, respectively (*p* < 0.0001) which is a fourfold difference. Furthermore, seven of the commercial samples contained less than the reported lower con‐

and can be used to categorise the national crop.

rescence marker for manuka honey.

**2.2. Lepteridine**

100 Honey Analysis

method.

(*R*<sup>2</sup>

The fluorescence generated by Leptosperin and Lepteridine at MM1 and MM2 display a rela‐ tively strong linear correlation in manuka honeys (*R*<sup>2</sup> = 0.8620). This indicates a degree of colinearity between these compounds. Whilst this may limit the use of both compounds in model development using data from more traditional techniques such as liquid chromatog‐ raphy or mass spectrometry, the use of two independent wavelength pairs in fluoro‐spectros‐ copy is expected to considerably strengthen this technique.

The fluorescence signal at both marker wavelengths in manuka field honeys (*n* = 27) is illus‐ trated along with the commercial samples (*n* = 17) (**Figure 4**). The commercial sample mean fluorescence was significantly lower than the field samples for both MM1 (*p* < 0.0001) and MM2 (*p* < 0.001), and this reflects the concentration of Leptosperin and Lepteridine that are present in these commercial honeys.

**Figure 4.** Correlation between MM1 and MM2 fluorescence signal in field and commercial manuka honeys.
