Simple and Rapid Evaluation of the Unique Manuka Factor in Manuka Honey Using Fluorescence Fingerprints and Principal Component Analysis

Formation of Protein-Bound Maillard Reaction Products during the Storage of Manuka Honey

Honey from the nectar of the Manuka tree (Leptospermum scoparium) grown in New Zealand contains high amounts of antibacterial methylglyoxal (MGO). MGO can react with proteins to form peptide-bound Maillard reaction products (MRPs) such as Nε-carboxyethyllysine (CEL) and "methylglyoxal-derived hydroimidazolone 1" (MG-H1). To study the reactions of MGO with honey proteins during storage, three manuka honeys with varying amounts of MGO and a kanuka honey (Kunzea ericoides) spiked with various MGO concentrations up to 700 mg/kg have been stored at 37 °C for 10 weeks, and the formation of protein-bound MRPs has been analyzed via high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS) following isolation of the protein fraction and enzymatic hydrolysis. During storage, contents of protein-bound CEL and MG-H1 increased continuously, directly depending on the MGO content. For honeys with large amounts of MGO, a slower formation of Nε-fructosyllysine (FL) was observed, indicating competing reactions of glucose and MGO with lysine. Furthermore, the lysine modification increased with storage independently from the MGO concentration. Up to 58-61% of the observed lysine modification was explainable with the formation of CEL and FL, indicating that other reactions, most likely the formation of Heyns products from lysine and fructose, may play an important role. Our results can contribute to the authentication of manuka honey.

Non-Targeted Detection and Quantification of Food Adulteration of High-Quality Stingless Bee Honey (SBH) via a Portable LED-Based Fluorescence Spectroscopy

Stingless bee honey (SBH) is rich in phenolic compounds and available in limited quantities. Authentication of SBH is important to protect SBH from adulteration and retain the reputation and sustainability of SBH production. In this research, we use portable LED-based fluorescence spectroscopy to generate and measure the fluorescence intensity of pure SBH and adulterated samples. The spectrometer is equipped with four UV-LED lamps (peaking at 365 nm) as an excitation source. Heterotrigona itama, a popular SBH, was used as a sample. 100 samples of pure SBH and 240 samples of adulterated SBH (levels of adulteration ranging from 10 to 60%) were prepared. Fluorescence spectral acquisition was measured for both the pure and adulterated SBH samples. Principal component analysis (PCA) demonstrated that a clear separation between the pure and adulterated SBH samples could be established from the first two principal components (PCs). A supervised classification based on soft independent modeling of class analogy (SIMCA) achieved an excellent classification result with 100% accuracy, sensitivity, specificity, and precision. Principal component regression (PCR) was superior to partial least squares regression (PLSR) and multiple linear regression (MLR) methods, with a coefficient of determination in prediction (R2p) = 0.9627, root mean squared error of prediction (RMSEP) = 4.1579%, ratio prediction to deviation (RPD) = 5.36, and range error ratio (RER) = 14.81. The LOD and LOQ obtained were higher compared to several previous studies. However, most predicted samples were very close to the regression line, which indicates that the developed PLSR, PCR, and MLR models could be used to detect HFCS adulteration of pure SBH samples. These results showed the proposed portable LED-based fluorescence spectroscopy has a high potential to detect and quantify food adulteration in SBH, with the additional advantages of being an accurate, affordable, and fast measurement with minimum sample preparation.

Methylnigakinone content determination and geographical origin discrimination for P. quassioides via fluorescence fingerprint and principal component analyses

Picrasma quassioides is used as a bittersweet stomach medicine. Because it is a natural product obtained from various geographical regions, the production area is important when P. quassioides is used as a crude drug. Herein, we developed a method to determine the content of methylnigakinone, one of the major active ingredients in P. quassioides, and a protocol for discriminating the geographical origin of this natural product using a fluorescence fingerprint analysis and principal component analysis (PCA). Because methylnigakinone is fluorescent (excitation wavelength: 352 nm, emission wavelength: 458 nm), the content of this molecule can be determined in the concentration range of 0.1-1 μg/mL. The quantification results of methylnigakinone obtained using the developed method were similar to those obtained from an HPLC analysis. Furthermore, the PCA of the fluorescence fingerprint of P. quassioides produced a score plot with the three different geographical origins (Kyushu island (Japan), Shikoku island (Japan), and China) plotted in the regions. Thus, it was possible to discriminate the geographical origin of the P. quassioides samples. The developed method is simple, quick, and has a minimal environmental impact. Therefore, the developed method will be useful for confirming the origin of P. quassioides.